Linear positive magnetoresistance and quantum interference in ferromagnetic metals.

Positive, linear in field, and isotropic magnetoresistance in fields up to 60 T is found in geometrically constrained ferromagnets, such as thin films of iron, nickel, and cobalt and their granular mixtures with nonmagnetic materials. The resistivity measured as a function of temperature shows a minimum at temperatures reaching a remarkably high 92 K, followed by logarithmic dependence at low temperatures. We propose to explain both phenomena by a modified version of the quantum electron-electron interaction theory. The agreement is only qualitative while the observed magnitude of the magnetoresistance slope is much larger than the calculated one.

Unusual symmetries in the Kugel-Khomskii Hamiltonian.

The Kugel-Khomskii Hamiltonian for cubic titanates describes spin and orbital superexchange interactions between d(1) ions having threefold degenerate t(2g) orbitals. Since orbitals do not couple along "inactive" axes, perpendicular to the orbital planes, the total number of electrons in |alpha> orbitals in any such plane and the corresponding total spin are both conserved. A Mermin-Wagner construction shows that there is no long-range spin ordering at nonzero temperatures. Inclusion of spin-orbit coupling allows such ordering, but even then the excitation spectrum is gapless due to a continuous symmetry. Thus, the observed order and gap require more symmetry breaking terms.