Phase coexistence and exchange-bias effect in LiMn2O4 nanorods

Abstract

In this paper, the magnetic properties of $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods with an average diameter of $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and length of $\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ are investigated. The temperature dependences of dc and ac susceptibility measurements show that $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods experience multiple magnetic phase transitions upon cooling, i.e., paramagnetic (PM), antiferromagnetic (AFM), canted antiferromagnetic (CAFM), and cluster spin glass (SG). The coexistence between a long-range ordered AFM phase due to a $\mathrm{M}{\mathrm{n}}^{4+}\text{\ensuremath{-}}\mathrm{M}{\mathrm{n}}^{4+}$ interaction and a cluster SG phase originating from frozen AFM clusters at low temperature in $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods is elucidated. Field-cooled hysteresis loops (FC loops) and magnetic training effect (TE) measurements confirm the presence of an exchange-bias (EB) effect in $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods below the N\'eel temperature $({T}_{\mathrm{N}}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}60\phantom{\rule{4pt}{0ex}}\mathrm{K})$. Furthermore, by analyzing the TE, we conclude that the observed EB effect originates completely from an exchange coupling interaction at the interface between the AFM and cluster SG states. A phenomenological model based on phase coexistence is proposed to interpret the origin of the EB effect below 60 K in the present compound. In turn, the appearance of the EB effect further supports the coexistence of AFM order along with a cluster SG state in $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods.