In this work, we measured N self-diffusion in the Co-N system and found an unexpected result that N diffuses out almost completely around 500 K, leaving behind fcc Co irrespective of the amount of N used to deposit Co-N. On the other hand, in previous attempts the Co 4 N phase has always been grown at 550 K or above. In view of our finding, it appears that fcc Co could have been mistaken for Co 4 N, probably due to the closeness of their lattice parameters (LP; fcc Co = 3.54 Å, Co 4 N= 3.74 Å). Therefore, Co 4 N -an interesting material for its high spin-polarization ratio and high magnetic moment remained unexplored. By bringing down the growth temperature, we report the growth of stoichiometric Co 4 N epitaxial thin films. Films were grown using a direct current reactive magnetron sputtering process on LaAlO 3 (LAO mismatch 1.4%) and MgO (mismatch 11.3%) substrates and their structural and magnetic properties were studied. Precise magnetic moment (M s ) of Co 4 N samples were measured using polarized neutron reflectivity and compared with bulk magnetization results. We found that the M s of Co 4 N is higher due to a magnetovolume effect. Unlike previous findings, we observed that substrates induce misfit strain and strain inhomogeneity is the cause of modifications in magnetic ensemble such as coercivity, saturation magnetization, and magnetic anisotropy. A consequence of incoherent strain present in our samples is also reflected in the magnetic anisotropy leading to a superposition of strong fourfold and a small fraction of uniaxial magnetic anisotropy. Obtained results are presented and discussed in this work.

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The present work reports the magnetic domain evolution during the magnetization reversal and the training effect in a polycrystalline Co/CoO exchange bias system. Co/CoO bilayers with different cobalt (Co) layer thicknesses are being studied. The measurements are carried out using the Kerr microscopy at different temperatures (≥ 80 K) after the field-cooling across the Néel temperature of the antiferromagnetic (AFM) CoO layer. It is observed that with the increasing ferromagnetic (FM) Co layer thickness, the exchange bias reduces and the training effect becomes weaker. Analysis of the temperature variation of the exchange bias field indicates an increasing disorder at the FM-AFM interface with the decreasing FM thickness. Two different training effects, viz., athermal and thermal mechanisms, are observed depending on the thickness of the FM layer. For the lower FM thickness an athermal training effect is observed, whereas for the thicker FM layer thermal training effect is dominated. The domain structure of FM layers drastically changes during the athermal training effect with the cycling and domain size significantly decreasing during the magnetization reversal process below blocking temperature (TB), whereas in the thermal training mechanism no significant changes are found in the magnetic domain evolution throughout the temperature range with cycling. These results are expected to provide key inputs to various theoretical models that are being used to study the exchange bias phenomena in the recent literature.

A hard/soft SmCo5/Fe nanocomposite magnetic bilayer system is fabricated on x-ray transparent 100–200 nm thin Si3N4 films by magnetron sputtering. The microscopic magnetic domain pattern and its behaviours during magnetization reversal in the hard and the soft magnetic phases are studied separately by element specific magnetic soft x-ray microscopy at a spatial resolution of better than 25 nm. We observe that the domain patterns for the soft and hard phases show coherent behaviours in varying magnetic fields. We derive local M(H) curves from the images of Fe and SmCo5 separately and find the switches for hard and soft phases to be the same.

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