Effect of mixing the low-valent transition metal atoms Y = Sc, Ti, V, Cr, Mn and Fe on the properties of quaternary Heusler compounds Co2-xYxFeSi (0≤x≤1)

Abstract

The realization of physical systems whose electronic and magnetic properties can be easily manipulated by playing with valence electrons are very promising from both the fundamental and applied perspectives. Half-metallic ferromagnets are ideal candidates for spintronic applications because of their high spin polarization of charge carriers at the Fermi level [1,2]. High Curie temperature is one of the important precondition of the half-metallic ferromagnets from the application point of view. Co2- based intermetallic Heusler alloys (L21 structure, space group Fm-3m, # 225) are among the most attractive half-metallic systems due to their high Curie temperatures, high spin polarization and the structural similarity to binary semiconductors [1,2]. The Co2FeSi (CFS) system is one such candidate of interest for spintronic applications due to its high Curie temperature crystallizing in the L21 structure but Fermi level falls on the edge of the minority conduction band (in both generalized gradient approximation (GGA) and GGA+U approach) making the system shy from being robust half metallic. High Tc = 1100K and large magnetic moment of 6.00 μB/f.u. is reported in Ref [3]. If one of the Co atom in CFS is replaced by low valence transition metal atom Y = Sc, Ti, V, Cr, Mn, and Fe, alloys with rich and useful properties can be tailored. The quaternary Heusler alloy CoFeMnSi [4] is observed theoretically and experimentally to be spin gapless semiconductor, CoFeCrSi [5] is reported to be half-metallic with some structural disorder, CoFeVSi and CoFeTiSi are reported theoretically to be nearly half-metallic and half-metallic respectively, but both are observed experimentally to show multi-phase behavior in bulk form. The Fermi level lies on the edge of the minority valence band in all Y substituted quaternary Heusler alloys, leading to an unstable half-metallicity. So, as we go from CFS to CoFeYSi, Fermi level is shifted from lower edge of conduction band to upper edge of valence band in minority spin channel which is in accordance to Ref. [2,6]. They have suggested that an expansion of the lattice should shift the Fermi level deeper in energy and the contraction should shift it higher in energy. Therefore, one can expect robust half metallicity with Fermi level exactly at the middle of the band gap for some intermediate Y concentrations in CFS due to the expansion of the lattice when Y with larger atomic radius substitutes for Co. The substitution of Y to Co may be also seen as d-electron deficiency.In this work, we have introduced the substitution of all low valence transition metal atoms Y = Sc, Ti, V, Cr, Mn, and Fe to Co atoms in the parent CFS system, and examined the structural, electronic, magnetic, and mechanical properties of quaternary Heusler compounds Co2-xYxFeSi (0≤x≤1) to get a global overview of the electronic, magnetic and mechanical properties, promising candidates for spintronics applications. All single phase alloys exhibit face centered cubic crystal structure with a strong tendency towards L21 ordering, as corroborated by X-ray diffraction. The low-temperature saturation magnetic moments agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. Fig.1 shows the linear decrease of the spontaneous magnetizations measured at 2K with the valence electron counts per formula units with element Y = Ti, V, Cr, Mn, Fe, and Co in Co2-xYxFeSi (x = 0.50). All alloys are soft ferromagnets with high Curie temperatures, ranging from 700K to 1100K, allowing for applications at room temperature and above. The Curie temperatures are observed to vary almost linearly with the saturation magnetic moment which is also expected in half-metallic systems.First-principles calculations also predicted finite band gap in the minority spin channel for most of the alloys in Co2-xYxFeSi series. Fig. 2 shows the calculated density of states (DOS) plots using GGA approach for majority and minority spin channels of Co2-xMnxFeSi (0≤x≤1) alloys, where the Fermi level is represented by the zero energy. It can be seen clearly that the system exhibits half-metallic behavior after Mn substitution. In the case of parent compound CFS, there is significant amount of DOS at the Fermi level on both the spin channels. After Mn substitution, the Fermi level is observed to shift deeper in energy-levels and falls at the middle of the energy gap on spin down channel for x = 0.50. Further increase of Mn content shifts the Fermi level towards the lower edge of energy gap due to lattice expansion. But, still Fermi level lies within energy gap making the system Half-metallic. In addition, conduction band and valence band in majority spin channel touch each other at Fermi level in x = 1, making the system spin gapless semiconductor. The energy gap is observed to increase with the increase of Mn content. Although, the calculated minority spin gap is the highest for x = 1, the half-metallic gap is greater for x = 0.50.Relatively high mechanical hardness values among the reported Heusler alloys are also observed, approaching 16 GPa for the most Ti-rich material [6-8]. **