Showing papers on "Relaxation (NMR) published in 2011"

Strong exchange and magnetic blocking in N₂³⁻-radical-bridged lanthanide complexes.

Abstract: Single-molecule magnets approach the ultimate size limit for spin-based devices. These complexes can retain spin information over long periods of time at low temperature, suggesting possible applications in high-density information storage, quantum computing and spintronics. Notably, the success of most such applications hinges upon raising the inherent molecular spin-inversion barrier. Although recent advances have shown the viability of lanthanide-containing complexes in generating large barriers, weak or non-existent magnetic exchange coupling allows fast relaxation pathways that mitigate the full potential of these species. Here, we show that the diffuse spin of an N(2)(3-) radical bridge can lead to exceptionally strong magnetic exchange in dinuclear Ln(III) (Ln = Gd, Dy) complexes. The Gd(III) congener exhibits the strongest magnetic coupling yet observed for that ion, while incorporation of the high-anisotropy Dy(III) ion gives rise to a molecule with a record magnetic blocking temperature of 8.3 K at a sweep rate of 0.08 T s(-1).

A N23– Radical-Bridged Terbium Complex Exhibiting Magnetic Hysteresis at 14 K

Abstract: The synthesis and magnetic properties of three new N23– radical-bridged dilanthanide complexes, {[(Me3Si)2N]2(THF)Ln}2(μ-η2:η2-N2)− (Ln = Tb, Ho, Er), are reported. All three display signatures of single-molecule-magnet behavior, with the terbium congener exhibiting magnetic hysteresis at 14 K and a 100 s blocking temperature of 13.9 K. The results show how synergizing the strong magnetic anisotropy of terbium(III) with the effective exchange-coupling ability of the N23– radical can create the hardest molecular magnet discovered to date. Through comparisons with non-radical-bridged ac magnetic susceptibility measurements, we show that the magnetic exchange coupling hinders zero-field fast relaxation pathways, forcing thermally activated relaxation behavior over a much broader temperature range.

Strong axiality and Ising exchange interaction suppress zero-field tunneling of magnetization of an asymmetric Dy2 single-molecule magnet.

Abstract: The high axiality and Ising exchange interaction efficiently suppress quantum tunneling of magnetization of an asymmetric dinuclear Dy(III) complex, as revealed by combined experimental and theoretical investigations. Two distinct regimes of blockage of magnetization, one originating from the blockage at individual Dy sites and the other due to the exchange interaction between the sites, are separated for the first time. The latter contribution is found to be crucial, allowing an increase of the relaxation time by 3 orders of magnitude.

Single Pyramid Magnets: Dy5 Pyramids with Slow Magnetic Relaxation to 40 K

Abstract: Single-molecule magnets: A square-pyramidal pentametallic dysprosium cluster was synthesized and showed slow magnetic relaxation at temperatures as high as 40 K. The thermal energy barrier to relaxation of magnetization of this single-molecule magnet was found at a temperature of 530 K and is the largest yet observed for any d- or f-block cluster compound.

Nuclear Magnetic Resonance: Petrophysical and Logging Applications

Abstract: Historical developments of NMR logging: Nuclear magnetic resonance NMR logging tool developments Applications in formation evaluation. NMR relaxation: Magnetic systems Relaxation times T1 measurements T2 measurements Diffusion in non-uniform fields Practical measurement problems. NMR properties of fluids: NMR properties of bulk fluids T2 in a magnetic field gradient Oil viscosity and diffusion constant. Petrophysical NMR measurements: NMR in porous media Diffusion in porous media Petrophysical applications MAS NMR NMR core imaging. NMR logging applications: Porosity estimation Irreducible water saturation Permeability prediction Residual oil determination Hydrocarbon typing Oil viscosity estimation Logging guidelines. NMR data acquisition and inversion: Data acquisition Data inversion. Theory of NMR in fluid-saturated porous media: Spin relaxation and diffusion PFGSE and its applications Random walker simulations. Appendices: Diffusion eigenstates and PFGSE amplitude Short time asymptotics.

The use of magnetic dilution to elucidate the slow magnetic relaxation effects of a Dy2 single-molecule magnet.

