Charge and Spin Currents in Ferromagnet-Insulator-Superconductor Tunneling Junctions Using Hg-1223 High- Superconductor

Charge and spin currents along the -axis in ferromagnet-insulator-superconductor (F/I/S) tunneling junctions have been studied within the framework of the tunneling Hamiltonian model. As a superconductor , HgBa2Ca2Cu3O8+δ (Hg-1223) with copper-oxide high- superconductor has been selected, and as a ferromagnet F, Fe metal with bcc structure has been selected for simplicity. The electronic structures of above materials have been calculated on the basis of the band theory using the spin-polarized self-consistent-field data for the atomic orbital energies and the universal tight-binding parameters (UTBP) for the interactions. For the and defined in the present paper, which are tunneling probabilities of the majority and the minority spin electrons, it is shown that the condition means the standard F/I/S tunneling junction with a nonmagnetic insulating layer, and the condition means the F/I/S tunneling junction with a magnetic insulating layer showing a detectable magnetization. We have found that the charge current and the differential conductance nearly remain the same as the change of , but the spin current is largely changed due to the change of . As an experimental method to detect the change of the spin current, the validity of an X-ray magnetic circular dichroism (XMCD) has been pointed out. 1. Introduction In 1982, Blonder, Tinkham, and Klapwijk (BTK) presented a pioneering paper for an interface at the normal (N) material and superconductor (S), in which they proposed a simple theory for the current-voltage ( - ) curves of normal (N)-superconducting (S) microconstriction contacts which describes the crossover from metallic to tunnel junction behavior [1]. Their model based on the Bogoliubov-de Gennes (BdG) equation, now called as “BTK model," worked well to understand the transmission and reflection of particle at N-S interface. Their results told us that the probability of Andreev reflection decreases with increasing the barrier strength at the interface and the BTK model reaches to the tunneling Hamiltonian model due to the increasing -value. Based on the BTK model, Kashiwaya et al. theoretically studied the origin of zero-bias conductance peaks (ZBCPs) observed in the YBCO high- superconductors and found that the calculation is in good agreement with the experiment [2]. The ZBCPs are observed at the - characteristics of the NS-interface. Within the framework of the BTK model, Kashiwaya et al. further studied the - characteristics of N/I/S and F/I/S junctions [3, 4]. Here it is noted that they have adopted the BTK model, so that the potential of