Giant exchange bias and the vertical shifts of hysteresis loops in γ-Fe2O3-coated Fe nanoparticles

We fabricated core/shell-structured Fe nanoparticles, in which the alpha-Fe core is about 5 nm in diameter and the gamma-Fe2O3 shell is about 3 nm thick, and systematically studied their structural and magnetic properties. The magnetic hysteresis (M-H) loops, measured at low temperatures, after the particles were cooled from 350 K in a 50 kOe field, show significant shifts in both horizontal and vertical directions. It has been found that the exchange-bias field can be as large as 6.3 kOe at 2 K, and that the coercive field is also enhanced greatly in the field-cooled (FC) loops. The large exchange bias and vertical shifts of the FC loops at low temperatures may be ascribed to the frozen spins in the shells. A simple model is proposed to interpret the observations. (C) 2004 American Institute of Physics.