Holmium induced enhanced functionality at room temperature and structural phase transition at high temperature in bismuth ferrite nanoparticles

Bi1-xHoxFeO3 (x = 0, 0.01, 0.03, 0.05, 0.07, 0.1) nanoparticles were synthesized using a soft chemical route combined with post synthesis annealing. The structural and magnetic properties were investigated in detail as a function of doping concentration and temperature. The room-temperature X-ray diffraction patterns indicate that with increasing Ho-concentration the lattice parameters decrease systematically up to x = 0.07 and exhibit a pronounced decrease at x = 0.1 due to their proximity to the rhombohedral-orthorhombic phase boundary. High temperature powder X-ray diffraction data revealed the onset of rhombohedral (R3c) to orthorhombic (Pnma) phase transition at high temperatures in the doped samples. These findings were corroborated using Raman spectroscopy measured up to temperatures as high as 873 K. Magnetic susceptibility data were analyzed using the two sublattice model. At room-temperature, the 'intrinsic' weak ferromagnetism of the Fe moments in BiFeO3 was found to show a significant enhancement upon Ho doping. The exchange bias, which is about 1 kOe in the pristine sample, vanishes completely upon initial Ho-doping but rejuvenates itself for higher Ho-concentration reaching an impressive value of similar to 400 Oe for the 10% Ho doped sample. High-temperature magnetization measurements were done for the pristine, 5 and 10% Ho-doped samples. We found that the antiferromagnetic ordering temperature of the Fe-sublattice (T-N similar to 640 K) remains robust against Ho substitution. Our investigations suggest that moderately Ho-doped BiFeO3 samples can be useful multifunctional materials because of their enhanced ferromagnetism, large exchange bias field and polar symmetry.