First principle study of the structural,electronic and magnetic properties of Fe,Co, Ni atomic nanochains encapsulated in single walled and double walled beryllium oxygen nanotubes

By using first-principles calculations within the density function theory, the structural, electronic and magnetic properties of transition metals TM (TM=Fe, Co and Ni) atomic chains wrapped in the single walled and double walled BeO nanotubes are investigated. It is found that all these TM chains @ BeONTs systems are ferromagnetic (FM) and a spin splitting between spin up and down is observed. The high magnetic moment and spin polarization of the TM @ BeONTs systems imply that it can be used as magnetic nanostructure and future applications in permanent magnetism, magnetic recording, and spintronics.     

Tailoring exchange interactions in engineered nanostructures: an ab initio study

We present a novel approach to spin manipulation in atomic-scale nanostructures. Our ab initio calculations clearly demonstrate that it is possible to tune magnetic properties of subnanometer structures by adjusting the geometry of the system. By the example of two surface-based systems we demonstrate the following. (i) The magnetic moment of a single adatom coupled to a buried magnetic Co layer can be stabilized in either a ferromagnetic or an antiferromagnetic configuration depending on the spacer thickness. It is found that a buried Co layer has a profound effect on the exchange interaction between two magnetic impurities on the surface. (ii) The exchange interaction between magnetic adatoms can be manipulated by introducing artificial nonmagnetic Cu chains to link them.     

Spin polarization and charge transfer of Co nanoclusters coated with CO molecules

We have investigated magnetic properties of Co clusters coated with CO molecules by first-principles density functional calculations. Total magnetic moment of the system decreases with the increase of CO molecule concentration regardless of adsorption sites for CO molecules. Spin polarization slightly increases as CO molecules are adsorbed on the most stable sites of Co cluster, which is caused by the increased local spin polarization of p-electrons due to charge transfer from Co clusters to CO molecules.     

Structural,electronic and magnetic properties of hcp Fe,Co and Ni nanowires encapsulated in zigzag carbon nanotubes

The structural, electronic and magnetic properties of hcp transition metal (TM = Fe, Co or Ni) nanowires TM₄ encapsulated inside zigzag nanotubes C(m, 0) (m = 7, 8, 9, 10, 11 or 12), along with TM _n (n = 4, 10 or 13) encapsulated inside C(12, 0), have been systematically investigated using the first-principle calculations. The results show that the TM nanowires can be inserted inside a variety of zigzag carbon nanotubes (CNTs) exothermically, except from the systems TM₄@(7, 0) and TM₁₃@(12, 0) which are endothermic. The charge is transferred from TM nanowires to CNTs, and the transferred charge increases with decreasing CNT diameter or increasing nanowire thickness. The magnetic moments of hybrid systems are smaller than those of the freestanding TM nanowires, especially for the atoms on the outermost shell of the nanowires. The magnetic moment per TM atom of TM/CNT system increases with increasing CNT diameter or decreasing nanowire thickness. Both the density of states and spin charge density analysis show that the spin polarization and the magnetic moments of all hybrid systems mainly originate from the TM nanowires, implying these systems can be applied in magnetic data storage devices.     

第一性原理研究Co2MnSi和Co2MnGe半金属与磁性的稳定性

基于广义梯度近似密度泛函和全势能线性缀加平面波方法,对Co2MnSi和Co2MnGe在晶格常数发生变化的情况下进行电子结构和磁矩的自旋极化计算,得到了它们的自旋态密度分布以及总磁矩和各原子磁矩。计算结果的分析表明：（1）Co2MnSi 和Co2MnGe具有半金属性质;（2）晶格常数的改变分别为-5%~ 4%和-6%~1%时,Co2MnSi 和 Co2MnGe仍保持稳定的半金属质性;（3）Co2MnSi 和Co2MnGe的总磁矩为5.00µB/formula。总磁矩主要来源于Mn和Co的原子磁矩,Si和Ge的原子磁矩对总磁矩的贡献极小而且为负值。（4）Co2MnSi 和 Co2MnGe的晶格常数变化分别为-6% ~ 6%和-7%~ 4%时,虽然各原子磁矩都发生了变化,但是它们总磁矩稳定于5.00µB/formula.     

