High Field Range Research Articles

Anisotropy of domain growth in epitaxial ferroelectric capacitors

Piezoresponse force microscopy (PFM) has been applied to investigate the switching kinetics in microscale epitaxial Pb(Zr,Ti)O3 capacitors. It is shown that transition from low to high field range brings about a qualitative change in domain growth kinetics, namely, laterally isotropic growth in the high fields as opposed to highly anisotropic growth in the low fields. It is suggested that anisotropy of domain growth can be attributed to orientational variations in the activation energy due to film microstructure. Fitting the switching kinetics using the Kolmogorov–Avrami–Ishibashi model shows excellent agreement with the PFM experimental data and yields the integer values of domain dimensionality.

Upper Critical Field Measurements up to 60 T in Arsenic-Deficient LaO0.9F0.1FeAs1−δ : Pauli Limiting Behavior at High Fields vs. Improved Superconductivity at Low Fields

We report resistivity and upper critical field B_c2(T) data for As deficient LaO_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and high field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T_c = 29 K and a significantly enlarged slope dB_(c2))/dT = -5.4 T/K near T_c which contrasts with a flattening of B_(c2)(T) starting near 23 K above 30 T. This flattening is interpreted as Pauli limiting behaviour (PLB) with B_(c2)(0) approx 63 T. We compare our results with B_(c2)(T)-data reported in the literature for clean and disordered samples. Whereas clean samples show no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of B_(c2)(T) for applied fields H || ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity.

Hyperfine coupling constant for 59Co estimated from a high-field susceptibility and high-field NMR shift in ferromagnetic Co2TiGa and Co2VGa

The hyperfine coupling constant for 59Co in the ferromagnetic state of a Heusler alloy Co2TiGa in the high-field range has been estimated to be 170 kOe per 1 μB of the moment of cobalt atom from the high-field magnetic susceptibility of 2.3 × 10−6 emu/gOe measured at 5 K in a field range from 20 to 70 kOe and a reported positive high-field NMR shift of +0.83 percent. The value of the hyperfine coupling constant has been found to be even larger in the case of Co2VGa. These features suggest that the positive hyperfine field at Co nucleus in the Co-based Heusler alloys is due to conduction electron polarization rather than a transferred hyperfine field.

Influence of Magnetic Field on Magnetostructural Transition in Ni<sub>46.4</sub>Mn<sub>32.8</sub>Sn<sub>20.8</sub> Heusler Alloy

A Mn-enriched Ni46.4Mn32.8Sn20.8 Heusler alloy has been prepared by the induction melting and casting method. The samples used were annealed at 900°C during 72 h with a subsequent water quench. The magnetostructural transformation (MST) in the vicinity of Tm=230K and Ta=250K has been found, and characterized by DSC, magnetization and resistivity measurements. Changes in the crystal structure due to the MST were observed using neutron diffraction at different temperatures. The crystal structure of martensitic phase shows a good fits to the 4-layered orthorhombic martensite with a Pmma space group. The linear shift of transformation temperatures was found in the entire high magnetic field range up 14T to be dTm/dB0 = -1.50 K/T and dTa/dB0 = -1.26 K/T, whereas in the low field range we found an increase of both characteristic temperatures. The entropy changes at the MST were evaluated using the Clausius-Clapeyron relationship. The transformation-induced magnetoresistance has been measured.

Evidence for Pauli-limiting behaviour at high fields and enhanced upper critical fields near [formula omitted] in several disordered FeAs based superconductors

We report resistivity and upper critical field Bc2(T) data for disordered (As-deficient) LaO0.9F0.1FeAs1-δ in a wide temperature and high field range up to 60T. These samples exhibit a slightly enhanced superconducting transition at Tc=28.5K and a significantly enlarged slope dBc2/dT=-5.4T/K near Tc which contrasts with a flattening of Bc2(T) starting near 23K above 30T. The latter evidences Pauli-limiting behaviour (PLB) with Bc2(0)≈63T. We compare our results with Bc2(T)-data from the literature for clean and disordered samples. Whereas clean samples show almost no PLB for fields below 60–70T, the hitherto unexplained pronounced flattening of Bc2(T) for applied fields H‖ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences are discussed in terms of disorder effects within the frames of (un)conventional superconductivity, respectively.

Hall effect in the pinned and sliding charge density wave state ofNbSe3

Results of Hall effect measurements are reported both below and above the threshold electric field,Et, for depinning the low temperature charge density wave (CDW) inNbSe3 in a wide temperature range. At low electric fields, belowEt, we have observed a change in the sign of the Hall voltage at all temperatures lower thanTp2. Comparison between the Hall effect and the magnetoresistance behavior indicates that then-type conductivity in the low magnetic field range differs qualitatively from the p-typeconductivity in the high field range. We demonstrate that at low temperature theCDW motion significantly alters the Hall voltage. These results indicate that, inNbSe3, the CDW in the sliding state interacts essentially with holes. Possible mechanisms of thiseffect are discussed.

