Crystallite size dependent exchange bias in MgFe2O4 thin films on Si(100)

Single phase materials showing large Exchange Bias (EB) at elevated temperatures are highly desirable for their technological advantage. However, the fundamental interactions leading to the effect are often very complex and ill-understood. Here, we report the observation of EB in single phase (111) oriented MgFe2O4 (MFO) thin films on MgO buffered Si(100). The effect is seen to persist till a significantly higher temperature of 110 K. Measurements on stoichiometric powder MFO showed the absence of EB, lower coercivity, and an enhanced saturation magnetization in comparison to thin films. This suggests the modified exchange interaction in the thin films possibly due to the presence of growth induced magnetic and structural disorder. Investigation on the nature of this EB produced some results which are akin to a conventional FM/AFM coupled system like the training effect. However, the same could not explain the observed variation in coercivity and the exchange bias field with temperature and cooling fields. We propose the existence of a disordered layer with random spin orientations at the crystalline grain boundaries of our film, promoting the AFM interaction between the ordered ferrimagnetic grains once field cooled below the spin freezing temperature. This generates a unidirectional anisotropy along the field direction which manifests as a loop shift in hysteresis measurements. A further study shows that the EB field can be varied systematically by changing the thickness of the thin films. This indicates a simple way to tune the disorder for achieving desirable magnetic properties in single phase magnetic thin films. Published by AIP Publishing.