Large magnetodielectric effects in orthorhombic HoMnO₃ and YMnO₃

We have found a remarkable increase (up to 60%) of the dielectric constant with the onset of magnetic order at 42 K in the metastable orthorhombic structures of YMnO₃ and HoMnO₃ that proves the existence of a strong magnetodielectric coupling in the compounds. Magnetic, dielectric, and thermodynamic properties show distinct anomalies at the onset of the incommensurate magnetic order and thermal hysteresis effects are observed around the lock-in transition temperature at which the incommensurate magnetic order locks into a temperature independent wave vector. The Mn³⁺ spins and Ho³⁺ moments both contribute to the magnetodielectric coupling. A large magnetodielectric effect was observed in HoMnO₃ at low temperature where the dielectric constant can be tuned by an external magnetic field resulting in a decrease of up to 8% at 7 T. By comparing data for YMnO₃ and HoMnO₃ the contributions to the coupling between the dielectric response and Mn and Ho magnetic moments are separated.