Th. Zeidler

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.

Structural and magnetic studies of FexCo1−x(001) alloy films on MgO(001) substrates

Thin epitaxial FexCo1−x alloy films sputtered on MgO(001) substrates were studied by x‐ray scattering and by the magneto‐optical Kerr effect in order to investigate the structural phase stability induced by the substrate and to relate the magnetic properties to the alloy film structure. Alloy films with a thickness of about 20 nm and concentrations from x=1 to x=0.3 exhibit a bcc (001) growth with the in‐plane [100] axis parallel to the MgO[110] axis. For pure Co (x=0) a fcc (001) structure with the Co[100] axis parallel to the MgO[100] axis was obtained. For x=0.15 no out‐of‐plane or in‐plane Bragg peak could be observed, possibly due to the bcc‐fcc structural phase transition. Kerr rotation angles in saturation as a function of x reveal a shape similar to the Slater–Pauling curve. Magnetic hysteresis measurements reveal a fourfold anisotropy with easy axes parallel to the FexCo1−x[100] crystal axis for 1≥x≥0.8, and parallel to the FexCo1−x[110] crystal axis for 0.7≥x≥0.3. In pure Co the easy axis lies p...

UNIAXIAL MAGNETIC ANISOTROPY OF CO FILMS ON SAPPHIRE

Abstract We have studied the magnetic properties of thin Co(0001) films on sapphire (11 2 0) substrates for thicknesses ranging from 10 to 40 nm. In this range the Co films, grown by molecular beam epitaxy techniques, exhibit a well defined orientational relationship to the sapphire substrate and a high structural coherence parallel to the film normal. Longitudinal magneto-optic Kerr effect measurements are presented which show that in addition to the usual in-plane anisotropy the magnetization strongly depends on the azimuthal angle between the magnetic field and an in-plane crystallographic direction. Instead of the expected six-fold crystal anisotropy we find a uniaxial anisotropy. This azimuthal anisotropy is solely due to an interface interaction and may be caused by internal oxidation of those Co atoms which are close to the sapphire substrate.

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.

Structural and magnetic properties of Co/face-centered-cubic Mn(001) multilayers

The structural and magnetic properties of Mn in Co/Mn (001) multilayers were investigated by x‐ray scattering, by magneto‐optical Kerr effect measurements, and with a Faraday balance. The multilayers were deposited by rf‐sputtering on MgO(001) substrates. Small angle x‐ray reflectivity scans and high angle Bragg scattering confirmed good layer quality and crystal coherence properties. In‐ and out‐of‐plane x‐ray scattering revealed a coherent growth of face‐centered‐cubic (fcc) (or fct) Mn on fcc Co(001). The stability limit of the metastable fct Mn structure is about 20 A or 10 atomic layers. The magnetic exchange coupling appears to be partly antiferromagnetic with a first maximum located between 20 and 30 A. Saturation magnetization measurements indicate no ferromagnetic moment for the fct Mn layers. Furthermore, the saturation moment per unit volume is reduced as compared to the bulk Co value, suggesting the presence of ‘‘dead layers’’ at the Co/Mn interfaces.

FMR study of MBE-grown Co films on Al2O3 and MgO substrates

Abstract FMR studies of epitaxial Co(0001) films on Al2O3(11 2 0) and Co(11 2 0) on MgO(001) substrates have been performed at the X-band and in the temperature range 80–400 K. For the samples on sapphire substrates the resonance field behavior as a function of the dc magnetic field orientation in the plane of the film exhibits a uniaxial anisotropy. In contrast, for the Co films on MgO substrates a fourfold anisotropy in the film plane has been observed. Both in-plane anisotropies are unexpected with regard to the Co film structure. The FMR analysis shows that the fourfold anisotropy for Co/MgO occurs due to a twinned film structure consisting of two Co domains with their c-axes in the film plane and perpendicular to each other.

Structure and magnetic properties of Fe films on Al2O3(112¯0) and MgO(001)

Abstract Structural and magnetic properties of Fe thin films (4–60 nm thick) on Al 2 O 3 (112¯0) and MgO(001) have been studied by X-ray scattering and Kerr effect measurements. Fe on Al 2 O 3 exhibits a strong in-plane uniaxial anisotropy, possibly due to the oxidation of Fe films on Al 2 O 3 substrates. In contrast, no interfacial anisotropy has been observed for Fe on MgO, indicating a very weak interaction of Fe with MgO.

Temperature dependence of noncollinear magnetic coupling in Fe/Cr(001) superlattices

Using polarized-neutron reflectivity, we have measured the temperature dependence of the noncollinear magnetic coupling of ferromagnetic iron across chromium interlayers in two superlattice samples. The first sample, (52 A Fe/17 A Cr)9, exhibits a room temperature remanent magnetic structure consisting of successive Fe layer moments aligned at 50° relative to each other. This magnetic structure remains unchanged upon cooling the sample to 42 K. In contrast, a superlattice with a thicker Cr interlayer, [44 A Fe/88 A Cr]30, passes in a 20 K-wide transition centered on TN=240 K from a room temperature state with Fe moments aligned at 90° to an uncoupled state. The formation of the incommensurate Cr spin density wave state below TN suppresses the interlayer coupling. The thinner Cr interlayers do not undergo a transition to this bulk structure, hence exhibit no suppression of coupling. In both samples, the coupling strength decreases for temperatures at and above room temperature.

Anisotropy studies of AFM coupled MBE grown Co/Cu(001) superlattices

We have studied the anisotropy behavior of antiferromagnetically (AFM) coupled fcc Co/Cu(001) sandwiches and superlattices. Magneto‐optical Kerr effect measurements on samples with Cu film thicknesses in the range of the second antiferromagnetic maximum reveal characteristic shapes of the hysteresis loops in close agreement with theoretical predictions. For the AFM coupled samples we infer antiparallel spin orientations in remanence perpendicular to the originally applied magnetic field direction. Changing the magnetic layer thickness and maintaining a constant thickness of the spacer completely alters the shape of the hysteresis loops. The loops of weakly coupled AFM layers show characteristic steps, indicative for a rather unusual nonsymmetric spin state. This behavior can be explained by the competing effects of anisotropy with the exchange coupling.

Structural and magnetic properties of Co/Cr(001) superlattices

Epitaxial Co/Cr(001) superlattices were prepared by Molecular‐Beam‐Epitaxy (MBE). The crystal structure was determined both with Reflection High Energy Electron Diffraction (RHEED) and x‐ray diffraction experiments. The Co layers grow in an hcp phase with their (11.0) planes oriented parallel to the Cr(001) planes. The magnetic anisotropy was measured using the Magneto‐optical Kerr Effect (MOKE) and Ferromagnetic Resonance (FMR) technique. It is found that the easy axis of the magnetization is perpendicular to the film plane below a Co thickness of about 14 A. The value of the interface anisotropy constant Ks is determined to (0.85±0.1)erg/cm2 assuming the bulk magnetization of hcp Co.