The mixed spin chain compound, $\mathrm{Li}\mathrm{Cu}{\mathrm{Fe}}_2({\mathrm{VO}}_4{)}_3$, reaches a magnetically ordered state in two steps at $T_{\mathrm{N}2}=9.8\mathrm{K}$ and $T_{\mathrm{N}1}=8.2\mathrm{K}$ detected in magnetic-susceptibility χ and specific-heat $C_p$ measurements. Dielectric permittivity ε evidences these transitions by a sharp peak at $T_{\mathrm{N}2}$ and a steplike anomaly at $T_{\mathrm{N}1}$, both easily suppressed by an external magnetic field. These features in permittivity are preceded by a frequency-dependent relaxation-type anomaly at $T^{\ast }$ insensitive to the magnetic field. Mössbauer spectroscopy reveals a wide distribution of a hyperfine field between $T_{\mathrm{N}2}$ and $T_{\mathrm{N}1}$ while the first-principles calculations provide values of magnetic-exchange-interaction parameters. For $\mathrm{Cu}$, the orbital moment aligned in the same direction as the spin moment is substantial. This large orbital moment is important in driving the polarization through an inverse Dzyaloshinksii-Moriya interaction in a situation where the spatial symmetry gets broken in the magnetic structure. The data obtained suggest the spin-order-induced ferroelectricity is inherent for improper multiferroics.

• Received 19 November 2017
• Revised 21 June 2018

DOI:https://doi.org/10.1103/PhysRevApplied.10.034008