Magnetic Anisotropy in Single-Crystal Ferric Ammonium Alum below Its Magnetic Critical Point

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Abstract
The adiabatic susceptibility of single crystals of ferric ammonium alum has been determined as a function of entropy and applied field below 0.1°K. Two methods of measurement have been employed: in addition to a standard ballistic bridge technique, a moving-coil device has been developed to yield static values. A ballistic galvanometer of period 4.6 seconds was used in both circuits. The two single crystals utilized were both shaped as 1.6:1 prolate ellipsoids, one with major dimension along a cubic axis (1,0,0) and the other with major dimension isogonal to the cubic axes (1,1,1). The susceptibility vs entropy results for the single crystals obtained by the ballistic bridge do not display any striking differences from those observed for powdered samples, but the moving coil measurements indicate that the region of nearly constant susceptibility below the magnetic critical point extends to even lower entropies. The differential susceptibility determined by the ballistic bridge in the presence of applied fields up to 300 gauss reveals the presence of magnetic anisotropy. The (1,1,1) data display a weak hump near 120 gauss, in contrast to the (1,0,0) results which reveal no hump and are consistently lower in magnitude. No adequate theoretical explanation of the ordering phenomena in ferric alum is as yet available.

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