Field-induced spin transitions in hematite powders as observed from Mo¨ssbauer spectroscopy

Mo¨ssbauer spectra for three Al-hematite powders were recorded at 4.2 K in applied field of 140 kOe. Particle sizes, as determined from X-ray diffraction line broadenings, are of order 100 nm. In zero field the powders are in a pure antiferromagnetic state. It is found that the applied-field is too low to produce a spin flop into the basal plane in all individual grains constituting the powders. Only portions undergo this transition, while remaining portions undergo a screw-rotation transition whereby the antiferromagnetic spin axis is canted away from the <f>c</f>-axis and rotates perpendicular to the external field. It is argued that the angle of the external field with respect to the <f>c</f>-axis determines which of the two distinct spin reorientation mechanisms takes place. A critical angle can be calculated from the numerical results of the spectral analyses. Its value depends on the field strength and on the Al substitution. [Copyright &y& Elsevier]