Low-field Behavior Research Articles

Magnetic domains and twin structure of the La0.7Sr0.3MnO3 single crystal

Using a magneto-optical indicator film imaging technique and an x-ray topography, we have observed a strong correlation between magnetic and twin structure of a single crystal of La0.7Sr0.3MnO3 (LSMO). The correlation between magnetic and twin domains can be understood in terms of a rhombohedral deformation of the cubic cell of LSMO which is accompanied by an occurrence of a magnetic anisotropy. Our data and analysis suggest that twin domains play a fundamental role in a low-field magnetization behavior and in magnetotransport properties of LSMO.

Magnetic behaviour of some amorphous Er–Co alloys

Some amorphous Er–Co alloys, have been studied by examining their normal magnetization curves with both hysteresis major and minor loops. Rayleigh parameters from hysteresis minor loops allowed to describe the low-field behaviour of the magnetization: (i) the increase of χ m is in a good agreement with Rayleigh laws but a weak deviation is observed on the thick samples at fields up to 100 Oe, which can be due to magnetostatic effects; (ii) the ratio αH c/ χ 0 is close to 1 for the thin sample (0.1 μm) as predicted by the Néel theory.

The magnetoresistance of compensated Ge:As at microwave frequencies in the vicinity of the metal-insulator phase transition

The magnetoresistance of heavily doped Ge:As near the metal-insulator phase transition has been studied both on the metal and insulator sides of the transition. Measurements were made at microwave frequencies using a noncontact technique of electron spin resonance. The field and temperature dependences of the magnetoresistance derivative in metallic samples reveal two main features of the phenomenon: a negative magnetoresistance at weak fields, due to the weak localization effect, and a positive magnetoresistance at strong fields, arising from the electron-electron interaction in the diffusion channel. Only a weak negative magnetoresistance with a characteristic low-field behavior is observed in insulating samples. The results are compared with the theory of quantum corrections.

Low-field flux creep behavior near second peak H 2p in Pr-doped Bi-2212 single crystal

Magnetic hysteresis M( H) and magnetic relaxation M( t) results on Pr-doped single-crystal Bi 2Sr 2Ca 0.89-Pr 0.11Cu 2O 8+ δ with optimum T c=88 K are reported. Superconducting H– T phase diagram with applied field parallel to single-crystal c-axis indicates that in the mixed state the anomalous second peak field and its onset field H 2p/ H on varies from 900/800 G at 15 K to 210/85 G at 40 K with a plateau around 20–30 K. Low-field magnetic relaxation M( t) data near H 2p/ H on indicates a distinct change of flux creep behavior. For H< H on, the relaxation mechanism follows the collective creep model M( t)= M 0[1+( μk B T/ U 0)ln( t/ t 0)] −1/ μ and crosses to the vortex glass model M( t)= M 0( t/ t 0) 1/[1−( Uo/ k B T)] when H> H on. Such a crossover behavior implies a change in vortex coupling mechanism from vortex quasi-lattice to entangled vortex glass.

Low-Field Magnetoresistance in La-Ca-Mn-O Films with Al2O3 Addition by D.C. Sputtering

The La0.67Ca0.33MnO3-x films doped with Al2O3 up to 25 at% have been grown on the (100)LaAlO3 substrates at 400 C by a d.c. magnetron sputtering process. The sputtered films have a polycrystalline structure, which shows a [100] preferential orientation. An Al2O3-rich second phase exists in the microstructure of films doped with Al2O3 more than 5%. It is found that the MR behavior of films is strongly influenced by the Al2O3 doping and annealing treatment. The film doped with 5% Al2O3, deposited at 400 C and annealed at 1000 C for 4 hr, shows a MR ratio as high as ∼4980% at 100 K and at 8 kOe which corresponds to a MR sensitivity of ∼0.6%/Oe. Such a huge low-field MR behavior can be ascribed to the spin-polarized tunneling across grain boundaries where an Al2O3-rich second phase is believed to exist.

Lower limit of method for the extraction of ionization coefficients from the electrical characteristics of heterojunction bipolar transistors

The error contributed by the Early effect to the ionization coefficient obtained from the characteristics of heterojunction bipolar transistors (HBTs) is calculated. For the case of InP-GaInAs HBTs, the Early effect does not introduce a significant error at low electric fields. For the case of HBTs with Ga/sub 0.28/In/sub 0.72/As/sub 0.61/P/sub 0.39/ (/spl lambda/=1.3 /spl mu/m) base and collector, the Early effect introduces a relatively large error at low electric fields, A procedure for correcting this error is outlined. It is found that the low field ionization coefficient in Ga/sub 0.28/In/sub 0.72/As/sub 0.61/P/sub 0.39/ exhibits an anomalous low field behavior, similar to that of Ga/sub 0.47/In/sub 0.53/As.

