A. Schreyer

Asymmetric magnetization reversal on exchange biased CoO/Co bilayers

We study magnetic hysteresis loops after field cooling of a CoO/Co bilayer by MOKE and polarized neutron reflectivity. The neutron scattering reveals that the first magnetization reversal after field cooling is dominated by domain wall movement, whereas all subsequent reversals proceed essentially by rotation of the magnetization. In addition, off-specular diffuse scattering indicates that the first magnetization reversal induces an irreversible change of the domain state in the antiferromagnet.

Direct Observation of Non-Collinear Spin Structures in Fe/Cr(001) Superlattices

We have studied the non-collinear interlayer exchange coupling in Fe/Cr(001) superlattices as a function of growth temperature using polarized neutron reflectometry with exit beam polarization analysis. We confirm that the occurrence of non-collinear spin structures is correlated with long-range lateral Cr thickness fluctuations, which, in turn, depend on the growth temperature. We find surprisingly strong coupling between the Fe layers. We explain our data using the recently proposed proximity magnetism model instead of the currently used theory of bilinear and biquadratic exchange coupling.

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

Proximity effects of 20 \AA 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 t_{Cr}. For t_{Cr} 1000 \AA 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.

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...

ADAM, the new reflectometer at the ILL

Abstract The new reflectometer ADAM at the ILL is described and some of the results obtained in the first year of operation are presented. These include a reflectivity of a Si wafer over eight orders of magnitude, a measurement of a thick [56Fe/57Fe] isotope superlattice, and a polarised reflectivity of a Co/Cu multilayer. The instrument is now available to outside users.

Neutron reflectometry on magnetic thin films

Abstract The current interest in the magnetism of ultrathin films is driven by their manifold applications in the nano-technology area, for instance as magnetic field sensors or as devices for information storage. Neutron scattering has played a dominant role for the determination of spin structures, phase transitions, and magnetic excitations in bulk materials. Today, its potential for the investigation of thin magnetic films has to be redefined. In the field of thin film magnetism, polarized neutron reflectivity (PNR) at small wave vectors can provide precise information on magnetization vectors in the film plane and on their variation from plane to plane. Therefore, neutron scattering remains the only method which allows to unravel the magnetization in thin films and superlattices independent of their thickness and depth below the surface. In addition, PNR is not only sensitive to structural interface roughness but also to the magnetic roughness. Some new developments will be discussed.

Spin polarized neutron reflectivity study of a Co/Cu superlattice

We present spin polarized neutron reflectivity data on a Co/Cu(111) superlattice and show how not only the magnitude but also the orientation of the average magnetic moment of each layer can be extracted by analyzing the polarization of the reflected beam. This method allows more detailed conclusions about the exchange coupling of magnetic layers across nonmagnetic interlayers and the magnetic in‐plane anisotropy in such systems. We present a theoretical fit to the spin‐flip and non‐spin‐flip data which leads to quantitative conclusions about the spin structure. These spin polarized neutron reflectivity results coincide well with the macroscopic magnetic properties which were measured using the magneto‐optic Kerr effect revealing a newly discovered uniaxial anisotropy in this system.

The new reflectometer ADAM at the ILL

Abstract The main characteristics of the new reflectometer ADAM at the ILL are described. A measured reflectivity of a Si wafer is shown, which demonstrates a dynamic range of up to 8 orders of magnitude. The instrument is now available to outside users.

Correlation between non-collinear exchange coupling and interface structure in Fe/Cr(001) superlattices

Abstract Using polarized neutron reflectometry with polarization analysis we have studied the non-collinear interlayer exchange coupling in Fe/Cr(0 0 1) superlattices as a function of growth temperature. From diffuse X-ray spectra we find that the occurence of non-collinear spin structures is correlated with long range lateral Cr-thickness fluctuations which, in turn, depend on the growth temperature. This finding leads the way to a better understanding of the origin of non-collinear coupling in Fe/Cr(0 0 1).

Direct observation of non-collinear spin structures in Fe/Cr (1 0 0) superlattices using spin polarized neutron reflectivity

Abstract Using the sensitivity of spin polarized neutron reflectivity with exit beam polarization analysis to the orientation of the in-plane magnetic moment, we have directly observed a non-collinear magnetization profile in Fe/Cr (1 0 0) superlattices which results from biquadratic coupling. A new approach is presented which allows the quantitative understanding of the data and which leads to detailed conclusions about the magnetization profile and magnetic domain structure in the sample. These conclusions are fully validated by domain observations with Kerr microscopy.