Griffiths phase-like behavior and spin-phonon coupling in double perovskite Tb₂NiMnO₆

Abstract

The Griffiths phase-like features and the spin-phonon coupling effects observed in Tb₂NiMnO₆ are reported. The double perovskite compound crystallizes in monoclinic P2₁/n space group and exhibits a magnetic phase transition at T_c ∼ 111 K as an abrupt change in magnetization. A negative deviation from ideal Curie--Weiss law exhibited by 1/χ(T) curves and less-than-unity susceptibility exponents from the power-law analysis of inverse susceptibility are reminiscent of Griffiths phase-like features. Arrott plots derived from magnetization isotherms support the inhomogeneous nature of magnetism in this material. The observed effects originate from antiferromagnetic interactions that arise from inherent disorder in the system. Raman scattering experiments display no magnetic-order-induced phonon renormalization below T_c in Tb₂NiMnO₆, which is different from the results observed in other double perovskites and is correlated to the smaller size of the rare earth. The temperature evolution of full-width-at-half-maximum for the stretching mode at 645 cm⁻¹ presents an anomaly that coincides with the magnetic transition temperature and signals a close connection between magnetism and lattice in this material.