Magnetic anisotropy of the spin-antiferromagnet GdNi2B2C probed by high-frequency ESR

A Gd3+ electron spin resonance study in a frequency range 10 - 350 GHz on a single crystal of the spin-only antiferromagnet GdNi2B2C is reported. The Korringa relaxation broadening of the Gd-ESR signal is surprisingly anisotropic implying anisotropic interactions between the localized Gd3+ spins and the conduction electrons. In the antiferromagnetic state at T <TN = 19.4 K we observe an out-of-plane anisotropy gap of ∼76 GHz. We also observe an in-plane gap with the same order of magnitude. Numerical analysis of the spin excitation modes in the ordered state assuming classical dipole-dipole interactions agrees with the experiment qualitatively well. However, quantitatively it yields appreciably higher gap values.