Magnetic ground states and magnetodielectric effect in $R$Cr(BO$_3$)$_2$ ($R$ = Y and Ho)

TL;DR

This study investigates the magnetic and magnetodielectric properties of layered perovskites RCr(BO3)2 (R=Y, Ho), revealing complex magnetic ordering, field-induced phase transitions, and strong magnetodielectric effects, especially in the Ho-compound due to magnetostriction.

Contribution

It provides detailed experimental insights into the magnetic ground states and magnetodielectric effects in RCr(BO3)2, including the first estimation of exchange interactions via spin wave theory.

Findings

Cr3+ spins order in a canted antiferromagnetic structure around 8-9 K.

Applying a magnetic field induces ferromagnetic order in both compounds.

The Ho-compound exhibits a stronger magnetodielectric response due to magnetostriction.

Abstract

The layered perovskites $R$Cr(BO$_3$)$_2$ ($R$ = Y and Ho) with magnetic triangular lattices were studied by performing ac/dc susceptibility, specific heat, elastic and inelastic neutron scattering, and dielectric constant measurements. The results show (i) both samples' Cr$^{3+}$ spins order in a canted antiferromagnetic structure with $T_N$ around 8-9 K, while the Ho$^{3+}$ ions do not order down to $T$ = 1.5 K in HoCr(BO$_3$)$_2$; (ii) when a critical magnetic field H$_{C}$ around 2-3 T is applied below $T_{N}$, the Cr$^{3+}$ spins in the Y-compound and both the Cr$^{3+}$ and Ho$^{3+}$ spins in the Ho-compound order in a ferromagnetic state; (iii) both samples exhibit dielectric constant anomalies around the transition temperature and critical field, but the Ho-compound displays a much stronger magnetodielectric response. We speculate that this is due to the magnetostriction which depends on both of the Cr$^{3+}$ and the Ho$^{3+}$ ions' ordering in the Ho-compound. Moreover, by using linear spin wave theory to simulate the inelastic neutron scattering data, we estimated the Y-compound's intralayer and interlayer exchange strengths as ferromagnetic J$_{1}$ = -0.12 meV and antiferromagnetic J$_{2}$ = 0.014 meV, respectively. The competition between different kinds of superexchange interactions results in the ferromagnetic intralayer interaction.