Films In Magnetic Field Research Articles

Designing high-performance superconductors with nanoparticle inclusions: Comparisons to strong pinning theory

One of the most promising routes for achieving high critical currents in superconductors is to incorporate dispersed, non-superconducting nanoparticles to control the dissipative motion of vortices. However, these inclusions reduce the overall superconducting volume and can strain the interlaying superconducting matrix, which can detrimentally reduce Tc. Consequently, an optimal balance must be achieved between the nanoparticle density np and size d. Determining this balance requires garnering a better understanding of vortex–nanoparticle interactions, described by strong pinning theory. Here, we map the dependence of the critical current on nanoparticle size and density in (Y0.77, Gd0.23)Ba2Cu3O7−δ films in magnetic fields of up to 35 T and compare the trends to recent results from time-dependent Ginzburg–Landau simulations. We identify consistency between the field-dependent critical current Jc(B) and expectations from strong pinning theory. Specifically, we find that Jc ∝ B−α, where α decreases from 0.66 to 0.2 with increasing density of nanoparticles and increases roughly linearly with nanoparticle size d/ξ (normalized to the coherence length). At high fields, the critical current decays faster (∼B−1), suggesting that each nanoparticle has captured a vortex. When nanoparticles capture more than one vortex, a small, high-field peak is expected in Jc(B). Due to a spread in defect sizes, this novel peak effect remains unresolved here. Finally, we reveal that the dependence of the vortex creep rate S on nanoparticle size and density roughly mirrors that of α, and we compare our results to low-T nonlinearities in S(T) that are predicted by strong pinning theory.

Characterization of Superconducting Nbn, WSi and MoSi Ultra-Thin Films in Magnetic Field

The system detection efficiency of superconducting nanowire single-photon detectors ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SNSPD</i> ) has rapidly improved in recent years. In this study, we fabricated NbN, WSi and MoSi ultra-thin films, which are widely used for SNSPD fabrication, employing magnetron sputtering. We characterized the films in the presence of a magnetic field to study the upper critical magnetic field H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> and the anisotropy. According to the results obtained in our experiments, we calculated the coherence length and diffusion coefficient of these films. The knowledge of these parameters will guide the modelling and the interpretation of the different features observed in SNSPD made out of NbN, WSi and MoSi.

Two-Dimensional Coulomb Glass as a Model for Vortex Pinning in Superconducting Films

A glass model of vortex pinning in highly disordered thin superconducting films in magnetic fields $B \ll H_{c2}$ at low temperatures is proposed. Strong collective pinning of a vortex system realized in disordered superconductors that are close to the quantum phase transition to the insulating phase -- such as $\mathrm{In O}_x$, $\mathrm{Nb N}$, $\mathrm{Ti N}$, $\mathrm{Mo Ge}$, nano-granular aluminium, and others -- is considered theoretically for the first time. Utilizing the replica trick developed for the spin glass theory, we demonstrate that such vortex system is in non-ergodic state of glass type with large kinetic inductance per square $L_K$. Distribution function of local pinning energies is calculated, and it is shown that it possesses a wide gap, i.e. the probability to find a weakly pinned vortex is extremely low.

Improvement of anisotropy of superconducting properties in Y-rich YBa2Cu3Oy film in magnetic fields

In order to put the REBa2Cu3Oy superconductor into practical use, it is necessary to more isotropically improve the critical current density (Jc) with respect to an applied magnetic field angle. Therefore, artificial pinning centers (APCs) which are like a particle shape should be introduced into the superconductor. In this study, nano-particule APCs were introduced by fabricating Y1+xBa2Cu3Oy (x = 0 ∼ 1.0) thin films with excess Y composition, and effect of x, substrate temperature (Tss) and pulsed laser repetition frequency (fLs) on the isotropy of Jc in the magnetic field was evaluated. As a result, these conditions proved to have a strong influence on isotropic improvement of Jc in the magnetic field.

Evolution of the Hysteresis Loops of Iron Garnet Films upon Deep Layered Etching

Dependences are studied for the parameters of the hysteresis loops of iron garnet monocrystalline films on film thickness as a result of their layered etching by ion-beam sputtering with an oxygen ion beam. The coercive force of the films in magnetic fields corresponding to the motion of domain walls does not exceed 0.6 Oe as a result of layered etching through as much as 90% of a film’s thickness. This is important when using films in devices based on domain walls and other nanoscale spin structures.

