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Showing papers on "Relaxation (NMR) published in 2013"


Journal ArticleDOI
TL;DR: Polylanthanide alkoxide cage complexes, and their doped diamagnetic yttrium analogues, are reported, in which competing relaxation pathways are observed and relaxation through the first excited state can be quenched, leading to energy barriers for relaxation of magnetization that exceed 800 K.
Abstract: Dysprosium alkoxides and dysprosium-doped yttrium alkoxides show very large energy barriers, greater than 800 K, to magnetic relaxation. These barriers arise from the presence of a strongly axial pseudo-octahedral crystal field, which switches off relaxation through the first excited state that typically occurs in single-molecule magnets, and favours a competitive pathway through higher-energy states.

606 citations


Journal ArticleDOI
TL;DR: In this paper, a series of two-coordinate complexes of iron(II) were prepared and studied for single-molecule magnet behavior, and the spin reversal barriers were fit by employing a sum of tunneling, direct, Raman and Orbach relaxation processes, resulting in spin reversal barrier of Ueff = 181, 146, 109, 104, and 43 cm−1 for 1−5, respectively.
Abstract: A series of two-coordinate complexes of iron(II) were prepared and studied for single-molecule magnet behavior. Five of the compounds, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*]2 (4), and Fe(OAr′)2 (5) feature a linear geometry at the FeII center, while the sixth compound, Fe[N(H)Ar#]2 (6), is bent with an N–Fe–N angle of 140.9(2)° (Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2). Ac magnetic susceptibility data for all compounds revealed slow magnetic relaxation under an applied dc field, with the magnetic relaxation times following a general trend of 1 > 2 > 3 > 4 > 5 ≫ 6. Arrhenius plots created for the linear complexes were fit by employing a sum of tunneling, direct, Raman, and Orbach relaxation processes, resulting in spin reversal barriers of Ueff = 181, 146, 109, 104, and 43 cm−1 for 1–5, respectively. CASSCF/NEVPT2 calculations on the crystal structures were performed to explore the influence of deviations from rigorous D∞h geometry on the d-orbital splittings and the electronic state energies. Asymmetry in the ligand fields quenches the orbital angular momentum of 1–6, but ultimately spin–orbit coupling is strong enough to compensate and regenerate the orbital moment. The lack of simple Arrhenius behavior in 1–5 can be attributed to a combination of the asymmetric ligand field and the influence of vibronic coupling, with the latter possibility being suggested by thermal ellipsoid models to the diffraction data.

492 citations


Journal ArticleDOI
TL;DR: An intuitive strategy based on a simple electrostatic method, capable of predicting the magnetic anisotropy of dysprosium(III) complexes, even in low symmetry, is presented.
Abstract: Understanding the anisotropic electronic structure of lanthanide complexes is important in areas as diverse as magnetic resonance imaging, luminescent cell labelling and quantum computing. Here we present an intuitive strategy based on a simple electrostatic method, capable of predicting the magnetic anisotropy of dysprosium(III) complexes, even in low symmetry. The strategy relies only on knowing the X-ray structure of the complex and the well-established observation that, in the absence of high symmetry, the ground state of dysprosium(III) is a doublet quantized along the anisotropy axis with an angular momentum quantum number mJ=±(15)/2. The magnetic anisotropy axis of 14 low-symmetry monometallic dysprosium(III) complexes computed via high-level ab initio calculations are very well reproduced by our electrostatic model. Furthermore, we show that the magnetic anisotropy is equally well predicted in a selection of low-symmetry polymetallic complexes.

488 citations


Journal ArticleDOI
TL;DR: TEKPol, a bulky derivative of bTbK with a molecular weight of 905 g·mol(-1), makes it a very efficient polarizing agent for DNP, yielding unprecedented proton enhancements of over 200 in both bulk and materials samples at 9.4 T and 100 K.
Abstract: A series of seven functionalized nitroxide biradicals (the bTbK biradical and six derivatives) are investigated as exogenous polarization sources for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and with ca. 100 K sample temperatures. The impact of electron relaxation times on the DNP enhancement (epsilon) is examined, and we observe that longer inversion recovery and phase memory relaxation times provide larger E. All radicals are tested in both bulk 1,1,2,2-tetrachloroethane solutions and in mesoporous materials, and the difference in E between the two cases is discussed. The impact of the sample temperature and magic angle spinning frequency on epsilon is investigated for several radicals each characterized by a range of electron relaxation times. In particular, TEKPol, a bulky derivative of bTbK with a molecular weight of 905 g.mol(-1), is presented. Its high-saturation factor makes it a very efficient polarizing agent for DNP, yielding unprecedented proton enhancements of over 200 in both bulk and materials samples at 9.4 T and 100 K. TEKPol also yields encouraging enhancements of 33 at 180 K and 12 at 200 K, suggesting that with the continued improvement of radicals large e may be obtained at higher temperatures.

