Unconventional Superparamagnetic Behavior In The Modified Cubic Spinel Compound Lini0.5mn1.5o4

Structural, electronic, and magnetic properties of modified cubic spinel compound LiNi0.5Mn1.5O4 are studied via x-ray diffraction, resistivity, dc and ac magnetization, heat capacity, neutron diffraction, Li-7 nuclear magnetic resonance, magnetocaloric effect, magnetic relaxation, and magnetic memory effect experiments. We stabilized this compound in a cubic structure with space group P4(3)32. It exhibits semiconducting character with an electronic band gap of Delta/k(B) similar or equal to 0.4 eV. The interaction within each Mn4+ and Ni2+ sublattice and 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 similar or equal to 125 K. The reduced values of frustration parameter (f) and ordered moments reflect magnetic frustration due to competing FM and AFM interactions. From the Li-7 nuclear magnetic resonance shift vs susceptibility plot, the average hyperfine coupling between Li-7 nuclei and Ni2+ and Mn4+ spins is calculated to be similar to 672.4 Oe/mu(B). A detailed critical behavior study is done in the vicinity of T-C using modified-Arrott plot, Kouvel-Fisher plot, and universal scaling of magnetization isotherms. The magnetic phase transition is found to be second order in nature and the estimated critical exponents correspond to the three-dimensional XY universality class. A large magnetocaloric effect is observed with a maximum value of isothermal change in entropy Delta S-m similar or equal to -11.3 J/Kg K and a maximum relative cooling power of RCP similar or equal to 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 similar or equal to 120 K. The Arrhenius behavior of frequency dependent T-f2 and the absence of zero-field-cooled memory confirm the existence of superparamagnetism in the ferrimagnetically ordered state.