Unconventional superparamagnetic behavior in the modified cubic spinel compound LiNi_0.5Mn_1.5O₄

LiNi_0.5Mn_1.5O₄ features a three-dimensional pyrochlore lattice and exhibits 1:3 cation order of Mn⁴⁺ and Ni²⁺ ions. The intra-sublattice interaction within each Mn⁴⁺ and Ni²⁺ sub-lattice and the inter-sublattice interaction between Mn⁴⁺ and Ni²⁺ sublattices is found to be ferromagnetic (FM) and antiferromagnetic (AFM) respectively. This leads to the onset of a ferrimagnetic transition at T_C ∼ 125 K. The reduced values of frustration parameter (f) and ordered moments reflect magnetic frustration due to competing FM and AFM interactions. A detailed critical behavior analysis nearT_C reveals that the magnetic phase transition is second order in nature and belongs to the 3D XY universality class. A large magneto-caloric effect is observed with a maximum value of isothermal change in entropy ΔS_m ∼ -11.3 J/Kg-K and a maximum relative cooling power of RCP ∼ 604 J/Kg for 9 T magnetic field change. The imaginary part of the AC susceptibility depicts a strong frequency dependent hump at T = T_f2 well below the blocking temperature T_b ∼ 120 K. The Arrhenius behaviour of frequency dependent T_f2 and the absence of ZFC memory confirm the existence of superparamagnetism in the ferrimagnetically ordered state.