102 research outputs found
High-Frequency Electron-Spin-Resonance Study of the Octanuclear Ferric Wheel CsFe
High-frequency ( = 190 GHz) electron paramagnetic resonance (EPR) at
magnetic fields up to 12 T as well as Q-band ( = 34.1 GHz) EPR were
performed on single crystals of the molecular wheel CsFe. In this molecule,
eight Fe(III) ions, which are coupled by nearest-neighbor antiferromagnetic
(AF) Heisenberg exchange interactions, form a nearly perfect ring. The
angle-dependent EPR data allow for the accurate determination of the spin
Hamiltonian parameters of the lowest spin multiplets with 4.
Furthermore, the data can well be reproduced by a dimer model with a uniaxial
anisotropy term, with only two free parameters and . A fit to the dimer
model yields = -15(2) cm and = -0.3940(8) cm. A rhombic
anisotropy term is found to be negligibly small, = 0.000(2) cm. The
results are in excellent agreement with previous inelastic neutron scattering
(INS) and high-field torque measurements. They confirm that the CsFe
molecule is an excellent experimental model of an AF Heisenberg ring. These
findings are also important within the scope of further investigations on this
molecule such as the exploration of recently observed magnetoelastic
instabilities.Comment: 21 pages, 8 figures, accepted for publication in Inorganic Chemistr
Nanoscale X-ray investigation of magnetic metallofullerene peapods
Endohedral lanthanide ions packed inside carbon nanotubes (CNTs) in a
one-dimensional assembly have been studied with a combination of high
resolution transmission electron microscopy (HRTEM), scanning transmission
X-ray microscopy (STXM), and X-ray magnetic circular dichroism (XMCD). By
correlating HRTEM and STXM images we show that structures down to 30 nm are
resolved with chemical contrast and record X-ray absorption spectra from
endohedral lanthanide ions embedded in individual nanoscale CNT bundles. XMCD
measurements of an ErN@C bulk sample and a macroscopic assembly of
filled CNTs indicates that the magnetic properties of the endohedral Er3+ ions
are unchanged when encapsulated in CNTs. This study demonstrates the
feasibility of local magnetic X-ray characterization of low concentrations of
lanthanide ions embedded in molecular nanostructures
Hysteresis enhancement on a hybrid Dy(III) single molecule magnet/iron oxide nanoparticle system
In this paper we report the synthesis and characterization of hybrid molecular-inorganic systems composed of superparamagnetic iron oxide nanoparticles coated with a shell of oleic acid (NP) and (Pr2NH2)5[Dy12(OH)16(SALO)4(SALOH)8(NO3)8(H2O)0.5]NO3 (Dy12) single-molecule magnets. The hybrid NP-Dy12 system presents an enhancement of the magnetization hysteresis with respect to the isolated components while retaining the morphological characteristics of the parent NPs
Tunneling, Remanence, and Frustration in Dysprosium based Endohedral Single Molecule Magnets
A single molecule magnet (SMM) can maintain its magnetization direction over
a long period of time [1,2]. It consists in a low number of atoms that
facilitates the understanding and control of the ground state, which is
essential in future applications such as high-density information storage or
quantum computers [3,4]. Endohedral fullerenes realize robust, nanometer sized,
and chemically protected magnetic clusters that are not found as free species
in nature. Here we demonstrate how adding one, two, or three dysprosium atoms
to the carbon cage results in three distinct magnetic ground states. The
significantly different hysteresis curves demonstrate the decisive influence of
the number of magnetic moments and their interactions. At zero field the
comparison relates tunneling of the magnetization, with remanence, and
frustration. The ground state of the tridysprosium species turns out to be one
of the simplest realizations of a frustrated, ferromagnetically coupled
magnetic system.Comment: 14 pages (latex file) + 3 seperate figures (jpeg
Network protocol scalability via a topological Kadanoff transformation
A natural hierarchical framework for network topology abstraction is
presented based on an analogy with the Kadanoff transformation and
renormalisation group in theoretical physics. Some properties of the
renormalisation group bear similarities to the scalability properties of
network routing protocols (interactions). Central to our abstraction are two
intimately connected and complementary path diversity units: simple cycles, and
cycle adjacencies. A recursive network abstraction procedure is presented,
together with an associated generic recursive routing protocol family that
offers many desirable features.Comment: 4 pages, 5 figures, PhysComNet 2008 workshop submissio
Self-Assembly and Magnetic Order of Bi-Molecular 2D Spin Lattices of M(II,III) Phthalocyanines on Au(111)
Single layer low-dimensional materials are presently of emerging interest, including in the context of magnetism. In the present report, on-surface supramolecular architecturing was further developed and employed to create surface supported two-dimensional binary spin arrays on atomically clean non-magnetic Au(111). By chemical programming of the modules, different checkerboards were produced combining phthalocyanines containing metals of different oxidation and spin states, diamagnetic zinc, and a metal-free 'spacer'. In an in-depth, spectro-microscopy and theoretical account, we correlate the structure and the magnetic properties of these tunable systems and discuss the emergence of 2D Kondo magnetism from the spin-bearing components and via the physico-chemical bonding to the underlying substrate. The contributions of the individual elements, as well as the role of the electronic surface state in the bottom substrate, are discussed, also looking towards further in-depth investigations
On-surface transmetalation of metalloporphyrins
Increasing the complexity of 2D metal-organic networks has led to the fabrication of structures with interesting magnetic and catalytic properties. However, increasing complexity by providing different coordination environments for different metal types imposes limitations on their synthesis if the controlled placement of one metal type into one coordination environment is desired. Whereas metal insertion into free-base porphyrins at the vacuum/solid interface has been thoroughly studied, providing detailed insight into the mechanisms at play, the chemical interaction of a metal atom with a metallated porphyrin is rarely investigated. Herein, the breadth of metalation reactions is augmented towards the metal exchange of a metalloporphyrin through the deliberate addition of atomic metal centers. The cation of Fe(ii)-tetraphenylporphyrins can be replaced by Co in a redox transmetalation-like reaction on a Au(111) surface. Likewise, Cu can be replaced by Co. The reverse reaction does not occur, i.e. Fe does not replace Co in the porphyrin. This non-reversible exchange is investigated in detail by X-ray absorption spectroscopy complemented by scanning tunneling microscopy. Density functional theory illuminates possible reaction pathways and leads to the conclusion that the transmetalation proceeds through the adsorption of initially metallic (neutral) Co onto the porphyrin and the expulsion of Fe towards the surface accompanied by Co insertion. Our findings have important implications for the fabrication of porphyrin layers on surfaces when subject to the additional deposition of metals. Mixed-metal porphyrin layers can be fabricated by design in a solvent-free process, but conversely care must be taken that the transmetalation does not proceed as an undesired side reaction.Instituto de Investigaciones FisicoquÃmicas Teóricas y Aplicada
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