Exchange Bias Induced by the Spin Glass-Like Surface Spins in Sputter Deposited Fe3O4 Thin Films

The exchange bias in the reactive sputtered polycrystalline Fe
₃
O
₄
thin films of thicknesses 50 and 150 nm is studied. X-ray diffraction, laser Raman, and selected area electron diffraction studies confirm the formation of the Fe
₃
O
₄
single phase. The high-resolution transmission electron microscope images show the presence of well-defined crystallites. The presence of the exchange bias effect is mainly due to the existence of a significant exchange coupling between the core spins and the spin glass-like surface spins of the grains. The temperature dependence of the magnitude of the exchange bias field H
_EB
shows two exponential regimes of which the lower temperature regime corresponds to the spin freezing effect below 50 K. The first magnetization curves measured after zero field cooling show S-shape below the spin freezing temperature. The presence of superparamagnetism and spin freezing has been investigated through the field cooled (FC) and zero FC magnetization measurements. Temperature dependence of coercivity also indicates the spin freezing effect. Hence, the observed large exchange bias of the samples at lower temperatures is due to the freezing of the surface spins.