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Register a new userPhase stability and the effect of lattice distortions on electronic properties and half-metallic ferromagnetism of Co2FeAl Heusler alloy: An ab initio study
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Density functional theory calculations within the generalized gradient approximation are employed to study the ground state of Co2FeAl. Various magnetic configurations are considered to find out its most stable phase. The ferromagnetic ground state of the Co2FeAl is energetically observed with an optimized lattice constant of 5.70 {AA}. Thereafter, the system was subjected under uniform and non-uniform strains to see their effects on spin polarization (P) and half-metallicity. The effect of spin orbit coupling is considered in the present study. Half-metallicity (and 100 % P) is only retained under uniform strains started from 0 to +4%, and dropped rapidly from 90% to 16% for the negative strains started from -1% to -6%. We find that the present system is much sensitive under tetragonal distortions as half-metallicity (and 100% P) is preserved only for the cubic case. The main reason for the loss of half-metallicity is due to the shift of the bands with respect to the Fermi level. We also discuss the influence of these results on spintronics devices.
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Co2FeSi, a Heusler alloy with the highest magnetic moment per unit cell and the highest Curie temperature, has largely been described theoretically as a half-metal. This conclusion, however, disagrees with Point Contact Andreev Reflection (PCAR) spectroscopy measurements, which give much lower values of spin polarization, P. Here, we present the spin polarization measurements of Co2FeSi by the PCAR technique, along with a thorough computational exploration, within the DFT and a GGA+U approach, of the Coulomb exchange U-parameters for Co and Fe atoms, taking into account spin-orbit coupling. We find that the orbital contribution (mo) to the total magnetic moment (mT) is significant, since it is at least 3 times greater than the experimental uncertainty of mT. Account of mo radically affects the acceptable values of U. Specifically, we find no values of U that would simultaneously satisfy the experimental values of the magnetic moment and result in the half-metallicity of Co2FeSi. On the other hand, the ranges of U that we report as acceptable are compatible with spin polarization measurements (ours and the ones found in the literature), which all are within approximately 40-60% range. Thus, based on reconciling experimental and computational results, we conclude that: a) spin-orbit coupling cannot be neglected in calculating Co2FeSi magnetic properties, and b) Co2FeSi Heusler alloy is not half-metallic. We believe that our approach can be applied to other Heusler alloys such as Co2FeAl.
The generalized gradient approximation (GGA) scheme in the first-principles calculations are used to study the effect of L21 and XA ordering on the phase stability, half-metallicity and magnetism of Co2FeAl (CFA) Heusler alloy. Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II were prepared under the conventional L21 and inverse XA phases by altering the atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of CFA is the most stable phase energetically among all the structures. The electronic structure calculations without U show the presence of half-metallic (HM) ground state only in L21-1 structure and the other structures are found to be metallic. However, the electronic structures of CFA are significantly modified in the presence of U, although the total magnetic moments per cell remained the same and consistent with the Slater-Pauling (SP) rule. The metallic ground states of CFA in L21-II and XA-II structures are converted into the half-metallic ground states in presence of U but remained the same (metallic) in XA-I structure. The results indicate that the electronic structures are not only dependent on the L21 and XA ordering of the atoms but also depend on the choice of U values. So experiments may only verify the superiority of GGA+U to GGA.
Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by chemical route. The effect of the size of NPs upon the structure and magnetization compared to its bulk counterpart was investigated. The structure and composition were determined from X-ray diffraction (XRD) and electron microscopy. XRD analysis shows that the samples are having single (A2-type) disordered phase. Magnetization measurements suggest that the samples are soft ferromagnetic in nature with very low coercivity. Enhanced magnetic properties like saturation magnetization, coercive force, retentivity, and Curie-temperature are observed with a decrease in particle size. The effect of particle size on hysteresis losses is also discussed. The smallest particles of size 16 nm exhibited the highest saturation magnetization and transition temperature of 180.73 emu/g and 1261 K, respectively. The origin of enhancement in the magnetization of Co2FeAl nano-alloy is attributed to the strong Co-Co exchange interaction due to disorder present in the systems.
A giant magnetocaloric effect across the ferromagnetic (FM) to paramagnetic (PM) phase transition was observed in chemically synthesized Co2FeAl Heusler alloy nanoparticles with a mean diameter of 16 nm. In our previous report, we have observed a significant enhancement in its saturation magnetization (Ms) and Curie temperature (Tc) as compared with the bulk counterpart. Motivated from those results, here, we aim to explore its magnetocaloric properties near the Tc. The magnetic entropy change shows a positive anomaly at 1252 K. Magnetic entropy change increases linearly with the magnetic field, and a large value of ~15 J/Kg-K is detected under a moderate field of 14 kOe. It leads to a net relative cooling power of 89 J/Kg for the magnetic field change of 14 kOe. To confirm the nature of magnetic phase transition, a detailed study of its magnetization is performed. The Arrott plot and nature of the universal curve conclude that FM to PM phase transition in the present system is of second-order.
This work reports on the elastic and electronic properties of the newly discovered superconductor Th2NiC2 (A .Machado, et al., Supercond. Sci. Technol. 25 (2012) 045010) as obtained within ab initio calculations. We found that Th2NiC2 is mechanically stable and it will behave as a ductile material exhibiting enhanced elastic anisotropy in shear and a rather low hardness Our data reveal that for Th2NiC2 the Fermi level is located in a deep DOS minimum and the experimentally observed increase in TC in the sequence Th2NiC2 -> Th1.8Sc0.2NiC2 may be explained by the growth of N(EF). We also speculate that (i) an increase in the hole concentration will promote exchange splitting of Ni 3d bands, therefore the hole-doped Th2NiC2 should have a certain concentration border, where a phase transition from the superconducting to the magnetic state will be expected, and (ii) an increase in N(EF) (and, probably, in TC) for Th2NiC2-based materials may be also achieved by an alternative way: by electron doping - for example, by partial substitution of V for Th or Cu for Ni, as well as by partial substitution of N for C with the formation of Th-Ni carbonitrides like Th2NiC2-xNx.
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