Broad Range Of Magnetic Fields Research Articles

Characterization of Fe(II) complexes exhibiting the ligand-driven light-induced spin-change effect using SQUID and magnetic circular dichroism

Abstract We present a study of the magnetic behavior of two isomeric derivatives of the photomagnetic Fe(II) complex, Fe(stpy)4(NCSe)2 (stpy = 4-styrylpyridine, either all-trans or all-cis), whose magnetic state depends on the conformation of the photoisomer stpy ligands, and hence can exhibit the so-called ligand-driven light-induced spin-change (LD-LISC) effect. We compare the results of SQUID magnetization measurements on microcrystalline powder samples with those from UV–vis magnetic circular dichroism (MCD) measurements of the complexes in dilutely doped polymer thin films over a broad range of magnetic field (0–8 T) and temperature (5–220 K). For the all-cis complex, which is high-spin (S = 2) over the whole temperature range investigated, we have measured the field and temperature dependence of the MCD signal including the saturation regime where information concerning the magnetic environment of the Fe(II) core can be gained. In contrast, the all-trans complex possesses a high-spin ↔ low-spin crossover, which is observed in the microcrystalline powder SQUID measurements with a transition temperature of T1/2 ∼ 163 K. We show that the temperature-dependent MCD signal also allows observation of this spin crossover for complexes in dilutely doped polymer thin films. The behavior of the MCD signal indicates a significantly lower temperature range for the SCO transition in the polymer host in comparison to the microcrystalline environment due to changes in intermolecular interactions, which has a significant impact on the operating temperature range for the LD-LISC effect. The results demonstrate that the MCD technique is a valuable tool for probing the magnetic state of LD-LISC complexes in dilute randomly-oriented molecular samples, with the use of optical techniques in principle allowing the extension to time-resolved studies of photomagnetic switching down to the excited-state time scale.

Magnetorheology for suspensions of solid particles dispersed in ferrofluids

In this work, the magnetorheological properties of suspensions of micron-sized iron particlesdispersed in magnetite ferrofluids were studied. With this aim, the flow properties ofthe suspensions in the steady-state regime were investigated using a commercialmagnetorheometer with a parallel-plate measuring cell. The effect of both magnetite andiron concentration on the magnitude of the yield stress was studied for a broad range ofmagnetic fields. In addition, the experimental values of the yield stress were compared withthe predictions from the chain model. With this purpose the values of the yield stresswere obtained by means of finite element calculations. Interestingly, it was foundthat the experimental yield stress increases with the concentration of magnetitenanoparticles in the ferrofluid. Unfortunately, this behaviour is not obtained fromcalculations based on the chain model, which predict just the opposite trend.

Critical Current Density in Superconducting<tex>$rm Nb-Ti$</tex>Strands in the 100 mT to 11 T Applied Field Range

The knowledge of the critical current density in a wide temperature and applied magnetic field range is a crucial issue for the design of a superconducting magnet, especially for determining both current and temperature margins. The critical current density of LHC-type Nb-Ti strands of 0.82 and 0.48 mm diameter was measured by means of critical current and magnetization measurements at both 4.2 K and 1.9 K and for a broad magnetic field range (up to 11 T). For the magnetic field range common to both measurement methods, critical current density values as extracted from transport current and from magnetization data are compared and found fairly consistent. Our experimental data are compared to other sets from literature and to scaling laws as well

Few‐electron systems in a quantum dot in a magnetic field: the Wigner molecule and paired electron phases

AbstractWe have developed a variational many‐body description for few‐electron systems in a parabolic quantum dot in an external magnetic field. The ground state energy of the electronic system has been theoretically investigated in a broad range of magnetic fields. The possibility of existence of a spin‐singlet paired state in a QD in an external magnetic field is discussed. Analytical expressions for the spin‐singlet phase and for the Wigner phase in a QD have been obtained and studied as a function of the magnetic field and QD confinement. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nuclear spin relaxation study of aqueous raffinose solution in the presence of a gadolinium contrast agent

Paramagnetic enhancement of nuclear spin-lattice relaxation rates (PREs) was measured in aqueous solution of the trisaccharide raffinose in the presence of a gadolinium(III) complex, GdDTPA-BMA, used as a magnetic resonance imaging contrast agent. The relaxation enhancement of aqueous protons was measured over a broad range of magnetic fields, using field-cycling apparatus in addition to conventional spectrometers. The nuclear magnetic relaxation dispersion profile thus obtained was interpreted with a recently developed model, allowing for both inner- and outer-sphere relaxation. The relaxation enhancement for the carbon-13 nuclei in raffinose was studied under high-resolution conditions at three magnetic fields, whereas the sugar proton PRE was measured at two fields. The PRE of the sugar nuclei could be interpreted in a consistent way, assuming that it was caused by the outer-sphere mechanism. The electron spin relaxation was found to be a less important source of modulation of the electron-nuclear dipole-dipole interaction than the mutual translational diffusion.