Giant vortex state in perforated aluminum microsquares

We investigate the nucleation of superconductivity in a uniform perpendicular magnetic field H in aluminum microsquares containing a few (two and four) submicron holes (antidots). The normal/superconducting phase boundary ${T}_{c}(H)$ of these structures shows a quite different behavior in low and high fields. In the low magnetic-field regime fluxoid quantization around each antidot leads to oscillations in ${T}_{c}(H),$ expected from the specific sample geometry, and reminiscent of the network behavior. In high magnetic fields, the ${T}_{c}(H)$ boundaries of the perforated and a reference nonperforated microsquare reveal cusps at the same values of $\ensuremath{\Phi}/{\ensuremath{\Phi}}_{0}$ (where $\ensuremath{\Phi}$ is the applied magnetic flux threading the total square area and ${\ensuremath{\Phi}}_{0}$ is the superconducting flux quantum), while the background on ${T}_{c}(H)$ becomes quasilinear, indicating that a giant vortex state is established. The influence of the actual geometries on ${T}_{c}(H)$ is analyzed in the framework of the linearized Ginzburg-Landau theory.

The Pre-Transitional Effect in Antiferroelectric Liquid Crystals: a Comparison between Theory and Experiment

We have investigated, experimentally and theoretically, the low field (pre-transitional) behaviour of an AFLC material. By measuring the light transmission through a device between crossed polarizers, as a function of both orientation and applied voltage, we determine the variation of both the effective optic axis tilt angle φ and the optical anisotropy Δn as a function of voltage. In order to model this behaviour we use an order parameter theory based on changes to the distribution of molecules within the AFLC layers rather than the soft mode description. With this model we have been able to describe the observed tilt and anisotropy behaviour.

Low Field Anomaly in the Specific Heat ofs-Wave Superconductors due to the Expansion of the Vortex Cores

The magnetic field dependence of the electronic specific heat C(H) in the s-wave superconductor NbSe_2 shows curvature at low fields, resembling the near H^1/2 term in C(H) which has been reported in high-T_c superconductors and attributed to a d-wave pairing state. In NbSe_2 we find that the low field behavior in C(H) is described quantitatively by the expansion of vortex cores and the field dependence of the magnetic induction B above H_c1. The associated change in the density of quasiparticle states localized in the vortex cores provides a simple explanation for the "low-field anomaly" in C(H) observed in s-wave superconductors.

Effects of gas damage on coupling and anisotropy in sputtered Co/Cu multilayers

The giant magnetoresistance (GMR) of sputtered Co/Cu multilayers is reduced when they are not grown in clean vacuum conditions and earlier work has shown that the most easily damaged parts of the multilayer are the Cu spacer layers. We have prepared samples with clean and dirty spacers and examined the magnetisation reversal mechanism as a function of applied field direction by the magneto-optic Kerr effect and Lorentz electron microscopy. Whilst an uniaxial anisotropy is induced during the growth process, its effect on clean films with high GMR is small. By contrast the low field behaviour of low GMR gas damaged films is highly anisotropic. Quite different domain configurations are observed in the two kinds of film, indicative of different dominant coupling mechanisms.

Optical and electrical properties of Al2O3 films containing silicon nanocrystals

Optical, electrical, and structural properties of Al2O3 films subjected to silicon-ion implantation and annealing were investigated by means of photoluminescence measurements, current-voltage measurements and transmission electron microscopy. Transmission electron microscopy revealed that silicon nanocrystals were epitaxially formed in ϑ-Al2O3. Visible photolum inescence was observed, for the first time, from Al2O3 films containing silicon nanocrystals. Observed visible photoluminescence seems to be related to quantum size effects in silicon nanocrystals as well as localized radiative recombination centers located at the interface between silicon nanocrystals and matrix, similar to porous Si and other Si nanostructures. The conduction mechanism in the samples was studied by using dc current-voltage measurements. The conduction properties depend on temperature and applied electric fields. The conduction behavior in low electric fields consists of thermally activated region dominated by the Schottky conduction and nonthermally activated region in which carrier transport is controlled by space-charge-limited currents. The conduction behavior under relatively high electric fields is almost independent of temperature and well fitted by space-charge-limited conduction.

