Exchange bias, and coercivity investigations in hematite nanoparticles

Hematite nanoparticles of average size of 20 nm were synthesized using sol-gel method and the structural characterisations were conducted using XRD and TEM. The XRD profile revealed the coexistence of small fraction of maghemite phase along with the main hematite phase. Magnetization versus applied field (M-H) measurements were performed between −5 and 5 T and respectively in the temperatures 2, 10, 30, 50, 70,100,150,200, and 300 K under zero field and 1, 2, 3, 4 T field cooling. At all field-cooling values, the coercivity was found to display a weak temperatures dependence below 150 K and a strong increase above 150 K reaching the largest value of 3352 Oe at 300 K for the field-cooling value of 3 T. Horizontal and vertical hysteresis loop shifts were observed at all temperatures in both the zero-field and field-cooled states. In the field-cooled state, both loop shifts where found to have significant and nonmonotonic field-cooling dependences. However, because saturation magnetization was not attained in all measurements our calculations were based on the minor hysteresis loops. M-H measurements were performed between −9 and 9 T at room temperature under zero field cooling and 1, 2, 3, 4, 5, 6 T field cooling. Saturation magnetization was not attained, and the loops displayed loop shifts similar to those for the ±5 T sweeping field. The highest coercivity value of 4400 Oe is observed for the 6 T field cooled MH loop. The ferromagnetic (FM) contribution towards the total magnetization was separated from the total magnetization and hysteresis loops displayed both horizontal and vertical shifts. The novel results of the temperature and field dependence of exchange bias were attributed mainly to the magnetic exchange coupling between the different magnetic phases (mainly the FM) and the spin-glass-like regions.