Quantum magnetoelectric effect in iron garnet

The magnetoelectric response and its quantum relaxation phenomenon have been investigated for a single crystal of yttrium iron garnet. The electric-dipole moments, built in by excess localized electrons forming Fe²⁺ sites, never freeze even at the lowest temperature and relax through a quantum tunneling process. Application of magnetic field enhances the dielectric relaxation strength and gives rise to a large magnetocapacitance effect (~13% at 10 K with 0.5 T). We show that this magnetically tunable quantum paraelectricity is associated with the Fe²⁺-based magnetoelectric centers in which the electric polarization depends on the magnetization vector via the spin-orbit coupling.