Strong Dipolar Interactions Research Articles

The role of dipolar interactions for the magnetic properties of ferromagnetic nanoring

We investigate numerically the effects of the dipolar interactions on magnetic properties in small ferromagnetic nanorings using a Monte Carlo technique. Our simulated results show that the strength of dipolar interaction in the magnetic nanoring has an important influence on the magnetization reversal processes and further the coercivity and the remanence. As the dipolar interaction increases, the transition of magnetization reversal processes from the onion-rotation state to the vortex state can occur, which results in an increase in coercivity and a decrease in remanence. On the other hand, it is found that the coercivity and the remanence depend more strongly on the strength of dipolar coupling for the relatively small size nanoring than for the large size nanoring in width. This can be attributed to the stable vortex state without core in smaller width nanoring in contrast to the metastable vortex state with core in larger width nanoring, induced by strong dipolar interactions. Additionally, the temperature dependence of coercivity and remanence in magnetic nanoring is also studied at a fixed dipolar interaction.

Rheology and Structure of Entangled Telechelic Linear and Star Polyisoprene Melts

We investigate the linear viscoelastic response of model telechelic linear and star (of varying functionality) polyisoprene melts with different molar masses above the entanglement limit in relation to their structure. We find that these systems self-assemble as a result of the strong dipolar interactions and form clusters that seem to depend primarily on the number of dipolar groups per star. The dynamics is rather complex, but some pertinent features are observed: the segmental dynamics is affected by the telechelic functionalization, especially for short arm lengths; this reflects the change of microstructure (and thus glass-transition temperature) with functionalization. The terminal relaxation is much slower compared to similar nonionic stars, reflecting the relaxation of clusters. Linear semitelechelic polymers (with only one end functionalized) aggregate in a star-like fashion. We further develop a tube model based on the time-marching algorithm for stars and linear chains, where we incorporate the association status of the chains via the dipolar interactions at each time step. The agreement of the predictions with the data, using two adjustable parameters (the average times when two dipolar pair remain associated or free, respectively), is remarkable and suggests design criteria for forming desired supramolecular assemblies.

Measurement of dipolar structure of 17O nuclear quadrupole resonance lines by three-frequency irradiation

A modification of nuclear quadrupole double resonance with coupled multiplet is proposed which can be used for the measurement of the dipolar structure of the 17O nuclear quadrupole resonance lines in case of a strong 1H17O dipolar interaction. The technique is based on magnetic field cycling between a high magnetic field and zero magnetic field and on the simultaneous application of three rf magnetic fields with the frequencies that are close to the three 17O NQR frequencies ν5/2−1/2>ν5/2−3/2⩾ν3/2−1/2 during the time spent in zero static magnetic field. When the sum of the two lower irradiation frequencies ν1+ν2 is not equal to the highest irradiation frequency ν, the three-frequency irradiation increases the proton relaxation rate in zero magnetic field and consequently decreases the proton NMR signal at the end of the magnetic field cycle. The new technique is theoretically analyzed and compared to the single-frequency and two-frequency irradiation techniques. It is shown that the sensitivity of the new technique exceeds the sensitivity of the two-frequency irradiation technique. As a test of the new technique we measured the shape of the highest-frequency 17O NQR line in paraelectric KH2PO4.

Effects of Interparticle Interaction in Ferromagnetic Nanoparticle Systems

The effect of interparticle interaction in ferromagnetic nanoparticle systems is reviewed focusing on metallic nanoparticles in an insulating host. Strong dipolar interparticle interactions may give rise to a superspin glass phase with dynamical properties radically different from the superparamagnetic relaxation displayed by non-interacting or only weakly interacting nanoparticles. Strong exchange coupling in nanoparticle systems with a volume concentration above the threshold for geometrical percolation induces ferromagnetism.

Thermal Evolution of Magnetic Interactions in Ni Nanowires Embedded in Polycarbonate Membranes by Ferromagnetic Resonance

Ni polycrystalline nanowires with diameters of 50, 80, and 100 nm were electrodeposited in cylindrical pores of track-etched polycarbonate membranes. Their magnetic properties were determined as a function of temperature using ferromagnetic resonance and magnetization measurements. At room temperature, the uniaxial anisotropy is equal to the shape anisotropy whereas an additional contribution is evidenced at low temperature. This additional contribution is attributed to magnetoelastic effects induced in the nanowires due to the different thermal expansion constants of Ni and polycarbonate. The analysis of magnetization processes in Ni nanowire arrays evidenced strong dipolar interactions inside the wires due to the domain structure. The coercive field of the nanowires was shown to be nearly a linear function of the temperature and could be accounted for temperature dependence of the uniaxial anisotropy.

