Training effect of the exchange bias in sputter deposited Fe3O4 thin films with varying thickness

Highlights: • Training effects are studied in Fe₃O₄ films having spin glass-like spins present. • Frozen and rotatable spin relaxation model gives the best description of training. • Reversible interface spins relax 7 times faster than the frozen spins at 2 K. • Relative relaxation rates of frozen and rotatable spins are thickness independent. • Above spin freezing temperature the exchange bias field vanishes after few cycles. The training effect property of the exchange bias in the reactively sputtered polycrystalline Fe₃O₄ thin films of varying thicknesses in the range 25–200 nm are studied. Structural studies by X-ray diffraction, X-ray photoelectron spectroscopy and selected area electron diffraction confirm the formation of single phase Fe₃O₄. The scanning electron spectroscopy images show that the grains are uniformly distributed. All the samples show clear and consistent exchange bias training behaviour due to the dynamics of the spins at the interface of the ferrimagnetic core and the spin glass-like surface of the grains. The analysis of the training effect data of the exchange bias field H_E measured at 2 K by using three different models show that the model based on the relaxation of the frozen and rotatable spin components at the interface gives the best description for all the samples. From this model, it is found that the reversible interface spins relax around 7 times faster than the frozen interface spins at 2 K for all the samples and that their relative relaxation rates are independent of the sample thickness. This constancy show that the relative relaxation rates of the interfacial frozen and rotatable spin components is a material dependent property. The frozen component of the interfacial spins of each sample is found to be dominated at the initial stage of the training. A direct equivalence between the H_E and remanence asymmetry M_E is observed. Above the spin freezing temperature, the training effect measurements at 75 K show that the H_E decreases sharply with successive field cycling as compared to the measurements made at 2 K and the H_E vanishes after first few cycles.