x-ray magnetic circular dichroism and element-selective magnetic hysteresis in fe/cu/co/cu...

ELSEVIER Journal of Magnetism and Magnetic Materials 166 (1997) 38-44

journal of magnetism and magnetic

~ H materials

X-ray magnetic circular dichroism and element-selective magnetic hysteresis in Fe/Cu/Co/Cu multilayers

S. Pizzini a,*, A. Fontaine a, L.M. Garcia a.l, J.-F. Bobo b, M. Piecuch b, F. Baudelet c, C. Malgrange d, A. Al imoussa e, E. Snoeck e,

M.J. Casanove e a Laboratoire Louis N~el, CNRS, BP 166X, F-38042 Grenoble Cedex, France

b Laboratoire de Physique du Solide, Universit£ Nancy 1, BP 239, F-54506 Vando~uvre Cedex, France c LURE, Centre Universitaire Paris-Sud, F-91405 Orsay Cedex, France

d Laboratoire de Mingralogie-Cristallographie, Unicersit~ Paris 6 et Paris 7, F-75252 Paris Cedex 05, France

¢ CEMES/CNRS, BP 4347, F-31055 Toulouse Cedex, France

Received 7 May 1996; revised 15 July 1996

Abstract

A F e / C u / C o / C u multilayer exhibiting an hysteresis loop with two characteristic coercive fields which can be assigned separately to the two magnetic 3d metals has been studied. XMCD measurements are able to follow the field dependent element-specific magnetisation. The photon helicity was reversed using a diamond quarter-wave plate in Laue geometry. XMCD and macroscopic magnetisation measurements are in agreement and show that in this multilayer system the Co layer reverses magnetisation direction at a lower field than the Fe layer.

Keywords: X-ray magnetic circular dichroism; Magnetic multilayers; Fe/Cu/Co/Cu multilayers; Magnetic hysteresis

1. Introduct ion

The investigation of microscopic magnetic properties of materials using the atomic selectivity intrinsic to X-ray absorption spectroscopy (XAS) constitutes the main goal and strength of X-ray magnetic circular dichroism (XMCD) [1]. This potential of this technique explains its rapid development and applications, and the wide interest and curiosity in this

* Corresponding author. Fax: +33-4-7688-1191; email: pizzini @grmag.polycnrs-gre.fr.

On leave from Departamento Fisica de la Materia Conden- sada, Facultad de Ciencias, Zaragoza, Spain.

new way of investigating magnetic compounds. A number of experiments have shown that XMCD is sensitive to very weak magnetic moments, and that it is particularly suited for studies of surfaces and thin films. Several measurements have been carried out on very thin metallic overlayers, multilayers, trilayers and diluted alloys [2-9].

The measure of the magnetisation as a function of the applied field is the first fundamental characterisation of a magnetic material. Classical macroscopic magnetisation techniques are unable to distinguish directly the contributions of the various magnetic components to the total magnetisation. In this sense the atomic selectivity of XAS can be used to gain insight into the magnetic properties of materials.

0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S0304- 8 85 3(96)00454-4

S. Pizzini et al. / Journal of Magnetism and Magnetic Materials 166 (1997) 38-44 39

The first XMCD measurements of element-selective hysteresis were carried out by Chen et al. on two C o / C u / F e trilayers deposited on glass [5]. The evolution of the Fe and Co L2. 3 edge absorption as a function of applied magnetic field allowed these authors to deconvolute the macroscopic hysteresis loops into two loops characteristic of the Fe and the Co layers magnetisations. The Co layer was found to exhibit a much larger coercive field ( ~ 200 Oe) than that of the Fe layer ( ~ 40 Oe). Moreover, a fraction of the Co layer was found to be coupled to Fe. The average moment of the Co atoms was found to be strongly reduced with respect to the bulk value.

