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Time-Resolved X-Ray Magnetic CircularDichroism A Selective Probeof Magnetization Dynamics on NanosecondTimescales

Stefania Pizzini, Jan Vogel, Marlio Bonfim, and Alain Fontaine

Laboratoire Louis Neel (CNRS),25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 09, France{pizzini,vogel,fontaine}@grenoble.cnrs.fr

Abstract. Many synchrotron radiation techniques have been developed in the last15 years for studying the magnetic properties of thin-film materials. The most at-tractive properties of synchrotron radiation are its energy tunability and its timestructure. The first property allows measurements in resonant conditions at an ab-sorption edge of each of the magnetic elements constituting the probed sample, andthe latter allows time-resolved measurements on subnanosecond timescales. In thisreview, we introduce some of the synchrotron-based techniques used for magnetic in-vestigations. We then describe in detail X-ray magnetic circular dichroism (XMCD)and how time-resolved XMCD studies can be carried out in the pump-probe mode.Finally, we illustrate some applications to magnetization reversal dynamics in spin

valves and tunnel junctions, using fast magnetic field pulses applied along the easymagnetization axis of the samples. Thanks to the element-selectivity of X-ray ab-sorption spectroscopy, the magnetization dynamics of the soft (Permalloy) and thehard (cobalt) layers can be studied independently. In the case of spin valves, thisallowed us to show that two magnetic layers that are strongly coupled in a staticregime can become uncoupled on nanosecond timescales.

1 Introduction

In recent years, the number of studies of magnetization reversal in thin filmsand nanostructures on nanosecond and subnanosecond timescales has in-creased considerably, motivated by the fast evolution of high-density mag-netic recording technology. In magnetic recording, the dynamics intervenesin both the read and the write processes. The core of the write head consistsof a small electromagnet that can deliver field pulses of the order of 1 T ina magnetic gap of 50 nm 200 nm. Bandwidths today are of the order of300 MHz which allows creating pulses less than 3 ns long to magnetize stor-age media. Magnetic bits are written if the pulsed field amplitude exceeds the

dynamic coercivity of the media. The fast switching of bit magnetization is Present address: Universidade Federal do Parana, Centro Politecnico CP 19011,

Curitiba - PR CEP 81531-990, Brazil

B. Hillebrands, K. Ounadjela (Eds.): Spin Dynamics in Confined Magnetic Structures II,Topics Appl. Phys. 87, 155185 (2003)c Springer-Verlag Berlin Heidelberg 2003


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156 Stefania Pizzini et al.

essential to ensure a high data transfer rate. The read heads use magneticallysensitive elements, typically spin valves, that exploit the giant magnetoresis-tance (GMR) effect [1,2]. The heads are placed close to the rotating mag-netized storage disk, therefore exposing the GMR element to the stray fieldof the magnetic bits. The typical bandwidth for the read head is also of theorder of 300400 MHz, corresponding to reversal times of the soft layer of lessthan 23 ns. If the present 40% per year increase in the data transfer rate ismaintained, the intrinsic magnetic response times could become a potentiallimitation to the ultimate bandwidth of magnetic data storage [3,4].

Most of the dynamic studies appearing in recent literature are dedicatedto Permalloy layers of (sub)micrometric size. This is related to the importanceof spin valves and tunnel junctions, which consist of two ferromagnetic layersseparated, respectively, by a nonmagnetic metallic or insulating layer. Usu-

ally, Permalloy layers constitute the soft, reactive element of these trilayers.The importance of nanostructures is explained by the evolution expected inthe magnetic recording industry toward using nanopatterned media in whicheach dot will constitute a bit. Patterned systems of submicron dimensions arealso the core of new magnetic random-access memories (M-RAM) which willprobably revolutionize the storage industry. The heart of these memories isa tunnel junction, where information is stored in the magnetization directionof the soft magnetic layer [5].

In applications to magnetic recording devices, one of the main subjects ofinterest is to predict and minimize the magnetic field and the time needed toreverse the magnetization of thin-film layers. Investigations of the switchingtimes of thin-film elements were initiated in the 1960s and continue today.In 1960, Suits and Pugh [6] applied magnetic field pulses of different dura-tions and consequently inspected the material by making magneto-opticalmeasurements to determine whether or not its magnetization had switched.Recently, Backet al. [7] performed a modern version of this experiment usinga much more sophisticated experimental approach. The relativistic electronbunches of the Stanford Linear Accelerator Center were used to generate ul-trashort magnetic field pulses. Finely focused electron pulses, which produce

picosecond-long magnetic fields of several tesla on a micrometer scale, wereused to reverse locally the magnetization of cobalt thin films. The inducedmagnetization reversal pattern was measured by spin-resolved scanning elec-tron microscopy. Because the field pulses were applied perpendicularly tothe magnetization direction, ultrafast reversal was induced by precessionalmotion of the magnetization vector around the applied field.

Clearly, the best way to study the dynamics of magnetization reversalexperimentally is to combine a high degree of spatial and temporal resolution.In this sense, the most fruitful experiments are at present the stroboscopic

magneto-optical microscopy studies of Freemans group at the Universityof Alberta [8]. Time resolution is obtained by synchronizing femtosecondlaser pulses with picosecond or nanosecond magnetic pulses produced by


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Time-Resolved X-Ray Magnetic Circular Dichroism 157

metallic strip lines. Images are taken in the scanning mode with typical spatialresolution of the order of 0.5 m. Using this method, the authors were ableto study, for example, the gyromagnetic effects intervening in the coherentreversal of the magnetization of micrometric Permalloy films [9,10]. Morerecently the effect of a transverse static field on the mode of reversal hasbeen studied in a micrometric Permalloy system excited by 10-ns magneticfield pulses applied in the easy magnetization direction [11]. The transversefield drastically changes the domain structure of the system and acceleratesthe reversal process.

Other experimental techniques have been used to study the average mag-netization dynamics of soft ferromagnetic layers. In Parkins group at IBM,the magnetization reversal dynamics of thin Permalloy films was studied bymeasuring the resistance of a tunnel junction on subnanosecond timescales.

The switching times were studied for several widths and amplitudes of ex-ternal magnetic pulses and compared with micromagnetic simulations basedon the LandauLifschitzGilbert equation. The oscillations observed in therelaxation curve were interpreted in terms of damped precession of the mag-netic moments around the hard magnetization axis [12]. In Hillebrands groupthe magneto-optical Kerr effect was used to measure magnetization dynamicsin a gadolinium gallium garnet (GGG) after the application of nanosecondcurrent pulses noncollinear with the magnetization direction. The authorsshowed that, to minimize switching time, the magnetization precession af-ter termination of the magnetic field pulse needs to be suppressed. Thissuppression can be obtained by appropriate adjustment of the field pulseparameters [13,14]. Silvas group at NIST published one of the rare studiesof magnetization reversal of a high coercivity magnetic system (a CoCrTa4alloy) used as media for magnetic recording [15].

