F. E. Luborsky

Particle Size Dependence of Coercivity and Remanence of Single‐Domain Particles

The coercive force and remanence of essentially spherical iron and iron‐cobalt alloy particles with diameters from 20 to 3000 A have been measured at 4°, 76°, and 207°K and compared to the theoretically predicted behavior. The remanence shows a broad, plateau‐like maximum while the coercive force has a rather sharp maximum. The maximum of the coercive force occurs at a much larger particle diameter than the maximum of the remanence. It is shown that these essential characteristics follow from the theory. Deviations from theory are seen in the smaller size range and can be accounted for by the distribution of particle sizes. A general treatment of the coercive force of mixtures of thermally stable, high coercive force particles with superparamagnetic and multidomain particles is given.

Development of Elongated Particle Magnets

The development of permanent magnet materials is briefly reviewed. The present status of fine particle magnets is discussed from the viewpoint of our present understanding and lack of understanding of their behavior. The present methods of preparation and the various theoretical descriptions of the properties of elongated particles are reviewed. New work is presented relating the parameters of preparation to the resulting diameter of the elongated particles prepared by electrolysis into mercury. Rotational hysteresis, coercive force, and coercive force as a function of orientation are reported for particle diameters from 130 A to 305 A. Their behavior is compared to various theoretical descriptions and found to correspond to a noncoherent magnetization reversal mechanism most similar to the chain-of-spheres fanning model rather than the curling, buckling or coherent rotation models. Iron and iron-cobalt alloy particle magnets are described with maximum energy products up to 4.3 and 6.5 million gauss-oe, r...

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.

Magnetic Anisotropy and Rotational Hysteresis in Elongated Fine‐Particle Magnets

Various aspects of the magnetic anisotropy of electrodeposited elongated single‐domain Fe and FeCo alloy particles are examined with a view to better understanding their process of magnetization. The initial increase of coercivity with increase of the angle between alignment direction and measuring field is in qualitative accord with the prediction of the chain‐of‐spheres model with fanning, as previously proposed, and in contrast to coherent rotation models. Analyses of high‐field torque curves and of the fields at which rotational hysteresis vanishes suggest that an anisotropy is present which is a little greater than predicted by the chain‐of‐spheres model but less than that predicted by the Stoner‐Wohlfarth ellipsoid model, for the observed dimensional ratios. Calculations of the rotational hysteresis in single domain‐particles are extended to the chain‐of‐spheres model. A study of the rotational hysteresis enables a relatively sensitive choice between several models of the magnetization process. Comp...

Crystallization of some FeNi metallic glasses

Abstract The beginning of crystallization was determined as a function of time and temperature for amorphous alloys of Fe40Ni40P14B6, Fe40Ni40B20, and Fe80B20. Both calorimetric and magnetic methods were used. The onset of crystallization was found to be a thermally activated process; the activation energies ΔE for the three alloys were 3.9 eV, 3.0 eV and 2.1 eV, respectively. These results, and all results available from the literature, show that ΔE increases with increase in the number of atomic species in the amorphous alloy. ΔE also increases with the increase in the difference between the temperature for the onset of crystallization and the glass temperature, Tx − Tg, both determined from scanning calorimetry. The Fe80B20 alloy was the least stable of the three alloys. Its projected life at 200 °C is 25 years, adequate for many, but not all magnetic applications. The temperature for the beginning of crystallization, from 2 h anneals, was also determined for amorphous alloys in the series FeyNi80−yP14B6, FeyNi80−yB20 and Fe40Ni40P20−zBz. These results are consistent with the structural relaxation model.

Stability of amorphous metallic alloys

In a previous paper it was concluded that the migration of phosphorus was causing the embrittlement of amorphous Fe40Ni40P14B6 at temperatures as low as 100 °C. In this paper we compare the stability of the above alloy to amorphous Fe40Ni40B20, Fe50Ni30P14B6, and Fe50Ni30B20. The stability is evaluated from brittleness measurements after annealing at temperatures between 100 and 400 °C. The results confirm the prediction that the removal of phosphorus suppresses the embrittlement at low temperatures; temperatures above 225 °C are required to cause embrittlement in the phosphorus‐free alloys. At higher temperatures the fracture strain approaches that of the phosphorus‐containing alloy. It is concluded that the replacement of P by B inhibits the low‐activation processes involved in the embrittlement process. The changes are not related to the changes in the glass transition temperatures.

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.

An Auger analysis of the embrittlement of the amorphous alloy Ni40Fe40P14B6

Abstract The amorphous alloy Ni40Fe40P14B6 becomes brittle when heated to temperatures in excess of about 100 °C for 2 h. Auger analysis of the fracture surface of ribbon samples heated at 325 °C and 350 °C showed phosphorus concentrations on the fracture surface greater than twice that of the expected bulk composition. Ion milling to a depth of ∼ 60 A reduced the phosphorus concentration to near that expected for the bulk. The concentrations of Fe, Ni and B on the fracture surface showed only small changes during ion milling. The results indicate that phosphorus has become segregated during the anneal to form discrete regions of high phosphorus concentration which are less than 60 angstrom in diameter. These high phosphorus regions are the cause of embrittlement and may be the nuclei for subsequent crystallization. Auger analysis of the ribbon surface indicates that the material is inhomogeneous, being high in phosphorus near the ribbon surface.

High gradient magnetic separation: Theory versus experiment

The experimental performance of a high gradient magnetic separator has been previously reported by other workers in some detail for a CuO/Al 2 O 3 slurry. Less detailed results were also reported for slurries of Mn 2 O 3 , Al, and α-Fe 2 O 3 particles with Al 2 O 3 representing a 20:1 range in particle sizes and a 200:1 range in magnetic susceptibility. Examination of these results indicates that many layers of particles build up on each filter fiber. Accordingly, in this paper we extend the original particle trajectory model for the calculation of filter performance, to include the build-up of multiple layers of particles on the fibers. Good agreement is obtained between the calculated recoveries and purities for all of the particles and the experimentally reported values using a filter packing efficiency and a mechanical trapping term, derived from the CuO data, as adjustable parameters.

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.