Jürgen Schnack

Classical and Quantum Magnetism in Giant Keplerate Magnetic Molecules

Complementary theoretical modeling methods are presented for the classical and quantum Heisenberg model to explain the magnetic properties of nanometer-sized magnetic molecules. Excellent quantitative agreement is achieved between our experimental data down to 0.1 K and for fields up to 60 Tesla and our theoretical results for the giant Keplerate species {Mo72Fe30}, by far the largest paramagnetic molecule synthesized to date.

Molecular Dynamics for Fermions

The time-dependent variational principle for many-body trial states is used to discuss the relation between the approaches of dierent molecular dynamics models to describe indistinguishable fermions. Early attempts to include eects of the Pauli principle by means of nonlocal potentials as well as more recent models which work with antisymmetrized many-body states are reviewed under these premises.

Macroscopic magnetization jumps due to independent magnons in frustrated quantum spin lattices.

For a class of frustrated spin lattices including the Kagomé lattice we construct exact eigenstates consisting of several independent, localized one-magnon states and argue that they are ground states for high magnetic fields. If the maximal number of local magnons scales with the number of spins in the system, which is the case for the Kagomé lattice, the effect persists in the thermodynamic limit and gives rise to a macroscopic jump in the zero-temperature magnetization curve just below the saturation field. The effect decreases with increasing spin quantum number and vanishes in the classical limit. Thus it is a true macroscopic quantum effect.

Rotational modes in molecular magnets with antiferromagnetic Heisenberg exchange

In an effort to understand the low-temperature behavior of recently synthesized molecular magnets, we present numerical evidence for the existence of a rotational band in systems of quantum spins interacting with nearest-neighbor antiferromagnetic Heisenberg exchange. While this result has previously been noted for ring arrays with an even number of spin sites, we find that it also applies for rings with an odd number of sites as well as for all of the polytope configurations we have investigated (tetrahedron, cube, octahedron, icosahedron, triangular prism, and axially truncated icosahedron). It is demonstrated how the rotational band levels can, in many cases, be accurately predicted using the underlying sublattice structure of the spin array. We illustrate how the characteristics of the rotational band can provide valuable estimates for the low-temperature magnetic susceptibility.

Exchange Interactions and Zero-Field Splittings in C3-Symmetric MnIII6FeIII: Using Molecular Recognition for the Construction of a Series of High Spin Complexes Based on the Triplesalen Ligand

The reaction of the tris(tetradentate) triplesalen ligand H6talen(t-Bu2), which provides three salen-like coordination environments bridged in a meta-phenylene arrangement by a phloroglucinol backbone, with Mn(II) salts under aerobic conditions affords, in situ, the trinuclear Mn(III) triplesalen complexes [(talen(t-Bu2)){Mn(III)(solv)n}3]3+. These can be used as molecular building blocks in the reaction with [Fe(CN)6]3- as a hexaconnector to form the heptanuclear complex [{(talen(t-Bu2)){Mn(III)(solv)n}3}2{Fe(III)(CN)6}]3+ ([Mn(III)6Fe(III)]3+). The regular ligand folding observed in the trinuclear triplesalen complexes preorganizes the three metal ions for the reaction of three facially coordinated nitrogen atoms of a hexacyanometallate and provides a driving force for the formation of the heptanuclear complexes [M(t)6M(c)]n+ (M(t), terminal metal ion of the triplesalen building block; M(c), central metal ion of the hexacyanometallate) by molecular recognition, as has already been demonstrated for the single-molecule magnet [Mn(III)6Cr(III)]3+. [{(talen(t-Bu2))(Mn(III)(MeOH))3}2{Fe(III)(CN)6}][Fe(III)(CN)6] (1) was characterized by single-crystal X-ray diffraction, FTIR, ESI- and MALDI-TOF-MS, Mössbauer spectroscopy, and magnetic measurements. The molecular structure of [Mn(III)6Fe(III)]3+ is overall identical to that of [Mn(III)6Cr(III)]3+ but exhibits a different ligand folding of the Mn(III) salen subunits with a helical distortion. The Mössbauer spectra demonstrate a stronger distortion from octahedral symmetry for the central [Fe(CN)6]3- in comparison to the ionic [Fe(CN)6]3-. At low temperatures in zero magnetic fields, the Mössbauer spectra show magnetic splittings indicative of slow relaxation of the magnetization on the Mössbauer time scale. Variable-temperature-variable-field and mu(eff) versus T magnetic data have been analyzed in detail by full-matrix diagonalization of the appropriate spin-Hamiltonian, consisting of isotropic exchange, zero-field splitting, and Zeeman interaction taking into account the relative orientation of the D tensors. Satisfactory reproduction of the experimental data has been obtained for parameters sets J(Mn-Mn) = -(0.85 +/- 0.15) cm(-1), J(Fe-Mn) = +(0.70 +/- 0.30) cm(-1), and D(Mn) = -(3.0 +/- 0.7) cm(-1). Comparing these values to those of [Mn(III)6Cr(III)]3+ provides insight into why [Mn(III)6Fe(III)]3+ is not a single-molecule magnet.

