Magnetoelectric coupling tuned by competing anisotropies in Mn$_{1-x}$Ni$_{x}$TiO$_3$

TL;DR

This study investigates how competing anisotropies in Mn$_{1-x}$Ni$_x$TiO$_3$ influence magnetoelectric coupling, revealing a spin reorientation transition and changes in electric polarization under magnetic fields, with effects diminishing at higher Ni concentrations.

Contribution

It demonstrates how anisotropy competition induces spin reorientation and modifies magnetoelectric responses in Mn$_{1-x}$Ni$_x$TiO$_3$, expanding understanding of tunable magnetoelectric coupling.

Findings

Electric polarization flips from c- to a-axis with magnetic field.

Critical magnetic field for polarization switch decreases with Ni doping.

Magnetoelectric effect vanishes at higher Ni concentrations due to new magnetic phases.

Abstract

A flop of electric polarization from $P$$\parallel$$c$ ($P_c$) to $P$$\parallel$$a$ ($P_a$) is observed in MnTiO$_3$ as a spin flop transition is triggered by a $c$-axis magnetic field, $H_{\|c}$=7 T. The critical magnetic field $H_{\|c}$ for $P_a$ is significantly reduced in Mn$_{1-x}$Ni$_x$TiO$_3$ (x=0.33). $P_a$ and $P_c$ have been observed with both $H_{\|c}$ and $H_{\|a}$. Neutron diffraction measurements revealed similar magnetic arrangements for the two compositions where the ordered spins couple antiferromagnetically with their nearest intra- and inter-planar neighbors. In the x=0.33 system, the uniaxial and planar anisotropies of Mn$^{2+}$ and Ni$^{2+}$ compete and give rise to a spin reorientation transition at $T_R$. A magnetic field, $H_{\|c}$, aligns the spins along $c$ for $T_R$$<$$T$$<$$T_N$. The rotation of the collinear spins away from the $c$-axis for $T$$<$$T_R$ alters the magnetic point symmetry and gives rise to a new ME susceptibility tensor form. Such linear ME response provides satisfactory explanation for the behavior of the field-induced electric polarization in both compositions. As the Ni content increases to x=0.5 and 0.68, the ME effect disappears as a new magnetic phase emerges.