328 citations


Journal ArticleDOI
TL;DR: Results highlight the utility of an equatorial ligand field for facilitating slow magnetic relaxation in the prolate Er(III) ion and the highest blocking temperature yet observed for a mononuclear complex and the second highest for any single-molecule magnet.
Abstract: The structures and magnetic properties of [K(18-crown-6)]+ (1) and [K(18-crown-6)(THF)2]+ (2) salts of the η8-cyclooctatetraenide sandwich complex [Er(COT)2]− (COT2– = cyclooctatetraene dianion) are reported. Despite slight differences in symmetry, both compounds exhibit slow magnetic relaxation under zero applied dc field with relaxation barriers of ∼150 cm–1 and waist-restricted magnetic hysteresis. Dc relaxation and dilution studies suggest that the drop in the magnetic hysteresis near zero field is influenced by a bulk magnetic avalanche effect coupled with tunneling of the magnetization. Through dilution with [K(18-crown-6)(THF)2][Y(COT)2] (3), these phenomena are substantially quenched, resulting in an open hysteresis loop to 10 K. Importantly, this represents the highest blocking temperature yet observed for a mononuclear complex and the second highest for any single-molecule magnet. A comprehensive comparative analysis of the magnetism of [K(18-crown-6)][Ln(COT)2] (Ln = Sm, Tb, Dy, Ho, Yb) reveals...

323 citations


Journal ArticleDOI
TL;DR: An air-stable star-shaped CoIICoIII3 complex with only one paramagnetic Co(II) ion in the D3 coordination environment has been synthesized from a chiral Schiff base ligand as mentioned in this paper.
Abstract: An air-stable star-shaped CoIICoIII3 complex with only one paramagnetic Co(II) ion in the D3 coordination environment has been synthesized from a chiral Schiff base ligand. Magnetic studies revealed that this complex exhibits slow magnetic relaxation in the absence of an applied dc field, which is one of the main characteristics of single-molecule magnets (SMMs). The relaxation barrier is as high as 109 K, which is quite large among transition-metal ion-based SMMs. The complex represents the first example of zero-field SMM behavior in a mononuclear six oxygen-coordinate Co(II) complex.

275 citations


Journal ArticleDOI
TL;DR: A reorientation of the easy axis of magnetization from perpendicular to parallel to the Ln-O bond of the apical water molecule is experimentally observed and theoretically predicted.
Abstract: Spotting trends: Upon going from Tb(III) to Yb(III) centers in the complexes of the DOTA(4-) ligand, a reorientation of the easy axis of magnetization from perpendicular to parallel to the Ln-O bond of the apical water molecule is experimentally observed and theoretically predicted (SMM=single-molecule magnet). Only ions with an odd number of electrons show slow relaxation of the magnetization.

266 citations


Journal ArticleDOI
TL;DR: It is shown that spin dephasing and relaxation can be largely suppressed, allowing for substantial spin squeezing under realistic experimental conditions.
Abstract: We propose and analyze a novel mechanism for long-range spin-spin interactions in diamond nanostructures. The interactions between electronic spins, associated with nitrogen-vacancy centers in diamond, are mediated by their coupling via strain to the vibrational mode of a diamond mechanical nanoresonator. This coupling results in phonon-mediated effective spin-spin interactions that can be used to generate squeezed states of a spin ensemble. We show that spin dephasing and relaxation can be largely suppressed, allowing for substantial spin squeezing under realistic experimental conditions. Our approach has implications for spin-ensemble magnetometry, as well as phonon-mediated quantum information processing with spin qubits.

