29 research outputs found
Evolution of the magnetic phase transition in MnO confined to channel type matrices. Neutron diffraction study
Neutron diffraction studies of antiferromagnetic MnO confined to MCM-41 type
matrices with channel diameters 24-87 A demonstrate a continuous magnetic phase
transition in contrast to a discontinuous first order transition in the bulk.
The character of the magnetic transition transforms with decreasing channel
diameter, showing the decreasing critical exponent and transition temperature,
however the latter turns out to be above the N\'eel temperature for the bulk.
This enhancement is explained within the framework of Landau theory taking into
consideration the ternary interaction of the magnetic and associated structural
order parameters.Comment: 6 pages pdf file, including 4 figures, uses revtex4.cl
ESR of MnO embedded in silica nanoporous matrices with different topologies
Electron spin resonance (ESR) experiments were performed with
antiferromagnetic MnO confined within a porous vycor-type glass and within
MCM-type channel matrices. A signal from confined MnO shows two components from
crystallized and amorphous MnO and depends on the pore topology. Crystallized
MnO within a porous glass shows a behavior having many similarities to the
bulk. In contrast with the bulk the strong ESR signal due to disordered
"surface" spins is observed below the magnetic transition. With the decrease of
channel diameter the fraction of amorphous MnO increases while the amount of
crystallized MnO decreases. The mutual influence of amorphous and crystalline
MnO is observed in the matrices with a larger channel diameter. In the matrices
with a smaller channel diameter the ESR signal mainly originates from amorphous
MnO and its behavior is typical for the highly disordered magnetic system.Comment: 7 pages pdf file, 5 figure
Structure of MnO nanoparticles embedded into channel-type matrices
X-ray diffraction experiments were performed on MnO confined in mesoporous
silica SBA-15 and MCM-41 matrices with different channel diameters. The
measured patterns were analyzed by profile analysis and compared to numerical
simulations of the diffraction from confined nanoparticles. From the lineshape
and the specific shift of the diffraction reflections it was shown that the
embedded objects form ribbon-like structures in the SBA-15 matrices with
channels diameters of 47-87 {\AA}, and nanowire-like structures in the MCM-41
matrices with channels diameters of 24-35 {\AA}. In the latter case the
confined nanoparticles appear to be narrower than the channel diameters. The
physical reasons for the two different shapes of the confined nanoparticles are
discussed.Comment: 8 pages, including 9 postscript figures, uses revtex4.cl
Superparaelectric phase in the ensemble of non-interacting ferroelectric nanoparticles
For the first time we predict the conditions of superparaelectric phase
appearance in the ensemble of non-interacting spherical ferroelectric
nanoparticles. The superparaelectricity in nanoparticle was defined by analogy
with superparamagnetism, obtained earlier in small nanoparticles made of
paramagnetic material. Calculations of correlation radius, energetic barriers
of polarization reorientation and polarization response to external electric
field, were performed within Landau-Ginzburg phenomenological approach for
perovskites Pb(Zr,Ti)O3, BiFeO3 and uniaxial ferroelectrics rochelle salt and
triglycine sulfate.Comment: 28 pages, 7 figures, 3 Appendices, to be submitted to Phys. Rev.
Superconductivity in Nanostructured Ga-Ag Eutectic Alloy
The dc and ac magnetizations were studied for the eutectic Ga-Ag alloy embedded into a porous glass with 7 nm mean pore size. The measurements were performed within a temperature range of 1.9 to 10 K which covers the superconducting transition for the nanocomposite. The onset of superconductivity at the magnetic field 10 Oe was found at about 7.1 K which is much higher than in bulk gallium. The phase diagram showed a positive curvature at low magnetic fields. The activation energy of vortex mobility was calculated at different magnetic fields using the ac data.
Keywords: Ga-Ag eutectic alloy, superconductivity, nanoconfinemen