External Dc Field Research Articles

Air-Stable Dy(III)-Macrocycle Enantiomers: From Chiral to Polar Space Group

Air-Stable Dy(III)-Macrocycle Enantiomers: From Chiral to Polar Space Group

A nonsymmetric Dy2 single-molecule magnet with two relaxation processes triggered by an external magnetic field: a theoretical and integrated EPR study of the role of magnetic-site dilution.

The 1 : 2 reaction between Dy(O2CMe)3·4H2O and 1-acetyl-2-napthol (LH) in MeOH has provided access to the complex [Dy2L6(MeOH)]·MeOH (1·MeOH) in a good yield. The structures of the isomorphous complex 1·MeOH and its doped diamagnetic yttrium analogue [Dy0.14Y1.86L6(MeOH)]·MeOH (Dy@Y2) have been determined by single-crystal X-ray crystallography and characterized based on elemental analyses, IR spectra, and powder X-ray patterns. Combined dc and ac magnetic susceptibility and the magnetization data for 1 suggest that this complex shows slow magnetic relaxation. Under a 0 Oe dc field, a single relaxation mechanism is seen while two magnetization relaxation processes are evident under a 1500 G external magnetic field. The fit to the Arrhenius law has been performed using the 1.8-10 K ac data, affording an effective barrier for the magnetization reversal of 13 K and 7 K under the external dc field. Theoretical studies have been performed using ab initio and density functional methodologies to understand the electronic structure and the magnetic relaxation dynamics resulting from the single DyIII ion as well as from the dinuclear exchange-coupled states. Rich powder EPR spectra at the X-band and Q-band were obtained from Dy@Y2, as well as from the 1·MeOH dimer, while simulation studies revealed the ferromagnetic nature of the interaction between the DyIII ions in accordance with theoretical studies.

Generation of unipolar pulses of terahertz radiation with a large electric area

A physical situation is proposed and theoretically analyzed, in which, in our opinion, it is possible to generate unipolar terahertz pulses with a large electric area. In this case, the gas in the tube is excited by a femtosecond IR laser pulse. In this case, the tube with gas is placed in a constant external electric field. The generation of a unipolar pulse is based on "three-step scheme" --- ionization of gas atoms by a femtosecond pulse, subsequent acceleration of a free electron in a dc external field and subsequent annihilation of an electron upon collision with a tube wall or another atom (ion). Keywords: unipolar pulses, ultrafast optics, electric pulse area, terahertz radiation, three-step model.

Генерация униполярных импульсов терагерцового излучения с большой электрической площадью

A physical situation is proposed and theoretically analyzed, in which, in our opinion, it is possible to generate unipolar terahertz pulses with a large electric area. In this case, the gas in the tube is excited by a femtosecond IR laser pulse. In this case, the tube with gas is placed in a constant external electric field. The generation of a unipolar pulse is based on ”three-step scheme" - ionization of gas atoms by a femtosecond pulse, subsequent acceleration of a free electron in a dc external field and subsequent annihilation of an electron upon collision with a tube wall or another atom (ion).

Paramagnetic 7Li NMR Shifts and Magnetic Properties of Divalent Transition Metal Silylamide Ate Complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co).