We present here the results of a field-dependent XMCD study of a F e / C u / C o / C u multilayer. The measurements were carried out at the Co K-edge and are therefore bulk-sensitive. The helicity of the X-ray photons was reversed using a diamond quarter-wave plate (QWP). One result of this investigation is that in this system the coercive field of the cobalt layer was found to be smaller than that of the iron layer, and much smaller than that reported in previous experiments. The results of our XMCD measurements confirm the indications of the macroscopic hysteresis cycles.

Before describing the experimental setup of our XMCD measurements and discussing the results, we first describe the macroscopic properties of the multilayer sample and in particular the results of the TEM and magnetic measurements.

2. Sample preparation, structure and macroscopic measurements

A multilayered sample with nominal sequence 30 X (Fe50~/Cu50~/COl00~/Cu50~) was deposited

on a 10 Ixm thick Si substrate using an Alcatel SCM650 sputtering setup. Typically 99.5% pure copper, iron and cobalt targets were mounted on the three cathodes of the chamber. The base pressure was 5 X 10 -7 mbar and the Ar working pressure was 3 X 10 -3 mbar. The rather low power densities (592 W / c m 2) led to deposition rates close to 1 ,~/s. Such sputtering conditions lead to smooth polycrystalline films and multilayers [10]. For best thickness uni- formity, the sample was deposited in dynamic mode, i.e. scanned over the cathodes during deposition.

In order to verify the quality of the stacking sequence and the interface structure, the multilayer was studied by transmission electron microscopy (TEM) on cross sectional specimens. These experiments were carried out on a Philips CM30/ST mi- croscope working at 300 kV with a point resolution of 1.9 A. The samples were first cut into two parts, glued face to face, and then mechanically polished. The final thinning to electron transparency was achieved by argon milling (5 kV, 5 mA, 15 °) at liquid nitrogen temperature. Since Fe, Co and Cu have very close Z numbers (26, 27 and 29, respectively) no strong contrast due to the different layers is expected to be observed in TEM experiments. However, complex Fresnel fringes can be obtained on micrographs taken at high under- and overfocus. These fringes are repeated in the growth direction with the same periodicity of the chemical periodicity of the sample. For complete characterisation of the interfaces (roughness and interdiffusion analyses) the experimental micrographs must be compared with the simulated ones for the same defocus values. These analyses are in progress and will be reported elsewhere [11]. Fresnel fringes running parallel to the interfaces show up on the micrograph in Fig. 1, obtained at an underfocus of 1.8 Ixm. This micro-

~X~F~ i ~ ~ ~ ¢ ~ i ~ ; ~ , ~ . . . . . . . , ~

si . . . . .

Fig. 1. Low-magnification micrograph of the Feso ~,/Cu 5o ~ / C o i 0o A/Cu 5o ~. multilayer revealing the continuity of the different layers and the chemical periodicity of the sample (250 ,~).

40 S. Pizzini et al. / Journal of Magnetism and Magnetic Materials 166 (1997) 38-44

graph shows the complicated structure of these fringes, and the measurement of their periodicity gives the expected value of 250 ,~. Moreover, this micrograph reveals that the different layers are continuous over a distance of at least 0.6 tzm.

Extended X-ray absorption fine structure (EX- AFS) m e a s u r e m e n t s carried out on the F e / C u / C o / C u multilayer reveal that Fe, Cu and Co layers have the local structure characteristic of the bulk phases (hcp for Co, bcc for Fe, and fcc for Cu).

The macroscopic magnetic properties of the multilayer were studied at room temperature with a vibrat- ing sample magnetometer (VSM) in both in-plane and out-of-plane field orientations. Concerning the in-plane magnetisation loops, we have found that our sample has a low uniaxial in-plane anisotropy. We attribute this to the dynamic deposition procedure which causes a slight oblique growth of the grains. All the following results have been obtained along the easy axis.

