Large magnetic excitation gap in antiferromagnetic YNiO₃ detected by NMR

The rare-earth nickelate perovskites (RENiO₃) are an archetypal family of strongly correlated electron materials. Their antiferromagnetic state with four nickel atoms per period remains poorly studied, even though it has been predicted to be a type-II multiferroic with large polarization. Currently, there is no agreement on the magnetic structure and low-energy spin model for bulk RENiO₃. We present ¹⁷O nuclear-spin–lattice relaxation measurements demonstrating the presence of a large gap of 30 meV in the magnetic excitation spectrum of YNiO₃.
NMR relaxation data on quadrupolar nuclei (spin > 1/2) such as ¹⁷O in a polycrystalline or powdered antiferromagnet often seem to contain an unphysical jump in the temperature-dependence of the apparent relaxation rate within the antiferromagnetic state. This jump occurs when the effects of the magnetic order and the electric quadrupole moment on the NMR spectrum become similar in magnitude. We have found a new theoretical expression, which allows us to determine the correct value of the relaxation time T1 for quadrupolar nuclei in antiferromagnetic powders, thus getting rid of this unphysical discontinuity.