252 citations


Journal ArticleDOI
TL;DR: In this article, the orientation of the anisotropic magnetic moments for compounds 2-5 and 8 was rationalized using a clear and succinct, chemically intuitive method based on the electrostatic repulsion of the aspherical electron density distributions of the lanthanides.
Abstract: The use of an amino-pyridyl substituted β-diketone, N-(2-pyridyl)-ketoacetamide (paaH), has allowed for the isolation of two new families of isostructural mononuclear lanthanide complexes with general formulae: [Ln(paaH*)2(H2O)4][Cl]3·2H2O (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5) and Y (6)) and [Ln(paaH*)2(NO3)2(MeOH)][NO3] (Ln = Tb (7), Dy (8), Ho (9) and Er (10)). The dysprosium members of each family (3 and 8) show interesting slow magnetic relaxation features. Compound 3 displays Single Molecule Magnet (SMM) behaviour in zero DC field with an energy barrier to thermal relaxation of Ea = 177(4) K (123(2) cm−1) with τ0 = 2.5(8) × 10−7 s, while compound 8 shows slow relaxation of the magnetization under an optimum DC field of 0.2 T with an energy barrier to thermal relaxation of Ea = 64 K (44 cm−1) with τ0 = 6.2 × 10−7 s. Ab initio multiconfigurational calculations of the Complete Active Space type have been employed to elucidate the electronic and magnetic structure of the low-lying energy levels of compounds 2–5 and 8. The orientation of the anisotropic magnetic moments for compounds 2–5 are rationalized using a clear and succinct, chemically intuitive method based on the electrostatic repulsion of the aspherical electron density distributions of the lanthanides.

201 citations


Journal ArticleDOI
TL;DR: Changing the ligand field and geometry with an additional terpy ligand leads to spin-crossover behavior in 3 with a gradual transition from high spin to low spin.
Abstract: The electronic and magnetic properties of the complexes [Co(terpy)Cl2 ] (1), [Co(terpy)(NCS)2 ] (2), and [Co(terpy)2 ](NCS)2 (3) were investigated. The coordination environment around Co(II) in 1 and 2 leads to a high-spin complex at low temperature and single-molecule magnet properties with multiple relaxation pathways. Changing the ligand field and geometry with an additional terpy ligand leads to spin-crossover behavior in 3 with a gradual transition from high spin to low spin.

196 citations


Journal ArticleDOI
TL;DR: For polyester ionomers based on polyethers and sulfonated phthalates with sodium counterions, a detailed comparison between LVE and DRS reveals that the α2 relaxation in DRS corresponds to a characteristic modulus of kT per ionic group, strongly suggesting that the molecular origin of the α 2 relaxation is the dissociation/association of ion pairs from/into the ionic clusters as mentioned in this paper.
Abstract: Linear viscoelastic (LVE) and dielectric relaxation spectroscopic (DRS) responses were examined for polyester ionomers based on polyethers and sulfonated phthalates with sodium counterions. For these ionomers, LVE shows both glassy and polymer relaxations increasingly delayed with increase of ionic content. DRS shows an α-relaxation process associated with the glass transition, followed by a ∼100× slower α2-relaxation process associated with ion rearrangements, before electrode polarization. A detailed comparison between LVE and DRS reveals that the α2 relaxation in DRS corresponds to a characteristic modulus of kT per ionic group, strongly suggesting that the molecular origin of the α2 relaxation is the dissociation/association of ion pairs from/into the ionic clusters. Based on this molecular picture, we can predict satisfactorily the LVE of the samples using a sticky Rouse model by setting the α2 relaxation time as the lifetime of ionic associations. For ionomers based on poly(ethylene oxide), the association energy is 8–13 kJ/mol, causing only a short delay of the terminal relaxation beyond the delay in the glassy relaxation. In contrast, for ionomers based on the weaker solvating poly(tetramethylene oxide), the delay spans nine decades of frequency, analogous to a highly entangled polymer, yielding an association energy of 58 kJ/mol.Linear viscoelastic (LVE) and dielectric relaxation spectroscopic (DRS) responses were examined for polyester ionomers based on polyethers and sulfonated phthalates with sodium counterions. For these ionomers, LVE shows both glassy and polymer relaxations increasingly delayed with increase of ionic content. DRS shows an α-relaxation process associated with the glass transition, followed by a ∼100× slower α2-relaxation process associated with ion rearrangements, before electrode polarization. A detailed comparison between LVE and DRS reveals that the α2 relaxation in DRS corresponds to a characteristic modulus of kT per ionic group, strongly suggesting that the molecular origin of the α2 relaxation is the dissociation/association of ion pairs from/into the ionic clusters. Based on this molecular picture, we can predict satisfactorily the LVE of the samples using a sticky Rouse model by setting the α2 relaxation time as the lifetime of ionic associations. For ionomers based on poly(ethylene oxide), the asso...

