850 research outputs found
Generalized calculation of magnetic coupling constants for Mott-Hubbard insulators: Application to ferromagnetic Cr compounds
Using a Rayleigh-Schr\"odinger perturbation expansion of multi-band Hubbard
models, we present analytic expressions for the super-exchange coupling
constants between magnetic transition metal ions of arbitrary separation in
Mott-Hubbard insulators. The only restrictions are i) all ligand ions are
closed shell anions and ii) all contributing interaction paths are of equal
length. For short paths, our results essentially confirm the
Goodenough-Kanamori-Anderson rules, yet in general there does not exist any
simple rule to predict the sign of the magnetic coupling constants. The most
favorable situation for ferromagnetic coupling is found for ions with less than
half filled d shells, the (relative) tendency to ferromagnetic coupling
increases with increasing path length. As an application, the magnetic
interactions of the Cr compounds RbCrCl, CrCl, CrBr and CrI
are investigated, all of which except CrCl are ferromagnets.Comment: 13 pages, 6 eps figures, submitted to Phys Rev
Spin-orbital physics for p orbitals in alkali RO_2 hyperoxides --- generalization of the Goodenough-Kanamori rules
We derive a realistic spin-orbital model at finite Hund's exchange for alkali
hyperoxides. We find that, due to the geometric frustration of the oxygen
lattice, spin and orbital waves destabilize both spin and p-orbital order in
almost all potential ground states. We show that the orbital order induced by
the lattice overrules the one favoured by superexchange and that this, together
with the large interorbital hopping, leads to generalized Goodenough-Kanamori
rules. They (i) lift the geometric frustration of the lattice, and (ii) explain
the observed layered C-type antiferromagnetic order in alkali hyperoxides. This
is confirmed by a spin-wave dispersion with no soft-mode behavior presented
here as a prediction for future experiments.Comment: 7 pages, 5 figures; accepted in EP
Magnetic Order and Dynamics in an Orbitally Degenerate Ferromagnetic Insulator
Neutron scattering was used to determine the spin structure and the magnon
spectrum of the Mott--Hubbard insulator YTiO. The magnetic structure is
complex, comprising substantial G-type and A-type antiferromagnetic components
in addition to the predominant ferromagnetic component. The magnon spectrum, on
the other hand, is gapless and nearly isotropic. We show that these findings
are inconsistent with the orbitally ordered states thus far proposed for
YTiO and discuss general implications for a theoretical description of
exchange interactions in orbitally degenerate systems.Comment: to appear in Phys. Rev. Let
Local Electronic and Magnetic Studies of an Artificial La2FeCrO6 Double Perovskite
Through the utilization of element-resolved polarized x-ray probes, the
electronic and magnetic state of an artificial La2FeCrO6 double perovskite were
explored. Applying unit-cell level control of thin film growth on SrTiO3 (111),
the rock salt double perovskite structure can be created for this system, which
does not have an ordered perovskite phase in the bulk. We find that the Fe and
Cr are in the proper 3+ valence state, but, contrary to previous studies, the
element-resolved magnetic studies find the moments in field are small and show
no evidence of a sizable magnetic moment in the remanent state.Comment: 3 pages, 4 figure
Spin--orbital physics in transition metal oxides
We present the main features of the spin-orbital superexchange which
describes the magnetic and optical properties of Mott insulators with orbital
degrees of freedom. In contrast to the SU(2) symmetry of spin superexchange,
the orbital part of the superexchange obeys the lower cubic symmetry of the
lattice and is intrinsically frustrated. This intrinsic frustration and
spin-orbital entanglement induce enhanced quantum fluctuations, and we point
out a few situations where this leads to disordered states. Strong coupling
between the spin and orbital degrees of freedom is discussed on the example of
the VO perovskites, with standing for rare-earth ion, La,...,Lu. We
explain the observed evolution of the orbital and N\'eel
transition temperature in the VO series with decreasing ionic radius
. A few open problems and the current directions of research in the field
of spin-orbital physics are pointed out.Comment: 19 pages, 5 figure
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