1,907 research outputs found
Classical and quantum magnetisation reversal studied in single nanometer-sized particles and clusters using micro-SQUIDs
Recent progress in experiment on quantum tunnelling of the magnetic moment in
mesoscopic systems will be reviewed. The emphasis will be made on measurements
of individual nanoparticles. These nanomagnets allow one to test the border
between classical and quantum behaviour. Using the micro-SQUID magnetometer,
waiting time, switching field and telegraph noise measurements show
unambiguously that the magnetisation reversal of small enough single
crystalline nanoparticles is described by a model of thermal activation over a
single-energy barrier. Results on insulating BaFeO nanoparticles show strong
deviations from this model below 0.4 K which agree with the theory of
macroscopic quantum tunnelling in the low dissipation regime.Comment: 6 pages, 2 figures, conference proceedings of LT22-Helsink
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
Magnetization Switching in Single-Domain Ferromagnets
A model for single-domain uniaxial ferromagnetic particles with high
anisotropy, the Ising model, is studied. Recent experimental observations have
been made of the probability that the magnetization has not switched. Here an
approach is described in which it is emphasized that a ferromagnetic particle
in an unfavorable field is in fact a metastable system, and the switching is
accomplished through the nucleation and subsequent growth of localized
droplets. Nucleation theory is applied to finite systems to determine the
coercivity as a function of particle size and to calculate the probability of
not switching. Both of these quantities are modified by different boundary
conditions, magnetostatic interactions, and quenched disorder.Comment: 4 pages, LaTeX, 2 figures, documentstyle{elsart} More fits and
Mathematica notebook at http://www.scri.fsu.edu/~novotny/magnetism.html To
appear in J.Mag.Mag.Mater. Conference Proceedings of 7th International
Conference on Magnetism Cairns, Australia, August, 199
On-chip SQUID measurements in the presence of high magnetic fields
We report a low temperature measurement technique and magnetization data of a
quantum molecular spin, by implementing an on-chip SQUID technique. This
technique enables the SQUID magnetometery in high magnetic fields, up to 7
Tesla. The main challenges and the calibration process are detailed. The
measurement protocol is used to observe quantum tunneling jumps of the S=10
molecular magnet, Mn12-tBuAc. The effect of transverse field on the tunneling
splitting for this molecular system is addressed as well.Comment: 7 pages, 3 figure
Micro-SQUID technique for studying the temperature dependence of switching fields of single nanoparticles
An improved micro-SQUID technique is presented allowing us to measure the
temperature dependence of the magnetisation switching fields of single
nanoparticles well above the critical superconducting temperature of the SQUID.
Our first measurements on 3 nm cobalt nanoparticle embedded in a niobium matrix
are compared to the Neel Brown model describing the magnetisation reversal by
thermal activation over a single anisotropy barrier.Comment: 3 pages, 4 figures; conference proceeding: 1st Joint European
Magnetic Symposia (JEMS'01), Grenoble (France), 28th August - 1st September,
200
Quantum phase interference (Berry phase) in single-molecule magnets of Mn12
Magnetization measurements of a molecular clusters Mn12 with a spin ground
state of S = 10 show resonance tunneling at avoided energy level crossings. The
observed oscillations of the tunnel probability as a function of the magnetic
field applied along the hard anisotropy axis are due to topological quantum
phase interference of two tunnel paths of opposite windings. Mn12 is therefore
the second molecular clusters presenting quantum phase interference.Comment: 3 pages, 4 figures, MMM'01 conference (12-16 Nov.
Molecular nanomagnets in ac magnetic field
The behavior of molecular nanomagnets in periodic magnetic field transverse
to the easy axis direction is investigated. It is shown that at sufficiently
strong field the tunneling time can be considerably reduced.Comment: on the basis of the article accepted for publication in JMM
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
Exact Activation Energy of Magnetic Single Domain Particles
I present the exact analytical expression for the activation energy as a
function of externally applied magnetic fields for a single--domain magnetic
particle with uniaxial anisotropy (Stoner--Wohlfahrt model), and investigate
the scaling behavior of the activation energy close to the switching boundary.Comment: published in 2004, posted here for general accessabilit
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