Electronic structure of fully epitaxial Co2TiSn thin films

Electronic structure of fully epitaxial Co 2 TiSn thin films Markus Meinert, ∗ Jan Schmalhorst, Hendrik Wulfmeier, and G¨ nter Reiss u Thin Films and Physics of Nanostructures, Department of Physics, Bielefeld University, 33501 Bielefeld, Germany Elke Arenholz Advanced Light Source, Lawrence Berkeley National Laboratory, CA 94720, USA Tanja Graf and Claudia Felser arXiv:1010.5754v1 [cond-mat.mtrl-sci] 27 Oct 2010 Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University, 55128 Mainz, Germany (Dated: October 28, 2010) In this article we report on the properties of thin films of the full Heusler compound Co 2 TiSn prepared by DC magnetron co-sputtering. Fully epitaxial, stoichiometric films were obtained by deposition on MgO (001) substrates at substrate temperatures above 600 ◦ C. The films are well ordered in the L2 1 structure, and the Curie temperature exceeds slightly the bulk value. They show a significant, isotropic magnetoresistance and the resistivity becomes strongly anomalous in the paramagnetic state. The films are weakly ferrimagnetic, with nearly 1 µ B on the Co atoms, and a small antiparallel Ti moment, in agreement with theoretical expectations. From comparison of x-ray absorption spectra on the Co L 3,2 edges, including circular and linear magnetic dichroism, with ab initio calculations of the x-ray absorption and circular dichroism spectra we infer that the electronic structure of Co 2 TiSn has essentially non-localized character. Spectral features that have not been explained in detail before, are explained here in terms of the final state band structure. PACS numbers: 75.70.-i, 78.70.Dm, 73.61.At, 81.15.Cd I. INTRODUCTION The materials class of Co 2 YZ Heusler compounds (with Y a transition metal and Z an sp element) has been the subject of extensive studies in the context of spintronics during the last decade. They are of interest because many of them are predicted as half-metallic fer- romagnets with full spin polarization at the Fermi edge. The Heusler compound Co 2 TiSn (CTS) is of partic- ular interest for applications. It is predicted to be a half-metallic ferromagnet with a magnetic moment of 2 µ B /f.u. and it has a high formation energy of the Co- Ti site-swap defect. 1,2 Making use of Heusler compounds which exhibit low disorder or high tolerance of the ground state properties against disorder is highly desired. Co 2 TiSn has been the subject of many experimental and theoretical studies. The ground state properties ob- tained by density functional theory (DFT) depend sensi- tively on the choice of the DFT method. 1–8 The poten- tial has strong non-spherical components and thus only a full-potential treatment in connection with the general- ized gradient approximation (GGA) to the density yields a half-metallic ground state. 1,4 Experiments conducted on bulk CTS find a lat- tice parameter of 6.07 ˚ , a magnetic moment of about A 1.95 µ B /f.u. and a Curie temperature (T C ) around 355 K. 1,9,10 Further, it is found to have a strongly anoma- lous temperature dependence of resistivity, the temper- ature coefficient becomes negative above the Curie tem- perature. A large negative magnetoresistance reveals the importance of spin fluctuations in the compound. 11 A rather new development aims at the magnetocaloric properties of Co 2 TiSn, which has a large and constant Seebeck coefficient of −50 µV/K above T C in the bulk. 10 There have been some efforts to understand the unusual transport properties of CTS by ab initio band structure and semi-classical transport theory. 10,12 These properties make CTS interesting for a possible application in spin caloritronics, which attempt to make use of the interac- tions between heat and spin. An implementation into thin films is of particular importance for such applica- tions. Only two studies on thin films of CTS are available as far as we know. Gupta et al. applied pulsed laser ablation to grow CTS on Si (001) substrates from a stoichiometric target at growth temperatures up to 200 ◦ C. 13 The au- thors found off-stoichiometric, polycrystalline films with (022) texture. Suharyadi et al. utilized an atomically controlled alternate deposition technique based on elec- tron beam evaporation. 14 They have grown (001) ori- ented, L2 1 ordered films on Cr buffered MgO (001) sub- strates at growth temperatures up to 600 ◦ C and investi- gated them by nuclear resonant scattering. In this paper we present a successful preparation tech- nique based on DC magnetron co-sputtering. We present data on the structural and magnetic properties of our films. Further, we characterize the electronic transport properties which make CTS a particularly interesting compound. Finally we discuss the electronic structure of our CTS films based on soft x-ray absorption spec- troscopy and ab initio electronic structure calculations.