196,018 research outputs found
Investigating the interfaces of the epiphyseal plate: An integrated approach of histochemistry, microtomography and SEM
No full textWe investigated the interfaces of the epiphyseal plate with over- and underlying bone segments using an integrated approach of histochemistry, microtomography and scanning electron microscopy (SEM) to overcome the inherent limitations of sections-based techniques. Microtomography was able to provide an unobstructed, frontal view of large expanses of the two bone surfaces facing the growth plate, while SEM observation after removal of the soft matrix granted an equally unhindered access with a higher resolution. The two interfaces appeared widely dissimilar. On the diaphyseal side the hypertrophic chondrocytes were arranged in tall columns packed in a sort of compact palisade; the interposed extracellular matrix was actively calcifying into a thick mineralized crust growing towards the epiphysis. Behind the mineralization front, histochemical data revealed a number of surviving cartilage islets which were being slowly remodelled into bone. In contrast, the epiphyseal side of the cartilage consisted of a relatively quiescent reserve zone whose mineralization was marginal in amount and discontinuous in extension; the epiphyseal bone consisted of a loose trabecular meshwork, with ample vascular spaces opening directly into the non-mineralized cartilage. On both sides the calcification process took place through the formation of spheroidal bodies 1-2 mu m wide which gradually grew by apposition and coalesced into a solid mass, in a way distinctly different from that of bone and other calcified tissues
In vivo biomedical applications of magnetic resonance and magnetic materials
No full textAn overview on the recent progress in the biomedical application of
magnetic materials and on the most used magnetic resonance imaging techniques is
presented. After briefly mentioning the basic aspects of Magnetic Resonance Imaging
(MRI), some of the most frequently used pulse sequences are illustrated with
particular emphasis on those used in Diffusion Tensor Imaging, Magnetic Resonance
Spectroscopy and functional MRI. Then the basis of the Dynamical Nuclear Polarization
technique, which allows to perform in vivo molecular imaging of the metabolic
processes, is presented. The physical properties of smart nanosized magnetic materials
which can further improve the potential of these diagnostic techniques are
described in the following sections. These magnetic nanoparticles may be used as
MRI contrast agents, for the magnetic transport and drug delivery or even for the
therapy of certain pathologies through magnetic fluid hyperthermia. The possible
combination of some of these functionalities into just one multifunctional nanoparticle
is also considered
Low-energy spin dynamics in the [YPc2]0 S=1/2 antiferromagnetic chain
No full text1H nuclear magnetic resonance (NMR) measurements in [YPc2]0, an organic compound formed by radicals stacking along chains, are presented. The temperature dependence of the macroscopic susceptibility of the NMR shift and of the spin-lattice relaxation rate 1/T1 indicate that the unpaired electron spins are not delocalized but rather form a S=1/2 antiferromagnetic chain. The exchange couplings estimated from those measurements are all in quantitative agreement. The low-energy spin dynamics can be described in terms of diffusive processes and the temperature dependence of the corresponding diffusion constant suggests that a spin gap at ~1 K might be present in this compound
Spin dynamics in the neutral rare-earth single-molecule magnets [TbPc2]0 and [DyPc2]0 from muSR and NMR spectroscopies
No full textThe spin dynamics in [TbPc2]0 and [DyPc2]0 single-molecule magnets have been investigated by means of muon and nuclear spin-lattice relaxation rate measurements. The correlation time for the spin fluctuations was found to be close to 0.1 ms already at 50 K, about 2 orders of magnitude larger than the one previously found in other lanthanide-based single-molecule magnets. In [TbPc2]0 two different regimes for the spin fluctuations have been evidenced: a high-temperature activated one involving spin fluctuations across a barrier around 880 K separating the ground and first excited states and a low-temperature regime involving quantum fluctuations within the twofold degenerate ground state. In [DyPc2]0 a high-temperature activated spin dynamics is also evidenced which, however, cannot be explained in terms of a single spin-phonon coupling constant