7Li NMR shifts and magnetic properties have been determined for three so-called ate complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co; e.g., named lithium-tris(bis(trimethylsilylamide))-manganate(II) in accordance with a formally negative charge assigned to the complex fragment [M{N(SiMe3)2}3]-, which comprises the transition metal). They are formed by addition reactions of LiN(SiMe3)2 and [M{N(SiMe3)2}2] and stabilized by Lewis base/Lewis acid interactions. The results are compared to those of the related "ion-separated" complexes [Li(15-crown-5)][M{N(SiMe3)2}3]. The ate complexes with the lithium atoms connected to the 3d metal atoms manganese, iron, or cobalt via μ2 nitrogen bridges reveal strong 7Li NMR paramagnetic shifts of about -75, 125, and 171 ppm, respectively, whereas the shifts for the lithium ions coordinated by the 15-crown-5 ether are close to zero. The observed trends of the 7Li NMR shifts are confirmed by density-functional theory calculations. The magnetic dc and ac properties display distinct differences for the six compounds under investigation. Both manganese compounds, [LiMn{N(SiMe3)2}3] and [Li(15-crown-5)][Mn{N(SiMe3)2}3], display almost pure and ideal spin-only paramagnetic behavior of a 3d5 high-spin complex. In this respect slightly unexpected, both complexes show slow relaxation behavior at low temperatures under applied dc fields, which is especially pronounced for the ate complex [LiMn{N(SiMe3)2}3]. Dc magnetic properties of the iron complexes reveal moderate g-factor anisotropies with small values of the axial magnetic anisotropy parameter D and a larger E (transversal anisotropy). Both complexes display at low temperatures and, under external dc fields of up to 5000 Oe, only weak ac signals with no maxima in the frequency range from 1 to 1500 s-1. In contrast, the two cobalt complexes display strong g-factor anisotropies with large values of D and E. In addition, in both cases, the ac measurements at low temperatures and applied dc fields reveal two, in terms of their frequency range, well separated relaxation processes with maxima lying for the most part outside of the measurement range between 1 and 1500 s-1.

Larger magnetocaloric effect and single molecule magnet behavior in dinuclear Ln(III)-based compounds constructed from Schiff base ligand

A series of Ln(III)-based compounds with the formula [Ln2(HL)2(acac)2(CH3OH)2] (Ln (III) = Gd (1), Tb (2), Dy (3), Ho (4) and Er (5), Hacac = acetylacetone, H3L = 3-hydroxy-N’-[(1E)-(2-hydroxy-3-methoxyphenyl)methylidene]pyridine-2-carbohydrazide) based on a flexible Schiff base ligand H3L have been successfully synthesized, and the structures and magnetic properties of 1–5 have been systematacially studied. The crystal structures study indicates that compounds 1–5 are isostructural and possess a parallelogram Ln2O2 core. Magnetic properties measurements reveal that an antiferromagnetic interaction is exsisted in 1 and ferromagnetic interactions are exsisted in compounds 3 and 4. Remarkably, compound 1 displays well cryogenic magnetic refrigeration property with a larger magnetic entropy (−ΔSm = 28.79 J K−1 kg−1, T = 2.0 K and ΔH = 70 kOe). Moreover, alternating-current (ac) magnetic susceptibility research reveals that compound 3 shows slow magnetic relaxation behavior under zero dc field. Furthermore, under an external dc field of 1500 Oe, compound 3 displays remarkable SMM behavior with the effective barrier (ΔE/kB) of 14.9 K.

Study of Possible Frequency Dependence of Small AC Fields on Magnetic Flux Trapping in Niobium by Polarized Neutron Imaging

Reducing the size of ambient magnetic flux trapping during cooldown in superconducting radio-frequency niobium cavities is essential to reaching the lowest power dissipation as required for continuous wave application. Here, it is suggested that applying an alternating magnetic field superimposed to the external DC field can potentially reduce the size of trapped flux by supporting flux line movement. This hypothesis is tested for the first time systematically on a buffered chemically polished (BCP) niobium sample before and after high temperature annealing, a procedure which is known to reduce flux pinning. External low-frequency (Hz-range) magnetic fields were applied to the samples during their superconducting transition and the effect of varying their amplitude, frequency and offset was investigated. A few results can be highlighted: The influence of the frequency and magnitude of the AC fields on the flux trapping in the untreated Nb sample cannot be neglected. The trapped flux seems to be homogeneously distributed, unlike the flux trapping in, e.g., lead (Pb), which is a type I superconductor. After annealing, the Nb sample shows practically no dependency of flux trapping on external AC fields. The trapped magnetic flux was measured by polarized neutron imaging, and calculations of trapped fields show good agreement with experimental results.