Study of the impact of HgO addition and low-field magnetic relaxation behaviour in granular high-T c superconductors

A series of high-T c superconductors have been prepared with the HgO addition/substitution. Significant improvement in theT conset as well asT c0 was observed in all the cases. Substitution of Hg at the Sr site and Ba site in the case of (Bi, Pb)-Sr-Ca-Cu-O and Y-Ba-Cu-O systems, respectively over a range 0·01–0·6 at% helps in constructing an entire spectrum: improvement ofT c0 up to 0·4 at % in the case of Bi-system and up to 0·03 at % in the case of Y123 system and slight drop inT c thereafter. Such improvement is the result of abundant supply of highly reactive nascent oxygen all through the bulk. HgO decomposes and provides oxygen which helps in maintaining proper oxygen stoichiometry throughout the bulk. No Hg or Hg-based impure phases were observed in the X-ray diffraction spectra. Low-field (10–100 Oe) magnetic relaxation studies reveal faster relaxation of the intergranular critical state in the case of silver added samples as the grain boundary coupling energyE J becomes quite uniform across the entire bulk which leads to smaller flux pinning energy. The distribution of the pinning energy is evaluated from the observed relaxation pattern and is found to be narrower in the case of silver added samples. It was also observed that the transportJ c ∼ exp[−ΔT c/T c0] and the flux pinning energyU ∼ ΔT c, where ΔT c is the transition width and is a measure of the inhomogeneity within the sample. Such relationships may help in devising a strategy for achieving highJ c, highU yet low ΔT c. Silver addition turns out to be an effective tool in tailoring the sample properties depending on requirement.

Magnetic properties of single-crystalline Mott insulator YVO3

Single crystals of YVO3 were grown using the floating-zone technique. YVO3 crystallizes in the orthorhombic Pbnm structure, and the structural parameters were determined by means of neutron diffraction. Using a superconducting quantum interference device magnetometer, we have studied the magnetic properties of YVO3 along the three principal directions. Our single crystal exhibits two magnetic transitions at about 115 and 75 K. For temperatures above the magnetic ordering, almost isotropic Curie–Weiss behavior is found for the magnetic susceptibility. Below the ordering, however, we find a highly anisotropic response with the c axis as the easy-magnetization direction. Both magnetic phases exhibit a weak ferromagnetic component along the c axis. A complex low-field magnetization behavior indicates that YVO3 orders ferrimagnetically for temperatures between 75 and 115 K.

Anisotropy of magnetoresistance in (110)La0.7Sr0.3MnO3 thin films

We have performed magnetization and magnetoresistance measurements on (110)La0.7Sr0.3MnO3 thin films in order to understand the role of in-plane uniaxial magnetic anisotropy in the magnetoresistance. Transport measurements with current flowing along the magnetically easy [001] and magnetically hard [11̄0] directions exhibit a linear dependence of magnetoresistance on field at high fields due to bulk colossal magnetoresistance. The low field behavior depends not only on the angle of the magnetization with the current but also the direction of the current in the crystal.

Superconducting Properties of CeRu2

We have measured the specific heat and the magnetization in the superconducting mixed state of CeRu 2 at low temperatures and in high magnetic fields. We have clarified a rich variety of superconducting properties such as the temperature dependence of the electronic specific heat and Maki parameters κ 1 , κ 2 and κ 3 . From these experimental results, CeRu 2 is concluded to be of the BCS-type in superconductivity. Nevertheless, interesting is the magnetic field dependence of the electronic specific heat at 0.5 K, indicating a \(\sqrt{H}\) like behavior in low fields and a H -linear dependence in high fields up to the upper critical field of 50 kOe. This characteristic specific heat is discussed on the basis of a microscopic theory by Brandt et al.

Particle Lagrangian Tracking with Hydrodynamic Interactions and Collisions

Hydrodynamic interactions between particle pairs are included in a Lagrangian approach for the simulation of the turbulent dispersion of a discrete particle. Several particles are simultaneously followed, and the set of equations of motion involves a coupling between particles through the use of a resistance matrix pertaining to the sedimentation theory. Results are presented for particle behavior in low turbulence fields, where collisions between particle pairs are assumed to be perfectly elastic. The influence of the turbulence anisotropy is considered and the anisotropy of the particle fluctuating motion is shown to be reduced by collisions.