These studies show that the magnetization dynamics of micron-size sys-tems on nanosecond timescales is a complex mixture of thermally activatedprocesses, nucleation of reverse domains and domain-wall propagation, andcoherent rotation. This is also shown by micromagnetic simulation [16] thathas become a powerful and reliable tool for these investigations.

In this chapter, we investigate nanosecond timescale magnetization rever-sal initiated by magnetic field pulses applied along the easy magnetizationaxis. In this case, thermally activated processes dominate. When the appliedmagnetic field keeps the energy barrier EA finite, reversal proceeds by ther-mally assisted hopping over this barrier. The relaxation times for the reversalfollow the NeelBrown law of the Arrhenius type = 0 exp(EA/kT) where0 is the average relaxation time in response to thermal fluctuations. One ofthe experimental consequences of thermally assisted reversal is the increase incoercivity (Hc) with magnetic sweep rate dH/dt. Phenomenological models

taking into account both domain-wall motion and nucleation processes [17,18]predict logarithmic dependence of the coercive field on the applied field sweeprate. These models, using a single relaxation time approximation, suppose


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that the energy barrier for magnetization reversal varies linearly with the ap-plied magnetic field. The activation volume the magnetization volume thatswitches for a single activation event [19] and the relaxation time can beobtained from measurements of Hc versus dH/dt. The Kerr measurementsby Raquetetal. [18] on Au/Co/Au sandwiches showed different linear ratesof change ofHc for low and high sweep rates dH/dt. The results of the mod-eling show that the two regimes are dominated, respectively, by domain-wallmotion (low dH/dt) and nucleation processes (high dH/dt). The value ofdH/dt for which the change of regime takes place depends strongly on thesystem under study. Camarero etal. [20], for example, investigated the dy-namic coercivity of thin cobalt layers in exchange-biased Co/NiO systems.The change of reversal regime was observed at a sweep rate of 10 MOe/ s

Fig. 1. Magnetization reversal dynamics of Co/NiO bilayers. (a) Longitudinal Kerrcurves measured for a NiO (25 nm)/Co (3 nm) bilayer along the easy axis for dif-

ferent applied field sweep rates. (b) Applied field sweep rate dependence of thecoercive field of exchange-coupled Co/NiO bilayers with 3 nm of Co and differentNiO thicknesses. The symbols are experimental data, and the lines are fits usinga phenomological model (see text and [20])


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for a Co(3 nm)/NiO(4 nm) bilayer (Fig. 1) and for decreasing values of largerNiO thicknesses. The large increase in coercivity that is generally obtainedfor high sweep rates (a factor of about 3 between dH/dt = 1 O e/ s anddH/dt = 30 MOe/ s in [20]) could become a serious challenge for write heads,even if high magnetization alloys were used in their magnetic circuits.

All of these studies have concentrated on single magnetic films or mi-crostructures, but real devices like spin valves and magnetic tunnel junctionsare complex heterostructures that include several ferromagnetic (FM) layers.For good understanding of the magnetization dynamics in these structures,the magnetization of the individual FM layers should be probed as well astheir mutual interaction. Magnetic coupling, either related to exchange anddipolar interactions or induced by surface roughness, strongly influences thestatic magnetization reversal of spin valves and tunnel junctions. The influ-

ence of magnetic coupling on magnetization dynamics has not been investi-gated because the experimental techniques used until now measure either thetotal magnetization or the average angle between the magnetizations of twoFM layers. The individual behavior of each layer is thus difficult to addressby using these techniques. For element selectivity, X-ray magnetic circulardichroism (XMCD) is the ideal technique to probe independently the mag-netization of separate FM layers. In the next section, we will introduce thistechnique and other X-ray based techniques used for magnetization studies.

2 Synchrotron Radiation Techniques for Magnetism

The interaction between visible polarized light and the magnetic and elec-tronic properties of a material have been known since the Faraday, Kerr,and Voigt effects were discovered in the nineteenth century. The extensionof these techniques to the X-ray energy range is much more recent. In 1972,De Bergevin and Brunel used a conventional X-ray source for the first X-raymagnetic scattering measurements [22]; magneto-optical effects in X-ray ab-sorption were first predicted in 1975 for the M2,3 edge of nickel [23]. In 1985,

much larger effects were predicted [24] and subsequently measured [25] in rareearths. These large magneto-optical effects manifest themselves around X-rayabsorption edges of magnetic elements [25,26,27]. The new synchrotron radi-ation techniques developed in the last 15 years exploit, in the best way, thehigh brilliance and the tunable polarization (linear, left, and right circularlypolarized) available in second- and especially third-generation synchrotronradiation storage rings. A good review of these studies can be found in [21].

Several synchrotron-based X-ray techniques have been developed forstudying magnetism, but very few have been applied so far to magnetiza-tion dynamics studies. This is surprising because synchrotron radiation hastwo properties that are particularly attractive for these investigations. Thefirst is the temporal structure of X-rays. Synchrotron radiation is generatedwhen bunches of electrons in a storage ring, moving at almost the velocity of


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light, are deviated by the magnetic field of a bending magnet or of an insertiondevice, which delivers a periodic magnetic field in a straight section of thering. The X-ray pulses keep the temporal structure of the electron bunches.Their repetition rate depends on the number of electron bunches and on thecircumference of the storage ring, and pulse duration depends on the lengthof the electron bunches. For example, at the European Synchrotron Radia-tion Facility (ESRF) the repetition rate in single bunch filling is 357 kHz (theseparation between two bunches is 2.8 s) and the duration of each bunch is100 ps. The pulsed character of X-rays allows using pump-probe techniques.The second advantage of synchrotron radiation is that the X-ray energy canbe tuned. This allows measurements in resonant conditions at the absorptionedge of each of the magnetic elements constituting the probed sample.

The first study of magnetization dynamics using synchrotron radiation

was based on photoelectron emission [28]. The measurements were performedat the second-generation synchrotron radiation source SuperACO-LURE inOrsay (France). The magnetization state of a Fe surface was monitored usinga Mott detector to measure the spin polarization of the secondary electronsemitted after illumination with non monochromatized X-rays. Real-time andpump-probe measurements were performed. In real-time measurements, thespin polarization was measured as a function of the external field withoutsynchronization of field and X-rays. In this case, the temporal resolution of120 ns was determined by the repetition rate of the X-ray bunches, 8.333 MHzin the two-bunch mode at SuperACO. In the pump-probe mode, where theexternal field pulses were synchronized with the X-ray pulses, the spin po-larization was measured as a function of the delay between field and X-raypulses; resolution was given by the pulse length of 500 ps. Because polychro-matic X-rays were used, this technique was not element-selective, but surfacesensitivity was provided by the low escape depth of secondary electrons. Us-ing this property, the authors were able to demonstrate a difference betweenthe magnetization dynamics of the surface and the bulk in thin layers of Fedeposited on an amorphous soft-ferromagnetic substrate [29].

More recently, the element-selectivity of time-resolved X-ray magnetic

circular dichroism (XMCD) was used to demonstrate the different magne-tization dynamics of the soft and hard ferromagnetic layers of spin valvesand tunnel junctions [30]. These measurements will be discussed in detailin Sect. 4. Among the synchrotron radiation techniques developed in recentyears, X-ray resonant magnetic scattering (XRMS) should provide a magneticcontrast large enough to allow time-resolved measurements. This techniqueand XMCD are discussed next.

