Low External Field Research Articles

Large enhancement of elastic light scattering due to random fluctuations of the magnetization

We have observed the field dependence and enhancement of the intensity of polarized elastic light scattering, from ultrathin magnetic films with perpendicular magnetic anisotropy (PMA). High-resolution Brillouin spectroscopy elucidates that this enhancement reaches its maximum at a critical field (Hcrit) where the magnetization starts to incline toward out-of-plane due to the strong PMA. We attribute this new effect to the microscopic instability due to random fluctuations of the magnetization near Hcrit, since this elastic scattering has no frequency shift from the excitation light and thus the correlation of each spin decays monotonically. The enhancement factor depends on the PMA strength as well as the film thickness, and we obtain an enhancement as large as a factor of 20 at room temperature with a low external field of 0.18 kOe.

Angular dependence of AC transport current losses in biaxially aligned Ag/YBCO-123/YSZ/Hastelloy coated conductor

AC transport losses in a biaxially aligned YBCO-123 tape (10 cm in length) were measured (at 77 K) as a function of frequency and amplitude of a sinusoidally varying transport current. In the frequency range of 30-320 and the transport current losses are hysteretic in nature. The measurements of the self-field losses caused by the AC transport current as a function of the magnitude and orientation of the applied low external DC magnetic field (up to 150 mT) are also reported. The external magnetic field was rotated in the range of +120 to -120 degrees with respect to the plane of the tapes, and was in all cases perpendicular to the transport current flow. Strong anomaly in angular critical current dependence on external magnetic field was observed.

Structure and Properties of Sr2Au3In4 and Eu2Au3In4

Sr2Au3In4 was synthesized by reacting the elements in glassy carbon crucibles under an argon atmosphere in a high-frequency furnace. Eu2Au3In4 was obtained in the same way but starting with EuAuIn as a precursor and admixing indium and gold. Both compounds were investigated by X-ray diffraction on powders and single crystals: anti-Hf2Co4P3 type, P62m, a=1501.6(9) pm, c=457.9(2) pm, wR2=0.0734, 1030 F2 values, 45 variables for Sr2Au2.85In4.15 and a=1489.7(3) pm, c= 454.90(7) pm, wR2=0.1222, 1051 F2 values, 45 variables for Eu2Au3In4. One gold site in the strontium compound shows a mixed occupancy with indium atoms indicating a small homogeneity range according to Sr2Au3−xIn4+x. Striking structural motifs of Sr2Au3In4 and Eu2Au3In4 are gold-centered trigonal prisms formed by the strontium (europium) and indium atoms. All gold atoms have coordination number 9 in the form of tricapped trigonal prisms. The indium atoms in Sr2Au3In4 and Eu2Au3In4 occupy the cobalt positions of the Hf2Co4P3 type and the more electronegative gold atoms take the phosphorus positions, indicating that Sr2Au3In4 and Eu2Au3In4 may be considered as ternary aurides. The gold and indium atoms form three-dimensional [Au3In4]4− polyanions in which the divalent strontium and europium atoms occupy distorted hexagonal tubes. Magnetic susceptibility measurements of Eu2Au3In4 show paramagnetic behavior above 50 K with an experimental magnetic moment of 7.88(5) μB/Eu indicating Eu2+. Antiferromagnetic ordering is detected at 9.5(5) K at low external fields. A metamagnetic transition is observed at 2 K at a critical field of 0.5(2) T. The experimental saturation moment at 2 K and 5.5 T is 5.95(5) μB/Eu. Electrical conductivity investigations exhibit metallic behavior with a room temperature value of 75±10 μΩcm for the specific resistivity. 151Eu Mössbauer spectroscopic measurements show a broadened line with an isomer shift of −10.5(5) mm/s at 78 K in accordance with several sites of divalent europium. Below 10 K magnetic hyperfine field splitting is observed.

