The magnetic properties of Fe2+ ions in Fe(NH4)2(SO4)26H2O at low temperatures

The published data on the specific heat and susceptibility of Fe(NH4)2(SO4)26H2O are analysed to find the energies, wavefunctions and magnetic axes appropriate to the Fe2+ ions at temperatures below 20 K. Two ambiguities are solved by experiment. The dependence on temperature of the crystal susceptibility axes is measured to decide between alternative directions for the ionic magnetic axes, with results which invalidate certain assumptions of previous authors. The problem of associating magnetic axes with a particular one of the two inequivalent types of paramagnetic site is solved by a spin-resonance study not of the Fe2+ ions themselves (which at low temperatures show no absorption at ordinary microwave frequencies), but rather of Cu2+ ions added as an impurity and in whose spectrum the effects of individual Fe2+ ions can be distinguished.

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