New Solution to High-Field Transport in Semiconductors: II. Velocity Saturation and Ballistic Transmission

High-field transport in semiconductor diodes at room temperature is analyzed in the reflection–transmission regime. The pseudo-one-dimensional Boltzmann equation with a constant electric field is transformed into a pair of carrier flux equations. They are analytically solved neither with the relaxation time approximation nor with the perturbation expansion. The carrier energy relaxation due to optical phonon emission is essential in high-field transport. The current- and velocity-field characteristics are closely related to flux transmission through a specific layer, in which the elastic scattering is dominant and the optical phonon emission is absent. If the transmission coefficient is much less than unity, the proportionality of the current to the field results as the Ohm's law in high-field range. The current and velocity tend to saturate when the coefficient approaches unity (ballistic transmission). This result provides simple insight into transport in nanoscale devices.

Quantitative evaluation of spin-orbit interaction in InAs quantum dots

Influence of spin-orbit interaction (SOI) on the energy spectrum is studied for single uncapped InAs self-assembled quantum dots (SAQDs) holding just a few electrons. We measure Coulomb oscillations with magnetic field, and identify lifting of spin doublets in the low field range and transitions in the ground states with keeping the total angular momentum in the high field range. Both of these features are induced by large Zeeman splitting in the present SAQDs. For the ground state transitions, anti-crossing due to the SOI hybridization between the ground and excited states with opposite spins is observed in the excitation spectra, and the SOI energy of about 0.1 meV is evaluated from the anti-crossing size. This energy is comparable to that of previous report on InAs nanowires, and reproduced by simple calculation using the g-factor also derived in the excitation spectra.

Electrical characteristics and conduction mechanisms of metal-insulator-metal capacitors with nanolaminated Al2O3–HfO2 dielectrics

Electrical characteristics of metal-insulator-metal capacitors with various Al2O3–HfO2 nanolaminates are investigated. The results indicate that the breakdown field decreases with increasing the HfO2 individual-layer (IL) thickness. Concerning the same dielectric composition, the insulator with a thinner HfO2 IL also exhibits a significant improvement in the electrical breakdown and leakage characteristics. This is attributed to enhanced crystallization of the thicker HfO2 ILs. The insulator with alternate 1nm Al2O3 and 5nm HfO2 exhibits a breakdown field of 3.85MV∕cm at 125°C, and a leakage current of 9.6×10−8A∕cm2 at 1MV∕cm and 200°C. Further, it is revealed that the conduction mechanism in the high field range is dominated by the Poole–Frenkel emission with intrinsic trap energy of 1.91eV.

Theoretical study of the electron field emission phenomena in the generation of a micrometer scale discharge

The phenomenon of field emission plays a significant role in the deviation of the breakdown voltage from that predicted by Paschen's law within the range of high electric fields. High fields obtained in small gaps may enhance the secondary electron emission and such enhancement could lead to a lowering of the breakdown voltage and a departure from the Paschen curve. In this paper, the dc breakdown characteristics of the discharge in the micrometric regime were extensively studied by the theoretical approach including ion-enhanced field emission. In addition, semi-empirical expressions for the dependence of the breakdown voltage on the gap spacing and on the pressure based on the numerical solutions of the equation that describes the dc breakdown criteria have been proposed.

Granular superconductivity in YBa 2Cu 3O 7−δ single crystals by divalent impurities doping: Possible role of valence and orbital symmetry

We have investigated the effect of divalent Ca, Sr and Zn atoms at the Y, Ba and Cu sites, respectively, of the YBa 2Cu 3O 7− δ single crystal on the magnetoconductivity and magnetic irreversibility properties for H|| c axis and J|| ab plane. The resistive transition is a clear two-step process. While the upper temperature step is only weakly affected by low applied fields, the lower temperature step is visibly broadened and shifted down to lower temperatures. Between upper and lower temperature steps lays the magnetic irreversibility line, T irr( H) which follows the power law predicted by the flux creep theories in most of the high field range. However, in a low field region, the T irr( H) exhibits two different regimes dominated by disorder and frustration and characterized by the de Almeida–Thouless and Gabay–Toulouse power law behaviors. We attribute the origin of the observed stepwise resistive transition and low field magnetic irreversibility regimes mainly to the granularity of the superconducting state of our doped samples and contrast our magnetic irreversibility present results with those of a pure YBa 2Cu 3O 7− δ single crystal. We suggest that the valance as well as the local breaking of the orbital symmetry by impurities plays an important role in the induction of this state. On the other hand, our results show that granular superconductivity can be induced in the YBa 2Cu 3O 7− δ system by divalent impurities doping in whatever site Y, Ba or Cu.

