Griffiths-like phase, large magnetocaloric effect, and unconventional critical behavior in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>NdSrCoFe</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math> disordered double perovskite

Abstract

A B-site disordered double perovskite $\mathrm{NdSrCoFe}{\mathrm{O}}_{6}$ was successfully synthesized by the conventional sol-gel method. Detailed experimental analyses revealed that $\mathrm{NdSrCoFe}{\mathrm{O}}_{6}$ crystallizes in the orthorhombic $Pnma$ space group, in which ${\mathrm{Co}}^{2+/3+}$ and ${\mathrm{Fe}}^{3+/4+}$ ions are randomly distributed at the BB\ensuremath{'} sites, and ${\mathrm{Sr}}^{2+}$ and ${\mathrm{Nd}}^{3+}$ ions are respectively ordered at the A and A\ensuremath{'} sites in an alternating arrangement along the $c$ direction. $\mathrm{NdSrCoFe}{\mathrm{O}}_{6}$ has a semimetallic-to-semiconducting transition nature, and a paramagnetic-ferromagnetic (FM) second-order phase transition originating from the complex hybridization between Co $3d$ and O $2p$ states is also found to occur at ${T}_{\mathrm{C}}\ensuremath{\approx}150$ K. Then the spin coupling between ${\mathrm{Fe}}^{4+}\ensuremath{\leftrightarrow}{\mathrm{Co}}^{3+}$ and ${\mathrm{Fe}}^{3+}\ensuremath{\leftrightarrow}{\mathrm{Co}}^{2+}$ randomly distributed on the B and B\ensuremath{'} sites leads to a FM cluster spin-glass behavior with characteristic parameters of $k=0.01, {T}_{\mathrm{SG}}=82.7$ K, $zv=1.89$, and ${\ensuremath{\tau}}_{0}=0.46\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ s. Additionally, Griffiths-like phase behavior was observed in the region ${T}_{\mathrm{C}}&lt;T&lt;{T}_{\mathrm{GP}}$, with ${T}_{\mathrm{GP}}=245$ K, consistent with the power law exponent of $\ensuremath{\lambda}=0.74$. The maximum isothermal magnetic entropy change $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{M}^{\mathrm{max}}\ensuremath{\approx}1.84\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{k}{\mathrm{g}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ and relative cooling power $\ensuremath{\approx}43.8\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{k}{\mathrm{g}}^{\ensuremath{-}1}$ under a field of 40 kOe also indicate a magnetocaloric coupling wherein fitted critical exponents $\ensuremath{\beta}=1.384, \ensuremath{\gamma}=0.621$, and $\ensuremath{\delta}=1.421$ are far from any conventional universality class. Density functional theory calculations demonstrated spin short- and long-range ordering competitions for Fe/Co at BB\ensuremath{'} sites, which arise predominantly from the stronger negatively charged ligand interaction with Co $3d$ orbitals and the weakest Fe $3d$ orbitals. This unconventional behavior is expected to be the main reason for the experimentally observed magnetic exchange distance decreasing with $J(r)\ensuremath{\approx}{r}^{\ensuremath{-}4.7}$.

Topics & Concepts