Cr(001) superlattices

Journal of Magnetism and Magnetic Materials 126 (1993) 397-399 North-Holland

MBE growth and structural and magnetic studies of high-quality single-crystal Co/Cr(001) superlattices N. Metoki, W. Donner, Th. Zeidler and H. Zabel lnstitut fiir Festkgrperphysik, Fakultiit fiir Physik und Astronomic, Ruhr-Unicersitiit Bochum, D-44780 Bochum, Germany

High-quality Co/Cr(001) superlattices have been grown by the MBE method. At growth temperatures 300-350 °C we get sharp interfaces with a total roughness of about 7 ,~. The Co layers exhibit a precursive structural change in direction to the metastable bcc phase as the Co layer thickness tco decreases. Ferromagnetic, antiferromagnetic and biquadratic interlayer coupling of Co spins with spin orientations perpendicular to the film plane are observed below a critical tco of about 15 A. The strong tcr-dependence of the interlayer coupling indicates the existence of short- and long-period oscillations similar to those of Fe/Cr(001).

1. Introduction Magnetic metal multilayers and superlattices are currently of great interest for fundamental and technical reasons. The oscillatory interlayer exchange coupling with short and long periodicities in Fe/Cr(001) have stimulated intensive studies [1]. As a similar system Co/Cr(001) has also attracted much interest. Many theoretical calculations [2] have been carried out to elucidate the electronic and spin structure in Co/Cr(001) in comparison with Fe/Cr(001). However, no (001)-oriented C o / C r superlattice has been grown so far because of the difficulties related to the growth process, so that the magnetic properties are unknown. Only a weak effect has been observed in polycrystalline or (ll0)-oriented samples [3]. In addition, stabilization of the metastable bcc Co structure presents another interesting challenge. The bcc Co phase has been reported for the first time for Co deposited on GaAs(ll0) for Co thicknesses too up to 357 ,~ [4], but the situation in C o / C r is conflicting. In (ll0)-oriented samples, bcc Co has been reported for tco< 15 ,~ [5], while no bcc phase has been observed by other groups [6]. A recent LEED study showed that bcc Co could be grown up to 20 atomic layers on Cr(001) single-crystal surfaces [7]. Recently we have successfully grown epitaxial single-crystal Co/Cr(001) superlattices by the MBE method [8]. Magneto-optical studies [9] show that the Co spins exhibit strong ferromagnetic (FM), antiferromagnetic (AFM) and biquadratic interlayer coupling with almost the same short and long periodicities as in Fe/Cr. The remarkable difference between the two Correspondence to: Prof. H. Zabel, Institut fiir Festk6rperphysik, Fakult~itfiir Physik und Astronomic, Ruhr-Universit~it Bochum, D-44780 Bochum, Germany.

structures, however, is that the coupling in Co/Cr(001) is associated with spin orientations perpendicular to the film plane below tco= 15 A,. X-ray scattering experiments have shown that the Co layers exhibit a precursive structural change to the metastable bcc phase with decreasing tCo,obut maintain hexagonal symmetry for tco down to 4 A. In this paper we report recent results on the structural and magnetic properties of this novel system.

2. Sample preparation Samples were grown by the MBE method under ultrahigh-vacuum conditions (base pressure < l0 -1° Torr). Sapphire A1203(1]02) substrates were rinsed in acetone and isopropanol, cleaned by sputtering with Ar + (600 eV, 1 ~ A / c m 2) and annealed at 1100°C. The Co/Cr(001) superlattices were grown at growth temperature T s = 300-350°C on a 500 ,~ thick Cr(001) buffer layer on a 500 ,~ Nb(001) layer on sapphire. The Cr and Nb buffer layers were grown at Ts = 450 and 900°C, respectively. The growth temperatures were optimized by RHEED and X-ray scattering experiments. At lower temperatures, island growth of Co was observed, whereas strong interdiffusion occured above 450°C. Samples were fully characterized by out-of-plane and grazing-incidence in-plane X-ray scattering experiments with Cu-Ka radiation. The magnetization curves were recorded via magneto-optical Kerr effect (MOKE) measurements. Our X-ray and MOKE apparatus have been described elsewhere [8,9].