The hysteresis curve measured along the easy magnetisation axis (Fig. 2) shows that the magnetisations of Co and Fe layers switch direction indepen-

1.o (a) c o

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~ 0.0 B c

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-150 -100 -50

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J i i 50 100 150

1.0 (b)

u~0"5 £ ~

0.0

-0.5 ~

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H

i i 50 1 O0 1 5 0

Oe)

Fig. 2. Hysteresis curve of the Fes0 ~/Cus0 i//COlo0, ~//C115o ~ multilayer measured with the applied field parallel to the in-plane easy magnetisation direction and deconvolution of this curve into two contributions with coercive fields at around 20 and 40 Oe.

dently for fields at around 20 and 40 Oe. One of the aims of this investigation is to show that using the element selectivity of XMCD we can determine which of the two magnetic layers corresponds to the two coercive fields. Three magnetisation regimes can be found as a function of magnetic field. At point A in Fig. 2 the magnetisations of Fe and Co films are parallel. At point B the applied field is larger than the saturation fields of one of the two magnetic films but smaller than the coercive field of the other. As a result, the magnetisation of successive magnetic layers is antiparallel and an antiferromagnetic state across the Cu spacer is obtained. At point C the other magnetic layers switch and a parallel alignment between Fe and Co magnetisations is restored. The antiferromagnetic state can be realised thanks to the thick Cu spacer layers, which weaken the strong ferromagnetic coupling between Fe and Co layers.

From the deconvolution of the magnetisation curve (Fig. 2b) we find that the contributions to the total magnetisation corresponding to coercive fields of 20 and 40 Oe, respectively, are in the ratio 1.49:1. If we assume that in the multilayer the magnetic moments of Co and Fe atoms are close to those of the bulk phases (1.65 /% for Co, 2.2 /xO for Fe) we find that the magnetisations of the 100 A thick Co layers and the 50 A thick Fe layers are in the ratio 1.5:1. This good agreement suggests that the Co film is the softest of the two films.

These results appear to be different from those found by Chen et al. in their XMCD study of a Fe102,~/Cu30~,/Co51 i trilayer [5]. These authors found values of the switching fields of 38 Oe for Fe and 200 Oe for Co. This latter field is much larger than that found in our study. Moreover, although the Fe moment was found to be 2.0-2.1 /z B, close to the bulk value, the Co moment was found to be strongly reduced with respect to the bulk (1.1-1.2 /x B versus 1.7 /XB). This is not the case for our multilayer film, where the magnetisations of both Co and Fe appear to be close to the bulk values. This appears to be consistent with the fact that the layers are suffi- ciently thick to have attained the structure of the bulk phases.

In order to confirm the indications of the macroscopic magnetisation measurements, we carried out a series of XMCD measurements at the Co K-edge. These measurements give us more insight into the

S. Pizzini et aL / Journal of Magnetism and Magnetic Materials 166 (1997) 38-44 41

magnetic properties of the cobalt layers in this sample.

3. Magnetic circular X-ray dichroism with quarter-wave plates

The element selectivity of XMCD should in prin- ciple allow us to probe selectively the different contributions to the hysteresis curve of the F e / C u / C o / C u multilayer. The Co K-edge measurements were carried out in transmission geometry on the energy-dispersive X-ray absorption beamline at LURE.

XMCD is the difference between the absorption of left and right circularly polarised X-ray photons (tr + - ~r-) by a ferro- (ferri)magnetic material. Since it is more complicated to reverse the polarisation of the X-ray beam, XMCD is in general obtained by fixing the X-ray polarisation (left or right) and by

I (B*-B')/2

6 : I a -5 B ~

O X

-1o '7 ~ 13+

I I I I

7680 7700 7720 7740 7760 7780 ENERGY (eV)

Fig. 3. Co K-edge (tr + - or- ) spectra measured for bulk cobalt by flipping the diamond QWP between offset angles of + 158 and - 158 arcsec, in a magnetic field of 1 T. In the lower part of the figure: Full line: signal obtained with field + 1 T (B ÷ ). Dia- monds: signal obtained the field - 1 T (B- ) . Doued line: average of the two previous spectra (B ÷ + B- ) /2 , which gives a background that does not depend on the magnetic field. In the upper part of the figure, the XMCD signal obtained from signal B ÷ after subtraction of the background signal.

switching the direction of the applied magnetic field from parallel to antiparallel to k in order to reverse the magnetisation of the sample.

