Exchange bias and anomalous vertical shift of the hysteresis loops in milled Fe/MnO2 material

The present article reports studies on structural and magnetic properties of nanostructured Fe/MnO2 materials prepared by mechanosynthesis method, with Fe to MnO2 ratios of 20/80, 50/50 and 60/40. X-ray diffraction patterns indicate that the milled materials have crystalline grain size in the nanoscale region. Mossbauer spectra of the milled materials suggest the presence of two Fe phases for each sample: a nanocrystalline alpha-Fe phase with a high degree of disorder/defects and small Fe-oxide particles. The magnetic hysteresis (M(H)) loops, measured at 4.2K, after the samples were cooled from 300K in plus/minus l O kOe fields, show unexpected large shifts in both horizontal and vertical directions for the 20/80 sample, while only horizontal shift was detected in the samples with higher Fe concentration. The anomalous vertical shift of the M(H) loop for the 20/80 sample, observed at low cooling field (10 kOe), is being associated with a large contribution from non-collinear magnetic structure of the particles surface. This surface magnetic contribution is strongly influenced by the field cooling magnitude. A simple model is proposed to interpret this result.