Nuclear Frequency Pulling in a Dzialoshinskii-Moriya-Type Weak Ferromagnet: MnCO3

When MnC${\mathrm{O}}_3$ is in the canted state, it may be treated as consisting of two electron and two nuclear sublattices. For the above crystal we have calculated four eigenfrequencies; two correspond to electron modes, the other two to nuclear modes. From the temperature dependence of the low-frequency electron resonance we have determined the anisotropy field in the (111) plane, which consists of an approximate temperature-independent term of 0.98±0.04 G and a temperature-dependent term of $\frac{(8.6\mathrm{}\pm 0.3)}{T_n}$ G, where $T_n$ is the nuclear spin temperature. We have observed the nuclear resonance indirectly by measuring its effect on the electron resonance at 6 Gc/sec. For intermediate rf power levels two electron resonance lines were observed at 4.2°K between 538 and 638 Mc/sec. One of the lines corresponds to the unshifted resonance line; the other was dependent on the frequency only. This shift is due to partial saturation of the nuclear spin system, which saturates at 640 Mc/sec. The lowest observed nuclear frequency was 538 Mc/sec, which is 102 Mc/sec lower than the frequency we would expect if the nuclear and electron systems are decoupled. Direct Mn NMR, observed recently, is in good agreement with the above results.