J. F. Cochran

FMR linebroadening in metals due to two‐magnon scattering

The theory of two‐magnon scattering is extended to disordered ferromagnetic metals. In order to treat properly the role of two‐magnon scattering in metals we have derived the formula for the rf susceptibility in the parallel (in plane) configuration by using the retarded Green’s function formalism. The roles of the elliptical polarization, the finite values of resonance k vectors, nonresonant spin‐wave interactions, eddycurrent effects as well as the contribution of spin‐orbit interaction to the intrinsic damping are included. We have evaluated the susceptibility in the presence of magnetostatic inhomogeneities, statistical variations of local properties and fluctuating exchange interactions.

Large surface anisotropies in ultrathin films of bcc and fcc Fe(001) (invited)

Large uniaxial anisotropies associated with interfaces are observed for ultrathin films (3‐28 ML) of bcc Fe(001) grown epitaxially on Ag(001) single‐crystal substrates and for epitaxial sandwiches of fcc Fe(001) grown with three layers of Fe using Cu as substrate and coverlayers. The uniaxial anisotropy is well described by a pseudosurface anisotropy term as theoretically predicted, yet that theory also predicts large in‐plane anisotropies that are not observed. Adequate treatment of spin‐orbit coupling in magnetic theories remains a challenge. Comparisons of ultrathin films of bcc Fe(001) on Ag(001) with different coverlayers of Ag or Au show subtle differences in magnetic behavior as studied by ferromagnetic resonance (FMR) and Brillouin light scattering (BLS). The FMR measurements were carried out at 9.6, 36.6, and 73 GHz microwave frequencies. The BLS measurements were performed using a six‐pass Fabry–Perot interferometer. The power of the techniques of molecular‐beam epitaxy (MBE) for producing well‐...

Magnetic anisotropies in single and multilayered structures (invited)

Ultrathin magnetic metallic structures provide a variety of systems in which unique magnetic properties can be engineered. The investigation of magnetic anisotropies in ultrathin structures has brought exciting results to the basic studies of magnetism in systems with reduced dimensionality and to the engineering of new magnetic materials. Molecular‐beam epitaxy (MBE) techniques turned out to be particularly useful in the formation of new stable and metastable structures whose sharply defined interfaces, lattice relaxations, and lattice reconstructions have resulted in a wide range of interesting magnetic properties. In this presentation the present authors’ recent studies of single and multilayered structures composed of bcc Fe(001), bcc Cu(001), lattice‐expanded Pd(001), fcc Co(001), and fcc Cu(001) will be summarized. Besides presenting their interesting magnetic properties the following techniques will be highlighted: reflection high‐energy electron diffraction (RHEED) and ferromagnetic resonance (FMR...

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asKus = 0.96, 0.63, and 0.3 ergs/cm2 respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K1∥S=0.012 ergs/cm2. Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.

Magnetic properties of sandwiches and superlattices of fcc Fe(001) grown on Cu(001) substrates

The magnetic properties of Cu/Fe epitaxial sandwiches and superlattices have been measured using Brillouin light scattering and ferromagnetic resonance. All of the samples are perpendicularly magnetized at room temperature, due to the presence of large uniaxial anisotropies with an easy axis perpendicular to the sample surface. Inclusion of a second‐order uniaxial anisotropy term is particularly important for one of the superlattice samples.

Interface alloying at Fe/Cr interfaces and its role in exchange coupling, angular resolved Auger electron, magneto‐optic Kerr effect, and Brillouin light scattering studies (invited)

Angular resolved Auger electron studies were carried out for Fe whisker/Cr(001) interfaces which were prepared at 100, 180, 246, and 296 °C. The Cr atoms penetrate progressively into the second (counting from the surface) atomic layer at 100, 180, and 246 °C. At 296 °C the Cr atoms enter the third atomic layer. No noticeable fraction of the Cr atoms was found in the fourth atomic layer. The exchange coupling was studied in Fe whisker/Cr/Fe(001) films which were grown in a nearly perfect layer by layer mode. Magneto‐optic Kerr effect and Brillouin light scattering measurements showed that the measured change in the phase of the short wavelength oscillations, the presence of a slowly varying exchange coupling bias, and the small measured values of exchange coupling are caused by the same mechanism: interface alloying. The exchange coupling in Fe whisker/Cr/nFe specimens, for n=10, 20, 30, and 40 ML, showed no obvious dependence on the Fe layer thickness.

Ferromagnetic resonance in ultrahigh vacuum of bcc Fe(001) films grown on Ag(001)

Ferromagnetic resonance studies carried out in ultrahigh vacuum at 16.88 GHz on bcc Fe (001) films 5–14.2 monolayers (ML) thick grown on Ag (001) substrates indicate that an ultrathin Fe film 5 ML thick should be magnetized perpendicular to the specimen plane at room temperature. Covering the bare Fe specimens with Ag causes a substantial reduction in the uniaxial surface anisotropy for all Fe film thicknesses and would put the moment of a 5‐ML film back into the plane. For a given Fe film thickness, the maximum obtainable uniaxial surface anisotropy depends on both the amount of oxygen contamination in the film and on the surface roughness.

Calculation of the intensity of light scattered from magnons in thin films

Abstract A computational scheme is described for the calculation of the intensity of light scattered from thermal magnons in a uniformly magnetized thin film. The frequencies and magnetization distributions corresponding to magnetic normal modes of the film are calculated. The mean amplitude of the normal mode is determined from the requirement that the mean energy stored in the mode be equal to the thermal energy of an oscillator having the same frequency. The oscillating magnetization distribution modulates the optical dielectric tensor of the film material at the normal mode frequencies and this modulation produces scattered light whose frequency is shifted from the incident optical frequency by the magnetic normal mode frequencies. It is anticipated that some workers may find the formalism described herein, which is based upon familiar concepts from statistical mechanics, easier to implement than the formalism, based upon an application of the fluctuation-dissipation theorem, which was used by Camley, Rahman and Mills to treat the same problem.

Analysis of bilinear and biquadratic exchange coupling in Fe/Ag/Fe(001) trilayers

Ferromagnetic resonance (FMR) and surface magneto‐optical Kerr effect (SMOKE) studies of the exchange coupling in Fe/Ag/Fe(001) structures are presented. The interfaces in these structures can be improved significantly by growing the first Fe(001) layer at a raised substrate temperature. The exchange couplings in Fe/Ag/Fe trilayers were studied as a function of the interlayer thickness and temperature. The improved interfaces in the Fe/Ag/Fe system revealed new features in the exchange coupling which were absent in samples grown entirely at room temperature. Quantitative data from the FMR and SMOKE measurements are compared. The magnetization loops for Fe/Ag/Fe trilayers can be explained well only by including the simultaneous presence of bilinear and biquadratic exchange coupling. It is shown that the exchange coupling through Ag(001) exhibits long wavelength oscillations.

Engineering magnetic materials on the atomic scale (invited)

Ultrahigh vacuum (UHV) systems and the use of atomic beams for deposition of atoms layer by layer combine to make possible the creation of new materials. The applications to metallic magnetism are gaining increasing attention. The building of sandwiches of magnetic and nonmagnetic layers should lead to increased understanding of the propagation of spin polarization through metals and the effects of finite thickness on the ground state properties and the thermodynamics of magnetic materials. The most important step in this process is in the first layer, i.e., the preparation of the substrate and the determination of the quality of the interface and of the overlayer. The techniques of surface science, e.g., residual gas analysis (RGA), reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES), and x‐ray photoemission spectroscopy (XPS) are essential for the characterization of the interface. Illustrations of these include our own work on body‐centered‐cubic Ni deposited epitaxia...