Magneto-optical Kerr Effect Hysteresis Loops Research Articles

Magnetization reversal of a thin polycrystalline cobalt film measured by the magneto-optic Kerr effect (MOKE) technique and field-dependent magnetic force microscopy

The magnetic properties of a 100-nm-thick cobalt film, deposited thermally on a glass substrate under high-vacuum conditions, were measured ex situ through the longitudinal magneto-optic Kerr effect (MOKE) technique and through field-dependent magnetic force microscopy (MFM). The MOKE measurements showed that the film had a uniaxial anisotropy in the plane of the sample. Two series of MFM images were measured under the presence of an external magnetic field, applied separately along the easy and hard axes, with magnitudes encompassing positive and negative saturation of the sample magnetization. Each series of images recorded the magnetization reversal clearly, revealing and explaining distinctions between the two reversal processes that correlate well with the macroscopic reversals recorded by the MOKE technique. The reversal of the gray-scale polarity with the change of the applied field from negative saturation to positive saturation suggests the presence of a component to the magnetic force that was related directly to the magnetization of the individual grains. A calculation of the easy-axis MFM gray-scale image differences showed a hysteresis-like behavior that agreed qualitatively with the corresponding measured MOKE hysteresis loops.

Magnetization reversal and magnetoresistance in a lateral spin-injection device

We have investigated the magnetization reversal and magnetoresistance (MR) behavior of a lateral spin-injection device. The device consists of a two-dimensional electron gas (2DEG) system in an InAs quantum well and two ferromagnetic (Ni80Fe20) contacts: an injector (source) and a detector (drain). Spin-polarized electrons are injected from the first contact and propagating through InAs are collected by the second contact. By engineering the shape of the permalloy film distinct switching fields (Hc) from the injector and the collector have been observed by scanning Kerr microscopy and MR measurements. Magneto-optic Kerr effect (MOKE) hysteresis loops demonstrate that there is a range of magnetic field (20–60 Oe), at room temperature, over which magnetization in one contact is aligned antiparallel to that in the other. The MOKE results are consistent with the variation of the magnetoresistance in the spin-injection device.

Magnetization reversal of nanostructured Co/Pt multilayer dots and films studied with magnetic force microscopy and MOKE

Large scale periodic arrays of magnetic nanostructures consisting of e.g. Co/Pt multilayer dots with perpendicular magnetic anisotropy are possible candidates for future high-density magnetic storage media. We have fabricated periodic dot-arrays by optical interference lithography with Ar/sup +/ ion lasers operating at wavelengths of 457 nm and 244 nm, respectively. Periodicities range from 125 nm to 1100 nm with dot diameters between 70 nm and 740 nm covering a total area of up to 20 cm/sup 2/ with a maximum dot density of about 4.1/spl times/10/sup 10/ dots/in/sup 2/. The global magnetic properties of these Co/Pt dot-arrays and of corresponding homogeneous films are investigated by the magneto-optical Kerr effect (MOKE) in polar geometry. Results of the magnetization reversal are compared to magnetic force microscopy (MFM) investigations in external magnetic fields of up to H=1 kOe perpendicular to the sample. A simple MFM-data analysis is described which allows to determine hysteresis loops from a series of MFM images obtained in various external magnetic fields. This then allows to measure the coercivity H/sub c/ of a given sample on small length scales. A comparison with MOKE hysteresis loops shows good agreement, although the unknown hysteretic behavior of the tip cannot be separated. The MFM-image analysis described therefore stands as a semi-quantitative method, which is however useful to measure relative changes of the coercivity on small length scales.

Influence of the top and bottom interface on perpendicular magnetic anisotropy in Tb-Fe-Ag multilayers

Mossbauer spectroscopy (CEMS), vibrating-sample magnetometry and the polar magneto-optic Kerr effect (MOKE) have been employed to investigate the effect of either "top" (Tb-deposited-on-Fe) or "bottom" (Fe-deposited-on-Tb) interfaces, or of both types of interfaces, on magnetic properties of UHV-grown Tb-bcc-Fe-Ag multilayers. All samples develop perpendicular Fe-spin texture, perpendicular magnetic anisotropy, polar remanence, and large polar coercivities upon cooling; however, the T-dependence is found to be different for different types of interfaces. The remanent-magnetized state of the Fe-layers at low T is either nearly a perpendicular single-domain state or a nearly perpendicular multidomain state, both states being characterized by canted Fe-spins or canted easy axes (cone state). The shape of MOKE hysteresis loops is found to be wavelength dependent. Qualitatively, all effects may be correlated with differences in Tb-Fe interface roughness.

