Surface magnetism, Morin transition, and magnetic dynamics in antiferromagnetic α-Fe2O3 (hematite) nanograins

The grain size of alpha-Fe2O3 decreases to similar to 20 nm by 64 h mechanical milling of the bulk sample. X-ray diffraction pattern suggested identical crystal structure in bulk and mechanical milled samples. Magnetic study (at temperatures of 100-900 K and fields of 0- (+) 15 kOe) showed many interesting features during the decrease in grain size in antiferromagnetic alpha-Fe2O3, e. g., suppression of Morin transition, enhancement in low temperature magnetization, magnetic blocking at high temperature, exchange bias effect, and unusual relaxation of magnetic spin moment. We understand the results in terms of core-shell spin structure of nanograins, where the core part essentially retained the magnetic structure of the bulk sample and the magnetic structure of the shell part is modified due to grain size reduction and surface modification during mechanical milling. Core-shell structure also plays an important role in exhibiting the increasing soft ferromagnetic character in the present hematite samples. The in field magnetic relaxation at room temperature revealed some interesting properties of the magnetic spin ordering in hematite system. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3327433]