Magnetic-field-induced reorientation in thin antiferromagnetic films: Spin-flop transition and surface-induced twist effects

Phenomenological equations for antiferromagnetic structures in a thin antiferromagnetic layer are derived and investigated in a broad range of magnetic fields and layer thicknesses. Corresponding phase diagrams magnetic field vs layer thickness are presented. Depending on thickness and values of the intrinsic and surface-induced interactions, the spin-flop transition occurs either as first-order transition (similar as in bulk materials) or continuously via twisted structures that are inhomogeneous across the layer thickness. The antiferromagnetic twisted phases have the same physical nature and display similar properties as spatially modulated phases observed in confined liquid crystals (Freedericksz effect) or in thin ferromagnetic films and multilayers.

Thermodynamic fluctuations in [formula omitted]-phase SmLa 0.8Sr 0.2CuO 4− δ single crystal

Series of M– T measurements have been conducted on the T ∗ -phase SmLa 0.8Sr 0.2CuO 4− δ single crystal ( T c on≈24 K) in a broad range of magnetic field (1000 Oe–10 kOe) applied along the crystal c-axis. The result clearly establishes the existence of a crossing point (M ∗ , T ∗) , signifying an intrinsic thermal fluctuations effect in this system. The effective CuO 2 interlayer distance deduced from this analysis is s≈15.50 Å, in a good agreement with the XRD refinement result. The effective dimensionality of fluctuations associated with the thermodynamic scaling properties of magnetization is analyzed on the basis of Ginzburg–Landau (GL) theory with the order parameter restricted to the lowest Landau level (GL-LLL) as well as the XY fluctuations model. It is shown that the data for the field above or equal to 8000 Oe exhibit distinct scaling behavior characteristic of the 2D GL-LLL fluctuation model. It also shows a clear departure from this and the corresponding 3D model in the lower field regime considered in this work.

A new high sensitive method for determining weak microinhomogeneities in semiconductors

A method is suggested for determining random irregularities in semiconductors. The method is based on the effect of abnormal decrease of transverse magnetoresistance in nonuniform samples (compared with homogeneous ones) in classically strong magnetic fields. An additional peculiarity of effective conductivity is predicted in the nonhomogeneous semiconductors when alternating electric field is applied. We have observed that the dispersion of conductivity starts at low frequencies. These frequencies are equal to the frequency of redistribution of the spatial charge linked with the inhomogeneities. This is sufficiently lower than in homogeneous samples. The high sensitivity of the proposed method was experimentally proved in broad range of magnetic field up to 40kG. The experimentally determined measure of the sample inhomogeneity was found to be close to the theoretically predicted value.

Strong Magnetoresistance Induced by Long-Range Disorder

We calculate the semiclassical magnetoresistivity $\rho_{xx}(B)$ of non-interacting fermions in two dimensions moving in a weak and smoothly varying random potential or random magnetic field. We demonstrate that in a broad range of magnetic fields the non-Markovian character of the transport leads to a strong positive magnetoresistance. The effect is especially pronounced in the case of a random magnetic field where $\rho_{xx}(B)$ becomes parametrically much larger than its B=0 value.

A method for determining microinhomogeneities in semiconductors

A method is suggested for determining random irregularities in semiconductors. The method is based on the anomalous sensitivity of the effective electrical conductivity of semiconductors with weak random irregularities to an applied strong magnetic field. The primary advantage of this method is the possibility to control the sensitivity of measurements of small values of concentration of random weak inhomogeneities changing an external magnetic field. Preliminary results are presented here of experiments with Si:B thin layers at He temperatures under a broad range of magnetic fields (0–30 kOe). The experimentally determined measure of the sample inhomogeneity was found to be close to the theoretical predicted value.

