Magnetic and Thermal Properties of Chromic Methylamine Alum Below 1°K

The earlier measurements of de Klerk and Hudson on a spherical mass of small crystals of chromic methylamine alum have been repeated, and extended, using a one-inch sphere ground from a large single crystal. The variation of susceptibility with entropy follows closely---down to 0.1^∘{}K---the predictions of the theory of Hebb and Purcell when a suitable choice is made for the value of δ, the splitting between the two spin-doublets in the ground level of the Cr⁺⁺⁺ ion. It is found that δk=0.269±0.003 degree.Measurements have been made of the ballistic susceptibility χ, remanent magnetic moment Σ, and the in-phase and out-of-phase components of the ac susceptibility, χ^' and χ^'', respectively (these latter being measured at both 150 cps and 210 cps). The variation of χ^' with entropy S is found to agree closely with that reported for the powder specimen, except in the region of the susceptibility maximum where the behavior appears to depend upon the rate of precooling the specimen.At approximately the same entropy as that corresponding to maximum χ^', χ^'' shows an extremely sharp maximum. The ballistic susceptibility depends upon the measuring field in the region of the maximum; remanence appears here and goes through a maximum as the entropy is reduced. The remanence maximum is found at a lower entropy the larger is the measuring field. Associated with this hysteresis are very marked "time-effects" in the adjustment of the magnetization to a change in measuring field, as encountered in tracing out hysteresis loops.Measurements of the susceptibility in superimposed steady magnetic fields of 0-500 oersted show unusual behavior below the Curie point and suggest the existence of anisotropy. For application of the field along a cubic axis, a magnetization of about one-half the saturation value appears to have special significance; this value is attained in fields as small as 150 oersted, but increasing the field up to about 500 oersted produces little further change in the magnetization.