The Hubbard model is a “highly oversimplified model” for electrons in a solid which interact with each other through extremely short ranged repulsive (Coulomb) interaction. The Hamiltonian of the Hubbard model consists of two pieces; Hhop which describes quantum mechanical hopping of electrons, and Hint which describes nonlinear repulsive interaction. Either Hhop or Hint alone is easy to analyz...

We describe the ferromagnetism in the manganese pyrochlores, such as Tl2Mn2O7 and Sc2Mn2O7 , in terms of the interplay between superexchange, Zener double exchange, and indirect exchange, the first being antiferromagnetic ~AF! while the last two are ferromagnetic. The tendency towards the antiferroalignment of the localized Mn spins is significantly weakened due to ~i! the presence of spin frus...

Using the constrained-path Monte Carlo method, we studied the magnetic properties of the two-dimensional periodic Anderson model for electron fillings between 1/4 and 1/2. We also derived two effective low-energy theories to assist in interpreting the numerical results. For 1/4 filling, we found that the system can be a Mott or a charge-transfer insulator, depending on the relative values of th...

Compositional, microstructural, and magnetic characterization of ZnO 30 Å /Mn x n multilayers prepared by sputtering is presented to study the observed ferromagnetism in the Mn-ZnO system. The nominal Mn layer thickness, x, is varied from 3 to 60 Å, while the number of bilayers, n, is increased to maintain the total amount of Mn constant. Microstructure information was deduced from x-ray reflec...

We report the observation of spatially separated Kondo scattering and ferromagnetism in anatase Ta0.06Ti0.94O2 thin films as a function of thickness (10-200 nm). The Kondo behavior observed in thicker films is suppressed on decreasing thickness and vanishes below ~25 nm. In 200 nm film, transport data could be fitted to a renormalization group theory for Kondo scattering though the carrier dens...

The structural, electronic and magnetic properties of Zn0.95Co0.05O, Zn0.95Fe0.05O and Zn0.90Fe0.05Co0.05O nanoparticles prepared by a sol-gel method are presented and discussed. Xray diffraction and optical analysis indicated that high spin Co ions substitute for the Zn ions in tetrahedral sites. Fe Mössbauer spectroscopy showed the presence of isolated paramagnetic Fe ions in both Fe doped an...

Using density functional theory calculations, we study the electronic and magnetic properties of the p-type monolayer II-VI semiconductors SrX (X = S,Se). The pristine SrS and SrSe monolayers are large band gap semiconductor with a very flat band in the top valence band. Upon injecting hole uniformly, ferromagnetism emerges in those system in a large range of hole density. By varying hole densi...

III-V ferromagnetic semiconductor has attracted great attention as a potential application for spintronics due to a successful demonstration of spin injection from ferromagnetic GaMnAs into semiconductor. A conclusion so far in the GaMnAs system is the maximum incorporation of Mn up to ~ 5% with ferromagnetism below ~110 K. Higher Mn incorporation produces MnAs precipitates, and which has rathe...

Keeping nickel, cobalt and iron in mind, we investigate the origin of the itinerant ferromagnetism. Recent experiments show that the systems should be in an intermediate coupling regime, where the band width and the interaction energy are of the same order of magnitudes. To treat such a situation, we generalize the Gutzwiller approximation. In that, we take account of the effect of the band deg...

The article presents, using Bi doped ZnO, an example of a heavy ion doped oxide semiconductor, highlighting a novel p-symmetry interaction of the electronic states to stabilize ferromagnetism. The study includes both ab initio theory and experiments, which yield clear evidence for above room temperature ferromagnetism. ZnBi(x)O(1-x) thin films are grown using the pulsed laser deposition techniq...