Field emission properties of silicon carbide and diamond-like carbon (DLC) films made by chemical vapour deposition techniques

Chemical vapour-deposited (CVD) amorphous silicon carbide (a-SiC) and diamond-like carbon (DLC) thin films were realised on p-type silicon field emitter arrays (FEAs) in order to investigate their field emission properties. The FEA geometry was investigated by scanning electron microscopy (SEM), the film morphology by scanning force microscopy (SFM) and by SEM, and the film structure by X-ray photoelectron spectroscopy (XPS). Field emission properties of FEAs were determined in high vacuum conditions. a-SiC/p-Si in comparison with DLC/p-Si emitter arrays have higher current emission at lower external fields. The upper limits of the field emission current densities were 2.4 mA/cm 2 for an electric field of 25 V/μm in the case of SiC and 0.8 mA/cm 2 for the electric field of 42 V/μm in the case of DLC films.

Shield-related signal instability in magnetoresistive heads

Magnetoresistive (MR) heads with various upper shield materials were fabricated and their read-write performance was tested to clarify the shield-related effect on the signal instability in MR heads. Comparison of a head with an upper shield layer of higher magnetostriction and one with lower magnetostriction showed that the latter had better stability in the output signal of a repeated read-write test. The output amplitude of a head with an upper shield layer of Co52Ni27Fe21 film, which had a high magnetostriction of about +3×10−6, was varied by applying a low external longitudinal field, which affected just the shield layers. This change in the output corresponded well to the output variation in the repeated read-write test. The spin scanning electron micrograph image of this head revealed a distinct domain wall in the air bearing surface near the MR sensor. These results indicated that instability of the domain structure in a shield layer was one of the causes of the signal instability in MR heads; an unusual bias field from a domain wall of the shield layer, which could be moved easily by a repeated writing operation, caused a variation in the biased state of the MR layer which resulted in the signal variation, and that low magnetostriction was required for a shield material to achieve a stable head.

A MONTE CARLO STUDY OF THE SPECIFIC HEAT OF DILUTED ANTIFERROMAGNETS

The three-dimensional diluted antiferromagnet in a magnetic field has a longe-range ordered state for sufficiently low temperatures and external fields. We study the irreversibilities at the phase transition from this state into the paramagnetic state at higher temperatures. Performing field cooling and zero field cooling simulation procedures we find a small irreversibility in the internal energy and the specific heat, although the behavior of the order parameter is irreversible and the phase transition is suppressed in the FC case. The specific heat shows a noncritical broad maximum above the transition temperature Tc. The results are compatible with recent experimental results obtained by Satooka et al., whereas our interpretation of the data is different.

Perpendicular magnetization and dipolar antiferromagnetism in double layer nanostripe arrays of Fe(110) on W(110)

Fe(110) nanostripe arrays, consisting of alternating monolayer and double layer stripes, have been grown by step flow on vicinal W(110) substrates. The magnetic easy axis switches from in-plane in the monolayer to perpendicular in the double layer stripes. The data strongly suggest that magnetostatic interactions induce antiferromagnetic order in the double layer nanostripe array. It can be switched into a ferromagnetic arrangement by low external fields.

Magnetomechanical instability in FeTb/Fe multilayers

Multilayers composed of alternate layers of iron–rare earth and Fe or FeCo alloys feature a high magnetostriction at low external fields and a coercive force as low as 5 mT. The coercive fields of as sputtered multilayers are significantly higher than that and the low values are attained after an anneal at 473 K. Since the hysteresis remaining after the heat treatment might be of magnetomechanical origin it is of interest to investigate the dynamic magnetomechanical characteristics of this class of multilayers. Experiments on multilayers of the composition (7 nm FeTb)/(8 nm/Fe) show a pronounced magnetomechanical damping maximum and an attendant decrease of Young’s modulus upon annealing. Both occur in the vicinity of the coercive field. Their evolution as a function of the annealing and magnetic field will be described and it will be shown that they reflect an instability principally caused by the magnetomechanical interaction of the component layers of the multilayer.

