O-NMR*

Because of their poorly understood high-temperature metal–insulator transition and their unusual low-temperature magnetic order, the rare-earth nickelate perovskites RENiO₃ have received a lot of attention in recent years. The antiferromagnetic order with a propagation vector q = (1/4, 1/4, 1/4) (w.r.t. the pseudocubic unit cell) breaks the crystalline inversion symmetry. The microscopic origin of this non-trivial magnetic order is still unclear. Our temperature dependent ¹⁷O-NMR relaxation measurements on isotopically enriched SmNiO₃ reveal the appearance of additional magnetic fluctuations below the metal–insulator transition, which then freeze out at the magnetic phase transition. We argue that this indicates that fluctuations with q = (1/4, 1/4, 1/4) are “switched on” only in the insulating phase, growing stronger upon cooling, and finally causing the magnetic phase transition.