2.1 X-ray Resonant Magnetic Scattering

An incoming X-ray beam can be scattered from a sample surface in differ-ent ways. If the photon wavelength matches some typical length scale of the


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sample material (the lattice spacing, the periodicity of a multilayer, the peri-odicity of a regular domain structure), Bragg or coherent diffraction can takeplace. Specular reflectivity is possible for all wavelengths.

The first nonresonant X-ray magnetic scattering experiments were per-formed by De Bergevin and Brunel in 1972 [22]. In an antiferromagnetic NiOsingle crystal, the authors observed two superlattice X-ray diffraction peaksthat disappeared above the Neel temperature. Their intensities, 4 108smaller than crystallographic Bragg peaks, agreed with those evaluated fromphotonelectron spin interaction. With the advent of tunable X-ray radia-tion, it was discovered that these effects are largely enhanced close to anabsorption edge of the magnetic element. The first X-ray resonant magneticscattering (XRMS) experiments were performed in the 410 keV energy re-gion by Gibbs etal. [26]. At these energies, the X-ray photon wavelengths

are in the range 0.30.1 nm, making it possible to match diffraction condi-tions for typical lattice spacings or short range order more generally. Theavailable absorption edges in this energy range are the K edges (1s 4ptransitions) of 3d transition metals (TM) and the L2,3 edges (2p 5d tran-sitions) of rare earths (RE), whose probe states are not directly responsiblefor the magnetism of these elements. The magnetic effects on X-ray scatteringare thus relatively small, even in resonant conditions. Much larger magneticeffects were subsequently observed at core resonances located in the softX-ray region (502000 eV) for 3d transition metals (TM) [27] and rare earths(RE) [31]. In this case, the magnetic orbitals are directly involved in thescattering process (3p, 2p 3d resonances in TM and 4d, 3d 4f in RE).Several soft X-ray experiments have since been performed in a diffractionmode, using multilayers where the chemical modulation period matches theappropriate photon wavelength [32,33,34]. Off-specular magnetic scatteringhas been used recently to address interesting problems like magnetic rough-ness [35] and order in closure domains [36]. In the latter experiment, it wasshown that it is possible to obtain information on magnetic domain structuresin thin films. Polarized X-rays were scattered off FePd thin films in which themagnetization self-organizes in magnetic stripe domains. These structures act

as a magnetic grating that produces well-defined satellites around the specu-lar reflected beam. By varying the angle of incidence, it was shown that onecan also obtain information on the domain structure below the surface, whichis very important because most domain imaging techniques probe only themagnetic profile of the stray fields just above the sample surface.

Recently, several groups have used the partial coherence of X-ray beamsin third-generation synchrotron radiation sources to observe magnetic speck-les [37]. In the hard X-ray range, magnetic speckles had already been reportedon a Bragg peak of UAs [38]. In the soft X-ray range, the first published exper-

iments were carried out on amorphous GdFe2 thin films [37]. The X-ray beamwas incident normally on a 35-nm thick film of amorphous GdFe2 showinga pattern of 110-nm wide meandering magnetic stripe domains. Off reso-


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nance, only the transmitted beam was observed. By tuning the X-ray energyto the Gd M5 resonance ( = 1.1 nm), the magnetic scattering cross-sectionwas enhanced by several orders of magnitude [39], and a clear first-order anda much weaker third-order magnetic diffraction ring appeared. The inten-sity in the ring showed strong spatial fluctuations corresponding to staticmagnetic speckles, a result of diffraction by a limited number of disorderedmagnetic domains in the coherent scattering volume. These speckles containall spatial information inside the illuminated volume on scales between thewavelength and the coherence length of the light. To retrieve the spatial in-formation, one has to measure the complete scattered field, i.e., intensity andphase. In principle, phase information can be measured only by mixing thescattered field with a reference beam, as done in holography, or by using anobjective lens. Algorithms exist to retrieve the phase information [40] and

have been used with some success.The most interesting application of speckles, however, lies in dynamic light

scattering. By measuring the time dependence of the intensity at a particularscattering angle during and after a magnetic field pulse, one could obtain thetime/field dependence of the spatial correlation length corresponding to thatangle.

In the specular reflectivity mode and for soft X-rays, the informationobtained is similar to that extracted from X-ray absorption (XAS) measure-ments. The absorption is given by the imaginary part of the complex refrac-tive index of the measured material. Real and imaginary parts are connectedthrough the KramersKronig relation, and reflectivity depends on both ac-cording to Fresnels equations. Due to the relatively large penetration andescape depth of X-rays, thin buried layers can be measured by this technique.It has been shown, for example, that magnetic moments for thin Fe layersdown to 2 A can be measured under 85 A of Cu [42].

Resonant reflectivity of soft X-rays depends strongly on energy and can berelatively large at a very grazing incidence and decrease fast going to largerangles. At the L3 edge of Ni, for example, the reflectivity goes from 50%at a grazing angle of 1 to

0.001% at 20 [41]. However, when circularly

polarized X-rays are used, asymmetry for different directions of magnetizationcan be very large, up to 80% of the signal [41]. Therefore, this signal couldbe exploited for time-resolved magnetization measurements. An advantage ofthis technique, as for all XRMS techniques, is that only photons are involvedin the process. Measurements can, therefore, be made under strong and/ortime-dependent magnetic fields and on insulating materials.

2.2 X-ray Magnetic Circular Dichroism

X-ray magnetic circular dichroism (XMCD) has the exceptional property ofbeing able to discern between the contributions to the total magnetizationof the different atoms of a complex sample and between the spin and or-bital components. For a magnetic material, XMCD is the difference between


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the absorption of left and right circularly polarized X-rays. This differencecan be related to the magnetic moment of the atoms involved in the absorp-tion process. In the soft X-ray range, the cross sections for X-ray absorptionspectroscopy (XAS), where a photon is absorbed by an atom to transfer anelectron from a core state to an empty state above the Fermi level, are ingeneral large. The L2,3 edges of TM and the M4,5 edges of RE have cross sec-

tions of the order of 103 and 104 barn/atom (1 barn = 1024 cm2 = 104 A2

),whereas only 110 barn/atom is typical for neutron scattering. The absorp-tion edges have energies that are characteristic for each element, and, due todipole selection rules, final states with different symmetries can be probedby choosing the correct initial state. The ensemble of these characteristicsmakes XAS a technique that can probe submonolayer samples in relativelyshort times (a few minutes) with element- and symmetry-selectivity.

