Weak Dipolar Interactions Research Articles

Pressure and compressibility factor of bidisperse magnetic fluids

In this work, we investigate the pressure and compressibility factors of bidisperse magnetic fluids with relatively weak dipolar interactions and different granulometric compositions. In order to study these properties, we employ the method of diagram expansion, taking into account two possible scenarios: (1) dipolar particles repel each other as hard spheres; (2) the polymer shell on the surface of the particles is modelled through a soft-sphere approximation. The theoretical predictions of the pressure and compressibility factors of bidisperse ferrofluids at different granulometric compositions are supported by data obtained by means of molecular dynamics computer simulations, which we also carried out for these systems. Both theory and simulations reveal that the pressure and compressibility factors decrease with growing dipolar correlations in the system, namely with an increasing fraction of large particles. We also demonstrate that even if dipolar interactions are too weak for any self-assembly to take place, the interparticle correlations lead to a qualitative change in the behaviour of the compressibility factors when compared to that of non-dipolar spheres, making the dependence monotonic.

Equilibrium phases of dipolar lattice bosons in the presence of random diagonal disorder

Ultracold gases offer an unprecedented opportunity to engineer disorder and interactions in a controlled manner. In an effort to understand the interplay between disorder, dipolar interaction and quantum degeneracy, we study two-dimensional hard-core dipolar lattice bosons in the presence of on-site bound disorder. Our results are based on large-scale path-integral quantum Monte Carlo simulations by the Worm algorithm. We study the ground state phase diagram at fixed half-integer filling factor for which the clean system is either a superfluid at lower dipolar interaction strength or a checkerboard solid at larger dipolar interaction strength. We find that, even for weak dipolar interaction, superfluidity is destroyed in favor of a Bose glass at relatively low disorder strength. Interestingly, in the presence of disorder, superfluidity persists for values of dipolar interaction strength for which the clean system is a checkerboard solid. At fixed disorder strength, as the dipolar interaction is increased, superfluidity is destroyed in favor of a Bose glass. As the interaction is further increased, the system eventually develops extended checkerboard patterns in the density distribution. Due to the presence of disorder, though, grain boundaries and defects, responsible for a finite residual compressibility, are present in the density distribution. Finally, we study the robustness of the superfluid phase against thermal fluctuations.

Structural and magnetic properties of multi-core nanoparticles analysed using a generalised numerical inversion method

The structural and magnetic properties of magnetic multi-core particles were determined by numerical inversion of small angle scattering and isothermal magnetisation data. The investigated particles consist of iron oxide nanoparticle cores (9 nm) embedded in poly(styrene) spheres (160 nm). A thorough physical characterisation of the particles included transmission electron microscopy, X-ray diffraction and asymmetrical flow field-flow fractionation. Their structure was ultimately disclosed by an indirect Fourier transform of static light scattering, small angle X-ray scattering and small angle neutron scattering data of the colloidal dispersion. The extracted pair distance distribution functions clearly indicated that the cores were mostly accumulated in the outer surface layers of the poly(styrene) spheres. To investigate the magnetic properties, the isothermal magnetisation curves of the multi-core particles (immobilised and dispersed in water) were analysed. The study stands out by applying the same numerical approach to extract the apparent moment distributions of the particles as for the indirect Fourier transform. It could be shown that the main peak of the apparent moment distributions correlated to the expected intrinsic moment distribution of the cores. Additional peaks were observed which signaled deviations of the isothermal magnetisation behavior from the non-interacting case, indicating weak dipolar interactions.

Crystal structure and absolute configuration of (3aR,3'aR,7aS,7'aS)-2,2,2',2'-tetra-methyl-3a,6,7,7a,3'a,6',7',7'a-octa-hydro-4,4'-bi[1,3-benzodioxol-yl], obtained from a Pd-catalyzed homocoupling reaction.

The absolute configuration, i.e. (3aR,3'aR,7aS,7'aS), of the title compound, C18H26O4, synthesized via a palladium-catalyzed homocoupling reaction, was determined on the basis of the synthetic pathway and was confirmed by X-ray diffraction. The homocoupled mol-ecule is formed by two chemically identical moieties built up from two five- and six-membered fused rings. The supra-molecular assembly is controlled mainly by C-H⋯O inter-actions that lead to the formation of hydrogen-bonded chains of mol-ecules along the [001] direction, while weak dipolar inter-actions and van der Waals forces hold the chains together in the crystal structure.