Manipulation of Molecular Qubits by Isotope Effect on Spin Dynamics

Controlling spin behavior via external stimuli is a key route to develop molecular spintronics devices. Photons, temperature, pressure, chemicals, and electric field are the possible stimuli. Herei...

X-ray Structure and Magnetic Properties of Heterobimetallic Chains Based on the Use of an Octacyanidodicobalt(III) Complex as Metalloligand

The assembly of [Co2III(μ-2,5-dpp)(CN)8]2− anions and [MII(CH3OH)2(DMSO)2]2+ cations resulted into the formation of two heterobimetallic 1D coordination polymers of formula [MII(CH3OH)2(DMSO)2(μ-NC)2Co2III(μ-2,5-dpp)(CN)6]n·4nCH3OH [M = CoII (1)/FeII (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine. The [Co2III(μ-2,5-dpp)(CN)8]2− metalloligand coordinates the paramagnetic [MII(CH3OH)2(DMSO)2]2+ complex cations, in a bis-monodentate fashion, to give rise to neutral heterobimetallic chains. Cryomagnetic dc (1.9–300 K) and ac (2.0–13 K) magnetic measurements for 1 and 2 show the presence of Co(II)HS (1) and Fe(II)HS (2) ions (HS – high-spin), respectively, with D values of +53.7(5) (1) and −5.1(3) cm−1 (2) and slow magnetic relaxation for 1, this compound being a new example of SIM with transversal magnetic anisotropy. Low-temperature Q-band EPR study of 1 confirms that D value is positive, which reveals the occurrence of a strong asymmetry in the g-tensors and allows a rough estimation of the E/D ratio, whereas 2 is EPR silent. Theoretical calculations by CASSCF/NEVPT2 on 1 and 2 support the results from magnetometry and EPR. The analysis of the ac magnetic measurements of 1 shows that the relaxation of M takes place in the ground state under external magnetic dc fields through dominant Raman and direct spin-phonon processes.

Permeability and Ferromagnetic Resonance Study for Magnetic Nanowires Substrate With Copper Layer

In this letter, a straightforward accurate method for magnetic nanowires (MNWs) ferromagnetic resonance (FMR) frequency determination and permeability extraction is proposed. FMR frequencies are obtained using a one-port coplanar waveguide (CPW) circuit system. Relative permeability, defined as ${\mu } = {\mu }{'}\mathrm {-j} {\mu }{'}{'}$ , of MNW samples is extracted by “four steps” method, commonly used for thin films (Bekker et al. , 2004; Liu et al. , 2005; Wei et al. , 2015). MNW samples with and without copper (Cu) layer are compared. Samples with a Cu layer show stronger FMR resonances, negligible influences from board resonances, and low distortion ${\mu }{'}$ compared to those without a Cu layer. From ${\mu }{'}{'}$ , the average linewidth of a cobalt sample with and without Cu layer is 4.96 and 5.6 GHz, respectively. From the peak of ${\mu }{'}{'}$ , the FMR frequency of an iron sample is higher at low dc field when compared to cobalt. This can be beneficial for microwave device design when external dc field is limited, and higher operating frequency is required.

Simulation of electrorheological plasmas with superthermal ion drift

By applying an external electric AC field to a dusty plasma, the micro-particles arrange in strings or chains caused by the wake potential between the micro-particles due to the ion drift in the plasma. In analogy to electrorheological fluids, such a system is called electrorheological plasma. In contrast, in the case of an external DC field, no string formation has been observed. Therefore, turning gradually the AC field into a DC field, a phase transition from a string-like electrorheological to an isotropic dusty plasma is expected. A molecular dynamics (MD) simulation assuming a superthermal ion drift is performed for studying this phase transition. A superthermal ion drift in an external electric field could be realized in a future microgravity experiment with complex plasmas proposed for the ISS. In this case, the MD simulation presented here predicts a crossover transition from a string fluid to an isotropic system if the AC field is turned into a DC field. The details of the transition depend on the gas pressure, particle size, and particle density. A microscopic explanation due to particle collisions for the phase transition could be identified from the simulation.

