Structural, And Magnetic Properties Of Co₃W Clusters:A Density Functional Theory Investigation

Magnetic nanoclusters are drawing considerable attentions due to their unique structural,electronic and magnetic properties, which are promising for wide applications in high-density storage, optical devices, magnetic refrigeration, and biomedicine. Especially, Co3 W alloy is a hard magnetic alloy which doesn’t consist of rare earth or precious metals. And it’s not only possessing excellent magnetic properties but also unrestricted by the limited resources. These will provide with the huge potential using and development. In experiments, there will be a high coercivity, saturation magnetization and anisotropy energy when the size of WCo3 alloys are in nanometer scopes. The coercivity is the one represents the quality of magnetic storage materials. Further more,WCo3 alloys are strong wear- and corrosion-resistance, which represents, as magnetic nano-materials, it will be applied under extreme conditions. At the same time, the knowledges of their detailed magnetic, physical and chemical stability, electronic properties, and geometrical structures are essential for further improving their performances in the field of magnetic. In this paper, based on the ratio of the component elements of the alloy block we have designed series of(7)(8)39nWCo(n=1,4,12) spherical nanoclusters. After that we perform a systemically research on the structural trend, electronic, geometrical and magnetic properties of Co3 W alloy nanoclusters by using the density functional theory(DFT) under the framework of generalized gradient approximation(GGA). The results show:1) The designing of the micro structure, researching on geometrical propertiesWe have built a large supercell of the hexagonal crystal WCo3 structure and then cut out spherical skeletons with different specific radius by Materials Studio software. These spherical skeletons will have the structure of short range order and the chemical compositions of the clusters exactly/or closely match the stoichiometry of bulk WCo3. In this study, three clusters sizes consisting of 9, 48, 144 atoms. Using the density functional theory(DFT) under the framework of generalized gradient approximation(GGA), this leads us to the conclusion that surface relaxation of bond lengths of nanospheres and the stability of nanoclusters increase gradually with increasing sizes of these clusters. The average bond lengths of the calculation are consistent with that of the experiment. The HOMO and LUMO orbitals show that are strongly locaized on the peripheralCo-Co bonds and partially on the inside Co atoms. It implies that the charges transfer from W atoms to Co atoms. Analying from Bader population reaches the same conclusion.2) The researching on electronic and magnetic properties of WCo3 alloy nanoclustersUsing spin polarization and spin-orbit coupling of DFT method, the spin and orbital magnetic moments, magnetic anisotropy energy(MAE) and spin magnetic moment influenced by spin-orbit coupling have been obtained. We have got the electronic structure information about numbers of charge population, density of states(DOS) as well as these occupied molecular orbitals.It turns out that there a strong hybridization between Co-3d orbitals and W-5d orbitals. Thus the spins of W atoms split under the influence of Co atoms with the spin splitting. It will result in an antiferromagnetic coupling between W and Co atoms with a small magnetic moment. Furthermore,Co atoms possess 0.398-1.173μBspin magnetic moment(0.018-0.049μBorbital magnetic moment)and W atoms possess 0.04-0.184μBspin magnetic moment(0.003-0.224μBorbital magnetic moment).After calculating different composition ratios of the Co-W nanoclusters, we come to the conclusion that the resulting moment per Co atom decreases(increases) with increasing the amount of W(Co)in the clusters. The conclusion we got that is consistent with the experiment. Among the nanosystems we have calculated, there is 0.049-0.133 me V/atom anisotropy energy in Co-W nanoclusters. It’s values closely match or exceed 0.0335 me V/atom anisotropy energy of the bulk.Based on the magnetic moment, anisotropy energy, and structural stability of Co-W clusters, we speculate that the small size of Co-W nanoparticles will possess potential to be used as a magnetic storage medium with their unique magnetic properties.