1,176 research outputs found
Ferromagnetism in defect-ridden oxides and related materials
The existence of high-temperature ferromagnetism in thin films and
nanoparticles of oxides containing small quantities of magnetic dopants remains
controversial. Some regard these materials as dilute magnetic semiconductors,
while others think they are ferromagnetic only because the magnetic dopants
form secondary ferromagnetic impurity phases such as cobalt metal or magnetite.
There are also reports in d0 systems and other defective oxides that contain no
magnetic ions. Here, we investigate TiO2 (rutile) containing 1 - 5% of iron
cations and find that the room-temperature ferromagnetism of films prepared by
pulsed-laser deposition is not due to magnetic ordering of the iron. The films
are neither dilute magnetic semiconductors nor hosts to an iron-based
ferromagnetic impurity phase. A new model is developed for defect-related
ferromagnetism which involves a spin-split defect band populated by charge
transfer from a proximate charge reservoir in the present case a mixture Fe2+
and Fe3+ ions in the oxide lattice. The phase diagram for the model shows how
inhomogeneous Stoner ferromagnetism depends on the total number of electrons
Ntot, the Stoner exchange integral I and the defect bandwidth W; the band
occupancy is governed by the d-d Coulomb interaction U. There are regions of
ferromagnetic metal, half-metal and insulator as well as nonmagnetic metal and
insulator. A characteristic feature of the high-temperature Stoner magnetism is
an an anhysteretic magnetization curve which is practically temperature
independent below room temperature. This is related to a wandering
ferromagnetic axis which is determined by local dipole fields. The
magnetization is limited by the defect concentration, not by the 3d doping.
Only 1-2 % of the volume of the films is magnetically ordered.Comment: 22 pages, 6 figure
Crystal field in nitrogenated rare-earth intermetallics
The crystal-field in Sm2Fe17N3- δ and Sm(Fe11Ti)N1- δ due to interstitial nitrogen has been investigated. Intrinsic parametrization in the superposition model allows separation of the crystal field created by a neighboring nitrogen atom from a purely geometrical factor, which is different for Sm2Fe17N3- δ and Sm( Fe11Ti) N1- δ Using published magnetic data, values for the intrinsic parameter A2 per nitrogen atom of A2=200± 60 Ka0-2 and A2=270±60 Ka0-2 for Sm2Fe17N3- δ and Sm(Fe11Ti)N1- δ, respectively, are obtained. Because of charge penetration, which is discussed in the form of an explicit crystal-field weight function, it is not possible to interpret A20 or à 2 as crystal-field parameters independent of the 4f ion
Charge dynamics in the half-metallic ferromagnet CrO\u3csub\u3e2\u3c/sub\u3e
Infrared spectroscopy is used to investigate the electronic structure and charge carrier relaxation in crystalline films of CrO2 which is the simplest of all half-metallic ferromagnets. Chromium dioxide is a bad metal at room temperature but it has a remarkably low residual resistivity (\u3c5 \u3eμΩ cm) despite the small spectral weight associated with free carrier absorption. The infrared measurements show that low residual resistivity is due to the collapse of the scattering rate at ω\u3c2000 \u3ecm-1. The blocking of the relaxation channels at low v and T can be attributed to the unique electronic structure of a half-metallic ferromagnet. In contrast to other ferromagnetic oxides, the intraband spectral weight is constant below the Curie temperature
Magnetic field screening and mirroring in graphene
The orbital magnetism in spatially varying magnetic fields is studied in
monolayer graphene within the effective mass approximation. We find that,
unlike the conventional two-dimensional electron system, graphene with small
Fermi wave number k_F works as a magnetic shield where the field produced by a
magnetic object placed above graphene is always screened by a constant factor
on the other side of graphene. The object is repelled by a diamagnetic force
from the graphene, as if there exists its mirror image with a reduced amplitude
on the other side of graphene. The magnitude of the force is much greater than
that of conventional two-dimensional system. The effect disappears with the
increase of k_F.Comment: 5 pages, 3 figure
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