Hongkuan Yuan

Density-functional study of small neutral and cationic bismuth clusters Bin and Bin+(n=2–24)

Density-functional theory with scalar-relativistic pseudopotential and a generalized gradient correction is used to calculate the neutral and cationic Bi(n) clusters (2< or =n< or =24), with the aim to elucidate their structural evolution, relative stability, and magnetic property. The structures of neutral Bi clusters are found to be similar to that of other group-V elemental clusters, with the extensively studied sizes of n=4 and 8 having a tetrahedron and wedgelike structure, respectively. Generally, larger Bi clusters consist of a combination of several stable units of Bi(4), Bi(6), and Bi(8), and they have a tendency to form an amorphous structure with the increase of cluster sizes. The curves of second order energy difference exhibit strong odd-even alternations for both neutral and cationic Bi clusters, indicating that even-atom (odd-atom) sizes are relatively stable in neutral clusters (cationic clusters). The calculated magnetic moments are 1micro (B) for odd-atom clusters and zero for even-atom clusters. We propose that the difference in magnetism between experiment and theory can be greatly improved by considering the orbital contribution. The calculated fragmentation behavior agrees well with the experiment, and for each cationic cluster the dissociation into Bi(4) or Bi(7) (+) subclusters confirms the special stability of Bi(4) and Bi(7) (+). Moreover, the bond orders and the gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital show that small Bi clusters would prefer semiconductor characters to metallicity.

The spin and orbital moment of Fen (n = 2–20) clusters

Complementary to the recent experimental finding that the orbital magnetic moment is strongly quenched in small Fe clusters [M. Niemeyer, K. Hirsch, V. Zamudio-Bayer, A. Langenberg, M. Vogel, M. Kossick, C. Ebrecht, K. Egashira, A. Terasaki, T. Möller, B. v. Issendorff, and J. T. Lau, Phys. Rev. Lett. 108, 057201 (2012)], we provide the theoretical understanding of the spin and orbital moments as well as the electronic properties of neutral and cation Fen clusters (n = 2-20) by taking into account the effects of strong electronic correlation, spin-orbit coupling, and noncollinearity of inter-atomic magnetization. The generalized gradient approximation (GGA)+U method is used and its effluence on the magnetic moment is emphasized. We find that without inclusion of the Coulomb interaction U, the spin (orbital) moments have an average value between 2.69 and 3.50 μB/atom (0.04 and 0.08 μB/atom). With inclusion of U, the magnetic value is between 2.75 and 3.80 μB/atom (0.10 and 0.30 μB/atom), which provide an excellent agreement with the experimental measurements. Our results confirm that the spin moments are less quenched, while the orbital moments are strongly quenched in small Fe clusters. Both GGA and GGA+U functionals always yield collinear magnetic ground-state solutions for the fully relaxed Fe structures. Geometrical evolution, as a function of cluster size, illustrates that the icosahedral morphology competes with the hexagonal-antiprism morphology for large Fe clusters. In addition, the calculated trends of ionization potentials, electron affinities, fragment energies, and polarizabilities generally agree with respective experimental observations.

Tunable magnetism and half-metallicity in bulk and (1 0 0) surface of quaternary Co2MnGe1−xGax Heusler alloy

The structural, magnetic and half-metallic properties of the bulk and (1u20090u20090) surface of quaternary Heusler alloy Co2MnGe1−xGax are investigated from the first-principles calculations. For the bulk, the lattice constant and total magnetic moment follow the Vegard law and Slater–Pauling rule well, respectively. Except for Co2MnGa, the Co2MnGe1−xGax series are half-metallic. Because the Fermi level of Co2MnGe0.5Ga0.5 is just located at the middle of the minority-spin gap, we predict that it bears the most robust half-metallicity as against remnant doped alloys. As for the Co2MnGe1−xGax(1u20090u20090) surface, the analyses on relaxed atomic positions and surface energies reveal that Co–Ge and Co–Ga bonding are more favourable than Co–Mn bonding and the terminations involving surface Mn atoms are more stable than CoCo terminations. By comparing with the bulk values, the surface Co and Mn magnetic moments are enhanced obviously. The calculated PDOS of all accessible ideal surfaces show that the half-metallicity observed in bulk has been destroyed by the surface states, which is a possible reason why the tunnel magnetoresistence steeply drops as temperature increases. However, in the pure atomic terminations the surface properties can be slightly adjusted by the Ga-doped concentrations in bulk through the dipolar interaction. As a result, in the MnMn termination of Co2MnGe0.5Ga0.5(1u20090u20090) the spin polarization of 1u20090u20090% is detected, indicating that in the pure Mn atomic termination the half-metallicity of the (1u20090u20090) surface can remain if the corresponding bulk presents excellent half-metallic stability. Thus we predict that this thin film will present a higher potential for applications in ferromagnetic electrodes.

