2,141 research outputs found
On the relationship between sigma models and spin chains
We consider the two-dimensional non-linear sigma model with
topological term using a lattice regularization introduced by Shankar and Read
[Nucl.Phys. B336 (1990), 457], that is suitable for studying the strong
coupling regime. When this lattice model is quantized, the coefficient
of the topological term is quantized as , with integer or
half-integer. We study in detail the relationship between the low energy
behaviour of this theory and the one-dimensional spin- Heisenberg model. We
generalize the analysis to sigma models with other symmetries.Comment: To appear in Int. J. MOd. Phys.
Control of the cation occupancies of MnZn ferrite synthesized via reverse micelles
Variations in cation occupancy in mixed metal ferrite systems can affect their electronic and magnetic properties. It is known that different synthesis parameters can lead to various cation distributions and the ability to tune these distributions is of great interest. This study uses the extended x-ray-absorption fine structure–IR relationship to investigate the effect of various Fe2+/Fe3+ ratios in initial synthesis conditions on cation distribution for manganesezincferrite (MZFO). Differences in the precipitated material before firing could lead to differences in the final material if fired under similar conditions. This work uses several different ratios of Fe3+/Fe2+, which will affect the initial cell potential for the reaction, to synthesize nano MZFO. All samples were fired for 5h at 500°C under flowing nitrogen. Transmission electron microscopy micrographs reveal highly crystalline uniform nanoparticles of 16±2nm. The x-ray diffraction revealed single phase crystalline MZFO with an average crystallite size of around 14nm. The saturation magnetization ranged from 43to68emu∕g as measured by vibrating-sample magnetometry. The Fourier transform infrared (FTIR) analysis was used to determine the cation occupancies while changing the initial Fe3+/Fe2+ ratios from 10∕90 to90∕10. The FTIRspectra revealed a shift in the first absorption region in the far IR from 566.98to549.62cm−1 corresponding to the octahedral occupancies. This shift corresponds to a change in the percentage of octahedral sites occupied by manganese from roughly 25% to 12%. This change in manganese occupancy is also observed in the iron occupancies, which in turn help to explain the variation in saturation magnetization
Spin transition in GdN@C, detected by low-temperature on-chip SQUID technique
We present a magnetic study of the GdN@C molecule, consisting of a
Gd-trimer via a Nitrogen atom, encapsulated in a C cage. This molecular
system can be an efficient contrast agent for Magnetic Resonance Imaging (MRI)
applications. We used a low-temperature technique able to detect small magnetic
signals by placing the sample in the vicinity of an on-chip SQUID. The
technique implemented at NHMFL has the particularity to operate in high
magnetic fields of up to 7 T. The GdN@C shows a paramagnetic
behavior and we find a spin transition of the GdN structure at 1.2 K. We
perform quantum mechanical simulations, which indicate that one of the Gd ions
changes from a state () to a state (), likely due to a charge transfer between the C cage and the ion
One parameter control of the size of iron oxide nanoparticles synthesized in reverse micelles
Iron oxide nanoparticles were synthesized via reverse micelle methods. The initial iron concentration was varied, while maintaining all other parameters constant, in order to investigate the effect of the iron concentration on the resultant iron oxide nanoparticle size. Increasing the iron concentration from 0.125M to 0.5M yielded an increase in average nanoparticle diameter from 4.71 to 7.95 nm, as measured by transmission electron microscopy. Three other concentrations between 0.125M and 0.5M showed corresponding size variations, all with statistical significance. Magnetic characterization by vibrating sample magnetometry and powder x-ray diffraction was performed to verify proper phase and material. Further insight into the reverse micelle method was acquired along with the ability to tune the nanoparticle size
Cation occupancy determination in manganese zinc ferrites using Fourier transform infrared spectroscopy
The magnetic and electric properties of ferrites are influenced by the cation distribution within the crystalline spinel lattice. Methods such as extended x-ray-absorption fine structure(EXAFS) have been used to determine cation occupancies within the crystalline structure of materials such as manganesezincferrite (MZFO); however, it is not practical to be used for daily analysis. Fourier transform infrared (FTIR)spectroscopy is another technique which has the potential to determine cation occupancy while offering speed and convenience. In the literature it has been demonstrated that in ferrite systems FTIR data can be correlated to cation percentages when comparing tetrahedral (Td) and octahedral (Oh) sites. FTIRspectra were collected on a series of MZFO nanoparticles in the range from 200 to 600cm−1 and two absorbance peaks were observed. The first absorption region shifted with changing sample composition as calculated from transmission EXAFS experiments and elemental analysis. The data was normalized to the maximum of the peak of interest and the shifts were correlated to cation occupancy
Photometric variability of the LAMOST sample of magnetic chemically peculiar stars as seen by TESS
High-quality light curves from space missions have opened up a new window on
the rotational and pulsational properties of magnetic chemically peculiar (mCP)
stars and have fuelled asteroseismic studies. They allow the internal effects
of surface magnetic fields to be probed and numerous astrophysical parameters
to be derived with great precision. We present an investigation of the
photometric variability of a sample of 1002 mCP stars discovered in the LAMOST
archival spectra with the aims of measuring their rotational periods and
identifying interesting objects for follow-up studies. TESS photometry was
available for 782 mCP stars and was analysed using a Fourier two-term frequency
fit to determine the stars' rotational periods. The rotational signal was then
subtracted from the light curve to identify non-rotational variability. A
pixel-level blending analysis was performed to check whether the variability
originates in the target star or a nearby blended neighbour. We investigated
correlations between the rotational periods, fractional age on the main
sequence, mass, and several other observables. We present rotational periods
and period estimates for 720 mCP stars. In addition, we identified four
eclipsing binary systems that likely host an mCP star, as well as 25 stars with
additional signals consistent with pulsation (12 stars with frequencies above
10 d and 13 stars with frequencies below 10 ). We find that more
evolved stars have longer rotation periods, in agreement with the assumption of
the conservation of angular momentum during main-sequence evolution. With our
work, we increase the sample size of mCP stars with known rotation periods and
identify prime candidates for detailed follow-up studies. This enables two
paths towards future investigations: population studies of even larger samples
of mCP stars and the detailed characterisation of high-value targets.Comment: 30 pages, 9 figures, 1 table. Accepted for publication in the Journal
of Astronomy and Astrophysics (A&A
Searching for Weak or Complex Magnetic Fields in Polarized Spectra of Rigel
Seventy-eight high-resolution Stokes V, Q and U spectra of the B8Iae
supergiant Rigel were obtained with the ESPaDOnS spectropolarimeter at CFHT and
its clone NARVAL at TBL in the context of the Magnetism in Massive Stars
(MiMeS) Large Program, in order to scrutinize this core-collapse supernova
progenitor for evidence of weak and/or complex magnetic fields. In this paper
we describe the reduction and analysis of the data, the constraints obtained on
any photospheric magnetic field, and the variability of photospheric and wind
lines.Comment: IAUS272 - Active OB Stars: Structure, Evolution, Mass Loss and
Critical Limit
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