Effect of Fe substitution on the structural, magnetic and electron-transport properties of half-metallic Co2TiSi

The structural, magnetic and electron-transport properties of Co2Ti1−xFexSi (x = 0, 0.25, 0.5) ribbons prepared by arc-melting and melt-spinning were investigated. The rapidly quenched Co2Ti0.5Fe0.5Si crystallized in the cubic L21 structure whereas Co2Ti0.75Fe0.25Si and Co2TiFe0Si showed various degrees of B2-type disorder. At room temperature, all the samples are ferromagnetic, and the Curie temperature increased from 360 K for Co2TiSi to about 800 K for Co2Ti0.5Fe0.5Si. The measured magnetization also increased due to partial substitution of Fe for Ti atoms. The ribbons are moderately conducting and show positive temperature coefficient of resistivity with the room temperature resistivity being between 360 μΩcm and 440 μΩcm. The experimentally observed structural and magnetic properties are consistent with the results of first-principle calculations. Our calculations also indicate that the Co2Ti1−xFexSi compound remains nearly half-metallic for x ≤ 0.5. The predicted large band gaps and high Curie tempe...

The structural, magnetic and electron-transport properties of Co 2 Ti 1x Fe x Si (x = 0, 0.25, 0.5) ribbons prepared by arc-melting and melt-spinning were investigated. The rapidly quenched Co 2 Ti 0.5 Fe 0.5 Si crystallized in the cubic L2 1 structure whereas Co 2 Ti 0.75 Fe 0.25 Si and Co 2 TiFe 0 Si showed various degrees of B2-type disorder. At room temperature, all the samples are ferromagnetic, and the Curie temperature increased from 360 K for Co 2 TiSi to about 800 K for Co 2 Ti 0.5 Fe 0.5 Si. The measured magnetization also increased due to partial substitution of Fe for Ti atoms. The ribbons are moderately conducting and show positive temperature coefficient of resistivity with the room temperature resistivity being between 360 µΩcm and 440 µΩcm. The experimentally observed structural and magnetic properties are consistent with the results of first-principle calculations. Our calculations also indicate that the Co 2 Ti 1x Fe x Si compound remains nearly half-metallic for x ≤ 0.5. The predicted large band gaps and high Curie temperatures much above room temperature make these materials promising for room temperature spintronic and magnetic applications. © 2017 Author(s). All

I. INTRODUCTION
Magnetic materials including half-metallic ferro-, ferri-, and antiferromagnets that conduct electrons of only one spin channel have recently attracted a lot of attention due to their potential for spintronic devices. [1][2][3][4][5][6] Half-metals with the metallic electronic band structure for one spin channel and an insulating band structure for the opposite spin channel are of special interest because they are expected to produce an electron current of only one spin orientation i.e., show nearly 100% transport spin-polarization. Recent theoretical and experimental investigations have indicated that some Co-based Heusler alloys including Co 2 MnSi, Co 2 FeSi, Co 2 FeAl have shown half-metallic band properties with high Curie temperature much above room temperature, making them potential candidates for room temperature spintronic applications. [7][8][9][10] One of the issues with these materials is the difficulty of synthesizing them in completely ordered L2 1 structures. Most experimentally reported compounds are either partially B2-type or A2-type disordered. Certain types of structural disorder are detrimental to half-metallic properties. 11 Further, for the robustness of half-metallic properties the materials need to have large band gap. These considerations stimulated this work where we aimed to synthesize Co 2 TiSi, which has been predicted to be half metallic with large band gap of 0.621 eV, with high degree of structural order. 7,12 Since the Curie temperature of Co 2 TiSi is close to room temperature, we replaced certain fraction of Ti with Fe to increase its Curie temperature. Experimentally, we investigated three samples with compositions Co 2 Ti 1x Fe x Si (x = 0, 0.25, 0.5). Prior report shows that a partial Fe substitution for Cr in Co 2 CrAl substantially increases its Curie temperature. 13 In this paper, we present our experimental results on the structural, magnetic and electron-transport properties of Co 2 Ti 1x Fe x Si (x = 0, 0.25, 0.5) compounds and compare the experimentally observed data with the results of our first-principle calculations.