Geometrical structure and spin order of Gd13 cluster

The spin-polarized generalized gradient approximation to the density-functional theory has been used to determine the lowest energy structure, electronic structure, and magnetic property of Gd(13) cluster. Our results show that the ionic bonding is combined with the covalent characteristics in stabilizing the Gd cluster. The ferrimagnetic icosahedron is found to be the lowest energy configuration, in which the centered Gd atom couples antiferromagnetically with the rest Gd atoms surrounding it. No spin non-collinear evidence has been detected in our calculations. It is identified that the local magnetic moments of Gd atom are about 8 μ(B) regardless of geometrical structure. Finally, the comprehensive electronic structure analyses show that the indirect long-range magnetic coupling between the polarized 4f is mediated by the polarization of 5d, 6s, and 6p conduction electrons, which is the typical Ruderman-Kittel-Kasuya-Yosida interactions.

Thermodynamic stability, magnetism and half metallicity of Mn2CoAl/GaAs(0 0 1) interface

Interface properties of the heterojunction which is composed of the inverse Heusler alloy MnCoAl and semiconductor GaAs are investigated by employing the first-principles density functional simulations. Two kinds of interface structures, namely the top-type and bridge-type structure by connecting termination of nine MnCoAl layers to the top of the As-terminated GaAs layer and bridge site between interface As atoms are respectively built. Our calculations reveal that, as for the structure with the same interface atoms, different atoms sitting directly on top of the interface As atom will lead to different interface magnetism and electronic structures. The calculated phase diagram reveals that the top-type structure including natural MnCo or MnAl termination is stable only when the interface Mn or interface Al atom directly locates on top of the As atom. Besides, bridge-type and top-type structures containing a pure Mn interface are always thermodynamically accessible regardless of values of the chemical potential of Mn and Co. The atom-resolved spin magnetic moments of most interface magnetic atoms are enhanced due to the rehybridization caused by symmetry breaking at the interface. Further analyses on electronic structures indicate that, owing to the interface effect, the interface half metallicity of all structures are completely destroyed. However, the top-type structure with MnAl termination where the interface Al atom directly sits on top of the As atom preserves the highest interface spin polarization of 80%, indicating that it has more advantages in spintronics application than other atomic terminations.

The effect of disorder on the electronic and magnetic properties of Mn2CoAl/GaAs heterostructures

We study the effect of disorder, including swap and antisite, on the electronic and magnetic properties of heterostructures by using extensive first-principles calculations within density functional theory. Thirteen kinds of swap disorders and sixteen kinds of antisite disorders are proposed and studied comprehensively. Our calculation reveals that disorders at the interface have low formation energies, indicating that disorders are most likely to appear at the interface instead of the deep layer. Among all kinds of disorders, Mn1(Al) (where the interface Mn is occupied by an Al atom) and Mn1(As) (where the interface Mn is occupied by an As atom from a GaAs slab) antisite disorders possess the lowest formation energies. This shows that the interface Mn has a higher probability of being replaced by an Al atom, and that an As atom from a GaAs slab easily diffuses into a Mn2CoAl slab and occupies the position of the interface Mn. Moreover, further study on the interface electronic structure reveals that interface spin polarization suffers dramatic reduction due to Mn1(Al) and Mn1(As) antisite disorders. It can be deduced that the interface state, together with Mn1(Al) and Mn1(As) antisite disorders, may be the main causes of the low TMR ratio of Mn2CoAl/GaAs heterostructures.

