Disordered Multiferroics

Abstract

Disordered multiferroic materials (type-III multiferroics) escape the conventional schematics oftype-Iandtype-IImultiferroics, where two types of ferroic long-range order are expected to coexist under different interdependences and promise to attain a maximized bilinear (αorEH)magnetoelectriceffect under special symmetry conditions. Nevertheless sizable higher orderMEresponse occurs also in disordered systems such as in the simultaneousdipolarandspin glasses(multiglass) Sr0.98Mn0.02TiO3and K0.94Mn0.03TaO3, thequantum paraelectric antiferromagnetEuTiO3, thespin glassandrelaxor ferroelectricPbFe0.5Nb0.5O3, and theantiferroelectric antiferromagnetic dipole glassCuCr1-xInxP2S6. They have in common to show large quadratic magneto-capacitance effects, ΔεH2, which are related to dominating third-orderE2H2terms in their free energies and do not require special symmetry conditions. The polarization controlled exchange coupling can achieve giant fluctuation-enhanced values in the vicinity of critical magnetic fields as observed,e.g., in EuTiO3. Exceptionally, even the first-orderEH-typemagnetoelectriceffect is observed whenever metastable homogeneous order parameters are induced by field cooling as in EuTiO3, or in the spin glass phase of the relaxor multiferroic Pb (Fe0.5Nb0.5)O3atT < Tg= 10.6 K.