High-temperature magnetoelectricity of terbium aluminum borate: The role of excited states of the rare-earth ion

Recently discovered magnetoelectricity in the rare-earth aluminum borates RAI sub(3)(BO sub(3)) sub(4 ) has attracted attention due to the large values of magnetoinduced electric polarization. We have observed for the first time the magnetoelectric polarization in TbAl sub(3)(BO sub(3)) sub( 4) exhibiting anomalous temperature dependence: an electric polarization induced by in-plane magnetic field (P||a, H[bottom]c axis) which is small at low temperatures (4K), remarkably increases by almost an order of magnitude at high temperatures (150-300 K). The observed nonmonotonic temperature behavior of the field-induced polarization, including a change of sign at ~ 65- 70K, is attributed to the competition of the ground and excited crystal-field states of Tb super(3+) ions. Quantum theory analysis, involving the combination of analytical and numerical methods, has enabled us to quantitatively describe the observed magnetic and magnetoelectric properties of TbAl sub(3) (BO sub(3)) sub(4).