The complex multiferroic phase diagram of Mn$_{1-x}$Co$_x$WO$_4$

TL;DR

This study maps the complex magnetic and multiferroic phase diagram of Mn$_{1-x}$Co$_x$WO$_4$, revealing multiple polarization flops driven by Co-induced anisotropy and discovering an intermediate collinear phase.

Contribution

It provides the first detailed phase diagram of Mn$_{1-x}$Co$_x$WO$_4$ and explains polarization flops through Co anisotropy effects on spin helix orientation.

Findings

Polarization direction changes twice with Co content.

An intermediate collinear phase is identified.

The origin of polarization flops is linked to Co anisotropy effects.

Abstract

The complete magnetic and multiferroic phase diagram of Mn$_{1-x}$Co$_{x}$WO$_4$ single crystals is investigated by means of magnetic, heat capacity, and polarization experiments. We show that the ferroelectric polarization $\overrightarrow{P}$ in the multiferroic state abruptly changes its direction twice upon increasing Co content, x. At x$_{c1}$=0.075, $\overrightarrow{P}$ rotates from the $b-$axis into the $a-c$ plane and at x$_{c2}$=0.15 it flips back to the $b-$axis. The origin of the multiple polarization flops is identified as an effect of the Co anisotropy on the orientation and shape of the spin helix leading to thermodynamic instabilities caused by the decrease of the magnitude of the polarization in the corresponding phases. A qualitative description of the ferroelectric polarization is derived by taking into account the intrachain ($c-$axis) as well as the interchain ($a-$axis) exchange pathways connecting the magnetic ions. In a narrow Co concentration range (0.1$\leq$x$\leq$0.15), an intermediate phase, sandwiched between the collinear high-temperature and the helical low-temperature phases, is discovered. The new phase exhibits a collinear and commensurate spin modulation similar to the low-temperature magnetic structure of MnWO$_4$.