1,356 research outputs found
Theory of spin-polarized scanning tunneling microscopy applied to local spins
We provide a theory for scanning tunneling microscopy and spectroscopy using
a spin-polarized tip. It it shown that the tunneling conductance can be
partitioned into three separate contributions, a background conductance which
is independent of the local spin, a dynamical conductance which is proportional
to the local spin moment, and a conductance which is proportional to the noise
spectrum of the local spin interactions. The presented theory is applicable to
setups with magnetic tip and substrate in non-collinear arrangement, as well as
for non-magnetic situations. The partitioning of the tunneling current suggests
a possibility to extract the total spin moment of the local spin from the
dynamical conductance. The dynamical conductance suggests a possibility to
generate very high frequency spin-dependent ac currents and/or voltages. We
also propose a measurement of the dynamical conductance that can be used to
determine the character of the effective exchange interaction between
individual spins in clusters. The third contribution to the tunneling current
is associated with the spin-spin correlations induced by the exchange
interaction between the local spin moment and the tunneling electrons. We
demonstrate how this term can be used in the analysis of spin excitations
recorded in conductance measurements. Finally, we propose to use spin-polarized
scanning tunneling microscopy for detailed studies of the spin excitation
spectrum.Comment: 12 pages, 4 figure, updated to match the published version, to appear
in the Phys. Rev.
Magnetism in Graphene Induced by Single-Atom Defects
We study from first principles the magnetism in graphene induced by single
carbon atom defects. For two types of defects considered in our study, the
hydrogen chemisorption defect and the vacancy defect, the itinerant magnetism
due to the defect-induced extended states has been observed. Calculated
magnetic moments are equal to 1 per hydrogen chemisorption defect and
1.121.53 per vacancy defect depending on the defect concentration.
The coupling between the magnetic moments is either ferromagnetic or
antiferromagnetic, depending on whether the defects correspond to the same or
to different hexagonal sublattices of the graphene lattice, respectively. The
relevance of itinerant magnetism in graphene to the high- magnetic
ordering is discussed.Comment: 5 pages, 6 figure
The electronic and magnetic properties of anion doped (C, N, S) GaFeO3; an ab initio DFT study
AbstractIn this study we present ab initio DFT calculations performed on stoichiometric and anion doped GaFeO3 substituting O by a C, N and S atom, respectively. Stoichiometric GaFeO3 has an antiferromagnetic (AFM) ground state. The Fe atoms of the sublattices Fe1 and Fe2 couple antiferromagnetically via the O atoms through the superexchange mechanism. Replacing the superexchange mediating O atom with p-elements of a different valence electron configuration changes the underlying magnetic exchange mechanism and influence the ground state properties. This may be used for tuning properties interesting for technical applications. Four different doping configurations were examined revealing a cell site dependent influence on the magnetic properties. Carbon, for example, changes the AFM coupling present in the Fe1–O–Fe2 configuration into a ferrimagnetic exchange for the Fe1–C–Fe2 bond. Depending on the respective cell site C substitution introduces a ferrimagnetic or AFM ground state. Nitrogen alters the ground state magnetic moment as well and sulfur introduces large structural distortions affecting the band gap and the overall AFM coupling inside the doped GaFeO3 simulation cell. We give a detailed discussion on the respective magnetic exchange mechanisms and electronic properties with regard to applications as photocatalysis and use the predictive power of ab initio DFT simulations that may trigger future experiments in the very promising field of tunable multifunctional devices
Electronic and Magnetic Properties of Fe 2
The electronic and magnetic properties of the nitrides Fe2N and hypothetical FeN were investigated by use of the ASW method. For both nitrides, the calculations were done in the orthorhombic structure
of Fe2N for the two nitrides. The magnetization is low (0.95 μB/at.) for Fe2N and vanishes for FeN. The decrease of the magnetization with increasing amount of N is assessed within the Fe-N system in a model derived from a Slater-Pauling type behavior. Accordingly, a trend from weak to strong ferromagnetism is suggested
Thermodynamic properties of the itinerant-boson ferromagnet
Thermodynamics of a spin-1 Bose gas with ferromagnetic interactions are
investigated via the mean-field theory. It is apparently shown in the specific
heat curve that the system undergoes two phase transitions, the ferromagnetic
transition and the Bose-Einstein condensation, with the Curie point above the
condensation temperature. Above the Curie point, the susceptibility fits the
Curie-Weiss law perfectly. At a fixed temperature, the reciprocal
susceptibility is also in a good linear relationship with the ferromagnetic
interaction.Comment: 5 pages, 5 figure
Spin waves in a Bose Ferromagnet
It is shown that the ferromagnetic transition takes place always above
Bose-Einstein condensation in ferromagnetically coupled spinor Bose gases. We
describe the Bose ferromagnet within Ginzburg-Landau theory by a "two-fluid"
model below Bose-Einstein condensation. Both the Bose condensate and the normal
phase are spontaneously magnetized. As a main result we show that spin waves in
the two fluids are coupled together so as to produce only one mixed spin-wave
mode in the coexisting state. The long wavelength spectrum is quadratic in the
wave vector , consistent with usual ferromagnetism theory, and the
spin-wave stiffness coefficient includes contributions from both the two
phases, implying the "two-fluid" feature of the system. can show a sharp
bend at the Bose-Einstein condensation temperature.Comment: 4 pages, 1 figur
First-principles study of thin magnetic transition-metal silicide films on Si(001)
In order to combine silicon technology with the functionality of magnetic
systems, a number of ferromagnetic (FM) materials have been suggested for the
fabrication of metal/semiconductor heterojunctions. In this work, we present a
systematic study of several candidate materials in contact with the Si surface.
We employ density-functional theory calculations to address the thermodynamic
stability and magnetism of both pseudomorphic CsCl-like Si (=Mn, Fe, Co,
Ni) thin films and Heusler alloy MnSi (=Fe, Co, Ni) films on Si(001).
Our calculations show that Si-termination of the Si films is energetically
preferable during epitaxy since it minimizes the energetic cost of broken bonds
at the surface. Moreover, we can explain the calculated trends in thermodynamic
stability of the Si thin films in terms of the -Si bond-strength and the
3d orbital occupation. From our calculations, we predict that ultrathin
MnSi films are FM with sizable spin magnetic moments at the Mn atoms, while
FeSi and NiSi films are nonmagnetic. However, CoSi films display itinerant
ferromagnetism. For the MnSi films with Heusler-type structure, the MnSi
termination is found to have the highest thermodynamic stability. In the FM
ground state, the calculated strength of the effective coupling between the
magnetic moments of Mn atoms within the same layer approximately scales with
the measured Curie temperatures of the bulk MnSi compounds. In particular,
the CoMnSi/Si(001) thin film has a robust FM ground state as in the bulk,
and is found to be stable against a phase separation into CoSi/Si(001) and
MnSi/Si(001) films. Hence this material is of possible use in FM-Si
heterojunctions and deserves further experimental investigations.Comment: 13 pages, 8 figure
Magnetic properties of 3d-impurities substituted in GaAs
We have calculated the magnetic properties of substituted 3d-impurities
(Cr-Ni) in a GaAs host by means of first principles electronic structure
calculations. We provide a novel model explaining the ferromagnetic long rang
order of III-V dilute magnetic semiconductors. The origin of the ferromagnetism
is shown to be due to delocalized spin-uncompensated As dangling bond
electrons. Besides the quantitative prediction of the magnetic moments, our
model provides an understanding of the halfmetallicity, and the raise of the
critical temperature with the impurity concentration
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