Exchange bias in magnetic thin films and nanoparticles : study of spin glass, spin dynamics and anisotropy

The spin valve structure has important applications in the hard disk drives, magnetic sensors, magnetoresistive random-access memory, etc. and consequently is actively researched. The materials having perpendicular magnetic anisotropy are in high demand for perpendicular magnetic recording and heat-assisted magnetic recording. Other than the magnetic thin films, the magnetic nanoparticles have strong contribution in the emerging technologies, such as, drug delivery, tagging or labeling of the desired biological entity and magnetic resonance imaging. The goal of this thesis is to explore different magnetic behaviours in the magnetic thin films and nanoparticles, i.e., exchange bias (EB), perpendicular magnetic anisotropy (PMA), spin glass (SG) phases, spin dynamics, magnetic interactions in the magnetic thin films and nanoparticles. Moreover, this thesis includes some new magnetic behaviours which have not been yet reported after the acreful literature survey. The PMA is studied in the thin films of CoPd alloy. Perpendicular EB is developed by using IrMn as an antiferromagnet. A transition in the EB is observed at low temperatures, where the in-plane (IP) EB field becomes larger than the out-of-plane (OP) EB field. It is found that with decreasing temperature there is a spin structure transition in the IrMn (111) layer related to a 3Q (favors OP spins component) to 2Q transition (favors IP spins component) in the bulk. This transition is responsible for the increase in the IP EB at low temperatures and has not been reported before. A spin reorientation transition from OP to IP (at higher temperatures) is also observed in the CoPd alloy, but is too weak to perturb the spin structure transition of IrMn, which dominates the temperature dependence of EB. The most interesting result is an increase in PMA of CoPd/IrMn with decreasing the thickness of CoPd layer, which is contrary to bare CoPd alloy. The bulk 3Q spin structure of IrMn and the angle between the CoPd and IrMn moments at the interface are the reasons of this discrepancy. Memory effect, aging, Morin transition and EB effect are studied in the agglomerated hematite nanoparticles of different shapes from spherical to spindle. The super spin glass (SSG) and surface SG phases are found in these nanoparticles generating EB field. The interesting behaviour is the sign change in the EB field along with Morin transition, where the rotation of Fe ions in the core changes the exchange coupling at interface resulting in positive EB field. Morin transition is diminished with increasing the aspect ratio of nanoparticles due to the enhanced lattice strain and surface defects. In order to eliminate agglomeration, silica coating is used around the nanoparticles. The silica coating enhances the surface disorder, decreases magnetization and depletes Morin transition. The sign of the EB field is changed to negative as compared to uncoated ones. Co1-xMgxFe2O4 nanoparticles are studied in quasi-free state and semi-compressed form. Spontaneous exchange bias (SEB) is observed along with conventional exchange bias (CEB) in the both states of nanoparticles. In the quasi-free state of nanoparticles, SEB is larger than CEB, while SEB becomes weak in semi-compressed form. The initial magnetic state has strong influence on SEB, where strong randomness contributes to the volume fraction of SG phase. The frustration and smooth movement of nanoparticles enhance SEB in quasi-free state of nanoparticles. The metastable domain states are developed in the system due to strain and dilution which perturbed by the rotation of the ferrimagnetic core moments with increasing temperature.The temperature driven sign reversal in Fe/Cr bilayers is studied and explained by the formation of interface alloying. The FeCr alloy at the interface transforms into SG phase during cooling. At low temperatures, the ferromagnetic interactions between frozen Fe clusters in alloy and adjacent Fe layer, results into enhanced negative EB field. The Cr rich alloy favours re-entrant SG to antiferromagnetic phase transition in alloy with increase in temperature. This phase transition changes the ferromagnetic coupling to antiferromagnetic between uncompensated Cr moments and Fe layer at interface resulting into positive EB.The novel results in the EB in different magnetic systems certainly encourage further probe into it by changing the thickness of IrMn layer for CoPd/IrMn system and the interparticle interactions