225 research outputs found
Exchange interactions in transition metal oxides: The role of oxygen spin polarization
Magnetism of transition metal (TM) oxides is usually described in terms of
the Heisenberg model, with orientation-independent interactions between the
spins. However, the applicability of such a model is not fully justified for TM
oxides because spin polarization of oxygen is usually ignored. In the
conventional model based on the Anderson principle, oxygen effects are
considered as a property of the TM ion and only TM interactions are relevant.
Here, we perform a systematic comparison between two approaches for spin
polarization on oxygen in typical TM oxides. To this end, we calculate the
exchange interactions in NiO, MnO, and hematite (Fe2O3) for different magnetic
configurations using the magnetic force theorem. We consider the full spin
Hamiltonian including oxygen sites, and also derive an effective model where
the spin polarization on oxygen renormalizes the exchange interactions between
TM sites. Surprisingly, the exchange interactions in NiO depend on the magnetic
state if spin polarization on oxygen is neglected, resulting in non-Heisenberg
behavior. In contrast, the inclusion of spin polarization in NiO makes the
Heisenberg model more applicable. Just the opposite, MnO behaves as a
Heisenberg magnet when oxygen spin polarization is neglected, but shows strong
non-Heisenberg effects when spin polarization on oxygen is included. In
hematite, both models result in non-Heisenberg behavior. General applicability
of the magnetic force theorem as well as the Heisenberg model to TM oxides is
discussed.Comment: 19 pages, 2 figure
Geometric, electronic and magnetic structure of FeO clusters
Correlation between geometry, electronic structure and magnetism of solids is
both intriguing and elusive. This is particularly strongly manifested in small
clusters, where a vast number of unusual structures appear. Here, we employ
density functional theory in combination with a genetic search algorithm,
GGA and a hybrid functional to determine the structure of gas phase
FeO clusters. For FeO cation clusters we also
calculate the corresponding vibration spectra and compare them with
experiments. We successfully identify FeO, FeO,
FeO, FeO and propose structures for
FeO. Within the triangular geometric structure of
FeO a non-collinear, ferrimagnetic and ferromagnetic state are
comparable in energy. FeO and FeO are
ferrimagnetic with a residual magnetic moment of 1~\muB{} due to ionization.
FeO is ferrimagnetic due to the odd number of Fe atoms. We
compare the electronic structure with bulk magnetite and find
FeO, FeO, FeO to be mixed
valence clusters. In contrast, in FeO and FeO
all Fe are found to be trivalent.Comment: 14 pages, 21 figure
Photoinduced superconducting nanowires in Gd-Ba-Cu-O films
We report the fabrication of high Tc superconducting wires by photodoping a
GdBa2Cu3O{6.5} thin film. An optical near-field probe was used to locally
excite carriers in the system at room temperature. Trapping of the
photogenerated electrons define a confining potential for the conducting holes
in the CuO planes. Spatially resolved reflectance measurements show the
photogenerated nanowires to be ~ 250 nm wide. Electron diffusion, before
electron capture, is believed to be responsible for the observed width of the
wires.Comment: 8 pages, 3 figures Submitted to Appl. Phys. Let
Enhancement of the Curie temperature in small particles of weak itinerant ferromagnets
Self consistent renormalization theory of itinerant ferromagnets is used to
calculate the Curie temperature of clusters down to approximately 100 atoms in
size. In these clusters the electrons responsible for the magnetic properties
are assumed to be (weakly) itinerant. It is shown that the Curie temperature
can be larger than in the bulk. The effect originates from the phenomenon of
level repulsion in chaotic quantum systems, which suppresses spin fluctuations.