Effect of surface roughness on magnetic properties of Co films on plasma-etched Si(100) substrates

Magnetic and morphological properties of ∼970-Å-thick Co films, deposited simultaneously on ten plasma-etched Si(100) substrates, were measured through the magneto-optic Kerr effect (MOKE) technique, ferromagnetic resonance (FMR), magnetic force microscopy (MFM), and atomic force microscopy. As the etch time t increased from 0 to 100 min, the vertical interface width w of Co films increased from ∼5 to ∼1400 Å; the lateral correlation length ξ, from ∼300 to ∼10 500 Å. The MOKE and FMR measurements gave the in-plane azimuthal angular dependence of the hysteresis loops and the ferromagnetic resonance absorption spectra, respectively. From MOKE and FMR, the smoother films showed uniaxial magnetic anisotropy (t⩽40 min). The uniaxial anisotropy decreased with the increase of the surface roughness and disappeared for the roughest films (t=60 and 100 min). The MOKE hysteresis loop measurements suggested that, with the increasing surface roughness, the magnetization reversal changed gradually from magnetization rotation dominated for the smoothest films to domain-wall motion dominated for the roughest films. The MFM images of the films showed anisotropic magnetic domain contrasts for the smooth film, and the subsequent fragmentation of these domains as the roughness parameters increased. This supports the results of the MOKE and FMR measurements.

Influence of the crystal structure on the magnetic property of Co/Cr superlattices

Co/Cr superlattices with Co layer thickness of 15 Å and Cr layer thickness of 10 Å have been successfully grown on different substrates, such as MgO, Si, and quartz, by molecular-beam-epitaxy. The crystal structure of all samples has been characterized by in situ reflection high-energy electron diffraction. Both magnetoresistance (MR) and magneto-optical Kerr effect (MOKE) measurements were used to analyze the magnetic properties of these samples. Excellent epitaxial growth of hcp-Co(11̄00)/bcc-Cr(211) superlattices grown on MgO(110) was observed. Co/Cr superlattices were polycrystalline when grown on Si and amorphous when grown on quartz. Isotropic MRs of less than 1% were observed for all polycrystalline and amorphous Co/Cr superlattices. For Co(11̄00)/Cr(211) superlattices grown on MgO(110), a transverse MR of 14.2% and longitudinal MR of 0.67% were observed. MOKE hysteresis loops show perpendicular magnetization for polycrystalline Co/Cr superlattices grown on Si, and in-plane isotropic magnetization for amorphous Co/Cr superlattices grown on quartz. MOKE hysteresis loops for Co/Cr superlattices grown on MgO(110) show strong anisotropic behavior between the hard and easy directions. The magnetic properties of Co/Cr superlattices are strongly influenced by the crystal structure of Co and Cr which is controlled by the substrate.

Fourfold anisotropy and structural behavior of epitaxial hcp Co/GaAs(001) thin films.

Thin Cr-capped Co films (thickness 50--150 \AA{}) have been grown epitaxially on GaAs(001) single-crystal substrates by molecular-beam epitaxy. In contrast to other investigations, transmission electron diffraction reveals that the epitaxial Co films have a hexagonal-close-packed (hcp) structure with the epitaxial relationships (1\ifmmode\bar\else\textasciimacron\fi{}21\ifmmode\bar\else\textasciimacron\fi{}0)[0001]Co\ensuremath{\parallel}(001)[110]GaAs and (1\ifmmode\bar\else\textasciimacron\fi{}21\ifmmode\bar\else\textasciimacron\fi{}0)[0001]Co\ensuremath{\parallel}(001)[1\ifmmode\bar\else\textasciimacron\fi{}10]GaAs, i.e., the c axis of each crystallite is assigned to either the in-plane [110] or the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction of the GaAs(001) substrate. In-plane magneto-optical Kerr-effect hysteresis loops and Brillouin light-scattering measurements show that the thinnest epitaxial hcp Co film (50 \AA{}) has a dominant fourfold anisotropy with easy axes along the in-plane 〈100〉 directions and that a uniaxial anisotropy becomes dominant with increasing film thickness. We are able to account for the magnetic anisotropy properties of these epitaxial Co films in terms of their oriented hcp microstructure.

