Possible multipolar transition in NdB₄

Abstract

Physical properties of neodymium tetraboride NdB4 which crystallizes in the ThB4-type structure belonging to space group D4h5 – P4/mbm have been studied. The specific heat measurement on single crystal of NdB4 shows that this material undergoes successive phase transitions at TQ = 17.2 K (second-order transition), TN1 = 7.0 K (second-order transition) and TN2 = 4.8 K (first-order transition). The magnetic entropy of approximately R ln2 and R ln 4 is released below TN1 and TC, respectively. The crystalline electric field (CEF) ground state of NdB4 is a pseudo-quartet consisting of two Kramers doublets. At around TQ, no anomalies of the magnetic susceptibility along the c-direction which is easy axis of magnetization are observed. Moreover, the susceptibility within the ab-plane shows only a very small peak. These results seem to indicate that NdB4 undergoes a quadrupolar (or higher rank multipolar) transition at TQ followed by two magnetic transitions since the behaviors are quite different from those of conventional magnet.