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Critical phenomena at phase transitions in multiferroic materials remain not fully understood, and quantum beams drive the elucidation of the phenomena from the viewpoint of microscopic dynamical properties. BiFeO$_3$ is one of the most promising multiferroic materials with high antiferromagnetic ($T_N$$\sim$650 K) and ferroelectric ($T_C$$\sim$1100 K) transition temperatures [1]. We aim to investigate local spin fluctuations in BiFeO$_3$ to study dynamics near the phase transition in systems with coexisting ferroelectric orderings.
$\space\space$ The temperature dependence of initial asymmetry $A_0$ and longitudinal relaxation rate $\lambda$ was obtained by the simple exponential-decay fit in the late time window between 2 and 15 $\mu$s. A sharp increase in $A_0$ and a sudden drop in $\lambda$ are observed at 650 K, indicating the Néel temperature of BiFeO$_3$. $\lambda$ is a parameter related to the fluctuations of the Fe$^{3+}$ electron spins. At the high temperature side above $T_N$, $\lambda$ shows a temperature dependence of $T^{1.24}$. On the other hand, at the lower temperature side of $T_N$, corresponding to the ferroelectric–antiferromagnetic ordered phase, $\lambda$ exhibits a non-power-law temperature dependence. This behavior is likely related to underlying electromagnetic coupling in the multiferroic dynamics and/or muon diffusion at elevated temperatures.
[1] P. Fischer et. al., J. Phys. C: Sol. Sta. Phys. 13, 1931 (1980).
| hirotaka.okabe@kek.jp | |
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