Strong anisotropies in MBE-grown Co/Cr(001): Ferromagnetic-resonance and magneto-optical Kerr-effect studies.

Ferromagnetic-resonance (FMR) and magneto-optical Kerr-effect (MOKE) studies of molecular-beam-epitaxy grown Co/Cr(001) superlattices and single layers are presented. The FMR measurements were performed in different sample orientations and in a broad frequency range (9--92 GHz). MOKE hysteresis loops were detected in polar and in longitudinal configuration. The in-plane anisotropy parameters, ${\mathit{K}}_{1}$ and ${\mathit{K}}_{2}$, the out-of-plane anisotropy (i.e., the effective magnetization including the surface anisotropy, ${\mathit{K}}_{\mathit{s}}$), and the g factor were determined for these samples, which exhibit an anomalous out-of-plane lattice expansion of the hcp Co(112\ifmmode\bar\else\textasciimacron\fi{}0) on bcc Cr(001) as a precursor of a structural phase transition. Due to the strong in-plane anisotropy the switching of the magnetization to out-of-plane easy axis can occur at lower thicknesses (\ensuremath{\approxeq}14 \AA{}) than the change of sign of the effective magnetization (\ensuremath{\approxeq}21 \AA{}). In the intermediate thickness range a peculiar situation can be found with the orientation of the magnetization along the surface normal lying energetically between the in-plane easy and hard axis. Taking into account these effects as well as a reduced magnetic moment, ${\mathit{K}}_{\mathit{s}}$\ensuremath{\approxeq}0.76 erg/${\mathrm{cm}}^{2}$ is found. For thin Co layers the g factor is found to be increased in comparison with the bulk value. This indicates modifications of the electronic properties accompanied with the considerable structural changes.

Structural dependence of the oscillatory exchange interaction across Cu layers.

A structure-dependent exchange coupling across wedge-shaped Cu layers in fcc (100) Co/Cu/Co and bcc (100) Fe/Cu/Fe epitaxial sandwiches has been determined from analysis of longitudinal magneto-optical Kerr effect hysteresis loops. In the fcc Co/Cu/Co system, the variation of the coupling with Cu thickness can be described as a superposition of two oscillatory terms, with periods of 2.6 and 8.0 monolayers (ML). The bcc Fe/Cu/Fe system shows oscillations with a period of 2 ML. These periodicities are related to the Fermi surface dimensions and topology of fcc and hypothetical bcc bulk Cu.

Magneto-optic Kerr effect hysteresis loop measurements on rf-sputtered iron oxide thin films

Hysteresis curves of Fe3O4 and γ-Fe2O3 films were obtained using magnetio-optic Kerr effect measurements. The polar Kerr rotation angles of Fe3O4 and γ-Fe2O3 at 632.8 nm were determined to be 4.8 and 3.0 min, respectively.

Magneto-optic Kerr effect hysteresis loop measurements on particulate recording media

A direct comparison between the surface and the bulk hysteretic behavior of particulate oxide recording media was made for the first time. The surface properties were measured using a magneto-optic Kerr effect apparatus operating in fields up to 10 kOe while the bulk hysteresis loops were measured with a vibrating sample magnetometer. Because of the very small Kerr rotations observed, typically an order of magnitude less than those of metal films, careful attention to experimental detail was required. Several different types of particulate recording media were measured. The surface coercivity was found to be about 10% higher than the bulk coercivity for some of the high energy tapes; however, it was virtually identical to the bulk coercivity in several other samples. A thorough discussion of the apparatus is given.