216 research outputs found
Spin-enhanced magnetocaloric effect in molecular nanomagnets
An unusually large magnetocaloric effect for the temperature region below 10 K is found for the Fe-14 molecular nanomagnet. This is to large extent caused by its extremely large spin S ground state combined with an excess of entropy arising from the presence of low-lying excited S states. We also show that the highly symmetric Fe-14 cluster core, resulting in small cluster magnetic anisotropy, enables the occurrence of long-range antiferromagnetic order below T-N=1.87 K
Enhanced magnetocaloric effect in frustrated magnetic molecules with icosahedral symmetry
We investigate the magnetocaloric properties of certain antiferromagnetic
spin systems that have already been or very likely can be synthesized as
magnetic molecules. It turns out that the special geometric frustration which
is present in antiferromagnets that consist of corner-sharing triangles leads
to an enhanced magnetocaloric effect with high cooling rates in the vicinity of
the saturation field. These findings are compared with the behavior of a simple
unfrustrated spin ring as well as with the properties of the icosahedron. To
our surprise, also for the icosahedron large cooling rates can be achieved but
due to a different kind of geometric frustration.Comment: 5 pages, 8 figures, more information at
http://obelix.physik.uni-osnabrueck.de/~schnack
Influence of antisymmetric exchange interaction on quantum tunneling of magnetization in a dimeric molecular magnet Mn6
We present magnetization measurements on the single molecule magnet Mn6,
revealing various tunnel transitions inconsistent with a giant-spin
description. We propose a dimeric model of the molecule with two coupled spins
S=6, which involves crystal-field anisotropy, symmetric Heisenberg exchange
interaction, and antisymmetric Dzyaloshinskii-Moriya exchange interaction. We
show that this simplified model of the molecule explains the experimentally
observed tunnel transitions and that the antisymmetric exchange interaction
between the spins gives rise to tunneling processes between spin states
belonging to different spin multiplets.Comment: 5 pages, 4 figure
Direct Observation of Quantum Coherence in Single-Molecule Magnets
Direct evidence of quantum coherence in a single-molecule magnet in frozen
solution is reported with coherence times as long as T2 = 630 ns. We can
strongly increase the coherence time by modifying the matrix in which the
single-molecule magnets are embedded. The electron spins are coupled to the
proton nuclear spins of both the molecule itself and interestingly, also to
those of the solvent. The clear observation of Rabi oscillations indicates that
we can manipulate the spin coherently, an essential prerequisite for performing
quantum computations.Comment: 5 Pages, 4 Figures, final version published in PR
Tunable dipolar magnetism in high-spin molecular clusters
We report on the Fe17 high-spin molecular cluster and show that this system
is an exemplification of nanostructured dipolar magnetism. Each Fe17 molecule,
with spin S=35/2 and axial anisotropy as small as D=-0.02K, is the magnetic
unit that can be chemically arranged in different packing crystals whilst
preserving both spin ground-state and anisotropy. For every configuration,
molecular spins are correlated only by dipolar interactions. The ensuing
interplay between dipolar energy and anisotropy gives rise to macroscopic
behaviors ranging from superparamagnetism to long-range magnetic order at
temperatures below 1K.Comment: Replaced with version accepted for publication in Physical Review
Letter
Relaxation dynamics in a Fe7 nanomagnet
We investigate the phonon-induced relaxation dynamics in the Fe
7
magnetic molecule, which is made of
two Fe
3
+
triangles bridged together by a central Fe
3
+
ion. The competition between different antiferromagnetic
exchange interactions leads to a low-spin ground state multiplet with a complex pattern of low-lying excited
levels. We theoretically investigate the decay of the time correlation function of molecular observables, such as the
cluster magnetization, due to the spin-phonon interaction. We find that more than one time contributes to the decay
of the molecular magnetization. The relaxation dynamics is probed by measurements of the nuclear spin-lattice
relaxation rate 1
/T
1
. The interpretation of these measurements allows the determination of the magnetoelastic
coupling strength and to set the scale factor of the relaxation dynamics time scales. In our theoretical interpretation
of 1
/T
1
data we also take into account the wipeout effect at low temperature
Tracking nuclear motion in single-molecule magnets using femtosecond X-ray absorption spectroscopy
The development of new data storage solutions is crucial for emerging digital technologies. Recently, all-optical magnetic switching has been achieved in dielectrics, proving to be faster than traditional methods. Despite this, single-molecule magnets (SMMs), which are an important class of magnetic materials due to their nanometre size, remain underexplored for ultrafast photomagnetic switching. Herein, we report femtosecond time-resolved K-edge X-ray absorption spectroscopy (TR-XAS) on a Mn(III)-based trinuclear SMM. Exploiting the elemental specificity of XAS, we directly track nuclear dynamics around the metal ions and show that the ultrafast dynamics upon excitation of a crystal-field transition are dominated by a magnetically active Jahn-Teller mode. Our results, supported by simulations, reveal minute bond length changes from 0.01 to 0.05 \uc5 demonstrating the sensitivity of the method. These geometrical changes are discussed in terms of magneto-structural relationships and consequently our results illustrate the importance of TR-XAS for the emerging area of ultrafast molecular magnetism
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