Asymmetric Berry-Phase Interference Patterns in a Single-Molecule Magnet

A Mn4 single-molecule magnet displays asymmetric Berry-phase interference patterns in the transverse-field (HT) dependence of the magnetization tunneling probability when a longitudinal field (HL) is present, contrary to symmetric patterns observed for HL=0. Reversal of HL results in a reflection of the transverse-field asymmetry about HT=0, as expected on the basis of the time-reversal invariance of the spin-orbit Hamiltonian which is responsible for the tunneling oscillations. A fascinating motion of Berry-phase minima within the transverse-field magnitude-direction phase space results from a competition between noncollinear magnetoanisotropy tensors at the two distinct Mn sites.Comment: 4 double-column page

Molecular nanomagnets

Nanomagnetism is a rapidly expanding area of research which appears to be able to provide novel applications. Magnetic molecules are at the very bottom of the possible size of nanomagnets and they provide a unique opportunity to observe the coexistence of classical and quantum properties. The discovery in the early 90's that a cluster comprising twelve manganese ions shows hysteresis of molecular origin, and later proved evidence of quantum effects, opened a new research area whichis still flourishing through the collaboration of chemists and physicists. This book is the first attempt to cove

Spin Chirality in a Molecular Dysprosium Triangle: the Archetype of the Non-Collinear Ising Model

Single crystal magnetic studies combined with a theoretical analysis show that cancellation of the magnetic moments in the trinuclear Dy3+ cluster [Dy3(OH)2L3Cl(H2O)5]Cl3, resulting in a non-magnetic ground doublet, originates from the non-collinearity of the single ion easy axes of magnetization of the Dy3+ ions that lie in the plane of the triangle at 120 (deg.) one from each other. This gives rise to a peculiar chiral nature of the ground non-magnetic doublet and to slow relaxation of the magnetization with abrupt accelerations at the crossings of the discrete energy levels.Comment: 4 pages and 5 figure

Linked Nickel Metallacrowns from a Phosphonate/2-Pyridyloximate Blend of Ligands: Structure and Magnetic Properties

In the present work, four new NiII clusters with nuclearities ranging between Ni4 and Na2Ni8 were synthesized, employing the versatile ligand phenylphosphonate and 6-methylpyridylaldoximate as the coligand. Crystallographic data show that the tetranuclear complex [Ni 4 (6-MepaoH)4(PhPO3)2(OH)2(MeOH)4](OH)2 (1) consists of two dimers linked by phosphonate bridges, whereas [Cs2Ni6(6-Mepao)6(PhPO3)3(OH)2(H2O)8] (2), Cs[Ni8(6-MepaoH)6(6-Mepao)6(PhPO3)3](ClO4)5 (3), and [Ni8Na2(BzO)6(6-Mepao)6(PhPO3)3] (4) are built from phosphonato-linked {Ni3(6-Mepao)3} metallacycles. The [9-MCNiII(6‑Mepao)-3] fragments in 2−4 show the unusual coordination of additional Cs+, Na+ , and/or NiII cations. Direct-current magnetic measurements were carried in the 300−2 K range. Analysis of the experimental data revealed a complex response with strong antiferromagnetic interactions mediated by the oximato bridges and weak interactions mediated by the phosphonate ones

Enhancement of the Magnetocaloric Effect in Geometrically Frustrated Cluster Spin Glass Systems

In this work, we theoretically demonstrate that a strong enhancement of the Magnetocaloric Effect is achieved in geometrically frustrated cluster spin-glass systems just above the freezing temperature. We consider a network of clusters interacting randomly which have triangular structure composed of Ising spins interacting antiferromagnetically. The intercluster disorder problem is treated using a cluster spin glass mean-field theory, which allows exact solution of the disordered problem. The intracluster part can be solved using exact enumeration. The coupling between the inter and intracluster problem incorporates the interplay between effects coming from geometric frustration and disorder. As a result, it is shown that there is the onset of cluster spin glass phase even with very weak disorder. Remarkably, it is exactly within a range of very weak disorder and small magnetic field that is observed the strongest isothermal release of entropy.Comment: 10 pages, 7 figures. Provisionally accepted in Journal of Physics: Condensed Matte

Low temperature magnetic properties and spin dynamics in single crystals of Cr8Zn antiferromagnetic molecular rings

Under the terms of the CC BY license.-- et al.A detailed experimental investigation of the effects giving rise to the magnetic energy level structure in the vicinity of the level crossing (LC) at low temperature is reported for the open antiferromagnetic molecular ring CrZn. The study is conducted by means of thermodynamic techniques (torque magnetometry, magnetization and specific heat measurements) and microscopic techniques (nuclear magnetic resonance line width, nuclear spin lattice, and spin-spin relaxation measurements). The experimental results are shown to be in excellent agreement with theoretical calculations based on a minimal spin model Hamiltonian, which includes a Dzyaloshinskii-Moriya interaction. The first ground state level crossing at μH = 2.15 T is found to be an almost true LC while the second LC at μH = 6.95 T has an anti-crossing gap of Δ = 0.19 K. In addition, both NMR and specific heat measurements show the presence of a level anti-crossing between excited states at μH = 4.5 T as predicted by the theory. In all cases, the fit of the experimental data is improved by introducing a distribution of the isotropic exchange couplings (J), i.e., using a J strain model. The peaks at the first and second LCs in the nuclear spin-lattice relaxation rate are dominated by inelastic scattering and a value of Γ ∼ 10 rad/s is inferred for the life time broadening of the excited state of the open ring, due to spin phonon interaction. A loss of NMR signal (wipe-out effect) is observed for the first time at LC and is explained by the enhancement of the spin-spin relaxation rate due to the inelastic scattering.This work was financially supported by the Italian FIRB Project No. RBFR12RPD1 of the Italian MIUR “New Challenges in Molecular Nanomagnetism: From Spin Dynamics to Quantum-Information Processing.”Peer Reviewe

