Ru asymmetric sandwich structures

Journal of Magnetism and Magnetic Materials 148 (1995) 145-147

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Competing anisotropies and magnetization processes in epitaxial Co/Ru asymmetric sandwich structures S. Zoll

a,b,*

, H . A . M . V a n den Berg b, K. Ounadjela a, D. Stoeffler ~, A. Dinia a a IPCMS-C-EMME, 23 rue du Loess, F-67037 Strasbourg, Frznce b SIEMENS AG, ZFE B T MR 13, P.O. Box 3220, D-91050 Erlangen, Germany

Abstract A model is presented on pairs of exchange-coupled magnetic layers that have interfaces with surface anisotropy. The interfaces are coupled to the bulk by twisted magnetization configurations. Experimental magnetization curves of C o / R u sandwiches are reproduced with precision. Most of the physical parameters used are in accordance with the literature. A difference in the anisotropy constants between both magnetic layers was found, which, in the present samples, can be explained by lattice misfit and interface roughness. The exchange coupling forces the magnetization of both layers to be along the same axis in the low field range.

During the last decade, magnetic multilayers have gained much interest. The giant magnetoresistanee (GMR) effect was discovered in F e / C r stacks in which the magnetization ( M ) of adjacent layers is antiferromagnetieally coupled. In other systems, the surface anisotropy plays an important role. Both these effects act on the interracial atoms [1,2] of the magnetic layers and domain-wall-like configurations frequently occur to match the often conflicting requirements at the interfaces and the bulk. Here we present the results of a model which can handle these interactions and verify its validity by comparing with experimental data on C o / R u stacks. The micromagnetic boundary conditions have to enter the model via additional tiny interface layers with only surface anisotropy. These layers represent the intermixing effects at the interfaces (Fig. 1). A twisted spin configuration matches the interface and bulk magnetization. The space not occupied by the walls is considered to be uniform. We have calculated the wall configuration by the Ritz method, implying that no additional minimization of parameters is required. We have confined ourselves to uniaxial anisotropies with symmetry axis normal to the stack. Then the contributions to lh¢ energy are the different bulk and surface anisotropies, the bulk exchange coupling, the interlayer

antiferromagnetic coupling (IAFC) and external field H . The total energy is minimized with respect to these parameters. C o / R u sandwiches were grown by UHV evaporation onto mica [3]. We present four samples with Ru thickness (tRu), of 12 ~,. The bottom Co layer has a constant thickness of L 2 = 32 A and the other one is L 1 = 10, 13, 16 and 32 A. The RHEED patterns obtained during the sample's growth show high crystalline quality with a (0001) hop

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* Corresponding author. Fax: steph_ [email protected].

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S. Zoll et a L / Journal of Magnetism and Magnetic Materials 148 (1995) 145-147

Table 1 Physical parameters used in the fits

Co32 Rtt12Co52 Co32 Rut2Cot6 Co32 RulzCot3 Co32 RUl2COl0 Error Literature

Jar (erg/cm 2)

Jf (10 -6 erg/cm)

K~ (erg/cm 2)

K~ (thin layer) (10 6 erg/cm 3)

K~ (thick layer) (10 6 erg/cm 3)

- 1.4 - 1.0 - 1.3 -- 1.4 + 0.1 - 6-0

0.38 0.5 0.5 0.5 4-0.1 0.1-2

0.55 0.42 0.40 0.42 + 0.01 0.4-0.5

8 5.5 4.5 4.5 + 0.5 5.2

9.8 9.5 I0 10.5 + 0.2 5.2

texture o f the Co and Ru layers. Further details on the structure of the stacks can be found elsewhere [3]. Co single layers have been grown separately to investigate the anisotropy and the saturation magnetisation. The direction ef easy magnetisation changes from perpendicular to parallel to the f'dmoPlane when the Co thickness increases from 10 to 16 A. We found that there is an effective thickness 8 of 2 ,A at each interface that carries no moment, which we shall refer as a dead layer. For all our samples, we traced the magnetisation curves at r o o m temperature using an A G F M . In Fig. 2, we can see a comparison between the calculated and experimental curves. The model gives very good results for the samples. With the same set of parameters, we can well reproduce the curves for the in-plane and perpendicular H. In Table 1 we summarize all the values o f the free parameters and those found in the literature [4]. Most parameters are close to the data from the literature. The Ru thickness is chosen such that it is not at the m a x i m u m of the antiferromagnetic coupling regime so that the values of Jaf cannot be compared. The bulk exchange constant Jf and the anisotropies affect the calculated saturation field H s and lead to corrections in ,Jraf compared with the well-known estimations o f J~f based on H s. Jf is of the right order of magnitude but relatively small, which might be caused by the small Co layer thieknesses and by intermixing o f the interfaces. The surface anisotropy, K~, is close to well known "calues. The bulk anisotropy, K b, o f the Co layer near the buffer is larger than its counterpart and is about twice the value o f bulk hop Co. The most likely reason f o r the increase is the magneto-elastic contribution due to the expansion of the Co in the lateral direction because o f the lattice misfit with Ru o f 6%. It is well known that the interracial roughness opposes the magneto-elastic contribution to K b of the strain of the bulk. Thus, the likely reasons for the lower K b of the second Co layer are its smaller thickness and larger roughness. With H parallel to the film, the rotation is in the plane for the Coa2Ru12Col~ and in a plane perpendicular to the stack for Coa2RutzColo. At low fields, the moments of the thick and thin layers are aligned antiparallel for both samples. The effective anisotropies, Keff = 2 K s + ( L - 2 ~ X K b 2,irMa) of the thin and thick layers are of opposite sign -

1.5 1.0 / Co~2RunCoI6 0.5 Hparallel 0.0 .0.5

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.I.0 "1"$.20 .15 -I0 -$ 0 5 10 15 20 It~,..,ot (koe) Fig. 2. Experimental (solid line) and calculated (open clmles) magnetisation curves for the stacks Co32RuI2COt6 and

Co32Ru12Co10.

S. Zoll et al. / Jog~rn~l of Afagnet~sm and Magnetic Materials 148 (199.5) 145-247

for Co32Ru12Co13: the I A F C is strong enough to force the thin layer to be opposite to the thicker one. This can he considered as a transfer o f Keff mediated b y the 1AFC. For the C032Ru12C010 sample, the plateau is large because of the small thickness ratio ( L ~ / / L 2 ) = R d and because Ke~f is of the same sign for both layers. For the C032Ru12C016 sample, a plateau exists for J£ in the plane of the film and its width is relatively small because R d is larger. Although it cannot be seen from the B - H curves, the moments of both Co layers o f Co32Rna2C010 tend to stay antiparallel in a relatively large window,, also when H is in the film plane. For C032Ru12C016 with a perpendicular field, this window is smaller [5].

147

References

[1] A. Thiaville and A. Fert, J. Magn. Magn. Mater. 113 (1992) 161. [2] J. Baraas and P. Griinberg, J. Magn. Magn. Mater. 98 (1991) 57. [3] D. Muller, K. Ounadjela, P. Vennegues, V. Pierr~n-Bohaes, A. Arbaoui, J.P. Jay, A. Dinia and P. Panissod, J. Magn. Magn. Mater. 104-107 (1992) 1873. [4] W.J.M. de Jonge, P.J.H. Bloemen and F.J.A. den Breeder, Ultra Thin Magnetic Structures (Springer, Berlin, 1992). [5] S. Zoll, H.A.M. Van den Berg, K. Ounadjela, D. Stoeffler and A. Din~a, to be published.