Understanding the multiferroicity in TmMn2O5 by a magnetically induced ferrielectric model

The magnetically induced electric polarization behaviors in multiferroic TmMn2O5 in response to varying temperature and magnetic field are carefully investigated by means of a series of characterizations including the high precision pyroelectric current technique. Here polycrystalline rather than single crystal samples are used for avoiding the strong electrically self-polarized effect in single crystals, and various parallel experiments on excluding the thermally excited current contributions are performed. The temperature-dependent electric polarization flop as a major character is identified for different measuring paths. The magneto-current measurements indicate that the electric polarization in the low temperature magnetic phase region has different origin from that in the high temperature magnetic phase. It is suggested that the electric polarization does have multiple components which align along different orientations, including the Mn3+-Mn4+-Mn3+ exchange striction induced polarization P-MM1 the Tm3+-Mn4+-Tm3+ exchange striction induced polarization P-TM1,P- and the low temperature polarization P-LT probably associated with the Tm3+ commensurate phase. The observed electric polarization flop can be reasonably explained by the ferrielectric model proposed earlier for DyMn2O5, where P-MM and P-TM are the two antiparallel components both along the b-axis and P-LT may align along the alpha-axis. Finally, several issues on the unusual temperature dependence of ferroelectric polarizations are discussed.