T. Schmitte

Interfacial domain formation during magnetization reversal in exchange-biased CoO/CO bilayers

We have carried out detailed experimental studies of the exchange bias effect of a series of CoO/Co(111) textured bilayers with different Co layer thickness, using the magneto-optical Kerr effect, SQUID magnetometry, polarized neutron reflectivity, x-ray diffraction, and atomic force microscopy. All samples exhibit a pronounced asymmetry of the magnetic hysteresis at the first magnetization reversal as compared to the second reversal. Polarized neutron reflectivity measurements show that the first reversal occurs via nucleation and domain wall motion, while the second reversal is characterized by magnetization rotation. Off-specular diffuse spin-flip scattering indicates the existence of interfacial magnetic domains. All samples feature a small positive exchange bias just below the blocking temperature, followed by a dominating negative exchange bias field with decreasing temperature. For very thin Co-films the coexistence of ferromagnetic domains with parallel and perpendicular magnetization directions leads to a peculiar shape of the hysteresis with an extended plateau like region of almost zero magnetization.

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.

Magneto-optical Kerr effects of ferromagnetic Ni-gratings

We report measurements of the magneto-optical Kerr effect (MOKE) on an optical grating of Ni-stripes on Si(111) and a grating of Al-stripes on a Ni thin film. The analysis of the MOKE signal at different orders of diffraction n reveals a change of the Kerr signal amplitude, which is periodic in n having the same periodicity as the total intensity of the diffracted light. For the grating of Ni-stripes we observe a definite change of the shape of the magnetic hysteresis loops also varying systematically with the order of diffraction n.

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...

Magneto-optical study of the magnetization reversal process of Fe nanowires

We discuss results of magneto-optical Kerr effect (MOKE) measurements performed on a thin Fe film of 13 nm thickness, which has been patterned into a periodic arrangement of nanowires by means of optical interference lithography. The resulting array of nanowires consist of stripes having a width of 150 nm and a periodicity of 300 nm. MOKE hysteresis loops are measured within magnetic fields which are aligned in different directions, both parallel and perpendicular with respect to the direction of the nanowires as well as for various angles in between. A particular arrangement of the longitudinal Kerr effect measurement allows us to identify both the longitudinal and the transverse component of the magnetization of Fe nanowires. From this both the angle and the magnitude of the magnetization vector are derived. For a non-parallel alignment of the nanowires with respect to the direction of the external magnetic field, the hysteresis loops consist of a plateau region with two coercive fields Hc1 and Hc2, which is discussed as resulting from an anisotropic pinning behaviour of magnetic domains in directions along and perpendicular to the nanowires.

Magneto-optical Kerr effect in the diffracted light of Fe gratings

The magneto-optical Kerr effect in longitudinal configuration is used to study hysteresis loops of ferromagnetic gratings patterned by electron beam lithography. We study the Kerr effect not only in specular reflection but also off-specular at the intensity maxima of the grating at different orders of diffraction n. The Kerr rotation in saturation increases linearly with the order of diffraction n if the incoming beam is at normal incidence. The shape of the hysteresis curves changes for special orders of diffraction, indicating an enhanced sensitivity for the formation of magnetic domains close to remanence. The nth order of diffraction is sensitive to the nth order Fourier component of the magnetization distribution.

Bragg magneto-optical Kerr effect measurements at Co stripe arrays on Fe(001)

We report on magneto-optical Kerr effect (MOKE) measurements of a patterned spin valve system, consisting of few-micrometers-wide Co stripes on a continuous Fe film separated by a 14-nm-thick Cr spacer layer. The spin valve array was studied by regular longitudinal MOKE in specular geometry as well as in Bragg MOKE geometry, using the diffraction spots from the grating for hysteresis measurements. We have investigated the shape of the hysteresis loops and the Kerr amplitude as a function of the diffraction order and for various grating periods. The hysteresis loops measured at the diffracted spots reveal an amplification of the Kerr signal in the field regime where the magnetization of the Fe and Co layers is antiparallel.

Magnetooptical Kerr effect of Fe-gratings

Abstract The magnetooptical Kerr effect in longitudinal configuration is used to measure hysteresis loops of ferromagnetic Fe-gratings grown on Al2O3 at different orders of diffraction. At even order of diffraction the hysteresis loops exhibit anomalies which can be attributed to the interference of the magnetic and non-magnetic parts of the grating. The Kerr angle in saturation increases linearly with the order of diffraction.

Magnetization reversal studies of magnetic wire arrays by polarized neutron scattering

Polarized neutron reflectivity has been used to investigate the peak pattern and the magnetization reversal process of laterally structured Fe- and CoFe-films. The peak pattern of the wire arrays was analyzed in the specular as well as in the off-specular regime. The intensities of the different cross-sections at the off-specular first-order peak were investigated as a function of external magnetic field. Magnetization reversals are discussed and compared to MOKE studies of the same system.

Kerr microscopy study on sputtered Fe stripes

Sputtered polycrystalline iron gratings with a varying stripe width between 0.5 and 3.7 /spl mu/m at a constant periodicity of 5 /spl mu/m were studied by Kerr microscopy. The magnetic anisotropy direction is aligned transverse to the stripe length axis. The observed domain processes are directly compared with hysteresis curve measurements. Domain nucleation and the resulting domain structure depend on the external magnetic field direction. Complicated multidomain states are observed down to a stripe width of 1.7 /spl mu/m. The most narrowly spaced stripes display a simple domain configuration with the average magnetization vector perpendicular to the stripes, along the full film anisotropy axis, indicating dipolar coupling between the elements. With decreasing stripe width and increasing stripe distance, a change to a Landau-Lifshitz-like domain pattern is found. For very narrow stripes, it transforms into a ripple modulated domain structure. Altogether, a continuous transition from full-film-like to single strip-like magnetization behavior with increasing stripe spacing is found. The results are explained by the existing uniaxial magnetic anisotropy together with the shape anisotropy increasing with reduced stripe width. The strong influence of coercivity on the magnetization patterns is shown.