The Orbital Moment And Anisotropy Energy Of Free Cobalt Nanoclusters

Nano-materials, because of their unique structural characteristics and excellent chemical and physical properties, not only can effectively improve the electrical, optical, and magnetic properties of functional components, but also can meet the urgent needs for the development of components miniaturization. Nanoclusters composed of several or even thousands of atoms are in the limit scale of low-dimensional materials, and structural materials consisted of transition metal elements with unfulfilled d shell usually leads to a certain magnetic moment. Recently, it’s reported that stronger magnetic moment and higher magnetic anisotropy have been found in transition metal nanoclusters than that in its bulk phase. Thus, the transition metal nanocluster has promised great potential opportunities for its popularization and application in high-density magnetic storage, bio-medicine, information storage density, micro-nano devices and other areas.Utilizing Stern-Gerlach deflection experiments and XMCD absorption spectrum experiments, recently, researchers have measured the atomic spin and orbital magnetic moments, the total magnetic moment of the transition metals(such as Fe, Co, Ni, etc.), just to found that the atomic orbital magnetic moment in clusters did not "quench" as it did in the bulk phase, the atomic spin in nanocluster is 1.4 to 2.4 times larger, and the atomic orbital magnetic moments is several times to dozens of times, or even close to a hundred times stronger than that in the bulk phase. On the one hand, different datas from experimental measurements are just qualitatively consistent with each other, but are quite different quantitatively, yet the experimentally measured values of the atomic orbital moment have not been reasonably comprehended in theory, urgently needing to master the evolution law of the orbital moment; On the other hand, according to Bruno formula MAE=-ξΔμorb/4μB, nanoclusters with large orbital magnetic should have strong magnetic anisotropy energy(MAE) in the role of spin-orbit coupling effect, this means that such small clusters consisted of ferromagnetic elements show great potential to be a magnetic memory cell.Basing on first-principles DFT method, and considering scalar relativistic DFT+SR and the spin-orbit coupling effect DFT +SOC, we have systematically studied in this thesis the dependence of cluster size on the structure evolution, the spin and orbit magnetic moment, the evolution of the magnetic anisotropy energy, as well as the influence of the spin-orbit coupling effect on the structure and magnetic properties of Co_n(n=10-24) clusters. Co_nsequently the following trends were identified:(1) For media sized clusters(n>10), structures of hexagonal close-packed(hcp) layer-like stacking pattern are more stable than their icosahedral evolution type based on structural unit of pentagonal bipyramid. Difference of average binding energe between them is about 0.01-0.11 e V/atom.(2) The average spin magnetic moment of all the structures we discussed is approximately 1.73-2.60μB/atom, to be more specific, The average spin magnetic moment of Co_n(n=2-9) and Co_n(n=10-24) clusters are 1.89-2.60μB/atom and 1.73-2.20μB/atom. It was weakly dependent on the sizes, the structural configurations, and the local environments of the clusters.(3) Extraordinary, the orbital magnetic moments of the clusters have a great dependency on the size, the direction of magnetization, the local coordination number,and the symmetry. Exceptionally, Co2 dimer, in the limit of cluster size, has a remarkablely large orbital magnetic moment, measure up to 0.78μB/atom. With the cluster size increasing, the average coordination number of Co atoms in clusters increases, then the average orbital moment decreases. When the size of the system is more than 10, the average orbital moment will reduced its dependence on its system size and geometric configuration. It will remain in a range of 0.09-0.13μB/atom, and slightly oscillate around 0.10μB/atom.(4) Spin-orbit coupling effect slightly influences the stability of the clushter system, but universally stretches the bond lengths between Co atoms(<0.01?), Meanwhile, the average binding energe will reduce entirely(~0.12 e V/atom), thus make a slight reduce of the stability.(5) Magnetic anisotropy calculations show that the cluster system generally gets large differences in its orbital moment in in different direction of magnetization, but share the same spin magnetic moment. For layer-like structures, the easy axis of magnetization inclines to parallel to layer plane, and the hard axis is perpendicular to the plane. Magnetic anisotropy energy of the clusters varies with cluster size and exhibits a considerable oscillation, but much larger than the bulk value.(6) The spin and orbital magnetic moments present a highly recognizable identity for the structures of layer-like and icosahedral evolution ones. The corresponding value of Layer-like structures oscillate more pronounced. Meanwhile, the icosahedral configurations prone to behave in stronger anisotropy, its anisotropy energy is generally greater than the layer-like configurations, that is a difference of 2-12 me V.