326 research outputs found
Quantum Hole Digging in Magnetic Molecular Clusters
Below 360 mK, Fe8 magnetic molecular clusters are in the pure quantum
relaxation regime. We showed recently that the predicted ``square-root time''
relaxation is obeyed, allowing us to develop a new method for watching the
evolution of the distribution of molecular spin states in the sample. We
measured the distribution P(H) of molecules which are in resonance at the
applied field H. Tunnelling initially causes rapid transitions of molecules,
thereby ``digging a hole'' in P(H). For small initial magnetisation values, the
hole width shows an intrinsic broadening which may be due to nuclear spins. We
present here hole digging measurements in the thermal activated regime which
may allow to study the effect of spin-phonon coupling.Comment: 3 pages, 2 figures, conference proceedings of LT22 (Helsinki,
Finland, August 4-11, 1999
Experimental determination of the frequency and field dependence of Specific Loss Power in Magnetic Fluid Hyperthermia
Magnetic nanoparticles are promising systems for biomedical applications and
in particular for Magnetic Fluid Hyperthermia, a promising therapy that
utilizes the heat released by such systems to damage tumor cells. We present an
experimental study of the physical properties that influences the capability of
heat release, i.e. the Specific Loss Power, SLP, of three biocompatible
ferrofluid samples having a magnetic core of maghemite with different core
diameter d= 10.2, 14.6 and 19.7 nm. The SLP was measured as a function of
frequency f and intensity of the applied alternating magnetic field H, and it
turned out to depend on the core diameter, as expected. The results allowed us
to highlight experimentally that the physical mechanism responsible for the
heating is size-dependent and to establish, at applied constant frequency, the
phenomenological functional relationship SLP=cH^x, with 2<x<3 for all samples.
The x-value depends on sample size and field frequency/ intensity, here chosen
in the typical range of operating magnetic hyperthermia devices. For the
smallest sample, the effective relaxation time Teff=19.5 ns obtained from SLP
data is in agreement with the value estimated from magnetization data, thus
confirming the validity of the Linear Response Theory model for this system at
properly chosen field intensity and frequency
Answer to the comment of Chudnovsky: On the square-root time relaxation in molecular nanomagnets
Answer to the comment of E. Chudnovsky concerning the following papers:
(1) N.V. Prokof'ev, P.C.E. Stamp, Phys. Rev. Lett.80, 5794 (1998).
(2) W. Wernsdorfer, T. Ohm, C. Sangregorio, R. Sessoli, D. Mailly, C.
Paulsen, Phys. Rev. Lett. 82, 3903 (1999).Comment: 1 page
Proton Spin Relaxation Induced by Quantum Tunneling in Fe8 Molecular Nanomagnet
The spin-lattice relaxation rate and NMR spectra of H in
single crystal molecular magnets of Fe8 have been measured down to 15 mK. The
relaxation rate shows a strong temperature dependence down to 400
mK. The relaxation is well explained in terms of the thermal transition of the
iron state between the discreet energy levels of the total spin S=10. The
relaxation time becomes temperature independent below 300 mK and is
longer than 100 s. In this temperature region stepwise recovery of the
H-NMR signal after saturation was observed depending on the return field of
the sweep field. This phenomenon is attributed to resonant quantum tunneling at
the fields where levels cross and is discussed in terms of the Landau-Zener
transition.Comment: 13 pages, 5 figure
Large transverse field tunnel splittings in the Fe_8 spin Hamiltonian
The spin Hamiltonian that describes the molecular magnet Fe has biaxial
symmetry with mutually perpendicular easy, medium, and hard magnetic axes.
Previous calculations of the ground state tunnel splittings in the presence of
a magnetic field along the hard axis are extended, and the meaning of the
previously discovered oscillation of this splitting is further clarified
Glauber slow dynamics of the magnetization in a molecular Ising chain
The slow dynamics (10^-6 s - 10^4 s) of the magnetization in the paramagnetic
phase, predicted by Glauber for 1d Ising ferromagnets, has been observed with
ac susceptibility and SQUID magnetometry measurements in a molecular chain
comprising alternating Co{2+} spins and organic radical spins strongly
antiferromagnetically coupled. An Arrhenius behavior with activation energy
Delta=152 K has been observed for ten decades of relaxation time and found to
be consistent with the Glauber model. We have extended this model to take into
account the ferrimagnetic nature of the chain as well as its helicoidal
structure.Comment: 4 pages, 4 figures (low resolution), 16 references. Submitted to
Physical Review Letter
Quantum tunneling in a three dimensional network of exchange coupled single-molecule magnets
A Mn4 single-molecule magnet (SMM) is used to show that quantum tunneling of
magnetization (QTM) is not suppressed by moderate three dimensional exchange
coupling between molecules. Instead, it leads to an exchange bias of the
quantum resonances which allows precise measurements of the effective exchange
coupling that is mainly due to weak intermolecular hydrogen bounds. The
magnetization versus applied field was recorded on single crystals of [Mn4]2
using an array of micro-SQUIDs. The step fine structure was studied via minor
hysteresis loops.Comment: 4 pages, 4 figure
Quantum Computing in Molecular Magnets
Shor and Grover demonstrated that a quantum computer can outperform any
classical computer in factoring numbers and in searching a database by
exploiting the parallelism of quantum mechanics. Whereas Shor's algorithm
requires both superposition and entanglement of a many-particle system, the
superposition of single-particle quantum states is sufficient for Grover's
algorithm. Recently, the latter has been successfully implemented using Rydberg
atoms. Here we propose an implementation of Grover's algorithm that uses
molecular magnets, which are solid-state systems with a large spin; their spin
eigenstates make them natural candidates for single-particle systems. We show
theoretically that molecular magnets can be used to build dense and efficient
memory devices based on the Grover algorithm. In particular, one single crystal
can serve as a storage unit of a dynamic random access memory device. Fast
electron spin resonance pulses can be used to decode and read out stored
numbers of up to 10^5, with access times as short as 10^{-10} seconds. We show
that our proposal should be feasible using the molecular magnets Fe8 and Mn12.Comment: 13 pages, 2 figures, PDF, version published in Nature, typos
correcte
Magnetic Hyperthermia and Radiation Therapy : Radiobiological Principles and Current Practice
Hyperthermia, though by itself generally non-curative for cancer, can significantly increase the efficacy of radiation therapy, as demonstrated by in vitro, in vivo, and clinical results. Its limited use in the clinic is mainly due to various practical implementation difficulties, the most important being how to adequately heat the tumor, especially deep-seated ones. In this work, we first review the effects of hyperthermia on tissue, the limitations of radiation therapy and the radiobiological rationale for combining the two treatment modalities. Subsequently, we review the theory and evidence for magnetic hyperthermia that is based on magnetic nanoparticles, its advantages compared with other methods of hyperthermia, and how it can be used to overcome the problems associated with traditional techniques of hyperthermia
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