Modulating Magnetic and Photoluminescence Properties in 2-Aminonicotinate-Based Bifunctional Coordination Polymers by Merging 3d Metal Ions.

Herein, the synthesis and study of bifunctional coordination polymers (CPs) with both magnetic and photoluminescence properties, derived from a heterometallic environment, are reported. As a starting point, three isostructural monometallic CPs with the formula [M(μ-2ani)2 ]n (MII =Mn (1Mn ), Co (3Co ) and Ni (4Ni ); 2ani=2-aminonicotinate), crystallise as chiral 2D-layered structures stacked by means of supramolecular interactions. These compounds show high thermal stability in the solid state (above 350 °C), despite which, in aqueous solution, compound 1Mn is shown to partially transform into a novel 1D chain CP with the formula [Mn(2ani)2 (μ-H2 O)2 ]n (2Mn ). A study of the direct current (dc) magnetic properties of 1Mn , 3Co and 4Ni reveals a spin-canted structure derived from antisymmetric antiferromagnetic weak exchanges along the chiral network (as confirmed by DFT calculations) and magnetic anisotropy of the ions, in such a way that long-range ordering is observed with variable magnitude for the spin carriers. Moreover, compounds 3Co and 4Ni show no frequency-dependent alternating current (ac) susceptibility curves under zero dc field; this is characteristic behaviour of a glassy state that may be partially supressed for 3Co by applying an external dc field. To overcome long-range magnetic ordering, CoII ions are diluted in a diamagnetic ZnII -based matrix, which enables single-molecule magnet behaviour. Interestingly, this strategy allows a bifunctional Cox Zn1-x 2ani material, which is imbued with a strong photoluminescent emitting capacity, as characterised by an intense blue light followed by a green afterglow, to be obtained.

The essential role of surface pinning in the dynamics of charge density waves submitted to external dc fields

A Charge Density Wave (CDW) submitted to an electric field displays a strong shear deformation because of pinning at the lateral surfaces of the sample. This CDW transverse pinning was recently observed but has received little attention from a theoretical point of view until now despite important consequences on electrical conductivity properties. Here, we provide a description of this phenomenon by considering a CDW submitted to an external dc electric field and constrained by boundary conditions including both longitudinal pinning due to electrical contacts and transverse surface pinning. A simple formula for the CDW phase is obtained in 3D by using the Green function and image charges method. In addition, an analytical expression of the threshold field dependence on both length and sample cross section is obtained by considering the phase slip process. We show that the experimental data are well reproduced with this model and that bulk pinning can be neglected. This study shows that the dynamical properties of CDW systems could be mainly driven by boundary effects, despite the comparatively huge sample volumes.

Propagation of the first order annular Bessel Gaussian beams in a uniaxial crystal along the optical axis governed by the Pockels effect.

We investigate the first order annular Bessel Gaussian beams propagating in a strontium barium niobate (SBN) crystal governed by the Pockels effect. The left-hand circularly polarized incident waves propagating along the optical axis in the crystal give rise to the right-hand circularly polarized vortex fields with a topological charge of 2. If an external dc field is applied along the optical axis of the SBN crystal, the anisotropy strength can be controlled rapidly, and the rotational invariance around the optical axis is still maintained. In this case, the normalized intensity in the focal region, the power exchange between left-hand and right-hand components, and the exchange between the spin and orbit contributions of the angular momentum flux can be manipulated. We also consider the case where the incident waves are linearly polarized along the x axis. The cylindrical symmetry of two Cartesian components is broken during propagation due to the anisotropy, and the intensity distribution as well as the polarization state of the beams can be regulated electrically.

