Structural and magnetic properties of Cu/Co and Au/Co multilayers

Transcription

1 Journal of Magnetism and Magnetic Materials 121 (1993) North-Holland Structural and magnetic properties of Cu/Co and Au/Co multilayers S. Pizzini a, F. Baudelet b, E. Dartyge a, m. Fontaine a Ch. Giorgetti a, J.F. Bobo c, M. Piecuch c and C. Marli~re d LURE, Bat. 209D, Universit[ Paris-Sud, Orsay, France b Laboratoire de Physique des Solides, Universit~ de Nancy I, Vandoeuvre les Nancy, France c Lab. Mixte CNRS-Saint Gobain, BP 109, Pont-k-Mousson, France d Institut d'optique, Universit Paris-Sud, Orsay, France The structural and magnetic properties of Cu//Co and Au//Co multilayers are studied by X-ray absorption spectroscopy. The results of EXAFS measurements show that thin cobalt layers in Cu/Co and Au/Co multilayers have a different structure imposed by the mismatch with Cu (2%) and Au (14%). The most interesting results for Cu/Co multilayers are provided by magnetic circular X-ray dichroism (MCXD) measurements. The presence of an MCXD signal at the Cu K edge indicates the presence of a p-band moment on copper. I. Introduction 2. Samples In this paper we describe the results of X-ray absorption spectroscopy (XAS and in particular EXAFS) measurements and the results of preliminary magnetic circular X-ray dichroism (MCXD) on Cu/Co multilayers. The potential of XAS in the structural characterization of metallic multilayers has been recently demonstrated [1]. Here we take advantage of the site-selectivity of this spectroscopy to probe the local structure around each of the chemical species which constitute the multilayer. We first concentrate on the structural information obtainable with EXAFS and in particular on the difference between the Co structure in the two multilayer systems. In the second part we describe the results of magnetic X-ray circular dichroism measurements on Cu/Co multilayers. Although MCXD is a relatively new technique [2] and a lot still remains to be understood about the information carried by the MCXD signal in the domain of hard X-rays, its potential for the investigation of the magnetic properties of materials is very wide. Correspondence to: Dr. Stefania Pizzini, LURE, Bat. 209D, Centre Universitaire Paris-Sud, Orsay Cedex, France. Tel.: ; telefax: Cu/Co and Au/Co multilayers for EXAFS were prepared by thermal evaporation from tungsten crucibles in an ultrahigh vacuum chamber [3]. Cu IoMLC IOML, CU5MLCOsML, Cu 3MLCO 3M L and AU21MLCO27ML, AU3MLCO15ML, AU3MLCO4ML, AUI2~LCOnM L multilayers were evaporated on a 250 A thick Au buffer deposited on a float glass substrate. The multilayers are polycrystalline and have a well-defined (111) texture. In all the Co/Cu multilayers Co has an fcc structure [4], with a large (38%) concentration of stacking faults having a local hcp environment. The interface roughness is less than 4 A for Cu/Co multilayers and about 6,~ for Au/Co multilayers. RHEED measurements on Au/Co multilayers [5] indicate that for Co thicknesses equivalent to 1-2 ML the Co free surface has the same hexagonal lattice cell as the (111) Au surface, with a large expansion of the Co lattice, up to 10-13% with respect to bulk Co. As the Co thickness increases, the expansion decreases, reaching = 5% for 5 monolayers (ML) and -- 2% for 15 ML. Cus~Co9A, Cu9ACOl0,~ and CUl3~COl0 ~ multilayers for MCXD measurements were prepared by sputtering technic~ues. Multilayers with a total thickness of A were deposited on a 40.~ Fe buffer deposited on a kapton substrate and /93//$ Elsevier Science Publishers B.V. All rights reserved

