281 research outputs found
Transition metal implanted ZnO: a correlation between structure and magnetism
Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors question the origin of this ferromagnetism, i.e. if the observed ferromagnetism stems from ferromagnetic precipitates rather than from carriermediated magnetic coupling of ionic impurities, as required for a diluted magnetic semiconductor. In this thesis, this question will be answered for transition-metal implanted ZnO single crystals. Magnetic secondary phases, namely metallic Fe, Co and Ni nanocrystals, are formed inside ZnO. They are - although difficult to detect by common approaches of structural analysis - responsible for the observed ferromagnetism. Particularly Co and Ni nanocrystals are crystallographically oriented with respect to the ZnO matrix. Their structure phase transformation and corresponding evolution of magnetic properties upon annealing have been established. Finally, an approach, pre-annealing ZnO crystals at high temperature before implantation, has been demonstrated to sufficiently suppress the formation of metallic secondary phases
Ferromagnetic InMnAs on InAs Prepared by Ion Implantation and Pulsed Laser Annealing
Ferromagnetic InMnAs has been prepared by Mn ion implantation and pulsed
laser annealing. The InMnAs layer reveals a saturated magnetization of 2.6
mu_B/Mn at 5 K and a perpendicular magnetic anisotropy. The Curie temperature
is determined to be 46 K, which is higher than those in previous reports with
similar Mn concentrations. Ferromagnetism is further evidenced by the large
magnetic circular dichroism.Comment: 9 pages, 3 figure
The importance of hole concentration in establishing carrier-mediated ferromagnetism in Mn doped Ge
In the present work, we have prepared Mn-doped Ge using different annealing
approaches after Mn ion implantation, and obtained samples with hole
concentrations ranging from 10^18 to 2.1x10^20 cm^-3, the latter being the
highest reported so far. Based on the magnetotransport properties of Mn doped
Ge, we argue that the hole concentration is a decisive parameter in
establishing carrier-mediated ferromagnetism in magnetic Ge.Comment: 7 pages, 3 figure
MnSi nanoparticles embedded in Si: Superparamagnetism with a collective behavior
The doping of Mn in Si is attracting research attentions due to the
possibility to fabricate Si-based diluted magnetic semiconductors. However, the
low solubility of Mn in Si favors the precipitation of Mn ions even at
non-equilibrium growth conditions. MnSi nanoparticles are the common
precipitates, which show exotic magnetic properties in comparison with the
MnSi bulk phase. In this paper we present the static and dynamic
magnetic properties of MnSi nanoparticles. Using the Preisach model, we
derive the magnetic parameters, such as the magnetization of individual
particles, the distribution of coercive fields and the inter-particle
interaction field. Time-dependent magnetization measurements reveal a
spin-glass behavior of the system.Comment: 11 pages, 6 figures, submitted to PR
Carrier induced ferromagnetism in the insulating Mn doped III-V semiconductor InP
Although InP and GaAs have very similar band-structure their magnetic
properties appear to drastically differ. Critical temperatures in (In,Mn)P are
much smaller than that of (Ga,Mn)As and scale linearly with Mn concentration.
This is in contrast to the square root behaviour found in (Ga,Mn)As. Moreover
the magnetization curve exhibits an unconventional shape in (In,Mn)P
contrasting with the conventional one of well annealed (Ga,Mn)As. By combining
several theoretical approaches, the nature of ferromagnetism in Mn doped InP is
investigated. It appears that the magnetic properties are essentially
controlled by the position of the Mn acceptor level. Our calculations are in
excellent agreement with recent measurements for both critical temperatures and
magnetizations. The results are only consistent with a Fermi level lying in an
impurity band, ruling out the possibility to understand the physical properties
of Mn doped InP within the valence band scenario. The quantitative success
found here reveals a predictive tool of choice that should open interesting
pathways to address magnetic properties in other compoundsComment: 5 pages and 5 figures, accepted for publication in Phys. Rev.
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
Defect-induced magnetism in graphite through neutron irradiation
We have investigated the variation in the magnetization of highly ordered
pyrolytic graphite (HOPG) after neutron irradiation, which introduces defects
in the bulk sample and consequently gives rise to a large magnetic signal. We
observe strong paramagnetism in HOPG, increasing with the neutron fluence. We
correlate the induced paramagnetism with structural defects by comparison with
density-functional theory calculations. In addition to the in-plane vacancies,
the trans-planar defects also contribute to the magnetization. The lack of any
magnetic order between the local moments is possibly due to the absence of
hydrogen/nitrogen chemisorption, or the magnetic order cannot be established at
all in the bulk form.Comment: 10 pages, 8 figure
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