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Anomalous hyperfine interaction in CoF2 investigated by high resolution neutron spectroscopy

We investigated the low energy excitations in CoF2 in the µeV range with a back-scattering neutron spectrometer. The energy scans on a CoF2 powder sample revealed inelastic peaks at E = 0.728 ± 0.008 µeV at T = 3.46 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at the electronic magnetic ordering temperature TN≈37 K. We interpret the inelastic peaks to be due to the transition between the hyperfine-split nuclear level of the 59Co isotopes with spin I = 7/2. We have shown that the energy of the inelastic peak or the hyperfine splitting in CoF2 can be treated as an order parameter of the antiferromagnetic phase transition and yields the critical exponent β = 0.313 ± 0.007, consistent with the neutron diffraction results and also the three-dimensional Ising character of the magnetic system. The determined hyperfine splitting in CoF2 deviates from the linear relationship between the ordered electronic magnetic moment and the hyperfine splitting in Co, Co–P amorphous alloys and CoO, presumably due to the presence of an unquenched orbital moment.