Unconventional superparamagnetic behavior in the modified cubic spinel compound LiNi0.5Mn1.5O4

Structural, electronic, and magnetic properties of modified cubic spinel compound ${\mathrm{LiNi}}_0.5{\mathrm{Mn}}_1.5{\mathrm{O}}₄$ are studied via x-ray diffraction, resistivity, dc and ac magnetization, heat capacity, neutron diffraction, $⁷\mathrm{Li}$ nuclear magnetic resonance, magnetocaloric effect, magnetic relaxation, and magnetic memory effect experiments. We stabilized this compound in a cubic structure with space group $P{4}₃32$. It exhibits semiconducting character with an electronic band gap of $\mathrm{\ensuremath{\Delta}}/{k}_{\mathrm{B}}\ensuremath{\simeq}0.4$ eV. The interaction within each ${\mathrm{Mn}}⁴⁺$ and ${\mathrm{Ni}}²⁺$ sublattice and between ${\mathrm{Mn}}⁴⁺$ and ${\mathrm{Ni}}²⁺$ sublattices is found to be ferromagnetic (FM) and antiferromagnetic (AFM), respectively. This leads to the onset of a ferrimagnetic transition at ${T}_{\mathrm{C}}\ensuremath{\simeq}125$ K. The reduced values of frustration parameter ($f$) and ordered moments reflect magnetic frustration due to competing FM and AFM interactions. From the $⁷\mathrm{Li}$ nuclear magnetic resonance shift vs susceptibility plot, the average hyperfine coupling between $⁷\mathrm{Li}$ nuclei and ${\mathrm{Ni}}²⁺$ and ${\mathrm{Mn}}⁴⁺$ spins is calculated to be $\ensuremath{\sim}672.4$ Oe/${\ensuremath{\mu}}_{\mathrm{B}}$. A detailed critical behavior study is done in the vicinity of ${T}_{\mathrm{C}}$ 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 $\mathrm{\ensuremath{\Delta}}{S}_m\ensuremath{\simeq}\ensuremath{-}11.3$ J/Kg K and a maximum relative cooling power of $\text{RCP}\ensuremath{\simeq}604$ J/Kg for 9 T magnetic field change. The imaginary part of the ac susceptibility depicts a strong frequency-dependent hump at $T={T}_{\mathrm{f}2}$ well below the blocking temperature ${T}_{\mathrm{b}}\ensuremath{\simeq}120$ K. The Arrhenius behavior of frequency dependent ${T}_{\mathrm{f}2}$ and the absence of zero-field-cooled memory confirm the existence of superparamagnetism in the ferrimagnetically ordered state.