Current carrying capability of multifilamentary (BiPb)2Sr2Ca2Cu3Ox tapes determined from transport and magnetization measurements

The magnetic field dependence of the critical current IC(B) and the remanent magnetization of several multifilamentary (BiPb)2Sr2Ca2Cu3Ox tapes were measured. The low field behavior of IC(B) is interpreted by the self-field effect, whereas at higher fields, the field dependence for all tapes was found to be identical except for a constant factor. Although self-field transport critical current densities jC0 differed widely for the tapes, the intergranular critical current densities determined from remanent magnetization were found to be equal. The results indicate that the reduction of jC0 by macroscopic defects is an important factor, whereas locally the filaments are of high quality.

A density matrix renormalization group study of low-energy excitations and low-temperature properties of alternating spin systems

We use the density matrix renormalization group (DMRG) method to study the ground and low-lying excited states of three kinds of uniform and dimerized alternating spin chain. The DMRG procedure is also employed to obtain low-temperature thermodynamic properties of these systems. We consider a 2N-site system with spins and alternating from site to site and interacting via a Heisenberg antiferromagnetic exchange. The three systems studied correspond to being equal to and ; all of them have very similar properties. The ground state is found to be ferrimagnetic with total spin . We find that there is a gapless excitation to a state with spin , and a gapped excitation to a state with spin . The DMRG analysis shows that the chain is susceptible to a conditional spin - Peierls instability. Furthermore, our studies of the magnetization, magnetic susceptibility and specific heat show strong magnetic field dependences. The product shows a minimum as a function of temperature T at low magnetic fields; the minimum vanishes at high magnetic fields. This low-field behaviour is in agreement with earlier experimental observations. The specific heat shows a maximum as a function of temperature, and the height of the maximum increases sharply at high magnetic fields. Although all three systems show qualitatively similar behaviour, there are some notable quantitative differences between the systems in which the site spin difference, , is large and small respectively.

Unique characteristics of cold cathode carbon-nanotube-matrix field emitters

The attributes of electron field emission from disordered matrix arrays of carbon nanotubes are studied and found to be quite reproducible in spite of the disorder, density, and quality variations from sample to sample. At low applied electric fields, the electron field emission current-voltage characteristics qualitatively follow conventional Fowler-Nordheim behavior up to a critical current density. However, the current rise at low applied fields is anomalously steep, suggesting that the Fowler-Nordheim model is not sufficient to quantitatively characterize the emission. In the high-field region, the emission characteristics have a more complex behavior. In that regime, the instantaneous field emission is reminiscent of the low-field behavior, but discrete switching events lead to an overall current suppression. We attribute the sudden and well-defined onset of the switching events to interactions between neighboring nanotube tips. By correlating the switching behavior to the current-voltage characteristics, we rule out other physical processes that cause similar effects.

Low-lying excited states and low-temperature properties of an alternating spin-1–spin-1/2 chain: A density-matrix renormalization-group study

We report spin wave and DMRG studies of the ground and low-lying excited states of uniform and dimerized alternating spin chains. The DMRG procedure is also employed to obtain low-temperature thermodynamic properties of the system. The ground state of a 2N spin system with spin-1 and spin-1/2 alternating from site to site and interacting via an antiferromagnetic exchange is found to be ferrimagnetic with total spin $s_G=N/2$ from both DMRG and spin wave analysis. Both the studies also show that there is a gapless excitation to a state with spin $s_G-1$ and a gapped excitation to a state with spin $s_G+1$. Surprisingly, the correlation length in the ground state is found to be very small from both the studies for this gapless system. For this very reason, we show that the ground state can be described by a variational ``ansatz'' of the product type. DMRG analysis shows that the chain is susceptible to a conditional spin-Peierls' instability. The DMRG studies of magnetization, magnetic susceptibility ($\chi$) and specific heat show strong magnetic-field dependence. The product $\chi T$ shows a minimum as a function of temperature($T$) at low-magnetic fields and the minimum vanishes at high-magnetic fields. This low-field behaviour is in agreement with earlier experimental observations. The specific heat shows a maximum as a function of temperature and the height of the maximum increases sharply at high magnetic fields. It is hoped that these studies will motivate experimental studies at high-magnetic fields.