Abstract: The magnetic dilution method was employed in order to elucidate the origin of the slow relaxation of the magnetization in a Dy(2) single-molecule magnet (SMM). The doping effect was studied using SQUID and micro-SQUID measurements on a Dy(2) SMM diluted in a diamagnetic Y(2) matrix. The quantum tunneling of the magnetization that can occur was suppressed by applying optimum dc fields. The dominant single-ion relaxation was found to be entangled with the neighboring Dy(III) ion relaxation within the molecule, greatly influencing the quantum tunneling of the magnetization in this complex.

Muon Spin Rotation, Relaxation, and Resonance: Applications to Condensed Matter

Abstract: In recent years the Muon Spin Rotation and Relaxation/ Rotation/Relaxation technique (widely known under the acronym of lSR) has become a powerful spectroscopic tool in condensed matter physics and material science. This is exemplified by the notable increase of proposals in the lSR facilities around the world and the remarkable raise of the number of manuscripts based on lSR measurements published in high-impact journals. As a rather new technique (high muon flux beamlines dedicated to lSR activities appeared more than 20 years after the development of highflux neutron reactors) many of former lSR textbooks appeared rather outdated and a book covering the recent applications while providing an exhaustive theoretical introduction was clearly lacking.Muon Spin Rotation, Relaxation and Resonance – Applications to Condensed Matter (Oxford University Press) written by A. Yaouanc and P. Dalmas de Réotier, two internationally recognized experts, fulfils this need and provides a clear, systematic and self-contained overview. The authors, both senior scientists at the CEA – Grenoble, are lSR pioneers who have widely contributed to the raise of the technique. The authors have chosen to divide this monograph in three parts. The first part with two chapters describes the basic principle of the technique including illustrative examples of research results. In addition, a description of a standard lSR experimental setup is given, as well as the principles of lSR data treatment. The second part represents the bulk of the monograph and is divided in six chapters. Starting from the basics, the authors provide a thorough treatment of the polarization functions not only for the muon in its diamagnetic state, but also for the muonium. The precise and systematic approach of the matter, coupled with a didactic and highly readable style, makes this authoritative part not only essential for newcomers in the field but also an invaluable reference for experienced lSR users. It is worth mentioning here that in addition to the usual field distribution approach to treat the polarization function, the authors also discuss the quantum approach in a dedicated chapter of this part. In the third part, selected examples of lSR studies are analyzed in five chapters. The specific physical information obtained by the lSR technique is clearly emphasized and discussed. The provided examples are recent and taken from the main fields where lSR has strongly contributed. A large share is dedicated to magnetism and superconductivity, which represent the bulk of the lSR condensed matter research. A chapter is also devoted to muonium centres in materials, where the muonium basically mimics a hydrogen impurity and another is dedicated to the diffusion properties of muons and muonium centres in solids. In the epilogue the authors attempt to identify the major trends for the future of the technique. To complete this comprehensive monograph, eight useful appendices are provided. To name a few, details are given on data analysis, mathematical concepts and formulas, Fourier transforms, correlation functions and linear response theory. In conclusion, there is no doubt that this book will be an important educative resource for students and researchers entering the field and at the same time an invaluable reference for experienced users.

Capping Ligand Perturbed Slow Magnetic Relaxation in Dysprosium Single-Ion Magnets

Abstract: Single-ion magnets 1 and 2 and their diamagnetic analogues 3 and 4 for magnetic-site dilution were obtained through substitution of the coordinated water molecules of [Ln(TTA)(3)-(H(2)O)(2)] (Ln=Dy (1, 2), Y (3, 4); TTA = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate) by 2,2'-bipyridine (1, 3) and 1,10-phenanthroline (2, 4) capping ligands. Their structures and magnetic properties were investigated with the goal of identifying features relevant to modulating relaxation dynamics of single-ion magnets. The metal ions in all complexes adopt an approximately square-antiprismatic (SAP) O(6)N(2) coordination environment. The SAP polyhedrons for both 1 and 2 show slight longitudinal compression, while the coordination sphere of 1 deviates more from an ideal SAP than that of 2, as indicated by the skew angles of the SAP environment. The similar values of U(eff) for the two magnetically diluted samples imply nearly the same distribution of low-lying states for their Dy(III) centers, which is consistent with the slight axial contraction observed for 1 and 2 and further corroborated by ligand-field analysis. The fast quantum tunneling rate tau(QTM) of 1, which is about ten times faster than that of 2, can presumably be associated with the larger rotation of the SAP surroundings. This distortion may result in a significant transverse anisotropy terms, and thus strongly affect the dynamic behavior of the system.

Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum

Abstract: The femtosecond-resolved evolution of the emission spectrum of the important conjugated polymer poly(3-hexylthiophene) (P3HT) is presented. Detailed fluorescence up-conversion spectroscopy was performed on P3HT solid-state films and on P3HT in chlorobenzene solution. Two excitation wavelengths and several emission wavelengths, covering the entire fluorescence spectrum, were used. The data were complemented by polarization-sensitive measurements. Our global analysis allowed a reconstruction of the time-resolved emission spectra with 200 fs temporal resolution, so that spectral changes due to the early relaxation processes following π–π* interband absorption in the pristine polymer could be comprehensively characterized. Absorption occurs in isolated polymer chains in solution and in the solid state (including interchain interactions) for the film. In both cases, we find evidence of delocalization of the electrons and holes formed in the energy bands directly after photoexcitation with excess energy. This i...

Ultrafast Transport of Laser-Excited Spin-Polarized Carriers in Au/Fe/MgO(001)

Abstract: Hot carrier-induced spin dynamics is analyzed in epitaxial Au=Fe=MgOð001Þ by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.

Importance of out-of-state spin-orbit coupling for slow magnetic relaxation in mononuclear Fe(II) complexes.

Abstract: Two mononuclear high-spin FeII complexes with trigonal planar ([FeII(N(TMS)2)2(PCy3)] (1) and distorted tetrahedral ([FeII(N(TMS)2)2(depe)] (2) geometries are reported (TMS = SiMe3, Cy = cyclohexyl, depe = 1,2-bis(diethylphosphino)ethane). The magnetic properties of 1 and 2 reveal the profound effect of out-of-state spin–orbit coupling (SOC) on slow magnetic relaxation. Complex 1 exhibits slow relaxation of the magnetization under an applied optimal dc field of 600 Oe due to the presence of low-lying electronic excited states that mix with the ground electronic state. This mixing re-introduces orbital angular momentum into the electronic ground state via SOC, and 1 thus behaves as a field-induced single-molecule magnet. In complex 2, the lowest-energy excited states have higher energy due to the ligand field of the distorted tetrahedral geometry. This higher energy gap minimizes out-of-state SOC mixing and zero-field splitting, thus precluding slow relaxation of the magnetization for 2.

Mechanism of the order–disorder phase transition, and glassy behavior in the metal-organic framework [(CH3)2NH2]Zn(HCOO)3

Abstract: Transitions associated with orientational order–disorder phenomena are found in a wide range of materials and may have a significant impact on their properties. In this work, specific heat and 1H NMR measurements have been used to study the phase transition in the metal-organic framework (MOF) compound [(CH3)2NH2]Zn(HCOO)3. This compound, which possesses a perovskite-type architecture, undergoes a remarkable order–disorder phase transition at 156 K. The (DMA+) cationic moieties that are bound by hydrogen bonds to the oxygens of the formate groups (N─H⋯O ∼ 2.9 A) are essentially trapped inside the basic perovskite cage architecture. Above 156 K, it is the orientations of these moieties that are responsible for the disorder, as each can take up three different orientations with equal probability. Below 156 K, the DMA+ is ordered within one of these sites, although the moiety still retains a considerable state of motion. Below 40 K, the rotational motions of the methyl groups start to freeze. As the temperature is increased from 4 K in the NMR measurements, different relaxation pathways can be observed in the temperature range approximately 65–150 K, as a result of a “memory effect.” This dynamic behavior is characteristic of a glass in which multiple states possess similar energies, found here for a MOF. This conclusion is strongly supported by the specific heat data.

Theory of the ground state spin of the NV- center in diamond: II. Spin solutions, time-evolution, relaxation and inhomogeneous dephasing

Abstract: The ground state spin of the negatively charged nitrogen-vacancy center in diamond has many exciting applications in quantum metrology and solid state quantum information processing, including magnetometry, electrometry, quantum memory and quantum optical networks. Each of these applications involve the interaction of the spin with some configuration of electric, magnetic and strain fields, however, to date there does not exist a detailed model of the spin's interactions with such fields, nor an understanding of how the fields influence the time-evolution of the spin and its relaxation and inhomogeneous dephasing. In this work, a general solution is obtained for the spin in any given electric-magnetic-strain field configuration for the first time, and the influence of the fields on the evolution of the spin is examined. Thus, this work provides the essential theoretical tools for the precise control and modeling of this remarkable spin in its current and future applications.

