Nonlinear Response Of The Giant Magnetoresistance

This thesis reports an analytic study on the giant- magnetoresistance of multi-layer sandwich structure and spin-valve structure by using Green function quantum statistics approach and nonlinear Kubo formula that was derived by sub-dynamics theory.Giant-magnetoresistance phenomenon has very important and wide application foreground. It becomes a hotspot in the area of condensed matter physics study. Then the problem about how to analytically deduce the giant-magnetoresistance of different practical devices also attracts wide attentions recently. In fifth century Kubo derived his theoretical linear formula for calculating non-local response function. In 1998 Vedyayev and cooperators calculated giant-magnetoresistance of multi-layer sandwich structure and spin-valve structure by using the linear response Kubo formula and Green function technology. However, because of the small size of nanosystems the ordinary external field must be viewed as strong field. That makes the conductivity formula considering nonlinear response very important. Thus Zeng Xiang-Hua and Bi Qiao deduced the nonlinear response Kubo formula by means of sub-dynamics theory. The research work reported in this thesis is the first time application of the nonlinear response Kubo formula in the calculation of the giant-magnetoresistance of practical devices. The authors deduced the analytic formula for the calculation of the giant-magnetoresistance of multi-layer sandwich structure and spin-valve structure, and also discussed what the calculation results implied.It is the consideration of nonlinear response that makes possible to discuss the influences of bias and temperature on giant-magnetoresistance. The calculation results of the thesis author show that for multi-layer sandwiches the giant-magnetoresistance becomes larger when the bias increases. It becomes smaller when the bias is negative. The variation of the giant-magnetoresistance is proportional to the bias. It does not change when the mean free path of theelectrons, those locate at the interfaces and have downward spins, increases. Comparing to the influence of the bias, the variation of the giant-magnetoresistance induced by change of temperature is smaller. It becomes important only in the situation when the variation of temperature is large. In the case when the temperature becomes near to 0?k, the decrease of temperature makes the giant-magnetoresistance smaller when the bias is positive, it makes the giant-magnetoresistance larger when the bias is negative. Again, the variation of the giant-magnetoresistance is proportional to temperature. It does not change when the mean free path of the electrons, those locate at the interfaces and have downward spins, increases. Similarly, for spin-valve structures giant-magnetoresistance becomes larger when the bias increases. It becomes smaller when the bias is negative. In the case when the temperature becomes near to 0?k, the decrease of temperature makes the giant-magnetoresistance larger when the bias is positive. The variation curve of the giant-magnetoresistance with the change of the thickness of the ferromagnetic layer, a, usually shows a maximum. At this place the influence of bias and temperature is larger than elsewhere.We have compared our calculation results with the experimental data and obtained a good agreement.