2,843 research outputs found
Absence of ferromagnetism in V-implanted ZnO single crystals
The structural and magnetic properties of V doped ZnO are presented. V ions
were introduced into hydrothermal ZnO single crystals by ion implantation with
fluences of 1.2*10^16 to 6*10^16 cm^-2. Post-implantation annealing was
performed in high vacuum from 823 K to 1023 K. The ZnO host material still
partly remains in a crystalline state after irradiation, and is partly
recovered by annealing. The V ions show a thermal mobility as revealed by depth
profile Auger electron spectroscopy. Synchrotron radiation x-ray diffraction
revealed no secondary phase formation which indicates the substitution of V
onto Zn site. However in all samples no pronounced ferromagnetism was observed
down to 5 K by a superconducting quantum interference device magnetometer.Comment: 13 pages, 4 figs, MMM conference 2007, accepted by J. Appl. Phy
High Curie temperature and perpendicular magnetic anisotropy in homoepitaxial InMnAs films
We have prepared the dilute magnetic semiconductor (DMS) InMnAs with
different Mn concentrations by ion implantation and pulsed laser melting. The
Curie temperature of the In1-xMnxAs epilayer depends on the Mn concentration x,
reaching 82 K for x=0.105. The substitution of Mn ions at the Indium sites
induces a compressive strain perpendicular to the InMnAs layer and a tensile
strain along the in-plane direction. This gives rise to a large perpendicular
magnetic anisotropy, which is often needed for the demonstration of electrical
control of magnetization and for spin-transfer-torque induced magnetization
reversal.Comment: 16 pages, 5 figure
Recommended from our members
Annual report 2007 // Institute of Ion Beam Physics and Materials Research
[no abstract available
Crystallographically oriented Co and Ni nanocrystals inside ZnO formed by ion implantation and postannealing
In the last decade, transition-metal-doped ZnO has been intensively
investigated as a route to room-temperature diluted magnetic semiconductors
(DMSs). However, the origin for the reported ferromagnetism in ZnO-based DMS
remains questionable. Possible options are diluted magnetic semiconductors,
spinodal decomposition, or secondary phases. In order to clarify this question,
we have performed a thorough characterization of the structural and magnetic
properties of Co- and Ni-implanted ZnO single crystals. Our measurements reveal
that Co or Ni nanocrystals (NCs) are the major contribution of the measured
ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed
and they exhibit superparamagnetic properties. The Co or Ni NCs are
crystallographically oriented with respect to the ZnO matrix. Their magnetic
properties, e.g., the anisotropy and the superparamagnetic blocking
temperature, can be tuned by annealing. We discuss the magnetic anisotropy of
Ni NCs embedded in ZnO concerning the strain anisotropy.Comment: 13 pages, 14 figure
System-environment correlations and Non-Markovian dynamics
We determine the total state dynamics of a dephasing open quantum system
using the standard environment of harmonic oscillators. Of particular interest
are random unitary approaches to the same reduced dynamics and
system-environment correlations in the full model. Concentrating on a model
with an at times negative dephasing rate, the issue of "non-Markovianity" will
also be addressed. Crucially, given the quantum environment, the appearance of
non-Markovian dynamics turns out to be accompanied by a loss of
system-environment correlations. Depending on the initial purity of the qubit
state, these system-environment correlations may be purely classical over the
whole relevant time scale, or there may be intervals of genuine
system-environment entanglement. In the latter case, we see no obvious relation
between the build-up or decay of these quantum correlations and
"Non-Markovianity"
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