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22 results on '"Simeoni, Giovanna G."'

1. The magnetic Hamiltonian and phase diagram of the quantum spin liquid Ca10Cr7O28

Author

Balz, Christian, Lake, Bella, Islam, A. T. M. Nazmul, Singh, Yogesh, Rodriguez-Rivera, Jose A., Guidi, Tatiana, Wheeler, Elisa M., Simeoni, Giovanna G., and Ryll, Hanjo

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

A spin liquid is a new state of matter with topological order where the spin moments continue to fluctuate coherently down to the lowest temperatures rather than develop static long-range magnetic order as found in conventional magnets. For spin liquid behavior to arise in a material the magnetic Hamiltonian must be "frustrated" where the combination of lattice geometry, interactions and anisotropies gives rise to competing spin arrangements in the ground state. Theoretical Hamiltonians which produce spin liquids are spin ice, the Kitaev honeycomb model and the Heisenberg kagome antiferromagnet. Spin liquid behavior however in real materials is rare because they can only approximate these Hamiltonians and often have weak higher order terms that destroy the spin liquid state. Ca10Cr7O28 is a new quantum spin liquid with magnetic Cr5+ ions that possess quantum spin number S=1/2. The spins are entirely dynamic in the ground state and the excitation spectrum is broad and diffuse as is typical of spinons which are the excitations of a spin liquid. In this paper we determine the Hamiltonian of Ca10Cr7O28 using inelastic neutron scattering under high magnetic field to induce a ferromagnetic ground state and spin-wave excitations that can be fitted to extract the interactions. We further explore the phase diagram by using inelastic neutron scattering and heat capacity measurements and establish the boundaries of the spin liquid phase as a function of magnetic field and temperature. Our results show that Ca10Cr7O28 consists of distorted kagome bilayers with several isotropic ferromagnetic and antiferromagnetic interactions where unexpectedly the ferromagnetic interactions are stronger than the antiferromagnetic ones. This complex Hamiltonian does not correspond to any known spin liquid model and points to new directions in the search for quantum spin liquid behavior.

Published
2016
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2. Physical realization of a quantum spin liquid based on a novel frustration mechanism

Author

Balz, Christian, Lake, Bella, Reuther, Johannes, Luetkens, Hubertus, Schönemann, Rico, Herrmannsdörfer, Thomas, Singh, Yogesh, Islam, A. T. M. Nazmul, Wheeler, Elisa M., Rodriguez-Rivera, Jose A., Guidi, Tatiana, Simeoni, Giovanna G., Baines, Chris, and Ryll, Hanjo

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. Such phenomena is usually sought in simple lattices where antiferromagnetic interactions and/or anisotropies that favor specific alignments of the magnetic moments are "frustrated" by lattice geometries incompatible with such order e.g. triangular structures. Despite an extensive search among such compounds, experimental realizations remain very few. Here we describe the investigation of a novel, unexplored magnetic system consisting of strong ferromagnetic and weaker antiferromagnetic isotropic interactions as realized by the compound Ca$_{10}$Cr$_7$O$_{28}$. Despite its exotic structure we show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid including coherent spin dynamics in the ground state and the complete absence of static magnetism., Comment: Modified version accepted in Nature Physics

Published
2016
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6. Magnetic Anisotropy Switch: Easy Axis to Easy Plane Conversion and Vice Versa

Author

Perfetti, Mauro, primary, Sørensen, Mikkel A., additional, Hansen, Ursula B., additional, Bamberger, Heiko, additional, Lenz, Samuel, additional, Hallmen, Philipp P., additional, Fennell, Tom, additional, Simeoni, Giovanna G., additional, Arauzo, Ana, additional, Bartolomé, Juan, additional, Bartolomé, Elena, additional, Lefmann, Kim, additional, Weihe, Høgni, additional, van Slageren, Joris, additional, and Bendix, Jesper, additional

Published
2018
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7. Chemical tunnel-splitting-engineering in a dysprosium-based molecular nanomagnet

Author

Sørensen, Mikkel A., primary, Hansen, Ursula B., additional, Perfetti, Mauro, additional, Pedersen, Kasper S., additional, Bartolomé, Elena, additional, Simeoni, Giovanna G., additional, Mutka, Hannu, additional, Rols, Stéphane, additional, Jeong, Minki, additional, Zivkovic, Ivica, additional, Retuerto, Maria, additional, Arauzo, Ana, additional, Bartolomé, Juan, additional, Piligkos, Stergios, additional, Weihe, Høgni, additional, Doerrer, Linda H., additional, van Slageren, Joris, additional, Rønnow, Henrik M., additional, Lefmann, Kim, additional, and Bendix, Jesper, additional