Thermodynamics of organic mixtures containing amines. X. Phase equilibria for binary systems formed by imidazoles and hydrocarbons: Experimental data and modelling using DISQUAC

(Solid + liquid) equilibrium (SLE) temperatures have been determined using a dynamic method for the systems (1 H -imidazole, + benzene, + toluene, + hexane, or + cyclohexane; 1 -methylimidazole + benzene, or + toluene, 2-methyl-1 H -imidazole + benzene, + toluene, or + cyclohexane, and benzimidazole + benzene). In addition (liquid + liquid) equilibrium (LLE) temperatures have been obtained using a cloud point method for (1 H -imidazole, + hexane, or + cyclohexane; 1 -methylimidazole + toluene, and 2-methyl-1 H -imidazole + cyclohexane). The measured systems show positive deviations from the Raoult’s law, due to strong dipolar interactions between amine molecules related to the high dipole moment of imidazoles. On the other hand, DISQUAC interaction parameters for the contacts present in these solutions and for the amine/hydroxyl contacts in (1 H -imidazole + 1-alkanol) mixtures have been determined. The model correctly represents the available data for the examined systems. Deviations between experimental and calculated SLE temperatures are similar to those obtained using the Wilson or NRTL equations, or the UNIQUAC association solution model. The quasichemical interaction parameters are the same for mixtures containing 1 H-imidazole, 1-methylimidazole, or 2-methyl-1 H-imidazole and hydrocarbons. This may be interpreted assuming that they are members of a homologous series. Benzimidazole behaves differently.

Fermi surface of a trapped dipolar Fermi gas

Under the framework of the semiclassical theory, we investigate the equilibrium-state properties of a spin polarized dipolar Fermi gas through full numerical calculation. We show that the Fermi surfaces in both real and momentum spaces are stretched along the attractive direction of dipolar interaction. We further verify that the deformed Fermi surfaces can be well approximated by ellipsoids. In addition, the deformation parameters slightly depend on the local real- and momentum-space densities. We also study the interaction strength dependence of the energy and real- and momentum-space densities. By comparing them with variational results, we find that the ellipsoidal ansatz usually generates accurate results for weak dipolar interaction, while under strong dipolar interaction limit, notable discrepancy can be observed. Finally, we map out the stability boundary of the system.

Magnetic resonance and ferromagnetic behaviour in Fe-implanted [formula omitted

The results of magnetic resonance and magnetization measurements of Fe-implanted strontium titanate (SrTiO 3) are presented. Electron paramagnetic resonance (EPR) spectra including resonance lines due to the paramagnetic Fe 3+–V o site with axially disturbed ligand field, and due to S -state Fe 3+ ions ( S = 5 / 2 and L = 0 ) substituted into the titanium site coordinated by six oxygen atoms in the Ti 4+-centered unit cell, were observed. It has been revealed that the implantation of Fe ions into SrTiO 3 produces a remarkable ferromagnetic behavior. The ferromagnetic resonance (FMR) and magnetization measurements revealed an out-of-plane uniaxial magnetic anisotropy in Fe-implanted SrTiO 3. The observed phenomena are discussed on the basis of strong magnetic dipolar interaction between Fe nanoparticles due to the decreasing interparticle distance with increasing implantation fluence.

HIGH-FREQUENCY RESPONSE AND VOLTAGE NOISE IN MAGNETIC NANOCOMPOSITES

We study the noise spectra and high-frequency permeability of inhomogeneous magnetic materials consisting of single-domain magnetic nanoparticles embedded into an insulating matrix. Possible mechanisms of 1/f voltage noise in phase-separated manganites is analyzed. The material is modelled by a system of small ferromagnetic metallic droplets (magnetic polarons or ferrons) in insulating antiferromagnetic or paramagnetic matrix. The electron transport is related to tunnelling of charge carriers between droplets. One of the sources of the 1/f noise in such a system stems from fluctuations of the number of droplets with extra electron. In the case of strong magnetic anisotropy, the 1/f noise can arise also due to the fluctuations of the magnetic moments of ferrons. The high frequency magnetic permeability of nanocomposite film with magnetic particles in insulating non-magnetic matrix is studied in detail. The case of strong magnetic dipole interaction and strong magnetic anisotropy of ferromagnetic granules is considered. The composite is modelled by a cubic regular array of ferromagnetic particles. The high-frequency permeability tensor components are found as a functions of frequency, temperature, ferromagnetic phase content, and magnetic anisotropy. The results demonstrate that magnetic dipole interaction leads to a shift of the resonance frequencies towards higher values, and nanocomposite film could have rather high value of magnetic permeability in the microwave range.

