Ferroelectricity of holmium manganese oxide HoMn2O5 with helical spin ordering

We report the results of dielectric, pyroelectric, magnetoelectric and specific heat measurements of HoMn2O5 single crystals, concluding that this oxide is ferroelectric below 38 K. The direction of the spontaneous polarization is along the b-direction of the orthorhombic structure. The presence of the ferroelectric transition is demonstrated by a peak in the temperature dependence of dielectric constant and also a shoulder in specific heat. The transition is accompanied by an appearance of spontaneous polarization and the quadratic (in magnetic field) magnetoelectric effect, which can be switched by electric field applied during cooling the crystal through the transition temperature. The crystal undergoes another phase transition around 22 K. HoMn2O5, like other RMn2O5 oxides, has several features typical to improper ferroelectrics, such as small spontaneous polarization, a low peak height of dielectric constant at the transition temperature, etc. We suppose that the ferroelectricity of RMn2O5 is related with the presence of long-range magnetic ordering in its origin.