Very Slow Cooling Dynamics of Photoexcited Carriers in Graphene Observed by Optical-Pump Terahertz-Probe Spectroscopy

Abstract: Using optical-pump terahertz-probe spectroscopy, we study the relaxation dynamics of photoexcited carriers in graphene at different substrate temperatures. We find that at lower temperatures the tail of the relaxation transients measured by the differential probe transmission become slower, extending beyond several hundred picoseconds below 50 K. We interpret the observed relaxation transients as resulting from the cooling of the photoexcited carriers via phonon emission. The slow cooling of the photoexcited carriers at low temperatures is attributed to the bulk of the electron and hole energy distributions moving close enough to the Dirac point that both intraband and interband scattering of carriers via optical phonon emission become inefficient for removing heat from the carriers. Our model, which includes intraband carrier scattering and interband carrier recombination and generation, agrees very well with the experimental observations.

Dilution-induced slow magnetic relaxation and anomalous hysteresis in trigonal prismatic dysprosium(III) and uranium(III) complexes.

Abstract: Magnetically dilute samples of complexes Dy(H2BPzMe22)3 (1) and U(H2BPz2)3 (3) were prepared through cocrystallization with diamagnetic Y(H2BPzMe22)3 (2) and Y(H2BPz2)3. Alternating current (ac) susceptibility measurements performed on these samples reveal magnetic relaxation behavior drastically different from their concentrated counterparts. For concentrated 1, slow magnetic relaxation is not observed under zero or applied dc fields of several hundred Oersteds. However, a 1:65 (Dy:Y) molar dilution results in a nonzero out-of-phase component to the magnetic susceptibility under zero applied dc field, characteristic of a single-molecule magnet. The highest dilution of 3 (1:90, U:Y) yields a relaxation barrier Ueff = 16 cm–1, double that of the concentrated sample. These combined results highlight the impact of intermolecular interactions in mononuclear single-molecule magnets possessing a highly anisotropic metal center. Finally, dilution elucidates the previously observed secondary relaxation process fo...

Magnetic multicore nanoparticles for hyperthermia—influence of particle immobilization in tumour tissue on magnetic properties

Abstract: When using magnetic nanoparticles as a heating source for magnetic particle hyperthermia it is of particular interest to know if the particles are free to move in the interstitial fluid or are fixed to the tumour tissue. The immobilization state determines the relaxation behaviour of the administered particles and thus their specific heating power. To investigate this behaviour, magnetic multicore nanoparticles were injected into experimentally grown tumours in mice and magnetic heating treatment was carried out in an alternating magnetic field (H = 25 kA m − 1, f = 400 kHz). The tested particles were well suited for magnetic heating treatment as they heated a tumour of about 100 mg by about 22 K within the first 60 s. Upon sacrifice, histological tumour examination showed that the particles form spots in the tissue with a mainly homogeneous particle distribution in these spots. The magnetic ex vivo characterization of the removed tumour tissue gave clear evidence for the immobilization of the particles in the tumour tissue because the particles in the tumour showed the same magnetic behaviour as immobilized particles. Therefore, the particles are not able to rotate and a temperature increase due to Brown relaxation can be neglected. To accurately estimate the heating potential of magnetic materials, the respective environments influencing the nanoparticle mobility status have to be taken into account.

Hyperthermic effects of dissipative structures of magnetic nanoparticles in large alternating magnetic fields

Abstract: Targeted hyperthermia treatment using magnetic nanoparticles is a promising cancer therapy. However, the mechanisms of heat dissipation in the large alternating magnetic field used during such treatment have not been clarified. In this study, we numerically compared the magnetic loss in rotatable nanoparticles in aqueous media with that of non-rotatable nanoparticles anchored to localised structures. In the former, the relaxation loss in superparamagnetic nanoparticles has a secondary maximum because of slow rotation of the magnetic easy axis of each nanoparticle in the large field in addition to the known primary maximum caused by rapid Neel relaxation. Irradiation of rotatable ferromagnetic nanoparticles with a high-frequency axial field generates structures oriented in a longitudinal or planar direction irrespective of the free energy. Consequently, these dissipative structures significantly affect the conditions for maximum hysteresis loss. These findings shed new light on the design of targeted magnetic hyperthermia treatments.

