1,273 research outputs found
The luminescence of yitria stabilized zirconia doped with Bi2O3
Yttria stabilized zirconia doped with Bi2O3 shows an efficient luminescence due to the Bi3+ ions. Two luminescent centres were observed which are ascribed to BiO8 and BiO7 groups
Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAlGaO: Lattice expansion and site disorder
The spinels CoBO with magnetic Co ions on the diamond lattice
A site can be frustrated because of competing near-neighbor () and
next-near neighbor () interactions. Here we describe attempts to tune the
relative strengths of these interactions by substitution on the non-magnetic
B-site. The system we employ is CoAlGaO, where Al is
systematically replaced by the larger Ga, ostensibly on the B site. As
expected, Ga substitution expands the lattice, resulting in Co atoms on the
A-site being pushed further from one other and thereby weakening magnetic
interactions. In addition, Ga distributes between the B and the A site in a
concentration dependent manner displacing an increasing amount of Co from the A
site with increasing . This increased inversion, which is confirmed by
neutron diffraction studies carried out at room temperature, affects magnetic
ordering very significantly, and changes the nature of the ground state.
Modeling of the magnetic coupling illustrates the complexity that arises from
the cation site disorder.Comment: 9 pages, 10 figure
Study of Magnetic Properties of A_2B^'NbO_6 (A=Ba,Sr, (BaSr): and B^'=Fe and Mn) double perovskites
We have studied the magnetic properties of Ba_2FeNbO_6 and Ba_2MnNbO_6. it is
seen that Ba_2FeNbO_6 is an antiferromagnet with a weak ferromagnetic behaviour
at 5K while Ba_2MnNbO_6 shows two magnetic transitions one at 45 K and the
other at 12K. Electron spin resonance (ESR) measurements at room temperature
show that the Mn compound does not show any Jahn-Teller distortion. It is also
seen that the Neel temperature of the A_2FeNbO_6 (A=Ba,Sr, BaSr) compounds do
not vary significantly. However variations in the average A-site ionic radius
influence the formation of short range correlations that persist above T_N.Comment: 10 oages, 5 figures, MMM, to appear in J.Appl.Phy
An effective spin-orbital Hamiltonian for the double perovskite SrFeW O: Derivation of the phase diagram
We formulate a superexchange theory of insulating double-perovskite compounds
such as SrFeWO. An effective spin-orbital Hamiltonian is derived in the
strong coupling limit of Hubbard model for d-electrons on Fe and W ions. The
relevant degrees of freedom are the spins S=2 and the three-fold orbital
degeneracy of Fe-ions. W-sites are integrated out by means of a
fourth-order perturbative expansion. The magnetically and orbitally ordered
ground states of the effective Hamiltonia n are discussed as a function of the
model parameters. We show that for realistic values of such parameters the
ground state is antiferromagnetic, as experimentally observed. The order found
is of type-II, consisting of \{111\} ferromagnetic planes stac ked
antiferromagnetically. The orbital order energy scale found is one order of
magnitude less than the spi n one.Comment: 12 pages, 4 figure
Fluorescent oxide nanoparticles adapted to active tips for near-field optics
We present a new kind of fluorescent oxide nanoparticles with properties well
suited to active-tip based near-field optics. These particles with an average
diameter in the range 5-10 nm are produced by Low Energy Cluster Beam
Deposition (LECBD) from a YAG:Ce3+ target. They are studied by transmission
electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), classical
photoluminescence, cathodoluminescence and near-field scanning optical
microscopy (NSOM). Particles of extreme photo-stability as small as 10 nm in
size are observed. These emitters are validated as building blocks of active
NSOM tips by coating a standard optical tip with a 10 nm thick layer of
YAG:Ce3+ particles directly in the LECBD reactor and by subsequently performing
NSOM imaging of test surfaces.Comment: Changes made following Referee's comments; added references; one
added figure. See story on this article at:
http://nanotechweb.org/cws/article/tech/3606
Magnetic phase evolution in the spinel compounds ZnCoCrO
We present the magnetic properties of complete solid solutions of
ZnCrO and CoCrO: two well-studied oxide spinels with very
different magnetic ground states. ZnCrO, with non-magnetic
cations occupying the A site and magnetic cations on the B site, is a
highly frustrated antiferromagnet. CoCrO, with magnetic cations
(three unpaired electrons) on the A site as well, exhibits both N\'eel
ferrimagnetism as well as commensurate and incommensurate non-collinear
magnetic order. More recently, CoCrO has been studied extensively for
its polar behavior which arises from conical magnetic ordering. Gradually
introducing magnetism on the A site of ZnCrO results in a transition
from frustrated antiferromagnetism to glassy magnetism at low concentrations of
Co, and eventually to ferrimagnetic and conical ground states at higher
concentrations. Real-space Monte-Carlo simulations of the magnetic
susceptibility suggest that the first magnetic ordering transition and features
of the susceptibility across are captured by near-neighbor self- and
cross-couplings between the magnetic A and B atoms. We present as a part of
this study, a method for displaying the temperature dependence of magnetic
susceptibility in a manner which helps distinguish between compounds possessing
purely antiferromagnetic interactions from compounds where other kinds of
ordering are present.Comment: 10 pages, 5 figures, 1 tabl
Structural and Luminescence Properties of Silica-Based Hybrids Containing New Silylated-Diketonato Europium(III) Complex
A new betadiketonate ligand displaying a trimethoxysilyl group as grafting function and a diketone moiety as complexing site (TTA-Si = 4,4,4-trifluoro-2-(3-trimethoxysilyl)propyl)-1-3-butanedione (C4H3S)COCH[(CH2)3Si(OCH3)3]COCF3) and its highly luminescent europium(III) complex [Eu(TTA-Si)3] have been synthesized and fully characterized. Luminescent silica-based hybrids have been prepared as well with this new complex grafted on the surface of dense silica nanoparticles (28 (+/-3 nm) or on mesoporous
silica particles. The covalent bonding of Eu(TTA-Si)3 inside the core of uniform silica
nanoparticles (40 (+/- 5 nm) was also achieved. Luminescence properties are discussed in relation to the europium chemical environment involved in each of the three hybrids. The general methodology proposed allowed high grafting ratios and overcame chelate release and tendency to agglomeration, and it could be applied to any silica matrix (in the core or at the surface, nanosized or not, dense or mesoporous) and therefore numerous applications such as luminescent markers and luminophors could be foreseen
Pressure-Induced Two-Color Photoluminescence in MnF2 at Room Temperature
A novel two-color photoluminescence (PL) is found in MnF2 at room temperature under high pressure.
Contrary to low-temperature PL, PL at room temperature is unusual in transition-metal concentrated
materials like MnF2, since the deexcitation process at room temperature is fully governed by energy
transfer to nonradiative centers. We show that room-temperature PL in MnF2 originates from two distinct
Mn2 emissions in the high-pressure cotunnite phase. The electronic structure and the excited-state
dynamics are investigated by time-resolved emission and excitation spectroscopy at high pressure
- …