Changes in Optical Properties of Azopolymers in an Electric Field

We report the preliminary results obtained for polymers incorporating the azobenzene side-group as an optically active molecule. The reversible change of the thin film absorption is observed when illuminating it with monochromatic linearly polarized light under the applied external DC field. The magnotude of a change depends on the angle between the light polarization and the DC electric field direction.

Asymmetric Dinuclear Lanthanide(III) Complexes from the Use of a Ligand Derived from 2-Acetylpyridine and Picolinoylhydrazide: Synthetic, Structural and Magnetic Studies

A family of four Ln(III) complexes has been synthesized with the general formula [Ln2(NO3)4(L)2(S)] (Ln = Gd, Tb, Er, and S = H2O; 1, 2 and 4, respectively/Ln = Dy, S = MeOH, complex 3), where HL is the flexible ditopic ligand N’-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide. The structures of isostructural MeOH/H2O solvates of these complexes were determined by single-crystal X-ray diffraction. The two LnIII ions are doubly bridged by the deprotonated oxygen atoms of two “head-to-head” 2.21011 (Harris notation) L¯ ligands, forming a central, nearly rhombic {LnIII2(μ-OR)2}4+ core. Two bidentate chelating nitrato groups complete a sphenocoronal 10-coordination at one metal ion, while two bidentate chelating nitrato groups and one solvent molecule (H2O or MeOH) complete a spherical capped square antiprismatic 9-coordination at the other. The structures are critically compared with those of other, previously reported metal complexes of HL or L¯. The IR spectra of 1–4 are discussed in terms of the coordination modes of the organic and inorganic ligands involved. The f-f transitions in the solid-state (diffuse reflectance) spectra of the Tb(III), Dy(III), and Er(III) complexes have been fully assigned in the UV/Vis and near-IR regions. Magnetic susceptibility studies in the 1.85–300 K range reveal the presence of weak, intramolecular GdIII∙∙∙GdIII antiferromagnetic exchange interactions in 1 [J/kB = −0.020(6) K based on the spin Hamiltonian Ĥ = −2J(ŜGd1∙ ŜGd2)] and probably weak antiferromagnetic LnIII∙∙∙LnIII exchange interactions in 2–4. Ac susceptibility measurements in zero dc field do not show frequency dependent out-of-phase signals, and this experimental fact is discussed for 3 in terms of the magnetic anisotropy axis for each DyIII center and the oblate electron density of this metal ion. Complexes 3 and 4 are Single-Molecule Magnets (SMMs) and this behavior is optimally observed under external dc fields of 600 and 1000 Oe, respectively. The magnetization relaxation pathways are discussed and a satisfactory fit of the temperature and field dependencies of the relaxation time τ was achieved considering a model that employs Raman, direct, and Orbach relaxation mechanisms.

4f-Metal Clusters Exhibiting Slow Relaxation of Magnetization: A {Dy7} Complex with An Hourglass-like Metal Topology.

The reaction between Dy(NO3)3∙6H2O and the bulky Schiff base ligand, N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH2), in the presence of the organic base NEt3 has led to crystallization and structural, spectroscopic and magnetic characterization of a new heptanuclear [Dy7(OH)6(OMe)2(NO3)1.5(nacb)2(nacbH)6(MeOH)(H2O)2](NO3)1.5 (1) compound in ~40% yield. Complex 1 has a unique hourglass-like metal topology, among all previously reported {Dy7} clusters, comprising two distorted {Dy4(μ3-OH)3(μ3-OMe)}8+ cubanes that share a common metal vertex (Dy2). Peripheral ligation about the metal core is provided by the carboxylate groups of four η1:η1:η1:μ single-deprotonated nacbH− and two η1:η1:η2:η1:μ3 fully-deprotonated nacb2− ligands. Complex 1 is the first structurally characterized 4f-metal complex bearing the chelating/bridging ligand nacbH2 at any protonation level. Magnetic susceptibility studies revealed that 1 exhibits slow relaxation of magnetization at a zero external dc field, albeit with a small energy barrier of ~5 K for the magnetization reversal, most likely due to the very fast quantum-tunneling process. The combined results are a promising start to further explore the reactivity of nacbH2 upon all lanthanide ions and the systematic use of this chelate ligand as a route to new 4f-metal cluster compounds with beautiful structures and interesting magnetic dynamics.