The dependence of singlet exciton relaxation on excitation density and temperature in polycrystalline tetracene thin films: Kinetic evidence for a dark intermediate state and implications for singlet fission

Abstract: The excited state dynamics of polycrystalline tetracene films are studied using femtosecond transient absorption in combination with picosecond fluorescence, continuing work reported in an earlier paper [J. J. Burdett, A. M. Muller, D. Gosztola, and C. J. Bardeen, J. Chem. Phys. 133, 144506 (2010)]. A study of the intensity dependence of the singlet state decay is conducted to understand the origins of the discrepancy between the broadband transient absorption and fluorescence experiments seen previously. High-sensitivity single channel transient absorption experiments allow us to compare the transient absorption dynamics to the fluorescence dynamics measured at identical laser fluences. At high excitation densities, an exciton-exciton annihilation rate constant of ~1 × 10(-8) cm(3) s(-1) leads to rapid singlet decays, but at excitation densities of 2 × 10(17) cm(-3) or less the kinetics of the transient absorption match those of the fluorescence. At these lower excitation densities, both measurements confirm that the initially excited singlet state relaxes with a decay time of 80 ± 3 ps, not 9.2 ps as claimed in the earlier paper. In order to investigate the origin of the singlet decay, the wavelength-resolved fluorescence dynamics were measured at 298 K, 77 K, and 4 K. A high-energy J-type emitting species undergo a rapid (~100 ps) decay at all temperatures, while at 77 K and 4 K additional species with H-type and J-type emission lineshapes have much longer lifetimes. A global analysis of the wavelength-dependent decays shows that the initial ~100 ps decay occurs to a dark state and not via energy transfer to lower energy bright states. Varying the excitation wavelength from 400 nm to 510 nm had no effect on the fast decay, suggesting that there is no energy threshold for the initial singlet relaxation. The presence of different emitting species at different temperatures means that earlier interpretations of the fluorescence behavior in terms of one singlet state that is short-lived due to singlet fission at high temperatures but long-lived at lower temperatures are probably too simplistic. The presence of a rapid singlet decay at all temperatures indicates that the initially created J-type singlet exciton decays to an intermediate that only produces free triplets (and delayed fluorescence) at high temperatures.

Structure and dynamics of water confined in silica nanopores

Abstract: We report the results of molecular simulation of water in silica nanopores at full hydration and room temperature. The model systems are approximately cylindrical pores in amorphous silica, with diameters ranging from 20 to 40 A. The filled pores are prepared using grand canonical Monte Carlo simulation and molecular dynamics simulation is used to calculate the water structure and dynamics. We found that water forms two distinct molecular layers at the interface and exhibits uniform, but somewhat lower than bulk liquid, density in the core region. The hydrogen bond density profile follows similar trends, with lower than bulk density in the core and enhancements at the interface, due to hydrogen bonds between water and surface non-bridging oxygens and OH groups. Our studies of water dynamics included translational mean squared displacements, orientational time correlations, survival probabilities in interfacial shells, and hydrogen bond population relaxation. We found that the radial-axial anisotropy in translational motion largely follows the predictions of a model of free diffusion in a cylinder. However, both translational and rotational water mobilities are strongly dependent on the proximity to the interface, with pronounced slowdown in layers near the interface. Within these layers, the effects of interface curvature are relatively modest, with only a small increase in mobility in going from the 20 to 40 A diameter pore. Hydrogen bond population relaxation is nearly bulk-like in the core, but considerably slower in the interfacial region.