Magnetic nanoparticles in fluid environment: combining molecular dynamics and Lattice-Boltzmann

Hydrodynamic interactions between magnetic nanoparticles suspended in the Newtonian liquid are accounted for using a combination of the lattice Boltzmann method and molecular dynamics simulations. Nanoparticle is modelled by the system of molecular dynamics material points (which form structure resembles raspberry) coupled to the lattice Boltzmann fluid. The hydrodynamic coupling between the colloids is studied by simulations of the thermo-induced rotational diffusion of two raspberry objects. It was found that for the considered range of model parameters the approaching of the raspberries leads to slight retard of the relaxation process. The presence of the weak magnetic dipolar interaction between the objects leads to modest decrease of the relaxation time and the extent of the acceleration of the diffusion is intensified along with magnetic forces.

Weak dipolar interaction between CoPd multilayer nanodots for bit-patterned media application

The morphology and magnetic performance of CoPd multilayer nanodots have been investigated by a combination of DC sputtering, anodic aluminum oxide(AAO) template method and Ar ion etching. The nanodots exhibit a normal switching field distribution(SFD) of 17%, which is comparative to Bit-patterned media (BPM) made by e-Beam. It is found that the first-order reverse curve (FORC) data of CoPd nanodots have a good match with the mean-field model, which shows dipolar interaction contributes as small as 8.4% to the total SFD. Magnetic force microscope(MFM) imaging at a certain location confirms the FORC results by observing the effect of dipolar interaction directly. Our CoPd multilayer nanodots can be a good candidate for BPM application.

Anomalous supersolidity in a weakly interacting dipolar Bose mixture on a square lattice

We calculate the mean-field phase diagram of a zero-temperature, binary Bose mixture on a square optical lattice, where one species possesses a non-negligible dipole moment. Remarkably, this system exhibits supersolidity for anomalously weak dipolar interaction strengths, which are readily accessible with current experimental capabilities. The supersolid phases are robust, in that they occupy large regions in the parameter space. Further, we identify a first-order quantum phase transition between supersolid and superfluid phases. Our results demonstrate the rich features of the dipolar Bose mixture, and suggest that this system is well-suited for exploring supersolidity in the experimental setting.

Interacting Superparamagnetic Iron(II) Oxide Nanoparticles: Synthesis and Characterization in Ionic Liquids.

Interacting superparamagnetic iron(II) oxide nanoparticles (NPs) with sizes of 5.3 ± 1.6 nm were prepared by simple decomposition of [Fe(COT)2] (COT = 1,3,5,7-cyclooctatetraene) with 5 bar of H2 in 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI·NTf2) ionic liquid (IL). The static and dynamic magnetic characterization revealed a superparamagnetic behavior with weak dipolar interactions of these NPs. In situ structural studies by X-ray absorption spectroscopy demonstrated that they consist of nanostructured FeO. This approach is an appropriate method to prepare and stabilize nanostructured FeO particles, where the presence of an IL proved to be fundamental to suppress the aggregation and usual overoxidation of the FeO NPs.

Monte-Carlo investigation of competition between uniaxial anisotropy, exchange and dipolar interactions in critical behavior of ultrathin magnetic films

The investigation includes the observation of ferromagnetic-paramagnetic phase transition for a weak dipolar interaction. The system with strong dipolar interaction reveals the striped domain structure. Bilayer model with dipolar interlayer interaction demonstrates its effective antiferromagnetic character. Memory effects in ultrathin magnetic films are studied on basis of analysis of non-equilibrium relaxation of the autocorrelation function and magnetization.

