Joseph J. Becker

Random anisotropy in amorphous ferromagnets

Random anisotropy is present in all amorphous magnetic materials, and depending on its strength, it can dramatically affect the magnetic behavior. We describe the effect of strong random anisotropy in materials such as amorphous Tb‐Fe and Dy‐Fe at low temperature and also examine the role which weak random anisotropy might play in even ideally homogeneous soft materials. Much of our analysis is based on the simple model proposed by Harris, Plischke and Zuckerman for a ferromagnet with random‐axis uniaxial anisotropy. We describe computer simulation results for this model and then develop scale length arguments which allow us to describe fluctuations in the magnetization direction. For a perfectly isotropic distribution of anisotropy axis we find that the conventional ferromagnetic ground state is unstable. The new ground state has large frozen in fluctuations but probably has a considerable moment and is, therefore, not spin glass‐like. This system does not support domain walls of the conventional type. F...

A Family of New Cobalt‐Base Permanent Magnet Materials

The magnetocrystalline anisotropy of several intermetallic phases of the type RCo5 (R = Y, Ce, Pr, Sm, Y‐rich and Ce‐rich mischmetals) has been investigated, and it is concluded that these alloys are promising candidates for fine‐particle permanent magnets. They have extremely high uniaxial anisotropy (K = 5.4 to 7.7 × 107 erg/cm3), single easy axis, high saturation (Bs = 8500 to 11 200 G) and Curie point (tc = 464° to 747°C). Approximate upper limits for the possible energy product lie between 18 and 31.3 MGOe. Experimentally, coercive forces of over 8000 Oe and (BH)max = 5.1 MGOe have been observed in SmCo5 merely ground at room temperature. Grinding of YCo5 and (Ce‐MM) Co5 produces an increase of MHc to 2200 and 2700 Oe, respectively, followed by a decrease as particle size continues to decrease.

Magnetic annealing of amorphous alloys

Amorphous alloys with nominal composition of Ni 40 Fe 40 P 14 B 6 are shown to respond to annealing in a magnetic field. Coercive forces are reduced by a factor of 10 to 50 during annealing of straight ribbons to values of 0.003 Oe, as low as ever reported for potentially useful materials. Concurrently the ratio of the magnetization in 1 Oe applied field, to saturation, increases from about 0.5 to 0.95. These changes during annealing correlate with measured stress relief changes. It thus appears that most of the strain-magnetostriction contribution to the anisotropy is removed during annealing. Magnetic annealing at temperatures as low as 100°C results in noticeable changes in properties. From measurements transverse to the magneticaliy induced anisotropy axis, the induced anisotropy is calculated to be about 800 ergs/cm3, considerably smaller than obtained in crystalline Ni 50 Fe 50 . This field-induced anisotropy is reversible in direction and magnitude by reheating the sample to its Curie temperature and then cooling in a field. Annealing of 1.5 cm diameter toroids, made from 50 μm thick tapes, increases the initial permeability by more than a factor of 10 and decreases losses by more than a factor of 10. Losses and permeabilities after heat treatment compare favorably to the Permalloys with similar saturation magnetizations.

Formation and magnetic properties of Fe-B-Si amorphous alloys

The magnetic properties and crystallization temperatures of alloys in the ternary Fe-B-Si system are reported. The Curie temperature increases slightly on replacement of boron by silicon. This results in a sharp ridge of relatively constant room-temperature saturation magnetization extending from Fe 80 B 20 to Fe 82 B 12 Si 6 . The coercivity exhibits a broad minimum, both before and after stress relief annealing, in the region around Fe 81 B 15 Si 4 and extending at least to Fe 77 B 13 Si 10 . The crystallization temperature increases with increasing silicon and with decreasing iron and boron. The alloys with silicon are generally easier to prepare in the amorphous state than the binary Fe-B alloys. Thus for the highest saturation magnetization alloy combined with ease of preparation, stability, and lowest losses, the alloys between Fe 81 B 17 Si 2 and Fe 82 B 12 Si 6 are preferred.

