Antiferromagnetic resonance in the double-layered system Rb₃Mn₂Cl₇

Millimetre-wave antiferromagnetic resonance (AFMR) experiments are reported on carefully grown and well characterised single-crystal samples of the double-layered antiferromagnet Rb₃Mn₂Cl₇. The zero-field frequency of the magnetic excitations is found to be 106 GHz. By comparison with the corresponding value in Rb₂MnCl₄, this result supports the model in which the anisotropy energy decreases almost proportionally to n in the compounds Rb_n+1Mn_nCl_3n+1. Some discrepancy with ordinary AFMR theory seems to be observed in the spin-flopped phase.