Spin-order-induced multiferroicity in LiCuFe2(VO4)3 and disorder effects in NaCuFe2(VO4)3

TL;DR

This study investigates how spin order induces multiferroicity in LiCuFe2(VO4)3 and explores the effects of disorder in NaCuFe2(VO4)3, revealing the importance of alkali metal mobility in multiferroic behavior.

Contribution

It demonstrates the role of spin order in inducing multiferroicity in Li compound and highlights the impact of alkali metal mobility on multiferroic properties in these compounds.

Findings

Li compound shows spin-order-induced ferroelectricity; Na does not.

Disorder effects are significant and linked to alkali metal mobility.

Similar magnetic interactions are found in both compounds.

Abstract

Mixed spin chain compounds, ACuFe2(VO4)3 (A= Li,Na), reach magnetically ordered state at TN ~ 11 K (Li) or ~ 9 K (Na) and experience further transformation of magnetic order at T* ~ 7 K (Li) or ~ 5 K (Na), evidenced in magnetic susceptibility chi and specific heat Cp measurements. While no anomaly has been detected in dielectric property of NaCuFe2(VO4)3, the step-like feature precedes a sharp peak in permittivity epsilon at TN in LiCuFe2(VO4)3. These data suggest the spin-order-induced ferroelectricity in Li compound and no such thing in Na compound. On the contrary, the Moessbauer spectroscopy study suggests similarly wide distribution of hyperfine field in between T* and TN for both the compounds. The first principles calculations also provide similar values for magnetic exchange interaction parameters in both compounds. These observations lead us to conclude on the crucial role of alkali metals mobility within the channels of the crystal structure needed to be considered in explaining the improper multiferroicity in one compound and its absence in other.