The simplest description of X-ray absorption spectra employs a single-electron model. In this picture, the core electron is excited to an unoccupiedcontinuum state of the system. According to Fermis Golden Rule, the tran-sition probability per unit time from a bound state to a continuum state canbe written as

w =2

h|k|P|c|2f(h Ec) (1)

where |k|P|c| is the matrix element of the electromagnetic field operator Pbetween core state

|c

and valence state|k. In the electric dipole approxi-

mation, P = p e, where e is the polarization vector and p is the electronmomentum operator. f is the density of valence states at energy Eabove theFermi level, and Ec is the coreelectron binding energy. Because the electricdipole operator is odd and acts only on the radial part of the electronic wavefunction, transitions can be made between states that have opposite parityand differ in angular momentum by one: l = 1 with s = 0. These arethe dipole selection rules. For instance, K edges (initial 1s state) have p finalstates and L2,3 edges have s and d final states. The core state |c is stronglylocalized on the nucleus, so the matrix element is also local to the atom.

In 1975, Erskine and Stern [23] predicted that if XAS is performed withpolarized light, it could supply magnetic information about the initial stateof the absorption process. Thole etal. [24] showed that for a rare-earth ionwhose ground state is split by a magnetic field, the line shape of the M4,5absorption edges depends on the relative orientation between the magneti-zation direction and the polarization vector of the X-rays. The experimentalproof of X-ray dichroism was given by Van der Laanetal. [25] who measuredterbium iron garnets by using linearly polarized light. The first experimentswith circularly polarized X-rays were performed in the hard X-ray range,

where Schutz et al. [43] found a small circular dichroism ( 0.1% of the totalsignal) at the K edge of Fe. In 1990, Chenet al. measured XMCD at the L2,3edges of Ni [44], Co, and Fe [45] and found effects as large as 20%. This is dueto the fact that transitions at these edges take place directly to the empty


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3d states, which are strongly polarized. XMCD has been largely used in thelast 10 years to measure the magnetism in thin TM layers, multilayers andin rare earths.

An intuitive way of understanding the presence of circular dichroism inthe X-ray absorption spectra of magnetic elements is the two-step modelproposed by Stohr and Wu [46] for the L2,3 edge of transition metals (Fig. 2).In the first step, spin polarized photoelectrons are excited from the 2p levelby a polarization that depends on the edge (L3 or L2). Using the selectionrules on orbital angular momentum (ml = +1 for left, and ml = 1 forright circular polarization), it can be demonstrated that at the L2 edge, left(right) circularly polarized light excites 25% (75%) spin-up electrons and 75%(25%) spin-down electrons. At the L3 edge, left (right) circularly polarizedlight excites 62.5% (37.5%) spin-up electrons and 37.5% (62.5%) spin-down

electrons. Because the transition probability depends on the density of emptystates in the d band, in the second step, the d band acts as a spin detector. Ina nonmagnetic material, the absorption of left and right circularly polarizedlight is the same. As soon as an unbalance exists between the number ofavailable spin-up and down states, the absorption of the two polarizations isdifferent; the difference is opposite for the L2 and L3 edges.

Comparing their XMCD data on Ni films with band structure calcula-tions, Chenet al. found that they could get values for the orbital and spinmagnetic moment which agreed well with values obtained by using other

Fig. 2. Two-step model for XMCD. In the first step, spin polarized electrons are

excited at the 2p level with a polarization depending on the edge (L3 or L2) and onthe polarization of the light. In the second step, the d band acts as a spin detector,depending on the number of available empty states of a given spin


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techniques [47]. Thole et al. [48] showed that the integrated intensities of theXMCD and of the total absorption spectra can be used directly to obtainthe values for the orbital and the spin moments of the measured final-stateelectronic shell.

From an experimental viewpoint, the XMCD signal of a ferromagneticlayer can be obtained as the difference of two absorption spectra measuredwith right and left circular polarizations, or equivalently as the difference oftwo spectra taken with fixed helicity but opposite magnetization direction.The latter condition is obtained by applying, alternately, an external field ofopposite direction. Often, to reduce the effects of beam instability and de-tection asymmetries, a combination of the four spectra is used to obtain theXMCD signal. X-ray absorption spectra in the soft X-ray range are usuallymeasured by detecting the total electron yield (which is proportional to ab-

sorption) or the fluorescence yield. The application of the sum rules [48] tothe XMCD signal allows us to obtain the average magnetic moments, pro-

jected in the direction of the incoming X-ray beam, of the different elementsof the probed sample. The approximations involved in the applications of thesum rules are such that orbital and spin moments can be calculated witherror bars of the order of 10%.

3 Time-Resolved X-Ray Magnetic Circular Dichroism(XMCD) Experiments

In this section, we describe how the temporal structure of synchrotron radi-ation can be used for XMCD measurements to obtain the element-selectivemagnetization dynamics of thin films. Our measurements were carried outin the pump-probe mode by synchronizing the magnetic field pulses createdby microcoils with the X-ray pulses of the European Synchrotron RadiationFacility (ESRF) in single bunch filling [30,49,50]. The microcoils and theexperimental method will be described in detail.

3.1 Microcoils for High Pulsed Fields

The generation of high-frequency magnetic field pulses to be used as a sourceof excitation of magnetic materials has been long the subject of theoreticaland applied investigations. Today, this field is of particular interest for appli-cations like write heads for magnetic recording that use short magnetic pulsesto magnetize storage media. The standard method for generating a magneticfield is to send a current pulse through a good conductor. When fields perpen-dicular to the sample surface are needed, the traditional coil is used. In this

case, the field is inhomogeneous across the coil surface. Strip-line geometry isadapted to in-plane fields, and in this case, the fields are much more homoge-neous, but their amplitudes are well below those produced with coils. Write


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heads use magnetic coils wound around a soft ferromagnetic material to con-centrate the field in a small region of the order of 50 nm 200 nm. By usingthis method, the maximum field is limited by the saturation magnetizationof the magnetic material (about 2 T for FeCo alloys).

At the Laboratoire Louis Neel, we have developed microcoils and powersupplies that can deliver magnetic fields of large amplitude (up to severalteslas), high repetition rates, and short rise times for applications to time-resolved Kerr and XMCD experiments [51]. For perpendicular fields, single-turn copper coils electrodeposited on silicon substrates were fabricated to de-liver very high magnetic fields (Fig. 3a). Silicon is the ideal substrate becausemost micromachining techniques have been developed around this materialand copper can be easily electroplated in thick films. Silicon is a relativelygood heat sink for copper, and adhesion of copper onto oxidized silicon is quite

good. The details of the fabrication of coils are reported in [51]. A first seriescopper coils 7 m thick, inner diameter 50 m, and outer diameter 150 mwas fabricated and tested. These coils could not support currents greaterthan 1500 A; they were rapidly destroyed both by the heating and the largestress. The second series of coils was 30 m thick and had a very large outerdiameter (5 mm) to increase adhesion. Radial slits were introduced into thecopper to reduce the effective diameter for the current to 150 m. The restof the copper acts as mass to increase the mechanical inertia and to helpdissipate heat between pulses. This geometry allowed higher levels of current(up to 4000 A for 30 ns) without damage. The field is inhomogeneous, and toguarantee field variations of less than 5%, the probed sample region has tobe included within a radius of 15 m from the center.

The U-shaped strip-line geometry shown in Fig. 3 was used to producelongitudinal pulsed fields. The sample is mounted inside the U to maximizethe field strength. The field is much more homogeneous than that in theprevious case.