2 S. Pizzini et al. / Structural and magnetic properties of Cu / Co multilayers 209 protected by 50,~ Cu. The use of a kapton substrate was required by the experimental setup for MCXD, which uses transmission geometry [6]. The multilayers show magnetic properties similar to those obtained for the multilayers on glass substrates [7]. However, for Cu thicknesses corresponding to anti-ferromagnetic coupling between Co layers (Cu8ACo9A), the magnetoresistance value (6%) is significantly smaller than that obtained for the same multilayer on glass (= 30%). The coupling is antiferromagnetic for the other two multilayer samples. 3. Experiments and data analysis X-ray absorption spectroscopy (XAS) measurements were carried out at the 1.85 GeV DCI synchrotron radiation source at the Laboratoire pour l'utilisation de Rayonnement Electromagnetique (LURE) at Orsay. The EXAFS spectra of the Cu/Co and Au/Co multilayers were measured using the electron yield mode. Differences between the crystal structure in the growth plane and in the growth direction were pointed out by recording the XAS spectra with the sample surface parallel and nearly perpendicular to the polarisation direction of the X-ray beam. The spectra measured for the Cu/Co multilayers, Fourier-filtered to isolate the contribution from the nearest neighbour shell, were fitted using a plane-wave approximation and experimental phase shifts. Details of experiments and data analysis can be found in ref. [1]. The spectra for the Au/Co samples were analysed with a fitting program using curved wave EXAFS theory [8]. MCXD measurements were carried out on the energy dispersive XAS beamline at LURE [6]. Experimentally MCXD consists in measuring the X-ray absoption of the sample positioned in a magnetic field applied respectively in the direction parallel (~+) and anti-parallel (/z-) to the X-ray beam propagation vector. The MCXD signal (/z +-/z-)/(/z++ ~-) differs from zero for a ferro(ferri)magnetic material. The MCXD spectra were recorded in grazing incidence geometry, with the sample surface almost parallel to the X-ray propagation vector. The intensity of the magnetic field acting on the samples was = 3.3 koe. In order to check that the signal was not contaminated by spurious effects (e.g. decay of the X-ray beam intensity, sample movements, etc.) the Cu K edge spectrum of the CusACo9A multilayer was also measured in a magnetic field of constant direction, which is supposed to give no MCXD signal 4. EXAFS measurements on Cu/Co and Au/Co muitilayers In fig. 1 the Fourier transforms (FT) of the Co K edge EXAFS spectra of the Cu/Co (a) and Au/Co (b) multilayers, recorded with in-plane I a j2 /4 ) R (A) I I I l I I I I R (A) Fig. 1. Fourier transform amplitudes of the Co K edge EX- AFS spectra of the Cu/Co and Au/Co multilayers, recorded with the sample surface parallel (in-plane) to the polarisation direction of the X-ray beam. (a) Cu/Co: (1) Cu bulk; (2) CH10MLCOIoML; (3) CU5MLCO5ML; (4) CU3MLCOaM L. (b) Au/Co: (1) Co bulk; (2) AU21MLCO27ML; (3) AU3MLCOIsML; (4) AU3MLCO4ML; (5) Au 12MLCO4ML.