A. Experimental methods
Ingots of Co 2 Ti 1x Fe x Si (x = 0, 0.25, 0.5) compounds were prepared by arc melting high-purity elements in an argon atmosphere. The ribbon samples, which are about 2 mm wide and 50 µm thick, were made by rapid quenching in a melt spinner where induction-melted pieces of the ingots were ejected from a quartz tube onto the surface of a copper wheel rotating with a speed of 25 m/s. The crystal structures of the samples were investigated using powder x-ray diffraction (XRD) in Rigaku Miniflex and PANalytical Empyrean diffractometers with copper Kα radiation. Magnetic properties and electron-transport properties were investigated with a Quantum Design VersaLab magnetometer and Physical Properties Measurement system (PPMS). The elemental compositions of the films were determined using the energy-dispersive X-ray spectroscopy (EDX) in FEI Nova NanoSEM450. In all magnetic measurements, the external magnetic field was applied parallel to the length of the ribbon.

B. Computational methods
We performed density functional calculations of electronic and magnetic structures of Heusler compounds, Co 16 Ti (8-x) Fe x Si 8 , using the projector augmented-wave method (PAW), 14 implemented in the Vienna ab initio simulation package (VASP) 15 within the generalized-gradient approximation (GGA). 16 The integration method 17 with a 0.05 eV width of smearing is used, along with the planewave cut-off energy of 500 eV and convergence criteria of 10 -2 meV for atomic relaxation, and 10 -3 meV for the total energy and electronic structure calculations. A k-point mesh of 12 × 12 × 12 is used for the Brillouin-zone integration. 16-atom cubic cell is used, with periodic boundary condition imposed. For all ground state calculations, unit cell geometry was fully optimized to obtain equilibrium structures. Some of the results are obtained using the MedeA R software package. 18  (111) and (002) superlattice peaks indicating that the samples mainly have cubic Heusler L2 1 structure. However, the absence of (111) peak in the pattern of Co 2 Ti 0.75 Fe 0.25 Si suggests that the sample contain strong B2-type disorder. In order to find the degree of L2 1 ordering, experimental peak intensities and the peak intensities of the fully ordered structure deduced from calculations were compared and ordering parameters were determined. The order parameters S L21 , and S B2 can be envisaged as a measure of closeness to ideal L2 1 and B2 type structures and their deviation from 100% represents the interatomic exchange between the sublattices involved. They can be estimated from the expressions where, I hkl , and I full order hkl are the experimental diffraction intensities for the (hkl) planes and the reference intensities calculated for the fully ordered alloys, respectively. 19 The calculated values of S L21 , and S B2 are 88 % and 85 % for x = 0.5, and 94 % and 74 % for x = 0, respectively. This suggests that Co 2 TiSi and Co 2 Ti 0.5 Fe 0.5 Si have mainly ordered L2 1 structures with small disorder. However, we found that the Co 2 Ti 0.75 Fe 0.25 Si has mainly B2-type disordered structure. The XRD patterns show a distinct shift to higher angles indicating that there is a lattice contraction due to Fe substitution for Ti. In addition, some weak impurity peaks have also been detected in these samples and we carried out the Rietveld analysis of the XRD patterns of all three samples in order to quantify them. The Rietveld analysis shows that the samples with x = 0, x = 0.25 and x = 0.5 respectively contain about 7 wt. %, 13 wt. % and 4 wt. % of Co 1-x Si x type impurity phases. The Rietveld fit along with the experimental data for the Co 2 Ti 0.5 Fe 0.5 Si is shown in Fig. 1(b) as an example. The lattice constants estimated from the Rietveld analysis are a = 5.729 Å for Co 2 TiSi, a = 5.690 Å for Co 2 Ti 0.75 Fe 0.25 Si, and a = 5.677 Å for Co 2 Ti 0.5 Fe 0.5 Si, respectively. These values of a are in good agreement with the calculated lattice constants as discussed below. Moreover, the elemental compositions as determined by EDX analysis are very close (within 4%) to the values estimated from the initial weight of the constituent elements. . This may be caused by the presence of a small amount of impurity phase as seen in the Rietveld analysis of XRD data. 21 Figure 2(b) shows thermomagnetic curves M(T) of Co 2 Ti 1x Fe x Si (x = 0, 0.25, 0.5) alloys measured at 10 kOe, where the magnetization gradually decreases before reaching the Curie temperature. The magnetic transition near the Curie temperature for Co 2 TiSi is sharp, whereas that for Co 2 Ti 0.75 Fe 0.25 Si and Co 2 Ti 0.5 Fe 0.5 Si is gradual similar to the one observed in some ferrimagnetic compounds. 20 The Curie temperature of Co 2 TiSi is 360 K, which increased with Fe substitution  reaching 450 K for Co 2 Ti 0.75 Fe 0.25 Si and about 800 K for Co 2 Ti 0.5 Fe 0.5 Si. We note that the Curie temperature of Co 2 FeSi is 1100 K, 21 which is one of the highest values for Heusler compounds. It has been found that the Curie temperature in Co 2 -based Heusler compounds varies linearly with their magnetic moments. 22 In other words, for these compounds, higher valence electron concentration corresponds to the higher value of Curie temperature. Since the magnetic moment in our Co 2 Ti 1x Fe x Si samples shows a systematic increase with the increase in Fe concentration, consistent with our first principles calculations, the increase in Curie temperature is expected. Similar increase in the Curie temperature has been observed in Fe doped Co 2 CrAl. 13 We may attribute this to the increased positive exchange interaction between the Co and Fe atoms. The temperature dependence of zero-field resistivity, ρ of Co 2 Ti 1x Fe x Si (x = 0, 0.25, 0.5) ribbons is shown in Fig. 3, where the ρ increases as temperature (T ) increases from 5 K to 300 K, showing a