Fig. 3. Schematic view of two types of microcoils: (a) 50-

m single-turn microcoilfor a perpendicular pulsed field and (b) U-shaped strip line for a longitudinal field


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Power supplies for pulsed currents were developed to operate on these twotypes of coils. To produce very high magnetic field pulses (up to 50 T), thecoil was placed in a RLC (resistor-inductor-capacitor) circuit, and the pulsewas produced by capacitor discharge, which was controlled by a spark gap.The capacitor bank consisted of twelve 1-nF ceramic capacitors connected inparallel between two copper plates. The spark gap consisted of two copperplates separated by a mixture of Ar and SF6. The relative concentrationsof the two gases determined the breakdown voltage and therefore the totalcharge stored in the capacitor. Currents of the order of 3500 A were sent inthe 50 m diameter coil to obtain fields up to 50 T in pulses 30 ns long [51].

The power supplies for the time-resolved XMCD experiments, which donot need extremely high fields, use MOSFET transistors as commuting ele-ments [49,50]. These are associated with a source of variable tension and re-

sistances that define the final current value. The major source of constraintsfor the ESRF measurements is the power dissipated by the circuit, which hasto be evacuated to avoid overheating. For this reason, the components weresoldered to an epoxy plate covered with a thick layer of an Al conductor. Thealuminum allows the heat to be transferred efficiently between the compo-nents of the circuit and the cooling element. This also acts as an electricalground for the circuit. Using water cooling and dissipated power of 7 W/ cm2

(Ipulse = 70 A, 30 ns, 357 kHz), the temperature of the electronic componentsdoes not exceed 70 C. The unipolar power supply model most adapted tothese applications supports up to 70 A at 357 kHz. The corresponding max-imum magnetic field for a U-shaped coil with an 0.8-mm slit is 56 mT witha maximum slope of 8 mT/ ns. A bipolar power supply was also developedto deliver fields up to 1.5 T using the 50-m microcoil. The rise time in thiscase is 80 ns.

3.2 Magnetization Dynamics Studied With XMCD

The magnetization state for each magnetic component of a probed thin-film system during and after a magnetic pulse generated by a microcoil is

determined by using XMCD. The field and the X-ray pulses for pump-probemeasurements are synchronized by using the radio-frequency signal of themachine (352.2 MHz) which one can access on each ESRF beam line.

In the single bunch mode at the ESRF, the frequency of revolution of theelectrons in the storage ring corresponds to this frequency divided by 992(357 kHz). The synchronization is accomplished by an electronic frequencydivider which gives a TTL output compatible with the input of the powersupply of the microcoils (Fig. 4). The phase is changed by imposing a divisionby 991 (go one step forward) or by 993 (go one step back) during a single

2.8-s period. This allows placing the magnetic pulse anywhere within the2.8 s time range between two photon pulses; a resolution of 10 ps is deter-mined by a delay generator inserted between the frequency divider and thepulsed-current power supply.


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Fig. 4. Schematic view of the synchronization between magnetic field and X-raypulses, using the radio-frequency signal of the ESRF in single bunch filling

To study magnetization dynamics in the application of magnetic fieldpulses, a series of XMCD measurements is carried out for several delaysbetween the pump and the probe to cover the time range from before themagnetic pulse to well after it. This is shown schematically in Fig. 5. Becausethe XMCD signal is proportional to the average magnetization of the probedmagnetic component, the result of each measurement represents one point inits dynamic response. This is repeated for all of the magnetic componentsof a complex system by adjusting the X-ray energy to the correspondingabsorption edge.

The time-resolved XMCD experiments were developed first on the hardX-rays energy-dispersive beam line (ID24) [49] and later on the soft X-rayrange beam line (ID12B) [30] of the ESRF. The K edges of first-row transi-tion metals and the L2,3 edges of rare earths, which probe the polarization ofthe 4p and 5d levels, respectively, are accessible in the hard X-ray range. Thecorresponding XMCD signals are small (some 103 of the total absorptionfor Fe, Co, and Ni; a few percent for rare earths) because the polarizationof the final states of the transition is weak. The feasibility of time-resolved

XMCD measurements was first demonstrated for a GdCo2.5 film, for whichthe magnetization dynamics after the application of pulses 22 ns long wasstudied for Gd at the Gd L3 edge [49]. Because the pulsed field was appliedalong a hard magnetization axis, the Gd magnetization turned by coherentrotation, and its amplitude in the pulse direction followed the field amplitudeexactly (Fig. 6). Time-resolved measurements at the Co Kedge, which wouldgive the dynamics of the 3d element, were not successful on ID24. Each spec-trum requires much longer acquisition times than the Gd L3 edge (typically,1 hour/spectrum) and excellent beam stability. A full set of measurementsdescribing the temporal evolution of magnetization could not be obtainedwithin the time between two refills of the machine (typically, 5 hours). Be-cause the major strength of XMCD is its element-selectivity, the impossibilityof obtaining both RE and TM dynamics strongly decreases interest in carry-


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Fig. 5. Schematic view of pump-probe XMCD measurements. Electrons circulate inthe storage ring at the speed of light. X-ray bunches reflect the temporal structureof electron bunches at the ESRF. They are separated by 2.8

s and they are 100 pslong. For a chosen absorption edge, the ensemble of the XMCD signals obtainedfor different delays (t0, t1, . . .) between pump and probe represent the dynamicresponse of the probed element to the applied field

Fig. 6. Magnetization reversal dynamics of a GdCo2.5 layer using the Gd L3-edgeXMCD. The field pulse is applied perpendicularly to the plane of the layer. Theinset shows the quasi-static hysteresis curve obtained with a vibrating sample mag-netometer (VSM)


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ing out time-resolved XMCD measurements in the hard X-ray range. Anotherlimitation is that the measurements have to be carried out in transmissiongeometry on thick layers (typically, 5 m). This obviously excludes the studyof all of the thin layer samples used for magnetic recording.

The soft X-ray range is a better choice for the study of magnetic 3delements, and in particular of thin multilayer systems like spin valves andtunnel junctions. The absorption cross sections as well as the XMCD signalsat the TM L2,3 edges are large so that very thin layers can be detected, andshort acquisition times can be used. For these reasons, an experimental setupwas developed to carry out XMCD measurements on the soft X-ray beamline of the ESRF (ID12B, now ID8).

A compact high-vacuum chamber was built to host the sample, the micro-coil, and the detection system. The sample is positioned at grazing incidence

in the 0.8-mm wide aperture of a U-shaped copper microcoil (see Sect. 3.1).X-ray absorption spectra are measured by fluorescence detection using a largesurface photodiode placed in front of the sample. A charged electron grid isplaced between the diode and the sample to prevent secondary electrons fromreaching the diode. In contrast to electron detection, fluorescence detectionallows studying deeply buried magnetic layers, and measurements can bedone in the presence of a varying magnetic field.