If the sample presents some hysteresis, a detailed XMCD study versus field requires the external magnetic field direction to be a free parameter. It therefore becomes necessary to change the helicity of the X-ray beam.

The usual way of obtaining circularly polarised X-rays from a bending magnet is to select the beam emitted off the orbit plane. An altemative solution is to transform the linear polarisation of the in-plane X-ray beam into circular polarisation using a quarter-wave plate (QWP) [12-17]. By inserting a diamond QWP in Laue geometry in the energy-dispersive spectrometer, we were able to obtain at the Ho L3-edge (8070 eV) a circular polarisation rate and a photon flux larger than that normally available by optimising I'r 2 in the off-plane configuration [16]. To demonstrate the wide range of applications of this QWP, XMCD spectra at rare earth L-edges were obtained for energies from 6.4 up to 8.6 keV [17].

Since the helicity of the X-ray photons delivered by the QWP depends on the sign of the 'offset' angle (i.e. of the incident angle with respect to the centre of the reflection profile of the diamond (0in c -- 0Bragg)) XMCD spectra may be measured by fixing the direction of the magnetic field and by alternating the QWP between two positions on either side of the Bragg profile. In a previous paper [16] we showed that the difference signal (or + - or-) obtained with this method is the superposition of a magnetic signal (the XMCD) and a non-magnetic background due to the different absorptions of the QWP in the two angular positions. The XMCD signal can be obtained as the half-difference of two (cr + - or-) spectra measured with magnetic fields B + and B- applied in opposite directions and large enough to saturate the magnetisation.

As an example, in Fig. 3 we show the two difference spectra obtained at the K-edge of bulk Co when alternating the QWP between offset angles of + 158 and - 158 arcsec, in a magnetic field of 1 T applied parallel (B +) and antiparallel to k (B-), respectively. The signal obtained after subtraction of the non-magnetic background [ ( B + - B - ) / 2 ] is equivalent to that obtained with the classical method,


i.e. by alternating the direction of the applied magnetic field.

The setup of this experiment is the same as that of our previous measurements at the Ho L3-edge; de- tails can be found in Ref. [14]. A 0.77 mm thick diamond crystal was installed on the energy-dispersive spectrometer of LURE after the curved Si poly- chromator. The non-dispersivity condition was met by coupling the (111) reflection of the curved Si crystal with the (111) asymmetric reflection of the phase plate, set in Laue geometry.

The F e / C u / C o / C u multilayer sample was set close to the focus point in glancing angle geometry (30 ° ) with respect to the direction of the applied magnetic field. The large absorption by the Si substrate strongly deteriorates the Co K-edge signal. Thinner Si substrates were unable to resist to the epitaxy-induced strains. An acceptable S / N ratio for the XMCD signal was obtained with acquisition times of more than 24 h.

| i i

12 Oe .". /%' "d. ~ ' . ' c , :~, . st. ~ ",~,

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Fig. 4. X M C D signals measured for the F e / C u / C o / C u multi-

layer as a function o f applied magnet ic field referenced to the

signal measured at saturation.

1.0

0.5

~ 0.0

-0.5

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J d

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Fig. 5. Exper imental peak- to-peak ampl i tude of the X M C D signal

as a function o f magnet ic field super imposed on the hysteresis

curve.

