635 research outputs found
Magnetic properties and spin dynamics in single molecule paramagnets Cu6Fe and Cu6Co
The magnetic properties and the spin dynamics of two molecular magnets have
been investigated by magnetization and d.c. susceptibility measurements,
Electron Paramagnetic Resonance (EPR) and proton Nuclear Magnetic Resonance
(NMR) over a wide range of temperature (1.6-300K) at applied magnetic fields,
H=0.5 and 1.5 Tesla. The two molecular magnets consist of
CuII(saldmen)(H2O)}6{FeIII(CN)6}](ClO4)38H2O in short Cu6Fe and the analog
compound with cobalt, Cu6Co. It is found that in Cu6Fe whose magnetic core is
constituted by six Cu2+ ions and one Fe3+ ion all with s=1/2, a weak
ferromagnetic interaction between Cu2+ moments through the central Fe3+ ion
with J = 0.14 K is present, while in Cu6Co the Co3+ ion is diamagnetic and the
weak interaction is antiferromagnetic with J = -1.12 K. The NMR spectra show
the presence of non equivalent groups of protons with a measurable contact
hyperfine interaction consistent with a small admixture of s-wave function with
the d-function of the magnetic ion. The NMR relaxation results are explained in
terms of a single ion (Cu2+, Fe3+, Co3+) uncorrelated spin dynamics with an
almost temperature independent correlation time due to the weak magnetic
exchange interaction. We conclude that the two molecular magnets studied here
behave as single molecule paramagnets with a very weak intramolecular
interaction, almost of the order of the dipolar intermolecular interaction.
Thus they represent a new class of molecular magnets which differ from the
single molecule magnets investigated up to now, where the intramolecular
interaction is much larger than the intermolecular one
High-quality epitaxial iron nitride films grown by gas-assisted molecular-beam epitaxy
Thin films of γ’-Fe4N were grown on polished (001) MgO substrates by molecular-beam epitaxy of iron in the presence of a gas flow from a rf atomic source. By means of x-ray diffraction, Mössbauer Spectroscopy, Rutherford backscattering/channeling, and scanning probe microscopy, it is shown that, with this method, single-phase, high-quality epitaxial thin films can be grown with a very smooth surface (root-mean-square roughness ∼0.4 nm). Magnetic measurements reveal square hysteresis loops, moderate coercivities (45 Oe for a 33 nm thick film) and complete in-plane orientation of the magnetization. These properties make the films interesting candidates for device applications
Hallmarks and detection techniques of cellular senescence and cellular ageing in immune cells
The ageing of the global population brings about unprecedented challenges. Chronic age-related diseases in an increasing number of people represent an enormous burden for health and social care. The immune system deteriorates during ageing and contributes to many of these age-associated diseases due to its pivotal role in pathogen clearance, tissue homeostasis and maintenance. Moreover, in order to develop treatments for COVID-19, we urgently need to acquire more knowledge about the aged immune system, as older adults are disproportionally and more severely affected. Changes with age lead to impaired responses to infections, malignancies and vaccination, and are accompanied by chronic, low-degree inflammation, which together is termed immunosenescence. However, the molecular and cellular mechanisms that underlie immunosenescence, termed immune cell senescence, are mostly unknown. Cellular senescence, characterised by an irreversible cell cycle arrest, is thought to be the cause of tissue and organismal ageing. Thus, better understanding of cellular senescence in immune populations at single-cell level may provide us with insight into how immune cell senescence develops over the life time of an individual. In this review, we will briefly introduce the phenotypic characterisation of aged innate and adaptive immune cells, which also contributes to overall immunosenescence, including subsets and function. Next, we will focus on the different hallmarks of cellular senescence and cellular ageing, and the detection techniques most suitable for immune cells. Applying these techniques will deepen our understanding of immune cell senescence and to discover potential druggable pathways, which can be modulated to reverse immune ageing
Growth and properties of Cu3N films and Cu3N/gamma '-Fe4N bilayers
Copper nitride films were grown by molecular-beam epitaxy of copper in the presence of nitrogen from a radio-frequency atomic source on (001) gamma'-Fe4N/(001)MgO or directly on MgO substrates. The structural properties of the Cu3N films were found to be very dependent on the substrate and on the deposition temperature. At optimal growth conditions, the Cu3N films grow epitaxial on both substrates. The Cu3N films grown on MgO were characterized optically to be insulators with an energy gap of 1.65 eV. On gamma'-Fe4N, Cu3N films with a thickness of only 6 nm, were grown as closed layers, epitaxial and rather smooth (root-mean-square roughness of 0.7 nm). This material has ideal properties to be used as a barrier in low resistance magnetic tunnel junctions. (C) 2002 American Institute of Physics
Structure and magnetism of single-phase epitaxial γ′-Fe4N
Single phase epitaxial pure γ′-Fe4N films are grown on MgO (001) by molecular beam epitaxy of iron in the presence of nitrogen obtained from a radio frequency atomic source. The epitaxial, single phase nature of the films is revealed by x-ray diffraction and by the local magnetic environment investigated by Mössbauer spectroscopy. The macroscopic magnetic properties of the γ′-Fe4N films are studied in detail by means of transverse Kerr effect measurements. The hysteresis loops are consistent with the cubic atomic structure, displaying easy [100] magnetization directions. The films are single domain at remanence, and the reversal is dominated by 180° or 90° domain wall nucleation and propagation, depending on the applied field direction. When 90° domain walls are responsible for the magnetization reversal, this proceeds in two stages, and the measured coercive fields vary accordingly. Magnetic domain observations reveal the two distinct reversal —driven by 180° or 90° domain walls— modes displaying large domains, of the order of mm. From magnetometer techniques, the saturation magnetization, μ0Ms, is measured to be 1.8 T. A magneto-optical torque technique is used to obtain a value of the anisotropy constant of 2.9×104J/m3.The authors acknowledge partial financing from EC project HIDEMAR G5RD-CT-2002-00731 and PHANTOMS network. The authors are indebted to A. Gupta and K. V. Rao from the department of Materials Science and
Engineering, KTH, Sweden for help with the low T SQUID measurements, and to L. Ballcels and M. A. García from Materials Science ICMM CSIC, Spain for high-T VSM measurements.
