Magnetism of Synthetic and Natural MnCO3

Abstract

Weakly ferromagnetic MnC${\mathrm{O}}_3$ powder with $T_N=32.2\mathrm{}$°K shows single-domain behavior with coercive force of about 3.5 kOe at 4.2°K. The differential susceptibility at dc and 10 MHz is less at low fields than at high fields for $T<\mathrm{}10\mathrm{}$°K; the remanent susceptibility is the same as the initial susceptibility. The measurements are consistent with a magnetization process involving spin flopping of the sublattices in the low-anisotropy basal plane. The measured remanent moment at 4.2°K indicates a spontaneous (ferromagnetic moment of 92 emu/mole in pure MnC${\mathrm{O}}_3$. Natural crystals of composition ${\mathrm{Mn}}_{0.95}$ ${\mathrm{Fe}}_{0.05}$C${\mathrm{O}}_3$ (2.3% Fe by weight) show no weak ferromagnetism below $T_N=35\mathrm{}$°K. Instead, the spins are aligned parallel to the trigonal crystal axis, resulting in normal uniaxial antiferromagnetism. The large anisotropy of the ${\mathrm{Fe}}^{++}$ impurities is assumed to be the cause.