An electromagnet is mounted outside the vacuum chamber to providea static bias field for the dynamic measurements. This also allows measuringelement-selective hysteresis cycles. Chemical selectivity is obtained by tuningthe X-ray photon energy to an absorption edge of (one of) the element(s) ofthe layer of interest. The large dichroism at the TM L3 edges allows followingthe magnetic state of the layer by monitoring the absorption intensity withX-ray photon energy fixed at the L3 maximum, where XMCD asymmetry islargest. To determine magnetization dynamics, the delay between the mag-netic pulse and the photon bunch is varied. This delay scan is performed forthe two helicities of the X-ray beam. The difference between the two scansgives the L3-XMCD intensity versus time and therefore the time-dependenceof the magnetization of the probed layer. The ultimate temporal resolution is

limited by the X-ray bunch width (100 ps at the ESRF). All edges of interestand several amplitudes of the magnetic pulse are measured.

To obtain a good signal-to-noise ratio, acquisition times of the order of0.5 s per point, corresponding to several thousands of photons and field pulses,were needed. This multibunch detection implies that only reproducible phe-nomena can be measured. Therefore, for all of the measurements, the samplewas first magnetically saturated in the negative direction by a static field.A negative bias field was then constantly applied during the time-resolvedmeasurements to guarantee that the system returned to negative saturation

after each positive field pulse was applied.


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4 Magnetization Dynamics of Spin Valvesand Tunnel Junctions Studiedwith Time-Resolved XMCD

In this section, we present the results of time-resolved XMCD measurementsobtained for a series of Co/X/Ni80Fe20 spin valves and tunnel junctions (withX = Cu or Al2O3), either in the form of continuous thin films or patternedin micron-sized squares. The magnetic field pulses in all cases were appliedalong the easy magnetization axis of the FM layers. From measurements ofthe magnetization dynamics of the Co and the Ni80Fe20 layers, informationis obtained on their coupling on a nanosecond timescale. The samples inves-tigated here display a variety of magnetic coupling behaviors [30].

The Co/Cu/Ni80Fe20 spin valves [52] and the Co/Al2O3/Ni80Fe20 tun-

nel junctions [53] were grown, respectively, by molecular beam epitaxy andRF sputtering on vicinal Si(111) substrates misaligned toward the [112]direction [54]. A step bunching mechanism is activated by thermal treat-ment, yielding silicon surfaces made of flat terraces separated by facets madeof gatherings of steps. Transmission electron microscopy observations showthat the substrate topology is well transmitted to the deposited layers. Theroughness of the two interfaces are therefore correlated and in phase.

This situation is at the origin of magnetostatic (orange peel-like) ferro-magnetic coupling between the two magnetic layers [55]. The coupling coef-

ficient can be expressed by the equation,

J=

2

2Lh1h2M1M2e

2d

L , (2)

where d is the distance between ferromagnetic layers, L is the spatial period-icity of the roughness, Mi are the volume magnetizations of the FM layers,and hi the amplitudes of the roughness of the two interfaces. This formulagives the coupling for a regular sinusoidal roughness which is not found inthe step-bunched samples. In this case, the coupling is not homogeneous butis concentrated in the regions of the steps. This inhomogeneous coupling has

an influence on the magnetization dynamics, as we will show later on.All of the layers display in-plane uniaxial magnetic anisotropy with the

easy axis parallel to the steps. Patterned structures consisting of square dotsof 1.3-m sides and a period of 4 m were fabricated by Ar ion etching of thecontinuous Co/Al2O3/Ni80Fe20 films.

The absorption spectra and the XMCD signals obtained at the L2,3 edgesof Fe, Ni, and Co for a Co/Cu/Ni80Fe20 spin valve are shown in Fig. 7.Chemical selectivity is obtained for the time-resolved measurements by tuningthe X-ray photon energy to the L3 white line of Co (h= 778 eV) to study the

cobalt layer and to the L3 white line of Ni (h= 853 eV) for the Permalloylayer. The time-resolved XMCD measurements were carried out by using theprocedures described in Sect. 3.2; the pulsed field and the bias field wereapplied along the easy magnetization axis of the samples.


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Fig. 7. L2,3 absorption edges and XMCD signals for Fe, Co, and Ni ina Co/Cu/NiFe spin valve. The energies are marked for which the XMCD signalis maximum and which were chosen for time-resolved XMCD measurements. TheNi L3 edge was chosen for the Permalloy layer because the XMCD signal is largerthan that of Fe

4.1 Magnetization Dynamics of Tunnel Junctions

The cobalt and Permalloy layers in the magnetic tunnel junction of compo-

sition Co(15 nm)/Al2O3(2 nm)/Ni80Fe20(5 nm) display weak ferromagneticorange-peel type coupling with a coupling energy of about 4 mJ/m2. Thequasi-static coercive fields of the Permalloy and Co layers are, respectively,1.6mT and 3mT.

Figure 8 shows the temporal evolution of both Co and Ni80Fe20 mag-netization for 30-ns field pulses of different amplitudes. A 5-mT static biasfield was applied in the direction opposite to the pulsed field. Consideringthe 10-ns rise time of the field pulse, the initial dead time corresponds tothe time required to overcome the reverse bias field plus the coercive fieldof the Ni80Fe20 layer. After this regime, the evolution on the rising edge is

almost linear with time; the slope increases with the pulse amplitude. TheNi80Fe20 magnetization completely reverses for a field of 9.5 mT, whereas16 mT is needed to reverse the Co layer. The effective coercivities for theseshort pulses are about five times larger than the quasi-static coercivities.

Let us define the switching time as the time needed to cancel the magne-tization of the Co or of the Permalloy layers. A plot of the inverse of versusthe amplitude of the pulsed field (inset in Fig. 8) shows that the relationship

1 = S1w (HH0) (3)

is verified for both magnetic layers, with different Sw values. Here H is theapplied field, H0 is a constant field related to the coercivity, and Sw is a co-efficient related to magnetic viscosity.


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Fig. 8. Temporal evolution of the magnetization of the Ni80Fe20 layer (top) andthe Co layer (bottom) of a Co/Al2O3/Ni80Fe20 tunnel junction during and afterapplication of 30-ns long magnetic pulses of different amplitudes. A continuousbias field of5 mT was applied during the measurements. The field amplitudes inthe legends are (Hpulse + Hbias). Inset: plot of the inverse of the reversal time (1/)

versus the field amplitude on the rising edge of the field pulse

This empirical relationship holds with different switching coefficients Swfor several mechanisms of magnetization reversal ranging from domain-wallpropagation (lower 1) to uniform rotation (higher 1) [4,56]. The valuesof Sw deduced from our measurements (1.2 1011 s T for Ni80Fe20 and4.01011 sT for Co layers), compared with those reported in the literature,point to magnetization reversal dominated by nucleation of reversed domains.The possibility of reversal by coherent rotation is ruled out by the fact that

the magnetic response is strongly delayed with respect to the pulsed field.Extrapolation of the fits to 1 = 0 gives field values three to four timeslarger than the quasi-static coercive fields of the Co and Ni80Fe20 layers.This result is not surprising because a larger Sw value [and therefore a smallerslope of (3)] is expected for the low field (large ) range, where domain-wallpropagation dominates.

