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Magnetic Exchange (magnetic + exchange)

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Chemistry80%

Terms modified by Magnetic Exchange

  • magnetic exchange interaction
    		•

    Selected Abstracts

    Towards a Better Understanding of Magnetic Interactions within m -Phenylene ,-Nitronyl Nitroxide and Imino Nitroxide Based Radicals, Part III: Magnetic Exchange in a Series of Triradicals and Tetraradicals Based on the Phenyl Acetylene and Biphenyl Coupling Units

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2005
    Laure Catala Dr.
    Abstract The present work completes and extends our previous reports1,,2 on the determination of the magnetic ground state and on the strength of the through bond exchange coupling within series of biradicals. This knowledge was subsequently exploited for the analysis of the magnetic interactions in their crystals. We report here the studies of series of triradicals incorporating ,-nitronyl nitroxides (NN) or ,-imino nitroxides (IN) as terminal radical fragments connected through a m -phenylene coupling unit in one case and a phenyl acetylene unit in other case. Tetraradical derivatives have also been studied. The studies of isolated molecules (EPR in solution and DFT calculations) allow the assessment of the magnetic interactions through the magnetic coupling unit. All triradical derivatives are found to exhibit a quartet ground state, whereas a singlet ground state is determined for the tetraradical. This last result reinforces previous findings that the singlet ground state is favoured in related biradicals involving similar m -phenylene couplers. Moreover, the through bond magnetic exchange coupling for the ortho,meta connectivity could be demonstrated as being ferromagnetic, thus ascertaining our previous hypotheses.1 The magnetic properties of the triradicals and tetraradicals in their solid state have been rationalized by using a previously proposed methodology,2 allowing to identify the most relevant magnetic pathways. Le présent travail complète et étend nos travaux précédents portant sur la détermination des couplages d'échange magnétique intramoléculaires de séries de biradicaux,1,,2 et partant de cette connaissance, de l'analyse des corrélations magnétostructurales dans l'état cristallin. Nous présentons ici l'étude de séries de triradicaux comprenant des radicaux , -nitronyl nitroxydes (NN) ou , -imino nitroxydes (IN) substitués en méta d'un groupement phénylène d'une part, et en para d'un groupement phénylène éthynylène d'autre part. Des dérivés tétraradicalaires ont aussi été synthétisés et étudiés. Les interactions d'échange magnétique à travers les liaisons sont estimées d'après l'étude de ces molécules isolées par RPE en solution ainsi que par des calculs effectués dans l'approximation de la fonctionnelle densité (DFT). Alors que l'ensemble des dérivés triradicalaires présente un état fondamental magnétique quartet de spin, un état singulet est proposé pour les tétraradicaux. Ce dernier résultat confirme nos travaux précédents1 ayant conclu à un état fondamental singulet pour des biradicaux basés sur le même coupleur. De plus, le couplage d'échange magnétique au travers du coupleur phénylène éthynylène pour des biradicaux substitués dans une topologie ortho,méta est ferromagnétique, confirmant ainsi les hypothèses antérieures.1 Suivant une méthodologie précédemment décrite,2 les propriétés magnétiques des triradicaux à l'état cristallin ont pu être analysées dans le détail et permettent de proposer des chemins d'interaction magnétique et des géométries de contacts intermoléculaires précises pour l'établissement d'interactions magnétiques bien identifiées. El presente trabajo completa y extiende nuestras anteriores estudios1,,2 sobre la determinacion del estado fundamental magnético y la fuerza del intercambio magnetico a través del enlace en una serie de biradicales. Esta información es luego usada en el análisis de las interacciones magneticas dentro de sus cristales. En este trabajo presentamos los estudios sobre una serie de triradicales que contienen , -nitroxidos nitronílicos (NN) o , -imino nitróxidos (IN) como fragmentos radicalarios terminales conectados a través de unidades acopladoras del tipo m -phenilénicas, en un caso, y fenil acetilénicas, en el otro. Se han estudiado también tetraradicales. Los estudios con moleculas aisladas (EPR en disolución y DFT) permiten evaluar las interacciones magneticas a través de las unidades acopladoras. Todos los triradicales se ve que presentan un estado fundamental del tipo cuadruplete, mientras que el cuadruplete se cree que tienen un estado fundamental singlete. Este último resultado refuerza las conclusiones de un estudio previo en el que se vió que el singlete es el estado fundamental en biradicales similares conectados por una unidad m -fenilénica. Además, el intercambio magnético a través del enlace para la conectividad orto y meta se demuestra que es ferromagnetica, confirmando hipótesis anteriores.1 Las propiedades magnéticas de los triradicales y tetraradicales en su estado sólido se han racionalizado empleando una matodología propuesta anteriormente,2 identificando los caminos magnéticos relevantes. [source]

