Radio-Frequency Susceptibilities of Weak Ferromagnets: MnCO3and NiF2

The rf susceptibilities of weakly ferromagnetic MnC${\mathrm{O}}_3$ and Ni${\mathrm{F}}_2$ were calculated and measurements were performed at about 10 and 20 Mc/sec. For both crystals we find that whenever the rf magnetic field is perpendicular to the ferromagnetic component and also in the plane which contains the magnetic moments of the sublattices, the susceptibility, ${\chi }^{\prime }$, is large provided only small static fields, $H$, are present. However, when the rf field is perpendicular to the plane which contains the magnetic moments, ${\chi }^{\prime }$ is small regardless of the magnitude or orientation of $H$. For MnC${\mathrm{O}}_3$ a maximum appears in ${\chi }^{\prime }$ which is a function of the orientation and magnitude of $H$ and which is related to an effective magnetic field in the (111) plane of about 60 G. The experimental results of MnC${\mathrm{O}}_3$ are in good agreement with the theory and are characteristic of a weak ferromagnet with anisotropic spin-spin interaction and a small anisotropy field in the plane which contains the spins. Similar results were obtained for Ni${\mathrm{F}}_2$ with a maximum in ${\chi }^{\prime }$ which occurs when $H$ in the (001) plane is approximately 0.7 kG. This and other results of Ni${\mathrm{F}}_2$ do not agree with the theory and are not understood at present.