Journal ArticleDOI
TL;DR: The effects of water compartmentalization on the MRI signal with the goal of extracting compartment-specific information is investigated and the potential application of these findings to the direct mapping of myelin content and assessment of WM fiber integrity with high field MRI is shown.

Journal ArticleDOI
TL;DR: Kinetic Monte Carlo simulations agree well with the temporal evolution of the magnetic state when including disorder, and the experimental results can be explained by considering the effective interaction energy associated with the separation of pairs of vertex excitations.
Abstract: We study the thermal relaxation of artificial spin ice with photoemission electron microscopy, and are able to directly observe how such a system finds its way from an energetically excited state to the ground state. On plotting vertex-type populations as a function of time, we can characterize the relaxation, which occurs in two stages, namely a string and a domain regime. Kinetic Monte Carlo simulations agree well with the temporal evolution of the magnetic state when including disorder, and the experimental results can be explained by considering the effective interaction energy associated with the separation of pairs of vertex excitations.

Journal ArticleDOI
TL;DR: An analytical solution of the Bloch–McConnell equations that describes the magnetization of coupled spin populations under radiofrequency irradiation is derived and it is shown that existing theoretical treatments for CEST are special cases of this approach.
Abstract: Chemical exchange observed by NMR saturation transfer (CEST) and spin-lock (SL) experiments provide an MRI contrast by indirect detection of exchanging protons. The determination of the relative concentrations and exchange rates is commonly achieved by numerical integration of the Bloch-McConnell equations. We derive an analytical solution of the Bloch-McConnell equations that describes the magnetization of coupled spin populations under radiofrequency irradiation. As CEST and off-resonant SL are equivalent, their steady-state magnetization and dynamics can be predicted by the same single eigenvalue: the longitudinal relaxation rate in the rotating frame R1ρ . For the case of slowly exchanging systems, e.g. amide protons, the saturation of the small proton pool is affected by transverse relaxation (R2b ). It turns out, that R2b is also significant for intermediate exchange, such as amine- or hydroxyl-exchange or paramagnetic CEST agents, if pools are only partially saturated. We propose a solution for R1ρ that includes R2 of the exchanging pool by extending existing approaches, and verify it by numerical simulations. With the appropriate projection factors, we obtain an analytical solution for CEST and SL for nonzero R2 of the exchanging pool, exchange rates in the range 1-10(4) Hz, B1 from 0.1 to 20 μT and arbitrary chemical shift differences between the exchanging pools, whilst considering the dilution by direct water saturation across the entire Z-spectra. This allows the optimization of irradiation parameters and the quantification of pH-dependent exchange rates and metabolite concentrations. In addition, we propose evaluation methods that correct for concomitant direct saturation effects. It is shown that existing theoretical treatments for CEST are special cases of this approach.

Journal ArticleDOI
TL;DR: In this paper, broadband dielectric spectroscopy (BDS) was used to investigate the filler effect on the molecular mobility of polyvinylidene fluoride (PVDF) polymer chains.
Abstract: Polymer nanocomposites based on polyvinylidene fluoride (PVDF) matrix filled with TiO 2 nanoparticles (1%, 2%, 3% and 5% by v/v%) were studied by broadband dielectric spectroscopy (BDS) in order to investigate the filler effect on the molecular mobility of the polymer chains. The formalism of electric modulus was used to analyze the dielectric response, thus three relaxation processes were observed. In fact, the first one, which is around −40 °C at 10 Hz, is attributed to the glass transition at low temperature. As for the second phenomenon, around 30 °C at 10 Hz, it is related to the dipolar relaxations in the crystalline phase. Regarding the third one, around 100 °C at 10 Hz, it can be due to the interfacial polarization (IP). The crystallinity ratio decreases and the electric modulus of the interfacial polarization increases with the increase of the TiO 2 content.