Density functional calculations for structural, electronic, and magnetic properties of gadolinium-oxide clusters

Gadolinium-oxide clusters in various sizes and stoichiometries have been systematically studied by employing the density functional theory with the generalized gradient approximation. The clusters in bulk stoichiometry are relatively more stable and their binding energies increase with the increasing size. Stoichiometric (Gd2O3)n clusters of n = 1–3 prefer cage-like structures, whereas the clusters of n = 4–30 prefer compact structures layered by wedge-like units and exhibit a rough feature toward the bulk-like arrangement with small disorders of atomic positions. The polyhedral-cages analogous to carbon-fullerenes are stable isomers yet not the minimum energy configurations. Their stabilities can be improved by embedding one oxygen atom or a suitable cage to form core-shell configurations. The mostly favored antiferromagnetic couplings between adjacent Gd atoms are nearly degenerated in energy with their ferromagnetic couplings, resulting in super-paramagnetic characters of gadolinium-oxide clusters. The...

Electronic structural and magnetic properties of Mn5Ge3 clusters.

Theoretical understanding of the stability, ferromagnetism, and spin polarization of Mn5Ge3 clusters has been performed by using the density functional theory with generalized gradient approximation for exchange and correlation. The magnetic moments and magnetic anisotropy energy (MAE) have been calculated for both bulk and clusters, and the enhanced magnetic moment as well as the enlarged MAE have been identified in clusters. The most attractive achievement is that Mn5Ge3 clusters show a fine half-metallic character with large energy scales. The present results may have important implications for potential applications of small Mn5Ge3 clusters as both emerging spintronics and next-generation data-storage technologies.

Magnetic moment and magnetic anisotropy of Ge3Mn5 thinfilms on Ge(111) substrate: A density functional study

Magnetism and magnetic anisotropy energy (MAE) of the Ge3Mn5 bulk, free-standing surface, and Ge3Mn5(001)|Ge(111) thinfilms and superlattice have been systemically investigated by using the relativistic first-principles electronic structure calculations. For Ge3Mn5 adlayers on Ge(111) substrates within Mn1 termination, the direction of magnetization undergoes a transition from in-plane at 1 monolayer (ML) thickness (MAE = -0.50 meV/ML) to out-of-plane beginning at 3 ML thickness (nearly invariant MAE = 0.16 meV/ML). The surficial/interfacial MAE is extracted to be 1.23/-0.54 meV for Mn1-termination; the corresponding value is 0.19/1.03 meV for Mn2/Ge-termination; the interior MAE is averaged to be 0.09 meV per ML. For various Ge3Mn5 systems, the in-plane lattice expansion and/or interlayer distance contraction would enhance the out-of-plane MAE. Our theoretical magnetic moments and MAEs fit well with the experimental measurements. Finally, the origination of MAE is elucidated under the framework of second-order perturbation with the electronic structure analyses.

[TM13@Bi20]− clusters in three-shell icosahedral matryoshka structure: being as superatoms

Using the density functional theory (DFT) method, the 33-atom intermetalloid [TM13@Bi20]− clusters (TM = 3d, 4d), which are composed of Bi20 pentagonal dodecahedra surrounding TM12 icosahedra with a single TM atom at the center, have been systematically examined to explore the possibility of clusters being as superatoms. The results show that most TM13 clusters can be attractively encapsulated into Bi20 cage to form a stable core–shell configuration, exhibiting an interesting progression of thermal stability along the 3d and 4d periods. Taking into account the structural stability (binding energy, embedding energy, and core–shell interaction) as well as the chemical stability (HOMO–LUMO gap), we proposed that [TM13@Bi20]− clusters with Ti and Mn doping in 3d series (Zr and Ag doping in 4d series) are specially stable and to be the protyle superatoms. For such systems, the molecular orbital shapes and energy alignments are in analogy with the atomic patterns, coinciding the general characters of superatomic orbitals. The closed core superatomic shell together with the partially-filled valence superatomic shell configuration leads to magnetic moment in stable [TM13@Bi20]−, e.g., [Mn13@Bi20]− cluster with the half-filled subshell can be assigned as a magnetic superatom owning to its modest HOMO–LUMO gap of 0.37 eV and large magnetic moment of 36 μB. The exchange-splitting in TM-3d states is found to be the driving force for the improvement of exchange-splitting of superatomic states.