Since the latter destroy the magnetic order the resulting Curie temperature
increases, contrary to expectations of the naive Stoner picture. The
calculations are done assuming that the energy levels of the cluster are
described by the Gaussian Orthogonal Ensemble of random matrix theory.Comment: Phys. Rev. B, accepted for publicatio
Kramers degeneracy and relaxation in vanadium, niobium and tantalum clusters
In this work we use magnetic deflection of V, Nb, and Ta atomic clusters to
measure their magnetic moments. While only a few of the clusters show weak
magnetism, all odd-numbered clusters deflect due to the presence of a single
unpaired electron. Surprisingly, for majority of V and Nb clusters an
atomic-like behavior is found, which is a direct indication of the absence of
spin-lattice interaction. This is in agreement with Kramers degeneracy theorem
for systems with a half-integer spin. This purely quantum phenomenon is
surprisingly observed for large systems of more than 20 atoms, and also
indicates various quantum relaxation processes, via Raman two-phonon and Orbach
high-spin mechanisms. In heavier, Ta clusters, the relaxation is always
present, probably due to larger masses and thus lower phonon energies, as well
as increased spin-orbit coupling.Comment: 7 pages, 5 figure
Magnetic properties of Co doped Nb clusters
From magnetic deflection experiments on isolated Co doped Nb clusters we made
the interesting observation of some clusters being magnetic, while others
appear to be non-magnetic. There are in principle two explanations for this
behavior. Either the local moment at the Co site is completely quenched or it
is screened by the delocalized electrons of the cluster, i.e. the Kondo effect.
In order to reveal the physical origin, we conducted a combined theoretical and
experimental investigation. First, we established the ground state geometry of
the clusters by comparing the experimental vibrational spectra with those
obtained from a density functional theory study. Then, we performed an analyses
based on the Anderson impurity model. It appears that the non-magnetic clusters
are due to a complete quenching of the local Co moment and not due to the Kondo
effect. In addition, the magnetic behavior of the clusters can be understood
from an inspection of their electronic structure. Here magnetism is favored
when the effective hybridization around the chemical potential is small, while
the absence of magnetism is signalled by a large effective hybridization around
the chemical potential.Comment: 14 pages, 8 figure
Quantum control on entangled bipartite qubits
Ising interaction between qubits could produce distortion in entangled pairs
generated for engineering purposes (as in quantum computation) in presence of
parasite magnetic fields, destroying or altering the expected behavior of
process in which is projected to be used. Quantum control could be used to
correct that situation in several ways. Sometimes the user should be make some
measurement upon the system to decide which is the best control scheme; other
posibility is try to reconstruct the system using similar procedures without
perturbate it. In the complete pictures both schemes are present. We will work
first with pure systems studying advantages of different procedures. After, we
will extend these operations when time of distortion is uncertain, generating a
mixed state, which needs to be corrected by suposing the most probably time of
distortion.Comment: 10 pages, 5 figure
Dynamics of pH-sensitive nitroxide radicals in water adsorbed in ordered mesoporous molecular sieves by EPR Spectroscopy
A spin pH probe technique was used to study the influence of the channel diameter on the EPR spectra of pH-sensitive nitroxide radicals (NR) located in the channels of the mesoporous molecular sieves MCM-41 and SBA-15 with diameters ranging from 2.3 to 8.1 nm. From EPR spectra analysis and the results of the NR retention by the mesoporous molecular sieves upon washing with an aqueous KCl solution, the regularities of NR molecular location inside the channels were studied. The obtained dependence of the fraction of the radical molecules in the fast motional regime (with the rotational correlation times, τc = 2 × 10-11 s-9 × 10-11s) in the channels of the mesoporous molecular sieves as a function of pH indicates that both NR in the fast and slow motional regime (with τc = 8 × 10 -9s-7 × 10-10s) may be used for estimation of the solution acidity inside the channels and of the near-surface electrical potential. © 2013 Elsevier Inc. All rights reserved
Correlation effects and orbital magnetism of Co clusters
Recent experiments on isolated Co clusters have shown huge orbital magnetic
moments in comparison with their bulk and surface counterparts. These clusters
hence provide the unique possibility to study the evolution of the orbital
magnetic moment with respect to the cluster size and how competing interactions
contribute to the quenching of orbital magnetism. We investigate here different
theoretical methods to calculate the spin and orbital moments of Co clusters,
and assess the performances of the methods in comparison with experiments. It
is shown that density functional theory in conventional local density or
generalized gradient approximations, or even with a hybrid functional, severely
underestimates the orbital moment. As natural extensions/corrections we
considered the orbital polarization correction, the LDA+U approximation as well
as the LDA+DMFT method. Our theory shows that of the considered methods, only
the LDA+DMFT method provides orbital moments in agreement with experiment, thus
emphasizing the importance of dynamic correlations effects for determining
fundamental magnetic properties of magnets in the nano-size regime
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