Magnetic properties and structural chemistry of the solid solution La 5B 2C 6–Gd 5B 2C 6

The borocarbides (La 1− x Gd x ) 5B 2C 6 with 0≤ x≤1 were prepared by arc melting elemental lanthanum, gadolinium, boron and carbon. From X-ray powder diffraction analysis, all alloys were found to be isostructural with the Ce 5B 2C 6 type. (La 1− x Gd x ) 5B 2C 6 (with 0≤ x≤1) forms a continuous solid solution. La 5B 2C 6 is a type I superconductor at T<6.9 K. The superconducting state is easily destroyed either by applying low external fields ( H crit=34 mT) or by replacing lanthanum by paramagnetic gadolinium (Gd 3+) ( n crit<1.8 atom%). Isotypical Gd 5B 2C 6 is an antiferromagnet with a Néel temperature T N=26 K. Within the solid solution (La 1− x Gd x ) 5B 2C 6 the ordering temperature decreases gradually with increasing x. The positive θ p values suggest that the spin structure contains ferromagnetic sheets which are antiferromagnetically coupled. For x<0.4 all samples remain paramagnetic above T>4.2 K.

Resonance fluorescence near a photonic band edge: Dressed-state Monte Carlo wave-function approach

We introduce a dressed-state Monte Carlo wave-function technique to describe resonance fluorescence in a broad class of non-Markovian reservoirs with strong atom-reservoir interaction. The method recaptures photon localization effects which are beyond the Born and Markovian approximations, and describes the influence of the driving field on the atom-reservoir interaction. Using this approach, we predict a number of fundamentally new features in resonance fluorescence near the edge of a photonic band gap. In particular, the atomic population exhibits inversion for moderate applied field intensity. For a low external field intensity, the atomic system retains a long-time memory of its initial state.

Two-color infrared detection using intersubband transitions in multiple step quantum wells with superlattice barriers

A dual-band multiple quantum well infrared photodetector has been fabricated for the near and mid-infrared detection. The structure consists of InGaAs/GaAs step quantum wells separated by AlGaAs/GaAs superlattice barriers. The step quantum wells are designed to have two bound states and a quasi-continuum above the superlattice barriers. The bound-to-bound and bound-to-continuum transitions are employed for the dual-band detection. The superlattice barriers are used to extract the photocurrent from the bound-to-bound transition with a relatively low external electric field, which keeps the dark current low. This work demonstrates the use of a new quantum well structure for the fabrication of multicolor quantum well infrared photodetectors.

Multilayer neutron polarizing mirrors working under a low external magnetic field less than 100 gausses

Abstract We have developed multilayer neutron polarizing mirrors which are controlled under an external magnetic field less than 100 gausses. Multilayers of Permalloy-Ge and Fe-Ge have been evaporated on Si wafers in the magnetic field of about 130 gausses parallel to the surface of the wafer. The polarizing efficiencies of Fe-Ge and Permalloy-Ge multilayer mirrors were 95% and 91% respectively under a low external magnetic field of 80 gausses. Under a very low external field of 50 gausses, the polarizing efficiency was 90% in case of Permalloy-Ge. A neutron pulse was obtained by applying a pulsed magnetic field of about 50 gausses parallel to the Permalloy-Ge polarizing mirror. The pulse rise time is the order of 100μs in the present pulsed magnetic devices.

Perpendicular induced magnetic moments of Ag in Ag/Fe multilayers

We have studied the induced magnetic moments of Ag in Ag/Fe multilayers with low temperature nuclear orientation. The experiments show that there are induced magnetic moments in the Ag spacer and these induced magnetic moments do not lie in the plane of the multilayer for 2 and 4 ML of Ag at low external field.

C-axis ac susceptibility in high-Tcsuperconductors

We have investigated the angle and magnetic field dependence of the ac susceptibility in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Ca${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$ and Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ single crystals at low external fields. The ac field was applied perpendicular to the Cu${\mathrm{O}}_{2}$ planes. The first and third harmonics of the ac susceptibility exhibit remarkably sharp features when the dc field component perpendicular to the Cu${\mathrm{O}}_{2}$ planes passes a threshold field ${H}_{\mathrm{th}}$. ${H}_{\mathrm{th}}$ is strongly temperature dependent, but is independent of the parallel field component. We propose a simple model which excellently explains the data. Within this model the peak structures are related to the irreversibility line. We discuss the implications of the model for the interpretation of the ac susceptibility.

