Journal of Magnetism and Magnetic Materials 156 (1996) 23-24 ~ Jeurnal of

inagnellc ELSEVIER , i ~ materials

Stabilization of the fcc Co structure in Co /Mn multilayers with very thin Mn layers

A. Michel a,*, V. Pierron-Bohnes a, S. Lefebvre b, M. Bessi~re b, H. Fischer b ~' IPCMS-GEMM, UMR46 CNRS-ULP, 23 rue du Loess. F-67037 Strasbourg. France

b LURE, Bat. 209D, F-91405 Orsay, France

Abstract In multilayers [Co x / M n 0.4 n m ] × N (0.75 < x < 6 nm), an fcc epitaxial, but incoherent growth of Co and Mn has been

revealed using X-ray diffraction and high resolution transmission electron microscopy. The stacking faults, numerous in the Co, are located mainly near the Mn layers.

C o / M n stands out as an interesting magnetic multi- layer system, as it alternates ferromagnetic Co layers with antiferromagnetic Mn spacers [1]. Giant magnetoresistance and antiferromagnetic coupling between the magnetic lay- ers have been observed in this kind of system [2]. Mn can exist in four different bulk structures, with a wide range of atomic w)lumes, whereas only the fcc and hcp structures are possible for Co. An extensive structural study of C o / M n multilayers grown at 0°C on a hcp Ru buffer deposited onto a mica substrate showed that the samples undergo a structural change when the Mn layer thickness is above a critical value t c = 1.2 nm [3]. Below t o both Co and Mn layers adopt a fcc stacking. Above t c, the Co partly relaxes toward the stable hop phase, whereas the Mn is divided into misoriented and deformed grains having fcc and Mn-a structures. We studied the microstructure of samples with very thin Mn layers [Co x / M n 0.4 nm] × N ((I.75 < x < 6 nm nominally) by high resolution X-ray diffraction and high resolution transmission electron mi- croscopy (HRTEM), in order to study the respective roles of strain and chemical interaction in this system.

Wide-angle 0 / 2 0 X-ray diffraction patterns, using contrast-enhancing anomalous dispersion, were obtained at LURE (Fig. 1). Simulating the three spectra with the same parameters, we deduce that the Mn lattice parameters are larger than those of the Co. both in-plane (about 15% misfit) and in the growth direction (2.2%). This is in agreement with in situ RHEED measurements made during growth, indicating an incoherent growth of Mn on Co. Moreover, the damping of the peaks shows that the Mn layers are intermixed over several planes.

* Corresponding author. Fax: + 33-8810-7249.

Asymmetric diffraction geometry scans gave informa- tion about the structure within the Co layers, for which we chose an area around the fcc 113 peak (similar to the area indicated in the electron diffraction pattern of Fig. 2) where stacking faults show up as diffuse lines parallel to the [111] direction. Fig. 3 displays the 2D scans obtained for samples with x = 0.75 and 6 nm. The thin Co layers exhibit an fcc peak, with a slight shift from the bulk position that reveals a strain of about 1% along the growth direction, whereas the thick Co layers, despite hcp and stacking fault contributions, are mostly fcc (about 66%) with relaxed parameters. Hence a long range stabilization of fcc Co by thin Mn layers is observed, which can be compared with the effect of dilute Mn in bulk Co (namely,

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0.41 0.45 0.49 0.53 0.57 Qz (~k t)

Fig. 1. 0 /20 patterns of[Co 2 nm/Mn 0.6 ran]×30: A= 1.8975 (a), 1.609 ,~ (b), 1.5406 A (c) and simulations. Lattice parame-

ters: Cco=2.045 A; eM,=2.09 ~.: ac, ,=2.5 A: tiM,=2.9 *; thicknesses eco = 1.92 nm; eMn = 0.28 nnl: thickness fluctuation error 5%; no intermixing. The disagreement for peak SL,,, 2 in spectrum (a) shows that there is intermixing.

0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)00771-7

24 A. Michel et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 23 24

Fig. 2. Selected area electron diffraction pattern obtained on the same region as Fig. 4. The area around the 10.3 spot corresponds to the maps of Fig. 3.

the h c p - f c c martensitic transformation temperature de- creases down to room temperature at 25% Mn) or with the effect of Cu in Co/Cu multilayers [4].

The microstructure of the sample with x = 6 nm has been studied by HRTEM (Fig. 4). Numerous regions of contrast are visible, indicating lattice deformations and stacking faults. Image processing with filters applied in Fourier space reveals that the strong horizontal fringes

Q x ( A -~) log (I / I m a x ) +1.5

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0.450

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0 .450 ~ ~ i

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Q z ( A " )

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Fig. 3. Diffraction mapping showing the 113 fcc Co Bragg peak (Qz = 0•815 , ~ - i ) the 10.3 hcp Co peak (Qz = 0.737 A - i ) and in the bottom-left comer the Ru buffer 10•3 peak• (a) [Co 0.75 n m / M n 0.4 nm]×65; (b) [Co 6 n m / M n 0.4 nm]× 12.

Fig. 4. TEM image of the upper part including capping layer of the [Co 6 n m / M n 0.4 nm]× 12 sample. Mn and Co cannot be distinguished due to the weak difference in scattering factors. The strong fault contrast indicated by black arrows arises where Mn layers are expected.

arise from accumulated stacking faults, locally building hcp-like stacking. These fringes correspond exactly to the expected location of the Mn layers. Image processing also revealed the occurrence of misorientation and stacking faults for the (111) planes at 70.5 ° from the (l l l) growth planes. The defects may thus be attributed to the release of strains introduced in the Co layers by the incommensurate Mn layers.

The observed microstmctures of these multilayers show competition between two effects: ( l ) the Co fcc structure is favoured by the presence of Mn; (2) however, elastic strains occur due to the incommensurate growth of both species• Although thicker Co layers still adopt fcc stack- ing, direct contact with the Mn layers creates numerous stacking faults in order to compensate for the large lattice parameter mismatch.

References

[1] Y. Henry, PhD Thesis, ULP Strasbourg 1995. [2] B. Heinrich and J.F. Cochran, Adv. Phys• 42 (1993) 523. [3] K. Ounadjela, P. Vennegues, Y. Henry, A. Michel, V.

Pierron-Bohnes and J. Arabski, Phys. Rev. B 49 (1994) 8561. [4] F.J. Lamelas, C.H. Lee, H. He, W. Vavra and R. Clarke, Phys.

Rev. B 40 (1989) 5837.