Strong electronic correlations in LixM(Pc,Nc) organic conductors near half flling
No full textEPR and NMR measurements in LixM(Pc,Nc) (M=Zn,Mg; Pc=phthalocyanine, Nc=naphthalocyanine) are presented. The changes of the electronic properties induced by the
substitution of the metal M and by the expansion of the organic core are discussed for the half filling of the conduction band, i.e. for x around 2. The comparison with Li2ZnPc, which behaves
as a Fermi liquid, suggests that the substitution of Zn with Mg does not lead to substantial effects. On the contrary in Li2:2MgNc, where a more two-dimensional packing of the molecules
is attained, a stronger tendency to localization and a low temperature phase transition towards a spin density wave ground state are observed
NMR as a Probe of the Relaxation of the Magnetization in Magnetic Molecules
No full textWe investigate the time autocorrelation of the molecular magnetization M(t) for three classes of magnetic molecules (antiferromagnetic rings, grids, and nanomagnets), in contact with the phonon heat bath. For all three classes, we find that the exponential decay of the fluctuations of M(t) is characterized by a single characteristic time τ(T, B) for not too high temperature T and field B. This is reflected in a nearly single-Lorentzian shape of the spectral density of the fluctuations. We show that such fluctuations are effectively probed by NMR, and that our theory explains the recent phenomenological observation by Back et al. [Phys. Rev. B 70, 134434 (2004)] that the Larmor-frequency dependence of 1/T1] data in a large number of AFM rings fits to a single-Lorentzian form
Low-energy excitations in electron-doped metal phthalocyanines
No full textMagnetization and NMR relaxation measurements in Lix(Mn,Zn)Pc were performed in order to study the evolution of the electronic
properties of metal phthalocyanines upon electron doping. For x = 2 the doubly degenerate lowest unoccupied molecular orbital
(LUMO) is half filled and the electrons are coupled in a singlet state owing to the Jahn–Teller distortions. The effects of the Jahn–Teller distortions on the spin excitations are discussed in the light of NMR relaxation measurements
Strong electronic correlations in LixZnPc organic metals
No full textNuclear magnetic resonance, electron paramagnetic resonance and magnetization measurements show that bulk LixZnPc are strongly correlated one-dimensional metals. The temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 and of the static uniform susceptibility chi_S on approaching room
temperature are characteristic of a Fermi liquid. Moreover, while for x around 2 the electrons are delocalized down to low temperature, for x-> 4 a tendency towards localization is noticed upon cooling, yielding an increase both in 1/T1 and chi_s. The x dependence of the effective density of states at the Fermi level D(EF) displays a sharp enhancement for x=2, at the half filling of the ZnPc lowest unoccupied molecular orbitals. This suggests that LixZnPc is on the edge of a metal-insulator transition where enhanced superconducting fluctuations could develo
Spin and charge dynamics in [TbPc2]0 and [DyPc2]0 single-molecule magnets
No full textMagnetization, ac-susceptibility, and muon spin-relaxation mSR measurements have been performed in
neutral phthalocyaninato lanthanide [LnPc2]0 single-molecule magnets in order to determine the low-energy
levels structure and to compare the low-frequency spin excitations probed by means of macroscopic techniques,
such as ac susceptibility, with the ones explored by means of techniques of microscopic character, such
as mSR. Both techniques show a high temperature thermally activated regime for the spin dynamics and a
low-temperature tunneling one. While in the activated regime the correlation times for the spin fluctuations
estimated by ac susceptibility and mSR basically agree, clear discrepancies are found in the tunneling regime.
In particular, mSR probes a faster dynamics with respect to ac susceptibility. It is argued that the tunneling
dynamics probed by mSR involves fluctuations which do not yield a net change in the macroscopic magnetization
probed by ac susceptibiliy. Finally resistivity measurements in [TbPc2]0 crystals show a high temperature
nearly metallic behavior and a low-temperature-activated behavior
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