Thermodynamic properties of the new multiferroic material (NH$_4$)$_2$[FeCl$_5$(H$_2$O)]

TL;DR

This study characterizes a new multiferroic material, (NH$_4$)$_2$[FeCl$_5$(H$_2$O)], revealing its magnetic, electric, and structural phase transitions and how magnetic fields influence its ferroelectric properties.

Contribution

It provides detailed magnetic, electric, and thermal phase diagrams of the new multiferroic compound, highlighting multiple phase transitions and the influence of magnetic fields on ferroelectricity.

Findings

Ferroelectricity appears below ~6.9 K within an antiferromagnetic state.

Magnetic fields can significantly alter the electric polarization.

Multiple multiferroic phases are identified depending on magnetic field direction.

Abstract

(NH$_4$)$_2$[FeCl$_5$(H$_2$O)], a member of the family of antiferromagnetic $A_2$[Fe$X_5$(H$_2$O)] compounds ($X$ = halide ion, $A$ = alkali metal or ammonium ion) is classified as a new multiferroic material. We report the onset of ferroelectricity below ~6.9 K within an antiferromagnetically ordered state ($T_N \sim 7.25 K$). The corresponding electric polarization can drastically be influenced by applying magnetic fields. Based on measurements of pyroelectric currents, dielectric constants and magnetization we characterize the magnetoelectric, dielectric and magnetic properties of (NH$_4$)$_2$[FeCl$_5$(H$_2$O)]. Combining these data with measurements of thermal expansion, magnetostriction and specific heat, we derive detailed magnetic field versus temperature phase diagrams. Depending on the direction of the magnetic field up to three different multiferroic phases are identified, which are separated by a magnetically ordered, but non-ferroelectric phase from the paramagnetic phase. Besides these low-temperature transitions, we observe an additional phase transition at ~79 K, which we suspect to be of structural origin.