363 research outputs found
Neutron and X-ray diffraction studies of the SrCr8Ga4O19 kagomè compound synthesised by the citrate route
No full textThe SrCrxGa12-xO19 (SCGO) has the magnetoplumbite structure with part of the Cr3+ (S=3/2) moments lying on the kagomè layers (12k sites) separated by Cr3+ triangular layers (4f and 2a sites). The strong geometric frustrations of the magnetic sublattices gives a high degeneracy of the ground state that is the origin of particular low temperature properties. The magnetic sublattice is diluted by a partial interchange of the Cr and Ga atoms that has been observed in all structural refinements published up to now. This dilution affects the magnetic properties and therefore it is important to investigate the Cr/Ga distribution in this system, as well as the synthesis conditions that control the structural properties.
We synthesized different samples of SCGO by solid state reaction and by citrate route. The last one is a wet method that is known to give homogeneous powders with high purity and controlled stoichiometry. For the last method we followed, by X-ray diffraction patterns, the formation of the crystalline phase starting from the amorphous precursor and calcining at different temperatures. The analysis of the X-ray spectra reveals the formation of the Cr2O3 at 600°C, and due to the high stability of this oxide, it is necessary to reach very high temperatures (like in the solid state reaction) to synthesize the SCGO. The complete crystallization of the compound is reached at 1150°C, but we obtained a very pure sample only at 1350°C.
Comparing the spectra for the samples prepared by the different methods, we observed a better crystallization in the sample from the citrate route, but no other differences are evident from the XRD spectra. We performed also high resolution neutron powder diffraction analysis on the D2B two axis diffractometer at the Institute Laue Langevin to investigate the differences in the Cr/Ga distributions on the 12k and 4f sites in order to relate them with the synthesis route
Magnetic properties of transition metal dimers probed by inelastic neutron scattering
No full textThe physical characterisation and understanding of molecular magnetic materials is one of the most important steps towards the integration of such systems in hybrid spintronic devices. Amongst the many characterisation techniques employed in such a task, Inelastic Neutron Scattering (INS) stands as one of the most powerful and sensitive tools to investigate their spin dynamics. Herein, the magnetic properties and spin dynamics of two dinuclear complexes, namely [(M(hfacac)2)2(bpym)] (where M = Ni2+, Co2+, abbreviated in the following as Ni2, Co2) are reported. These are model systems that could constitute fundamental units of future spintronic devices. By exploiting the highly sensitive IN5 Cold INS spectrometer, we are able to gain a deep insight into the spin dynamics of Ni2 and to fully obtain the microscopic spin Hamiltonian parameters; while for Co2, a multitude of INS transitions are observed demonstrating the complexity of the magnetic properties of octahedral cobalt-based systems
Phonon control of magnetic relaxation in the pyrochlore slab compounds SrCr9xGa12-9xO19 and Ba2Sn2ZnCr7xGa10-7xO22
No full textInternational audienceWe are interested in the phonon response in the frustrated magnets SrCr9xGa12-9xO19 (SCGO) and Ba2Sn2ZnCr7xGa10-7xO22 (BSZCGO). The motivation of the study is the recently discovered, phonon-driven, magnetic relaxation in the SCGO compound [Mutka , Phys. Rev. Lett. 97, 047203 (2006)] pointing out the importance of a low-energy (h omega similar to 7 meV) phonon mode. In neutron-scattering experiments on these compounds, the phonon signal is partly masked by the magnetic signal from the Cr moments and we have therefore examined in detail the nonmagnetic isostructural counterparts SrGa12O19 (SGO) and Ba2Sn2ZnGa10O22 (BSZGO). Our ab initio lattice-dynamics calculations on SGO reveal a peak in the vibrational density of states matching with the neutron observations on SGO and SCGO. A strong contribution in the vibrational density of states comes from the partial contribution of the Ga atoms on the 2b and 12k sites, involving modes at the M point of the hexagonal system. These modes comprise dynamics of the Kagomeacute planes of the pyrochlore slab magnetic sublattice, 12k sites, and therefore can drive magnetic relaxation via spin-phonon coupling. Both BSZCGO and BSZGO show a similar low-energy Raman peak but no corresponding peak in the neutron-determined density of states of BSZGO is seen. However, a strong non-Debye enhancement of low-energy phonon response is observed. We attribute this particular feature to the Zn/Ga disorder on the 2d site, already evoked earlier to affect the magnetic properties of BSZCGO. We propose that this disorder-induced phonon response explains the absence of a characteristic energy scale and the much faster magnetic relaxation observed in BSZCGO
Inelastic neutron scattering study and Hubbard model description of the antiferromagnetic tetrahedral molecule Ni 4Mo 12
No full textNehrkorn J, Höck M, Brueger M, Mutka H, Schnack J, Waldmann O. Inelastic neutron scattering study and Hubbard model description of the antiferromagnetic tetrahedral molecule Ni4Mo12. European Physical Journal B. 2010;73(4):515-526.The tetrameric Ni(II) spin cluster Ni4Mo12 has been studied by INS. The data were analyzed extensively in terms of a very general spin Hamiltonian, which includes antiferromagnetic Heisenberg interactions, biquadratic 2-spin and 3-spin interactions, a single-ion magnetic anisotropy, and Dzyaloshinsky-Moriya interactions. Some of the experimentally observed features in the INS spectra could be reproduced, however, one feature at 1.65 meV resisted all efforts. This supports the conclusion that the spin Hamiltonian approach is not adequate to describe the magnetism in Ni4Mo12. The isotropic terms in the spin Hamiltonian can be obtained in a strong-coupling expansion of the Hubbard model at half-filling. Therefore detailed theoretical studies of the Hubbard model were undertaken, using analytical as well as numerical techniques. We carefully analyzed its abilities and restrictions in applications to molecular spin clusters. As a main result it was found that the Hubbard model is also unable to appropriately explain the magnetism in Ni4Mo12. Extensions of the model are also discussed
