60 research outputs found
Antiferromagnetic interactions in single crystalline Zn1-xCoxO thin films
In a rather contradictory situation regarding magnetic data on Co-doped ZnO,
we have succeeded in fabricating high-quality single crystalline Zn1-xCoxO
(x=0.003-0.07) thin films. This gives us the possibility, for the first time,
to examine the it intrinsic magnetic properties of ZnO:Co at a quantitative
level and therefore to address several unsolved problems, the major one being
the nature of the Co-Co interaction in the ZnO structure.Comment: 4 pages, 4 figures,accepted for publication in PR
Spin-Exchange Interaction in ZnO-based Quantum Wells
Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit
unprecedented tunability of the short-range spin exchange interaction. In the
context of large exciton binding energies and electron-hole exchange
interaction in ZnO, this tunability results from the competition between
quantum confinement and giant quantum confined Stark effect. By using
time-resolved photoluminescence we identify, for well widths under 3 nm, the
redistribution of oscillator strengths between the A and B excitonic
transitions, due to the enhancement of the exchange interaction. Conversely,
for wider wells, the redistribution is cancelled by the dominant effect of
internal electric fields, which dramatically reduce the exchange energy.Comment: 14 pages, 3 figure
Shifting donor-acceptor photoluminescence in N-doped ZnO
We have grown nitrogen-doped ZnO films grown by two kinds of epitaxial
methods on lattice-matched ScAlMgO substrates. We measured the
photoluminescence (PL) of the two kinds of ZnO:N layers in the
donor-acceptor-pair transition region. The analysis of excitation-intensity
dependence of the PL peak shift with a fluctuation model has proven that our
observed growth-technique dependence was explained in terms of the
inhomogeneity of charged impurity distribution. It was found that the
inhomogeneity in the sample prepared with the process showing better electrical
property was significantly smaller in spite of the similar nitrogen
concentration. The activation energy of acceptor has been evaluated to be
meV, which is independent of the nitrogen concentration.Comment: 4 pages, 3 figures, 1 table, RevTeX4, to appear in the July issue of
J. Phys. Soc. Jp
Magnetic Anisotropy of Co2+ as Signature of Intrinsic Ferromagnetism in ZnO:Co
We report on the magnetic properties of thoroughly characterized Zn1-xCoxO
epitaxial thin films, with low Co concentration, x=0.003-0.005. Magnetic and
EPR measurements, combined with crystal field theory, reveal that isolated Co2+
ions in ZnO possess a strong single ion anisotropy which leads to an "easy
plane" ferromagnetic state when the ferromagnetic Co-Co interaction is
considered. We suggest that the peculiarities of the magnetization process of
this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co
materials.Comment: 4 pages, 4 figure
Influence of s,p-d and s-p exchange couplings on exciton splitting in (Zn,Mn)O
This work presents results of near-band gap magnetooptical studies on
(Zn,Mn)O epitaxial layers. We observe excitonic transitions in reflectivity and
photoluminescence, that shift towards higher energies when the Mn concentration
increases and split nonlinearly under the magnetic field. Excitonic shifts are
determined by the s,p-d exchange coupling to magnetic ions, by the
electron-hole s-p exchange, and the spin-orbit interactions. A quantitative
description of the magnetoreflectivity findings indicates that the free
excitons A and B are associated with the Gamma_7 and Gamma_9 valence bands,
respectively, the order reversed as compared to wurtzite GaN. Furthermore, our
results show that the magnitude of the giant exciton splittings, specific to
dilute magnetic semiconductors, is unusual: the magnetoreflectivity data is
described by an effective exchange energy N_0(beta-alpha)=+0.2+/-0.1 eV, what
points to small and positive N_0 beta. It is shown that both the increase of
the gap with x and the small positive value of the exchange energy N_0 beta
corroborate recent theory describing the exchange splitting of the valence band
in a non-perturbative way, suitable for the case of a strong p-d hybridization.Comment: 8 pages, 8 figure
Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies
We studied structural, optical and magnetic properties of high-quality 5 and
15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on
(0001)-sapphire substrates. Magnetic force microscopy (MFM) and magnetic
measurements with SQUID magnetometer show clear ferromagnetic behavior of the
films up to room temperature whereas they are antiferromagnetic below 200 K
approximately. Temperature dependence of the carrier mobility was determined
using Raman line shape analysis of the longitudinal-optical-phonon-plasmon
coupled modes. It shows that the microscopic mechanism for ferromagnetic
ordering is coupling mediated by free electrons between spins of Co atoms.
These results bring insight into a subtle interplay between charge carriers and
magnetism in MBE-grown Zn(1-x)CoxO films.Comment: 10 pages, 9 figures, 2 table
Bearing defect diagnosis and acoustic emission.
Acoustic emission (AE) was originally developed for non-destructive testing of
static structures, however, over the years its application has been extended to
health monitoring of rotating machines and bearings. It offers the advantage of
earlier defect detection in comparison to vibration analysis. However,
limitations in the successful application of AE technique for monitoring
bearings have been partly due to thedifficulty in processing, interpreting and
classifying the acquired data. The investigation reported in this paper was
centered on the application of standard acoustic emissions (AE) characteristic
parameters on a radially loaded bearing. An experimental test-rig was modified
such that defects could be seeded onto the inner and outer races of a test
bearing. As the test-rig was adapted for this purpose it offered high background
acoustic emission noise providing a realistic test for fault diagnosis. In
addition to a review of current diagnostic methods for applying AE to bearing
diagnosis, the results of this investigation validated the use of r.m.s,
amplitude, energy and AE counts for diagnosis. Furthermore, this study
determined the most appropriate threshold level for AE count diagnosis, the
first known attempt
Quantitative evaluation of the impurity content of grease for low-speed heavy-duty bearing using an acoustic emission technique
Single-ion anisotropy in Mn-doped diluted magnetic semiconductors
International audienceMotivated by recent developments in spintronics, we propose an explanation of the single-ion anisotropy of Mn-doped diluted magnetic semiconductors using as an example high-quality ZnO:Mn thin films for which X-band electron-paramagnetic-resonance studies were performed. We derive an analytic formula for the axial parameter D and we prove its validity by the exact diagonalization method. We demonstrate a quantitative agreement between the experimental data and our model. These results bring insights into a long-standing problem of single-ion anisotropy in magnetic solids
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