Search

Tunability of structural, magnetic and electronic properties of Co

Spherical nanoparticles (NPs) of size 14 nm, made of intermetallic Fe2CoAl (FCA) Heusler alloy, are synthesized via the co-precipitation and thermal deoxidization method. X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns confirm that the present nanoalloy is crystallized in A2-disordered cubic Heusler structure. Magnetic field (H) and temperature (T) dependent magnetization (M) results reveal that the NPs are soft ferromagnetic (FM) with high saturation magnetization (Ms) and Curie temperature (Tc). Fe2CoAl nanoalloy does not follow the Slater Pauling (SP) rule, possibly because of the disorder present in the system. We also investigate its magnetic phase transition (MPT) and magnetocaloric (MC) properties. The peak value of the magnetic entropy change vs T curve at a magnetic field change of 20 kOe corresponds to about 2.65 J/kg-K, and the observed value of refrigeration capacity (RCP) is as large as 44 J/kg, suggesting a large heat conversion in magnetic refrigeration cycle. The Arrott plot and the nature of the universal curve accomplish that the FM to paramagnetic (PM) phase transition in Fe2CoAl nanoalloy is of second-order. The present study suggests that the Fe2CoAl nanoscale system is proficient, useful and a good candidate for the spintronics application and opens up a window for further research on full-Heusler based magnetic refrigerants., Comment: 25 pages, 9 figures, 2 tables

Experimental and computational studies were performed on Co2FeGe Heusler alloy. It was found that the alloy has very high experimental magnetic moment of 6.1 muB/f.u., curie temperature of 1073K and very high spin-wave stiffness constant of 10.4 nm2-meV, which indicates that the magnetic moment is very high and do not vary with change in temperature in the range 0-300K. The alloy strictly follows Slater-Pauling (SP) rule and the minor experimental deviation from its SP value is justified by doing full-potential density functional calculations which gives more accurate result when electron-electron correlation parameter (U) is taken into account with conventional GGA method. Effect of lattice strain and electron correlation on individual atomic moments, total magnetic moment and spin-polarization is studied in detail and can be concluded that they have a role in the deviation of the experimental results from the expected theoretical values.

Tunability of structural, magnetic and electronic properties of Co2FeGe Heusler alloy is experimentally demonstrated by doping Ti in the Fe site (i.e. Co2Fe1−x Ti x Ge), followed by in-depth first principle calculations. Co2FeGe in its pure phase shows very high saturation magnetization, Curie temperature and spin-wave stiffness constant which were reported in our earlier work. With gradual increase in Ti doping concentration (x = 0.5 and 0.75), the experimental saturation magnetization is found to be decreased to 4.3 μ B/f.u. and 3.1 μ B/f.u. respectively as compared to the parent alloy (x = 0) having the saturation magnetization of 6.1 μ B/f.u. Variation of spinwave stiffness constant is also studied for different x and found to be decreasing from peak value of 10.4 nm2 meV (for x = 0) to the least value of 2.56 nm2 meV for x = 0.5. Justification of the experimental results is given with first principle calculations. Computational phase diagram of the alloys is found in terms of formation energy showing that the doping in Fe site (i.e. Co2Fe1−x Ti x Ge) is more stable rather than in Co site (i.e. Co2−x FeTi x Ge). The change in magnetic moment and half-metallicity with Ti doping concentration is better explained under GGA + U approach as compared to GGA approach signifying that the electron–electron correlation (U) has a distinct role to play in the alloys. Effect of variation of U for Ti atom is studied and optimized with reference to the experimental results. The dynamical stability of the Co2Fe1−x Ti x Ge alloy crystal structure is explained in terms of phonon dispersion relations and the effect of U on the phonon density of states is also explored. Close agreement between the experimental and theoretical results is observed.

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., 12 pages, 6 Figures

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., 23 pages, 7 figures, one Table

In this report, Fe2CoAl Heusler alloy is selected as a target to study site-preferences of atoms by first-principles calculations using WIEN2k code. It has been observed that Fe2CoAl (FCA) Alloy tend to form XA-I type structure (Hg2CuTi-type) which is more energetically favorable and also much stable compared to XA-II and XA-III type structures. Further, we observed that the total magnetic moment Mt (μB/f.u.) shows a strong dependence on site preferences of Co in all available Wyckoff sites. However, this compound does not show a half-metallic character but a metallic behavior has been observed for both spin direction at Fermi level EF, still, it can be used as the spin-polarized material for spintronics application.