68 research outputs found
Modified Spin-Wave Theory for Nanomagnets : Application to the Keplerate Molecule Mo(72)Fe(30)
We adapt Takahashi's modified spin-wave theory to the context of
nano-magnets, and apply it to the molecular compound based on the giant
magnetic molecule Mo(72)Fe(30). This involves solving numerically the
mean-field equations and then forcing the sublattice magnetizations to zero by
means of local chemical potentials for the magnons. We have thus constructed a
quantum state with no local magnetization at all temperatures, appropriate to a
finite-size system, but with strong correlations. We compare theoretical
results to specific heat and ESR measurements.Comment: 12 pages, 11 figure
Transverse magnetization and transient oscillations in the quantum tunneling of molecular magnets
We calculate the response of a molecular magnet subject to a time-varying
magnetic field and coupled to a heat bath. We propose that observations of
calculated oscillations transverse to the field direction may be an effective
way of demonstrating quantum tunneling and thus probing the details of level
repulsion. The effective model of a triangle of Heisenberg spins and weak
anisotropies, as has been used to model the molecular magnets V15 and Cu3, is
used to illustrate this
Why RKKY exchange integrals are inappropriate to describe ferromagnetism in diluted magnetic semiconductors
We calculate Curie temperatures and study the stability of ferromagnetism in
diluted magnetic materials, taking as a model for the exchange between magnetic
impurities a damped Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and a shor
t range term representing the effects of superexchange. To properly include
effects of spin and thermal fluctuations as well as geometric disorder, we
solve the effective Heisenberg Hamiltonian by means of a recently developed
semi-analytical approach. This approach, ``self-consistent local Random Phase
Approximation (SC-L RPA)'', is explained. We show that previous mean-field
treatments, which have been widely used in the literature, largely overestimate
both the Curie temperatures and the stability of ferromagnetism as a function
of carrier density. The discr epancy when compared to the current approach was
that effects of frustration in RKKY oscillations had been strongly
underestimated by such simple mea n-field theories. We argue that the use, as
is frequent, of a weakly-disordered RKKY exchange to model ferromagnetism in
diluted III-V systems is inconsistent with the observation of ferromagnetism
over a wide region of itinerant carrier densities. This may be puzzling when
compared to the apparent success of calculations based on {\it ab-initio}
estimates of the coupling; we propose a resolution to this issue by taking
RKKY-like interactions between resonant states close to the Fermi level.Comment: Accepted for publication in Physical Review B. 22 pages, 7 figure
Model for vacancy-induced d ferromagnetism in oxide compounds
We propose a model with few parameters, for vacancy-induced ferromagnetism
based on a correlated model for anions (oxygen orbitals) with random potentials
that represent cation vacancies. There is a range of potential strength for
which moments appear on oxygen sites neighboring the vacancies. By using
techniques which allow us to treat the randomness exactly, we calculate the
magnetic couplings between the total moment around each vacancy, the Curie
temperature and dynamical correlations as a function of the effective stren gth
of the vacancy potential and doping properties, the density of vacancies, as
well as the correlation and band width of the host. For physically reasonable
parameters this predicts Curie temperatures well above room temperature for
small concentrations of vacancies and appropriate parameters. We disc uss our
results in relation to questions of stability and reproducibility raised in
experiments. To circumvent the difficulties of controlling intrinsic defects,
we propose specific non-magnetic host doping that could be, for example,
substituted for cations in HfO or ZrO.Comment: 4 figures + tex files. Accepted for publication in Physical Review
Letters. Note that the title has changed and the text is slightly modifie
Magnetic Structures and Spin-wave Excitations in Rare-Earth Iron Garnets near the Compensation Temperature
We introduce a simple model for the ferrimagnetic non-collinear ``magnetic
umbrella" states of rare-earth iron garnets (REIG), common when the rare-earth
moments have non-zero orbital angular momentum. The spin-wave excitations are
calculated within linear spin wave theory and temperature effects via
mean-field theory. This could be used to determine the magnetic polarization of
each mode and thereby the spin currents generated by thermal excitations
including the effects of mixed chirality. The spectra reproduce essential
features seen in more complete models, with hybridization between the rare
earth crystal field excitations and the propagating mode on the iron moments.
By the symmetry of the model, only one rare earth mode hybridizes, inducing a
gap at zero wave number and level repulsion at finite frequency. At the
compensation point, the hybridization gap closes and finally, as we approach
the N\'eel temperature, the hybridization gap appears to reopen. The chirality
of the lowest mode changes its sign around the frequency at which the level
repulsion occurs. This is important to estimate the spin current generation in
REIGs.Comment: 6 pages, 8 figure
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