Field induced transition of the AC susceptibility in high-Tc superconductors

We have investigated the angle and field dependence of the ac susceptibility in Bi2Sr2CaCu2O8 single crystals at low external fields. The ac field was applied perpendicular to the CuO2 planes. The first and third harmonic of the ac susceptibility exhibit a remarkably sharp transition when the dc field component perpendicular to the CuO2 planes crosses a threshold field Hth. Hth is strongly temperature dependent, but is independent of the parallel field component. We prove that these structures are related to the irreversibility line and propose a simple model which excellently explains the measurement data.

Free-ion yield and electron-ion recombination rate in liquid xenon

A recently proposed re-encounter model for free-ion yeild at low-LET is applied to liquid xenon. For an initial electron-ion separation given by the thermalization distance (≈ 4000–5000 nm), good agreement with measured escape probability is obtained for an encounter recombination probability ≈0.01–0.03. Analysis of homogeneous recombination at low external fields places the electron-ion recombination rate in the range of (4.0–8.5) × 10 15 M −1 s −1.

Photoacoustic investigation of the temperature and magnetic-field dependence of the specific-heat capacity and thermal conductivity near the Curie point of gadolinium.

The photoacoustic technique was used as a simple but accurate tool to simultaneously determine the specific-heat capacity C and thermal conductivity \ensuremath{\kappa} of gadolinium in the neighborhood of the Curie point. One polycrystalline sample and two single crystals of different quality have been investigated as a function of temperature and external magnetic field B. One of the single-crystal samples was the same as the one for which Bednarz et al. [Phys. Rev. B 47, 14 247 (1993)] obtained high-resolution C(T,B=0) data by means of ac calorimetry. Our photoacoustic results are in full agreement with the Bednarz et al. data, thus confirming the complex crossover behavior found by these authors. The photoacoustic results in the presence of low external fields clearly were affected by demagnetization effects resulting in the occurrence of kink temperatures in the ${\mathit{C}}_{\mathit{B}}$(T) curves below the Curie temperature. Information on the temperature dependence of the spontaneous magnetization could be deduced from the experimentally obtained kink temperatures. Scaling properties for the B dependence of the specific-heat capacity have also been investigated. The thermal conductivity \ensuremath{\kappa}(T) for B=0 shows a clear minimum at the Curie point for the best sample. The minimum is diminished with poorer sample quality and in the presence of an external magnetic field.

Compact Steady State Tokamaks

The small aspect ratio tokamak combines the attractive features of the tokamak, reverse field pinch and spheromak to produce a compact, stable, high-{Beta} configuration with low external fields. Experiments on the Small Tight Aspect Ratio Tokamak (START) at Culham have demonstrated the production of stable, high temperature (T{sub e} 30%) and self-consistent steady-state equilibria using pressure and beam driven currents. Such steady-state solutions exist for devices at the Mega Amp level, compact component test facilities and fusion power plants of modest size (Ro=2-3m). 5 refs., 5 figs.

1−t [formula omitted], a basic function to quantify magnetic measurements in high-Tc superconductors at low external magnetic fields

The magnetization of high-T c superconductors as a function of temperature at low external magnteic fields is well described by curves that are based on the function 1− t 2 (1−α) . We discuss three modela that generate this function; the two-fluid model, a field cooling remanent magnetization model and a zero field cooling magnetization model. The basic concepts of each model are different.

Dielectric measurements on Li + doped KCl

We report on an investigation of the temperature dependence of the complex dielectric constant of KCl single crystals containing Li + tunneling defects. Our results partially agree with the theoretical prediction for resonant interaction between a low frequency external field and tunneling systems with an energy splitting of 1.1 K. Additionally to resonant contributions, we find significant contributions from relaxation processes. They depend on the defect concentration and on frequency. The results suggest the presence of a broad density of states at energy splittings below 100 mK.