Exchange bias and inhomogeneous spin states in La 1.5 Sm 0.5 NiMnO 6

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The presence of antisite disorder in double perovskites manifests various intriguing properties like the spin-glass state, exchange bias, and memory effect. Here, we report the synthesis of a ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ compound that crystallizes in a monoclinic ($P2{}_{1}/n$) structure. The presence of multiple oxidation states of Ni(Mn) cations induces competing (ferromagnetic and antiferromagnetic) exchange interactions that originate an inhomogeneous spin state, as evident from observed magnetic anomalies in temperature-dependent magnetization measurements. A spin-glass (SG) state is evolved that manifests field cooling ($H{}_{\text{CF}}$ = 500 Oe) induced exchange bias ($H{}_{\text{EB}}\ensuremath{\sim}153$ Oe) below spin-glass temperature $T{}_{\text{SG}}$ (65 $\ifmmode\pm\else\textpm\fi{}$ 1 K). The strength of the exchange bias is reduced after successive magnetization reversal cycles performed at 5 K. The reported magnetic training effect is explained within the frameworks of metastable magnetic disorder across frozen antiphase boundaries in the frustrated SG state. Measurements of frequency-dependent ac-susceptibility $\ensuremath{\chi}(\ensuremath{\omega})$ suggest critical slowing dynamics and memory effect in the proximity of $T{}_{\text{SG}}$, which is described using a critical slowing model resulting in relaxation exponent $\mathrm{z}\ensuremath{\nu}$ = 1.99 $\ifmmode\pm\else\textpm\fi{}$ 0.04 and ${\ensuremath{\tau}}_{0}$ = 8.91 x ${10}^{\ensuremath{-}7}\mathrm{s}$. Employing first-principles calculations, we find the insulating ferromagnetic ground state of ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ in the ordered phase where Ni(Mn) appears to be in the $2+(4+$) state. Further, the presence of antisite disorder eventually results in lower magnetic moments per formula unit, which is well corroborated by experimental observations. Our findings provide a pathway for designing host materials with inhomogeneous spin-frustrated systems and variable electronic states.

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