Reorientation of Spin Density Waves in Cr(001) Films Induced by Fe(001) Cap Layers

Abstract

Proximity effects of 20Å thin Fe layers on the spin density waves (SDWs) in epitaxial Cr(001) films are revealed by neutron scattering. Unlike in bulk Cr we observe a SDW with its wave vector Q pointing along only one {100} direction which depends dramatically on the film thickness tCr. For tCr < 250Å the SDW propagates out-of-plane with the spins in the film plane. For tCr > 1000Å the SDW propagates in the film plane with the spins out-of-plane perpendicular to the in-plane Fe moments. This reorientation transition is explained by frustration effects in the antiferromagnetic interaction between Fe and Cr across the Fe/Cr interface due to steps at the interface.PACS numbers: 75.25.+z, 75.30.Fv While the incommensurate spin density wave (SDW) antiferromagnetism is well established for bulk Cr [1], it is presently of high interest to analyze how the magnetic properties of Cr are altered either by reduced dimensionality in thin films or by proximity effects to ferromagnetic (FM) layers. The magnetic state of Cr is particularly interesting since ultrathin Cr films play an important role in exchange coupled Fe/Cr superlattices exhibiting giant magneto-resistance effects [2,3]. Also for theoretical treatments of the exchange coupling it is uncertain whether the Cr spacer layer should be treated as a paramagnet, an antiferromagnet, or as a proximity induced antiferromagnet [4]. In this context, the role of the Fe/Cr interface is a matter of intense study [5,6]. Magnetic domain imaging of an Fe layer deposited on a wedge shaped Cr layer on an Fe whisker shows a domain pattern switching between parallel and anti-parallel alignments having a periodicity of two Cr(001) monolayers and a phase shift consistent with a SDW state [7]. More recently, neutron scattering and perturbed angular correlation spectrocopy (PACS) have been used on Fe/Cr(001) superlattices to investigate the magnetic structure of Cr directly for Cr film thicknesses t Cr of about 30 -400Å [8][9][10]. Although some inconsistencies still remain, these experiments show that the SDW state collapses for Cr films well below the period Λ of the SDW.The aim of the present work is to gain a basic understanding of the effect of FM proximity layers on the magnetic properties of thin Cr(001) films in the SDW phase. The thickness range of the Cr films (200 -3000 A) is chosen such that the question of the presence of a SDW state is not an issue. Using neutron scattering we find that the propagation direction of the SDW depends dramatically on the Cr film thickness. Our experimental results are rationalized by computer simulations using a Heisenberg model which takes realistic Fe/Cr interfaces with interfacial roughness and interdiffusion into account. Complementary experiments with synchrotron radiation will be discussed elsewhere.We have grown epitaxial Fe/Cr(001) bilayers by molecular beam epitaxy on Al 2 O 3 (1102) substrates with a 500Å thick Nb(001) buffer layer, following well established growth recipies [11,12]. Cr(001) films with thicknesses from 200 -3000Å ...

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