Magnetic hysteretic phenomena in multiferroic HoMnO₃ single crystals and polycrystals with nano- and micrometer particle size

We report on the magnetic properties of multiferroic hexagonal HoMnO ₃ single crystals and polycrystalline samples with micrometer and nanometer particle size. We have studied the in-plane and out-of-plane magnetization of HoMnO₃ single crystals under low applied magnetic fields in the temperature range below T_Néel = 72 K and observe the bifurcation of zero-field-cooled and field-cooled curves at the Mn spin reorientation transition temperature at 34 K. In addition, the c-axis magnetization shows a ferrimagnetic-like behavior which may relate to the magnetic Ho³⁺ and Mn³⁺ domain boundary structures and sensitively respond to changes of the magnetic structure such as spin rotations at the phase transitions near 5 and 34 K. We also studied the particles' size dependent magnetic behavior in the HoMnO₃ polycrystalline samples and observe the presence of a net magnetic moment at the surface due to the large surface/volume ratio. Below the Ho³⁺ ordering temperature of 5 K, magnetization curves as a function of applied magnetic field, in contrast to those for the single crystal, show hysteresis behavior with coercivity, which increases with diminishing particle size.