Function Of External Magnetic Field Research Articles

Dependence of energy gap on magnetic field in semiconductor nano-scale quantum rings

We study the electron and hole energy states for a complete three-dimensional (3D) model of semiconductor nano-scale quantum rings in an external magnetic field. In this study, the model formulation includes: (i) the position dependent effective mass Hamiltonian in non-parabolic approximation for electrons, (ii) the position dependent effective mass Hamiltonian in parabolic approximation for holes, (iii) the finite hard wall confinement potential, and (iv) the Ben Daniel–Duke boundary conditions. To solve this 3D non-linear problem, we apply the non-linear iterative method to obtain self-consistent solutions. We find a non-periodical oscillation of the energy band gap between the lowest electron and hole states as a function of external magnetic fields. The result is useful in describing magneto-optical properties of the nano-scale quantum rings.

Magnetization Reversal Studies of Magnetic Wire Arrays by Polarized Neutron Scattering

Polarized neutron reflectivity has been used to investigate the peak pattern and the magnetization reversal process of laterally structured Fe- and CoFe-films. The peak pattern of the wire arrays was analyzed in the specular as well as in the off-specular regime. The intensities of the different cross-sections at the off-specular first-order peak were investigated as a function of external magnetic field. Magnetization reversals are discussed and compared to MOKE studies of the same system.

Spin properties of quantum wells with magnetic barriers. II. Inverted band ordering and spin polarized interface states

The electronic band-edge spectrum of the interface states in the magnetic semiconductor quantum wells based on narrow-gap semiconductors with mutually inverted band arrangement is studied within the envelope-function formalism. Interface states are shown to appear in these structures in the case of overlapping bulk bands of the constituents. The hybridization between the bare $\mathrm{sp}$-electron states and the d states of the Mn atoms leads to spin splitting. The spin-splitting effect of the interface states as a function of external magnetic field, well width, band offsets, and fraction of the magnetic atoms, is studied. One essential result is that one can design a structure where the states localized at the interfaces only have one spin direction. The results give evidence of the perspective for using the magnetic semiconductor structures in spin electronics.

Spin properties of quantum wells with magnetic barriers. I. Ak⋅panalysis for structures with normal band ordering

The electronic band-edge spectrum of magnetic semiconductor quantum wells containing a diluted magnetic semiconductor as one of the constituents is studied within the envelope-function formalism. The effects of Mn d electrons are explicitly included in a k p Hamiltonian which in the first approximation of perturbation theory is shown to be reduced to an effective Kane model. The sp-d hybridization leads to a spin-splitting effect. The results are applied to the system Cd 1 - x Mn x Te/CdTe. The spin-splitting effect is studied as a function of external magnetic field, well width, valence band offset and fraction of the magnetic atoms. The numerical results are in accord with experimental data.

Magnetic properties of Fe-based nanoparticle assembly

The magnetic properties of an assembly of very small Fe nanoparticles with an average diameter of the order of 3–5 nm are studied both experimentally and theoretically. The particles are dispersed in a polyethylene matrix with mass concentration 5%. The stray magnetic field of the samples with sizes 1×1×0.2 mm is measured by means of scanning SQUID microscope at 77 K as a function of external magnetic field. The presence of a weak remanent magnetization indicates a magnetic ordering in the samples studied at rather high temperatures. A distribution of the particle volumes or partial agglomeration of the particles within the assembly is suggested to be responsible for this unexpected behavior.

Rotation angle sensors based on spin valve structures: A modeling approach

Spin valve systems are widely considered in magnetic rotation angle sensors. We present a simple magnetostatic model to simulate the electrical and magnetic signal of typical spin valve systems as a function of external magnetic field and rotation angle. To match simulation and experiment only a few parameters are needed, which can be determined by a single measurement. Two types of systems are treated and discussed: a simple spin valve and a spin valve with an “artificial antiferromagnet,” often used to enhance the exchange bias. The model shows a very good correspondence of the predictions to the experimental results of the systems investigated. In order to quantitatively compare experiment and model, we introduce a useful concept to quantify the quality of the angle rotation curves: the total harmonic distortion.

Positive Ion Mobility in Normal Liquid 3He Under High Magnetic Fields

Positive ion mobility in normal liquid 3He has been measured as a function of external magnetic fields up to 15 T at various pressures and temperatures down to 3 mK. At 20 mK, a monotonic decrease of the mobility with increasing the magnetic field has been observed at the pressures above 10 bars. At 3.2 mK, the field dependence for pressures above 20 bars is found to exhibit a pressure dependent broad peak, followed by a big decrease. Possible origins for these anomalous behaviors are discussed.

