Abstract
AbstractSince the discovery of giant magnetoresistance (GMR) in Fe/Cr super lattices by Baibich et al. [1], the GMR has received great interest over the last few years for applications in high-density magnetic recording heads. Spin-valve with multilayers is one of the promising magnetic devices and has been applied in the advanced magnetoresistive read head. However, it is not easy to deposit homogeneous ultra thin films to fabricate a multiplayer spin valve. In addition, long-term operations under a high current density also enhance the interlayer diffusion of the constituent atoms, which can significantly degrade the GMR performance. In our study, we first collect and develop the proper HRTEM technique for the analysis utilizing the spin valve structure of GMR read head device. Then study the interdiffusion in detail among the multiplayer. Moreover, we probe into the dependence between interdiffusion and GMR properties. Furthermore, another spin valve sample was used to directly discuss the relation among the GMR properties, anneal temperatures, and interdiffusion. Finally, the analytical results was fitted and compared by Dieny model.Cap/NiFe/CoFe/Cu/CoFe/Ru/CoFe/NiFe/NiMn/seed layer after the life stress test of the spin-valves read head at 200 o C for 1000 hr is investigated by using high resolution transmission electron microscopy (HRTEM) coupling with nanobeam energy dispersive spectrometer x-ray (EDX) and electron energy loss spectrometer (EELS). Investigation was focused on the interdiffusion among interlayers in the nanometer scale. Accumulated sum technique was applied to analyze the lattice-spacing variation across the ultra-thin film from HRTEM micrographs. A Wiener filter was used to remove the electron beam broadening effect from nanobeam EDX data so that the spatial resolution of composition profile is improved. Furthermore, interface roughness was extracted by utilizing EELS map. Combination of these techniques (lattice variation measurement, the deconvolution of nanobeam EDX, and EELS maps) allows the insight of interdiffusion among ultra-thin films in the spin-valve device to be revealed. The compositional analysis demonstrated that Ni diffused into the Cu/Co bilayer. The analytical result also suggests that the Ru layer may not only act as the spacer of the synthetic antiferromagnet but also behave as a good diffusion barrier for Ni and Mn element in the spin valve structure. To further study the diffusion mechanism of constituent elements in the spin valve device, the diffusion coefficients of constituent elements are simply investigated using the Boltzmann-Matano method. The diffusion mechanisms of Cu in Co layer, Co in Cu layer, and Ni in Co layer were primarily dominated by the grain boundary.The spin valves, seed layer/NiFe/CoFe/Cu/CoFe/Ru/CoFe/PtMn/cap, were annealing at several temperatures (220~400 oC) for 20 hr in the parallel magnetic field. Accordingly, it is shown that the magnetoresistance ratio (MR ratio) was degraded with increasing annealing temperature. Surprisingly, there was a demixing phenomenon in the intermixing region of CoFe/Cu bilayer during the annealing at 250 oC for 20 hr. It is consequently shown that the GMR was enhanced due to the sharper and more pure interface of CoFe/Cu and Cu/CoFe. Besides, regarding to the interdiffusion of another constituent elements, Ni diffused into the CoFe/Cu bilayer after 300 oC annealing. Moreover, the analytical result also suggests that the Ru layer may not only act as the spacer of the synthetic antiferromagnet but also behave as a good diffusion barrier for Pt and Mn element in the spin valve structure up to 350 oC annealing. The annealing temperature raised sequentially to 400 oC, Ru layer could also behave as the diffusion barrier for Mn.