Published
2018
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8. Magnetic Anisotropy Switch:Easy Axis to Easy Plane Conversion and Vice Versa

Author

Perfetti, Mauro, Sørensen, Mikkel Agerbæk, Hansen, Ursula Bengård, Bamberger, Heiko, Lenz, Samuel, Hallmen, Philipp P., Fennell, Tom, Simeoni, Giovanna G., Arauzo, Ana, Bartolome, Juan, Bartolome, Elena, Lefmann, Kim, Weihe, Høgni, van Slageren, Joris, Bendix, Jesper, Perfetti, Mauro, Sørensen, Mikkel Agerbæk, Hansen, Ursula Bengård, Bamberger, Heiko, Lenz, Samuel, Hallmen, Philipp P., Fennell, Tom, Simeoni, Giovanna G., Arauzo, Ana, Bartolome, Juan, Bartolome, Elena, Lefmann, Kim, Weihe, Høgni, van Slageren, Joris, and Bendix, Jesper

Published
2018

9. Chemical tunnel-splitting-engineering in a dysprosium-based molecular nanomagnet

Author

Sørensen, Mikkel Agerbæk, Hansen, Ursula Bengård, Perfetti, Mauro, Pedersen, Kasper Steen, Bartolome, Elena, Simeoni, Giovanna G., Mutka, Hannu, Rols, Stephane, Jeong, Minki, Zivkovic, Ivica, Retuerto Millan, Maria, Arauzo, Ana, Bartolome, Juan, Piligkos, Stergios, Weihe, Høgni, Doerrer, Linda H., van Slageren, Joris, Rønnow, Henrik Moodysson, Lefmann, Kim, Bendix, Jesper, Sørensen, Mikkel Agerbæk, Hansen, Ursula Bengård, Perfetti, Mauro, Pedersen, Kasper Steen, Bartolome, Elena, Simeoni, Giovanna G., Mutka, Hannu, Rols, Stephane, Jeong, Minki, Zivkovic, Ivica, Retuerto Millan, Maria, Arauzo, Ana, Bartolome, Juan, Piligkos, Stergios, Weihe, Høgni, Doerrer, Linda H., van Slageren, Joris, Rønnow, Henrik Moodysson, Lefmann, Kim, and Bendix, Jesper

Published
2018

10. Magnetic anisotropy switch: Easy axis to easy plane conversion and vice versa

Author

Paul Scherrer Institute (Switzerland), Agencia Estatal de Investigación (España), Danish Agency for Science, Technology and Innovation, German Research Foundation, Oticon Foundation, Augustinus Foundation, University of Stuttgart, Ministerio de Economía y Competitividad (España), European Commission, Ministerio de Ciencia, Innovación y Universidades (España), Perfetti, Mauro, Sørensen, Mikkel A., Hansen, Ursula B., Bamberger, Heiko, Lenz, Samuel, Hallmen, Philipp P., Fennell, Tom, Simeoni, Giovanna G., Arauzo, Ana B., Bartolomé, Juan, Bartolomé, Elena, Lefmann, Kim, Weihe, Høgni, Slageren, Joris van, Bendix, Jesper, Paul Scherrer Institute (Switzerland), Agencia Estatal de Investigación (España), Danish Agency for Science, Technology and Innovation, German Research Foundation, Oticon Foundation, Augustinus Foundation, University of Stuttgart, Ministerio de Economía y Competitividad (España), European Commission, Ministerio de Ciencia, Innovación y Universidades (España), Perfetti, Mauro, Sørensen, Mikkel A., Hansen, Ursula B., Bamberger, Heiko, Lenz, Samuel, Hallmen, Philipp P., Fennell, Tom, Simeoni, Giovanna G., Arauzo, Ana B., Bartolomé, Juan, Bartolomé, Elena, Lefmann, Kim, Weihe, Høgni, Slageren, Joris van, and Bendix, Jesper

Abstract

The rational design of the magnetic anisotropy of molecular materials constitutes a goal of primary importance in molecular magnetism. Indeed, the applications of molecular nanomagnets, such as single-molecule magnets and molecular magnetic refrigerants, depend on the full control over this property. Axially anisotropic magnetic systems are frequently classified as easy axis or easy plane, depending on whether the lowest energy is obtained by application of a magnetic field parallelly or perpendicularly to the unique axis. Here, the magnetic aniso-tropy of three lanthanide complexes is studied as a function of magnetic field and temperature. It is found that for two of these the type of magnetic aniso-tropy switches as a function of these parameters. Thus, this paper experimen-tally demonstrates that the magnetic anisotropy is not uniquely defined by the intrinsic electronic structure of the systems in question but can also be reversibly switched using external stimuli: temperature and magnetic field.