The different switching times measured on the falling edge of the pulsefor the two magnetic layers can be explained by (3). For the Ni80 Fe20 layer,the effective reverse field (Heff = Hbias +Hdip) is much larger than the static

coercive field, leading to fast reversal (less than 2 ns). Heff for cobalt is closerto the static coercivity, leading to slower dynamics.


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4.2 Static and Dynamic Couplingin Co/Cu/Ni80Fe20 Spin Valves

In spin valves of composition Co(5 nm)/Cu/Ni80Fe20(5 nm), the magnetiza-

tions of the two magnetic layers are strongly coupled for copper thicknessesless than 10 nm. The quasi-static magnetization cycles, measured by clas-sical magnetometry as well as by XMCD (top of Fig. 9), show that, forcopper thicknesses of 6 and 8 nm, Co and Ni80Fe20 reverse simultaneouslywith a static coercive field of 45 mT. For 10 nm of Cu, the magnetic layersbecome nearly uncoupled, and two separate hysteresis curves, with Hc of 3and 5 mT, are measured for Ni80Fe20 and Co layers, respectively.

Fig. 9. Static and dynamic measurements in Co(5 nm)/Cu/Ni80Fe20(5 nm) trilayers(tCu = 6, 8, and 10 nm). Top: Site-selective hysteresis loops measured by XMCD

for the Co (open dots) and Ni80Fe20 (squares ) layers. Bottom: Dynamic responseof the Permalloy layer (top) and cobalt layer (bottom) to fields (Hpulse + Hbias) ofamplitude 9 to 23 mT and width 30 ns (Hbias = 5mT)


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The results of time-resolved XMCD measurements obtained with 30-nspulses are shown at the bottom of Fig. 9. The 10-nm Cu sample displaysbehavior qualitatively similar to the magnetic tunnel junctions (MTJ) de-scribed above. The cobalt magnetization does not switch for pulses up to23 mT. The Ni80Fe20 switches completely for fields around 10 mT, and itsreversal speed increases with pulse amplitude. The two magnetic layers areclearly decoupled, as expected from quasi-static measurements.

For 6 nm of Cu, the Co and Ni80Fe20 layers switch in the same pulsed field,and the dynamic responses of the two layers are exactly superimposed. ThePermalloy layer magnetization relaxes with the same long switching times asthe Co layer, to which it is strongly coupled.

The 8-nm Cu sample displays the most interesting behavior. While in thequasi-static regime, the two FM layers reverse simultaneously in a field of

5 mT; with 30-ns pulses, the two metallic layers reverse in different fields.Similarly to the MTJ, on the rising edge of the pulse, the two layers reversewith different speeds; the Ni80Fe20 magnetization reverses well before that ofCo. On the falling edge, we retrieve slower magnetization decay for Co. Theseresults indicate that, on this timescale, the two layers are mostly uncoupled.The tail of the magnetization decay of Ni80Fe20 shows, however, that a frac-tion (about 15%) of its magnetization remains coupled to Co. These datashow that, whereas in the static regime, the Co and Ni80Fe20 magnetizationsare always parallel, for short pulses, there is a field range where a nearlyantiparallel configuration can be achieved.

As a complement to these measurements, the dynamic hysteresis cyclesof the Co(5 nm)/Cu(10 nm)/Ni80Fe20(5 nm) spin valve were measured [57]by the Kerr effect as a function of dH/dt (Fig. 10). In the quasi-static case,the minor hysteresis curves of the Ni80Fe20 layer are shifted by about 5 Oedue to the orange-peel type magnetic coupling with the Co layer. As dH/dtincreases, the Co and Permalloy coercivities increase, and the reversal transi-tions widen. The linear dependence ofHc on ln(dH/dt) and square hysteresiscycles are observed for a low sweep rate. This is consistent with magnetiza-tion reversal dominated by domain-wall propagation [18]. For sweep rates

larger than 10 MOe/ s, the cycles widen, and a change in the slope of Hcversus ln(dH/dt) is observed. In agreement with previous work [18,20], wesuggest that this is due to a change in the magnetization reversal process forhigher sweep rates, where nucleation of reversed domains starts to dominate.The two curves for Ni80Fe20 in Fig. 10 (right) give the HC values taken fromthe major (filled circles) and minor (open circles) hysteresis loops. The mag-

netic coupling HE is equal to (HmajC HminC ). Figure 10 shows that HE is

practically independent of dH/dt for low sweep rates and decreases drasti-cally starting from dH/dt values that correspond to the change of regime in

the magnetization reversal process. The direct XMCD observation, for theCo/Cu(8 nm)/Ni80Fe20 sample, of a much lower coupling in the nanosecond


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Fig. 10. Dynamic Kerr measurements on the Co/Cu(10 nm)/Ni80Fe20 spin valve.Both major and minor hysteresis cycles have been measured as a function of dH/dt,from 0.1 to 10MOe/ s. On the left, we show representative hysteresis cycles forvalues of dH/dt of 10 kOe/ s, 1MOe/ s, and 5 MOe/ s. On the right, we show thecoercive field for the Ni80Fe20 layer, as a function of ln(dH/dt), obtained from bothmajor and minor cycles. The shift of the minor curves, given by HmajC H

minC , is

also plotted

regime with respect to the quasi-static regime, is consistent with the resultsof these Kerr measurements.

A plausible explanation of this behavior may be given in terms of the inho-mogeneous orange peel-like coupling between the two FM layers across thecopper spacer [57]. For the 6-nm Cu sample, the orange-peel coupling betweenFM layers is very strong, and the layers reverse together in both the staticand dynamic regimes. For the 10-nm samples, much weaker coupling existsbetween Co and Ni80Fe20, and the two layers reverse independently in thetwo regimes. The 8-nm Cu layer is intermediate between these two extreme

situations. In the quasi-static regime, a few nucleation centers are generatedin Permalloy, and the rest of the layer is then reversed by propagation ofthe domain walls created. The domain walls are pinned at the step bunches,where the coupling is strong, until the Co layer switches as well. For 30-nspulses in the XMCD measurements, nucleation of reversed domains domi-nates. Many nucleation centers can be formed and all of the terraces, wherecoupling is weaker, can switch their magnetization. The Permalloy coerciv-ity is therefore smaller than that of Co. The 15% volume of Ni80Fe20 thatswitches with the Co would then correspond to the strongly coupled region

at the steps.


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4.3 Patterned Co/Al2O3/Ni80Fe20 Tunnel Junctions

As a last illustration of time-resolved XMCD, we focus on square magneticdots of sides 1.3 m patterned from the Co(15 nm)/Al2O3(2 nm)/Ni80Fe20

(5 nm) MTJ studied above. The main interest in this kind of patterned struc-ture is its possible application to nonvolatile magnetic memories (M-RAM).In contrast with the continuous film, the two FM layers display strong an-tiparallel coupling in a zero field due to the strong dipolar field acting throughthe sides of the squares. This antiparallel coupling is clearly shown in quasi-static hysteresis loops measured by XMCD for the two FM layers (inset ofFig. 11).

