Investigating spin reversal and other anomalies in magnetic, transport and specific heat measurements of NdFeO3 and NdFe0.5Ni0.5O3 ortho-perovskites

Abstract

Polycrystalline NdFeO₃ (NFO) and NdFe_0.5Ni_0.5O₃ (NFNO) compounds were synthesized by the standard solid state reaction route and were characterized to understand their structural, electrical, magnetic and thermal properties, and electronic structure. Rietveld refinements of the X-ray diffraction pattern confirm the single phase nature of both these compounds with an orthorhombic structure having the space group Pbnm. The temperature dependent resistivity follows the variable range hopping (VRH) model. From this model, the parameters such as density of states (DOS) at the Fermi level N(E_F), hopping distance R_h, hopping energy E_h etc. were calculated. Electrical, magnetic and specific heat [C_p(T)] properties of the NFO and NFNO compounds were systematically studied in the temperature range of 2 to 300 K to understand the spin reorientation transition, magnetic intersection and magnetic inversion anomalies. NFO and NFNO exhibit an interesting phenomenon of spin reversal at low temperatures. The magnetic behaviour at low temperatures and fields is explained based on the competition among magnetic moments of rare earth (Nd) and transition metal ions (Fe/Ni). At a low magnetic field (500 Oe), the zero field cooled (ZFC) and field cooled (FC) branches intersect at low temperatures T_X of 8.75 K and 17.05 K for NFO, and NFNO respectively. Below T_X, the magnetization is negative. The FC curve crosses the M = 0 axis at 4.7 K for NFO, and 5.2 K for NFNO. The spin reorientation transitions are observed in the temperature range of 100–200 K for NFO and NFNO samples with a transition of the Fe³⁺/Ni³⁺ magnetic moments from Γ₄(G_x,F_z)-type ordering (at high temperatures) to Γ₂(G_z,F_x)-type ordering (at low temperatures). The isothermal hysteresis loops show a decrease of magnetization and increase of coercivity with the temperature and complements the magnetization versus temperature data. The Schottky and crossing point anomalies and the effect of an applied magnetic field over them at low temperatures of these compounds were also investigated. In addition to the above properties and magnetic and the thermal anomalies of NFO and NFNO instigated by the spin reorientation were also discussed.