J. D. Livingston

Magnetic domains in sintered Fe-Nd-B magnets

Magnetic domain structures in sintered Fe‐Nd‐B magnets from several suppliers were studied using the polar Kerr effect. Most grains were multidomain in the thermally demagnetized state. Measurement of average surface domain width in several large grains yielded a domain‐wall energy of 25 ergs/cm2 for the Fe14Nd2B primary phase. After saturation and subsequent demagnetization in reverse field, most grains were again multidomain. Consistent with this observation, thermal remagnetization was small. A Fe‐Nd‐B‐based magnet with much higher coercivity showed a much higher percentage of single‐domain grains in the field‐demagnetized state and a much higher thermal remagnetization, i.e., a behavior more nearly similar to earlier results on sintered Co5Sm magnets.

Microstructure of aged (Co,Cu,Fe)7Sm magnets

The microstructure of (Co,Cu,Fe)7Sm magnets has been characterized at various states of aging by transmission electron microscopy. At peak magnet properties, the magnet consists of cells of 17 : 2 phase about 500 A in diameter, separated by a thin continuous fully coherent 5 : 1 boundary phase. This structure evolved by precipitation of ordered 17 : 2 particles from a disordered 5 : 1 matrix, and coarsens with aging until full coherence is lost and the cellular structure breaks up. The overaged alloy consists of semicoherent 5 : 1 particles in a 17 : 2 matrix, with a single set of interface dislocations accommodating the lattice mismatch. The effect of these various microstructural changes on domain‐wall pinning and coercivity is discussed.

A review of coercivity mechanisms (invited)

Mechanisms of magnetization reversal and factors determining coercivity are reviewed for various permanent‐magnet materials.

Domain‐wall energy in cobalt‐rare‐earth compounds

We have used polarized light metallography to study magnetic domain configurations in Co5Sm, Co5Y, Co5Ce, and Co5Pr as a function of crystal thickness. From measurements of equilibrium domain widths on thin crystals, we estimate the domain‐wall energy in these compounds to be 85, 35, 25, and 40 erg/cm2, respectively. From surface domain observations on bulk crystals, we have also made crude estimates of wall energy for Co5Nd, Co5La, and Co5Gd. From these wall energies and published anisotropy constants, we have calculated exchange constants, domain‐wall thicknesses, and critical single‐domain particle sizes. We find this critical particle size to be larger for Co5Sm and Co5Gd than for the other compounds, and suggest that this may partly explain why high coercive forces are more easily attained in powders of these two compounds.

Domains in sintered Co‐Cu‐Fe‐Sm magnets

Magnetic domain behavior in a sintered Co‐Cu‐Fe‐Sm magnet was studied using the Kerr effect. Magnetic reversal initiated in thin bands along most grain boundaries and extended from these bands into the interior of the grains. Domain walls were immobile at low fields. Transmission electron microscopy and microprobe analysis were used to supplement the domain studies.

Effect of Ta additions to bronze-processed Nb 3 Sn superconductors

The results of reaction between Nb(Ta) filaments and a Cu(Sn) matrix have been studied for filament compositions ranging from 0 to 34 at pct Ta. Microprobe measurements show that the Ta is fully incorporated into the reaction layer, producing (Nb,Ta) 3 Sn ternary A-15 compounds. High Ta concentrations lead to a decrease in reaction kinetics. Superconducting critical temperatures decrease with increasing Ta content more rapidly than reported earlier for powder samples. However, small Ta concentrations produce a decreased field dependence of critical currents and an increase in critical currents at high fields. These effects are believed to result from increased critical fields produced by a decreased electron mean free path, and may prove to be of technological significance for the production of high-field superconducting magnets.

Domain studies on amorphous ribbons with transverse or oblique magnetic anisotropy

SEM techniques are used to study domain structures in amorphous ribbons with transverse or oblique magnetic anisotropy. Domain widths are much finer than in ribbons with longitudinal anisotropy, and vary with ribbon dimensions and anisotropy constant. The effects of applied magnetic fields and applied stresses are also discussed.

SEM studies of magnetic domains in amorphous ribbons

Static and dynamic observations of magnetic domains in amorphous Fe-B-Si and Fe-B-Si-C ribbons have been made employing a 200 kV scanning electron microscope (SEM) and a special stage allowing in situ application of mechanical stresses and dc and ac magnatic fields. Average domain width in the demagnetized state is found to decrease with increasing demagnetizing frequency, and to be relatively insensitive to tensile stress. Under ac conditions, a great range in the mobility of individual walls is often observed. Some domain walls are strongly pinned, in many cases by obvious surface defects. The implications of these and other observations on hysteresis and eddy-current losses, coercivity, and permeability are discussed.

Magnetic domains in amorphous metal ribbons (invited)

The results of various static and dynamic studies of domains in amorphous metals by SEM and Bitter techniques are reviewed. Observations include domain widths and orientations, wall mobility and pinning, domain‐wall energy measurements, and the effects of various distributions of residual or applied stresses on domain patterns. Domain structures resulting from longitudinal, transverse, and perpendicular anisotropies are contrasted.

Precipitation and Superconductivity in Lead‐Tin and Lead‐Cadmium Alloys

The effects of precipitation from solid solution on the superconducting properties of lead‐tin and lead‐cadmium alloys have been studied by magnetization measurements at 4.2°K. Microstructural changes have been followed by optical and electron microscopy. These alloys as‐quenched are superconductors of the second kind, their upper critical fields enhanced by quenched‐in solute. At various stages in the precipitation process, the upper critical field continues to be determined primarily by the remanent solute. Magnetic hysteresis and trapped flux result from the interaction between precipitates and the flux filaments of the mixed state, a given precipitate distribution causing much more flux trapping if the matrix is a superconductor of the second kind than if it is a superconductor of the first kind. Several instances are seen where superconducting magnetization measurements yield useful metallurgical information.