The Low Temperature Transformation in Ferrites

The general behavior of magnetite (iron ferrite) at its low temperature transformation ($\mathrm{ca}$ 118°K) is reviewed. One of the properties which undergoes a large change is the magnetocrystalline anisotropy, which is larger in magnitude and has a lower symmetry below the transformation. The magnetic symmetry axis below the transition temperature can be predetermined by cooling a single crystal through the transition in a magnetic field, and can be changed from one cube edge to another below the transformation by means of a strong magnetic field.

Directional pressure also appears to be effective in establishing the axis. Hysteresis loop measurements show that if a compressive stress is applied along the length of a rod-shaped specimen cut parallel to [100] as it cools through the transition, the specimen is more easily magnetized in this direction. No such effect was observed along [110] or [111] directions.

Changes in dimension occurring at the transition have been measured by means of resistance strain gages cemented along the principal crystallographic directions. Data were obtained as the crystal warmed through the transition in the demagnetized state after having been cooled in various conditions of biasing magnetic field and pressure. It is concluded that the large changes in magnetic symmetry accompany small changes in crystal structure. The low temperature form appears to be orthorhombic, the maximum distortion from cubic symmetry, along [110], amounting to $\mathrm{ca}0.07\mathrm{}$ percent. All three orthorhombic axes can be established unambiguously by cooling the crystal through the transformation in a magnetic field along [100] with pressure applied simultaneously along [011]. The change in direction of easy magnetization above the transition from [111] to [100] is accompanied by a small dimension change ($\mathrm{ca}0.002\mathrm{}$ percent).

Similarities with the behavior of barium titanate at its ferroelectric Curie temperature are pointed out. Evidence for the existence of similar transformations in the other ferrites is discussed briefly.