Excess-electron induced polarization and magnetoelectric effect in yttrium iron garnet

Magnetoelectric (ME) properties in yttrium iron garnet (YIG:Y₃Fe₅O₁₂), including both the first-order and second-order effects, have long been under dispute. In particular, the conflict between observations of the first-order ME effect and the centrosymmetric lattice structure has remained as a puzzling issue. As a key to solve the problem, we found that YIG shows quantum ME relaxation; the dielectric relaxation process is correlated closely with the magnetic one and has characteristic features of quantum tunneling. An application of magnetic field enhances the dielectric relaxation strength (by 300% at 10 K with 0.5 T), which gives rise to the large second-order ME (magnetocapacitance) effect critically dependent on the magnetization direction. The temperature and magnetic-field dependence of dielectric relaxation strength is well described by the noninteracting transverse-field Ising model for the excess-electron or Fe²⁺ center with the quantum tunneling and spin-orbit coupling effects. We could also spectroscopically identify such a ME Fe²⁺ center in terms of linear dichroism under a magnetic field along the specific direction. On this basis, the fictitious first-order ME effect---the magnetic-field induced electric polarization without the presence of external electric field---as observed for the electric-field cooled sample is ascribed to the combined effect of the above large second-order ME effect and the poling induced charge accumulation. The correlation between the ME effect and the thermally stimulated depolarization current indicates that hopping electrons freeze below around 125 K and the frozen-in dipoles generate an internal electric field (i.e., an electret-like effect). Investigation of electron-compensating doping effect on dielectric relaxation phenomena gives compelling evidences that excess electrons forming Fe²⁺ ions play a critical role in the charge accumulation as well as in the ME effect in YIG.