Probing a single dipolar interaction between a pair of two-level quantum system by scatterings of single photons in an aside waveguide

Weak dipolar interactions exist widely in various atomic, nuclear and molecular systems, and could be utilized to implement the desired quantum information processings. However, these interactions are relatively weak and hard to be measured precisely. Here, we propose an approach to detect such a weak interaction by probing the transport of a single waveguide-photon scattered by two aside qubits with a single dipolar exchange-interaction. By a full quantum theory of photon transports in optical waveguide, we show that the dipolar interaction between the aside two qubits significantly influence the transmitted spectra of the photon traveling along the one-dimensional waveguide. Thus, probing the relevant changes in the transmitted spectra and the transmission probability distribution specifically for the resonant photons, compared with those scattered by the two individual qubits, the information of the single dipolar interaction between the qubits could be extracted. The feasibility of the proposal is also discussed.

Low-temperature1/fnoise in microwave dielectric constant of amorphous dielectrics in Josephson qubits

The accurate analytical solution for the low temperature $1/f$ noise in a microwave dielectric constant of amorphous films containing tunneling two-level systems (TLSs) is derived within the standard tunneling model including the weak dipolar or elastic TLS-TLS interactions. The results are consistent with the recent experimental investigations of $1/f$ noise in Josephson junction qubits including the power law increase of the noise amplitude with decreasing temperature at low temperatures $T<0.1$K. The long time correlations needed for $1/f$ noise are provided by the logarithmic broadening of TLS absorption resonances with time due to their interaction with neighboring TLSs. The noise behavior at higher temperatures $T>0.1$K and its possible sensitivity to quasi-particle excitations are discussed.

Magnetic Interactions and Energy Barrier Enhancement in Core/Shell Bimagnetic Nanoparticles

In this work, we studied the dynamic and static magnetic properties of ZnO-core/CoFe2O4-shell and CoO-core/CoFe2O4-shell nanoparticles. Both systems are formed by a core of ∼4 nm of diameter encapsulated in a shell of ∼2 nm of thickness. The mean blocking temperature changes from 106(7) to 276(5) K when the core is diamagnetic or antiferromagnetic, respectively. Magnetic remanence studies revealed the presence of weak dipolar interparticle interactions, where Hint is approximately −0.1 kOe for ZnO/CoFe2O4 and −0.9 kOe for CoO/CoFe2O4, playing a minor role in the magnetic behavior of the materials. Relaxation experiments provided evidence that the magnetization reversal process of CoFe2O4 is strongly dependent on the magnetic order of the core. At 10 K, activation volumes of ∼46(6) and ∼69(5) nm3 were found for CoO/CoFe2O4 and ZnO/CoFe2O4 nanoparticles, respectively, corresponding to one-third and one-fifth of the total shell volume. While the magnetic behavior of ZnO/CoFe2O4 nanoparticles is strongly affe...

Classical dipoles on the kagome lattice

Motivated by recent developments in magnetic materials, frustrated nanoarrays, and cold atomic systems, we investigate the behavior of dipolar spins on the frustrated two-dimensional kagome lattice. By combining the Luttinger-Tisza approach, numerical energy minimization, spin-wave analysis, and parallel tempering Monte Carlo, we study long-range ordering and finite-temperature phase transitions for a Hamiltonian containing both dipolar and nearest-neighbor interactions. For antiferromagnetic exchange and both weak and moderate dipolar interactions, the system enters a three-sublattice long-range ordered state with each triangle having vanishing dipole and quadrupole moments; whereas for dominating dipolar interactions we uncover ferrimagnetic three-sublattice order. These are also the ground states for $XY$ spins. We discuss excitations of, as well as phase transitions into, these states. We find behavior consistent with Ising criticality for the ${120}^{\ensuremath{\circ}}$ state, whereas the ferrimagnetic state appears to be associated with drifting exponents. The celebrated flat band of zero-energy excitations of the kagome nearest-neighbor Heisenberg model is lifted to finite energies but acquires only minimal dispersion as dipolar interactions are added.

