Electronic structure and magnetic properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>3</mml:mn><mml:mi>d</mml:mi></mml:mrow><mml:mtext>−</mml:mtext><mml:mrow><mml:mn>4</mml:mn><mml:mi>f</mml:mi></mml:mrow></mml:math> double perovskite material

Abstract

Double-perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. In this paper, we report the electronic structure and magnetic properties of an ordered double perovskite material, ${\mathrm{Ho}}_{2}{\mathrm{CoMnO}}_{6}$. In the double perovskites with general class ${A}_{2}BB{}^{\ensuremath{'}}{\mathrm{O}}_{6}$ ($A$ = rare-earth ions; $B$, $B{}^{\ensuremath{'}}$ = transition metal ions), the octahedral $B$ and $B{}^{\ensuremath{'}}$ sites have a distinct crystallographic site. The Rietveld refinement of x-ray diffraction data reveals that ${\mathrm{Ho}}_{2}{\mathrm{CoMnO}}_{6}$ crystallizes in the monoclinic $P{2}_{1}/n$ space group. X-ray photoelectron spectroscopy confirms the charge state of cations present in this material. The temperature dependence of magnetization and specific heat exhibits a long-range ferromagnetic ordering at ${T}_{\mathrm{c}}\ensuremath{\sim}76$ K owing to superexchange interaction between ${\mathrm{Co}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ moments. Furthermore, the magnetization isotherm at 5 K shows a hysteresis curve that confirms the ferromagnetic behavior of this double perovskite. We observed a reentrant glassy state in the intermediate-temperature regime, which is attributed to inherent antisite disorder and competing interactions. A large magnetocaloric effect has been observed much below the ferromagnetic transition temperature. Temperature-dependent Raman spectroscopy studies support the presence of spin-phonon coupling and short-range order above ${T}_{\mathrm{c}}$ in this double perovskite. The stabilization of magnetic ordering and charge states is further analyzed through electronic structure calculations. The latter also infer the compound to be a narrow-band-gap insulator with the gap arising between the lower and upper Hubbard Co $d$ subbands. Our results demonstrate that antisite disorder and complex $3d\text{\ensuremath{-}}4f$ exchange interactions in the spin lattice account for the observed electronic and magnetic properties in this promising double perovskite material.

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