A method for improved quantification of 1H NMR signals under low-resolution conditions for solids

Accurate determination of 1H NMR signal intensities is useful for quantitative analysis of the hydrogen content and also to determine the relative peak intensity ratios in different application scenarios. To this end we have investigated the reliability and sources of intensity errors in 1H solid-state MAS NMR. If sufficient resolution can be achieved by very high spinning speeds and high magnetic fields, quantification is straight forward. However, for poorly resolved spectra we show that small phase errors add a considerable amount of uncertainty. An analytical expression for the phase induced intensity-errors allowed us to suggest a robust and reliable recipe which is based on a combination of the spin-echo experiment, an extrapolation technique and a deconvolution algorithm which includes fitting of the signal phase. It significantly reduces errors caused by phase distortions, homonuclear dipolar dephasing, the receiver dead time delay and baseline rolling. The method was validated experimentally on samples with strong homonuclear dipolar interactions.

Toward charge-neutral ‘soft scorpionates’: Coordination chemistry and Lewis acid promoted isomerization of tris(1-organo-imidazol-2-ylthio)methanes

Two tris(1-organo-imidazol-2-ylthio)methanes, HC(S-tim R) 3 (R = organo = methyl, tert-butyl), have been prepared by a triphasic reaction between chloroform, the appropriate heterocycle, and saturated aqueous solutions of Na 2CO 3, in the presence of a phase transfer agent, (NBu 4)(Br). These ligands have been characterized both spectroscopically and by single crystal X-ray diffraction. The reaction chemistry of these potentially N,N,N-tripodal ligands with AgBF 4 was also explored where simple 1:1 coordination complexes could be isolated from reactions performed in THF solution at room temperature. The derivative {Ag[HC(S-tim Me) 3]}(BF 4) was structurally characterized which showed that the ligand binds in a μ –κ 2 N, κ 1 N-mode to give a coordination polymer with an interesting layered supramolecular structure. Surprisingly, heating CH 3CN solutions of the silver complexes at reflux resulted in decomposition of the complex and concomitant isomerization of the ligands to give metal-free tris(3-organo-1-imidazole-2-thione)methane, HC(N-imt R) 3; the heretofore elusive charge-neutral analogues of the well-studied ‘soft scorpionate’ Tm R− anions. The solution isomerization of HC(S-tim R) 3 to HC(N-imt R) 3 was found to be general, occurring in a variety of solvents with any of a host of different Lewis acids [ para-toluenesulfonic acid, KPF 6, and M(CO) 5Br (M = Mn, Re)] but did not occur by heating in the absence of Lewis acid. The compound HC(N-imt Me) 3 exhibited unusually low solubility in common organic solvents. Single crystal X-ray diffraction of HC(N-imt Me) 3 revealed a remarkable honeycomb supramolecular structure with ca. 5 Å channels filled with solvent. The robust nature of this solid is a result of strong dipolar stacking interactions of molecules into polymer chains bolstered by concerted π–π and CH–π interactions involving the heterocycles, holding the chains together in the remaining two dimensions.

Solvothermal Synthesis and Characterization of Fe3O4 and γ-Fe2O3 Nanoplates

γ-Fe2O3 nanoplates were prepared by a simple solution process and could be transformed to Fe3O4 nanoplates while keeping the size and morphology unchanged after reduction in hydrazine solution. The crystal structure and surface organic groups of the nanoplates were characterized in detail. The nanoplates exhibited good dispersibility in polar solvents except water and strong dipolar interactions, which favored the formation of one-dimensional chainlike structure. The poly(vinylpyrrolidone) molecules in this system played a key role for the growth of nanoplates, and it was supposed that the formation of nanoplates was a kinetically controlled process. The γ-Fe2O3 and Fe3O4 nanoplates with strong dipolar interactions are ferromagnetic at room temperature.

Understanding the Chemistry of Cytosine Methylation Effects

Proteins bind to DNA to initiate several important biological processes including DNA replication and transcription of genes, these processes are also highly regulated. A common method of bioregulation is cytosine methylation. Recent research suggests that cytosine methylation quenches DNA dynamics, preventing the DNA deformation often necessary for efficient protein-DNA complex formation. Using 31P NMR, we have shown that methylation induced rigidity may stem from conformational changes in the backbone from BI to BII conformation. To explore the mechanism of these conformational changes, we have turned to the study of cytosine fluorination. Through comparison of FTIR and NMR spectra, respectively, one can qualitatively and quantitatively observe the effects of methylation and fluorination on DNA structure and dynamics. Preliminary results demonstrate that these different covalent modifications of the cytosine base affect the structure and dynamics in different ways. Both modifications alter the BI/BII ratio but fluorination does not appear to affect the dynamics as significantly as methylation. We attribute the enhanced quenching of backbone dynamics by methylation to the formation of a strong dipolar interaction between the negatively charged backbone oxygen with the hydrogens of the methyl group whose proximity to the backbone is closer in BII than BI. Comparisons of fluorination and methylation effects on sequences containing both the EcoRI and Cre binding sites, as well as their modulation due to sequence context, will be presented.

