2,005 research outputs found
Magnitude and crystalline anisotropy of hole magnetization in (Ga,Mn)As
Theory of hole magnetization Mc in zinc-blende diluted ferromagnetic
semiconductors is developed relaxing the spherical approximation of earlier
approaches. The theory is employed to determine Mc for (Ga,Mn)As over a wide
range of hole concentrations and a number of crystallographic orientations of
Mn magnetization. It is found that anisotropy of Mc is practically negligible
but the obtained magnitude of Mc is significantly greater than that determined
in the spherical approximation. Its sign and value compares favorably with the
results of available magnetization measurements and ferromagnetic resonance
studies.Comment: 5 pages, 3 figure
Origin of ferromagnetism in (Zn,Co)O from magnetization and spin-dependent magnetoresistance
In order to elucidate the nature of ferromagnetic signatures observed in
(Zn,Co)O we have examined experimentally and theoretically magnetic properties
and spin-dependent quantum localization effects that control low-temperature
magnetoresistance. Our findings, together with a through structural
characterization, substantiate the model assigning spontaneous magnetization of
(Zn,Co)O to uncompensated spins at the surface of antiferromagnetic nanocrystal
of Co-rich wurtzite (Zn,Co)O. The model explains a large anisotropy observed in
both magnetization and magnetoresistance in terms of spin hamiltonian of Co
ions in the crystal field of the wurtzite lattice.Comment: 6 pages, 6 figure
Spin-related magnetoresistance of n-type ZnO:Al and Zn_{1-x}Mn_{x}O:Al thin films
Effects of spin-orbit coupling and s-d exchange interaction are probed by
magnetoresistance measurements carried out down to 50 mK on ZnO and
Zn_{1-x}Mn_{x}O with x = 3 and 7%. The films were obtained by laser ablation
and doped with Al to electron concentration ~10^{20} cm^{-3}. A quantitative
description of the data for ZnO:Al in terms of weak-localization theory makes
it possible to determine the coupling constant \lambda_{so} = (4.4 +-
0.4)*10^{-11} eVcm of the kp hamiltonian for the wurzite structure, H_{so} =
\lambda_{so}*c(s x k). A complex and large magnetoresistance of
Zn_{1-x}Mn_{x}O:Al is interpreted in terms of the influence of the s-d
spin-splitting and magnetic polaron formation on the disorder-modified
electron-electron interactions. It is suggested that the proposed model
explains the origin of magnetoresistance observed recently in many magnetic
oxide systems.Comment: 4 pages, 4 figure
A new magnetic field dependence of Landau levels on a graphene like structure
We consider a tight-binding model on the honeycomb lattice in a magnetic
field. For special values of the hopping integrals, the dispersion relation is
linear in one direction and quadratic in the other. We find that, in this case,
the energy of the Landau levels varies with the field B as E_n(B) ~
[(n+\gamma)B]^{2/3}. This result is obtained from the low-field study of the
tight-binding spectrum on the honeycomb lattice in a magnetic field (Hofstadter
spectrum) as well as from a calculation in the continuum approximation at low
field. The latter links the new spectrum to the one of a modified quartic
oscillator. The obtained value is found to result from the
cancellation of a Berry phase.Comment: 4 pages, 4 figure
Magnetic properties of undoped Cu2O fine powders with magnetic impurities and/or cation vacancies
Fine powders of micron- and submicron-sized particles of undoped Cu2O
semiconductor, with three different sizes and morphologies have been
synthesized by different chemical processes. These samples include nanospheres
200 nm in diameter, octahedra of size 1 micron, and polyhedra of size 800 nm.
They exhibit a wide spectrum of magnetic properties. At low temperature, T = 5
K, the octahedron sample is diamagnetic. The nanosphere is paramagnetic. The
other two polyhedron samples synthesized in different runs by the same process
are found to show different magnetic properties. One of them exhibits weak
ferromagnetism with T_C = 455 K and saturation magnetization, M_S = 0.19 emu/g
at T = 5 K, while the other is paramagnetic. The total magnetic moment
estimated from the detected impurity concentration of Fe, Co, and Ni, is too
small to account for the observed magnetism by one to two orders of magnitude.
Calculations by the density functional theory (DFT) reveal that cation
vacancies in the Cu2O lattice are one of the possible causes of induced
magnetic moments. The results further predict that the defect-induced magnetic
moments favour a ferromagnetically coupled ground state if the local
concentration of cation vacancies, n_C, exceeds 12.5%. This offers a possible
scenario to explain the observed magnetic properties. The limitations of the
investigations in the present work, in particular in the theoretical
calculations, are discussed and possible areas for further study are suggested.Comment: 20 pages, 5 figures 2 tables, submitted to J Phys Condense Matte
Origin of bulk uniaxial anisotropy in zinc-blende dilute magnetic semiconductors
It is demonstrated that the nearest neighbor Mn pair on the GaAs (001)
surface has a lower energy for the [-110] direction comparing to the [110]
case. According to the group theory and the Luttinger's method of invariants,
this specific Mn distribution results in bulk uniaxial in-plane and
out-of-plane anisotropies. The sign and magnitude of the corresponding
anisotropy energies determined by a perturbation method and ab initio
computations are consistent with experimental results.Comment: 5 pages, 1 figur
Electromodulation of the Magnetoresistance in Diluted Magnetic Semiconductors Based Heterostructures
We study the properties of heterostructures formed by two layers of diluted
magnetic semiconductor separated by a nonmagnetic semiconductor layer. We find
that there is a RKKY-type exchange coupling between the magnetic layers that
oscilles between ferromagnetic and antiferromagnetic as a function of the
different parameters in the problem. The different transport properties of
these phases make that this heterostructure presents strong magnetoresistive
effects. The coupling can be also modified by an electric field. We propose
that it is possible to alter dramatically the electrical resistance of the
heterostructure by applying an electric field. Our results indicate that in a
single gated sample the magnetoresistance could be modulated by with an
electrical bias voltage.Comment: 4 pages, 5 figures include
Spin separation in digital ferromagnetic heterostructures
In a study of the ferromagnetic phase of a multilayer digital ferromagnetic
semiconductor in the mean-field and effective-mass approximations, we find the
exchange interaction to have the dominant energy scale of the problem,
effectively controlling the spatial distribution of the carrier spins in the
digital ferromagnetic heterostructures. In the ferromagnetic phase, the
majority and minority carriers tend to be in different regions of the space
(spin separation). Hence, the charge distribution of carriers also changes
noticeably from the ferromagnetic to the paramagnetic phase. An example of a
design to exploit these phenomena is given.Comment: 4 pages, 3 figures. Submitted to Phys. Rev.
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