Metal insulator transition in TlSr2CoO5 from orbital degeneracy and spin disproportionation

To describe the metal insulator transition in the new oxide TlSr2CoO5 we investigate its electronic structure by LDA and model Hartree-Fock calculations. Within LDA we find a homogeneous metallic and ferromagnetic ground state, but when including the Coulomb interaction more explicitly within the Hartree-Fock approximation, we find an insulating state of lower energy with both spin and orbital order. We also interpret our results in terms of a simple model.Comment: 8 pages, 9 figure

Inhomogeneous States in a Small Magnetic Disk with Single-Ion Surface Anisotropy

We investigate analytically and numerically the ground and metastable states for easy-plane Heisenberg magnets with single-ion surface anisotropy and disk geometry. The configurations with two half-vortices at the opposite points of the border are shown to be preferable for strong anisotropy. We propose a simple analytical description of the spin configurations for all values of a surface anisotropy. The effects of lattice pinning leads to appearance of a set of metastable configurations.Comment: 10 pages, 7 figures; submitted to Phys. Rev.

Invar, AF-Invar, Anti-Invar and Martensite in FCC Fe-Based Alloys : An Attempt to Order this Chaos

Systematics concerning magnetic and structural phase transitions within Fe-based sytems can be achieved if experimental data are analyzed as a function of composition and the atomic volume. The findings can be understood from calculations of the exchange coupling of pure fcc Fe as a function of the n.n. distance. Decisive is that the volume increase going along with Invar, AF-Invar, Anti-Invar as well as martensite transitions originates from a charge transfer between antibonding electronic orbitals with t2g symmetry and AF exchange to non-bonding orbitals with eg symmetry and FM exchange. Which of the effects is leading at a given composition is a question of the position of the Fermi energy relative to these electronic levels. The question of how an effect manifests itself as a function of temperature is a question of the different strength of the t2g- and eg-electron-phonon coupling

Invar Behavior and Martensitic Transitions in FeNiPt Alloys

We determined the structural and the magnetic phase diagram for the disordered, ternary system FeNiPt in the Fe-rich corner in the vicinity of the γ(fcc)-α(bbc) transition region along the quasi binary cross section Fe1-x(Pt50Ni50)x with x=0.28, 0.3, 0.31, 0.32 and 0.35. All alloys are characterized by X-ray investigations, magnetization measurements M(T,H) (4.2≤T≤850K), as well as resistivity R(T) and thermal expansion measurements α(T) below 300K. We observe for x≤0.31 a non reversible, γ-α martensitic phase transition. In the alloys with x> 0.31 the γ(fcc) phase is stable down to 4.2K, and pronounced magneto-volume effects (Invar-like behavior) is observed. For the alloys close to the γ-α transition, the magnetic behavior M(T,H) is also discussed