Field-Induced Single-Ion Magnet Phenomenon in Hexabromo- and Hexaiodorhenate(IV) Complexes

Two mononuclear ReIV complexes of general formula (PPh4)2[ReX6] [PPh4+ = tetraphenylphosphonium cation, X = Br (1) and I (2)] have been prepared and structurally and magnetically characterised. Both compounds crystallise in the triclinic system with space group Pī. Their structures are made up of hexahalorhenate(IV), [ReX6]2−, anions, and bulky PPh4+ cations. Each ReIV ion in 1 and 2 is six-coordinate and bonded to six halide ions in a quasi regular octahedral geometry. In their crystal packing, the [ReX6]2− anions are well separated from each other through the organic cations, generating alternated anionic and cationic layers, and no intermolecular Re−X···X−Re interactions are present. Variable-temperature dc magnetic susceptibility measurements performed on microcrystalline samples of 1 and 2 show a very similar magnetic behaviour, which is typical of noninteracting mononuclear ReIV complexes with S = 3/2. Ac magnetic susceptibility measurements reveal the slow relaxation of the magnetisation in the presence of external dc fields for 1 and 2, hence indicating the occurrence of the field-induced single-ion magnet (SIM) phenomenon in these hexabromo- and hexaiodorhenate(IV) complexes.

Dilution effect on the slow relaxation of a luminescent dysprosium Metal-Organic Framework based on 2,5-dihydroxyterephthalic acid

A new dysprosium based Metal-Organic Framework with {[Dy(dhbdc)1.5(DMF)2]·DMF}n formula has been obtained from solvothermal reaction with 2,5-dihydroxyterephthalic acid ligand and dysprosium chloride. This coordination polymer has been characterized and its crystal structure has been solved by X-ray diffraction methods elucidating a three-dimensional network. Magnetic studies of this compound reveal the existence of weak antiferromagnetic interactions among the metal ions with θ value of −0.26 K. Dynamic ac magnetic susceptibility measurements were carried out under an external dc field of 1 kOe, highlighting that at high frequencies two relaxation processes can be observed. However, when studying the diamagnetically diluted analogue 1Y, a single relaxation process was detected highlighting the effect of the weak but not negligible exchange interactions. Finally, photoluminescence measurements were performed at different temperatures with the aim of getting a more representative characterization of the emissive performance of the material for potential applications in lighting and thermometry.

Two Ln(III)2 (Ln = Gd and Dy) compounds showing magnetic refrigeration and slow magnetic relaxation

Two new Ln2 compounds, namely [Ln2(L)2(CH3OH)2] (Ln = Gd (1) and Dy (2)), have been successfully synthesized and further characterized by crystallography. Structural analysis shows that both 1 and 2 are binuclear compounds, and each LnIII ion is eight-coordinated model and the coordination polyhedron of centre LnIII ion can be represented as a distorted dodecahedron with triangular faces. The magnetic studies reveal that weak antiferromagnetic exchange is existed between the two neighbouring Gd(III) ions in 1. Furthermore, for 1, it exhibits magnetic refrigeration property (magnetic entropy changes −ΔSm = 24.75 J kg−1 K−1 for magnetic field intensity ΔH = 7.0 T at 3.0 K); while for 2, it displays slow magnetic relaxation behavior under zero dc field. When under an external dc field (dc = 3000 Oe), remarkable single-molecule-magnet (SMM) behavior was observed in 2 with the anisotropic barrier (ΔE/kB) of 16.65 K and pre-exponential factor (τ0) of 3.12 × 10−6 s.