Library

Notes: We present the observation of antiferromagnetic interlayer coupling between two ferromagnetic layers of Fe(100) and EuS(100), which are in direct contact. The coupling strength J is about 0.2 mJ/m2 at 5 K, it decreases with increasing tempeature and becomes negligibly small near the Curie temperature of the EuS layer. The samples were molecular-beam epitaxy-grown on the GaAs(100) single crystal substrates using a silver buffer layer. The structural and chemical properties of the layers were characterized by in situ low-energy electron diffraction and Auger electron spectroscopy and by ex situ Rutherford backscattering. Superconducting quantum interference device (SQUID) magnetometry was used for measurements of the absolute magnetic moments as a function of temperature or magnetic field. Hysteresis measurements were performed by means of the magneto-optical Kerr effect (MOKE). We found evidence of antiferromagnetic coupling between the two different ferromagnetic layers both from zero field cooling experiments through Tc(EuS) as well as directly from the shape of the hysteresis loops at low temperatures. The temperature dependence of the interlayer coupling strength shows that J is proportional to the EuS magnetization. The observed onset temperature of ferromagnetic order in EuS is up to three times higher than the Curie temperature of bulk EuS. We discuss a mechanism in which this enhancement is connected to the existence of structural defects in the EuS film, caused by the growth. The defects produce additional band carriers, which in turn enhance the Curie temperature of the layer due to a very large indirect s(d)–f exchange.© 1997 American Institute of Physics.

Notes: The static and spin wave properties of regular square lattices of magnetic dots of 0.5–2 μm dot diameter and 1–4 μm periodicity patterned in permalloy films have been investigated by Brillouin light scattering. The samples have been structured using x-ray lithography and ion beam etching. The Brillouin light scattering spectra reveal both surface and bulk spin wave modes. The spin wave frequencies can be well described taking into account the demagnetization factor of each single dot. For the samples with smallest dot separation of 0.1 μm a fourfold in-plane magnetic anisotropy with the easy axis directed along the pattern diagonal is observed, indicating anisotropic coupling between the dots. © 1997 American Institute of Physics.

Notes: The first experimental observation of a spacer-thickness dependent oscillatory exchange bias effect in ferromagnet(FM)/spacer/antiferromagnet trilayers is reported. The period of the oscillatory exchange bias field is found to be half of the period of the oscillatory interlayer coupling in the corresponding FM/spacer/FM systems with the same spacer, indicating that the observed effect is caused by an analogous coupling mechanism, being, however, sensitive to the absolute value of the coupling strength and not on its sign. © 2000 American Institute of Physics.

Notes: An overview of the current status of the study of spin wave excitations in arrays of magnetic dots and wires is given. We describe both the status of theory and recent inelastic light scattering experiments addressing the most important issues; the quantization of localized spin waves due to the in-plane confinement of spin waves in elements, dipolar coupling between the quantized modes, and the localization of the modes within rectangular elements due to an inhomogeneous demagnetizing field. © 2001 American Institute of Physics.

Notes: We report on the observation of spin wave quantization in tangentially magnetized Ni80Fe20 disks by means of Brillouin light scattering spectroscopy. For a large wave vector interval, several discrete, dispersionless modes with a frequency splitting up to 2.5 GHz were observed. The modes are identified as being magnetostatic surface spin wave modes quantized by their lateral confinement in the disk. For the lowest modes, dynamic magnetic dipolar coupling between the disks is found for a disk spacing of 0.1 μm. © 2000 American Institute of Physics.

Notes: A new advanced space- and time-resolved Brillouin light scattering technique is used to study diffraction of two-dimensional beams and pulses of dipolar spin waves excited by strip-line antennas in tangentially magnetized garnet films. The technique is an effective tool for investigations of two-dimensional spin wave propagation with high spatial and temporal resolution. Nonlinear effects such as stationary and nonstationary self-focusing are investigated in detail. It is shown that nonlinear diffraction of a stationary backward volume magnetostatic wave (BVMSW) beam, having a finite transverse aperture, leads to self-focusing of the beam at one spatial point. Diffraction of a finite-duration (nonstationary) BVMSW pulse leads to space–time self-focusing and formation of a strongly localized two-dimensional wave packet (spin wave bullet). © 2000 American Institute of Physics.