    New Molecular Charge-Transfer Salts of TM-TTF and BMDT-TTF with Thiocyanate and Selenocyanate Complex Anions [TMTTF = Tetramethyltetrathiafulvalene; BMDT-TTF = Bis(methylenedithio)tetrathiafulvalene]

    EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 4 2003
    Marta Mas-Torrent
    Abstract Three new charge-transfer salts of tetrathiafulvalene (TTF)-based donors with thiocyanato- or selenocyanato-metal complex anions have been synthesised. The salts isolated were [BMDT-TTF]4[Cr(NCS)6] (1), [TM-TTF]4[Cr(NCS)6]·2CH3CN (2) and [TM-TTF]4[Cr(NCSe)6]·2CH3CN (3) [BMDT-TTF = bis(methylenedithio)tetrathiafulvalene and TM-TTF = tetramethyltetrathiafulvalene]. Single crystals of compound 1 crystallise in the monoclinic C2/c space group with a = 37.286(3), b = 10.0539(6), c = 21.069(2) Å, , = 124.348(4)°, V = 6520.9(9) Å3 and Z = 4. Compound 3 was also suitable for an X-ray diffraction study, however the anionic part, [Cr(NCSe)6], was highly disordered and the best solution gave a final R factor of 16.4%. A solution was found for the monoclinic space group C2/m with a = 13.787(3), b = 19.507(3), c = 14.735(5) Å, , = 102.90(3)°, V = 3862.9(17) Å3 and Z = 2. For compound 1 there are several S···S close atomic contacts between the donors and acceptors, but there is no discernible magnetic exchange between ions. Such an interaction was previously observed in related salts such as [TTF][Cr(NCS)4(phenanthroline)2] and [donor][M(NCS)4(isoquinoline)2] [M = Cr, Fe and donor = TTF, BEDT-TTF or TM-TTF (tetramethyltetrathiafulvalene)]. Compounds 1 to 3 are all paramagnetic semiconductors in which the magnetic susceptibility is dominated by the Cr-containing anions. The structure-function relationship, along with a comparison with related compounds, indicates that there is no long-range magnetic order because there are no ,-stacking interactions between donor and acceptor; these types of interactions are seen in all of the bulk magnets of this type in which the donor spin is magnetically coupled to the anion. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

    Recent Progress in Exploring Magnetocaloric Materials

    ADVANCED MATERIALS, Issue 45 2009
    B. G. Shen
    Abstract The magnetic refrigeration technique based on the magnetocaloric effect (MCE) has attracted increasing interest because of its high efficiency and environment friendliness. In this article, our recent progress in exploring effective MCE materials is reviewed with emphasis on the MCE in the LaFe13,xSixbased alloys discovered by us. These alloys show large entropy changes over a wide temperature range near room temperature. The effects of magnetic rare-earth doping, interstitial atoms and high pressure on the MCE have been systematically studied. Special issues, such as appropriate approaches to determining the MCE associated with the first-order magnetic transition, the depression of magnetic and thermal hysteresis, and the key factors determining the magnetic exchange in alloys of this kind, are discussed. The applicability of giant MCE materials to magnetic refrigeration near ambient temperature is evaluated. A brief review of other materials with significant MCE is also presented. [source]

    Tuning the Magnetic Properties of LixCrTi0.25Se2 (0.03,x,0.7) by Directed Deintercalation of Lithium

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 16 2008
    Malte Behrens Dr.
    Abstract X-ray diffraction (XRD), in situ energy-dispersive X-ray diffraction (EDXRD), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and magnetic measurements were applied to investigate the effects of lithium deintercalation on pseudolayered Li0.70CrTi0.25Se2. A detailed picture of structural changes during the deintercalation process was obtained by combining the results of EDXRD and EXAFS. Removal of Li from the host,guest complex leads to anisotropic contraction of the unit cell with stronger impact on the c axis, which is the stacking axis of the layers. The EDXRD experiments evidence that the shrinkage of the lattice parameters with decreasing xLi in LixCrTi0.25Se2 is nonlinear in the beginning and then becomes linear. Analysis of the EXAFS spectra clearly shows that the Cr/TiSe distances are affected in a different manner by Li removal. The CrSe bond lengths decrease, whereas the TiSe bonds lengthen when the Li content is reduced, which is consistent with XRD data. Magnetic measurements reveal a change from predominantly antiferromagnetic exchange (,p=,300,K) interactions for the pristine material to ferromagnetic exchange interactions (,=25,K) for the fully intercalated material. Thus, the magnetic properties can be altered under ambient conditions by directed adjustment of the dominant magnetic exchange. The unusual magnetic behavior can be explained on the basis of the variation of the metal,metal distances and the Cr-Se-Cr angles with x, which were determined by Rietveld refinements. Owing to competing ferromagnetic and antiferromagnetic exchange interactions and disorder, the magnetic ground state of the intercalated materials is characterized by spin-glass or spin-glass-like behavior. [source]