Structural, dielectric, magnetic, magnetodielectric and impedance spectroscopic studies of multiferroic BiFeO3–BaTiO3 ceramics

Abstract: Polycrystalline (1−x)BiFeO3–xBaTiO3 (x = 0.00, 0.10, 0.20 and 0.30) ceramics have been prepared via mixed oxide route. The effect of BaTiO3 substitution on the dielectric, ferroelectric and magnetic properties of the BiFeO3 multiferroic perovskite was studied. From XRD analysis it revealed that BaTiO3 substitution does not affect the crystal structure of the (1−x)BiFeO3–xBaTiO3 system up to x = 0.30. Improved dielectric properties were observed in the prepared system. An anomaly in the dielectric constant (ɛ) was observed in the vicinity of the antiferromagnetic transition temperature. Experimental results suggest that in the (1−x)BiFeO3–xBaTiO3 system, the increase of BaTiO3 concentration leads to the effective suppression of the spiral spin structure of BiFeO3, resulting in the appearance of net magnetization. The dependence of dielectric constant and loss tangent on the magnetic field is a evidence of magnetoelectric coupling in (1−x)BiFeO3–xBaTiO3 system. The impedance analysis suggests the presence of a temperature dependent electrical relaxation process in the material, which is almost similar for all the concentrations in the present studies. The electrical conductivity has been observed to increase with rise in temperature showing a typical negative temperature coefficient of the resistance (NTCR) behaviors analogous to a semiconductor and suggests a non-Debye type of electrical relaxation.

One-dimensional exciton diffusion in perylene bisimide aggregates.

Abstract: The dynamics and mobility of excitons in J-aggregates of perylene bisimides are investigated by transient absorption spectroscopy with a time resolution of 50 fs. The transient spectra are compatible with an exciton delocalization length of two monomers and indicate that vibrational and configurational relaxation processes are not relevant for the spectroscopic properties of the aggregates. Increasing the pump pulse energy and in that way the initial exciton density results in an accelerated signal decay and pronounced exciton−exciton annihilation dynamics. Modeling the data by assuming a diffusive exciton motion reveals that the excitons cannot migrate freely in all three directions of space but their mobility is restricted to one dimension. The observed anisotropy supports this picture and points against direct Forster-transfer-mediated annihilation between the excitons. A diffusion constant of 1.29 nm2/ps is deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96...

Singlet nuclear magnetic resonance of nearly-equivalent spins

Abstract: Nuclear singlet states may display lifetimes that are an order of magnitude greater than conventional relaxation times. Existing methods for accessing these long-lived states require a resolved chemical shift difference between the nuclei involved. Here, we demonstrate a new method for accessing singlet states that works even when the nuclei are almost magnetically equivalent, such that the chemical shift difference is unresolved. The method involves trains of 180° pulses that are synchronized with the spin–spin coupling between the nuclei. We demonstrate experiments on the terminal glycine resonances of the tripeptide alanylglycylglycine (AGG) in aqueous solution, showing that the nuclear singlet order of this system is long-lived even when no resonant locking field is applied. Variation of the pulse sequence parameters allows the estimation of small chemical shift differences that are normally obscured by larger J-couplings.

Rietveld analysis, dielectric and magnetic properties of Sr and Ti codoped BiFeO3 multiferroic

Abstract: Polycrystalline Bi0.8Sr0.2Fe1 − xTixO3 (x = 0.0, 0.1, 0.2) multiferroics were synthesized via the conventional solid state reaction method. The structure, dielectric relaxation, and magnetic properties of as prepared samples were investigated. The crystal structure examined via XRD and Rietveld analysis confirmed a single phase rhombohedral (space group R3c no. 161) structure. In the Rietveld refinement, good agreement between the observed and calculated pattern was observed. The dielectric response of these multiferroics was analyzed in the frequency range of 10 Hz to 5 MHz at different temperatures. All the samples showed dispersion in dielectric constant (έ) and dielectric loss factor (tan δ) values. The temperature dependence of έ and tan δ showed broad peaks. A reduction in the values of έ and tan δ was observed upon the incorporation of Ti. Magnetic measurements were carried out at room temperature up to a field of 20 kOe. Magnetic hysteresis loops revealed a significant increase in magnetization with Ti substitution. A remnant magnetization (Mr) of 33.428 memu/g and a coercive field (Hc) of 1.724 kOe were observed in the sample with x = 0.2.

Beyond power-law density scaling: Theory, Simulation, and Experiment

Abstract: Supercooled liquids are characterized by relaxation times that increase dramatically by cooling or compression. Many liquids have been shown to obey power-law density scaling, according to which the relaxation time is a function of density to some power over temperature. We show that power-law density scaling breaks down for larger density variations than usually studied. This is demonstrated by simulations of the Kob-Andersen binary Lennard-Jones mixture and two molecular models, as well as by experimental results for two van der Waals liquids. A more general form of density scaling is derived, which is consistent with results for all the systems studied. An analytical expression for the scaling function for liquids of particles interacting via generalized Lennard-Jones potentials is derived and shown to agree very well with simulations. This effectively reduces the problem of understanding the viscous slowing down from being a quest for a function of two variables to a search for a single-variable function.