Mechanical Properties of Poly(L-Lactide) Films Controlled by Blending with Polyesters of Lignin-Derived Stable Metabolic Intermediate, 2-Pyrone-4,6-Dicarboxylic Acid (PDC)

Two kinds of polyesters containing 2-pyrone-4,6-dicarboxylic acid (PDC) moieties in poly(L-lactic acid) (PLLA) and poly(butylene succinate) (PBS) main chains were for the first time employed as an additive to PLLA for improvement of the mechanical properties. Solution casting of a mixture of PLLA and PDC polyesters provided the blend films, which were comprehensively characterized by thermal analyses, tensile measurements, and crystallinity evaluation. Differential scanning calorimetry measurements revealed that PDC incorporation into polymer main chain dramatically improves the miscibility with PLLA. PDC-incorporated PBS showed better miscibility than PDC-incorporated PLLA. Optical microscopy measurements also suggested the superior orientation of PDC-incorporated PBS over PDC-incorporated PLLA in the blend films. The fracture strain of the blend films could be linearly increased by a draw ratio in the region of low PDC polyester content. On the contrary, the fracture strain decreased at high PDC polyester content due to the restricted strain caused by the strong dipolar interactions between PDC-moieties. Elongation at break was significantly varied as a function of PDC polyester content, whereas tensile modulus was almost independent of the additive content and draw ratios. It is concluded that the blend films containing <10 wt% of PDC incorporated PBS as an additive show the best tensile property.

Ultracold polar molecules near quantum degeneracy

We report the creation and characterization of a near quantum-degenerate gas of polar 40K-87Rb molecules in their absolute rovibrational ground state. Starting from weakly bound heteronuclear KRb Feshbach molecules, we implement precise control of the molecular electronic, vibrational, and rotational degrees of freedom with phase-coherent laser fields. In particular, we coherently transfer these weakly bound molecules across a 125 THz frequency gap in a single step into the absolute rovibrational ground state of the electronic ground potential. Phase coherence between lasers involved in the transfer process is ensured by referencing the lasers to two single components of a phase-stabilized optical frequency comb. Using these methods, we prepare a dense gas of 4 x 10(4) polar molecules at a temperature below 400 nK. This fermionic molecular ensemble is close to quantum degeneracy and can be characterized by a degeneracy parameter of T/T(F) = 3. We have measured the molecular polarizability in an optical dipole trap where the trap lifetime gives clues to interesting decay mechanisms. Given the large measured dipole moment of the KRb molecules of 0.5 Debye, the study of quantum degenerate molecular gases interacting via strong dipolar interactions is now within experimental reach. PACS numbers: 37.10.Mn, 37.10.Pq.

Phosphine Polymerization by Nitric Oxide: Experimental Characterization and Theoretical Predictions of Mechanism

A yellow solid material [P(x)H(y)] has been obtained in the reaction of phosphine (PH(3)) and nitric oxide (NO) at room temperature and characterized by thermogravimetric analysis mass spectrometry (TGA-MS) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. In this work using complete basis set (CBS-QB3) methods a plausible mechanism has been investigated for phosphine polymerization in the presence of nitric oxide (NO). Theoretical explorations with the ab initio method suggest (a) instead of the monomer the nitric oxide dimer acts as an initial oxidant, (b) the resulting phosphine oxides (H(3)P=O <--> H(3)P(+)O(-)) in the gas phase draw each other via strong dipolar interactions between the P-O groups, and (c) consequently an autocatalyzed polymerization occurs among the phosphine oxides, forming P-P chemical bonds and losing water. The possible structures of polyhydride phosphorus polymer were discussed. In the calculations a series of cluster models was computed to simulate polymerization.