Magnetic moments and curie temperatures of (Fe, Ni) 80 (P, B) 20 amorphous alloys

The magnetic moment per transition metal atom at 0°K and the Curie temperature were obtained for a series of (Fe, Ni) 80 (P, B) 20 amorphous quenched alloy ribbons. Fe/Ni and P/B compositions were varied separately. The moment data can be fitted well by assigning 2.1 Bohr magnetons per Fe atom and 0.6 per Ni atom, with the moment being lowered by 0.3 per B atom and 1.0 per P atom. Alternatively, moments varying with composition, as shown by neutron diffraction in crystalline alloys, combined with a lowering of 1.2 per B atom and 2.1 per P atom, also fit well. For a given P/B composition, T c shows a broad maximum at Fe:Ni of about 3:1. For a given transition metal composition, T c increases with increasing B content.

Potential of amorphous alloys for application in magnetic devices

Amorphous alloys have potential applications in all types of magnetic devices, in both the electronic and power areas of application. For electronic devices, the properties are comparable to those of commercial alloys and the materials offer potentially much lower cost. In power applications such as transformers, losses are far lower than in materials used at present. This results in a potential favorable trade‐off between first cost and a substantial energy savings throughout the life of the device. Although power applications have not been emphasized up to now, they appear to hold great promise, especially as wider amorphous tapes become available.

A Domain‐Boundary Model for a High Coercive Force Material

The coercive force of aligned Co5 (rare earth) powders, while always much less than 2K/M, varies greatly with magnetizing field, even up to 50 kOe. The remanence is much more nearly constant. The coherent rotation model for noninteracting particles is not adequate to describe this behavior. A model is presented based on the idea that domain boundaries may be present and move easily, but that a sufficiently large field will drive them out so that they must be renucleated. Two experimental findings support this model. First, the measured rotational hysteresis integral value depends on whether or not a large field is applied before each measurement, as the model predicts. Second, magnetization jumps corresponding to nucleation have been directly observed in single particles. Calculations of the behavior of aggregates based on this single‐particle model show the type of variation of coercive force and remanence that has been observed.

Reversal mechanism in copper-modified cobalt-rare-earths

The magnetization reversal in copper-containing cobalt-rare-earth permanent magnet materials can be completely dominated by pure bulk wall-pinning. This is shown by the form of hysteresis loops in varying magnetizing fields, whose appearance is in extreme contrast to those of unmodified cobalt-rare earths, in which nucleation is the dominant factor in magnetization reversal. In some copper-modified materials it is possible to show both kinds of behavior in the same sample, with a sudden transition at a particular value of magnetizing field.

Rapidly quenched metals for permanent magnet materials (invited)

When magnetically soft amorphous metals are annealed until crystallization occurs, coercive forces sometimes arise that are high enough to suggest the possibility of permanent magnet materials. This paper begins with a brief review of results in alloys not containing rare earths. Then earlier results on rare‐earth‐modified Fe‐B alloys and on Fe‐R alloys quenched at intermediate rates are reviewed. Recent experiments indicate that intrinsic coercive forces of many kOe combined with high Ms and Mr are possible in rapidly quenched alloys containing only iron, light rare earths, and glass formers. Recent work and our current results in these alloys are presented and discussed, including effects of composition, quenching rate, annealing, and etching, as well as thermomagnetic effects and phase identification.

Unusual Magnetic Behavior of Disordered Ni3Mn

Measurements were made of the magnetization of disordered Ni3Mn for various fields between 945 and 7550 oe and at many temperatures between 1.5 and 300°K. The magnetization vs temperature curve for any given field is found to have a pronounced maximum at about 25°K and a long concave‐upward section extending from about 140°K to temperatures above 300°K. By means of a new method of analysis, the Curie point is deduced to be 132°K. An abnormally large induced magnetization, greatest at about 50°K, is also found. All the experimental results are consistent with the occurrence of a ferrimagnetic‐ferromagnetic transition at low temperatures.