The two-current model
The physical origin of GMR is the electron spins effect on the electronic transport in ferromagnetic conductors. The first proposal by Mott described the spin splitting of the energy bands in the ferromagnetic spin states induces the specific transport behavior. Then, Fert and Grünberg experimentally demonstrated the spin dependent conduction in ferromagnetic metals and a series of iron- and nickel-based alloys. The experimental results could be explained by the so called “two current model”, as schematically shown in Fig. 3. In the low temperature limit, the spin flip scattering of the conduction electrons due to magnons is frozen out, thus the spin mixing rate is much smaller than the momentum relaxation rate. The electrical current can be described by the two independent parallel channels: spin↑ (majority) and spin↓ (minority) electrons. The resistivity can be expressed as where and are the resistivity of spin ↑ and spin ↓ channels, respectively.
There are several kinds of contributions to the difference between and . These contributions are either intrinsically related to the spin dependence of the electrical conductivity parameter, where , τv is the spin relaxation time, mv is the effective mass, and n(EF) is the density of states at the Fermi level nν(EF), or extrinsically to the spin dependence of the impurity or defect potential. The latter case has been experimentally proved by Fert and Campbell. They have demonstrated that the asymmetry ratio α of spin↑and↓resistivities, defined by , can be as large as 20, for instance, when 1% of Co or Fe impurities are introduced in a Ni sample. The higher spin asymmetry ratio results in the manifest magnetoresistance effect.
The spin-valve type GMR effect
In 1990, S.S.P. Parkin et al. reported an oscillatory GMR behavior as the thickness of the nonmagnetic layer increases for Fe/Cr, Co/Ru and Co/Cr multilayers. In fact, the behavior is a result of the interlayer exchange coupling variation between ferromagnetic and antiferromagnetic coupling. The GMR effect can be observed only for the antiferromagnetic interlayer coupling and the behavior is related to the so-called RKKY effect.
Although the magnetic multilayers of ferromagnetic- and nonmagnetic-metal have shown very large MR ratio, the large saturation field hinders the practical applications in electronic devices. Therefore, the spin-valve type giant magnetoresistance is explored as illustrated in Fig.
Since 1991, the spin-valve type structure has been included in the GMR systems and now extensively used in magnetic recording devices such as read head for hard disk. The structure consists of the free ferromagnetic layer (lowest), nonmagnetic layer, pinned ferromagnetic layer (FeMn or IrMn) and antiferromagnetic layer (top).The free ferromagnetic layer in this structure can switch at very lower filed range, but the magnetization of the pinned ferromagnetic layer is fixed along the negative field direction due to the interface induced exchange-biased effect. Thus, the significant MR ratio occurs at lower field is very suitable for practical applications. On the other hand, the spin-valve type structure is restricted to trilayer devices and cannot be extended to multilayers with the highest GMR ratio. In fact, a trilayer can be equivalent to a multilayer by improving the electron specular reflectivity at the outer surfaces. So, the spin-valve type structure also demonstrates very large GMR effect for applications.
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