On the origin of the giant magnetic moments of Fe and Co in Cs Films

To unravel the mystery of the recently observed giant magnetic moments of Fe and Co in Cs films, orbital-polarization corrected relativistic spin density functional calculations have been performed. Unlike other transition–metal systems where the orbital magnetic moments are quenched, Fe and Co in Cs as well as in other alkali metals are found to possess a giant orbital moment of 2–3 μ_B along with a large spin moment. Also, these free atom-like spin and orbital magnetic moments in Cs would not be squashed under large lattice contractions up to 23% around the impurity atoms. The induced moments on the host atoms are small. The results offer an explanation for the origin of the giant magnetic moments of Fe and Co in Cs films.     

Effects of oxygen vacancy location on the electronic structure and spin density of Co-doped rutile TiO₂ dilute magnetic semiconductors

According to density functional theory (DFT) using the plane wave base and pseudo-potential, we investigate the effects of the specific location of oxygen vacancy (VO) in a (Ti,Co)O 6 distorted octahedron on the spin density and magnetic properties of Co-doped rutile TiO2 dilute magnetic semiconductors. Our calculations suggest that the V O location has a significant influence on the magnetic moment of individual Co cations. In the case where two Co atoms are separated far away from each other, when the V O is located at the equatorial site of a Co-contained octahedron, the ground state of the two Co cations is d6 (t3 2g ↑, t 3 2g ↓) without any magnetic moment. However, if the V O is located at the apical site, these two Co sites have different ground states and magnetic moments. The spin densities are also observed to be modified by the exchange coupling between the Co cations and the location of V O . Some positive spin polarization is induced around the adjacent O ions.     

CoAlN(N=2-11)团簇基态结构的稳定性和磁性

采用密度泛函理论中的广义梯度近似(GGA),对CoAlN(N=2-11)团簇进行构型优化和磁性计算.在考虑自旋多重度的情况下得到了团簇的平衡构型及基态结构,并重点讨论了Co原子的sp-d杂化效应对体系稳定性和磁性的影响.结果表明:N为偶数时,团簇的基态为自旋双重态,Co原子磁矩随N增加整体上呈减小趋势;N为奇数时(除N=5为自旋三重态外),团簇的基态为自旋单重态,Co原子磁矩为零.N≥8时,Co原子陷入主团簇内部,体系的对称性降低,稳定性增强;轨道杂化增强了体系的稳定性,但减小了双重态和三重态体系中Co原子的磁矩;N=3,8,10是团簇CoAlN(N=2.11)的幻数.     

x射线磁性圆二色吸收谱分析Co膜厚度对原子磁矩和自旋磁矩的影响

利用软x射线磁性圆二色吸收谱(XMCD)研究了Si衬底上沉积的不同厚度的Co膜的轨道磁矩和 自旋磁矩.样品是磁控溅射方法制备的，膜的厚度分别是2nm,10nm和30nm，并在表面覆盖0.8 —1nm厚的金膜防止样品的氧化.根据XMCD求和定则计算得到的轨道磁矩和自旋磁矩分别是0. 249—0.195μ_B(玻尔磁子)和1.230—1.734μ_B.随着膜厚的减小，C o原子的轨道磁矩增加，而自旋磁矩下降.轨道磁矩与总磁矩的比值由0.101上升至0.168，即 2nm膜中Co原子的轨道磁矩对总磁矩的贡献比30nm膜中Co原子的大了83%. 关键词： x射线磁性圆二色 磁性薄膜 轨道磁矩和自旋磁矩 厚度效应     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Doped coupled frustrated spin-1/2 chains with four-spin exchange

The role of various magnetic interchain couplings is investigated by numerical methods in doped frustrated quantum spin chains. A nonmagnetic dopant introduced in a gapped spin chain releases a free spin-1/2 soliton. The formation of a local magnetic moment is analyzed in terms of soliton confinement. A four-spin coupling which might originate from cyclic exchange is shown to produce such a confinement. Dopants on different chains experience an effective space-extended nonfrustrating pairwise spin interaction.     

Observing spin polarization of individual magnetic adatoms

We have used spin-polarized scanning tunneling spectroscopy to observe the spin polarization state of individual Fe and Cr atoms adsorbed onto Co nanoislands. These magnetic adatoms exhibit stationary out-of-plane spin polarization, but have opposite sign of the exchange coupling between electron states of the adatom and the Co island surface state: Fe adatoms exhibit parallel spin polarization to the Co surface state while Cr adatoms exhibit antiparallel spin polarization. First-principles calculations predict ferromagnetic and antiferromagnetic alignment of the spin moment for individual Fe and Cr adatoms on a Co film, respectively, implying negative spin polarization for Fe and Cr adatoms over the energy range of the Co surface state.     

