Colossal magneto-nonlinear-dielectric effect in magnetoelectric antiferromagnet tetracobalt diniobate

Abstract

The magnetic field effect on nonlinear dielectric susceptibilities of magnetoelectric antiferromagnet tetracobalt diniobate (Co4Nb2O9) around Néel temperature (TN) is studied. Under a magnetic field of H = 5 T, the real part of second order nonlinear dielectric susceptibility χ'2 shows a valley at TN, whereas those of linear and third order nonlinear dielectric susceptibilities (χ'1 and χ'3) show peaks. The magnetic field has much more colossal effects on χ'2 and χ'3 than on χ'1. The fact that χ'3 keeps positive around TN under H = 5 T suggest discontinuousness of the magnetic field induced ferroelectric transition, but absence of thermal hysteresis on heat capacity indicates continuousness of transition. The reason why nonlinear dielectric spectra fail to identify the type of ferroelectric transition can be explained by the deviation of critical exponent β from the value for the Landau universality class.