Microstructure changes in radiochromic films due to magnetic field and radiation

To correlate the dose response and changes in microscopic structures of the radiochromic films exposed to the clinical magnetic field in the range 1.5-3 T with standard and flattening filter-free (FFF) photon beams. The radiochromic film was cut into 5 × 5 cm2 sheets/samples from one batch. These samples were exposed to a 1.5-T and/or 3-T B-fields from an MRI scanner using an abdominal sequence for 7 min before and after irradiation with 6 MV and/or 6 MV FFF beams. Films were placed in a reference condition at 5 cm depth in a solid water phantom and exposed up to 20 Gy. The sample orientation was maintained the same during exposure, readout, and scanning electron microscopic (SEM) analysis. The samples were scanned with an Epson Expression 11000XL in a 48-bit RGB color mode at 300 dpi with red channel. Scanned images were processed in Image J and red channel mean intensity values were recorded. The samples were then coated with 6 nm gold and imaged by SEM Teneo (5 kV, 13 pA) under 2000, 2500, and 3000 magnifications for texture analysis. The changes in the microstructure of the films in magnetic fields (1.5- and 3.0-T) were dose dependent. The orientation and granular size of samples at higher doses were altered compared to the controls. Needle-shaped structures of the active layer were longer and aligned for samples exposed to higher doses and magnetic field. However, no significant changes in optical density due to the presence of a magnetic field pre/postirradiation up to 20 Gy were observed. Fine structures of the film represent the polymerization characteristics that are affected by the radiation dose in the magnetic field. Upon exposure to radiation, diacetylene monomers undergo polymerization that forms longer chains with a temporal response. Even though this study did not notice any significant changes in optical density due to the presence of magnetic field, this should be studied in simultaneous application of the magnetic field during treatment in a dedicated MR-linac unit.

Multistep Accelerated Aging of Magnetic Field Sensors Based on Nanostructured La–Sr–Mn–O Thin Films

We present results of investigation of accelerated aging influence on electrical resistivity and magnetoresistance (MR) of nanostructured La-Sr-Mn-O films used for magnetic field sensors, which could be applied in advanced scientific and industrial devices. The 400-nm-thick manganite films were grown by metal organic chemical vapor deposition technique. To investigate the effectiveness of accelerated aging, the three methods were tested: 1) thermal treatment of the sensor samples at 100 °C temperature in Ar atmosphere when film surface was uncoated; 2) thermal treatment of the samples coated prior to treatment by polymer; and 3) multistep aging including both previous methods: thermal treatment of the samples with uncoated surface in Ar atmosphere and additional thermal treatment of the coated samples. The sensor samples were studied without and in pulsed magnetic field up to 20 T. To describe the underlying processes, the kinetics of resistivity change during accelerated aging was analyzed using a stretched exponent relaxation function. It was found that the best long-term stability of resistivity in time demonstrated samples treated by a multistep accelerated aging method. All three aging methods resulted in minor influence (<;1%) on MR of the films in magnetic fields up to 20 T.

Transport properties of heteroepitaxial films based on bismuth telluride in strong magnetic fields

The temperature and magnetic-field dependences of the galvanomagnetic properties of n-Bi2Te3 heteroepitaxial films in magnetic fields to 14 T at low temperatures are studied. It is shown that steps in the magnetic-field dependences of the quantum Hall effect and the plateau in the temperature dependences of the magnetoresistance of films are caused by topological surface states of Dirac fermions.

Pinning effects on hot-electron vortex flow instability in superconducting films

The hot-electron vortex flow instability in superconducting films in magnetic field B at substrate temperature T0 ≪ Tc is theoretically considered in the presence of pinning. The magnetic field dependences of the instability critical parameters (electric field E*, current density j*, resistivity ρ*, power density P* and vortex velocity v*) are derived for a cosine and a saw-tooth washboard pinning potential and compared with the results obtained earlier by M. Kunchur [Phys. Rev. Lett. 89 (2002) 137005] in absence of pinning. It is shown that the B-behavior of E*, j* and ρ* is monotonic, whereas the B-dependence of v* is quite different, namely dv*/dB may change its sign twice, as sometimes observed in experiments. The simplest heat balance equation for electrons in low-Tc superconducting films is considered within the framework of the two-fluid model. A theoretical analysis reveals that the instability critical temperature T* ≈ 5Tc/6 at T0 < T*/2 with T* being independent of B.

Superconductors with spin–orbit interactions

The effect of spin-orbit (SO) interaction on the formation of the critical states in thin superconducting films in magnetic field oriented along the film is investigated. Hereby, the case of interband pairing is considered. It was found that eight branches exist in the plane of two parameters [Formula: see text] determined by the value of magnetic field and SO interaction. Six modes leads to inhomogeneous states with different values of the impulse [Formula: see text]. Each state is doubly degenerate over direction of impulse [Formula: see text]. The parameter values at critical point are found for all eight branches in explicit form for zero temperature. The optimal two branches are estimated, corresponding to largest critical magnetic field value for given SO interaction.

