Fundamental obstacle for electrical spin injection from a ferromagnetic metal into a diffusive semiconductor

Abstract

(Accepted by PRB Rap. com.)We have calculated the spin-polarization effects of a current in a two dimensional electron gas which is contacted by two ferromagnetic metals. In the purely diffusive regime, the current may indeed be spin-polarized. However, for a typical device geometry the degree of spin-polarization of the current is limited to less than 0.1%, only. The change in device resistance for parallel and antiparallel magnetization of the contacts is up to quadratically smaller, and will thus be difficult to detect.Spin-polarized electron injection into semiconductors has been a field of growing interest during the last years [1][2][3][4]. The injection and detection of a spin-polarized current in a semiconducting material could combine magnetic storage of information with electronic readout in a single semiconductor device, yielding many obvious advantages. However, up to now experiments for spininjection from ferromagnetic metals into semiconductors have only shown effects of less than 1% [5,6], which sometimes are difficult to separate from stray-field-induced Hall-or magnetoresistance-effects [2]. In contrast, spininjection from magnetic semiconductors has already been demonstrated successfully [7,8] using an optical detection method.Typically, the experiments on spin-injection from a ferromagnetic contact are performed using a device with a simple injector-detector geometry, where a ferromagnetic metal contact is used to inject spin polarized carriers into a two dimensional electron gas (2DEG) [5]. A spin-polarization of the current is expected from the different conductivities resulting from the different densities of states) for spin-up and spin-down electrons in the ferromagnet. For the full device, this should result in a conductance which depends on the relative magnetization of the two contacts [1].A simple linear-response model for transport across a ferromagnetic/normal metal interface, which nonetheless incorporates the detailed behaviour of the electrochemical potentials for both spin directions was first introduced by van Son et al. [9]. Based on a more detailed (Boltzmann) approach, the model was developed further by Valet and Fert for all metal multilayers and GMR [10]. Furthermore, it was applied by Jedema et al. to superconductor-ferromagnet junctions [11]. For the interface between a ferromagnetic and a normal metal, van Son et al. obtain a splitting of the electrochemical potentials for spinup and spindown electrons in the region of the interface. The model was applied only to a single contact and its boundary resistance [9]. We now apply a similar model to a system in which the material properties differ considerably.Our theory is based on the assumption that spinscattering occurs on a much slower timescale than other electron scattering events [12]. Under this assumption, two electrochemical potentials µ ↑ and µ ↓ , which need not be equal, can be defined for both spin directions at any point in the device [9]. If the current flow is one dimensional in the x-direction, the electroc...

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