Doping effects on the ferroelectric transition of multiferroic Y(Mn,Al/Ga)O3

Multiferroic hexagonal manganites ${\mathrm{RMnO}}₃$ have a very high ferroelectric transition temperature around and above 1200 K, depending on the rare-earth elements, and a reasonably large electric polarization of about $5.5\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}²$ at room temperature. It is generally believed that the ferroelectric transition is driven by the combination of $\text{R-}{\mathrm{O}}_{\mathrm{P}}$ displacement and ${\mathrm{MnO}}₅$ tilting, and hence called improper ferroelectric. In order to better understand the improper ferroelectric transition, we studied doping effects, using two elements with the same valence but a different ionic size: Al and Ga on the Mn site of ${\mathrm{YMnO}}₃$. Through detailed structural studies and nanoscale measurements of piezoresponse force microscopy (PFM) we conclude that there is a drastic doping effect for Al, whose ionic size is much smaller than Mn. It is in stark contrast with our observation that Ga, having a slightly smaller ionic size with Mn, does not change the ferroelectric transition up to 50% doping. This drastic difference in the doping effect is due to local strain induced by the difference in the ionic size of Al and Mn as compared with that of Mn, and sheds light on the intriguing nature of the improper ferroelectric transition.