E. M. Gyorgy

Structure and magnetic properties of epitaxial spinel ferrite thin films

We have grown epitaxial spinel ferrite thin films of (Mn,Zn) Fe2O4 and CoFe2O4 on (100) and (110) SrTiO3 and MgAl2O4 buffered by spinel structure buffer layers. High quality spinel ferrite films were grown at 400 °C on buffer layers that were grown at 600 °C and postannealed at 1000 °C. Although (Mn,Zn) Fe2O4 grown directly on SrTiO3 and MgAl2O4 shows mediocre structural and magnetic properties, ferrite films grown on (100) and (110) SrTiO3 and MgAl2O4 buffered with CoCr2O4 exhibit excellent crystallinity and bulk saturation magnetization values, thus indicating the importance of lattice match and structural similarity between the film and the immediately underlying layer.

Effect of oxidation on the magnetic properties of unprotected TbFe thin films

Amorphous Tb‐Fe thin films prepared by dual magnetron cosputtering were exposed to air at 200 °C in order to investigate the evolution of the films as they oxidize. Magnetic properties of the films were measured using a vibrating‐sample magnetometer and torque magnetometer and are interpreted in light of the structure of the films as revealed by Auger electron spectroscopy and composition‐depth profiling. This leads to a detailed and self‐consistent description of the oxidation process. At first a uniform and homogeneous oxidation layer grows from the surface toward the substrate. This layer has a high magnetization and low intrinsic anisotropy and consists of an intimate mixture of oxidized Tb and metallic TbxFe(1−x). The initially high intrinsic anisotropy of the unoxidized region decreases relatively quickly, while the composition changes only slowly as this region shrinks. When the oxidation layer reaches the substrate, two oxide phases (Fe2O3 and Tb2O3) begin to grow at the surface exposed to air.

Intrinsic anisotropy of Tb‐Fe films prepared by magnetron Co sputtering

Amorphous Tb‐Fe thin films have been produced by dual‐gun dc magnetron cosputtering, which allows a moderate range of compositions (spanning about 20 at. %) to be deposited in a single run under identical conditions. The films exhibit clean M‐H loops and torque curves which are somewhat anomalous but show consistent trends. Films deposited at room temperature with a composition in the range 17–30 at. % Tb showed a positive intrinsic anisotropy of roughly 3×106 erg/cm3, resulting in an easy axis of magnetization that is perpendicular to the plane of the film, while films with >37 at. % Tb had a small negative intrinsic anisotropy (ca. −2×105 erg/cm3). The temperature dependence of the anisotropy indicates that neither uniaxial stress nor shape anisotropy can account for most of the intrinsic uniaxial anisotropy observed, so we hypothesize that the large anisotropy must be due to pair ordering or a local anisotropy field.

Flux Reversal in Soft Ferromagnetics

To describe the flux reversal process in soft ferromagnetics, at least three mechanisms are required. A domain wall motion process is satisfactory only when the drive field is slightly larger than the coercive force. In the intermediate drive region, a nonuniform rotation model is necessary to be consistent with the experimental observations. In particular such a model predicts the observed linear relationship between drive and the inverse of the reversal time, the observed shape of the flux reversal transient and the observed value of the switching coefficient. In the high drive region with a transverse field applied, a coherent rotation model provides the best explanation of the experimental observations. The three regions can be demonstrated with a single sample in the case of the thin film.

Rotational Model of Flux Reversal in Square Loop Ferrites

The present paper analyzes a rotational model of flux reversal in ferromagnetic materials. A brief review of the experimentally established details of the flux reversal process in square loop ferrites is given. The discrepancies between the experimental results and the predictions of the domain wall motion theory are discussed. The switching coefficient Sw is examined in detail.The rotational model is based on the solution of the modified Landau and Lifshitz equation and predicts the shape of the output voltage pulse, the relationship between the flux reversal time and the applied field, and the minimum switching coefficient that can be obtained for a given ferrite. The switching coefficients determined experimentally are within a factor of three of the minimum value of Sw predicted by this model.

Domain Wall Mobility in Single-Crystal Yttrium Iron Garnet

Reasonable agreement between the damping constant inferred from the resonance linewidth and that obtained from the domain wall mobility has previously been found for ferrites. However, the recent linewidths reported for yttrium iron garnet (YIG) by Spencer, LeCraw, and Clogston are substantially narrower than those for ferrites. On using this narrow linewidth to calculate the YIG wall mobility, one obtains about 106 cm/sec oe. We have measured the YIG wall mobility as a function of temperature and find it to be less than 104 cm/sec oe. This discrepancy can be interpreted in terms of a model for domain wall motion which is based on a type of energy conversion process different from that usually assumed. The customary treatment requires that the domain wall dissipate entirely to the lattice all of the magnetostatic energy stored in a given region during the time that the wall passes through this region. For small damping such as exists in YIG, this requirement appears to be unreasonable. The wall motion mod...

dc magnetron- and diode-sputtered polycrystalline Fe and amorphous Tb(FeCo) films: Morphology and magnetic properties

We have prepared polycrystalline Fe and amorphous Tb(FeCo) films using both dc diode‐ and magnetron‐sputtering techniques. Magnetic properties and aging characteristics of these films were measured by a vibrating sample magnetometer and an automatic torque magnetometer. Film morphologies were studied by transmission electron microscopy. The magnetic and aging characteristics are closely correlated to the film morphology. It was also found that different sputtering methods can be tailored to produce a similar film morphology. Among the sputtering parameters, the Ar pressure during deposition at room or lower temperature strongly influences the film morphology.

Aging effects on amorphous Tb–transition‐metal films prepared by diode and magnetron sputtering

We have studied and compared the aging characteristics of the amorphous Tb(FeCo) films from both dc getter diode and magnetron sputtering. No protecting layer such as SiO2 or Al2O3 was put on the films. Fresh films of 1.0‐μm thickness from diode sputtering showed a perpendicular anisotropy and well‐behaved magnetic properties using a vibrating sample magnetometer and an automatic torque magnetometer. We noticed a decrease in perpendicular anisotropy when the films were exposed to the air. However, this aging phenomenon was not observed in films of the same thickness prepared by magnetron sputtering even after seven months’ exposure to the air. The reasons for the difference are due to two distinct reaction mechanisms: bulk reaction in diode‐sputtered films and surface reaction in magnetron‐sputtered films. A variation of the microstructure is believed to attribute to this. The surface reaction was studied in thinner magnetron‐sputtered films of 10 and 25 nm, where the surface‐to‐bulk ratio is larger. A de...

Magnetic properties of single crystal rare-earth Gd-Y superlattices

Abstract The first coherent single crystal rare earth superlattices of Gd n T m were grown by metal molecular beam epitaxy. The magnetic results based on magnetization measurements are consistent with a model in which the central Gd region in each array behaves like an ideal ferromagnetic thin film even for n as small as 5. The two interfacial Gd layers at the sides appear not to order ferromagnetically at low temperatures and have a susceptibility independent of n . The temperature dependence of the magnetic moment of the isolated Gd arrays (3⩽ n ⩽21) is similar to the behavior seen in bulk Gd with reduced Curie temperatures.

Interlayer exchange coupling in amorphous/crystalline NiFe2O4 thin‐film bilayers

Amorphous/crystalline bilayers of NiFe2O4 exhibit interlayer magnetic exchange coupling, which results from an interaction between a spin‐glass material and a ferrimagnetic material. The observed effect is reminiscent of the well‐known exchange coupling effect between an antiferromagnet and a ferromagnet, which is widely used in applications where field biasing of thin magnetic films is desirable.