Published
2018

11. Chemical tunnel-splitting-engineering in a dysprosium-based molecular nanomagnet

Author

Danish Council for Independent Research, Agencia Estatal de Investigación (España), European Commission, Oticon Foundation, University of Stuttgart, German Research Foundation, Danish Research Council, Swiss National Science Foundation, Ministerio de Ciencia, Innovación y Universidades (España), Ministerio de Economía y Competitividad (España), Augustinus Foundation, Sørensen, Mikkel A., Hansen, Ursula B., Perfetti, Mauro, Pedersen, Kasper S., Bartolomé, Elena, Simeoni, Giovanna G., Mutka, Hannu, Rols, Stéphane, Jeong, Minki, Zivkovic, Ivika, Retuerto, María, Arauzo, Ana B., Bartolomé, Juan, Piligkos, Stergios, Weihe, Høgni, Doerrer, Linda H., Slageren, Joris van, Rønnow, Henrik M., Lefmann, Kim, Bendix, Jesper, Danish Council for Independent Research, Agencia Estatal de Investigación (España), European Commission, Oticon Foundation, University of Stuttgart, German Research Foundation, Danish Research Council, Swiss National Science Foundation, Ministerio de Ciencia, Innovación y Universidades (España), Ministerio de Economía y Competitividad (España), Augustinus Foundation, Sørensen, Mikkel A., Hansen, Ursula B., Perfetti, Mauro, Pedersen, Kasper S., Bartolomé, Elena, Simeoni, Giovanna G., Mutka, Hannu, Rols, Stéphane, Jeong, Minki, Zivkovic, Ivika, Retuerto, María, Arauzo, Ana B., Bartolomé, Juan, Piligkos, Stergios, Weihe, Høgni, Doerrer, Linda H., Slageren, Joris van, Rønnow, Henrik M., Lefmann, Kim, and Bendix, Jesper

Abstract

Total control over the electronic spin relaxation in molecular nanomagnets is the ultimate goal in the design of new molecules with evermore realizable applications in spin-based devices. For single-ion lanthanide systems, with strong spin-orbit coupling, the potential applications are linked to the energetic structure of the crystal field levels and quantum tunneling within the ground state. Structural engineering of the timescale of these tunneling events via appropriate design of crystal fields represents a fundamental challenge for the synthetic chemist, since tunnel splittings are expected to be suppressed by crystal field environments with sufficiently high-order symmetry. Here, we report the long missing study of the effect of a non-linear (C4) to pseudo-linear (D4d) change in crystal field symmetry in an otherwise chemically unaltered dysprosium complex. From a purely experimental study of crystal field levels and electronic spin dynamics at milliKelvin temperatures, we demonstrate the ensuing threefold reduction of the tunnel splitting.

Published
2018

13. Physical realization of a quantum spin liquid based on a complex frustration mechanism

Author

Balz, Christian, primary, Lake, Bella, additional, Reuther, Johannes, additional, Luetkens, Hubertus, additional, Schönemann, Rico, additional, Herrmannsdörfer, Thomas, additional, Singh, Yogesh, additional, Nazmul Islam, A. T. M., additional, Wheeler, Elisa M., additional, Rodriguez-Rivera, Jose A., additional, Guidi, Tatiana, additional, Simeoni, Giovanna G., additional, Baines, Chris, additional, and Ryll, Hanjo, additional

Published
2016
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15. From a one-dimensional crystal to a one-dimensional liquid: A comprehensive dynamical study of C 60 peapods

