A. S. Arrott

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.

Point singularities and magnetization reversal in ideally soft ferromagnetic cylinders

The principles of micromagnetics are used to describe the magnetization processes and magnetic configurations in a cylinder of finite length. The cylinder is of radius large compared to the exchange radius. Magnetostatic terms dominate in the limit that crystalline anisotropy is negligible. The exchange energy is minimized for boundary conditions primarily determined by magnetostatics. The point singularities required by topology are essential to understanding the process of magnetization. Complete reversal of magnetization is made possible by propagating point singularities down the axis of the cylinder. These propagating singularities arise from pair creation in a unit vector field.

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.

Magnetic properties of CoxFe3−xO4 during conversion from normal to inverse spinel particles

Cobalt ferrite, CoxFe3−xO4 with 0.55≤x≤0.8, is produced by precipitation from aqueous solutions. Drying and annealing at temperatures below 300 °C results in a coercivity that increases with increasing temperature. Mossbauer effect spectroscopy and magnetic measurements are consistent with the interpretation that these Co ferrites have a (mostly) normal spinel structure as made. These convert to a (mostly) inverse spinel structure on thermal aging. No significant morphological changes occur during this transformation. Coercivities as high as μ0Hc=0.2 T are observed.

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...

In-Situ Techniques for Studying Epitaxially Grown Layers and Determining their Magnetic Properties

Ultrathin films of bcc Fe (001) on Ag (001) and Fe/Ni (001) bilayers on Ag were grown by molecular beam epitaxy. A wide range of surface science tools (RHEED, REELFS, AES, and XPS) 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 the easy axis perpendicular to the film surface were observed in all ultrathin structures studied. In sufficiently thin samples the saturation magnetization was oriented perpendicular 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. Fe/Ni bilayers were also investigated. Ni grows in the pure bcc structure for the first 3–6ML and then transforms to a new structure which exhibits unique magnetic properties. Transformed ultrathin bilayers possesses large in-plane 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.

Surface magneto‐optical Kerr effect for ultrathin Ni‐Fe bilayers

Surface magneto‐optical Kerr effect is used to measure magnetic hysteresis loops for Ni‐Fe bilayers previously studied by ferromagnetic resonance [Phys. Rev. 38, 12879 (1988)]. The approach to saturation in the [11] direction confirms the existence of enhanced fourfold in‐plane magnetic anisotropy, attributed to the effect of arrays of misfit dislocations arising during the epitaxial growth of bcc Ni overlayers on ultrathin layers of bcc (001)Fe. The [11] magnetization curves for 10 monolayers (ML) of Ni on 7 ML of Fe shows a breaking of symmetry between positive and negative fields. This is ascribed to spin configurations frozen during growth.

Application of magnetization measurements in iron to high temperature thermometry

It is proposed to use iron whiskers as elements for magnetic thermometers. The sharpness and reproducibility of the large second harmonic response that occurs just as the centre of the whisker saturates depends upon temperature through Ms(T). As a thermometer the whisker has a built‐in fixed point which is reproducible to parts in 106. A one point calibration is needed to determined the demagnetizing factor. Suggestions are made for incorporating magnetic thermometers into thermocouples and resistance thermometers.