P. J. Flanders

Influence of Crystallite Size on the Magnetic Properties of Acicular γ‐Fe2O3 Particles

Magnetic properties have been measured for a number of samples of acicular γ‐Fe2O3 particles of the type used in magnetic recording tapes. The average particle size and shape were approximately the same for all samples. However, the average crystallite sizes of the samples, as determined from x‐ray line broadening, ranged from 50 to 700 A. All magnetic properties measured showed a strong dependence on average crystallite size. Saturation magnetization at room temperature, σ, decreased sharply with decreasing crystallite size. An excellent fit to the σ vs crystallite size data was made by assuming that the crystallites were separated by a nonmagnetic grain boundary on the order of 6 A wide. Room‐temperature coercive force decreased with decreasing crystallite size, and the ratio Hc(83°K)/Hc(293°K) increased sharply with decreasing crystallite size.The data support a particle model in which the constituent crystallites interact magnetostatically across nonmagnetic grain boundaries. A critical crystallite di...

Collection, measurement, and analysis of airborne magnetic particulates from pollution in the environment (invited)

Magnetic particulates in the environment, collected from surfaces and directly from the atmosphere, are reported. A significant percentage of magnetic material in many air‐borne samples consists of spherical magnetite from coal‐burning utilities and from iron/steel manufacturing with particle diameters 2–10 μm which correspond to average coercive fields of ∼100 Oe and remanence to saturation ratios of ∼0.1. The amount of air‐borne magnetic material that settles to the ground varies inversely with distance from its source.

An alternating‐gradient magnetometer (invited)

This paper describes a type of vibrating‐sample magnetometer capable of sensitivity exceeding 10−8 emu. The instrument is 1000 times more sensitive than a conventional VSM with comparable working space, and is much quicker to use than a SQUID magnetometer, which generates point‐by‐point data. The magnetic sample is mounted on the end of a cantilevered rod that incorporates a piezoelectric element. The sample is magnetized by a dc field (variable in magnitude), and is simultaneously subjected to a small alternating field gradient. The alternating field gradient exerts an alternating force on the sample, proportional to the magnitude of the field gradient and to the magnetic moment of the sample. The resulting deflection of the cantilever rod is measured by the voltage output of the piezoelectric element. By operating at or near a mechanical resonance frequency of the cantilever, the output signal is greatly amplified. In practice, the operating frequency is 100–1000 Hz, with mechanical Q values of 25–250. ...

An analysis of time‐dependent magnetization and coercivity and of their relationship to print‐through in recording tapes

Time‐dependent magnetic phenomena in oxide particulate recording materials are observed by measuring the change of magnetization with time in the presence of a constant field or by measuring the dependence of the hysteresis loop on the field sweep rate. Either type of experiment yields a measured value of the coercivity as a function of the characteristic measurement time. For a constant‐field experiment, this time is simply the interval needed for the applied field to reduce the magnetization from saturation remanence to zero. A corresponding time, approximately inverse to the sweep rate, can be associated with the coercivity measured in a swept‐field experiment. A time‐dependence parameter that measures the strength of the dependence of magnetization on time at constant field and also that of coercivity on sweep rate in a swept‐field experiment can be determined. This parameter can be interpreted, using simple kinetic theory, to estimate the volume of the magnetic switching unit. Volumes estimated in th...

Changes in Curie temperature, physical dimensions, and magnetic anisotropy during annealing of amorphous magnetic alloys

We have measured the effects of annealing in air on various properties of several amorphous alloys. Reported here are results on the changes in Curie temperature, in the physical dimensions of lengths of amorphous ribbon, and in the magnetic anisotropy. Increases in Curie temperature up to 35°C have been measured. All the alloys examined show a steady increase in T c on annealing at low temperatures, but some compositions show a smaller increase in T c on annealing near the crystallization temperature than on annealing at lower temperatures. There appear to be two competing mechanisms influencing T c . All the alloys examined show a clearly measurable decrease in length on annealing; we interpret this as an increase in bulk density. The kinetics of the annealing are similar to those of the stress relaxation. Finally, annealing experiments on the shape of 60 Hz hysteresis loops show a decrease in the anisotropy associated with non-uniform internal stresses, but in some cases also show the slow development of a fairly strong uniaxial anisotropy with its easy axis perpendicular to the ribbon axis. This uniaxial anisotropy is tentatively ascribed to the development of an oxide layer during annealing, which in turn produces a uniform compressive stress due to differential thermal contraction and therefore a stress-magnetostriction anisotropy. The changes in Curie temperature and in sample dimensions cannot be ascribed to oxidation. All the results described above are for annealing treatments that do not cause crystallization. The time for crystallization at various temperatures has been measured, and activation energies for crystallization derived.

