Effect of Si substitution on electronic structure and magnetic properties of Heusler compounds Co 2 TiAl 1  x Si x Seyyed Mojtaba Zareii a,b , Hadi Arabi a,n , Reza Sarhaddi a a Magnetism and Superconducting Research Lab., Department of Physics, Faculty of Science, University of Birjand, P.O. Box 97175-615, Birjand, Iran b Department of Basic Sciences, Birjand University of Technology, Birjand, Iran article info Article history: Received 10 December 2011 Received in revised form 1 March 2012 Accepted 10 April 2012 Available online 19 April 2012 Keywords: Heusler alloys Density functional calculations Electronic structure Magnetic moments abstract The electronic structures of Co-based Heusler compounds CoTiAl 1x Si x (x ¼0, 0.25, 0.5, 0.75 and 1) are calculated by first-principles using the full potential linearized augmented plane wave (FP-LAPW) method within GGA and LSDAþU scheme. Particular emphasis was put on the role of the main group elements. In recent years, the GGA calculations of Co 2 TiAl (x ¼0) and Co 2 TiSi (x ¼1) indicated that they are half-metallic, but the electronic structure of this compound with x ¼0.25, 0.5 and 0.75 has not been reported yet, neither theoretically nor experimentally. The calculated results reveal that these are half-metallic and exhibit an energy gap in the minority spin state and also show 100% spin polarization. The substitution of Al by Si leads to an increase in the number of valence electrons, with increasing x. Our calculated results clearly show that with the Si doping, the lattice parameter linearly decreases; bulk modulus increases, and the total magnetic moment increases. The calculated energy gap in the minority spin state, using GGA scheme, was smaller than that obtained by using LSDAþU scheme. The outcomes of this research also show that the Co-3d DOS and therefore, the magnetic properties of compounds are dependent on electron concentration of the main group elements and it will affect the degree of p-d orbital occupation. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Half-metallic ferromagnets (HMFs) [1] have attracted scientific and technological interest due to their potential use as a highly spin-polarized current source in spintronics, magneto-electronic devices and ferromagnetic shape memory effect [2–9]. HMFs exhibit a strongly metallic behavior in one spin band (spin-up) at the Fermi energy level, E F , and at the other spin band (spin- down), they behave like a semiconductor, which results in a 100% spin polarization at E F [10]. They serve as a model for the understanding of band-magnetism of d-electrons due to their simple crystallographic structure and a series of interesting diverse magnetic phenomena like itinerant and localized magnet- ism, antiferromagnetism, helimagnetism, Pauli paramagnetism or heavy fermion behavior can be seen in these compounds [11]. The Heusler compounds as the half metallic ferromagnet are usually ternary compounds with the stoichiometric composition X 2 YZ where X and Y represent transition metals and Z represents a main group element. They crystallized in the L2 1 structure (space group Fm3m) which is depicted, as an example for Co 2 TiAl compound, in Fig. 1. Besides ternary X 2 YZ compounds, there exist large assortments of substitutional quaternary alloys of the type X 2 Y 1x Y 0 x Z or X 2 YZ 1x Z 0 x [12]. The L2 1 crystal structure consists of four fcc sublattices as X at (0,0,0) and (½, ½, ½), Y at (¼, ¼, ¼) and Z at (3/ 4, 3/4, 3/4) [13]. However, these alloys can transform into the B 2 (disorder between Y and Z atoms) or A 2 (disorder between X, Y and Z atoms) structure or a combination of the above structures, depending on their thermodynamic equilibrium [14]. Such a mixture of atoms in Y and Z positions can destroy the half-metallic properties [15]. In the Heusler compounds, the electronic structure plays an important role in determining the magnetic properties and predicting half-metallic ferromagnetism as the temperature stability of the energy gap in the minority (spin-down) spin states is one of the important questions for materials to be used in applications. Depending on the situation of E F , several different effects may destroy the half-metallicity at finite temperatures such as spin-rotation, the lattice parameter and defect densities changed at elevated temperatures [16]. Amongst some kinds of the Heusler alloys, the Co-based Heusler alloys Co 2 YZ are especially attractive for their predicted high spin polarization and high Curie temperature [17] as well as varying magnetic moments at Co sites depending on the consti- tuents Y and Z [18–20], making them attractive for various industrial application. As we know, Co provides the highest Curie temperature among all 3d transition metals [21]. In these com- pounds, the magnetic moment per Co atom ranges from 0.3 to 1.0m B in contrast with Mn-based Heusler alloys X 2 MnZ (where X is not Co) with almost constant moments of approximately 4m B on the Mn atom [18]. It is found that the Curie temperature (T C ) of the Heusler compounds decreases linearly as the total spin moment decreases [22]. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.04.019 n Corresponding author. Tel.: þ98 561 2502023; fax: þ98 561 2502041. E-mail address: arabi_h@yahoo.com (H. Arabi). Physica B 407 (2012) 3339–3346