Improved magnetic performance at low and high temperatures in non-exchange-coupling CoFe2O4/CoFe2 nanocomposites

The dispersive and uniform CoFe2O4 nanoparticles were synthesized by thermal decomposition of metal organic salt in organic solvent with high boiling point, and subsequently reduced in H2 ambient for 1.5, 4 and 8h to obtain the CoFe2O4/CoFe2 composite. The M(H) loops of all composites were measured at 10, 50, 100, 150, 200, 250, 300 and 390K, respectively. The jumps around the zero magnetic field in M(H) loops below 200K signify that no exchange-coupling takes place between soft and hard species. At 10K, the maximum Hc and Mr/Ms ratio are 16kOe and 0.81 for the sample reduced for 1.5h, the much larger value than that previously observed. When the measuring temperature increases, the Hc value monotonously decreases to 1.8, 1.7, 1.2kOe, and correspondingly the Mr/Ms ratio decreases to 0.55, 0.45 and 0.40 at T=300K for samples reduced for 1.5, 4 and 8h, respectively; these higher values at room temperature has not been obtained previously in CoFe2O4/CoFe2 composites as far as we know. We suggest that it is important to keep high anisotropy and weak dipolar interaction for improving Hc and Mr/Ms and that the Mr/Ms will be further improved if the exchange coupling was really realized between soft and hard magnets. Further investigations are in process.

Real-time observation of domain fluctuations in a two-dimensional magnetic model system.

Domain patterns of perpendicularly magnetized ultra-thin ferromagnetic films are often determined by the competition of the short range but strong exchange interaction favouring ferromagnetic alignment of magnetic moments and the long range but weak antiferromagnetic dipolar interaction. Detailed phase diagrams of the resulting stripe domain patterns have been evaluated in recent years; however, the domain fluctuations in these pattern forming systems have not been studied in great detail so far. Here we show that domain fluctuations can be observed in ultra-thin two-dimensional ferromagnetic Fe/Ni/Cu(001) films with perpendicular magnetization in the stripe domain phase. Non-stroboscopic time-resolved threshold photoemission electron microscopy with high temporal resolution allows analysing the dynamic fingerprint of the topological excitations in the nematic domain phase. Furthermore, proliferation of domain ending defects in the vicinity of the spin reorientation transition is witnessed.

Superparamagnetic properties of carbon nanotubes filled with NiFe2O4 nanoparticles

Multi walled carbon nanotubes (MWCNTs) were successfully synthesized using custom-made 80 nm pore-size alumina templates, and were uniformly filled with nickel ferrite (NFO) nanoparticles of 7.4 ± 1.7 nm diameter using a novel magnetically assisted capillary action method. X-ray diffraction confirmed the inverse spinel phase for the synthesized NFO. Transmission electron microscopy confirms spherical NFO nanoparticles with an average diameter of 7.4 nm inside MWCNTs. Magnetometry indicates that both NFO and NFO-filled MWCNTs present a blocking temperature around 52 K, with similar superparamagnetic-like behavior, and weak dipolar interactions, giving rise to a super-spin-glass-like behavior at low temperatures. These properties along with the uniformity of sub-100 nm structures and the possibility of tunable magnetic response in variable diameter carbon nanotubes make them ideal for advanced biomedical and microwave applications.

Probing the Secondary Structure of Membrane Proteins with the Pulsed EPR Technique: Electron Spin Ehco Envelope Modulation (ESEEM)

Despite the importance and large number of membrane proteins, relatively limited structural information is known about them. New biophysical techniques are needed to probe their structural properties. Their hydrophobic nature and low expression yields cause difficulties for traditional structural techniques such as x-ray crystallography and solution NMR. ESEEM spectroscopy indirectly observes NMR transitions through an electron spin coupled to a nearby NMR active nucleus. ESEEM can detect weak dipolar interactions between a NMR active nucleus and a spin label out to a distance of approximately 8 Å. The modulation depth for weakly coupled nuclei is scaled by 1/r6. A novel approach is being developed to probe the secondary structure of membrane proteins and peptides qualitatively utilizing the three-pulse Electron Spin Echo Envelope Modulation (ESEEM) pulse sequence. In order to demonstrate the practicality of this biophysical technique, the M2delta subunit of AChR (α-helical) and KIGAKI(β-sheet) peptides were incorporated into phospholipids bicelle to probe their secondary structure with ESSEM spectroscopy. Utilizing site-directed spin-labeling (SDSL) coupled with deuterated amino acid labeling of the peptides, the corresponding ESEEM spectra reveal characteristic patterns for α-helix and β-sheet structures. This ESSEM secondary structural approach can be used with different deuterated amino acids and provide pertinent qualitative structural information on membrane proteins in a short period of time (10 minutes) with small amounts of sample (30 µg).

