Single-axis dependent structural and multiferroic properties of orthorhombic RMnO$_3$ (R = Gd - Lu)

TL;DR

This study investigates how modifying a single lattice parameter in orthorhombic RMnO$_3$ films affects their multiferroic properties, revealing that ferroelectric polarization remains robust against changes in the $b$-axis.

Contribution

It demonstrates a method to selectively control a single lattice parameter using epitaxy and chemical pressure, providing insights into the structure-property relationship in multiferroic materials.

Findings

Ferroelectric polarization is mostly unaffected by $b$-axis variations.

The $b$-axis change shifts oxygen positions in the $ca$-plane.

Multiferroic order is robust when the $ca$-plane dimensions are fixed.

Abstract

Controlling material properties by modulating the crystalline structure has been attempted using various techniques, e.g., hydrostatic pressure, chemical pressure, and epitaxy. These techniques succeed to improve properties and achieve desired functionalities by changing the unit cell in all dimensions. In order to obtain a more detailed understanding on the relation between the crystal lattice and material properties, it is desirable to investigate the influence of a smaller number of parameters. Here, we utilize the combination of chemical pressure and epitaxy to modify a single lattice parameter of the multiferroic orthorhombic RMnO$_3$ (R = rare-earth, o-RMnO$_3$) system. By growing a series of o-RMnO$_3$ (R = Gd - Lu) films coherently on (010)-oriented YAlO$_3$ substrates, the influence of chemical pressure is reflected only along the $b$-axis. Thus, a series of o-RMnO$_3$ with $a$ ~ 5.18 {\AA}, 5.77 {\AA} < $b$ < 5.98 {\AA}, and $c$ ~ 7.37 {\AA} were obtained. Raman spectra analysis reveals that the change of the $b$-axis parameter induces a shift of the oxygen in the nominally "fixed" $ca$-plane. Their ferroelectric ground state is independent on the $b$-axis parameter showing polarization of ~ 1 $\mu$C cm$^{-2}$ along the $a$-axis for the above-mentioned range, except for $b$ ~ 5.94 {\AA} which corresponds to TbMnO$_3$ showing ~ 2 $\mu$C cm$^{-2}$. This result implies that multiferroic order of o-RMnO$_3$ is almost robust against the $b$-axis parameter provided that the dimension of the $ca$-plane is fixed to 7.37 {\AA} $\times$ 5.18 {\AA}.