PHYSICAL REVIEW B 83, 195112 (2011) Hard x-ray photoemission study of near-Heusler Fe x Si 1-x alloys A. X. Gray, 1,2 J. Karel, 3 J. Min´ ar, 4 C. Bordel, 5,6 H. Ebert, 4 J. Braun, 4 S. Ueda, 7 Y. Yamashita, 7 L. Ouyang, 8 D. J. Smith, 8 K. Kobayashi, 7 F. Hellman, 2,3,5 and C. S. Fadley 1,2 1 Department of Physics, University of California, Davis, California 95616, USA 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 3 Department of Materials Science and Engineering, University of California Berkeley, Berkeley, California 94720, USA 4 Dept. Chemie und Biochemie, Physikalische Chemie, Universit¨ at M ¨ unchen, Butenandtstrasse 5-13, 81377 M¨ unchen, Germany 5 Department of Physics, University of California Berkeley, Berkeley, California 94720, USA 6 Groupe de Physique des Mat´ eriaux, UMR CNRS 6634, Universit´ e-INSA de Rouen, Avenue de l’Universit´ e - BP12, 76801 Saint Etienne du Rouvray, France 7 NIMS Beamline Station at SPring-8, National Institute for Materials Science, Sayo, Hyogo 679-5198, Japan 8 Department of Physics, Arizona State University, Tempe, Arizona 85287, USA (Received 15 February 2011; published 9 May 2011) The structural and electronic properties of epitaxial and amorphous Fe x Si 1−x alloys with x = 0.72 and 0.67 near the binary Heusler composition of x = 0.75 were determined using hard x-ray photoelectron spectroscopy (HXPS). By performing the measurements at a photon energy of 5950.3 eV, the bulk-sensitivity of the measurement is enhanced by a factor of 4–7 compared to conventional soft x-ray photoelectron spectroscopy at about 1000 keV. HXPS probes, on average, as far as 76 ˚ A into the Fe x Si 1−x samples. Via core-level spectra, it is found in the amorphous alloy that, in spite of the disordered structure that could lead to a broad distribution of chemical environments, the Si environment is mostly unique. Valence-band spectra reveal a clear distinction between the contributions of the two inequivalent Fe sites of the most highly ordered (x = 0.72, D0 3 ) epitaxial sample. The valence-band spectra are compared to results of fully relativistic coherent potential approximation calculations performed in the framework of the one-step model of photoemission, which reveal details of the atomic-orbital makeup of various features, and generally exhibit good agreement with experiment. DOI: 10.1103/PhysRevB.83.195112 PACS number(s): 79.60.Dp, 71.20.Be, 71.20.Eh, 71.20.Gj I. INTRODUCTION Ferromagnetic Fe 3 Si has attracted significant interest due to its potential as a spin injector into semiconductors. 1,2 This stoichiometric compound can be viewed as Fe 2 FeSi, which is a binary Heusler alloy having two different Fe sites 3 with different moments. It was theoretically predicted to have a significant spin polarization at the Fermi energy, due to the splitting in energy of the majority- and minority-spin channels, 4,5 but the experimentally observed degree of spin polarization remains low, which is an obstacle for spintronics applications. 6 Up to now, significant work has been devoted to Fe 3 Si, 1,2, 6–9 but very little is known about nonstoichiometric alloys 10 or amorphous alloys. 11 In the composition range 0.55 < x < 0.75, a two-phase region of the bulk equilibrium phase diagram, thin-film growth can be used to produce homo- geneous alloys with varying degrees of structural and chemical ordering. This ordering affects the physical properties of the material, including, in particular, the electrical resistivity and magnetic properties, which can be significantly tuned. The D0 3 crystal structure is the equilibrium structure of stoichiometric Fe 3 Si. The unit cell has an fcc Bravais lattice and can be thought of as eight bcc-like subunits with Fe on the cube corners (Fe II ), and Fe (Fe I ) and Si alternating in the body centers. 12 In this structure, Fe II have four-nearest-neighbor Fe and four-nearest-neighbor Si, while Fe I have eight-nearest-neighbor Fe. In epitaxial samples with off-stoichiometric compositions (0.55 < x < 0.75), three different chemical orderings are possible. The additional Si can preferentially substitute for Fe in the body-centered (Fe I ) positions to maintain a D0 3 ordered structure, 12,13 causing a decreased number of Fe I and a decrease in the ratio of Fe/Si neighbors for Fe II . The alloy can also form in the closely related B2 (CsCl) structure, which at x = 0.5 has Fe on cube corners and Si in the body centers, with all Fe atoms surrounded by Si; this structure has distinct symmetry, but could be thought of as the limit of D0 3 where the number of Fe I and the ratio of Fe/Si neighbors for Fe II have both gone to zero. For x between 0.5 and 0.75, there is, however, an important distinction between D0 3 and B2, which lies in whether Fe I and Si in the body-centered positions are randomly arranged or preserve a long-range alternating structure with occasional substitution of Si at Fe I sites. For the same composition, both structures have the same nearest-neighbor arrangements for all body centers (all Si and Fe I atoms are surrounded by eight Fe atoms), but the next nearest neighbors (which are important for both magnetic and electronic properties) are different. The Fe II atoms statistically have the same average neighbor ratio, but the width of the distribution is much wider for B2 than D0 3 . The third possibility is an A2 structure, which is a random bcc solid solution with Fe and Si randomly occupying both corner and body-centered positions. The last possibility is a structurally disordered amorphous material. These differences in chemical and structural ordering produce different magnetic and electrical properties. The aim of this study is to investigate the role of composition as well as structural and chemical ordering on the electronic properties of homogeneous metastable Fe x Si 1−x samples. For this study, we used hard x-ray photoelectron spec- troscopy (HXPS) to measure core and valence electronic levels for three samples—epitaxial Fe 0.72 Si 0.28 (epi-Fe 0.72 Si 0.28 ), 195112-1 1098-0121/2011/83(19)/195112(10) ©2011 American Physical Society