The Spin Structure Of Magnetic Multilayer Systems, The Exchange Bias And Magnetoresistance Effect

The exchange interaction at the interface between a ferromagnet (FM) and an antiferromagnet (AF) may create a preferential direction in the spin orientation of the FM, namely, a unidirectional anisotropy (exchange anisotropy). The most commonly observed consequences of the exchange anisotropy are a shift of the hysteresis loop toward negative field values (exchange bias He), and a coercivity enhancement. Since the design of giant magnetoresistive spin-valve based on exchange bias, there has been considerable interest in exchange-biased magnetic multilayers because it can be useful in controlling the magnetization in devices, such as magnetic sensors and magnetic memory devices.The phases of the antiferromagnetic magnetization and the corresponding exchange bias and coercivity in exchange-biased bilayers (FM/AF) with a interface quadratic and biquadratic exchange coupling have been studied comprehensively. The results show that there are possible four phases for the antiferromagnetic magnetization, namely the reversible recovering case, irreversible half-rotating case, irreversible reversing and irreversible half-reversing cases. However, the realization of the cases strongly depends on interface quadratic coupling, interface biquadratic coupling and AF thickness. In the reversible recovering case the exchange coupling between FM and AF results in the exchange bias, and there is no exchange bias in the other cases. The magnetic phase diagram in term of the AF thickness tAF, the interfacial bilinear coupling J1 and the spin-flop coupling J2 has been constructed. The corresponding critical parameters in which the exchange bias will occur or approach saturation have been also presented. The results show that the bilinear and spin-flop couplings are two competitive factors for controlling the value of He. Specially, for exchange-biased bilayers without interface quadratic coupling, there exists a critical value of the biquadratic coupling constant J2crit, below which the positive exchange bias appears, otherwise there is no exchange bias, and the coercivity can be enhanced. For the FM/AF bilayers with interface quadratic and biquadratic coupling, the interface biquadratic coupling can weaken or eliminate the exchange bias, but always enhance the coercivity.In addition, By investigating the spin configuration in ferromagnetic multilayer under stress field, the magnetoresistance (MR) dependence upon the stress field in the ferromagnetic multilayer has been discussed. The results show that the stress field can induce MR, and the MR strongly depends on both the value and the direction of the stress field. Generally, when the stress field deviates from the magnetic easy axis to the magnetic hard axis, the MR first increases slowly, then decreases rapidly. For a given direction of stress field, the MR first increases rapidly, later decreases slowly with stress field increasing. Moreover, when the direction of stress field approaches that of the hard axis, MR dependence upon both the value and the direction of the stress field becomes obvious. Specially, if the direction of the stress field is perpendicular to the easy axis, change of MR can become double value with the stress field increasing. When the stress field Hλmeets to 8πM /5≤Hλ≤18πM/5,MR can increase monotonously, and the giant magnetoresistance (GMR) occurs. In addition, the stress field dependence of MR is close related with both the magnetostriction and the magnetocrystalline anisotropy of ferromagnetic layer.