Author

Bousige, Colin, Rols, Stéphane, Ollivier, Jacques, Schober, Helmut, Fouquet, Peter, Simeoni, Giovanna G., Agafonov, Viatcheslav, Davydov, Valery, Niimi, Yoshiko, Suenaga, Kazutomo, Kataura, Hiromichi, Launois, Pascale, Simeoni, Giovanna, Laboratoire de Physique des Solides (LPS), Centre National de la Recherche Scientifique (CNRS)-Université Paris-Sud - Paris 11 (UP11), Institut Laue-Langevin (ILL), ILL, Heinz Meier-Leibnitz Zentrum and Physics Department (MLZ), GREMAN (matériaux, microélectronique, acoustique et nanotechnologies) (GREMAN - UMR 7347), Institut National des Sciences Appliquées - Centre Val de Loire (INSA CVL), Institut National des Sciences Appliquées (INSA)-Institut National des Sciences Appliquées (INSA)-Université de Tours (UT)-Centre National de la Recherche Scientifique (CNRS), Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Nanotechnology Research Institute (NRI), Université Paris-Sud - Paris 11 (UP11)-Centre National de la Recherche Scientifique (CNRS), Université de Tours-Institut National des Sciences Appliquées - Centre Val de Loire (INSA CVL), and Institut National des Sciences Appliquées (INSA)-Institut National des Sciences Appliquées (INSA)-Centre National de la Recherche Scientifique (CNRS)

Subjects
Materials science, Thermal fluctuations, Nanotechnology, 02 engineering and technology, Carbon nanotube, 6865−k, 01 natural sciences, Molecular physics, law.invention, Crystal, 6580−g, law, Speed of sound, 6148−c, 0103 physical sciences, 010306 general physics, [PHYS]Physics [physics], Plane (geometry), Scattering, Anharmonicity, Atmospheric temperature range, 021001 nanoscience & nanotechnology, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, 0210 nano-technology, number(s): 6105F−
Abstract

International audience; We report an inelastic neutron-scattering investigation of the longitudinal acoustic modes of C 60 chains confined inside single walled carbon nanotubes. We take advantage of the orientations of the chains within the plane of the pellet sample to isolate their scattering signatures in the (Q,ω) space, which we follow as a function of temperature from 260 K up to 1100 K. The results show the progressive evolution of the confined chain from a one-dimensional (1D) crystal to a linear liquid, the transition occurring within a temperature range of ∼150 K centered around 600 K. The comparison of the data obtained on monomer and polymer peapods allows extracting the speed of sound in the monomer crystalline chains (v mono = 3.5 km s −1 , v poly /v mono = 1.7). We find that the sound velocity is further reduced by half in the liquid state which reveals that the melting is not only due to harmonic additive thermal fluctuations, but that anharmonic terms in the intermolecular potential play an important role at high temperatures.

Published
2013
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17. Cyanide single-molecule magnets exhibiting solvent dependent reversible 'on' and 'off' exchange bias behavior

Author

Pinkowicz, Dawid, Southerland, Heather I., Avendaño, Carolina, Prosvirin, Andrey, Sanders, Codi, Wernsdorfer, Wolfgang, Pedersen, Kasper Steen, Dreiser, Jan, Clérac, Rodolphe, Nehrkorn, Joscha, Simeoni, Giovanna G., Schnegg, Alexander, Holldack, Karsten, Dunbar, Kim R., Pinkowicz, Dawid, Southerland, Heather I., Avendaño, Carolina, Prosvirin, Andrey, Sanders, Codi, Wernsdorfer, Wolfgang, Pedersen, Kasper Steen, Dreiser, Jan, Clérac, Rodolphe, Nehrkorn, Joscha, Simeoni, Giovanna G., Schnegg, Alexander, Holldack, Karsten, and Dunbar, Kim R.