3. Structural properties Fig. 1 shows out-of-plane X-ray scattering spectra for various tco and tcr. For larger layer thicknesses strong satellite peaks were observed around the bulk bcc Cr(002) and hcp Co(110) positions. From model

0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. (North-Holland)

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calculations for the X-ray diffraction spectra [10] the d-spacingoOf the Co layers was found to be 1.26 A for too = 50 A, which is in goodoagreement with those for bulk hcp C o ( l l . 0 ) (d = 1.25 A), indicating that the hcp structure of Co prevails at this layer thickness. The double-maximum feature in the X-ray scattering spectra becomes weaker with decreasing tco, but a strong asymmetry is visible around the Cr(002) position for the thinnest tco. Our model calculations indicate a remarkable expansion of the Co lattice spacing with d = 1.30 ,~ for too = 10 ,~. This d-spacing is too large to be explained by the Poisson expansion of the Co layers, which would be caused by the epitaxial strain from the Cr(001) plane, while it is too small for the metastable bcc Co structure (d = 1.41 A [4]). On the other hand, the d-spacing of the Cr layers (d = 1.44 A) exhibit no t o dependence, implying a high structural stability of the Cr layers. In small-angle X-ray scattering experiments strong satellite peaks and many film thickness oscillations were observed, indicating sharp interfaces between Co and Cr layers. From a simple Debye-Waller-type reflectivity model we estimate a root mean square roughness of about 7 A. From the positions of the in-plane spots observed in R H E E D and grazing incidence X-ray scattering experiments we find that the c-axis of hcp Co is oriented parallel to the Cr[ll0] for T s = 40-450°C. This orientation gives us the smallest misfits of about - 0 . 2 % and 6% along the Co[00.1] and Co[11.0] axes, respectively. The additional peaks which appear only in the Co

layers can exclusively be observed from hcp(l 1.0) structures and are indexed as Co{11.1}. Thus the in-plane observations confirm the out-of-plane conclusions of the hcp structure of Co. The intensities of these spots decrease very rapidly with decreasing too, but they are observable down to t c o = 4 ,~. Our structural results are inconsistent with recent L E E D studies [7]. At present we have no explanation for this descrepancy. However it appears from our experience with L E E D that it is rather difficult to identify uniquely the crystal structure of Co with this technique, since the reflections peculiar to the hcp structure (i.e. hcp(ll.1) reflections) are very weak. Grazing-incidence X-ray scattering experiments, which sample the entire Co layer thickness instead of only the surface as in L E E D case, are more sensitive to the Co structure.

4. Magnetic properties Fig. 2 shows representative hysteresis curves from C o / C r superlattices with a constant to, = 12 A and tcr = 10-20 A. Note that the applied fields were perpendicular to the film plane, i.e. in a polar configura-

