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 TN 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 TN, 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.