Strain-induced effects on antiferromagnetic ordering and magnetocapacitance in orthorhombic HoMnO₃ thin films

We investigated the magnetic and ferroelectric properties of c-axis oriented orthorhombic phase HoMnO ₃ (o-HMO in Pbnm symmetry setting) thin films grown on Nb-doped SrTiO ₃ (001) substrates. The o-HMO films exhibit an antiferromagnetic ordering near 42K, irrespective of the orientation of the applied field. However, an additional magnetic ordering occurring around 35K was observed when the field was applied along the c-axis of o-HMO, which was absent when the field was applied in the ab-plane. The magnetocapacitance measured along the c-axis showed that although there is evidence of dielectric constant enhancement when the temperature is below 35K the expected abrupt change in dielectric constant appears at a much lower temperature and reaches maximum around 13.5K, indicating that the low-temperature c-axis polarization might be related to the ordering of the Ho ³⁺ moment. The lattice constant analyses using x-ray diffraction and the observation of a slight magnetization hysteresis suggest that the weak second magnetic transition along the c-axis at 35K might be more relevant to the strain-induced effect on antiferromagnetism.