A Landau-Ginzburg Description of Sb Overlayers
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A LANDAU-GINZBURG DESCRIPTION OF Sb OVERLAYERS R. SKOMSKI,1 T. KOMESU,1 H.-K. JEONG,1 C. N. BORCA,1 P. A. DOWBEN,1 D. RISTOIU2 and J. P. NOZIERES2 1
Department of Physics and Astronomy and Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68508. 2 CNRS, Laboratoire Louis Néel, Avenue des Martyrs BP 166, 38042 Grenoble, CEDEX 09, France. ABSTRACT The spin polarization of Sb overlayers on the semi-Heusler alloy NiMnSb is investigated in terms of the Landau-Ginzburg approach. The half-metallic semi-Heusler alloy NiMnSb acts as a ferromagnetic perturbation and induces a spin polarization in the semimetallic Sb overlayer. Using a Gaussian approximation, the propagation of the spin perturbation in the overlayer is calculated. The results are compared with spin-polarized inverse photoemission spectroscopy (SPIPES) results and with recent spin-dependent envelope-function approximation (SDEFA) predictions. The Landau-Ginzburg parameters are both band-structure and temperature dependent, and it is argued that thermal spin excitations lead to an injection depth decreasing as 1//T law at high temperatures.
INTRODUCTION The spin structure at interfaces is key to understanding spin electronics. Of particular interest are interfaces between different classes of materials, such as interfaces between magnetically ordered and semiconducting materials. Here we focus on the interface between a halfmetallic high-polarization ferromagnet and a semimetal. Halfmetallic materials are ferromagnets characterized by a ↓ subband which is completely filled, whereas the ↑ electrons provide metallic conductivity. Semimetals are reminiscent of ordinary paramagnetic semiconductors, except that they exhibit a negative 'energy gap'. This work deals with NiMnSb layers covered by Sb overlayers. NiMnSb is a halfmetallic semi-Heusler alloy crystallizing in the cubic C1b structure. It may be considered as a derivate of the parent Heusler alloy Ni2MnSb and has a ↓ band gap of less than about 0.5 eV [1]. Antimony is a semimetal characterized by a very small negative energy gap [2]; the overlap and Fermi energies are about 180 meV and 90 meV, respectively, and the electron and hole carrier densities are of comparable magnitude (about 5 × 10-19 cm-3) [2]. The preparation and characterization of the sputtered NiMnSb films considered in this work has been described elsewhere [3-5]. The Sb grows epitaxially on NiMnSb, with a orientation, a cubic structure, a 3.1 Å lattice constant [6]. U7.10.1
The magnetism of the Sb overlayer films was investigated by a combination of Xray absorption spectroscopy (XAS) and spin-polarized inverse photoemission spectroscopy (SPIPES) experiments [4,6]. Our approach yields a layer-specific analysis of the spin polarization, in contrast to methods such as that used in [7], where spin injection is probed indirectly, by considering the exchange coupling through a semiconducting medium. The measured spin asymmetry exhibits an oscillatory behavior and extends quite well into the Sb layer (up to about 1 n
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