The surface composition and spin polarization of NiMnSb epitaxial thin films

– The composition in the surface region of the Heusler alloy NiMnSb has been studied using angle-resolved X-ray photoemission spectroscopy, low-energy electron diffraction and spin-polarized inverse photoemission. The results are consistent with a generally Mn-Sb terminal layer, though the surface composition is very sensitive to preparation. The surface composition has a critical influence on the polarization near the Fermi level in spin-polarized inverse photoemission. Under some conditions, the polarization at the Fermi level for normal incidence inverse photoemission can reach values very close to 100% above background. Mn-based Heusler alloys such as NiMnSb and PtMnSb have been predicted to be [1–7] half-metallic ferromagnets (metallic in spin majority and semiconducting in spin minority), e.g. fully spin polarized. Unfortunately, experimental evidence of 100% spin polarization near the Fermi level is sadly absent. Using Andreev reflection [8] the spin polarization of NiMnSb was measured to be 58± 2.3%, in good agreement with a polarization of about 50% obtained from spin polarized photoemission [9] and consistent with a small perpendicular magnetoresistance for NiMnSb in a spin valve structure [10]. Other bulk measurements have provided evidence of half-metallic character. Indirect measurements using positron annihilation have, however, been interpreted as a proof of full spin polarization at low temperature (10 K) in bulk crystals [11]. A detailed study of the magnetic and transport properties in bulk single crystals has also suggested that NiMnSb is half-metallic at low temperature, with a transition to a metallic state around T = 80K [12]. Two explanations have been put forward to explain the smaller than expected polarization in measurements that are sensitive to electron transport across a surface or interface. The first possibility is that the gap in spin minority is smaller than the expected 0.5 eV [7, 9, 13]. The other possibility is that there is surface segregation [9,14] making the surface a different material from the bulk. Furthermore, few of these studies on NiMnSb have been undertaken on single crystals or epitaxial thin films. (∗) E-mail: [email protected]