Unconventional superparamagnetic behavior in the modified cubic spinel compound LiNi${}_{0.5}$Mn${}_{1.5}$O${}_4$

Abstract

Structural, electronic, and magnetic properties of modified cubic spinel compound LiNi${}_{0.5}$Mn${}_{1.5}$O${}_4$ are studied via x-ray diffraction, resistivity, DC and AC magnetization, heat capacity, neutron diffraction, ${}^7$Li nuclear magnetic resonance, magnetocaloric effect, magnetic relaxation, and magnetic memory effect experiments. We stabilized this compound in a cubic structure with space group $P{4}_{3}32$. It exhibits semiconducting character with an electronic band gap of $\Delta /k_{\mathrm{B}}\simeq 0.4$ eV. The interaction within each Mn${}^{4+}$ and Ni${}^{2+}$ sub-lattice and between Mn${}^{4+}$ and Ni${}^{2+}$ sublattices is found to be ferromagnetic (FM) and antiferromagnetic (AFM), respectively which leads to the onset of a ferrimagnetic transition at $T_{\mathrm{C}}\simeq 125$~K. The reduced values of frustration parameter ($f$) and ordered moments reflect magnetic frustration due to competing FM and AFM interactions. From the ${}^7$Li NMR shift vs susceptibility plot, the average hyperfine coupling between ${}^7$Li nuclei and Ni${}^{2+}$ and Mn${}^{4+}$ spins is calculated to be $\sim 672.4$~Oe/${\mu }_{\mathrm{B}}$. A detailed critical behaviour study is done in the vicinity of $T_{\mathrm{C}}$ using modified-Arrott plot, Kouvel-Fisher plot, and universal scaling of magnetization isotherms. The estimated critical exponents correspond to the 3D XY universality class. A large magneto-caloric effect is observed with a maximum isothermal change in entropy $\Delta S_m\simeq -11.3$~J/Kg-K and a maximum relative cooling power $RCP\simeq 604$~J/Kg for 9~T magnetic field change. The imaginary part of the AC susceptibility depicts a strong frequency dependent hump at $T=T_{f2}$ well below the blocking temperature $T_{\mathrm{b}}\simeq 120$~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.

arXiv comment23 pages, 22 figures, and 4 tables; Phys. Rev. B (Accepted)

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