Ch. Rehm

Magnetic and structural properties of Fe/Nb multilayers

Abstract MBE grown Fe/Nb multilayers were characterized by means of X-ray diffraction in small and high angle regions, and neutron reflectivity measurements. It can be observed that this system has an oscillating ferro/antiferromagnetic ground state depending on the Nb thickness. The coherent long range magnetic order was found to be irreversible, i.e. it gets lost if one cycles the films in an external magnetic field. The influence of hydrogen charging on the magnetic and structural properties of the multilayers was studied.

Magnetic depth profile of Au/Fe/Ni trilayers

Abstract Magnetization measurements of a series of Au/Fe/Ni trilayers were carried out by spin-polarized neutron reflectometry and SQUID magnetometry. The bottom layer of Ni had the same thickness for all samples (140 A) while the top layer of Fe varied from sample to sample (16–90 A). The magnetic moment of iron was found to be dependent on the layers thickness. For Fe layers of 32 A or less, the Fe magnetization was an order of magnitude smaller than that of bulk BCC Fe, indicating that Fe is either in a non-magnetic state or that its ordering is antiferromagnetic. For Fe layers of 60 A or more, the Fe moment was found to be in the plane of the film, constant throughout its entire layer, and with a value (2.0 μ B /atom) approaching that of bulk iron. This magnetic transition probably can be attributed to a structural phase transition from the FCC to the BCC structure.

Magnetic coupling in Fe/Nb multilayers

Abstract Fe/Nb multilayers are studied by neutron small-angle reflectivity measurements. It is shown that for small thicknesses of the Nb spacer layers ( t Nb A ) an oscillating behaviour of the indirect exchange coupling is present. For larger Nb thicknesses the strength of this RKKY-like interaction decays rapidly and merges into a non-oscillatory antiferromagnetic background presumably resulting from a dipole contribution to the coupling energy.

Time-dependent measurements at the SNS liquids reflectometer

Abstract The greatly increased flux that will be available at the Spallation Neutron Source (SNS) should enhance our ability to carry out time-resolved reflectivity measurements. Construction of horizontal- and vertical-surface reflectometers is well underway and the instruments will be commissioned in early 2006. We consider how to utilize the intrinsic wavelength bandwidth of the SNS to perform “snapshot” reflectivity measurements, using the outgassing of a hydrogen-charged Fe/Nb multilayer as a test case. At the expected SNS source flux of 2 MW, we determine that kinetics in our rather favorable test case can be observed with as short as a 1-s time resolution.

Hydrogen-induced switching of the magnetic coupling in an Fe/Nb multilayer

Abstract We show that the magnetic coupling of a [26 A Fe/15 A Nb]∗18 multilayer is changes from antiferro- to ferromagnetic in a continuous and reversible way by introducing hydrogen into the sample. The magnetic structure and its change during hydrogen charging (and the following decharging) of the Nb layers is measured in situ by neutron reflectivity.

In-situ hydrogen concentration measurements in multilayers using neutron reflectivity

Abstract We show that neutron reflectivity is very useful for in situ hydrogen concentration measurements in thin films, as will be demonstrated for Fe/Nb multilayers. The samples consisting of [ 26 A Fe /X A Nb ] ∗ n with X = 15 A –40 A were charged with hydrogen from the gas phase at different pressures at 473 K. The hydrogen concentration in the Nb layers (no hydrogen is dissolved in Fe) can be determined from the change in the scattering contrast between Nb and Fe and the expansion of the Nb lattice due to the uptake of hydrogen. Both features are clearly visible in the reflectivity diagrams.