3 210 S. Pizzini et al. / Structural and magnetic properties of Cu/Co multilayers Table 1 Results of the modelling of the Cu K and Co K edge EXAFS spectra recorded for the Cu/Co multilayers with the polarisation direction parallel (in-plane) and perpendicular (out-of-plane) to the polarisation of the X-ray beam. R 1 are the nearest neighbour distances. DW l are the Debye-Waller factors, relative to those of the model compounds used to obtain experimental phase shifts: bulk Cu for Cu K edge spectra and bulk Co for the Co K edge spectra. Typical error bars are 5:0.0005,~2 for DW 1 and ,~ for R 1. The coordination number for the nearest neighbour shell has been fixed at 12, as for an fcc or hcp structure Sample R1 (.~) DW1 (~2) R1 (~) DW 1 (~k2) (in-plane) (in-plane) (out-of-plane) (out-of-plane) Cu K edge Cu (bulk) Cu 10MLCO 10ML CU5MLCO5ML CU 3MLCOaM L Co K edge Co (bulk) Cu 10MLCO 10ML CUsMLCOsM L CD 3MLCO3M L X-ray polarisation, are compared with the FT of bulk Co. The FT provide a pseudo-radial distribution function around Co atoms in the direction parallel to the metallic layers. For Cu/Co multilayers, the FT keep, only slightly reduced in intensity, all the structures typical of Co metal. This shows that the layers are able to accomodate the small lattice mismatch (2%) between Co and Cu without inducing a large disorder. On the other hand, for Au/Co multilayers the peaks of the FT are significantly reduced with respect to the bulk material. For the 4 ML thick Co layers, the large distance order is completely destroyed, as indicated by the absence of FT peaks beyond the first neighbour shell. This seems to imply that the lattice is not able to accomodate the large mismatch (14%) and responds with the creation of defects. Table 1 shows the pseudo-radial distribution functions around Co and Cu atoms in Cu/Co multilayers, obtained by fitting the experimental EXAFS data. In Co the in-plane nearest neighbour Co-Co distance is 2.52 A ( ,~) and the out-of-plane distance 2.50.~ ( A). For Cu layers, the in-plane Cu-Cu distance is 2.54,~ ( A) and the out-of-plane distance 2.55.~ ( ~). In the plane of the layers the Co and Cu lattices are strained with respect to the bulk Table 2 Results of the modelling of the Co K edge EXAFS spectra recorded for the Au/Co multilayers with the polarisation direction parallel (in-plane) and perpendicular (out-of-plane) to the polarisation of the X-ray beam. R 1 are the nearest neighbour distances. DW 1 are the Debye-Waller factors. The numbers in brackets are the coordination numbers Sample R1 (,~) DW l (,~2) R1 (.~) DW 1 (~2) (in-plane) (in-plane) (out-of-plane) (out-of-plane) AU21MLCO27ML 2.51 (11) (11) (1) (1) AU3MLCO15ML 2.50 (11) (11) (1) (1) AU3MLCO4M L 2.49 (8) (10) (4) (2) 0.027

4 S. Pizzini et al. / Structural and magnetic properties of Cu/Co multilayers 211 layers (the first neighbour distance is 2.51,~ in Co and ~ in Cu). This in-plane strain tends to decrease the mismatch between the two lattices. The deformation tends naturally to expand (by less than 1%) the Co layers, which have a smaller bulk lattice parameter, and to contract by a similar amount the Cu layers. In the direction perpendicular to the Cu/Co interfaces and in both Cu and Co layers, the nearest neighbour distances are unchanged with respect to the bulk. The values of the Debye-Waller factors, which are a measure of the degree of disorder in the material, indicate that the Co layers are significantly more disordered than the Cu layers. The results of the fits of the EXAFS data for the Au/Co multilayers (table 2) indicate that for the thicker Co layers o (15 and 27 ML) the Co-Co distance in 2.51 A, as in Co metal, with a very small contribution of dilated Co-Co distance. For the 4 ML thick layer a significant (= 30%) part of Co atoms is characterised by a much larger nearest neighbour distance ( = 2.80,~). This is the case for both the in-plane and the out-ofplane directions. The average nearest neighbour distance in the 4 ML thick Co layers (2.57 A), is significantly smaller than that obtained from RHEED measurements on free Co surfaces (2.67,~). This indicates that, as expected, the process of relaxation towards the bulk structure is slower for the free surface. The EXAFS results may be explained by the presence of small crystallites within which the Co lattice is not expanded. The long Co-Co distances found by the simulation may arise from the interracial areas between the various crystallites, which should occupy a non negligible portion of the Co layer's volume. 