Abstract

The syntheses, structures, and magnetic properties of four new complex salts, (PPN){[Mn(III)(salphen)(MeOH)]2[M(III)(CN)6]}·7MeOH (Mn2M·7MeOH) (M = Fe, Ru, Os and Co; PPN(+) = bis(triphenylphosphoranylidene)ammonium cation; H2salphen = N,N'-bis(salicylidene)-1,2-diaminobenzene), and a mixed metal Co/Os analogue (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)0.92Os(III)0.08(CN)6]}·7MeOH were undertaken. It was found that all compounds exhibit switchable single-molecule magnet (SMM) and exchange-bias behavior depending on the interstitial methanol content. The pristine (PPN){[Mn(salphen)(MeOH)]2[Os(CN)6]}·7MeOH (Mn2Os·7MeOH) behaves as an SMM with an effective barrier for the magnetization reversal, (Ueff/kB), of 17.1 K. Upon desolvation, Mn2Os exhibits an increase of Ueff/kB to 42.0 K and an opening of the hysteresis loop observable at 1.8 K. Mn2Os·7MeOH shows also exchange-bias behavior with magnetic hysteresis loops exhibiting a shift in the quantum tunneling to 0.25 T from zero-field. The Fe(III) and Ru(III) analogues were prepared as reference compounds for assessing the effect of the 5d versus 4d and 3d metal ions on the SMM properties. These compounds are also SMMs and exhibit similar effects but with lower energy barriers. These findings underscore the importance of introducing heavy transition elements into SMMs to improve their slow relaxation of the magnetization properties. The (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)(CN)6]}·7MeOH (Mn2Co·7MeOH) analogue with a diamagnetic Co(III) central atom and the mixed Co/Os (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)0.92Os(III)0.08(CN)6]}·7MeOH (Mn2Co/Os·7MeOH) "magnetically diluted" system with a 9:1 Co/Os metal ratio were prepared in order to further probe the nature of the energy barrier increase upon desolvation of Mn2Os. In addition, inelastic neutron scattering and frequency-domain Fourier-transform THz electron paramagnetic resonance spectra obtained on Mn2Os·7MeOH and Mn2Os in combination with the magnetic data revealed

Published
2015

18. Cyanide Single-Molecule Magnets Exhibiting Solvent Dependent Reversible “On” and “Off” Exchange Bias Behavior

Author

Pinkowicz, Dawid, primary, Southerland, Heather I., additional, Avendaño, Carolina, additional, Prosvirin, Andrey, additional, Sanders, Codi, additional, Wernsdorfer, Wolfgang, additional, Pedersen, Kasper S., additional, Dreiser, Jan, additional, Clérac, Rodolphe, additional, Nehrkorn, Joscha, additional, Simeoni, Giovanna G., additional, Schnegg, Alexander, additional, Holldack, Karsten, additional, and Dunbar, Kim R., additional

Published
2015
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19. Ferrimagnetism and spin excitation in a Ni–Mn partially inverted spinel prepared using a modified polymeric precursor method

Author

Ferreira, R.A., Tedesco, Julio Cesar G., Birk, Jonas Okkels, Kalceff, W., Yokaichiya, F., Rasmussen, Nina, Peña, Octavio, Henry, Paul F., Simeoni, Giovanna G., Nunes Bordallo, Heloisa, Lisboa-Filho, Paulo N., Ferreira, R.A., Tedesco, Julio Cesar G., Birk, Jonas Okkels, Kalceff, W., Yokaichiya, F., Rasmussen, Nina, Peña, Octavio, Henry, Paul F., Simeoni, Giovanna G., Nunes Bordallo, Heloisa, and Lisboa-Filho, Paulo N.

Published
2014

21. From a one-dimensional crystal to a one-dimensional liquid: A comprehensive dynamical study of C60 peapods.

Author

Bousige, Colin, Rols, Stéphane, Ollivier, Jacques, Schober, Helmut, Fouquet, Peter, Simeoni, Giovanna G., Agafonov, Viatcheslav, Davydov, Valery, Niimi, Yoshiko, Suenaga, Kazutomo, Kataura, Hiromichi, and Launois, Pascale

Subjects
*NEUTRON scattering, *CARBON nanotubes, *SCATTERING (Physics), *ANHARMONIC motion, *SCATTERING cross sections (Physics), *MULTIPLE scattering (Physics), *COLLISIONS (Physics), *NANOTUBES
Abstract

We report an inelastic neutron-scattering investigation of the longitudinal acoustic modes of C60 chains confined inside single walled carbon nanotubes. We take advantage of the orientations of the chains within the plane of the pellet sample to isolate their scattering signatures in the (Q,ω) space, which we follow as a function of temperature from 260 K up to 1100 K. The results show the progressive evolution of the confined chain from a one-dimensional (ID) crystal to a linear liquid, the transition occurring within a temperature range of ∼150 K centered around 600K. The comparison of the data obtained on monomer and polymer peapods allows extracting the speed of sound in the monomer crystalline chains (vmono = 3.5 km s-1, vpoly/vmono= 1.7). We find that the sound velocity is further reduced by half in the liquid state which reveals that the melting is not only due to harmonic additive thermal fluctuations, but that anharmonic terms in the intermolecular potential play an important role at high temperatures. [ABSTRACT FROM AUTHOR]

Published
2013
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