Amorphous metallic alloys

REVIEW ARTICLE: Glassy metals

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. For a system with both a macroscopic anisotropy axis and random anisotropy, we can have domain walls. We present a theory for the intrinsic coercivity which gives values of about 10−6 Oe for Fe‐metalloids and 0.2 Oe for Gd‐Co‐Mo. This indicates that inhomogeneities of larger than atomic scale are limiting the behavior of present materials. A model illustrating aspects of magnetic resonance behavior is also described.

Random anisotropy in amorphous ferromagnets

Rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials. Hysteresis loops that are constricted in the middle section, but are wider above and below the middle section, are commonly observed in mixed magnetic assemblages. Such “wasp-waisted” hysteresis loops have been widely documented, particularly with respect to rare earth permanent magnets, basaltic lava flows, remagnetized Paleozoic carbonate rocks, and an increasingly wide range of other rocks. Our modelling, combined with a review of previous work, indicates that there are several conditions that give rise to, as well as magnetic properties that are characteristic of, wasp-waisted hysteresis loops. First, at least two magnetic components with strongly contrasting coercivities must coexist. This condition can arise from either mixtures of grain sizes of a single magnetic mineral, or a combination of magnetic minerals with contrasting cocrcivities, or a combination of these two situations. Second, materials that give rise to wasp-waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force (B cr /B c ) because B0 is controlled by the soft (low coercivity) component, whereas Bcris controlled by the hard (high coercivity) component. Third, values of B cr /B c ? 10 usually only occur for strongly wasp-waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains. Fourth, a given mixture of superparamagnetic and single-domain (SD) grains is more likely to give rise to wasp-waisted hysteresis loops than an equivalent mixture of SD and multidomain grains. Fifth, our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component, in the absence of significant magnetic interaction between particles. Thus to contribute significantly to wasp-waisted behavior, a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock. As a result, minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp-waistedness in materials that also contain ferrimagnetic minerals. We outline a method for determining the magnetic components that can give rise to wasp-waisted hysteresis loops. This method is based on high- and low-temperature magnetic measurements that are used to identify the dominant remanence-bearing mineral/s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states. The method is illustrated with several examples from archaeological, geological, and synthetic materials.

Wasp-waisted hysteresis loops: Mineral magnetic characteristics and discrimination of components in mixed magnetic systems

Magnetic properties of Co/Pd and Co/Pt multilayers were studied as a function of sputtering gas pressure. It was found that the magnetic properties of the films depended upon the sputtering gas pressure, and large coercivity (several kOe) and a perfect squareness of the perpendicular hysteresis loop were attained by the deposition at the high gas pressure. These multilayers are suitable for perpendicular magnetic recording media. In the Co/Pd, the origin of the perpendicular magnetic anisotropy is mainly an interfacial anisotropy, although there exists a stress‐induced anisotropy for the films with the relatively short periodicity. The perpendicular magnetic anisotropy of the Co/Pt can be concerned with the Co‐Pt mixture formed at the interface of the multilayer structure. The magnetostriction of the Co/Pd films with the relatively short periodicity is extraordinarily large with a negative sign, since Co atoms adjacent to Pd atoms at the interface could induce larger magnetostriction than that of pure Co....

Perpendicular magnetic anisotropy and magnetostriction of sputtered Co/Pd and Co/Pt multilayered films

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.

Magnetic annealing of 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.

Formation and magnetic properties of Fe-B-Si 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.

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

Amorphous magnetic metal alloys are processed by annealing at temperatures sufficient to achieve stress relief and cooling in directed magnetic fields or in zero magnetic fields. The ac and dc properties of magnetic cores produced in accordance with the processes of the invention may be tailored to match those of a wide range of magnetic alloys. Alloys processed in accordance with the invention provide improved performance in inductors, transformers, magnetometers, and electrodeless lamps.

Treatment of amorphous magnetic alloys to produce a wide range of magnetic properties

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.

Joseph J. Becker

Papers

Magnetic annealing of amorphous alloys

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

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

Treatment of amorphous magnetic alloys to produce a wide range of magnetic properties

Reversal mechanism in copper-modified cobalt-rare-earths