Antiferromagnetic Order of NdT2Zn20(T = Co and Rh) with the Kramers Gamma(6) Doublet Ground State

摘要

Electrical resistivity rho, magnetization M, and specific heat C were measured for 4f(3) Kramers systems NdT2Zn20(T = Co and Rh), in which Nd3+ ions form a diamond sublattice. The magnetic susceptibility chi = M/B follows the Curie-Weiss law in the temperature range from 300 to 10 K, from which the paramagnetic Curie temperatures theta(p) are evaluated to be -7.1 and -6.3 K for T= Co and Rh, respectively. A broad maximum in the magnetic specific heat divided by temperature C-m/T at around 13 K is ascribed to the excitations from the crystalline electric field ground state of the magnetic Gamma(6) doublet to the excited Gamma(8) quartets. Sharp peaks in C-m(T) at T-N = 0.53 and 0.94 K for T = Co and Rh, respectively, coincide with the drops in the resistivity rho(T), and both temperatures decrease with increasing magnetic field. Moreover, chi(T) for T = Co decreases on cooling at T-N, indicating the manifestation of an antiferromagnetic order at T-N. rho(T) shows upward curvature for T-N < T< 4 K. Taking account of the significant reduction of the magnetic entropy at T-N from the expected value R In 2/Nd, we attribute the anomalous behavior in rho(T) to the c-f hybridization of the Gamma(6) doublet ground state and/or magnetic frustration due to the diamond structure of the Nd sublattice.