Journal of Magnetics 12(2), 77-80 (2007) ⓒ 2007 Journal of Magnetics Electronic Spin Filter via Spin Superlattice Jae-ho Han, H.-W. Lee*, and Chun-Yeol You 1 Department of Physics, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Korea 1 Department of Physics, Inha University, Incheon 402-751, Korea (Received 22 March 2007) Recently there was a proposal for a spin filter by using the spin superlattice structure. In a certain energy range, the proposed structure exhibits a high spin filtering efficiency close to 100%. Unfortunately such energy range turns out to be narrow. In this paper, we report a method to widen the energy range by using an analogy to optical anti-reflection coating. In optics, it is well known that a stack of alternating layers of two dielectric materials can function as a highly transmissive or reflective filter for wide range of wavelength. Since electrons also have wave character as light, it would be possible to make an electronic analog of an optical filter. We dem- onstrate that alternating layers of two materials with different g-factors can function as a spin filter that allows electrons to be transmitted only when their spins point towards a certain particular direction. This spin-super- lattice-based spin filter operates in wide energy ranges, curing the problem in the previous proposal. Keywords : spin filter, spin superlattice 1. Introduction There are growing interests in using the spin degree of freedom for electronic devices. It has been suggested that the utilization of the spin degree of freedom may facilitate the realization of non-volatile, higher speed, and lower powered devices compared to conventional charge-only devices [1]. One of major concerns in this spin-based technology or spintronics is to find a way to generate highly spin-polarized current within semiconductors. There were many attempts to make the spin-polarized current. Attempts to inject a highly spin-polarized current from conventional metallic ferromagnets into semiconductors suffer from the conductance mismatch problem [2] and ways to overcome the problem are under investigation [3]. There are also attempt to generate spin-polarized currents by optical methods. Recently a still alternative approach [4, 5] has been proposed based on the wave nature of electrons. In the proposal in Ref. 5, an asym- metrical-shaped profile of the magnetic fringe field is generated by ferromagnetic strips deposited on the 2D electron gas system. Since such magnetic field profile serves as a spin-dependent potential energy profile, the resonant transport through such potential structure can lead to the spin filtering. However since this proposal utilizes fringe field of the ferromagnets, it can be applied only to the thin film. On the other hand, the proposal in Ref. 4 based on spin superlattice is applicable to bulk as well. However a shortcoming of this proposal is that the energy window of the high spin-polarization is narrow. In this paper, we revisit the spin superlattice structure and demonstrate that the energy window for high spin filtering efficiency can be considerably widened by using the analogy to the anti-reflection film coating in optics. Our analysis indicates that this method is superior to those in Refs. [4, 5]. 2. Spin Superlattice In optics, the dielectric coating is a widely used tool for high reflection or transmission of light in optical instrument. Alternating coating of two materials with the different reflective indices leads to this result [6]. Likewise, stack of alternating semiconductors may serve as an “anti- reflection coating” for electrons. In order to achieve the spin filter functionality, however, the stack should serve as an “anti-reflection coating for one particular spin direction only. One way to achieve this spin-selectivity is to apply a uniform external magnetic field. Due to the Zeeman coupling, the field then generates the spin-depen- dent potential energy profile. This is the main idea of the *Corresponding author: Tel: +82-54-279-2092, Fax: +82-54-279-3099, e-mail: hwl@postech.ac.kr