Mechanical Stress in the Superconducting Film Under External Magnetic Field

In this paper, the mechanical response is studied for different superconducting films in magnetic field. When the superconductors are under the magnetic field, the electromagnetic body force will act on the sample. High mechanical stress can affect the mechanical and electromagnetic properties of the superconductors. Based on Maxwell's equations and the fast Fourier transform, the Lorentz force is obtained for the superconducting film during magnetization. We take into account the square and circular superconducting films with a finite-element method. In addition, the effects of notches and inclusion on the mechanical stress are presented. Numerical results indicate that square and circular films have similar stress distributions. In the same magnetic field, the stress along the axis is smaller in the superconducting film with notches or inclusion. Finally, the superconducting film with substrate is considered. Mechanical stability can be enhanced by the substrate.

Enhancement of the magnetocapacitance effect in an external electric field in La x Bi1-x FeO3 films

The goal of the study is to determine the value of magnetocapacitance under the substitution of lanthanum for bismuth in thin LaxBi1-xFeO3 films and to find out the effect of an external bias electric field on the magnetocapacitance effect. To solve this task, the dielectric permittivity, the magnetic permeability, and the loss tangent are measured in LaxBi1-xFeO3 films in magnetic fields of up to 8 kOe in the range of temperatures 100 K < T < 1000 K. Maxima of the permittivity and permeability at low temperatures and the dependence of the permeability on the prehistory of a sample are found. An increase in the magnetocapacitance due to the substitution of lanthanum for bismuth is observed. A giant enhancement of magnetocapacitance in an external electric bias field is revealed. These phenomena are attributed to the rearrangement of the domain structure.

Magneto-Conductance Effect of Natural Porphyrin Thin Films Fabricated by Spin Coated Method

We present magneto conductivity measurements in natural porphyrin thin films in magnetic fields of 100 Oe at room-temperature. The films used in this experiment were isolated from natural spirulina sp by chromatography procedure with n-hexane. Each layer fabricated by spin coater methods with rotation speed of 2500 rpm for 60 second is followed post heating at temperature of 60°C for 300 second. The procedure is repeated N times. Magneto-conductance effect is presented at room temperature with induce magnetic field in plane of films. The results show that ratio magneto-conductance decreases with the increase of voltage provided. The magneto-conductance ratio increase with the increase of N repetition number. Finally, both the results indicate that magneto-conductance effect could be realized due to annihilation of carrier charge.

Determinant for Self-Organization of BaMO&lt;sub&gt;3&lt;/sub&gt; Nanorods Included in Vapor-Phase-Grown &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\mbox{REBa}_{2}\mbox{Cu}_{3}\mbox{O}_{y}$&lt;/tex-math&gt;&lt;/inline-formula&gt; Films

BaMO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (BMO; M = Zr, Sn, and Hf) self-organizes into nanorod shape within REBCO films grown by vapor phase epitaxy. Controlling the number density of the BMO nanorods is effective for an improvement of critical current density of the REBCO films in magnetic fields. We focused on the growth mechanism of the BMO nanorods in this paper. BaHfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (BHO)-doped REBCO films were fabricated under various conditions such as volume fraction of BHO and substrate temperature. We checked the number density and diameter of the BHO nanorods. We found that the volume fraction of BHO increased the number density, whereas the diameter was not affected by the volume fraction so much. The low substrate temperature contributed to the high number density in the films. Additionally, we carried out the 2-D Monte Carlo simulations for the nucleation and the selforganization of the BMO. The REBCO and BMO nuclei occurred randomly. However, due to small amount of BMO relative to REBCO, it was difficult for the BMO nuclei to get fat. We changed the molar fraction of the BMO and the substrate temperature in the simulations and evaluated the number density and the diameter of the resulting BMO nanorods.

Laser induced spin precession in highly anisotropic granular L1 FePt

The dynamic magnetic properties of a highly anisotropic, granular L10 FePt thin film in magnetic fields up to 7 T are investigated using time-resolved magneto-optical Kerr effect measurements. We find that ultrashort laser pulses induce coherent spin precession in the granular FePt sample. Frequencies of spin precession up to over 400 GHz are observed, which are strongly field and temperature dependent. The high frequencies can be ascribed to the high value of the magnetocrystalline anisotropy constant Ku leading to large anisotropy fields Ha of up to 10.7 T at 170 K. A Gilbert damping parameter of α ∼ 0.1 was derived from the lifetimes of the oscillations.