Time-resolved XMCD measurements were carried out for several valuesof the static bias field. Here, we report on the results obtained for a negative

bias of 8 mT after negative saturation of the Co layer (Fig. 11). This field cor-

Fig. 11. Dynamic response of the Permalloy and cobalt layers to pulsed fields ofincreasing amplitudes in the presence of a negative bias field of 8 mT for a series of1.3

m large square dots of Co/Al2O3/Ni80Fe20. The field amplitudes in the legendsare (Hpulse + Hbias). Inset: Hysteresis loops measured by XMCD for the Co andNi80Fe20 layers. Note that for weak fields, the two cycles are opposite due to theantiparallel alignment induced by strong dipolar coupling


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responds to the static coercive field of the Ni80Fe20 layer which is essentiallydefined by the dipolar field induced by Co.

The time-resolved data indicate that the switching process in the mi-crostructured film is very different from that observed in continuous films(Fig. 8). The time-dependent magnetization obtained for the cobalt layerfollows the time-dependence of the pulsed field. For each field pulse, partof the magnetization reverses and, after a time of around 10 ns, saturatesat a value that increases as the field amplitude increases. We attribute thisbehavior to a distribution of coercive fields among the magnetic dots. Asthe field amplitude increases, more and more dots reverse, and because theswitching time is shorter than the pulse duration, a plateau is found in themagnetic response.

When the pulse amplitude is not high enough to affect cobalt, the switch-

ing behavior of Ni80Fe20 simply follows the field pulse, as described abovefor the Co layer. For an intermediate range of pulse amplitudes, Ni80Fe20dynamics shows complex behavior that is determined by coupling with theCo layer. Ni80Fe20 magnetization switches first, followed by the Co layer. Thedipolar field from Co acting on Ni80Fe20 is now opposite to the pulsed field,and part of the Ni80Fe20 magnetization is pulled back. A stable state is thenmaintained until the end of the pulse, where a symmetrical situation is real-ized due to the faster response of Ni80Fe20 to the changing field. These resultsshow that in such coupled magnetic structures, dynamic reversal is a complexphenomenon and time-resolved XMCD is a unique probe to disentangle themagnetic response of each FM layer.

5 Conlusions and Perspectives

In conclusion, we have shown that time-resolved XMCD is an exceptionaltool for studying the dynamic reversal of magnetization with chemical se-lectivity. Although this probe cannot be competitive with other ultrafasttechniques due to the limitation on temporal resolution ( 100 ps in third-generation X-ray sources), its strength has to be found in its unique capabilityfor studying independently the magnetization dynamics of each of the mag-netic components of a complex heterostructure. We have used this propertyto investigate the magnetization dynamics of the individual FM layers in spinvalves and tunnel junctions presenting different types of coupling. Differentmagnetic coupling between FM layers has been measured in the static anddynamic regimes for a spin valve.

The decrease in the X-ray pulse length to subpicosecond length and theaddition of spatial resolution for time-resolved magnetic imaging will maketime-resolved X-ray studies even more powerful. Several scientific projectshave been proposed or are being realized around the world to generate in-tense X-ray pulses with durations as short as 100 fs. The most advancedprojects (TESLA in Hamburg, Germany and LCLS at SLAC, Stanford) are


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based on the free electron laser (XFEL) or self-amplified spontaneous emis-sion (SASE) principle. The large coherence of the X-ray beams produced bythese machines will make them suited to diffraction and magnetic speckleexperiments. Femtosecond X-ray pulses have been successfully generated atthe ALS synchrotron source in Berkeley [58], and a similar project exists atBESSY II (Berlin). A typical experiment which would become possible withthese sources is element-selective measurements of magnetization precession.The decrease in pulse length will, however, be accompanied by a decrease inpulse current. This can be partly compensated for by a higher repetition rate,but in general the expected beam current will be a factor of 10100 lowerthan that of third-generation sources. We do not think that this will be a realdrawback because an increase in the measuring time of even a factor of 30would still allow measuring a complete time-resolved magnetization curve in

23 hours.Magnetic imaging using photoemission electron microscopy (PEEM) is

a second development that will be of great importance for investigatingelement-selective magnetization dynamics [59,60]. This technique will allowstudy of the inhomogeneous distribution of the magnetization of a complexsample as a function of time. The micromagnetic simulations by Miltat etal.[16] show that the ideal probe of magnetization dynamics in micron-size sys-tems should have a spatial resolution well below 100 nm and a temporalresolution below 30 ps. Magneto-optical microscopy using visible light can-not satisfy these spatial resolution requirements. PEEM constructs an im-age in which secondary electrons escape from a sample after the absorptionof monochromatic X rays. The number of electrons emitted from a certainspot of the sample is proportional to the absorption at that spot. By tuningthe X rays to the absorption edge of a certain element, one can obtain animage of the distribution of this element in the sample. Using circularly po-larized X rays, the magnetic image can be obtained for each element in thesample. This technique has been used to investigate the coupling betweenthe ferro- and antiferromagnetic layer in Co/NiO(001) bilayers [61] and thespin reorientation transition as a function of spacer thickness in Co/Cu/Ni

trilayers [62]. In the spin valves studied by XMCD, time-resolved PEEM im-ages will hopefully confirm that the difference in magnetic coupling on thetwo timescales is actually due to a change in the reversal regime. Presently,the best spatial resolution of PEEM experiments is 30 nm [63], but severalprojects to improve the resolution of the microscope are on their way. TheSMART project in BESSY II [64] aims at a resolution of 2 nm. The maindifficulty for time-resolved PEEM imaging is that the secondary electrons en-tering the microscope are highly influenced by the applied magnetic pulses.This means that images will be difficult to acquire during magnetic field

pulses. This technique will, however, provide unprecedented sensitivity andspatial and temporal resolution for layer-resolved imaging of magnetizationdynamics that takes place after the field pulse. Even better results would be


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possible if photon-in, photon-out X-ray techniques like reflectivity were to bedeveloped for magnetic imaging.

Ackowledgements

The first time-resolved XMCD measurements were carried out on beam lineID24 of the ESRF. We thank T. Neisius and S. Pascarelli for their valu-able help and interest. Soft X-ray measurements have been proposed byG. Ghiringhelli who developed with M. Bonfim the instrumentation and thesoftware for the measurements on beam line ID12B. We thank N. Brookes ofID12B (now ID8) for his constant support and encouragement. The measure-ments on spin valves and tunnel junctions were realized on samples grown atCNRS-Thales (Orsay) by F. Petroffs group. We thank him and F. Montaigne

for their positive contribution to the experiments. We also thank J. Camareroand Y. Pennec of the Laboratoire Louis Neel Laboratory for their constanthelp and ideas and for performing the Kerr measurements.

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Index

activation volume, 158

domain wall, 157

magnetic speckle, 161microcoil, 166

nucleation, 157

PEEM, 179

pump-probe, 160

spin valve, 156, 159strip line, 166synchrotron, 155, 159

thermally activated, 157tunnel junction, 156, 159

XMCD, 159, 162, 165

pizzini PEEM

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