A density functional theory study of the electronic structures and magnetic properties of Fe（1-x）Cox alloy nanowires encapsulated in （10,0） carbon nanotubes

Under the generalized gradient approximation, the electronic structures and magnetic properties of Fe_(1-x)Co_x alloy nanowires encapsulated inside zigzag (10,0) carbon nanotubes (CNTs) are investigated systematically using firstprinciple density functional theory calculations. For the fully relaxed Fe_(1-x)Co_x/CNT structures, all the C atoms relax outwards, and thus the diameters of the CNTs are slightly increased. Formation energy analysis shows that the combining processes of all Fe_(1-x)Co_x/CNT systems are exothermic, and therefore the Fe_(1-x)Co_xalloy nanowires can be encapsulated into semiconducting zigzag (10,0) CNTs and form stable hybrid structures. The charges are transferred from the Fe_(1-x)Co_xnanowires to the more electronegative CNTs, and the Fe-C/Co-C bonds formed have polar covalent bond characteristics. Both the spin polarization and total magnetic moment of the Fe_(1-x)Co_x/CNT system are smaller than those of the corresponding freestanding Fe_(1-x)Co_xnanowire, and the magnetic moment of the Fe_(1-x)Co_x/CNT system decreases monotonously with increasing Co concentration, but the Fe_(1-x)Co_x/CNT systems still have a large magnetic moment, implying that they can be utilized in high-density magnetic recording devices.     

Theoretical study of the magnetic moments and anisotropy energy of CoRh nanoparticles

The role of size, structure and chemical order on the magnetic moments and magnetic anisotropy energy (MAE) of CoRh nanoparticles are studied in the framework of a self-consistent real-space tight-binding method. Our results show that a Rh core in a geometry having a large surface/volume ratio and with Co–Rh mixing at the interface is the most likely chemical arrangement. A local analysis reveals that the orbital and spin moments at the Co–Rh interface are largely responsible for the increase of the magnetic moments and magnetic anisotropy. Moreover, the local moments induced at the Rh atoms, which amount to about 20% of the moment per Co atom [ μ_Rh = (0.2–0.3) μ_B] and the orbital moments of Co atoms play a crucial role on the interpretation of experiment. The results are discussed in the context of the interplay between chemical order and magnetic properties.     

Synthesis,structure, and properties of Ba9Co3Se(15) with one-dimensional spin chains

A new compound with one-dimensional spin chains, Ba9Co3Se(15), was synthesized under high pressure and high temperature conditions and systematically characterized via structural, transport and magnetic measurements. Ba9Co3Se(15) crystallizes in a hexagonal structure with the space group P-6c2(No. 188) and lattice constants of a = b = 9.6765 ? and c = 18.9562 ?. The structure consists of trimeric face-sharing octahedral CoSe6 chains, which are arranged in a triangular lattice in the ab-plane and separated by Ba atoms. The distance of the nearest neighbor of CoSe6 chains is very large, given by the lattice constant a = 9.6765 ?. The Weiss temperature Tθ associated with the intra-chain coupling strength is about -346 K. However, no long-range magnetic order but a spin glass transition at ~ 3 K has been observed. Our results indicate that the spin glass behavior in Ba9Co3Se(15) mainly arises from the magnetic frustration due to the geometrically frustrated triangular lattice.     

Effect of Coulomb repulsion on spin and orbital magnetism of cobalt–benzene complex: A relativistic density functional study

We have demonstrated electronic configurations and magnetic properties of single Co adatom on benzene (Bz) molecule in the framework of relativistic density functional theory. A sequence of fixed spin moment (FSM) calculations were carried out with and without Coulomb repulsion (U). We have investigated that varying the strength of Coulomb repulsion results to different equilibrium positions for the Co adatom on benzene molecule. It was shown that inclusion of the on-site Coulomb repulsion in the Co 3d orbitals affects significantly the geometry of Co–Bz complex. We also found two stable low-spin and high-spin multiplicities for the complex. The nature of the high-spin configuration was explained according to the Hubbard electron–electron correlation in 3d shell of the Co adatom. Our FSM results indicate that the high-spin state is a global minimum in the presence of Hubbard parameter U. The relativistic spin–orbit coupling and using orbital polarization correction induce considerable orbital magnetism in both low and high spin states of the Co–Bz complex. We have also calculated magnetic anisotropy energies for two spin states and we found out that an out-of-plane magnetic orientation of Co adatom is more favorable in the low spin state whereas the Coulomb repulsion (U = 2 eV and U = 4 eV) predicts an in-plane magnetic orientation for Co adatom. Our findings can be implicitly taken into account for the extended system of added single Co atom on graphene.     

