Revisiting the chain magnetism in Sr14Cu24O41: Experimental and numerical results

We study the magnetism of the hole doped CuO2 spin chains in Sr14Cu24O41 by measuring the Electron Spin Resonance (ESR) and the static magnetization M in applied magnetic fields up to 14 T. In this compound, the dimerized ground state and the charge order in the chains are well established. Our experimental data suggest that at low temperatures the Curie-like increase of M as well as the occurrence of the related ESR signal are due to a small amount of paramagnetic centers which are not extrinsic defects but rather unpaired Cu spins in the chain. These observations qualitatively confirm recent ab initio calculations of the ground state properties of the CuO2 chains in Sr14Cu24O41. Our complementary quantum statistical simulations yield that the temperature and field dependence of the magnetization can be well described by an effective Heisenberg model in which the ground state configuration is composed of spin dimers, trimers, and monomers.Comment: revised versio

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

Frustration effects in magnetic molecules

Besides being a fascinating class of new materials, magnetic molecules provide the opportunity to study concepts of condensed matter physics in zero dimensions. This contribution will exemplify the impact of molecular magnetism on concepts of frustrated spin systems. We will discuss spin rings and the unexpected rules that govern their low-energy behavior. Rotational bands, which are experimentally observed in various molecular magnets, provide a useful, simplified framework for characterizing the energy spectrum, but there are also deviations thereof with far-reaching consequences. It will be shown that localized independent magnons on certain frustrated spin systems lead to giant magnetization jumps, a new macroscopic quantum effect. In addition a frustration-induced metamagnetic phase transitions will be discussed, which demonstrates that hysteresis can exist without anisotropy. Finally, it is demonstrated that frustrated magnetic molecules could give rise to an enhanced magnetocaloric effect.Comment: 6 pages, 4 figures; submitted to J. Low. Temp. Phys.; proceedings of the Sixth International Symposium on Crystalline Organic Metals, Superconductors, and Ferromagnets (ISCOM

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

For a class of frustrated spin lattices including the kagome 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 kagome 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.Comment: 4 pages, 4 figures, accepted by Phys.Rev.Let

Quantum rotational band model for the Heisenberg molecular magnet Mo72Fe30

We derive the low temperature properties of the molecular magnet Mo72Fe30, where 30 Fe(3+) paramagnetic ions occupy the sites of an icosidodecahedron and interact via isotropic nearest-neighbour antiferromagnetic Heisenberg exchange. The key idea of our model (J.S. & M.L.) is that the low-lying excitations form a sequence of rotational bands, i.e., for each such band the excitation energies depend quadratically on the total spin quantum number. For temperatures below 50 mK we predict that the magnetisation is described by a staircase with 75 equidistant steps as the magnetic field is increased up to a critical value and saturated for higher fields. For higher temperatures thermal broadening effects wash out the staircase and yield a linear ramp below the critical field, and this has been confirmed by our measurements (R.M.). We demonstrate that the lowest two rotational bands are separated by an energy gap of 0.7 meV, and this could be tested by EPR and inelastic neutron scattering measurements. We also predict the occurrence of resonances at temperatures below 0.1 K in the proton NMR spin-lattice relaxation rate associated with level crossings. As rotational bands characterize the spectra of many magnetic molecules our method opens a new road towards a description of their low-temperature behaviour which is not otherwise accessible.Comment: 7 pages, 6 figures, accepted for Europhysics Letter

Application of the finite-temperature Lanczos method for the evaluation of magnetocaloric properties of large magnetic molecules

We discuss the magnetocaloric properties of gadolinium containing magnetic molecules which potentially could be used for sub-Kelvin cooling. We show that a degeneracy of a singlet ground state could be advantageous in order to support adiabatic processes to low temperatures and simultaneously minimize disturbing dipolar interactions. Since the Hilbert spaces of such spin systems assume very large dimensions we evaluate the necessary thermodynamic observables by means of the Finite-Temperature Lanczos Method.Comment: 7 pages, 10 figures, invited for the special issue of EPJB on "New trends in magnetism and magnetic materials

Oviposición intrasexual en Belostoma oxyurum. Una nota sobre sus determinantes ecológicos

Intrasexual egg laying was detected in a female giant water bug, Belostoma oxyurum (Dufour) which was capturad carrying six fertilized eggs on its back. The location of the eggs does not belong to the pattern known for incubant males of this species. This finding is attributed to the absolute shortage of a sexual resource constituted by the male back space. Such environmental circuinstance was recorded in the field when this female was captured. The adult population from which it was taken included a hundred percent of both, gravid females and incubant males, and sex ratio was female biased though without statistical significance. The observed phenomenon has also been recorded in the Nearctic species Belostoma flumineum Say. Comparative considerations referred to bothspecies are included.Un caso de oviposición intrasexual fue detectado en una población de Belostoma oxyurum (Dufour) localizada en un limnótopo léntico de Pinamar, Provincia de Buenos Aires. El fenómeno observado es atribuido a la limitación del recurso sexual constituido por el área de postura, es decir, la superficie dorsal de los machos reproductivos. Tal circunstancia ambiental fue registrada en el campo cuando la “hembra incubante” era extraída de su habitat. La población de adultos incluía entonces, el máximo porcentaje de hembras grávidas y machos incubantes, ostentando elevados valores de fecundidad potencial y real, y un predominio, aunque no significativo, de hembras sobre machos