Research On Exchange Bias And Spontaneous Exchange Bias Mechanism

Conventional exchange bias(CEB)manifests itself as a shift in the hysteresis loops of a magnetic system consisting of different magnetic phases when the system is cooled with an applied field.Due to the pinning effect of exchange bias on the ferromagnetic moment,this phenomenon has become a cornerstone of the design and operation of spintronic devices such as tunnel junctions and spin valves.Later,Wang et al.obtained an exchange bias in Ni Mn In alloys at low temperatures under the zero-field cooled condition,which is called spontaneous exchange bias(SEB).Since there is no need for an additional cooling field to induce unidirectional magnetic anisotropy,the SEB devices are more easily manipulated by electric fields and have more practical application value.However,their underlying mechanisms still remain elusive due to the presence of complex magnetic disorder and the technological limitations involved in directly observing spin structures at the interface between two different magnetic phases.Interestingly,some of the earliest observations of CEB were in spin glasses.Later,it was surmised that CEB may involve aspects of glassy behaviour,and a number of experiments have been modelled as composite systems consisting of spin glasses and other magnetic phases.On the other hand,all SEB materials reported so far have the reentrant spin glass-like behavior as a common feature.These experimental results suggest that the presence of CEB and SEB is always accompanied by glassy behavior.Thus,we expect the study of these two experimental phenomena using spin glasses could provide a much clearer picture about the formation of the unidirectional anisotropy.The research for this thesis includes the following primary parts:(1)Study of exchange bias in CuMn alloys.The Cu_100-xMn_x(5??20)films were epitaxially grown on Si(111)substrates by molecular beam epitaxy.The magnetic behavior was measured by a superconducting quantum interference device,confirming that the magnetic properties of CuMn alloys are all spin glasses.Moreover,we found that a CEB effect was observed in all samples,and the related characteristics are consistent with the parametric behavior of CEB in ferromagnetic/antiferromagnetic systems,which verifies that glassy dynamics are intertwined with CEB.(2)A phenomenological model for the CEB and SEB using a spin glass.We obtained SEB in CuMn alloys by changing the size of the Mn concentration in the alloys and varying the maximum field applied in the hysteresis loop measurements.We found that CEB is attributed to the frustrated and disordered structure of magnetic moments in the materials,while SEB is related to the magnetic system that consists of two or more magnetic phases with a specific coupling dominated by different exchange interactions.(3)Study on the applications of CEB and SEB in CuMn alloy.First,by varying the magnitude of the initial field and cooling filed applied in the hysteresis loop measurements,we manipulated the SEB and CEB.Second,the transport measurements show that the anomalous Hall effect loops are dominated by the biased magnetic hysteresis loops.we show that the magnetic anisotropy causing the SEB can be resolved by a linear superposition of a unidirectional and a uniaxial parts.The former provides a source of net magnetic moments,whereas the latter provides a mechanism for its pinning,which induces the exchange anisotropy for the SEB during virgin magnetization process.