B. Computational results
In order to analyze the electronic, structural, and magnetic properties of Co 2 Ti 1x Fe x Si compounds, we performed series of calculations with Co 16 Fe x Ti (8-x) Si 8 supercell varying x between 0 and 7. First, we analyzed how the electronic structure of this material changes as a function of iron concentration. Figure 4 shows the site-projected densities of states (DOS) of Co 2 Ti 0.5 Fe 0.5 Si (i.e. 50% of Fe concentration) in the ground state, where we see that the Fermi level is pinned at the edge of the minority-spin conduction band, which is composed of Co, and to a lesser degree Fe states. Also, the bottom of the minority-spin band gap is entirely composed of Co states, with no contribution from Fe. The spin-resolved total DOS for Co 2 Ti 1x Fe x Si compound with various Fe concentrations are shown in Fig. 5. These materials show almost half-metallic band structures for low concentration of Fe, i.e., for x < 0.5.
The change in Fe concentration is also found to affect the lattice constant. As shown in the Fig. 6, increasing Fe concentration results in a decrease of the lattice parameter a (black squares), consistent with the XRD results. For smaller Fe concentrations, the decrease in a is small but the change becomes more prominent for x > 0.5. Our calculations also indicate that increasing Fe concentration results in a steady increase of the total magnetic moment (the atomic contributions are ≈ 1.2 µ B per Co, and 3.0 µ B per Fe), as shown on the Fig. 6 (blue circles). This result is in good agreement with our experimental measurements.

IV. CONCLUSIONS
In summary, a combined experimental and theoretical investigation of the structural, magnetic and electronic band properties of Co 2 Ti 1x Fe x Si (x = 0, 0.25, 0.5) Heusler alloys has been carried out. XRD analysis shows that the rapidly quenched Co 2 Ti 0.5 Fe 0.5 Si crystallized in the cubic L2 1 structure whereas Co 2 Ti 0.75 Fe 0.25 Si and Co 2 TiFe 0 Si showed various degrees of B2-type disorder. Fe doping increases the saturation magnetization in our samples, and this result is consistent with our calculations. The Curie temperature is enhanced due to Fe substitution from 360 K for Co 2 TiSi to about 800 K for Co 2 Ti 0.5 Fe 0.5 Si. The samples are moderately conducting and show metallic electron transport. The first principles calculations show that Fe doped materials are nearly half-metallic for x ≤ 0.5. These interesting results are expected to stimulate further research on the thin films of these materials.