First-Principles Study of Magnetic Properties of TM_(13) and TM_(13)@Au_(32) Clusters(TM = Mn,Co)

The structural and magnetic properties of TM_(13 )and TM_(13)@Au_(32 )clusters(TM=Mn,Co)are studied by firstprinciples calculations.We find that the Au_(32 )cluster can tune not only the magnetic moment but also the magnetic coupling properties between the TM atoms of the TM cluster.The Au_(32 )cluster can increase the net magnetic moment of Mn_(13 )clusters while reducing that of Co_(13 )clusters.The interaction between Au and Mn atoms induces more Mn atoms to form spin parallel coupling,resulting in an increase of the total magnetic moment of Mn_(13 )clusters,while for the Co_(13 )clusters,the interaction between Au and Co atoms does not change the magnetic coupling states between the Co atoms,but reduces the magnetic moment of the Co atoms,leading to a decrease of the total magnetic moment of this system.Our findings indicate that the encapsulation of Au_(32 )clusters can not only raise the chemical stability of TM clusters,but also can tune their magnetic coupling properties and magnetic moment,which enables such systems to be widely applied in fields of spintronics and medical science.     

X-ray magnetic circular dichroism study of the induced spin polarization of Cu in Co/Cu and Fe/Cu multilayers

We present an x-ray magnetic circular dichroism (XMCD) study of Co/Cu and Fe/Cu multilayers, finding that the Cu atoms in these structures exhibit an induced magnetic moment in the d shell. The average Cu spin moment is shown to fall-off inversely with the thickness of the Cu layer. Further, for comparable Cu layer thicknesses, the Cu moments in Fe/Cu and Co/Cu multilayers are found to be nearly equal, despite the fact that the Cu layers in the Co/Cu multilayers are shown to be fee while those in the Fe/Cu structures are bcc. These observations suggest that the induced moment is primarily situated at the Co/Cu and Fe/Cu interfaces and is resultant from short range chemical hybridization between the ferromagnetic and Cu atoms. Results from a local spin density functional theory are presented and found to be in excellent agreement with experimental observations. These results indicate that the Cu d electrons play a central role in mediating the exchange coupling between successive ferromagnetic layers.     

Co含量对Fe3Si合金磁学性能的影响

采用基于密度泛函理论（DFT）的赝势平面波法,对过渡金属Co掺杂Fe₃Si进行几何结构优化,计算Co含量对Fe₃Si合金磁学性质的影响.研究表明：Fe_3-xCo_xSi的磁性主要来源于过渡金属元素Fe和Co,并且相对于A、C位的Fe和Co原子,B位Fe的原子磁矩较大;在0≤x≤0.75范围内,随着Co含量的增大,Fe_3-xCo_xSi的总磁矩缓慢减小,在0.75≤x≤1.5时,其总磁矩迅速地增大;A、C位Fe原子的磁矩和合金总磁矩的变化趋势相同,Co原子的磁矩随着Co含量的增大缓慢地增大,原子的磁矩变化与自旋向上和向下方向的电荷转移有关.     

Effects of oxygen vacancy location on the electronic structure and spin density of Co-doped rutile TiO2 dilute magnetic semiconductors

According to density functional theory （DFT） using the plane wave base and pseudo-potential, we investigate the effects of the specific location of oxygen vacancy （Vo） in a （Ti,Co）06 distorted octahedron on the spin density and magnetic properties of Co-doped rutile Ti02 dilute magnetic semiconductors. Our calculations suggest that the Vo location has a significant influence on the magnetic moment of individual Co cations. In the case where two Co atoms are separated far away from each other, when the Vo is located at the equatorial site of a Co-contained octahedron, the ground state of the two Co cations is d6（t3g↑, t23g ↓） without any magnetic moment. However, if the Vo is located at the apical site, these two Co sites have different ground states and magnetic moments. The spin densities are also observed to be modified by the exchange coupling between the Co cations and the location of Vo. Some positive spin polarization is induced around the adjacent O ions.