1,802,451 research outputs found
Ferromagnetism in Mn doped GaAs due to substitutional-interstitial complexes
While most calculations on the properties of the ferromagnetic semiconductor
GaAs:Mn have focussed on isolated Mn substituting the Ga site (Mn), we
investigate here whether alternate lattice sites are favored and what the
magnetic consequences of this might be. Under As-rich (Ga-poor) conditions
prevalent at growth, we find that the formation energies are lower for
Mn over interstitial Mn (Mn).As the Fermi energy is shifted towards
the valence band maximum via external -doping, the formation energy of
Mn is reduced relative to Mn. Furthermore, under epitaxial growth
conditions, the solubility of both substitutional and interstitial Mn are
strongly enhanced over what is possible under bulk growth conditions. The high
concentration of Mn attained under epitaxial growth of p-type material opens
the possibility of Mn atoms forming small clusters. We consider various types
of clusters, including the Coulomb-stabilized clusters involving two Mn
and one Mn. While isolated Mn are hole killers (donors), and therefore
destroy ferromagnetism,complexes such as Mn-Mn-Mn) are found
to be more stable than complexes involving Mn-Mn-Mn. The
former complexes exhibit partial or total quenching of holes, yet Mn in
these complexes provide a channel for a ferromagnetic arrangement of the spins
on the two Mn within the complex. This suggests that ferromagnetism in
Mn doped GaAs arises both from holes due to isolated Mn as well as from
strongly Coulomb stabilized Mn-Mn-Mn clusters.Comment: 7 figure
Antiferromagnetic order in (Ga,Mn)N nanocrystals: A density functional theory study
We investigate the electronic and magnetic properties of (Ga,Mn)N
nanocrystals using the density functional theory. We study both wurtzite and
zinc-blende structures doped with one or two substitutional Mn impurities. For
a single Mn dopant placed close to surface, the behavior of the empty
Mn-induced state, hereafter referred to as "Mn hole", is different from bulk
(Ga,Mn)N. The energy level corresponding to this off-center Mn hole lies within
the nanocrystal gap near the conduction edge. For two Mn dopants, the most
stable magnetic configuration is antiferromagnetic, and this was unexpected
since (Ga,Mn)N bulk shows ferromagnetism in the ground state. The surprising
antiferromagnetic alignment of two Mn spins is ascribed also to the holes
linked to the Mn impurities located close to surface. Unlike Mn holes in
(Ga,Mn)N bulk, these Mn holes in confined (Ga,Mn)N nanostructures do not
contribute to the ferromagnetic alignment of the two Mn spins
Mechanism of magnetostructural transformation in multifunctional MnGaC
MnGaC undergoes a ferromagnetic to antiferromagnetic, volume
discontinuous cubic-cubic phase transition as a function of temperature,
pressure and magnetic field. Through a series of temperature dependent x-ray
absorption fine structure spectroscopy experiments at the Mn K and Ga K edge,
it is shown that the first order magnetic transformation in MnGaC is
entirely due to distortions in Mn sub-lattice and with a very little role for
Mn-C interactions. The distortion in Mn sub-lattice results in long and short
Mn-Mn bonds with the longer Mn-Mn bonds favoring ferromagnetic interactions and
the shorter Mn-Mn bonds favoring antiferromagnetic interactions. At the first
order transition, the shorter Mn-Mn bonds exhibit an abrupt decrease in their
length resulting in an antiferromagnetic ground state and a strained lattice.Comment: Accepted in J. Appl. Phys. Please contact authors for supplementary
informatio
Protostellar Jet and Outflow in the Collapsing Cloud Core
We investigate the driving mechanism of outflows and jets in star formation
process using resistive MHD nested grid simulations. We found two distinct
flows in the collapsing cloud core: Low-velocity outflows (sim 5 km/s) with a
wide opening angle, driven from the first adiabatic core, and high-velocity
jets (sim 50 km/s) with good collimation, driven from the protostar.
High-velocity jets are enclosed by low-velocity outflow. The difference in the
degree of collimation between the two flows is caused by the strength of the
magnetic field and configuration of the magnetic field lines. The magnetic
field around an adiabatic core is strong and has an hourglass configuration.
Therefore, the low-velocity outflow from the adiabatic core are driven mainly
by the magnetocentrifugal mechanism and guided by the hourglass-like field
lines. In contrast, the magnetic field around the protostar is weak and has a
straight configuration owing to Ohmic dissipation in the high-density gas
region. Therefore, high-velocity jet from the protostar are driven mainly by
the magnetic pressure gradient force and guided by straight field lines.
Differing depth of the gravitational potential between the adiabatic core and
the protostar cause the difference of the flow speed. Low-velocity outflows
correspond to the observed molecular outflows, while high-velocity jets
correspond to the observed optical jets. We suggest that the protostellar
outflow and the jet are driven by different cores (the first adiabatic core and
protostar), rather than that the outflow being entrained by the jet.Comment: To appear in the proceedings of the "Protostellar Jets in Context"
conference held on the island of Rhodes, Greece (7-12 July 2008
Spatial structure of Mn-Mn acceptor pairs in GaAs
The local density of states of Mn-Mn pairs in GaAs is mapped with
cross-sectional scanning tunneling microscopy and compared with theoretical
calculations based on envelope-function and tight-binding models. These
measurements and calculations show that the crosslike shape of the Mn-acceptor
wavefunction in GaAs persists even at very short Mn-Mn spatial separations. The
resilience of the Mn-acceptor wave-function to high doping levels suggests that
ferromagnetism in GaMnAs is strongly influenced by impurity-band formation. The
envelope-function and tight-binding models predict similarly anisotropic
overlaps of the Mn wave-functions for Mn-Mn pairs. This anisotropy implies
differing Curie temperatures for Mn -doped layers grown on differently
oriented substrates.Comment: 4 pages, 4 figure
Electronic structure and magnetism of Mn doped GaN
Mn doped semiconductors are extremely interesting systems due to their novel
magnetic properties suitable for the spintronics applications. It has been
shown recently by both theory and experiment that Mn doped GaN systems have a
very high Curie temperature compared to that of Mn doped GaAs systems. To
understand the electronic and magnetic properties, we have studied Mn doped GaN
system in detail by a first principles plane wave method. We show here the
effect of varying Mn concentration on the electronic and magnetic properties.
For dilute Mn concentration, states of Mn form an impurity band completely
separated from the valence band states of the host GaN. This is in contrast to
the Mn doped GaAs system where Mn states in the gap lie very close to the
valence band edge and hybridizes strongly with the delocalized valence band
states.
To study the effects of electron correlation, LSDA+U calculations have been
performed.
Calculated exchange interaction in (Mn,Ga)N is short ranged in contrary to
that in (Mn,Ga)As where the strength of the ferromagnetic coupling between Mn
spins is not decreased substantially for large Mn-Mn separation. Also, the
exchange interactions are anisotropic in different crystallographic directions
due to the presence or absence of connectivity between Mn atoms through As
bonds.Comment: 6 figures, submitted to Phys. Rev.
Diluted manganese on the bond-centered site in germanium
The functional properties of Mn-doped Ge depend to large extent on the lattice location of the Mn impurities. Here, we present a lattice location study of implanted diluted Mn by means of electron emission channeling. Surprisingly, in addition to the expected substitutional lattice position, a large fraction of the Mn impurities occupies the bond-centered site. Corroborated by ab initio calculations, the bond-centered Mn is related to Mn-vacancy complexes. These unexpected results call for a reassessment of the theoretical studies on the electrical and magnetic behavior of Mn-doped Ge, hereby including the possible role of Mn-vacancy complexes
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