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, I present the occurrence of a linear magnetoelectric effect and magnetic field-induced ferromagnetism and ferroelectricity below the 4f-3d coupled magnetic state in the orthorhombic green phases R₂BaCuO₅ (R = Dy and Ho). They undergo a long-range antiferromagnetic ordering of Cu²⁺ (  = 18.5 K and  = 17.5 K) and R³⁺ ions (  = 10.7 K and  = 8 K) for Dy and Ho compounds, respectively. Neutron diffraction study reveals that these compounds undergo a first-order magnetic transition from the high-temperature centrosymmetric antiferromagnetic phase ( ) to the low-temperature noncentrosymmetric phases,  (Dy) and  (Ho), which allow linear magnetoelectric coupling. This is consistent with field-induced electric polarization, below, which varies linearly up to ~1.2 T. Above a critical field (H_c > 1.2 T), both compounds exhibit metamagnetic transitions with magnetization close to the saturation value, M_s ~ 10.1 µ_B/f.u. (Dy) and ~ 11.8 µ_B/f.u. (Ho) at 7 T. Above the metamagnetic transition, a new polar state appears with large electric polarization indicating field-induced ferroelectricity. I discuss the important role of 4f-3d coupling in determining the ground state magnetic structure responsible for the magnetoelectric coupling in both compounds.