J. F. Dillon

Optical Properties of Several Ferrimagnetic Garnets

All of the ferrimagnetic garnets studied to date have been found to be transparent to visible light down to wavelengths of about 5200 A. Thin sections (e.g., 0.005 cm) have been examined to determine the absorption and the specific Faraday rotation through that part of the visible spectrum where they transmit light. These measurements have been made at temperatures down to 4.2°K. They are interpreted as giving us information on electronic energy levels. A magnetic birefringence and a circular dichroism have been observed. The Faraday rotation permits observation of the domain structure in these crystals at any temperature below the Curie point. Possible microwave‐optical experiments are discussed briefly.

Magnetostatic Modes in Disks and Rods

Ferrimagnetic‐resonance experiments have been performed using disk‐ and rod‐shaped samples cut from single crystals of a manganese ferrite and of yttrium iron garnet. A multiplicity of absorption maxima were observed which can be identified as magnetostatic modes. The excitation of different sets of modes by rf magnetic field configurations of varying symmetries aids in assigning indices to the modes observed. By far the narrowest lines were observed in thin disks with the steady field normal to the plane of the disk.

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.u2009%) 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.u2009% 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.u2009% 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.

Domain Wall Motion and Ferrimagnetic Resonance in a Manganese Ferrite

From a single crystal of manganese ferrite, specimen cores have been cut which are capable of supporting single domain walls. These cores have been proportioned in two ways, one favoring a movable (110) wall, the other favoring a wall system perpendicular to the (110) plane. The basic experiment consists in applying rectangular current pulses to a winding, and observing the rate of change of flux as presented by the voltage in another winding. At low driving fields, the wall, once torn free of the surface, moves through the crystal at a velocity proportional to the applied field less an internal coercive force. At high fields the pulse shapes indicate complex domain structure.Ferrimagnetic resonance experiments have also been performed on samples cut from the same crystal, and the results of these are given. It is particularly interesting to compare the loss parameter for the equation of motion which we derive from the resonance experiments with that obtained from the wall motion experiment.

Observation of Domains in the Ferrimagnetic Garnets by Transmitted Light

It has been found that single crystals of the ferrimagnetic garnets are transparent to visible light. Thin sections (e.g., 0.005 cm thick) transmit sufficient light for microscopic examination. The transmitted light undergoes a nonreciprocal rotation similar to the Faraday effect. By virtue of this rotation many features of the domain structure are readily visible. A magnetic birefringence has also been observed which can be used to differentiate domains whose magnetization lies perpendicular to the line of sight. It is possible to study domain structure at any temperature below the Curie point. The effects of strains set up by the polishing procedure are discussed. Finally, mention is made of possible combined microwave‐optical experiments, as well as experiments aimed at understanding the switching process.

Diffraction of Light by Domain Structure in Ferromagnetic CrBr3

Thin sections of the ionic ferromagnet CrBr3 are transparent to visible light. Below the Curie point, in the demagnetized condition, these sections possess a fine‐grained domain structure in which the characteristic distance is comparable to the wavelength of visible light. We find that near 5000 A the optical properties of this material are closely associated with the magnetic properties, and the modulation in magnetization direction which constitutes the domain structure has an associated modulation in the optical properties. This modulation constitutes a grating which gives rise to readily observed diffraction rings. Such a large fraction of the green light is diffracted that meaningful absorption measurements can only be made if the domains are removed by saturation. Spacings deduced from the size of the diffraction rings may be compared with domain theory, as well as used in the detailed interpretation of nuclear magnetic resonance results. The study of these rings constitutes a new technique for the...

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...

Ferromagnetic Resonance in CrBr3

Ferromagnetic resonance has been observed in single crystals of anhydrous chromium tribromide using frequencies of 20 to 27 kMc. At 1.5°K a g value of 2.006 is found along with a uniaxial anisotropy, K=9.4×105 ergs/cm3. Linewidths as narrow as 3.5 oe have been observed at 1.5°K.

Ferrimagnetic Resonance in Impurity Doped Yttrium Iron Garnet (YIG)

A research program aimed at understanding our earlier line width and anisotropy measurements on YIG has led to a study of ferrimagnetic resonance in doped crystals. YIG crystals have been grown containing appropriate concentrations (∼.01 to 5.0 atomic percent) of various impurities, including the 4f rare earth elements, members of the iron transition group, and several non-magnetic elements. This paper gives new results for the variation of the field for resonance with crystal direction at several temperatures in the liquid hydrogen and liquid helium range. In the case of holmium, sharp peaks in the Hres vs angle curve are seen which vary rapidly with temperature. The corresponding curves for the dysprosium containing sample do not show such sharp variations in (110), and at least in part do not vary with temperature below about 4°K. Though Kittels recent theoretical results should apparently apply to the peaks in the holmium case, his results for the temperature dependence of the anomaly height and widt...

New Characteristic in the Temperature Dependence of Ferrimagnetic Resonance Linewidth in Some Rare-Earth Doped Yttrium Iron Garnets

Measurements have been made of the ferrimagnetic resonance linewidth of YIG doped with each of the rare earths. These results will be presented in full elsewhere. For the most part there is a peak in the ΔH(T) curve at some temperature between 20° and 150°K. The position of this peak varies with the impurity, the frequency, and the crystal direction of the steady field. However, several cases have been observed in which the linewidth increases very rapidly with decreasing temperature down to the lowest temperature reached, about 1.5°K. This behavior occurs over a very narrow angular range. For ytterbium-doped YIG, it is associated with a very mild peak in the field for resonance. For the case of europium there appears to be no anomaly in Hres, though a plot of linewidth against angle shows an increase of about fifteen fold with a half-width of perhaps 1.5°. It is suggested that this anomalous linewidth vs angle and temperature curve, as well as the more generally encountered peak in ΔH(T), represent the e...