Absence of Antiferromagnetism in NbSe2 and TaSe2

Abstract

NbSe2 and TaSe2 are layered metallic compounds in which antiferromagnetic ordering has previously been deduced from anomalies in magnetic and electrical behavior. The anomalies consist of magnetic susceptibility maxima, Hall coefficient sign reversals and for TaSe2, an electrical resistivity kink, at T∼30°K and T∼110°K for the two compounds respectively. We have now studied the nuclear magnetic resonances of 93Nb in NbSe2 and of 77Se in TaSe2 above and below these temperatures. Observation of resonances from T=4.2°K to T=300°K shows that the materials are definitely not magnetically ordered. In NbSe2, an axially symmetric 93Nb quadrupole powder pattern is observed at 77° and 300°K but at 4.2°K is found to be further split. This implies that the transition is instead a low-temperature structural distortion. This distortion may be responsible for the previously observed anomalies and for the absence of magnetic order. On the basis of the observed change of the electric field gradient, the corresponding change of the niobium d-orbitals occupancy is calculated.