Journal ArticleDOI
TL;DR: Ac susceptibility data reveal the individual molecular nature of the slow magnetic relaxation and indicate that the quantum tunneling pathway observed at low temperatures is likely mediated by intermolecular dipolar interactions.
Abstract: The mononuclear Co(II) complex dmphCoBr (dmph = 2,9-dimethyl-1,10-phenanthroline) was obtained and X-ray structurally characterized as a distorted tetrahedron environment that is responsible for the moderately strong positive anisotropy of high spin Co(II). In combination with variable-field magnetic susceptibility data at low temperature, high-field electron paramagnetic resonance (HF-EPR) spectroscopy reveals the presence of easy-plane anisotropy (D > 0) in complex dmphCoBr. Slow magnetic relaxation effects were observed for dmphCoBr in the presence of a dc magnetic field. At very low temperatures, ac magnetic susceptibility data show the magnetic relaxation time, τ, to be temperature-independent, while above 2.4 K thermally activated Arrhenius behavior is dominated with Ueff = 22.8(8) cm−1 and τ0 = 3.7(5) × 10−10 s. Upon dilution of the complex within a matrix of the isomorphous compound dmphZnBr, ac susceptibility data reveal the individual molecular nature of the slow magnetic relaxation and indicate that the quantum tunneling pathway observed at low temperatures is likely mediated by intermolecular dipolar interactions.

Journal ArticleDOI
TL;DR: In this article, a five-pulse double electron-electron spin resonance (DEER) was used for biomolecular long-distance measurements, where the position of the extra pulse was fixed relative to the three pulses of the detection sequence, which significantly reduced the effect of nuclear spin-diffusion on the electron-spin phase relaxation.
Abstract: We describe significantly improved long-distance measurements in biomolecules by use of the new multipulse double electron–electron spin resonance (DEER) illustrated with the example of a five-pulse DEER sequence. In this sequence, an extra pulse at the pump frequency is used compared with standard four-pulse DEER. The position of the extra pulse is fixed relative to the three pulses of the detection sequence. This significantly reduces the effect of nuclear spin-diffusion on the electron-spin phase relaxation, thereby enabling longer dipolar evolution times that are required to measure longer distances. Using spin-labeled T4 lysozyme at a concentration less than 50 μM, as an example, we show that the evolution time increases by a factor of 1.8 in protonated solution and 1.4 in deuterated solution to 8 and 12 μs, respectively, with the potential to increase them further. This enables a significant increase in the measurable distances, improved distance resolution, or both.

Journal ArticleDOI
TL;DR: There exist two parts in ILs: one with the ordered nanostructure exhibiting the slow relaxation, and the other with disordered structure showing faster relaxation, according to neutron diffraction and neutron spin echo techniques.
Abstract: We have investigated structure and relaxation phenomena for ionic liquids 1-octyl-3-methylimidazolium hexafluorophosphate (C8mimPF6) and bis(trifluoromethylsulfonyl)imide (C8mimTFSI) by means of neutron diffraction and neutron spin echo (NSE) techniques. The diffraction patterns show two distinct peaks appeared at scattering vectors Q of 0.3 and 1.0 A–1. The former originates from the nanoscale structure characteristic to ionic liquids and the latter due to the interionic correlations. Interestingly, the intensity of the low-Q peak drastically grows upon cooling and keeps growing even below the glass transition temperature. The NSE measurements have been performed at these two Q positions, to explore the time evolution of each correlation. The relaxation related to the ionic correlation (ionic diffusion) is of Arrhenius-type and exhibits nonexponential behavior. The activation energy (Ea) of the ionic diffusion, which is linked to viscosity, depends on the type of anion; the larger is the anion size, the ...

Journal ArticleDOI
TL;DR: In this paper, the authors carried out atomistic simulations of water at the nanoscale and investigated the dielectric response of the liquid as a function of the distance between hydrophobic confining surfaces.
Abstract: We carried out atomistic simulations of water at the nanoscale, and we investigated the dielectric response of the liquid as a function of the distance between hydrophobic confining surfaces. We found that dipolar fluctuations are modified by the presence of surfaces up to strikingly large distances, i.e., tens of nanometers. Fluctuations are suppressed by approximately an order of magnitude in the z direction, perpendicular to the interface, and they are enhanced (up to 50%) in the x–y plane, giving rise to strong anisotropies in the components of the dielectric relaxation. Such anisotropies originate from the very presence of interfaces, and not from the details of the interaction between water and the hydrophobic surfaces. Our findings are consistent with recent terahertz and ultrafast infrared pump–probe spectroscopy experiments and bear important implications for the study of solvation under confinement.

