Evidence for cluster spin glass phase with precursor short-range antiferromagnetic correlations in the B -site disordered Ca(Fe1/2Nb1/2)O3 perovskite

Abstract

The origin of the spin glass (SG) phase in the well-known multiferroic $\mathrm{Pb}(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ compound remains controversial due to the complications introduced by the coexistence of SG and long-range ordered (LRO) antiferromagnetic (AFM) phases. We have addressed this controversy through a comprehensive study on a Pb-free system $\mathrm{Ca}(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ (CFN) which does not exhibit LRO AFM transition. The SG transition in CFN is confirmed by the appearance of a cusp in the temperature dependence of dc magnetization $M(T)$ with a SG freezing temperature ${T}_{\mathrm{f}}\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and bifurcation of the zero-field-cooled and field-cooled magnetization $M(T)$ below the irreversibility temperature ${T}_{\mathrm{irr}}\ensuremath{\sim}27\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Using ac susceptibility $[\ensuremath{\chi}(\ensuremath{\omega},T)]$ measurements, we show that the spin dynamics follows power/Vogel-Fulcher law-type critical dynamics which diverges at ${T}_{\mathrm{SG}}\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with an attempt time ${\ensuremath{\tau}}_{\mathrm{o}}\ensuremath{\sim}{10}^{\ensuremath{-}6}\phantom{\rule{0.28em}{0ex}}\mathrm{s}$ suggesting cluster spin glass (CSG) behavior. The field dependence of ${T}_{\mathrm{f}}(H)$ and ${T}_{\mathrm{irr}}(H)$ is shown to follow the de Almeida--Thouless line which separates the ergodic and nonergodic phases in the $H\ensuremath{-}T$ plane and gives ${T}_{\mathrm{f}}(H=0)\ensuremath{\sim}25\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which is in close agreement with ${T}_{\mathrm{SG}}$ obtained from $\ensuremath{\chi}(\ensuremath{\omega},T)$. The existence of the glassy phase below ${T}_{\mathrm{SG}}$ is further confirmed by the observation of slow nonexponential decay of thermoremanent magnetization with time, memory and rejuvenation effects, and unidirectional exchange-bias effect in the $M\ensuremath{-}H$ hysteresis loop of field-cooled samples. The neutron powder-diffraction patterns reveal the absence of any magnetic peak due to LRO AFM phase but show a broad diffuse peak due to the presence of $\ensuremath{\sim}2\ensuremath{-}\mathrm{nm}$-size AFM spin clusters which are responsible for the CSG freezing in CFN.