Xiaohui Wei

Magnetism of TiO and TiO2 nanoclusters

Nanoclusters of rocksalt TiO, anatase TiO2, and rutile TiO2 were produced by cluster deposition and examined with transmission-electron microscopy, x-ray diffraction, and magnetization measurements. The clusters are all magnetic at room temperature, but the magnetization is structure- dependent. The hysteresis loops show coercivities that are of the order of 100 Oe and all films show a preferential in-plane magnetization direction. The size dependence of the magnetization was investigated for rutile clusters with average sizes from about 15 to 40 nm. The analysis of the measurements indicates that the magnetism is predominantly located near the surface of the clusters and characterized by a nominal value of 7.6 μB/nm2.

Synthesis and magnetic characterizations of manganite-based composite nanoparticles for biomedical applications

We report chemically synthesized highly crystalline lanthanum strontium manganite (LaSrMnO3) and Eu-doped Y2O3 and their composites. The synthesis yields nanoparticles of size 30–40nm. Magnetic measurements performed on nanoparticles and composites show magnetic transition at about 370K with a superparamagnetic behavior at room temperature. The ferromagnetic resonance studies of the nanoparticles show large linewidth due to surface strains. The composite nanoparticles also display luminescent behavior when irradiated with ultraviolet light. The manganites as well their composite with the luminescent nanoparticles may be very useful for biomedical applications.

Magnetic properties of nickel hydroxide nanoparticles

The magnetic properties of 10 nm size Ni(OH)2 nanoparticles prepared by sol-gel method have been studied. The magnetic moments increase with decreasing temperature in a low applied field, which is due to the spin-frozen-like state at low temperatures, and the metamagnetic transition is not clearly observed even in an applied field of 70 kOe due to the size effect. Furthermore, the transition from paramagnetic to antiferromagnetic in the Ni(OH)2 nanoparticles occurs at lower temperature (22 K).

Structural and magnetic evolution of bimetallic MnAu clusters driven by asymmetric atomic migration

The nanoscale structural, compositional, and magnetic properties are examined for annealed MnAu nanoclusters. The MnAu clusters order into the L1(0) structure, and monotonic size-dependences develop for the composition and lattice parameters, which are well reproduced by our density functional theory calculations. Simultaneously, Mn diffusion forms 5 Å nanoshells on larger clusters inducing significant magnetization in an otherwise antiferromagnetic system. The differing atomic mobilities yield new cluster nanostructures that can be employed generally to create novel physical properties.

Theoretical and Experimental Characterization of Structures of MnAu Nanoclusters in the Size Range of 1–3 nm

Relative stabilities of MnAu magic-number nanoclusters with 55, 147, 309, and 561 atoms and highly symmetric morphologies (cuboctahedron, icosahedron, onion-like, and core-shell, respectively) are investigated based on density functional theory methods. Through an extensive search, spin arrangements on Mn atoms that give rise to lowest-energy clusters are predicted. The antiferromagnetic spin configurations are found to be the most favorable for all morphologies investigated. The energy rankings among MnAu nanoclusters with the same size and Mn/Au ratio but different morphologies are also determined. The L1(0) structure is found to be increasingly favorable as the size increases from 1.0 to 2.9 nm, consistent with experimental measurements of MnAu nanoparticles in the size range of 1.8-4.6 nm. The decahedron L1(0) morphology is found to be energetically more preferred when the Mn/Au ratio is close to 1:2, whereas the cuboctahedron L1(0) morphology is more preferred when the Mn/Au ratio is close to 1:1. The calculated lattice constants are in excellent agreement with high-resolution TEM measurements for MnAu nanoparticles of similar size. Magnetic states of MnAu nanoclusters are predicted to be stable at room temperature based on estimated Curie or Neél temperature.

Proteresis in Co:CoO core-shell nanoclusters

The magnetism of ultrasmall Co:CoO core-shell nanoclusters is investigated. The structures, produced by cluster-beam deposition, have Co core sizes ranging from 1to7nm and a CoO shell thickness of about 3nm. Hysteresis loops as well as zero-field-cooled and field-cooled magnetization curves have been measured and a striking feature of the nanostructures is the existence of proteretic (clockwise) rather than hysteretic loops in the core-size range from 3to4nm. The proteretic behavior and its particle-size dependence reflect a subtle interplay between various anisotropies and exchange interactions in the Co and CoO phases and at the Co–CoO interface.

Permittivity and permeability of Fe(Tb) nanoparticles and their microwave absorption in the 2-18 GHz range

The permittivity and permeability of the Tb-doped and undoped Fe core-shell nanoparticles were investigated for frequencies from 2 to 18 GHz. The particles were synthesized by arc discharge and contain some oxygen, probably in the form of Fe2O3, Fe3O4, and Tb2O3, whereas the core material is Fe. Both the electromagnetic materials constants and the morphology of the Fe nanoparticles are changed by Tb addition, which gives rise to the shifts to higher frequencies and thinner thicknesses of the maximum microwave absorption in the Tb-doped Fe nanoparticles.

Magnetization Reversal in Cubic Nanoparticles With Uniaxial Surface Anisotropy

The effect of surface anisotropy on the magnetization reversal in small magnetic particles is investigated. The model considers particles of cubic shape cut from a tetragonal crystal with cube faces in the (001) and equivalent planes. In particles having diameters of less than about 10 nm, the coercivity approaches the Stoner-Wohlfarth limit, but the anisotropy field differs from that of the bulk of the particles. With increasing particle size, the nucleation modes acquire the character of magnetic surface or bulk modes that reduce the coercivity

Experimental and theoretical studies of hydroxyl-induced magnetism in TiO nanoclusters.

A main challenge in understanding the defect ferromagnetism in dilute magnetic oxides is the direct experimental verification of the presence of a particular kind of defect and distinguishing its magnetic contributions from other defects. The magnetic effect of hydroxyls on TiO nanoclusters has been studied by measuring the evolution of the magnetic moment as a function of moisture exposure time, which increases the hydroxyl concentration. Our combined experiment and density-functional theory (DFT) calculations show that as dissociative water adsorption transforms oxygen vacancies into hydroxyls, the magnetic moment shows a significant increase. DFT calculations show that the magnetic moment created by hydroxyls arises from 3d orbitals of neighboring Ti sites predominantly from the top and second monolayers. The two nonequivalent hydroxyls contribute differently to the magnetic moment, which decreases as the separation of hydroxyls increases. This work illustrates the essential interplay among defect structure, local structural relaxation, charge redistribution, and magnetism. The microscopic differentiation and clarification of the specific roles of each kind of intrinsic defect is critical for the future applications of dilute magnetic oxides in spintronic or other multifunctional materials.

Structure and magnetism of MnAu nanoclusters

Equiatomic MnAu clusters with average sizes of 4 and 10 nm are produced by inert-gas condensation. As-produced clusters are used to form both dense cluster films and films with clusters embedded in a W matrix with a cluster volume fraction of 25%. Both structure and magnetism are size-dependent. Structural analysis of the 10 nm clusters indicate a distorted tetragonal body-centered cubic structure with lattice parameters a = 0.315 and c = 0.329 nm. The 4 nm clusters have a partially ordered tetragonal L10 structure with lattice parameters a = 0.410 nm and c = 0.395 nm. Magnetic properties of the clusters show evidence at low temperatures of mixed ferromagnetic and antiferromagnetic interactions and ordering as well as paramagnetic spins. Saturation moments are as large as 0.54 µB per average Mn atom. The results are compared with earlier theoretical calculations on bulk MnAu.