5. MXD measurements on Cu/Co multilayers The Co K edge MCXD spectra for Co metal and for the Cu8,~C09,~, Cu 9,~Co 10/~ and CUl3.~COl0 A multilayers are reported in fig. 2a. The Co K edge signal recorded for the multilayers is comparable to that measured for metallic Co. This shows that the Co moments are saturated and that the Co layers are ferromagnetically coupled due to the presence of the large (3.3 koe) applied magnetic field Cu8~~_ E-E o (ev) "10" I E- E o (ev) Fig. 2. MCXD signal for Co/Co multilayers: (a) Co K edge; (b) Cu K edge. E 0 is the energy of the K edge absorption edges (E 0 = 7710 ev for Co K and E 0 = 8979eV for Cu K edge). Fig. 2b shows the Cu K edge MCXD spectra recorded for Cu metal and for the three Cu/Co multilayers. The spectrum obtained for Cus~Co9A in a field of constant direction does not show any signal, confirming that no spurious contributions are measured. The MCXD spectrum recorded for bulk Cu bears no signal, as expected for a nonmagnetic solid. On the other hand, the MCXD spectra for the three multilayers show a negative signal (amplitude of = 4-6 x 10-4 and an S/N ratio of = 3) for energies close to the absorption edge. These results indicate that, due to the proximit3' of Co atoms at the interfaces, Cu p-states close to the Fermi level are spin-polarised. The presence of an MCXD signal at the Cu K edge indicates the presence of a p-band moment on copper. The spin-polarisation may be explained in terms of the model proposed by Ortega and Himpsel [9]. These authors demonstrate that, for Cu(100) thin films on Co(100), spin-polarised quantum well states are created by quantizing the momentum of the s, p band states perpendicular to the surface. The origin of the spin polarisation of these states may lie in the spin-dependent boundary conditions at the interface with the ferromagnet. t

5 212 S. Pizzini et al. / Structural and magnetic properties of Cu/Co multilayers The integrated MCXD signal for Cu is approximately five times smaller than for Co in Cu8ACo9A and Cu9ACOlo A multilayers and approximately ten times smaller than Co in the CUl3/~COl0/~ multilayer. The Cu K edge MCXD signal is significantly smaller for the Cu13ACo10 A sample, where the Cu layer is the thickest. This may suggests that the range of the magnetic interaction through the Cu/Co interfaces is relatively short and only copper atoms close to the interfaces "feel" the presence of cobalt. However, this indication may be confirmed only by a systematic study of the MCXD signal in a series of samples with constant Co thickness and variable Cu thickness. Finally, the sign of the MXD signal is consistent with the presence of ferromagnetic coupling between Cu and Co moments [6]. References [1] S. Pizzini, F. Baudelet, D. Chandesris, A. Fontaine, H. Magnan, J.M. George, F. Petroff, A. Barth61emy, A. Fert, R. Loloee and P.A. Schroeder, Phys. Rev. B (Rapid Commun.) 46 (1992) [2] G. Schlitz, W. Wagner, W. Wilhem, P. Kienle, R. Zeller, R. Frahm and G. Materlik, Phys. Rev. Lett. 58 (1987) 737. [3] C. Marli~re, D. Renard and J.P. Chauvineau, Thin Solid Films 201 (1991) 317. [4] K. Le Dang, P. Veillet, Hui Lee, F.J. Lamelas, C.H. Lee and R. Clarke, Phys. Rev. B 41 (1990) [5] J.P Renard, P. Beauvillain, C. Dupas, K. Le Dang, P. Veillet, E. Velu, C. Marli~re and D. Renard, J. Magn. Magn. Mater. 115 (1992) L147. [6] F. Baudelet, E. Dartyge, A. Fontaine, C. Brouder, G. Krill, J.P. Kappler and M. Piecuch, Phys. Rev. B 43 (1991) [7] J.F.. Bobo et al., J. Magn. Magn. Mater. 121 (1993) [8] S.J. Gurman, J. Phys. C 21 (1988) [9] J.E. Ortega, F.J. Himpsel, Phys. Rev. Lett. 69 (1992) 844.