High-temperature conduction behaviors of HfO2/TaN-based metal-insulator-metal capacitors

High-temperature (∼90–150 °C) conduction mechanisms of metal-insulator-metal (MIM) capacitors with atomic-layer-deposited HfO2 dielectric are studied. In the low field range, the Schottky emission current is dominant, and the deduced dielectric constant is close to the static one of HfO2. In the high field range, the resulting leakage current complies with the Poole-Frenkel (PF) emission, which is demonstrated by the fact that the extracted dielectric constant equals the optical frequency one (i.e., square of refractive index) of HfO2. The underlying mechanisms are discussed based on carrier velocities under different electric fields. Further, the deduced Schottky barrier height is ∼0.251–0.274 eV in the low field range, which relates to the contributions from high density traps in the HfO2 film and the nonideal TaN/HfO2 interface, etc. The extracted trap potential well depth for the PF effect is ∼1.11–1.37 eV in the high field range.

Electrical studies on sputtered CuCl thin films

CuCl is a wide-direct band gap semiconductor, lattice matched to Si and it possesses excellent ultra violet (UV) emission properties. It is thus a promising candidate for the next generation Si based UV optoelectronics. CuCl films were deposited using RF magnetron sputtering technique. X-ray diffraction analysis reveals that the grains are strongly oriented. Triangular crystallites of CuCl were observed in the AFM surface topograph. Au–CuCl–Si–Au structures were fabricated and field dependent electrical studies were carried out in the electric field range of 1.25 × 106 to 2.5 × 107 V/m. I–V characteristics show that ohmic conduction prevails in low electric fields up to 2.5 × 106 V/m. In the higher field range, from 2.5 × 106 to 2.5 × 107 V/m, the conduction mechanism was Schottky emission controlled. There was no trap related charge transport observed at higher electric fields. Preliminary electrical studies are reported in this article.

Superconducting Properties of Films Deposited From Micro-, Nanocrystalline and Optimally BZO-Doped YBCO Targets

Thin YBa2Cu3O6+x (YBCO) films are prepared by pulsed laser deposition (PLD) from pure micro- (mu) and nanocrystalline (n) targets and a n-target optimally doped with 3.9 wt% BaZrO3 (BZO). Effects of target grain size and doping on superconducting (SC) properties of the films are investigated using a SQUID magnetometer up to 5 T and transport measurements in pulsed magnetic fiefds up to 30 T. In comparison with mu-films, clearly increased values of jc are observed in n-films. Optimal doping with BZO still increases this difference in high magnetic field range and lengthens the low-field plateau of jc(B) and increases the accommodation field B* which are typically around 110 mT, 160 mT and 500 mT at 5 K in mu-, n- and BZO-doped films, respectiveiy. Transport measurements at T > 65 K show significant improvement of irreversibiiity line (IRL) and jc(B) in films prepared from n-target and BZO-doped target. These improvements of the superconducting properties in high magnetic fiefds make pure and BZO-doped n-films attractive for high field applications.

Nonlinearity of strain and strain hysteresis in morphotropic LaSr-doped lead zirconate titanate under unipolar cycling with high electric fields

The strain behavior of morphotropic, 1La2Sr-doped lead zirconate titanate was investigated. Particular attention was paid to the electric field induced strain (S-E characteristic) in the high field range for morphotropic materials having different Zr∕Ti ratios. Unipolar electrical cycles with maximum fields between 1 and 3kV∕mm were applied to the poled materials under load-free conditions. Strain curves were recorded and the data were evaluated with respect to field dependence of strain and strain hysteresis. Nonlinearity of strain was detected for all materials. Depending on the composition, marked differences were found with respect to the types of nonlinearity. The corresponding data on strain hysteresis indicate that the nonlinearity was due to a loss generating strain mechanism. Therefore the nonlinearity was attributed to domain switching. A phenomenological model is suggested, which is based on the assumptions of a distribution of domain switching over a field range. This field range is characterized by the activation field parameter Ea, which is specific for each composition. The model is capable of explaining the types of nonlinearity in strain that have been observed.

