Successive magnetic transitions and multiferroicity in layered honeycomb BiCrTeO$_{6}$

Abstract

Low-dimensional magnetic systems based on honeycomb lattices provide a promising platform for exploring exotic quantum phenomena that emerge from the intricate interplay of competing spin, orbital, lattice, and dipolar degrees of freedom. Here, we present a comprehensive study of the layered honeycomb lattice antiferromagnet BiCrTeO$_6$ using magnetization, specific heat, muon spin--relaxation ($μ$SR) spectroscopy, dielectric, pyrocurrent, and high-resolution synchrotron X-ray diffraction (SXRD) measurements. Our results reveal an array of intriguing and strongly correlated phenomena, including two successive antiferromagnetic transitions at $T_{\rm N1}\approx16$ K and $T_{\rm N2}\approx11$ K, a pronounced magnetodielectric coupling effect, and ferroelectric order at $T_{\rm N2}$. Consequently, this compound emerges as a new spin-driven multiferroic system. The SXRD analysis reveals a magnetoelastic-coupling-induced structural phase transition at $T_{\rm N2}$, characterized by a symmetry lowering from P$\bar{3}$1c (163) to P31c (159), which likely triggers the onset of ferroelectricity. In addition to its low-temperature multiferroic behavior, the system exhibits dielectric relaxor characteristics at higher temperatures within the paramagnetic region ($T<50$ K), which is intrinsically linked to the antisite disorder of Cr and Te atoms.

Figures (11)

• Figure 1: (a) SXRD pattern at $T = \text{290 K}$ (red circles) with the fitted and differences profiles (black and blue lines). Upper and lower green bars indicate the Bragg peak positions for the main phase of BiCrTeO$_6$ and the impurity phase (less than 1 wt.%), Bi$_6$Te$_2$O$_{15}$, respectively. The impurity phase peak is marked with a green $\Diamond$ symbol. (b) Crystal structure of BiCrTeO$_6$ as viewed perpendicular to the ab plane. (c) Corresponding polyhedral structure depicting the edge-shared Cr/TeO$_{6}$ octahedra that form the honeycomb structure. (d) Representation of the honeycomb lattice formed by Cr/Te atoms. (e) Two consecutive honeycomb layers stacked along the c axis.
• Figure 2: (a) and (b) depicts $\chi$($T$) ZFC (shown in blue) and FC (shown in red) curves under $H$= 1000 Oe and its closer view near transition temperatures, respectively. The corresponding Curie-Weiss fitting of “$1/\chi$ vs. $T$” curve is shown in the inset of Fig. (a). $M$($H$) curves collected at $T$ = 2, 7, 12, and 50 K are shown in the inset of Fig.(b).
• Figure 3: (a) Temperature dependence of the total specific heat $C_p(T)$ (blue circles) along with the fitted phononic contribution using the Debye–Einstein model (red solid line). The inset shows an expanded view at low temperature. (b) $T$-variation of $C_{\text{mag}}$/$T$ (right $y$-axis) and the corresponding magnetic entropy change ($S_{\text{mag}}$; left $y$-axis). Long-range magnetic ordering is demonstrated by the $\lambda$-like anomaly at $T_{N2} \approx 11$ K, whereas the shoulder-like anomaly near $T_{\rm N1} \approx 16$ K indicates another magnetic transition.
• Figure 4: (a) Time evolution of the muon spin asymmetry collected at various selected temperatures under an applied weak transverse field $H_{\rm wTF}=30$ Oe. The solid lines refer to the theoretical fits as described in the text. Panel (b) shows the temperature dependence of the wTF-asymmetry $A_{\rm TF}$, while panel (c) presents the variation of $\lambda_{\rm TF}$ with temperature. (d) depicts thermal variations of $A_{\mathrm{TF}}^{f}$ and $A_{\mathrm{TF}}^{s}$. Dashed vertical lines mark the transition temperatures $T_{\rm N1}$ and $T_{\rm N2}$.
• Figure 5: (a) Zero-field (ZF) muon spin asymmetry of $\mathrm{BiCrTeO_6}$ at shorter time scales for selected temperatures above and below $T_{\rm N1}$ and $T_{\rm N2}$. The solid lines represent the corresponding fits using the model described in the text. (b) Temperature dependence of the two oscillation frequencies, which likely correspond to different internal fields sensed by muons below $T_{\rm N1}$. (c) Time dependence of the ZF muon spin asymmetry at longer time scales for several representative temperatures. The solid lines correspond to the sum of exponential functions as described in the text. (d) Temperature dependence of the asymmetry associated with the non-oscillatory component observed at longer time scales. Inset: Temperature dependence of the fast ($\lambda_{\mathrm{ZF}}^{f}$) and slow ($\lambda_{\mathrm{ZF}}^{s}$) relaxation rates at longer time scale.