Resonance at 70 Gc/sec Near the Néel Point in RbMnCl3

Abstract

Magnetic resonance has been observed at 70 Gc/sec as a function of temperature near the Néel point (∼88°K) in RbMnCl₃. Below the ordering temperature, the antiferromagnetic resonance line moves rapidly downfield with decreasing temperature. At zero field, the line broadens and disappears. This resonance is identified as the high‐frequency mode of a hard‐axis antiferromagnet; it has not been observed at lower frequencies. At X band and at K band, the paramagnetic resonance line broadened and disappeared above T_N such that continuous observation of the resonance through the Néel point was not possible. The 70‐Gc/sec measurements in this temperature region display an interesting hysteresis effect. Below T_N, the AFMR line was observed at higher fields for an increasing field sweep than for a decreasing field sweep. Above 88°K, the field for resonance is independent of temperature but the amplitude of the PMR depends upon the direction in which the field is swept through resonance. Over a temperature interval of 1.5° below 88°K, the sample appeared to be ordered during a decreasing field sweep; but only the paramagnetic line was observed as the field was swept in the opposite direction. Possible explanations for this behavior are presented.