Magnetization dynamics in the inertial regime: Nutation predicted at short time scales

Abstract: The dynamical equation for magnetization has been reconsidered by enlarging the phase space of the ferromagnetic degrees of freedom to the angular momentum The generalized Landau-Lifshitz-Gilbert equation that includes inertial terms, and the corresponding Fokker-Planck equation, are then derived in the framework of mesoscopic nonequilibrium thermodynamics theory A typical relaxation time $\ensuremath{\tau}$ is introduced describing the relaxation of the magnetization acceleration from the inertial regime toward the precession regime defined by a constant Larmor frequency For time scales larger than $\ensuremath{\tau}$, the usual Gilbert equation is recovered For time scales below $\ensuremath{\tau}$, nutation and related inertial effects are predicted The inertial regime offers new opportunities for the implementation of ultrafast magnetization switching in magnetic devices

Dynamics of braided coronal loops - II. Cascade to multiple small-scale reconnection events

Abstract: Aims. Our aim is to investigate the resistive relaxation of a magnetic loop that contains braided magnetic flux but no net current or helicity. The loop is subject to line-tied boundary conditions. We investigate the dynamical processes that occur during this relaxation, in particular the magnetic reconnection that occurs, and discuss the nature of the final equilibrium. Methods. The three-dimensional evolution of a braided magnetic field is followed in a series of resistive MHD simulations. Results. It is found that, following an instability within the loop, a myriad of thin current layers forms, via a cascade-like process. This cascade becomes more developed and continues for a longer period of time for higher magnetic Reynolds number. During the cascade, magnetic flux is reconnected multiple times, with the level of this "multiple reconnection" positively correlated with the magnetic Reynolds number. Eventually the system evolves into a state with no more small-scale current layers. This final state is found to approximate a non-linear force-free field consisting of two flux tubes of oppositely-signed twist embedded in a uniform background field.

Nuclear magnetic relaxation of liquids in porous media

Abstract: Nuclear magnetic relaxation is useful for probing physical and chemical properties of liquids in porous media. Examples are given on high surface area porous materials including calibrated porous silica glasses, granular packings, plaster pastes, cement-based materials and natural porous materials, such as sandstone and carbonate rocks. Here, we outline our recent NMR relaxation work for these very different porous materials. For instance, low field NMR relaxation of water in calibrated granular packings leads to striking different pore-size dependencies of the relaxation times T1 and T2 when changing the amount of surface paramagnetic impurities. This allows separation of the diffusion and surface limited regimes of relaxation in these macroporous media. The magnetic field dependence of the nuclear spin-lattice relaxation rate 1/T1(!0) is also a rich source of dynamical information for characterizing the molecular dynamics of liquids in porous media. This allows a continuous characterization of the evolving microstructure of various cementitious materials. Our recent applications of two-dimensional (2D) T1-T2 and T2-z-store-T2 correlation experiments have evidenced the water exchange in connected micropores of cement pastes. The direct probing of water adsorption time on a solid surface gives access to an original characterization of the surface nano-wettability of porous plaster pastes. We show that such a parameter depends directly on the physical chemistry of the pore surfaces. Lastly, we outline our recent measurements of wettability in oil/brine/reservoir carbonate rocks.

[U(Tp(Me2))2(bipy)]+: a cationic uranium(III) complex with single-molecule-magnet behavior.

Abstract: The addition of 2,2′-bipyridine to [U(TpMe2)2I] (1) results in the displacement of the iodide and the formation of the cationic uranium(III) complex [U(TpMe2)2(bipy)]I (2). This compound was isolated as a dark-green solid in good yield and characterized by IR and NMR spectroscopies, and its molecular structure was determined by single-crystal X-ray diffraction. Studies of its magnetic properties revealed a frequency dependence of magnetization with a blocking temperature of 4.5 K and, at lower temperatures, a slow relaxation of magnetization with an energy barrier of 18.2 cm–1, characteristic of single-molecule-magnet behavior.