Effects of frustration on magnetic molecules: a survey from Olivier Kahn until today

In magnetism, of which molecular magnetism is a part, the term frustration is used rather sloppily. Sometimes one gains the impression that if the reason for some phenomenon is not quite clear then it is attributed to frustration. In this paper a discussion of the effects of frustration that are relevant for the field of molecular magnetism is presented. As will become clear later these effects indeed lead to a variety of unusual magnetic properties.

Quantum signatures of a molecular nanomagnet in direct magnetocaloric measurements.

Geometric spin frustration in low-dimensional materials, such as the two-dimensional kagome or triangular antiferromagnetic nets, can significantly enhance the change of the magnetic entropy and adiabatic temperature following a change in the applied magnetic field, that is, the magnetocaloric effect. In principle, an equivalent outcome should also be observable in certain high-symmetry zero-dimensional, that is, molecular, structures with frustrated topologies. Here we report experimental realization of this in a heptametallic gadolinium molecule. Adiabatic demagnetization experiments reach ~200 mK, the first sub-Kelvin cooling with any molecular nanomagnet, and reveal isentropes (the constant entropy paths followed in the temperature-field plane) with a rich structure. The latter is shown to be a direct manifestation of the trigonal antiferromagnetic net structure, allowing study of frustration-enhanced magnetocaloric effects in a finite system.

Fermionic Molecular Dynamics for Ground States and Collisions of Nuclei

Abstract The antisymmetric many-body trial state which describes a system of interacting fermions is parametrized in terms of localized wave packets. The equations of motion are derived from the time-dependent quantum variational principle. The resulting fermionic molecular dynamics (FMD) equations include a wide range of semi-quantal to classical physics extending from deformed Hartree-Fock theory to newtonian molecular dynamics. Conservation laws are discussed in connection with the choice of the trial state. The model is applied to heavy-ion collisions with which its basic features are illustrated. The results show a great variety of phenomena including deeply inelastic collisions, fusion, incomplete fusion, fragmentation, neck emission, promptly emitted nucleons and evaporation.

Magnetic characterization of the frustrated three-leg ladder compound [(CuCl2tachH)3Cl]Cl2

We report the magnetic features of a one-dimensional stack of antiferromagnetically coupled equilateral copper(II) triangles. High-field magnetization measurements show that the interaction between the copper triangles is of the same order of magnitude as the intratriangle exchange although only coupled via hydrogen bonds. The infinite chain turns out to be an interesting example of a frustrated cylindrical three-leg ladder with competing intra- and inter-triangle interactions. We demonstrate that the ground state is a spin singlet which is gaped from the triplet excitation.

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.