38 research outputs found
Low-energy excitations in electron-doped metal phthalocyanine from NMR in LiMnPc
Li and H NMR and magnetization measurements in \lpc
(PcCHN), recently proposed as a strongly correlated
metal, are presented. Two different low-frequency dynamics are evidenced. The
first one, probed by H nuclei gives rise to a slowly relaxing magnetization
at low temperature and is associated with the freezing of MnPc spins.
This dynamic is similar to the one observed in pristine -MnPc and
originates from Li depleted chain segments. The second one, evidenced by Li
spin-lattice relaxation rate, is associated with the hopping of the electrons
along Li-rich chains. The characteristic correlation times for the two dynamics
are derived and the role of disorder is briefly discussed.Comment: 7 two-columns pages, 11 figure
Spin dynamics in rare earth single molecule magnets from muSR and NMR in [TbPc] and [DyPc]
The spin dynamics in [TbPc] and [DyPc] 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 two orders
of magnitude larger than the one previously found in other lanthanide based
single molecule magnets. In [TbPc] two different regimes for the
spin fluctuations have been evidenced: a high temperature activated one
involving spin fluctuations across a barrier separating
the ground and first excited states and a low temperature regime involving
quantum fluctuations within the twofold degenerate ground-state. In
[DyPc] a high temperature activated spin dynamics is also evidenced
which, however, cannot be explained in terms of a single spin-phonon coupling
constant.Comment: 4 pages, 4 figure
Spin and charge dynamics in [TbPc] and [DyPc] single molecule magnets
Magnetization, AC susceptibility and SR measurements have been performed
in neutral phthalocyaninato lanthanide ([LnPc) 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 SR. 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 SR basically agree, clear
discrepancies are found in the tunneling regime. In particular, SR probes
a faster dynamics with respect to AC susceptibility. It is argued that the
tunneling dynamics probed by SR involves fluctuations which do not yield a
net change in the macroscopic magnetization probed by AC susceptibiliy. Finally
resistivity measurements in [TbPc crystals show a high temperature
nearly metallic behaviour and a low temperature activated behaviour.Comment: 8 pages, 12 figure
Strong electronic correlations in LiZnPc organic metals
Nuclear magnetic resonance, electron paramagnetic resonance and magnetization
measurements show that bulk LiZnPc are strongly correlated one-dimensional
metals. The temperature dependence of the nuclear spin-lattice relaxation rate
and of the static uniform susceptibility on approaching room
temperature are characteristic of a Fermi liquid. Moreover, while for the electrons are delocalized down to low temperature, for a
tendency towards localization is noticed upon cooling, yielding an increase
both in and . The -dependence of the effective density of
states at the Fermi level displays a sharp enhancement for , at the half filling of the ZnPc lowest unoccupied molecular orbitals. This
suggests that LiZnPc is on the edge of a metal-insulator transition where
enhanced superconducting fluctuations could develop.Comment: 5 pages, 4 figure
NMR as a probe of the relaxation of the magnetization in magnetic molecules
We investigate the time autocorrelation of the molecular magnetization
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 , associated with
the irreversible exchange of energy with the heat bath, is characterized by a
single characteristic time for not too high temperature and
field . 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 Baek et al. (PRB70, 134434) that the
Larmor-frequency dependence of data in a large number of AFM rings fits
to a single-lorentzian form.Comment: Published as Phys. Rev. Letters 94, 077203 (2005) in slightly reduced
for
Spin dynamics in the single-ion magnet [Er(W5O18)2]9−
In this work we present a detailed NMR and \u3bc+SR investigation of the spin dynamics in the new hydrated sodium salt containing the single-ion magnet [Er(W5O18)2]9-. The H1NMR absorption spectra at various applied magnetic fields present a line broadening on decreasing temperature which indicates a progressive spin freezing of the single-molecule magnetic moments. The onset of quasistatic local magnetic fields, due to spin freezing, is observed also in the muon relaxation curves at low temperature. Both techniques yield a local field distribution of the order of 0.1-0.2 T, which appears to be of dipolar origin. On decreasing the temperature, a gradual loss of the H1NMR signal intensity is observed, a phenomenon known as wipe-out effect. The effect is analyzed quantitatively on the basis of a simple model which relies on the enhancement of the NMR spin-spin, T2-1, relaxation rate due to the slowing down of the magnetic fluctuations. Measurements of spin-lattice relaxation rate T1-1 for H1NMR and of the muon longitudinal relaxation rate \u3bb show an increase as the temperature is lowered. However, while for the NMR case the signal is lost before reaching the very slow fluctuation region, the muon spin-lattice relaxation \u3bb can be followed until very low temperatures and the characteristic maximum, reached when the electronic spin fluctuation frequency becomes of the order of the muon Larmor frequency, can be observed. At high temperatures, the data can be well reproduced with a simple model based on a single correlation time \u3c4=\u3c40exp(\u394/T) for the magnetic fluctuations. However, to fit the relaxation data for both NMR and \u3bc+SR over the whole temperature and magnetic field range, one has to use a more detailed model that takes into account spin-phonon transitions among the Er3+ magnetic sublevels. A good agreement for both proton NMR and \u3bc+SR relaxation is obtained, which confirms the validity of the energy level scheme previously calculated from an effective crystal field Hamiltonian
In vivo biomedical applications of magnetic resonance and magnetic materials
An 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
In vivo biomedical applications of magnetic resonance and magnetic materials
An 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
1H 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