Low−field magnetic properties of ferromagnetic amorphous alloys

The low−field magnetic properties of some amorphous magnetic alloy ribbons have been measured. The as−prepared specimens have low coercive field but rather low remanence. The application of elastic stress greatly increases the remanence, and can also decrease the coercive field. Some of the improvement due to stress can be made permanent by annealing under stress. These alloys should be useful in various magnetic devices.

High-speed switching in magnetic recording media

Recent measurements of the high-speed switching behavior of a wide range of commercial and experimental magnetic tapes are summarized. The salient result is the variation with material in the increase in the remanence coercivity HCR(τ) as the field pulse width τ decreases to <1 ns. For high-viscosity materials, the results are in good agreement with the thermal switching model proposed by Sharrock when 1/τ≤f0, the attempt frequency. For low-viscosity samples, the results are in reasonable agreement with the Landau-Lifshitz damping-limited switching model but only if values of the damping constant α ≈ 1 are assumed, in conflict with the experimentally reported values in some samples. The fundamental relationship between the two models through the fluctuation-dissipation theorem is emphasized and the need for a comprehensive model identified. Finally, a strong empirical correlation is obtained between α and a figure of merit SW/HC, where SW is the switching constant and HC is the coercivity. The implications for future media where thermal effects will become more important are significant.

Kinetics of formation of induced magnetic anisotropy in a zero‐magnetostriction amorphous alloy

The kinetics of formation of magnetic annealing anisotropy have been measured in the zero‐magnetostriction amorphous alloys (Fe0.063Co0.937)75Si15B10 over the temperature range 165–380 °C by analysis of torque curves on single disk samples. Only at temperatures above 330 °C was it possible to reach equilibrium saturated values of the anisotropy in reasonable times (less than 20 h). By assuming that the equilibrium saturated value of the induced anisotropy is linear with temperature, it is possible to deduce an activation energy for the magnetic annealing process in a well‐annealed sample. The value is about 2.0±0.1 eV. This is considerably higher than the value of 1.4 eV found by Luborsky or 1.7 eV reported by Chambron and Chamberod for Fe40Ni40P14B6. The higher value in this work is presumably due to the relatively high glass transition temperature and crystallization temperature of the FeCoBSi alloy. The kinetic data reveal the existence of range or relaxation times, which can be analyzed assuming a log...

Microstructure, relaxation, and print‐through in γ‐Fe2O3 particles

Four samples of γ‐Fe2O3 representing a range of Hc, particle size, and print‐through behavior were examined for their structure. All particles were single crystals with symmetric cross sections and predominant [110] longitudinal axis orientation. The sizes obtained from x‐ray line broadening measurements were shown to relate to average particle diameters rather than average crystallite sizes within a particle. Neither Hc nor print‐through seemed to correlate with porosity as determined from TEM. Print‐through (PT) was highest in the sample with the smallest average particle size. This suggests that print‐through is strongest in samples with the largest fraction of particles small enough to show magnetization instability. The data for some samples were not completely consistent with this view. The sample with the highest print‐through also had the largest concentration of anti‐phase boundaries (APB). The manner in which APB’s can lower the threshold for magnetization instability, thereby increasing print‐t...

Magnetostriction in some magnetic oxide compacted powders

Magnetostriction measurements have been made on a series of compacted powder samples of oxide pigments used in magnetic recording, to provide information for the evaluation of the role of magnetostriction in the recording process. Two composition series were measured at 294 K: one from γFe 2 O 3 through a range of partial reductions to Fe 3 O 4 , and another from 0 to 4 at. % Co in γFe 2 O 3 ; also samples of CrO 2 . In addition, a 2 at. % Co-doped Fe 3 O 4 powder was measured as a function of field and temperature from 77 to 300 K. Magnetostriction was measured using a semiconductor strain gage on a rotating sample, in fields up to 9 kOe.