Journal ArticleDOI
TL;DR: Injection of spin-polarized electrons into a superconductor leads to both spin and charge imbalance, and if charge relaxation occurs faster than spin relaxation, it is possible to observe excess spin at almost no extra charge as discussed by the authors.
Abstract: Injection of spin-polarized electrons into a superconductor leads to both spin and charge imbalance. If charge relaxation occurs faster than spin relaxation, it is possible to observe excess spin at almost no extra charge.

Journal ArticleDOI
TL;DR: In this paper, a combination of several solid-state nuclear magnetic resonance (NMR) techniques was used to precisely probe short-range as well as long-range Li+ dynamics in Li6PS5Br from an atomic-scale point of view.
Abstract: The development of safe and long-lasting all-solid-state lithium-ion batteries needs electrolytes with exceptionally good transport properties. Here, we report on the combination of several solid-state nuclear magnetic resonance (NMR) techniques which have been used to precisely probe short-range as well as long-range Li+ dynamics in Li6PS5Br from an atomic-scale point of view. NMR data clearly reveal an extraordinary high Li diffusivity. This manifests in so-called diffusion-induced spin–lattice relaxation NMR rate peaks showing up at temperatures as low as 260 K. From a quantitative point of view, at ambient temperature the Li jump rate is of the order of 109 s–1 which corresponds to a Li+ conductivity in order of 10–3 to 10–2 S/cm, thus, indicating “liquid-like” Li+ diffusion behavior in Li6PS5Br.

Journal ArticleDOI
TL;DR: In this article, the authors show a strong dependence of the carrier relaxation time on the film thickness of Bi2Se3, which may result from the hybridization of Dirac cone states at the opposite surfaces for the thinnest films.
Abstract: Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi2Se3 reveal a strong dependence of the carrier relaxation time on the film thickness. For thicker films, the relaxation dynamics are similar to those of bulk Bi2Se3, where the contribution of the bulk insulating phase dominates over that of the surface metallic phase. The carrier relaxation time shortens with decreasing film thickness, reaching values comparable to those of noble metals. This effect may result from the hybridization of Dirac cone states at the opposite surfaces for the thinnest films.

Journal ArticleDOI
TL;DR: Frequency-dependent alternating-current susceptibility signals were clearly observed, indicating slow magnetic relaxation, and complex 1 behaves as a single-ion magnet.
Abstract: A MnIII-salen-type complex with a diamagnetic [CoIII(CN)6]3– moiety, [MnIII(5-TMAM(R)-salmen)(H2O)CoIII(CN)6]·7H2O·MeCN [1; 5-TMAM(R)-salmen = (R)-N,N′-(1-methylethylene)bis(5-trimethylammoniomethylsalicylideneiminate], was prepared. From direct-current magnetic susceptibilities, magnetization, and high-field and multifrequency electronic spin resonance measurements on powdered samples, 1 has a significant uniaxial anisotropy. Frequency-dependent alternating-current susceptibility signals were clearly observed, indicating slow magnetic relaxation. Thus, complex 1 behaves as a single-ion magnet.

Journal ArticleDOI
TL;DR: In this article, the switching of rare-earth-based ferrimagnets triggered by thermal excitation is investigated on the basis of an atomistic spin model beyond the rigid-spin approximation, distinguishing magnetic moments due to electrons in d and f orbitals of the rare earth.
Abstract: (Received 10 December 2012; revised manuscript received 10 June 2013; published 24 July 2013) The switching of rare-earth-based ferrimagnets triggered by thermal excitation is investigated on the basis of an atomistic spin model beyond the rigid-spin approximation, distinguishing magnetic moments due to electrons in d and f orbitals of the rare earth. It is shown that after excitation of the conduction electrons a transient ferromagneticlike state follows from a dissipationless spin dynamics where energy and angular momentum are distributed between the two sublattices. The final relaxation can then lead to a new state with the magnetization switched with respect to the initial state. The time scale of the switching event is to a large extent determined by the exchange interaction between the two sublattices. The quest for ever increasing speed of data procession has its bottleneck in data storage with current hard disk writing events being on the time scale of nanoseconds. Much quicker writing schemes have been demonstrated based on all-optical magnetization reversal mechanisms using circularly polarized laser light, with the helicity of the light determining the direction of magnetization in the written area. 1‐5 Most surprisingly, it has been demonstrated that even linearly polarized light can trigger a thermally driven switching in ferrimagnetic GdFeCo compounds 6,7 via a so-called “ferromagneticlike state,” where the rare-earth (RE) and transition-metal (TM) sublattice magnetizations are aligned parallel on a picosecond time scale. With these experiments the theoretical understanding of magnetization dynamics in terms of the macroscopic LandauLifshitz-Gilbert (LLG) equation of motion has reached its limits. The short time scale of the laser pulse in connection with the high electron temperatures following the excitation lead to nonequilibrium processes where longitudinal magnetization dynamics becomes pronounced. 2,8‐13 Af ull theoretical explanation of the thermally driven switching process in ferrimagnets and, particularly of the transient ferromagneticlike state, is still missing, though first attempts of a description of longitudinal magnetization dynamics in two-sublattice systems have been proposed recently. 10,14

