Evidence for the magnetoelectric effect in nickel-bromine boracite, Ni₃B₇O₁₃ Br

Abstract

A ferromagnetic/ferroelectric/ferroelastic phase, consistent with point group m'm2’ and T_{c(f. m.)}= 17K, has been identified by measurements of the magnetoelectric coefficient α₃₂, peak value at T_c(7 ps/m), and the ferromagnetic coercive field, H_c, versus temperature. A magnetoelectric effect persists above 17K up to ∼ 30K indicating a phase of still unknown nature. The dielectric constant ε₃₃ is insensitive to magnetic ordering, but a striking anomaly of spontaneous polarization, ^→P_s, occurs in the magnetic ordering range and is attributed to a spontaneous magnetoelectric interaction. Furthermore, ^→P_s has its maximum value (290 nC/cm²) at T_{C(f. e)}