Notes: The oxidation mechanisms of the formation of thin insulating barriers in magnetic tunnel junctions are studied. The barriers are prepared by means of an oxidation technique using a highly dissociated low-energy (30–80 eV) ionized oxygen atom beam. The barrier formation is investigated using two independent in situ techniques, resistance and an optical reflectivity method. The oxidation depth varies from 1.5 to 1.9 nm depending on the initial ion energy in agreement with performed Monte Carlo simulations. Two different oxidation mechanisms (ion embedding and diffusion) are identified. The electrical and magnetotransport properties of the junctions are studied. © 2001 American Institute of Physics.

Notes: We report on investigations of the crystallographic structure and the magnetic anisotropies of epitaxial iron films deposited onto periodically stepped Ag(001) surfaces using low energy electron diffraction, x-ray diffraction, second harmonic generation (SHG), as well as the Brillouin light scattering (BLS) technique. The focus of the present study lies on the interrelation between the surface morphology of the buffer layers and the magnetic properties of the Fe films, epitaxially grown onto them. Especially the symmetry breaking at the atomic steps is found to create an uniaxial magnetic anisotropy measured by BLS and a very strong anisotropic signal in magnetic SHG. © 2000 American Institute of Physics.

Notes: Static and dynamic magnetic properties of epitaxial Fe(001) films with Pd overlayers have been measured by means of Brillouin light scattering and magneto-optic Kerr effect magnetometry. From low energy electron diffraction studies, the initial growth of Pd [dPd〈2 monolayer (ML)] is identified as a self-formation process of quasione-dimensional Pd structures. Auger electron spectroscopy examinations proved that for dPd≥2 ML the Pd overlayers cover the Fe films almost completely. The measured frequencies of the dipolar spin wave mode and the derived anisotropy constants show a linear change with increasing dPd up to 5 ML with following saturation. The M(H) curves show pronounced anomalies in the coercive field for dPd〈2 ML accompanied by large tails in the hysteresis loops. The results are interpreted as a strong pinning of domain walls caused by large local changes in the surface anisotropy due to the inhomogeneous Pd cover layer in this regime. © 1997 American Institute of Physics.

Notes: We have investigated the propagation of surface and backward volume spin waves (BVSW) with the in-plane wavevector k(parallel)=10–900 cm−1 excited by microwaves in YIG and Lu2.04Bi0.96Fe5O12 (LBIG) films with the film thicknesses 5 and 1.5 μm, respectively. In all cases the magnetization and k(parallel) were in the film plane. The frequencies of the spin wave excitations as well as the spatial distribution of their intensities in the linear and in the nonlinear excitation regime were measured by means of a fully automated, high-stabilization, small-angle Brillouin light scattering (BLS) setup with a spatial resolution of 30 μm. It is well known1 that in the case of BVSW modes, the Lighthill criterion2 for modulational instability is fulfilled for both the longitudinal and the transverse perturbations of the initially constant-amplitude wave beam. Therefore, this mode is very much suitable for the investigation of the evolution of a plane-front, constant-amplitude initial beam, and for a direct experimental study of two-dimensional nonlinear diffraction effects of the beam leading to self-focusing. The garnet films with in-plane dimensions of 2×10 mm2 were grown by liquid phase epitaxy onto a single crystalline (111)-oriented gallium gadolinium garnet substrate. Two strip antennas, 35 μm wide and situated at the ends of the films, were used for the excitation of the spin waves and for monitoring. The working frequency was 8.10 GHz. Special efforts were taken to minimize the beam divergence due to the finite length of the input antenna. For all nonlinear studies the initial angular beam divergence was not larger than aitch-thetamax=0.05 rad. In the linear regime the attenuation factor was measured, and the reflection of the spin waves from the sample boundaries was studied. We also observed the interference between the two lowest order lateral modes having different initial spatial distributions of the magnetization. The BLS technique was shown to provide a signal to noise ratio exceeding 60 dB. In the nonlinear regime the propagation of the BVSW mode was thoroughly investigated. We have found clear evidence for the self-focusing effect, which exists in a narrow interval of input powers. In these studies overheating of the sample by the microwave power was carefully avoided by using an intermittent source of maximum average power of 100 mW working at a frequency of 4 kHz. That allowed us to claim that the observed focusing of the spin waves was not an artifact caused by sample heating. The distance between the focusing maximum and the antenna was 1.5 mm for LBIG sample and 6 mm for YIG sample. These values are in a good agreement with our theoretical calculations, if one takes into account the thicknesses of the films. For a comparison of the observed values of the width and the length of the focusing spot and their dependence on the microwave power with the theory, more numerical calculations are needed. Our results demonstrate the wide applicability of an advanced combined microwave-BLS technique for a two-dimensional mapping of the spin wave amplitudes in the media. © 1997 American Institute of Physics.