This work was part of the research program of the Foundation for Fundamental Research on Matter-FOM, The Netherlands. J.M.G.M. acknowledges financing through the
Ramón y Cajal program from the Spanish MCyT.Peer reviewe
^{7}Li NMR Study of Heavy Fermion LiV2O4 Containing Magnetic Defects
We present a systematic study of the variations of the ^{7}Li NMR properties
versus magnetic defect concentration up to 0.83 mol% within the spinel
structure of polycrystalline powder samples and a collection of small single
crystals of LiV2O4 in the temperature range from 0.5 to 4.2 K. We also report
static magnetization measurements and ac magnetic susceptibility measurements
at 14 MHz on the samples at low temperatures. Both the NMR spectrum and nuclear
spin-lattice relaxation rate are inhomogeneous in the presence of the magnetic
defects. The NMR data for the powders are well explained by assuming that (i)
there is a random distribution of magnetic point defects, (ii) the same heavy
Fermi liquid is present in the samples containing the magnetic defects as in
magnetically pure LiV2O4, and (iii) the influences of the magnetic defects and
of the Fermi liquid on the magnetization and NMR properties are separable. In
the single crystals, somewhat different behaviors are observed. Remarkably, the
magnetic defects in the powder samples show evidence of spin freezing below T ~
1.0 K, whereas in the single crystals with similar magnetic defect
concentration no spin freezing was found down to 0.5 K. Thus different types of
magnetic defects and/or interactions between them appear to arise in the
powders versus the crystals, possibly due to the substantially different
synthesis conditions of the powders and crystals.Comment: 18 pages typeset in 2 columns, 16 figures; submitted to PR
Proton NMR for Measuring Quantum-Level Crossing in the Magnetic Molecular Ring Fe10
The proton nuclear spin-lattice relaxation rate 1/T1 has been measured as a
function of temperature and magnetic field (up to 15 T) in the molecular
magnetic ring Fe10. Striking enhancement of 1/T1 is observed around magnetic
field values corresponding to a crossing between the ground state and the
excited states of the molecule. We propose that this is due to a
cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir
of the Zeeman levels of the molecule. This effect provides a powerful tool to
investigate quantum dynamical phenomena at level crossing.Comment: Four pages, to appear in Phys.Rev.Let
Theory of Spin Fluctuations in Striped Phases of Doped Antiferromagnetic Cuprates
We study the properties of generalized striped phases of doped cuprate planar
quantum antiferromagnets. We invoke an effective, spatially anisotropic,
non-linear sigma model in two space dimensions. Our theoretical predictions are
in quantitative agreement with recent experiments in La_{2-x}Sr_xCuO_4 with . We focus on (i) the magnetic correlation length, (ii) the
staggered magnetization at and (iii) the N\'eel temperature, as functions
of doping, using parameters determined previously and independently for this
system. These results support the proposal that the low doping
(antiferromagnetic) phase of the cuprates has a striped configuration.Comment: 4 pages, Revtex. To appear in the Proceedings of the International
Conference "Stripes, Lattice Instabilities and High Tc Superconductivity",
(Rome, Dec. 1996
Phase Separation of the Two-Dimensional t-J model
The boundary of phase separation of the two-dimensional t-J model is
investigated by the power-Lanczos method and Maxwell construction. The method
is similar to a variational approach and it determines the lower bound of the
phase separation boundary with in the limit . In
the physical interesting regime of high T_c superconductors where
there is no phase separation.Comment: LaTex 5 pages, 4 figure
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