Journal ArticleDOI
TL;DR: The collective behavior of cold atomic and molecular ensembles can be similar to that found in soft condensed-matter systems, and the evolution towards equilibrium in one and two dimensions is studied.
Abstract: We show that the dynamics of a laser driven Rydberg gas in the limit of strong dephasing is described by a master equation with manifest kinetic constraints. The equilibrium state of the system is uncorrelated but the constraints in the dynamics lead to spatially correlated collective relaxation reminiscent of glasses. We study and quantify the evolution towards equilibrium in one and two dimensions, and analyze how the degree of glassiness and the relaxation time are controlled by the interaction strength between Rydberg atoms. We also find that spontaneous decay of Rydberg excitations leads to an interruption of glassy relaxation that takes the system to a highly correlated nonequilibrium stationary state. The results presented here, which are in principle also applicable to other systems such as polar molecules and atoms with large magnetic dipole moments, show that the collective behavior of cold atomic and molecular ensembles can be similar to that found in soft condensed-matter systems.

Journal ArticleDOI
TL;DR: A novel technique to tune cut-off frequency exceeding the natural resonance frequency limit of monodisperse Fe3O4 nanoparticles via superparamagnetic relaxation is adopted, which provides a new approach to enhance the resonance frequency beyond the Snoek's limit.
Abstract: Magnetic nanoparticles have attracted much research interest in the past decades due to their potential applications in microwave devices. Here, we adopted a novel technique to tune cut-off frequency exceeding the natural resonance frequency limit of monodisperse Fe3O4 nanoparticles via superparamagnetic relaxation. We observed that the cut-off frequency can be enhanced from 5.3 GHz for Fe3O4 to 6.9 GHz forFe3O4@SiO2 core-shell structure superparamagnetic nanoparticles, which are much higher than the natural resonance frequency of 1.3 GHz for Fe3O4 bulk material. This finding not only provides us a new approach to enhance the resonance frequency beyond the Snoek's limit, but also extend the application for superparamagnetic nanoparticles to microwave devices.

Journal ArticleDOI
TL;DR: The Stokes-Einstein ratio, Dη∕T, is found to be constant across a broad range of temperatures and densities for the two classes of systems investigated here, and commonly-invoked assumptions should be critically evaluated across a wide spectrum of systems and thermodynamic conditions.
Abstract: We investigate numerically the temperature and density dependence of the Stokes-Einstein ratio, Dη/T, and of two commonly-used variants thereof, Dτ and Dτ/T, where D is a diffusivity, η the shear viscosity, and τ a structural relaxation time. We consider a family of atomic binary mixtures with systematically-softened repulsive interactions, and the Lewis-Wahnstrom model of ortho-terphenyl (OTP). The three quantities grow significantly as the temperature decreases in the supercooled regime, a well-known phenomenon. At higher temperatures, Dτ exhibits negative violations of Stokes-Einstein behavior, i.e., decrease upon cooling, for the atomic systems, though not for OTP. We consider two choices for the relaxation time, one based on the decay of the self-intermediate scattering function, and the other on the integral of the stress autocorrelation function. The instantaneous shear modulus exhibits appreciable temperature dependence for the two classes of systems investigated here. Our results suggest that commonly-invoked assumptions, such as τ ∼ η and τ ∼ η/T, should be critically evaluated across a wide spectrum of systems and thermodynamic conditions. We find the Stokes-Einstein ratio, Dη/T, to be constant across a broad range of temperatures and densities for the two classes of systems investigated here.

