Residual Stresses and Magnetoelastic Coupling in Ultrathin Fe Films Deposited on GaAs(001)
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RESIDUAL STRESSES AND MAGNETOELASTIC COUPLING IN ULTRATHIN Fe FILMS DEPOSITED ON GaAs(001). P. GERGAUD1, C. LALLAIZON2, M. PUTERO1, B. LÉPINE2, O. THOMAS1 and A. GUIVARC'H2 1 TECSEN, CNRS, FST St Jérôme, Univ Aix-Marseille III, 13397 Marseille, France 2 EPSI, UMR CNRS 6627, Bât.11C, Campus de Beaulieu, 35042 Rennes cedex, France. ABSTRACT The growing interest in the behavior of magnetic thin films on semiconductor substrates is due in part to their potential application in spin-sensitive heterostructure devices. High-quality epitaxial Fe(001) thin films can be grown on GaAs(001) substrates because of the small lattice parameter mismatch (-1.4%). Magnetic measurements performed on Fe films thinner than 3 nm have shown that such films exhibit an in-plane uniaxial magnetic anisotropy although an ideal bcc Fe(001) film should have fourfold symmetry. The source of this uniaxial component remains an open question and one of the mechanisms which may contribute to this is the epitaxial strain, through magnetoelastic coupling. Very small strains anisotropies are able to modify the magnetic anisotropy of iron thin films. Moreover the sign and magnitude of the magnetoelastic coupling seem to depend on the film thickness or film strain [1]. In this study, the strain tensor components in two Fe thin films (1.7 and 3.0 nm thick) has been measured by XRD. The magnetic free energy has been derived, using the strain tensor components. INTRODUCTION The growing interest in the behavior of magnetic thin films on semiconductor substrates is due to their potential for spin-sensitive heterostructure devices. Many studies have been devoted to bcc (001)Fe grown epitaxially on (001) surfaces of GaAs. The high quality of the Fe layers is due to the fact that the bcc Fe lattice parameter (a0=2.8664 Å) is close to half that of zinc-blende GaAs (a0=5.6538 Å), leading to a small lattice parameter misfit ((aGaAs – 2aFe )/ 2aFe = -1.37%. Ex situ magnetic measurements of thin Fe layers (< 3 nm) have shown that such thin films often have an in-plane uniaxial component to the magnetic anisotropy, although an ideal bcc Fe (001) film grown on a GaAs (001) substrate should exhibit a fourfold symmetry. The origin of this anisotropy remains an open question. Several mechanisms could induce this magnetic anisotropy : shape anisotropy, magneto – elastic coupling effects or magneto-crystalline anisotropy. This study is focused on the influence of residual strain on the magnetic properties of these Fe layers. Effectively, the magnetic energy density is strain-dependent through the contribution of the magneto-elastic coupling coefficients (MECC). The magnitude of both film strain and MECC determine how film strain alters the magnetic anisotropy. In cubic systems, these MECC are few orders of magnitude larger than the magneto-crystalline coefficients (MCC), and even small strains are likely to modify the magnetic anisotropy considerably. The easy axis of magnetization are given by the minima of the free energy density [1] : f (α i , ε j ) = f mc (α i ) + f me (α
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