Galvanomagnetic phenomena in layered conductors

The dependence of the resistance and the Hall field in a layered conductor with a quasi-two-dimensional electron energy spectrum of arbitrary shape on the magnitude and orientation of the magnetic field in relation to the layers is analyzed. It is found that when current flows perpendicular to the layers, the resistance of the specimen strongly depends on the angle ϑ between the normal and the vector of a strong magnetic field. The Kapitza law is shown to hold within a fairly broad range of magnetic fields in the plane of the layers, i.e., the resistance increases linearly with the magnetic field strength. The Hall field proves to be insensitive to the emergence of open sections of the Fermi surface, and the Hall constant in strong magnetic fields is the same for any orientation of the magnetic field and the current.

Variational studies of the quasi-two-dimensional D- centre in magnetic fields

We present a simple variational scheme for calculating the binding energies and the lowest dipole optical transition energies of D-impurities in the middle of quantum wells at moderate and high magnetic fields. The objective was to make the model and the method as simple as possible but to be able to explain many of the experimental features observed. The simple form of the electronic wavefunctions used enables physical insight into the properties of the D- system in quantum wells. The scheme, based on the parabolic approximation for the conduction band, gives results compatible with those of more sophisticated approaches. A simple renormalization procedure makes the results compatible also with the experimental results for GaAs quantum wells for which band non-parabolicity proved to be important. The energies for the lowest dipole optical transitions were calculated for a broad range of magnetic fields and well widths.

Magnetic field distribution in the vortex lattice of a type-II superconductor in the Ginzburg-Landau approach and experimental validation of theory by the μSR-method

The distribution of magnetic fields in the vortex lattices of type-II superconductors has been investigated. This has been done in the framework of Ginzburg-Landau (GL) theory for isotropic and anisotropic (HTSC) superconductors. Analytical and numerical results obtained show the way to verify experimentally the superconductivity theory and to determine the area of applicability of various GL theory approximations. It is shown that in some cases the system of GL equations for an anisotropic superconductor can be reduced to that of an isotropic one. The numerical solutions of GL equations were obtained. New approaches are suggested to approximate the exact solution in the whole range of magnetic fields. The exact solutions of GL equations are compared with the most frequently used approximations in the broad range of magnetic fields and κ parameter. The influence of nuclear magnetic moments upon the observable distributions of internal magnetic fields is considered.

Transversal magnetoresistance in La2CuO4

Abstract It is shown that in compounds whose Fermi Surface (F.S.) is a square based cylinder, the magnetoresistance is temperature independent (from 4 to 300 K) for a broad range of magnetic field ( B = 0 to 200 kG). From this kind of experiment the sharpness of the corners of the F.S. for the compound La 2 CuO 4 and similar superconductors with the layer-like structure of K 2 NiF 4 can be estimated.

Effects of nitroxides on the magnetic field and temperature dependence of 1/T1 of solvent water protons.

We report a study of the longitudinal NMRD profiles (proton longitudinal relaxation rates as a function of field strength) over a broad range of magnetic field (0.01 to 50 MHz proton Larmor frequency) and temperature (-9.6 to 37 degrees C) for aqueous solutions of (i) a fatty acid-nitroxide/albumin complex and (ii) 10 low molecular weight nitroxides. Analysis of the NMRD profile for the fatty acid-nitroxide/albumin complex provides a lower bound estimate for the rotational correlation time of the complex, which permits the calculation of an upper bound on the inner sphere contribution to relaxation of the uncomplexed nitroxides. Inner sphere processes, ostensibly due to water molecules hydrogen bonded to the nitroxide moiety, dominate the relaxation effects of the slowly rotating macromolecular nitroxide/albumin complex. By extrapolation, the contribution of these inner sphere processes are negligible for rapidly tumbling nitroxides free in solution, which affect solvent proton relaxation almost entirely through outer sphere processes (i.e., translational diffusion). A comparison of the relaxation data for aqueous solutions of the uncomplexed nitroxides with the theory of outer sphere relaxation of J.H. Freed (J. Chem. Phys. 68, 4034 (1978] yields values for the distance of closest approach of the water and nitroxide molecules, as well as for their relative diffusion constants, at five different temperatures. Our results indicate that the rather modest relaxivities of aqueous solutions of nitroxides increase substantially with increased solvent viscosity and with protein binding, supporting the potential utility of nitroxides for enhancement of contrast in nuclear magnetic resonance images.