Paramagnetic resonance and non-resonant microwave absorption in iron niobate

An electron paramagnetic resonance (EPR) study of FeNbO 4 powder samples in monoclinic phase (wolframite-type) at X-band (8.8–9.8 GHz), in the 90–300 K temperature range, is presented. For all the temperatures, the EPR spectrum shows a single line associated with Fe 3+ ions. Changes in the lineshape of the EPR spectrum, which can be attributed to Fe 2+ ions, are detected at low temperatures. This behavior can be ascribed to a strong magnetic dipolar interaction between Fe 2+ and Fe 3+ ions. The non-resonant microwave absorption techniques: magnetically-modulated microwave absorption spectroscopy (MAMMAS) and low-field microwave absorption spectroscopy (LFMAS), were used for a further knowledge on this material. MAMMAS response suggests also the presence of Fe 2+ ions, that originates a change in microwave absorption regime for T < T p (=140 K), associated with the presence of short-range magnetic correlations. LFMAS spectra showed a linear behavior with positive slope and non-hysteretic traces. The profiles obtained by plotting the slope vs. temperature of the LFMAS line are similar to those detected by the MAMMAS technique, confirming that both types of measurement show the same processes of absorption.

Interparticle Interactions Effects on the Magnetic Order in Surface of Fe3O4 Nanoparticles

We report interparticle interactions effects on the magnetic structure of the surface region in Fe3O4 nanoparticles. For that, we have studied a desirable system composed by Fe3O4 nanoparticles with (d) = 9.3 nm and a narrow size distribution. These particles present an interesting morphology constituted by a crystalline core and a broad (approximately 50% vol.) disordered superficial shell. Two samples were prepared with distinct concentrations of the particles: weakly-interacting particles dispersed in a polymer and strongly-dipolar-interacting particles in a powder sample. M(H, T) measurements clearly show that strong dipolar interparticle interaction modifies the magnetic structure of the structurally disordered superficial shell. Consequently, we have observed drastically distinct thermal behaviours of magnetization and susceptibility comparing weakly- and strongly-interacting samples for the temperature range 2 K < T < 300 K. We have also observed a temperature-field dependence of the hysteresis loops of the dispersed sample that is not observed in the hysteresis loops of the powder one.

NMR evidence of LiF coating rather than fluorine substitution in Li(Ni 0.425Mn 0.425Co 0.15)O 2

A series of “Li 1+ z /2(Ni 0.425Mn 0.425Co 0.15) 1− z /2O 2− z F z ” materials was prepared by a coprecipitation route and their structure was characterized using X-ray diffraction (XRD), as well as 7Li and 19F Magic Angle Spinning (MAS) NMR spectroscopy. Two hypotheses were considered: (i) formation of layered oxyfluoride materials and (ii) formation of a mixture between the layered material and LiF. Structural parameters were refined by the Rietveld method, using XRD diffraction data. The refinement results did not allow us to choose between these two hypotheses: no significant change in crystallinity and structural parameters was observed irrespective of the fluorine ratio. 7Li and 19F MAS NMR analyses showed signals with isotropic positions characteristic of LiF, but envelopes characteristic of very strong dipolar interactions with the electron spins of the material, demonstrating that LiF was not incorporated into the layered oxide structure but was instead present as a coating.

Magnetic Langmuir−Blodgett Films of Bimetallic Coordination Nanoparticles of Cs0.4Ni[Cr(CN)6]0.9

Magnetic Langmuir−Blodgett (LB) films of bimetallic coordination nanoparticles of the Prussian blue analogue Cs0.4Ni[Cr(CN)6]0.9 were prepared by using the adsorption properties of a monolayer of dioctadecyldimethylammonium bromide (DODA). The molecular organization of the DODA monolayer is strongly affected by the presence of the water-dispersable Cs0.4Ni[Cr(CN)6]0.9 nanoparticles in the subphase as has been shown by π−A isotherms and Brewster angle microscopy (BAM) images at the air−water interface. BAM images suggest that a layer of inorganic nanoparticles is adsorbed under the monolayer of DODA, for high surface pressure and medium concentration of nanoparticles in the subphase and for all surface pressures at high concentrations of nanoparticles. Transfer of the monolayer onto different substrates allowed the preparation of multilayers containing Cs0.4Ni[Cr(CN)6]0.9 nanoparticles. Magnetic properties of these LB films show a spin-glass-like behavior that can be explained by the strong magnetic dipolar interactions between the nanoparticles due to the small interparticle distances within the LB film. The magnetic properties of these LB films are very sensitive to the organization of the nanoparticles. They can be tuned by changing parameters such as the concentration of the subphase, the surface pressure, or the use of mixed monolayers. Thus, the Curie temperature of these LB films ranges from 60 K for the LB films prepared on the more concentrated subphase to 30 K for those prepared on the more diluted subphases. This is explained by the increase of interparticle distances and the subsequent weakening of the dipolar interactions between the nanoparticles within the LB film prepared on a subphase with a lower concentration of nanoparticles.