Antiferromagnetic Resonance in NiO in Far-infrared Region

Using the far-infrared spectrometer constructed recently in Osaka University, measurements of the optical absorption were made on the single crystal of NiO (Néel temperature=523°K) in the wave-length range of 210 to 385 microns and at temperatures 90° to 470°K. No magnetic field was applied. An absorption peak was clearly observed whose wave-length varied with temperature. With increasing temperature it shifted toward the longer wave-length side and at the same time the peak height diminished. At room temperature (291°K) the peak was at 293±5 microns (34.1±0.6 cm -1 ) and the wave-length value extrapolated to absolute zero was 274 microns (36.5 cm -1 ). This absorption peak is attributed to the antiferromagnetic resonance of the mode in which the antiparallel sublattice magnetizations oscillate perpendicularly to the easy plane of magnetization, (111). The constant, K , of the anisotropy energy which constrains the sublattice magnetizations in the easy plane is calculated to be 4.96×10 6 erg/cc at 0°K from the theoretical formula ω=( g e /2 m c )(2 K /χ ⊥ ) 1/2 , assuming g =2.23. This can be compared with theoretical value, 7.01×10 6 erg/cc, based on the magnetic dipolar interaction between Ni ++ ions, but the experimental value is smaller.