Journal ArticleDOI
TL;DR: X-ray diffraction, Raman spectroscopy, and temperature-dependent dielectric permittivity data do not indicate potential noncentrosymmetry in the crystal or concomitant ferroelectricity, and strong correlations between the magnetic and dielectrics properties were not encountered, and microwave-synthesized (RE)CrO(3) may not be classified as magnetoelectric or multiferroic materials.
Abstract: The full rare-earth (RE) chromites series (RE)CrO3 with an orthorhombic distorted (Pnma) perovskite structure and the isostructural compound YCrO3 can be synthesized through a simple microwave-assisted technique, yielding high-quality materials. Magnetization measurements evidence that the Neel temperature for antiferromagnetic Cr3+–Cr3+ ordering strongly depends on the RE3+ ionic radius (IOR), and a rich variety of different magnetic spin interactions exists. Dielectric spectroscopy on sintered pellets indicates electronic inhomogeneity in all samples as manifested by the presence of at least two dielectric relaxation processes associated with grain boundary and grain interior bulk contributions. X-ray diffraction, Raman spectroscopy, and temperature-dependent dielectric permittivity data do not indicate potential noncentrosymmetry in the crystal or concomitant ferroelectricity. Strong correlations between the magnetic and dielectric properties were not encountered, and microwave-synthesized (RE)CrO3 may...

Journal ArticleDOI
TL;DR: Two distinct high-temperature regimes of magnetic relaxation are observed with mechanisms that correspond to two distinct single-excitation Orbach processes within the ground-state spin-orbit coupled manifold of the iron(I) ion.
Abstract: The iron-57 Mossbauer spectra of the linear, two-coordinate complexes, [K(crypt-222)][Fe(C(SiMe3)3)2], 1, and Fe(C(SiMe3)3)2, 2, were measured between 5 and 295 K under zero applied direct current (dc) field. These spectra were analyzed with a relaxation profile that models the relaxation of the hyperfine field associated with the inversion of the iron cation spin. Because of the lifetime of the measurement (10(-8) to 10(-9) s), iron-57 Mossbauer spectroscopy yielded the magnetization dynamics of 1 and 2 on a significantly faster time scale than was previously possible with alternating current (ac) magnetometry. From the modeling of the Mossbauer spectral profiles, Arrhenius plots between 5 and 295 K were obtained for both 1 and 2. The high-temperature regimes revealed Orbach relaxation processes with U(eff) = 246(3) and 178(9) cm(-1) for 1 and 2, respectively, effective relaxation barriers which are in agreement with magnetic measurements and supporting ab initio calculations. In 1, two distinct high-temperature regimes of magnetic relaxation are observed with mechanisms that correspond to two distinct single-excitation Orbach processes within the ground-state spin-orbit coupled manifold of the iron(I) ion. For 2, Mossbauer spectroscopy yields the temperature dependence of the magnetic relaxation in zero applied dc field, a relaxation that could not be observed with zero-field ac magnetometry. The ab initio calculated Mossbauer hyperfine parameters of both 1 and 2 are in excellent agreement with the observed hyperfine parameters.

Journal ArticleDOI
TL;DR: A fortran77 code based on an effective electrostatic model of point charges around a rare earth ion, designed for real systems that need not bear ideal symmetry, and able to determine the easy axis of magnetization is presented.
Abstract: This work presents a fortran77 code based on an effective electrostatic model of point charges around a rare earth ion. The program calculates the full set of crystal field parameters, energy levels spectrum, and wave functions, as well as the magnetic properties such as the magnetization, the temperature dependence of the magnetic susceptibility, and the Schottky contribution to the specific heat. It is designed for real systems that need not bear ideal symmetry and it is able to determine the easy axis of magnetization. Its systematic application to different coordination environments allows magneto-structural studies. The package has already been successfully applied to several mononuclear systems with single-molecule magnetic behavior. The determination of effective point charge parameters in these studies facilitates its application to new systems. In this article, we illustrate its usage with two example studies: (a) an ideal cubic structure coordinating a lanthanoid ion and (b) a system with slow relaxation of the magnetization, LiHo(x)Y((1-x))F(4).