Pulse electron paramagnetic resonance studies of the interaction of methanol with the S2 state of the Mn4O5Ca cluster of photosystem II.

The binding of the substrate analogue methanol to the catalytic Mn4CaO5 cluster of the water-oxidizing enzyme photosystem II is known to alter the electronic structure properties of the oxygen-evolving complex without retarding O2-evolution under steady-state illumination conditions. We report the binding mode of (13)C-labeled methanol determined using 9.4 GHz (X-band) hyperfine sublevel-correlation (HYSCORE) and 34 GHz (Q-band) electron spin-echo electron nuclear double resonance (ESE-ENDOR) spectroscopies. These results are compared to analogous experiments on a mixed-valence Mn(III)Mn(IV) complex (2-OH-3,5-Cl2-salpn)2Mn(III)Mn(IV) (salpn = N,N'-bis(3,5-dichlorosalicylidene)-1,3-diamino-2-hydroxypropane) in which methanol ligates to the Mn(III) ion ( Larson et al. (1992) J. Am. Chem. Soc. , 114 , 6263 ). In the mixed-valence Mn(III,IV) complex, the hyperfine coupling to the (13)C of the bound methanol (Aiso = 0.65 MHz, T = 1.25 MHz) is appreciably larger than that observed for (13)C methanol associated with the Mn4CaO5 cluster poised in the S2 state, where only a weak dipolar hyperfine interaction (Aiso = 0.05 MHz, T = 0.27 MHz) is observed. An evaluation of the (13)C hyperfine interaction using the X-ray structure coordinates of the Mn4CaO5 cluster indicates that methanol does not bind as a terminal ligand to any of the manganese ions in the oxygen-evolving complex. We favor methanol binding in place of a water ligand to the Ca(2+) in the Mn4CaO5 cluster or in place of one of the waters that form hydrogen bonds with the oxygen bridges of the cluster.

Magnetic nanocomposites of periodic mesoporous silica: The influence of the silica substrate dimensionality on the inter-particle magnetic interactions

Magnetic nanocomposites consisting of iron oxide (hematite, α-Fe2O3) nanoparticles loaded into the pores of the periodically ordered mesoporous silica with hexagonal (SBA-15) or cubic (SBA-16) symmetry were investigated. The characterization of the samples was carried out by N2 adsorption/desorption, Small-angle X-ray scattering (SAXS), High-energy X-ray diffraction (HE-XRD) and HRTEM measurements. The magnetic properties of the prepared nanocomposites were investigated by the SQUID magnetometry. It was shown, that in spite of its non-magnetic nature the silica matrix significantly influences the magnetism of the samples. The magnetic properties are strongly affected by the strength of inter-particle interactions and dimensionality of the porous matrix. Weak dipolar interactions between superparamagnetic (SPM) hematite nanoparticles were observed in the nanocomposite with hexagonally ordered silica channels (α-Fe2O3@SBA-15), while the strong interactions between hematite nanoparticles, suggesting the superspin glass behavior (SSG), were observed in the nanocomposite with silica matrix of cubic symmetry (α-Fe2O3@SBA-16).

Spin dynamics of two bosons in an optical lattice site: A role of anharmonicity and anisotropy of the trapping potential

We study the spin dynamics of two magnetic chromium atoms trapped in a single site of a deep optical lattice in a resonant magnetic field. Dipole-dipole interactions couple spin degrees of freedom of two particles to their quantized orbital motion. A trap geometry combined with two-body contact $s$-wave interactions influence a spin dynamics through the energy spectrum of the two-atom system. Anharmonicity and anisotropy of the site results in a ``fine'' structure of two-body eigenenergies. The structure can be easily resolved by weak magnetic dipole-dipole interactions. As an example we examine the effect of anharmonicity and anisotropy of the binding potential on demagnetization processes. We show that weak dipolar interactions provide a perfect tool for precision spectroscopy of the energy spectrum of the interacting few particle system.