P. Bödeker

Epitaxial oxide formation on Cr(110) films

Abstract We have investigated the oxidation of Cr(110) at 330°C and high oxygen exposures by surface sensitive X-ray scattering. Single crystal Cr(110) films prepared by molecular beam epitaxy (MBE) were used. Via in-situ X-ray reflectivity measurements the exact oxide thickness could be determined. New information about the metal/oxide interface was obtained: the oxidation is proceeding without roughening of the interface and the Cr(110) layers are oxidized in a layer-by-layer fashion. The number of Cr(110) planes can be counted before and after oxidation from in-situ Laue oscillation measurements of the Cr(110) Bragg peak. This reveals detailed information about the material consumption by the oxidation process. Additional ex-situ in-plane Bragg scans under grazing incidence show that an epitaxial, orthorhombic Cr2O3 layer in (0001) orientation is formed. The in-plane structure of the oxide layer can be improved by annealing in ultra-high vacuum.

Neutron scattering on magnetic thin films: Pushing the limits (invited)

Neutron scattering has been the scattering technique of choice for the analysis of magnetic structures and their dynamics for many decades. The advent of magnetic thin film systems has posed new challenges since such samples have inherently small scattering volumes. By way of examples, recent progress in the application of neutron scattering for the study of both magnetic structure and dynamics in magnetic thin film systems will be presented. First, a combined high angle neutron scattering and polarized neutron reflectivity investigation of the magnetic order of Cr and its influence on the exchange coupling between the Fe layers in Fe/Cr superlattices is discussed. It is shown that in the whole thickness range up to 3000 A, the magnetic structure is governed by frustration effects at the Fe/Cr interfaces. Second, it is demonstrated that it is now possible to investigate the dynamic properties of magnetic thin films with neutron scattering. Unlike, e.g., Brillouin light scattering, inelastic neutron scatte...

Epitaxial growth of Co films and Co/Cu superlattices on sapphire substrates with and without buffer layers

Co(0001)/(111) films and Co/Cu(111) multilayers were grown by MBE on sapphire (1120) substrates. Co grows in a good epitaxial manner directly on the sapphire substrate as well as on a Cu(111)/Nb(110) buffer layer system. Co/Cu(111) superlattices were grown on the same buffer system. Extensive RHEED and X-ray characterizations demonstrate the high film quality.

FMR studies of magnetic properties of Co and Fe thin films on Al2O3 and MgO substrates

The effect of substrates on the magnetic properties has been studied for Co and Fe films both on Al2O3 (1120) and MgO (001) substrates by using ferromagnetic resonance techniques. For Fe(001)/MgO(001) samples the thickness dependence of the magnetocrystalline constant and of the effective magnetization values have been determined from the in‐plane angular variation of the resonance field H0. Different reasons for the thickness dependencies of these parameters are discussed. For Co(111)/Al2O3(1120) the angular variation of H0 exhibits an uniaxial anisotropy, for which several causes are discussed. For Co(1120)/MgO(100) a four‐fold in‐plane anisotropy was observed which is due to the twinned structure of these samples.

Magnetic in-plane anisotropy of MBE grown Co/Cu(111) superlattices

Abstract The magnetic in-plane anisotropy of molecular-beam epitaxy (MBE) grown Co/Cu(111) superlattices is investaged by ferromagnetic resonance (FMR). It is shown that the sixfold anisotropy contribution expected for fcc(111) and hcp(0001) oriented films is small, but nevertheless can be detected. This is mainly due to the resonance condition being governed by the derivatives of the free energy density, so that contributions from higher order terms enter with higher weighting. Evidence is found for a correlation between the in-plane anisotropy parameters and the structural quality as well as the magnetic coupling behaviour. It is proposed to use the sixfold in-plane anisotropy as a very sensitive test for the structural in-plane coherence of thin films and superlattices containing Co grown on a (111)-oriented plane.

NMR studies of 59Co in Cu/Co/Cu(111) trilayer systems grown by MBE

Abstract NMR using the 59 Co nucleus has been measured for a series of Cu/Co/Cu(111) trilayers grown by MBE with Co thicknesses ranging from 25 to 178 A. The Co layer retains the Cu fcc structure up to a thickness of ∼ 60 A at which point the hcp structure is favoured. This structural transition is not identical for each sample suggesting that the point at which the transition occurs depends on the exact growth conditions including the thickness of the Cu underlayer. Our results suggest that for very thin (15 A) Cu underlayers the transition to hcp occurs at thicknesses less than 60 A.

Anisotropy studies of molecular‐beam‐epitaxy‐grown Co(111) thin films by ferromagnetic resonance

The magnetic anisotropy of Co/Cu(111) thin films has been investigated using the ferromagnetic resonance (FMR) technique. The films were prepared by molecular‐beam epitaxy in ultrahigh vacuum on sapphire substrates with niobium as a buffer layer. In situ RHEED investigations, ex situ x‐ray low‐angle reflectivity measurements, and high‐angle Bragg scans confirm the structural quality of the films. Angular dependent FMR measurements are performed in the plane of the films and out‐of‐plane. The angular dependence of the line positions in‐plane shows a competition between the sixfold anisotropy, which is expected for bulk Co, and a twofold anisotropy contribution. It is demonstrated that by FMR one can detect even small values of the higher‐order anisotropy terms. In the analysis, we put emphasis on the relationship between magnetic and structural properties.

Effect of Fe cap layers on the spin density waves in epitaxial Cr(001) films

We have investigated proximity effects of Fe layers on the spin density waves (SDW) and the concomitant charge density waves or strain waves in thin epitaxial Cr[001] films using synchrotron and neutron scattering. Unlike in bulk Cr we observe a strong anisotropic occupation of the three possible SDWs with their wave vectors Q pointing along the {001} directions. In a pure 3000 A thick Cr[001] film, the SDW exhibits an almost complete out-of-plane orientation, whereas in a Cr film of the same thickness capped by a 20 A Fe layer the SDW becomes completely reoriented with Q now propagating in the plane. This SDW is preserved over the entire temperature range from 10 K up to the Neel temperature of about 311 K.

Study of the interface interaction of epitaxial Fe/Cr layers

Abstract Investigations on the magnetic behavior of expitaxially grown Fe films on Cr were performed by ferromagnetic resonance (FMR) and by SQUID DC-magnetometry. The magnetic Fe parameters show a significant deviation from the expected temperature dependence which is attributed to a change of the interface coupling between Fe and Cr. The observed anomalies are related to the spin–flip in Cr where the orientation of the magnetic moments flips from out-of-plane to in-plane alignment. The spin–flip transition temperature depends on the Cr thickness. The spin–flip is suppressed for Cr thicknesses comparable to the wavelength of the sinusoidal modulated antiferromagnetic groundstate. In addition, the influence of the interface conditions on the formation of standing spin waves in Fe on Cr has been studied.

Temperature dependence of the interface interactions on Fe/Cr studied by ferromagnetic resonance and SQUID

To study the interface interactions between ferromagnetic ordered Fe and antiferromagnetic ordered Cr we used Ferromagnetic Resonance (FMR) Spectroscopy and SQUID magnetometry. For a defined Cr thickness range the magnetic parameters of the Fe films show an anomalous temperature behavior which can be related to a change of the intrinsic spin structure of the chromium, the so-called spin-flip transition. The transition temperature decreases continuously with decreasing Cr thickness, ceasing in a suppression of the transition for Cr films below 60 /spl Aring/.