STUDY OF THE CHARGE SPIN AND HEAT IN THE METAL/MAGNETIC INSULATOR NANO-STRUCTURE
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This dissertation focuses on spin caloritronics, which studies the interplay between heat, spin and charge currents, and classic spintronics, which studies the generation and transport of a pure spin current without the accompaniment of a net charge current. When a temperature gradient is applied across the yttrium iron garnet (YIG), a pure spin current is generated, and this is the spin Seebeck effect. Usually, a thin layer of Pt film is attached to detect the pure spin current generated in YIG. In the study of the Pt/YIG structure, we have observed the magnetic proximity effect in the nonmagnetic metal Pt, which compromises the suitability of Pt as a pure spin current detector. The signature of the magnetic proximity effect is the magnetoresistance and anomalous Hall like behavior. More interestingly, the magnetoresistance in the Pt/YIG structures exhibits an unusual magnetic field angular dependence unlike any known magnetoresistance. Because of the magnetic proximity effect in Pt/YIG, one needs to search for a better pure spin current detector for the conclusive establishment of the spin Seebeck effect. We show that Au, with negligible magnetic proximity effect when in contact with YIG, is a better, indeed the best material to date. By varying the Au thin film thicknesses on YIG, we have conclusively demonstrated the intrinsic spin Seebeck behavior of YIG without appreciable contamination of any other effect. The observation of the intrinsic spin Seebeck effect allows us to inject a pure spin current into various materials of interest, such as the 5d heavy metals, to study the inverse spin Hall effect, where a spin current is converted to a charge current, and obtain the two key parameters of spin Hall angle, which is the conversion efficiency between spin current and charge current, and spin diffusion length, which quantifies the distance a spin can travel without losing its information, in pure spin current phenomena. Prior to our work, the inverse spin Hall effect has only been observed in non-magnetic metals. We have demonstrated the inverse spin Hall effect in ferromagnetic metals such as permalloy (Py) and Co, as well as antiferromagnetic metal of Cr. To our surprise, we found that the inverse spin Hall effect is independent of the magnetic ordering. By decoupling the magnetization of Co and YIG, we show that the direction of the Co magnetization has no effect on the spin index of the spin current from YIG. We have also found that the inverse spin Hall effect in antiferromagnetic Cr is apparently independent of the antiferromagnetic ordering since the inverse spin Hall effect remains the same below and above the Cr Neel temperature. Moreover, we have determined the spin Hall angles in these magnetic materials, and found their values to be comparable to those of the 5d metals, which exhibit some of largest values. In fact, we have found Cr, although only a 3d metal, possesses the largest spin Hall angle of all metals. The new physical phenomena and materials established in this dissertation, including the unique magnetoresistance, the intrinsic spin Seebeck effect, the inverse spin Hall effect in ferromagnetic and anitferromagnetic metals, the large spin Hall angle in magnetic materials, and independent of their magnetic ordering, the largest spin Hall angle in Cr, may help us build superior spintronic devices, especially pure spin current devices. This work was performed under the guidance of Professor C. L. Chien at Johns Hopkins University.