Triplet excitations in low-Hc spin-gap systems KCuCl3 and TlCuCl3: An inelastic neutron scattering study
No full textElementary excitations in valence bond magnets have a finite spin gap Delta gmu B H c to well defined triplet states. The degeneracy of the triplet states is lifted in the presence of an external field, according to the Zeeman interaction term. The energy and intensity of the excitation spectra in S 1 2 valence bond KCuCl3 and TlCuCl3 are investigated by inelastic neutron scattering at finite external fields. Experimental observations along representative directions of reciprocal space are addressed up to H H c similar to0.6 and H H c similar to0.9, respectively. A comprehensive analysis of the split Zeeman modes is reported, which shows excellent agreement with first principles. The obtained results extend former characterizations at H 0 and are of importance in the context of the field driven quantum criticality realized in the sister compounds KCuCl3 and TlCuCl
Rotational disorder in lithium borohydride
No full textLiBH4 has been discussed as a promising hydrogen storage material and as a solid-state electrolyte in lithiumion batteries. It contains 18.5 wt% hydrogen and undergoes a structural phase transition at 381K which is associated with a large increase in rotational disorder of the [BH4](-) anion and the increase of [Li](+) conductivity by three orders of magnitude. We investigated the [BH4](-)anion dynamic in bulk LiBH4, in LiBH4-LiI solid solutions and in nano-confined LiBH4 by quasielastic neutron scattering, complemented by DFT calculations. In all cases the H-dynamics is dominated by thermally activated rotational jumps of the [BH4](-) anion in the terahertz range. The addition of LiI as well as nano-confinement favours the disordered high temperature phase and lowers the phase transition below room temperatures. The results are discussed on the basis of first principles calculations and in relation to ionic conductivity of [Li](+)
Morphologic and functional correlates of synaptic pathology in the cathepsin D knockout mouse model of congenital neuronal ceroid lipofuscinosis
No full textMutations in the cathepsin D (CTSD) gene cause an aggressive neurodegenerative disease (congenital neuronal ceroid lipofuscinosis) that leads to early death. Recent evidence suggests that presynaptic abnormalities play a major role in the pathogenesis of CTSD deficiencies. To identify the early events that lead to synaptic alterations, we investigated synaptic ultrastructure and function in presymptomatic CTSD knockout (Ctsd) mice. Electron microscopy revealed that there were significantly greater numbers of readily releasable synaptic vesicles present in Ctsd mice than in wild-type control mice as early as postnatal day 16. The size of this synaptic vesicle pool continued to increase with disease progression in the hippocampus and thalamus of the Ctsd mice. Electrophysiology revealed a markedly decreased frequency of miniature excitatory postsynaptic currents (mEPSCs) with no effect on paired-pulse modulation of the evoked excitatory post synaptic potentials in the hippocampus of Ctsd mice. The reduced mEPSCs frequency was observed before the appearance of epilepsy or any morphologic sign of synaptic degeneration. Taken together, these data indicate that CTSD is required for normal synaptic function and that a failure in synaptic trafficking or recycling may bean early and important pathologic mechanism in Ctsd mice; these presynaptic abnormalities may initiate synaptic degeneration in advance of subsequent neuronal loss
Spin dynamics of molecular nanomagnets unravelled at atomic scale by four-dimensional inelastic neutron scattering
No full textMolecular nanomagnets are among the first examples of finite-size spin systems and have been test beds for addressing several phenomena in quantum dynamics. In fact, for short-enough timescales the spin wavefunctions evolve coherently according to an appropriate spin Hamiltonian, which can be engineered to meet specific requirements. Unfortunately, so far it has been impossible to determine these spin dynamics directly. Here we show that recently developed instrumentation yields the four-dimensional inelastic-neutron scattering function in vast portions of reciprocal space and enables the spin dynamics to be determined directly. We use the Cr 8 antiferromagnetic ring as a benchmark to demonstrate the potential of this approach which allows us, for example, to examine how quantum fluctuations propagate along the ring or to test the degree of validity of the Néel-vector-tunnelling framework. © 2012 Macmillan Publishers Limited. All rights reserved
Modification of the magnetic properties of a heterometallic wheel by inclusion of a Jahn-Teller distorted Cu(II) ion
No full textAn investigation into the physical consequences of including a Jahn-Teller distorted Cu(II) ion within an antiferromagnetically coupled ring, [R(2)NH(2)][Cr(7)CuF(8)((O(2)C(t)Bu)(16))] is reported. Inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR) spectroscopic data are simulated using a microscopic spin Hamiltonian, and show that the two Cr-Cu exchange interactions must be inequivalent. One Cr-Cu exchange is found to be antiferromagnetic and the other ferromagnetic. The geometry of the Jahn-Teller elongation is deduced from these results, and shows that a Jahn-Teller elongation axis must lie in the plane of the Cr(7)Cu wheel; the elongation is not observed by X-ray crystallography, due to positional disorder of the Cu site within the wheel. An electronic structure calculation confirms the structural distortion of the Cu site
Quantum Phase Interference and Ne.acte.el-Vector Tunneling in Antiferromagnetic Molecular Wheels
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