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N. Metoki et al. / MBE growth of single-crystal Co / Cr(O01) tion. For tCr = 10 ,~ a typical A F M hysteresis curve is observed. For tc~ = 12 A the hysteresis curve has a four-step structure, which can be explained by a biquadratic interlayer coupling. For tcr = 15 A a weak but perfect A F M coupling is again observed. Then the A F M couplingodisappears and F M coupling is observed for tCr = 20 A. The Cr layer thicknesses with biquadratic or A F M coupling are quite similar to those for F e / C r . The strong tcr-dependence of the coupling indicates the existence of short-period oscillations. In fact, short and long-period oscillations appeared in preliminary experiments using wedge-shape samples. A F M coupling with perpendicular anisotropy (PA) has been observed in C o / C u ( l l l ) [11], F e / C u ( 0 0 1 ) [12] for 1-3 atomic layers, but not in superlattices with strong PA such as in C o / P d , Pt superlattices. The biquadratic coupling with P A has been observed for the first time in this study. The spin structure in C o / C r ( 0 0 1 ) poses a highly interesting problem. The short-period oscillation may suggest that the Cr spins are oriented perpendicular to the film plane as opposed to F e / C r in which the Cr as well as the Fe spins are in-plane, or an intermediate spin structure might occur. The i n d u c e d / r e d u c e d magnetization and coupling of Co and Cr spins at the interface are also interesting problems that remain to be studied. The different spin structure in C o / C r as compared with F e / C r could explain the weak magnetoresistivity effect observed by Parkin et al. [3]. From our results it is quite clear that the theoretical studies that assumed or concluded that Co spins are oriented parallel to the film plane do not appropriately explain the present observations. The large critical tco for the perpendicular orientation, 15 A, indicates that the PA is quite strong, but the origin of this is not yet clear. We expect that it is due to an electronic-driven interface anisotropy associated with the structural change in direction to the Co bcc phase. Estimates show that the P A cannot be explained from the magneto-elastic energy assuming bulk magnetostriction constants. Sato concluded that the P A observed in his C o / C r ( l l 0 ) superlattices was due to the cylindrical texture of the films [6], but in our

399

case this mechanism is unlikely because of the different crystal orientations and growth techniques.

Acknowledgements: We thank W. Oswald and J. Podschwadek for their technical help. We also thank I.K. Schuller and Y. Bruynseraede for providing the fitting program for the X-ray scattering spectra. This work was supported by the Deutsch Forschungsgemeinschaft (SFB 166) and the Ministerium fiir Wissenschaft und Forschung N R W (Germany). References [1] J. Unguris, R.J. Celotta and D.T. Pierce, Phys. Rev. Lett. 67 (1991) 140; ibid, 69 (1992) 1125; S.T. Purcell, W. Folkerts, M.T. Johnson, N.W.E. McGee, K. J~iger, J. aan de Stegge, W.B. Zeper, W. Hoving and P. Griinberg, ibid, 67 (1991) 903. [2] F. Herman, P. Lambin and O. Jepsen, Phys. Rev. B31 (1985) 4394; H. Hasegawa and F. Herman, ibid, B38 (1988) 4863; H. Hasegawa, ibid, B43 (1991) 10803; D. Stoeffier and F. Gautier, ibid, B44 (1991) 10389; Surf. Sci. 251+252 (1991) 31. [3] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304; P. Griinberg, S. Demokritov, A. Fug, M. Vohl and J.A. Wolf, J. Appl. Phys. 69 (1991) 4789. [4] G.A. Prinz, Phys. Rev. Lett. 54 (1985) 1051; Y.U. Ydzerda, W.T. Elam, B.T. Jonker and G.A. Prinz, ibid, 62 (1989) 2480. [5] P. Boher, F. Giron, Ph. Houdy, P. Beauvillain, C. Chappert and P. Veillet, J. Appl. Phys. 70 (1991) 5507. [6] N. Sato, J. Appl. Phys. 61 (1987) 1979. [7] F. Scheurer, B. Carriere, J.P. Deville and E. Beaurepaire, Surf. Sci. 245 (1991) L175. [8] W. Donner, N. Metoki, A. Abromeit and H. Zabel, Phys. Rev. B, submitted. [9] N. Metoki, W. Donner, Th. Zeidler and H. Zabel, in preparation. [10] E.E. Fullerton, I.K. Schuller, H. Vanderstraeten and Y. Bruynseraede, Phys. Rev. B45 (1992) 9292. The fitting curves in fig. 1 are omitted for clarity since they are almost identical with the data points. [11] J. Kohlhepp, S. Cordes, H.J. Elmers and U. Gradmann, J. Magn. Magn. Mater. 111 (1992) L231. [12] W.R. Bennett, W. Schwarzacher and W.F. Egelhoff, Jr, Phys. Rev. Lett. 65 (1990) 3169.