Anomalous magnetic-field dependence of positive ion mobility in normal liquid (3)He.

The positive ion mobility in normal liquid (3)He has been measured as a function of external magnetic field up to 15 T at temperatures down to 3 mK. At 3.2 mK, the field dependence is found to exhibit a pressure-dependent broad peak followed by a large decrease at pressures above 20 bars. On the other hand, at 20 mK, a monotonic decrease with increasing the magnetic field has been observed in the same pressure region. Possible origins for these anomalous behaviors are discussed.

Bloch walls in a nickel single crystal

We present a consistent theory for the dependence of the magnetic structure in bulk samples on external static magnetic fields and corresponding experimental results. We applied the theory of micromagnetism to this crystal and calculated the Bloch wall thickness as a function of external magnetic fields. The theoretical results agree well with the experimental data, so that the Bloch wall thickness of a 〈110〉 71\ifmmode^\circ\else\textdegree\fi{} nickel single crystal was definitely determined with some hundred of nanometer.

Nuclear polarization of hydrogen molecules from recombination of polarized atoms.

The nuclear polarization of H(2) molecules formed by recombination of polarized H atoms on a Cu surface was measured as a function of external magnetic field and of temperature of the surface. The proton polarization of the molecules was determined by scattering of a longitudinally polarized 203-MeV proton beam in the Indiana University Cyclotron Facility storage ring. The nuclear polarization of the molecules, relative to the polarization of the atoms before recombination, increased from near zero in a weak magnetic field to 0.42 +/- 0.02 in a 0.66 T field. A simple model of the relaxation accounts quantitatively for the observations.

Correlation between electroresistance and magnetoresistance in La0.82Ca0.18MnO3 single crystal

The resistivity of La0.82Ca0.18MnO3 single crystal has been investigated as a function of external magnetic field and separately under an applied current flow. The measurements were carried out at various temperatures below and above the ferromagnetic transition temperature TC. It has been found that the dynamic electroresistance exhibits stunning similarities to the colossal magnetoresistance at the corresponding temperatures. The correlation observed between the electric- and magnetic-field effects is attributed to electrically induced magnetoresistance.

Extended reduced s–d model with anisotropic interactions

An anisotropic generalization of the reduced s–d model with antiferromagnetically coupled impurities is proposed. This extension consists in introducing different coupling constants of s–d interaction and interimpurity coupling for spin components along the c-axis and perpendicular to the c-axis. It is shown that the Hamiltonian of the anisotropic version of the reduced s–d model (AERs–dM), constructed in this manner, is equivalent in the thermodynamic limit to the corresponding mean-field Hamiltonian. Equations for the mean-field parameters are derived. The discussion of the possible solutions of these equations is carried out. The predictions of the AERs–dM are compared with experimental data for magnetization of uranium Kondo compounds UAsS, UAsSe, as function of external magnetic field and temperature. Appropriate adjustment of the parameters of the model allows the theory to reproduce basic features of the substances discussed, viz., c-axis oriented spontaneous magnetization of the system which does not change significantly even in strong external magnetic field perpendicular to this axis.

Spin Properties of Quantum wells Incorporating Semimagnetic Semiconductors

ABSTRACTThe electronic band-edge spectrum of magnetic semiconductor quantumwells containing a diluted magnetic semiconductor as one of the constituents is studied within the envelope-function formalism. Quantum wells with normal and mutually inverted band arrangements are considered. The sp – d hydribization between the bare sp-electron states and the d-states of the Mn atoms is shown to lead to a spin-splitting effect. The spin-splitting effect is studied as a function of external magnetic field, well width, valence band offset and fraction of magnetic atoms. The results have bearing on the perspective for using the magnetic semicondutor structures in spin electronics.

Distribution of the magnetic field and current density in superconducting films of finite thickness

An 1D equation describing the distribution of the effective vector potential A(y) across a film width, which holds for thin ( d< λ) and thick ( d> λ) films alike, is derived based on the analysis of a 2D Maxwell–London equation for superconducting films in a perpendicular magnetic field. For a finite λ case, the distributions of the local magnetic field and current density are found both inside and outside superconductors. An approximation dependence A(y) , finite (with all of its derivatives) across the entire film width, is found for films in the Meissner state. The flux-entry field is evaluated for a film of arbitrary thickness. An approximation expression is obtained for the distribution of the sheet current density in the mixed state of a pin-free superconducting film with an edge barrier. The latter approximation allows one to estimate magnetic field concentration factor at the film edge as a function of external magnetic field and geometrical parameters of the sample.

