Electric polarization induced by phase separation in magnetically ordered and paramagnetic states of RMn2O5 (R=Gd, Bi)

Abstract

The electric polarization hysteresis loops and remanent polarization were revealed in multiferroics RMn$_2$O$_5$ with $R = Gd$ and $Bi$ at wide temperature interval from 5 K up to 330 K. Until recently, the long-range ferroelectric order having an exchange-striction magnetic nature had been observed in RMn$_2$O$_5$ only at low temperatures ($T\leq T_C = 30$ -- 35 K). We believe that the polarization we observed was caused by the frozen superparaelectric state which was formed by the restricted polar domains resulting from phase separation and charge carriers self-organization. At some sufficiently high temperatures $T\gg T_C$ the frozen superparaelectric state was destroyed, and the conventional superparaelectric state occurred. This happened when the potential barriers of the restricted polar domain reorientations become equal to the kinetic energy of the itinerant electrons (leakage). The hysteresis loops were measured by the so-called PUND method which allowed us to correctly subtract the contribution of conductivity from the measured polarization. The correlations between properties of the phase separation domains and polarization were revealed and studied. The high-temperature polarization also had a magnetic nature and was controlled by the magnetic field because the double exchange between pairs of Mn ions with different valences (Mn$^{3+}$ and Mn$^{4+}$) in RMn$_2$O$_5$ was the basic interaction resulting in phase separation.