Relaxation Effects in the Magnetization of Chromic Methylamine Alum below Its Néel Point

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Abstract
A moving-coil apparatus is described which makes possible the measurement of static susceptibilities of paramagnetic salts cooled by adiabatic demagnetization to temperatures below 1°K. The sample studied was a 2:1 ellipsoid of revolution, with major dimension along a cubic axis, shaped from a single crystal of chromic methylamine alum. In contrast to data obtained by the customary mutual inductance bridge method with ballistic galvanometer detection under comparable conditions, the new technique made possible the direct study of a long relaxation time in the magnetizing process, which was observed whenever the crystal was cooled below its Néel point. An additional sharp peak in the susceptibility as a function of entropy was also found in this region. It is pointed out that the theory of magnetic ordering in the chromic alums can successfully account for the general dependence of susceptibility on entropy.