Variation of magnetic order for the 2-D systems of strongly correlated electrons with external magnetic field

Based on our earlier mean field approach (H. Doh et al., Phys. Rev. B 55 (1997) 14084), we present the variation of antiferromagnetic (staggered magnetization) order as a function of external magnetic field for the two-dimensional systems of strongly correlated electrons at half-filling. In the present work, we consider both the Zeeman and the Aharonov–Bohm effects in association with the Hubbard model. It is shown that the local minima of staggered magnetization and the undulatory variation of antiferromagnetic order remain unchanged up to a critical magnetic field and temperature. Further, we demonstrate that the Aharonov–Bohm (Berry) effect causes reduction in the critical magnetic field as a consequence of enhanced destruction of the antiferromagnetic order.

Transport in Two Dimensional Periodic Magnetic Fields

Ballistic transport properties in a two dimensional electron gas are studied numerically, where magnetic fields are perpendicular to the plane of two dimensional electron systemsand periodically modulated both in $x$ and $y$ directions. We show that there are three types of trajectories of classical electron motions in this system; chaotic, pinned and runaway trajectories. It is found that the runaway trajectories can explain the peaks of magnetoresistance as a function of external magnetic fields, which is believed to be related to the commensurability effect between the classical cyclotron diameter and the period of magnetic modulation. The similarity with and difference from the results in the antidot lattice are discussed.

Application of the interferometric noise measurement technique for the study of intrinsic fluctuations in microwave isolators

An ultrasensitive noise measurement system has been used to study the intrinsic fluctuations in microwave circulators and isolators. The key element of the measurement system is a carrier suppression interferometer which contains the device under test in one of its arms. The interferometer is used to cancel the carrier at the input of a low noise microwave amplifier in the readout system. Fluctuations in the device under test are then registered by the readout system which has an effective noise temperature of 360 K and is almost equal to its physical temperature. This corresponds to a phase noise of with 20 dBm of microwave power incident on the device under test. We show that this is sufficiently low to measure intrinsic fluctuations in low noise microwave components such as ferrite circulators and isolators. The noise of the microwave isolators was shown to be independent of both, microwave power and temperature in the region +10 to and -10 to . We have also measured the phase shift in microwave isolators as a function of external magnetic fields and temperature to determine the effects of environmental fluctuations.

Development of magnetic order in CeCu 2(Si 1− xGe x) 2 as a function of hybridization

In the alloy system CeCu 2(Si 1− x Ge x ) 2 phase A, i.e., antiferromagnetic order at x > 0, evolves from a heavy-fermion state at x = 0, where T A → 0 and non-Fermi-liquid effects are observed. Here, we present the results of specific-heat measurements on a CeCu 2(Si 0.9Ge 0.1) 2 polycrystal as a function of external magnetic field ( 0 ⩽ B ⩽ 8T) and hydrostatic pressure ( 0 ⩽ p ⩽ 1GPa). In this sample a magnetic ( T A = 1.4K) and a superconducting (sc) phase ( T C = 0.21K) have been observed at p = 0. Our finite-pressure results indicate a subtle difference between steric and chemical effects. At p = 1.0GPa superconductivity sets in at T C ⩽ 0.55K, while T A remains almost unchanged.

Plasma resonances and vortex dynamics in La 2− xSr xCuO 4

We studied the c-axis Josephson plasma edge of the La 2− x Sr x CuO 4 system for various doping levels as a function of external magnetic fields up to 25 T, both in Faraday and Voigt configuration, using broad band Fourier Transform spectrometers and monochromatic mm-wave sources. Only in the underdoped regime we find an appreciable shift of the plasma resonance with applied magnetic field. We compare the experimental results with recent theoretical predictions. In the long wavelength regime we find a new second plasma resonance which disappears in relatively low magnetic fields.

Sample size dependence of the Josephson plasma resonance in Bi 2Sr 2CaCu 2O 8+δ

The Josephson plasma resonance in single crystalline Bi 2Sr 2CaCu 2O 8+δ has been investigated at a microwave frequency of 35 GHz in a TE 102 cavity as functions of external magnetic field ( H ext //c- axis) and sample dimension L in the ab-plane. A sharp resonance is observed in E rf //c , whereas multi-peaks are found in H rf //ab . The former is independent of the sample dimension, while the latter shifts to higher fields as the sample size L is reduced, and disappears when L≤ L c . The size dependence of the resonance field observed in H rf //ab can be explained by the strong dispersion relation of the transverse Josephson plasma, and the size independent resonance can be described by the longitudinal Josephson plasma as predicted by the recent theory of Josephson plasma by Tachiki et al.