Large Thermoelectric Voltage in Point Contacts of Ni Ferromagnetic Metals
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Large Thermoelectric Voltage in Point Contacts of Ni Ferromagnetic Metals Kenji Kondo1, Hideo Kaiju1, 2, and Akira Ishibashi1 1 Laboratory of Quantum Electronics, Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0020, Japan. 2 PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan. ABSTRACT Recently, we have proposed a spin quantum cross structure (SQCS) device toward the realization of novel spintronics devices. In this paper, we have investigated thermoelectric effects in point contacts (PCs) of Ni ferromagnetic metals using SQCS devices, theoretically and experimentally. The calculated results show that the thermoelectric voltage Vq changes from 0.48 mV to 2.12 mV with the temperature difference of PCs increasing from 10 K to 50 K. Also, the magnitude of the theoretical thermoelectric voltage agrees very well with that of the experimental result. PCs of SQCS devices with Ni electrodes can serve as spin dependent thermobatteries. INTRODUCTION Many researchers have paid much attention to point contacts (PCs) of ferromagnetic metals since conductance quantization in units of e2/h was discovered at PCs of Ni ferromagnetic metals [1]. Also, some researchers report that PCs of Ni ferromagnetic metals show large magnetoresistance in the ballistic regime [2]. However, thermal characteristics in PCs of ferromagnetic metals have almost not been studied even though those in PCs of normal metals have been extensively studied. In this paper, we have theoretically investigated thermoelectric effects in PCs of Ni ferromagnetic metals using spin quantum cross structure (SQCS) devices, and we have verified the thermoelectric effects in PCs of Ni ferromagnetic metals experimentally. The SQCS device consists of two ferromagnetic metal thin films with their edges crossing, and sandwiches a few molecules and atoms [3-6]. The junction area made of two edges can be scaled down to nanometer size due to the good resolution in making films by the metal-deposition rate, ranging from 0.01 nm/s to 1 nm/s. We have already realized the SQCS device with Ni ferromagnetic electrodes [6]. THEORY AND EXPERIMENTAL RESULTS The models of SQCS devices with two-dimensional (2D) electrodes are shown in figure 1(a). We have analyzed the spin dependent thermoelectric characteristics within the framework of the Anderson Hamiltonian. We study the current-voltage (I-V) characteristics of SQCS devices with a molecule sandwiched between two Ni magnetic electrodes. The molecule is assumed to have two energy levels. We also assume that the molecule does not flip the spin of an electron passing through the energy level. The energy diagram for the SQCS device model is also shown in figure 1(b). Notice that this model can be applied to SQCS devices without any molecules sandwiched between the two Ni magnetic electrodes. In the case that there are no sandwiched molecules, both electrodes are in contact with each other directly and the conductance channels (energy levels) are formed in the gap between the electrodes. The energy
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