The measurements were made by alternating the diamond crystal plate between two offset positions of + 158 and - 158 arcsec. The magnetisation of the multilayer is at saturation for + 114 and - 114 Oe. The resulting spectrum consists of two contributions, the XMCD and a non-magnetic background which can be eliminated by subtracting to the signal measured for each magnetic field that measured at the saturation field of - 1 1 4 Oe. The difference signals are shown in Fig. 4. In Fig. 5 the peak-to-peak amplitudes are plotted versus the magnetic field. In order to restore the symmetry of the experiment, the peak-to-peak amplitude measured at - 1 1 4 Oe was subtracted from each point.

4. Results and discussion

The difference spectra reported in Fig. 4 show a clear evolution as a function of magnetic field. The vanishing signal for 8 and 12 Oe indicates that the cobalt magnetisation has the same direction and amplitude as for - 1 1 4 Oe. For larger fields (e.g. 20 Oe) the signal clearly increases, indicating that the cobalt magnetisation starts to reverse. Above this field the magnetisation of the whole cobalt layer has changed direction. The field associated with the Co magnetisation reversal is clearly at around 20 Oe, as suggested by the deconvolution of the macroscopic hysteresis curves.

We can now try to formulate an hypothesis to explain the magnetic behaviour of this F e / C u /

S. Pizzini et al. / Journal of Magnetism and Magnetic Materials 166 (1997) 38-44 43

Co/Cu multilayer. The most striking result is that in the multilayer described here (and in other multilayers prepared similarly with different Co and Fe thicknesses) the Co layers exhibit coercive fields ( ~ 20-30 Oe) much smaller than those reported in the literature for Co layers in C o / C u / F e or Co / C u /NiF e sandwiches and multilayers [5,18,19]. This coercive field is even smaller than that measured for the Fe layers in this system. Chaiken et al. [18] found H c° = 250 Oe in a C 0 6 0 ~ , / C u v 3 ~ / F e 5 5 ~

trilayer deposited on glass, and Chen et al. [5] found H c° =200 Oe in a Co51~/Cu30/,/Fe102~ trilayer also evaporated on glass. In both systems Hc Fe is of the order of 30-40 Oe, similar to our multilayers. We believe that the reason for the small Co coercive field in our multilayers is essentially the sample preparation conditions. Kawawake and Sakakima [18] recently showed that the magnetisation curve of a Co30~,/Cu60~,/NiFe30~, multilayer depends strongly on the deposition conditions. The film deposited epitaxially on Si(100) by UHV evaporation gives rise to a two-step hysteresis cycle that can be decon- voluted into two square loops. When the same film is deposited on a glass substrate the hysteresis loop characteristic of the Co layer becomes wider and less square, and is characteristic of a disordered medium where wall motion is pinned by defects. Similar behaviour can be found in the Co magnetisation curves of Refs. [5,18], where C o / C u / F e multilayers were grown on glass.

The soft magnetic behaviour of Co and the square hysteresis cycles measured for our multilayer can be explained in the light of these observations. The slow rate of deposition on a Si substrate leads to smooth, continuous layers and small grains, as shown by the TEM measurements. The homogeneity of the film, together with the fine grain size, favour the decrease in the coercive field. The presence of a slight uniaxial anisotropy in the applied field direction also plays in the same direction.

In conclusion, this study demonstrates that bulk- sensitive XMCD at K-edges of transition metals can be used to study element-selective hysteresis in magnetic multilayers. Nevertheless, it should be noted that this experiment has been carried out under ex- treme conditions: (i) the K-edge XMCD signals are two orders of magnitude smaller than the L-edge signals, which makes the acquisition times much

longer; (ii) on an energy-dispersive spectrometer transmission geometry is imposed and the photon flux is largely reduced by the multilayer substrate (Si in this case); and (iii) the use of a diamond quarter- wave plate to change the photon helicity introduces a background signal which, in the case of K-edges, is of the same order of magnitude as the magnetic signal. In this sense, the use of third-generation synchrotron radiation sources (smaller beam, higher flux) should be extremely useful for such applications.

References

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x-ray magnetic circular dichroism and element-selective magnetic hysteresis in fe/cu/co/cu...

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