C.D. Graham

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.

The origin of crystallographic texture produced during hot deformation in rapidly-quenched NdFeB permanent magnets

The authors present a model for the development of crystallographic texture by hot deformation in NdFeB magnets produced by rapid solidification. Favourably oriented grains, with their c-axis parallel to the compression axis, grow by mass transport through a Nd-rich liquid grain boundary phase, forming large platelet grains. The driving force is the elastic energy difference between grains with c-axes making different angles with the compression axis. Numerical calculations based on the known properties and conditions of these materials are consistent with a liquid diffusion model. >

Magnetic properties of amorphous magnetic alloys

Low-field magnetic properties of amorphous ferromagnetic alloys have been previously reported mainly at d.c. The alloys in ribbon form generally show square hysteresis loops, with coercive fields less than 0.1 Oe, and with saturation magnetization up to 17000 G. Maximum magnetization is obtained in low fields only when the ribbons are under tensile stress. This paper reports further measurements on amorphous magnetic alloys, including differential scanning calorimeter data on the crystallization of three compositions and a variety of a.c. measurements on toroidal samples at frequencies up to 2000 Hz. The coercive fields and magnetic losses under a.c. excitation are disappointingly high. This is attributed to an increase in coercive field when straight ribbon is wound into toroidal form, and to a further increase in H c with frequency due to an apparent decrease in the number of domain walls participating in the magnetization reversal. Neither effect is necessarily inherent in amorphous magnetic materials.

Magnetic properties of amorphous ribbon

The magnetic properties of transition-metal amorphous alloys are discussed, with emphasis on methods to increase the value of the saturation induction at room temperature, and on the factors that control the low-field magnetic properties that are important in most engineering applications.

Effect of added Cu on the Nd-rich phase in hot-deformed NdFeB magnets

It is found that the addition of Cu to NdFeB magnet material changes the composition of the grain-boundary phase from NdFe based phase to binary NdCu. It is also found that when the grain boundary phase is liquid, it can dissolve significant amounts of the 2-14-1 phase, and when liquid phase is extruded from the sample during deformation, the 2-14-1 phase is transported outside the original sample volume. During cooling, grains of the 2-14-1 phase nucleate and grow from the liquid. This process provides clear evidence that the constituents of 2-14-1 phase can be transported by liquid diffusion in the grain boundary phase at elevated temperatures. >

Experimental Demagnetizing Factors for Disk Samples Magnetized Along a Diameter

Demagnetizing factors for disk samples of a commercial 80Ni permalloy have been measured using a vibrating sample magnetometer with fields provided by a Helmholtz coil pair. The range of values of m (diameter/thickness) is from 12.5 to 385. The experimental values of N are significantly higher than those calculated for an ellipsoid, but lower than those calculated assuming uniform magnetization

High-field magnetostriction measurements on Fe 14 Nd 2 B magnets

Magnetostriction measurements have been made on aligned polycrystalline Fe 14 Nd 2 B magnets from two US manufacturers at applied fields up to 144 kOe at room temperature. The ordinary magnetostriction is comparable to that of iron (10 × 10-6). The high field forced magnetostriction is positive for all orientations of the field and of the measured strain, strongly anisotropic, and similar in magnitude to normal ferromagnetic materials. The forced volume magnetostriction is 1.3 × 10-9Oe-1with the field parallel to the alignment axis, and 2.1 × 10-9Oe-1with the field perpendicular.

Effects of application of axial fields on asymmetric magnetization reversal

Abstract We observed that asymmetric magnetization reversal (AMR) can be destroyed partially or totally by magnetizing the sample in a sufficiently high magnetic field. The single AMR is not altered for applied fields below ± 3Oe. For applied fields between ± 3 and ± 250Oe, the single AMR is increasingly altered by higher fields, but can be restored close to the original state. For fields greater than ± 300Oe, the single AMR is destroyed and cannot be restored to its original state by applying a field at room temperature. The experimental results can be explained by means of a metastable domain structure characterized by the induced anisotropy and the exchange anisotropy.

Effects of chemical etching on the asymmetric magnetization reversal in amorphous magnetic alloy

Intensive etching experiments have been performed in order to investigate the origin of asymmetric magnetization reversal (AMR), which can be developed in amorphous ferromagnetic ribbons annealed in a controlled magnetic field. The variation of the characteristics of AMR with etching time was analyzed from dc magnetic hysteresis loops measured in situ during etching. The AMR characteristics were altered very rapidly and severely just after most of the outer oxide layer was removed, but did not disappear until 30% of the volume of the as-annealed sample had been etched away. From Auger depth profile analysis and domain observations, we conclude that the major origin of AMR resides in a high coercive layer between the oxide layer and the bulk material. >

Apparent Image Effect in Closed-Circuit Magnetic Measurements

Magnetic measurements made under closed-circuit conditions are generally considered to be free from errors associated with the image effect. However, magnetic measurements on permanent magnet sample cylinders clamped between the pole pieces of an electromagnet show an apparent drop in magnetization with increasing applied field that is similar to the image effect found in open-circuit measurements. The closed-circuit effect has been measured for a range of sample geometries and materials, and occurs in both hard and soft magnetic materials. The drop in apparent magnetization is greater, and appears at lower fields, as the saturation magnetization of the sample increases. The effect increases as the length-to-diameter ratio L/D of the sample decreases, and becomes minimal for L/D > 1.8 . In addition to the apparent decrease in measured magnetization, there is a decrease in the measured magnetic field. We attribute the effect to a non-uniform magnetization of the electromagnet pole pieces, where local saturation distorts the magnetic flux distribution around the sample.