Characterization of a ferromagnetic porous silicon-based Ni/Si nanocomposite with a novel strong high-field anisotropy

Ferromagnetic (Ni) filaments are embedded into a porous silicon template by an electrochemical deposition process. During cathodic deposition using NiCl 2 as electrolyte the channels of the meso-/macroporous silicon structure are filled with metallic Ni. The resulting nanocomposite system consists of silicon as base material as well as of implemented Ni-structures, especially of highly oriented Ni-wires perpendicular to the surface showing an exceptionally high aspect ratio (>300) and is of interest for applications in microtechnology. The length of the Ni-wires is in the range of a few tens of micrometers. Concomitant with the growth of wires, spheres or ellipsoidally shaped particles are formed during the Ni-filling procedure, whose spatial frequency and distribution become tunable. Structural investigations of this system, using SEM and EDX as well as investigations of the magnetic behaviour using SQUID-magnetometry, demonstrate the dependency of the magnetic properties on the filling status of the samples. The hysteresis loops in the low-field regime up to 500 Oe as well as magnetization curves in the high-field range of a few tesla display a strong magnetic anisotropy due to magnetic rearrangements. At fields around 5 T, a decline of the magnetization followed by a steep increase is observed. This magnetic field-induced anisotropy depends on the detailed growth of the Ni within the pores which can be controlled by the deposition process. It is governed by yet unknown antiferromagnetic exchange between the wires, and inherently connected with the shape of the magnetic nano-objects.

Development of a Current Fit Function for NbTi to be Used for Calculation of Persistent Current Induced Field Errors in the LHC Main Dipoles

A new fit function for the critical current density of superconducting NbTi cables for the LHC main dipoles is presented. Existing fit functions usually show a good matching of the very low field range, but produce a current density which is significantly too small for the intermediate and high field range. Consequently the multipole range measured at cold is only partially reproduced and loops from current cycling do not match. The presented function is used as input for the field quality calculation of a complete magnet cross-section including arbitrary current cycling and all hysteresis effects. This way allows to trace a so-called finger-print of the cable combination used in the LHC main bending magnets. The finger-print pattern is a consequence of the differences of the measured superconductor magnetization of cables from different manufacturers. The simulation results have been compared with measurements at cold obtained from LHC main dipoles and a very good agreement for low and intermediate field values could be observed

Martensitic Transformation in Ni–Mn–Ga Alloy Under High Magnetic Fields

An effect of magnetic field on the martensitic transformation (MT) temperature in ferromagnetic shape memory Ni 49.4 Mn 27.7 Ga 22.9 single crystal with the MT temperature 285 K is studied by measuring and theoretical treatment of the magnetization versus temperature dependencies in a wide range of the high magnetic fields higher than saturating one. In this way, a linear approximation of the field dependence of MT temperature was proved for the high-field range. The linear increase of transformation temperature is characterized by the slope of 3.5 x 10 -2 K/kOe. This value is essentially larger than the value 2 x 10 -2 K/kOe reported by Gonzales-Comas et al. [Phys. Rev. B 60 (1999) 7085-7090] for the Ni 49.5 Mn 25.4 Ga 25.1 alloy with a low MT temperature. This difference is in an agreement with the predictions of the Landau theory.

Nanomagnetic Ni-Array in Porous Silicon as a Possible Magnetic Field Sensor in the High Field Range up to 7 T

AbstractA highly doped n-type silicon wafer is anodized in an aqueous hydrofluoric acid solution to generate a porous silicon skeleton with a convenient morphology. After drying, the mesoporous sample is exposed to an electrodeposition process in which the pores are filled with the ferromagnetic metal Ni. Anodization and deposition of Ni lead to a ferromagnetic nanoscopic system which shows an interesting behaviour in the high field range (> 3 T). In addition to the expected low field switching below 500 Oe a second switching at fields of a few Tesla with a steep slope is also present. This feature with its extremely high sensitivity for changes of the external magnetic field gives rise to high magnetic field sensor applications based on a silicon technology.

Two-Mode Magnetic Switching of a Ferromagnetic Nanowire-Granule-Array Embedded in a Porous Silicon Matrix

AbstractA low-cost method has been developed to produce metallic, perpendicularly oriented ferromagnetic nanowires embedded in crystalline silicon. The mesoporous silicon structure consists of channels with a diameter of about 60 nm and about 30 μm in depth. The electrochemically prepared ferromagnetic nanosystem is a composition of granules and wires which leads to a peculiar magnetic behaviour. The interesting magnetic properties of this bimodal system are investigated by SQUID-magnetometry. The hysteresis loop shows two switching fields at different magnetic fields, one in the low field range and the other one in the high field range. The first switching field at about 500 Oe is due to the magnetization of the granules, the second switching field nearby 5 T (at room temperature) is caused by dipolar coupling of the nanowires which become single domain at high fields due to Bloch Wall motions. This promising ferromagnetic nanocomposite system is not only interesting in basic research but gives raise to a lot of silicon based applications like spin-injection devices and magnetic field sensors.