Twisted molecular magnets

Journal article (post-print)The use of derivatised salicylaldoximes in manganese chemistry has led to the synthesis of a family of approximately fifty hexanuclear ([MnIII6]) and thirty trinuclear ([MnIII3]) Single-Molecule Magnets (SMMs). Deliberate, targeted structural distortion of the metallic core afforded family members with increasingly puckered configurations, leading to a switch in the pairwise magnetic exchange from antiferromagnetic to ferromagnetic. Examination of both the structural and magnetic data revealed a semi-quantitative magneto-structural correlation, from which the factors governing the magnetic properties could be extracted and used for predicting the properties of new family members and even more complicated structures containing analogous building blocks. Herein we describe an overview of this extensive body of work and discuss its potential impact on similar systems.IRCSE

Synthetic, structural, spectroscopic and theoretical study of a Mn(III)-Cu(II) dimer containing a Jahn-Teller compressed Mn ion

The heterobimetallic complex [Cu(II)Mn(III)(L)(2)(py)(4)](ClO4)center dot EtOH (1) built using the pro-ligand 2,2'-biphenol (LH2), contains a rare example of a Jahn-Teller compressed Mn(III) centre. Dc magnetic susceptibility measurements on 1 reveal a strong antiferromagnetic exchange between the Cu(II) and Mn(III) ions mediated through the phenolate O-atoms (J = -33.4 cm(-1)), with magnetisation measurements at low temperatures and high fields suggesting significant anisotropy. Simulations of high-field and high frequency powder EPR data suggest a single-ion anisotropy D-Mn(III) = +4.45 cm(-1). DFT calculations also yield an antiferromagnetic exchange for 1, though the magnitude is overestimated (J(DFT) = -71 cm(-1)). Calculations reveal that the antiferromagnetic interaction essentially stems from the Mn(d(x2-y2))-Cu(d(x2-y2)) interaction. The computed single-ion anisotropy and cluster anisotropy also correlates well with experiment. A larger cluster anisotropy for the S = 3/2 state compared to the single-ion anisotropy of Mn(III) is rationalised on the basis of orbital mixing and various contributions that arise due to the spin-orbit interaction

New topologies in pentanuclear nickel/oximato clusters: structural and magnetic characterization

In the present work, five new Ni5 clusters employing the versatile 2-pyridylcyanoxime ligand have been synthesized and chemically, structurally, and magnetically characterized. The crystallographic examination of these Ni5 clusters together with those already published in the literature, giving a total number of 14 complexes, exhibiting up to 8 different topologies for which the relationship between topology, reaction conditions and magnetic response has been analyzed. DC magnetic measurements were carried in the 300-2 K range for the new complexes and the analysis of the experimental data revealed an antiferromagnetic response for the oximato mediated interactions with a variety of ground states (S = 0, 1, 3) as function of the cluster topology

Templated Hydroxyapatite Nucleation and Growth at Physiological Conditions onto Self-assembled Elastin-Like Nanoparticles.

Studying materials in the nanosize scale lead to develop new synthetic approaches and discover a lot of new properties, and therefore manipulating to develop new materials that are used for different applications. In the nanoscale, physics, chemistry, biology, material science and engineering converge toward the same principles and tools. In this work, nanoparticles that developed genetically have been used to form hydroxyapatites. Polymers such as elastin-like polymers can be manipulated by the bottom-up approach to form nanoparticles and also by top-down to form patterns in scales of nanometers on the polymer hydrogel. The first aim of this work is to synthesis a nanoparticles and characterizing it. These nanoparticles are synthesized from amphiphilic elastin-like copolymers that exhibit a lower critical solution temperature, (LCST), and under the effect of the environmental stimuli could show a transition from soluble phase to insoluble phase. Below this transition temperature, in aqueous solution, the polymers chains are hydrated and extended by the hydrophobic interaction. Above this transition temperature, the chains can be assembled to form a phase separated state and adopt a dynamic, regular and nonrandom structure. Due to the self-assembly properties of thess amphiphilic elastin-like polymers at low temperature, recombinant DNA genetic engineering has been used to recombine it with SNA15 fragment. The SNA15 fragment is the first fifteen amino acid from salivary protein statherin wherein the two phosphoserine amino acid residues at positions 2 and 3 have been substituted by L- aspartic acid. This fragment has a negative charge and a helical structure in all solvents that has a high affinity to nucleate and promote the crystallization of hydroxyapatites. The amphiphilic block copolymer has been recombined with three and six fragments of SNA15. These polymers have been characterized in ultra-pure deionized water by using differential scanning calorimetry and light scattering; furthermore, have been characterized in the solid state by Fourier Transform Infrared Spectroscopy (FTIR). The second aim of this work is using these nanoparticles as a template to form nanoparticles of calcium phosphates under physiological conditions. The elastin-like polymers assembled to nanoparticles that are able to form calcium phosphate in SBF at 37oC. Electron microscopy used to study the formed nanoparticles of calcium phosphate by performing electron diffraction and elemental analysis over the nanoparticles. Also, the calcium phosphates formed have been investigated using X-ray to identify the formed phases of calcium phosphate. Furthermore; the formed nanocalcium phosphates and its effect on polymer structure have been studied by FTIR spectroscopy.Física de la Materia Condensada, Cristalografía y MineralogíaMáster en Nanociencia y Nanotecnología Molecula