Magnetic analysis of the precipitation of iron from beta brass

Abstract

Single crystals of beta brass containing 0.07 wt. % iron were annealed at 300°C and 400°C to precipitate the iron. At various stages in the anneals, magnetization curves, hysteresis loops and torque curves were measured. These magnetic data were analysed in terms of the concepts of fine particle ferromagnetism to derive size and shape distributions of the iron precipitate particles and the orientations of their erystallographic and shape symmetry axes. In the superparamagnetic size range average diameters were determined up to 140 Å. These particles showed no shape anisotropy, and had the same saturation magnetization and magnetocrystalline anisotropy symmetry as bulk iron. The crystal habit of the particles coincided with that of the beta brass matrix. With further annealing single domain properties appeared, and the particles showed increasing elongation in the 〈100〉 directions as determined by analysis of the torque data. Distributions of shapes and sizes of the single domain particles were calculated from remanent torque data. These calculations were consistent with the conclusions from remanence measurements. The shapes of the hysteresis loops and the spread in annealing time between maximum coercive force and maximum remanence could be simply interpreted on the basis of mixtures of superparamagnetic, single and multidomain particles. When the particles were principally in the multidomain (> 300 Å diameter) range, little information about size distributions could be derived, although the data indicated that the particles become progressively elongated in the 〈100〉 directions. Electron micrographs of replicas gave qualitative confirmation of the conclusions from the magnetic analysis. © 1960.