Magnetic-field-induced ferroelectric states in centrosymmetric R2BaCuO5 ( R=Dy and Ho)

The linear magnetoelectric effect and multiferroicity phenomena occur independently due to breaking inversion symmetry below the magnetically ordered state of either transition metal or rare-earth ions. Here, we report the occurrence of a linear magnetoelectric effect and magnetic field-induced ferromagnetism and ferroelectricity below ${T}_{\mathrm{N}}^{R}$ in the orthorhombic green phases ${R}_{2}\mathrm{BaCu}{\mathrm{O}}_{5}$ ($R=\mathrm{Dy}$ and Ho). They undergo a long-range antiferromagnetic ordering of ${\mathrm{Cu}}^{2+}$ (${T}_{\mathrm{N}}^{\mathrm{Cu}}=18.5$ and 17.5 K) and ${R}^{3+}$ ions (${T}_{\mathrm{N}}^{\mathrm{Dy}}=10.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}}^{\mathrm{Ho}}=8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). The neutron diffraction study reveals that these compounds undergo a first-order magnetic transition from the high-temperature centrosymmetric antiferromagnetic phase (${P}_{\mathrm{b}}{112}_{1}/n$) to the low-temperature noncentrosymmetric phases $Pn{m}^{\ensuremath{'}}a$ (Dy) and $P{112}_{1}^{\ensuremath{'}}/a$ (Ho), which allow linear magnetoelectric coupling. This is consistent with field-induced electric polarization, below ${T}_{\mathrm{N}}^{R}$, which varies linearly up to \ensuremath{\sim}1.2 T. Above a critical field $({H}_{\mathrm{c}}>1.2\phantom{\rule{0.16em}{0ex}}\mathrm{T})$, both compounds exhibit metamagnetic transitions with magnetization close to the saturation value ${M}_{s} \ensuremath{\sim}10.1\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ (Dy) and $\ensuremath{\sim}11.8\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ (Ho) at 7 T. Above the metamagnetic transition, a new polar state appears with large electric polarization, indicating field-induced ferroelectricity. We discuss the important role of $4f\text{\ensuremath{-}}3d$ coupling in determining the ground state magnetic structure responsible for the magnetoelectric coupling in both compounds.