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

摘要

We report on the magnetic properties of multiferroic hexagonal HoMnO3 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-Neel = 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.