Effect of Ca doping on enhancement of ferroelectricity and magnetism in HoMnO₃ multiferroic system

This paper reports an alternative approach to achieve orthorhombic HoMnO₃ (HMO) structure by Ca doping at Holmium site. It is observed that pure HMO undergoes hexagonal (P6 ₃ cm) to orthorhombic (Pbnm) structural transformation when Ca is substituted at Ho site through high temperature sintering at 1,450 °C for 10 h. With the substitution of Ca in HMO (Ho_1−xCa_xMnO₃, x = 0.05), hexagonal unit cell volume has been contracted significantly. However with further increase of Ca dopant concentration (x = 0.1, 0.2), unit cell volume for each composition are marginally enhanced. Spontaneous polarization (P_s), remanent polarization (P_r) values are observed to be enhanced up to the limiting doping value for x = 0.1. Further substitution of Ca in HMO (x > 0.2) reduces the P_s and P_r value marginally. Similarly dielectric constant (ε_r) and dielectric loss values are increased up to x = 0.1 and then decreased up to x = 0.2. These observations are expected to be linked with the structural and microstructural changes with the variation of x. At x = 0.1, larger grains are developed, with enhanced value of polarization and dielectric constant. In addition to above mentioned enhanced ferroelectric polarization and dielectric behaviour, we have observed peculiar magnetic property from M to H curve. In all Ho_1−xCa_xMnO₃ (x = 0–0.2) samples, a switching behaviour in low field was observed in field dependence of magnetization at room temperature and the behaviour found to be enhanced with Ca concentration up to 0.1 and then decreases.