The Exchange Bias Effect In Disordered Systems

The advent of information age makes greater demand on the information processing andstoring ability. Magnetic recording is one of the most popular data storage technics due to itslow cost, large capacity, low energy consumption, stable performance and rewritablity. Asone of the foundations of magnetic recording, the exchange bias effect has attracted muchattention mostly focusing on its origin and manipulation. The exchange bias (EB) effect ischaracterized by a shift of the hysteresis loop along the magnetic field axis in interfacecoupling systems, such as a ferromagnetic (FM) film coupling to an antiferromagnetic (AFM)material. In this dissertation we investigate the effect of disorder in the pinning layer, pinnedlayer and at the interface on exchange bias effect and coercivity in different EB systems.Firstly the effect of disorder in pinning layer is studied by introducing ferromagneticbonds into the antiferromagnet. With increasing bond dilution, the spin frustration in thepinning layers increases, a transition from an antiferromagnetic domain state (DS) to a spinglass (SG) is experienced, and the EB field increases. It is found that the EB field growsexponentially, with the Hamming distance being a measure of the degree of quenchedinterfacial spin disorder, which is the key ingredient of the EB effect. Meanwhile, it is foundthat, comparing to the FM/DS system, the larger coercivity in FM/SG system results from alarger amount of reversible spins in the SG layer. As for the FM/DS system, large AFMdomains have been observed with uncompensated magnetization which remains stable duringfield looping resuling in the exchange bias effect. However, due to the spin frustrations, thespins in small AFM domains and at the domain boundary are reversible, which enchences thecoercivity of the system. In both FM/DS and FM/SG systems, the dependence of coercivityon the reversible interface magnetization of the pinning layer follows two separate straightlines below and above their blocking temperatures tB. It may suggest an abrupt change in thestrength of interface coupling at the blocking temperature.Secondly, the effect of magnetic orders in pinned layer is investigated. Remarkableexchange bias features including the hysteresis-loop shift, the coercivity enhancement, andthe training effect are confirmed unambiguously in a diluted-antiferromagnetic (DAF) layerexchange coupling to an antiferromagnetic (AFM) layer system. In contrast to the conventional EB systems, the EB effect of the DAF/AFM bilayer system does not require anyuncompensated spins in the AFM pinning layer. Through analyzing the DAF-layer thicknessdependence of the EB parameters and the spin reversal patterns, we reveal that the EBoriginates from the AFM domains of the DAF layer that pinned by the compensated AFMinterface. However, no clear loop shift can be observed in a SG/AFM system no mater howthe cooling fields, measuring temperatures, exchange coupling strengths in SG and at theinterface change.Thirdly, the effect of disorder in the interface exchange coupling on EB is studied byintroducing bond dilution into the interface interactions. The interface bond dilution leads tofrustrations in the DS interface layer, which enchance the EB field and its blockingtemperature simultaneously. The EB enhancement is more significant in systems with thinnerDS layer, indicating that the domain structure of the pinning layers can mediate EB byinfluencing the interface spin disorder. It is suggested that the EB effect is determined notonly by the strength of interface interactions but also the distribution of interface coupling.Finally, the investigations of effects of disorder in core/shell structure nanoparticle onEB are carried out by introducing non-magnetic defects. It is found that the EB can be tunedby defects of different positions. Defects at both AFM and FM interface reduce the EB fieldwhile enhance the coercive field by decreasing the effective interface coupling. However theEB field and the coercive field show respectively a non-monotonic and a monotonicdependence on the defect concentration when the defects locate inside the AFM shell,indicating a similar microscopic mechanism proposed in the domain state model. Theseresults suggest a way to optimize the EB effect for applications.