    Towards a Better Understanding of Magnetic Interactions within m -Phenylene ,-Nitronyl Nitroxide and Imino Nitroxide Based Radicals, Part III: Magnetic Exchange in a Series of Triradicals and Tetraradicals Based on the Phenyl Acetylene and Biphenyl Coupling Units

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2005
    Laure Catala Dr.
    Abstract The present work completes and extends our previous reports1,,2 on the determination of the magnetic ground state and on the strength of the through bond exchange coupling within series of biradicals. This knowledge was subsequently exploited for the analysis of the magnetic interactions in their crystals. We report here the studies of series of triradicals incorporating ,-nitronyl nitroxides (NN) or ,-imino nitroxides (IN) as terminal radical fragments connected through a m -phenylene coupling unit in one case and a phenyl acetylene unit in other case. Tetraradical derivatives have also been studied. The studies of isolated molecules (EPR in solution and DFT calculations) allow the assessment of the magnetic interactions through the magnetic coupling unit. All triradical derivatives are found to exhibit a quartet ground state, whereas a singlet ground state is determined for the tetraradical. This last result reinforces previous findings that the singlet ground state is favoured in related biradicals involving similar m -phenylene couplers. Moreover, the through bond magnetic exchange coupling for the ortho,meta connectivity could be demonstrated as being ferromagnetic, thus ascertaining our previous hypotheses.1 The magnetic properties of the triradicals and tetraradicals in their solid state have been rationalized by using a previously proposed methodology,2 allowing to identify the most relevant magnetic pathways. Le présent travail complète et étend nos travaux précédents portant sur la détermination des couplages d'échange magnétique intramoléculaires de séries de biradicaux,1,,2 et partant de cette connaissance, de l'analyse des corrélations magnétostructurales dans l'état cristallin. Nous présentons ici l'étude de séries de triradicaux comprenant des radicaux , -nitronyl nitroxydes (NN) ou , -imino nitroxydes (IN) substitués en méta d'un groupement phénylène d'une part, et en para d'un groupement phénylène éthynylène d'autre part. Des dérivés tétraradicalaires ont aussi été synthétisés et étudiés. Les interactions d'échange magnétique à travers les liaisons sont estimées d'après l'étude de ces molécules isolées par RPE en solution ainsi que par des calculs effectués dans l'approximation de la fonctionnelle densité (DFT). Alors que l'ensemble des dérivés triradicalaires présente un état fondamental magnétique quartet de spin, un état singulet est proposé pour les tétraradicaux. Ce dernier résultat confirme nos travaux précédents1 ayant conclu à un état fondamental singulet pour des biradicaux basés sur le même coupleur. De plus, le couplage d'échange magnétique au travers du coupleur phénylène éthynylène pour des biradicaux substitués dans une topologie ortho,méta est ferromagnétique, confirmant ainsi les hypothèses antérieures.1 Suivant une méthodologie précédemment décrite,2 les propriétés magnétiques des triradicaux à l'état cristallin ont pu être analysées dans le détail et permettent de proposer des chemins d'interaction magnétique et des géométries de contacts intermoléculaires précises pour l'établissement d'interactions magnétiques bien identifiées. El presente trabajo completa y extiende nuestras anteriores estudios1,,2 sobre la determinacion del estado fundamental magnético y la fuerza del intercambio magnetico a través del enlace en una serie de biradicales. Esta información es luego usada en el análisis de las interacciones magneticas dentro de sus cristales. En este trabajo presentamos los estudios sobre una serie de triradicales que contienen , -nitroxidos nitronílicos (NN) o , -imino nitróxidos (IN) como fragmentos radicalarios terminales conectados a través de unidades acopladoras del tipo m -phenilénicas, en un caso, y fenil acetilénicas, en el otro. Se han estudiado también tetraradicales. Los estudios con moleculas aisladas (EPR en disolución y DFT) permiten evaluar las interacciones magneticas a través de las unidades acopladoras. Todos los triradicales se ve que presentan un estado fundamental del tipo cuadruplete, mientras que el cuadruplete se cree que tienen un estado fundamental singlete. Este último resultado refuerza las conclusiones de un estudio previo en el que se vió que el singlete es el estado fundamental en biradicales similares conectados por una unidad m -fenilénica. Además, el intercambio magnético a través del enlace para la conectividad orto y meta se demuestra que es ferromagnetica, confirmando hipótesis anteriores.1 Las propiedades magnéticas de los triradicales y tetraradicales en su estado sólido se han racionalizado empleando una matodología propuesta anteriormente,2 identificando los caminos magnéticos relevantes. [source]