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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 ...

Hydrogen in yttrium is of fundamental interest as a model system for driving metal-insulator transitions including switchable optical properties from reflecting to transparent in the visible region. We report on the structural properties of hydrogenated and deuterated thin, monocrystalline Y͑0001͒ films grown by molecular beam epitaxy on Nb/Al 2 O 3 substrates. X-ray diffraction reveals the response of the host metal lattice upon hydrogen loading. The structural coherence in all three spatial directions as well as the epitaxial relation to the substrate are maintained, though the sample undergoes structural phase transitions between the different hydride phases. With neutron reflectivity measurements we have determined the hydrogen and deuterium content since the critical angle for total reflection depends on their concentration in the sample. Measurements on hydrogenated and deuterated films show that H and D are completely interchangeable within the trihydride phase. Neutron scattering also allows us to determine the position of the deuterium atoms within the yttrium matrix. All structural information gained on thin films is in agreement with the space group P3 c1 which was previously determined from powder samples.

The phonon-dispersion curves of the high-temperature bcc phase of La (y-La) have been measured at 1163 K. A characteristic valley of transverse and low-energy phonons extends along the [ggO] and [g'2g] propagation directions. The eigenvectors of these modes are in the direction of the displacements needed for phase transitions to low-temperature close-packed structures. On the one hand they are indicative of the low potential barrier of the bcc phase for displacements toward these close-packed structures, whereas on the other hand the bcc structure is stabilized by the contribution of these lowenergy modes to the lattice entropy. All these modes are strongly damped and have lifetimes of a few vibrational periods. Interference effects due to multiphonons can be the origin of the observed pronounced alterations of the one-phonon scattering law.