Far-infrared electrodynamics of thin superconducting NbN film in magnetic fields

We studied a thin superconducting NbN film in magnetic fields up to 8 T above the zero-temperature limit by means of time-domain terahertz and scanning tunneling spectroscopies in order to understand the vortex response. Scanning tunneling spectroscopy was used to determine the optical gap and the upper critical field of the sample. The values obtained were subsequently used to fit the terahertz complex conductivity spectra in the magnetic field in the Faraday geometry above the zero-temperature limit. These spectra are best described in terms of the Coffey–Clem self-consistent solution of a modified London equation in the flux creep regime.

Superconducting Properties of BaHfO3-doped Nd1+xBa2-xCu3Oy Films Prepared by Alternating-targets Technique

In order to clarify the influence of RE/Ba substitution on superconducting properties on REBCO films in magnetic fields, we fabricated BHO-doped Nd1+xBa2-xCu3Oy films in which Nd composition were controlled by PLD method using alternating-targets technique. The Tc of NdBCO(x = 0.01, 0.08) films were about 92K. For the field applied parallel to the c-axis, a Jc and a characteristic magnetic field of the maximum pinning force of the NdBCO(x = 0.08) film increased compared with those of the NdBCO(x = 0.01) film. Additionally, a Jc of a BHO-doped NdBCO(x = 0.08) film improved for all applied angles of magnetic field in 1 T. The forms of Jc-θ curves for the BHO-NdBCO films with x = 0.01 and 0.08 were similar form. Therefore, it is considered that the reason of Jc increase in the BHO-doped NdBCO(x = 0.08) film is introduction of three-dimensional pinning centers of fine Nd/Ba solid solutions and the solid solution do not influence on the BHO growth.

Formation of ordered films of axially bridged aluminum phthalocyanine [(tBu)4PcAl]2O via magnetic field-induced reaction

The μ-(oxo)bis[tetra-tert-butylphthalocyaninato] aluminum(III) [(tBu)4PcAl]2O films with the crystallites oriented preferably in one direction were obtained via chemical transformation of tetra-tert-butylsubstituted chloroaluminum(III) phthalocyanine (tBu)4PcAlCl film upon its annealing in magnetic field. A comparative analysis of the influence of post-deposition annealing process without and under applied magnetic field of 1 T, on the orientation and morphology of (tBu)4PcAlCl and [(tBu)4PcAl]2O films, has been carried out by the methods of UV-vis, Infrared and Raman spectroscopies, XRD as well as atomic force microscopy. The formation of [(tBu)4PcAl]2O films with elongated crystallites having preferential orientation was observed upon heating of the films in magnetic field while annealing without magnetic field under the same conditions does not demonstrate any effect on the structure and morphology of these films. The reasons of the sensitivity of this reaction to the presence of such magnetic field is discussed and studied by electronic paramagnetic resonance spectroscopy.

Quantum capacitance of an ultrathin topological insulator film in a magnetic field

We present a theoretical study of the quantum magnetocapacitance of an ultrathin topological insulator film in an external magnetic field. The study is undertaken to investigate the interplay of the Zeeman interaction with the hybridization between the upper and lower surfaces of the thin film. Determining the density of states, we find that the electron-hole symmetry is broken when the Zeeman and hybridization energies are varied relative to each other. This leads to a change in the character of the magnetocapacitance at the charge neutrality point. We further show that in the presence of both Zeeman interaction and hybridization the magnetocapacitance exhibits beating at low and splitting of the Shubnikov de Haas oscillations at high perpendicular magnetic field. In addition, we address the crossover from perpendicular to parallel magnetic field and find consistency with recent experimental data.

Flux pinning properties of YBCO films with nano-particles by TFA-MOD method

Nano-particles were doped into YBCO films as pinning centers by a metal organic deposition (MOD) method using trifluoroacetates. Two types of initial solution with a cation ratio of Y: Ba: Cu = 1: 1.5: 3 were prepared; one with the dispersion of SnO2 particles with the size of 15-25 nm and the other one with the dispersion of smaller ZrO2 particles with the size of under 8 nm, then the solution was spin-coated on CeO2/Gd2Zr2O7/Hastelloy substrates. The coated films were calcined at 430 °C in oxygen atmosphere and crystallized at 780 °C in low oxygen atmosphere. From the results of X-ray diffraction analysis (XRD), peaks of BaSnO3 were observed clearly in the YBCO film by the starting solution with SnO2. On the other hands, little peaks corresponding to BaZrO3 were observed in the film by the solution with ZrO2. Many CuO segregations were recognized at the surface of SnO2 doped YBCO film in comparison to the YBCO film with ZrO2 doping. From these results, it is indicated that most of SnO2 particles in precursors are react with Ba during heating. Critical current density ( J C) of the YBCO films by both solutions showed higher performance than that of pure YBCO film in magnetic fields.