Magnetic Flux Avalanches Research Articles

Laser nanostructured metasurfaces in Nb superconducting thin films

Superconducting Nb thin films have been nanostructured by means of a femtosecond UV laser. Laser induced periodic surface structures (LIPSS) with lateral modulation of ≈250nm and depth smaller than amplitude (≲20nm from crest to trough) are obtained for optimized laser scanning conditions over the film surface, i.e. power, frequency, scanning speed and polarization. This provides a fast and scalable procedure of surface control at the nanoscale. In thin films, control over the kinetics of the LIPSS formation process has been crucial. Untreated and laser-patterned samples have been characterized by electron and atomic force microscopy as well as by local and global magnetometry. The superconducting properties reveal anisotropic behavior in accordance with the observed topography. The imprinted LIPSS define channels for anisotropic current flow and flux penetration. At low temperatures, magnetic flux avalanches are promoted by the increased critical current density, though flux tends to be channeled along the LIPSS. In general, directional flux penetration is observed, being a useful feature in fluxonic devices. Scalability allows us to pattern areas of the order of cm2/min.

Catastrophic magnetic flux avalanches in NbTiN superconducting resonators

Macroscopic superconducting components are an important building block of various quantum circuits. Since several of the envisioned applications require exposure to magnetic fields, it is of utmost importance to explore the impact of magnetic fields on their performance. Here we explore the complex pattern of magnetic field penetration and identify its impact on the resonance frequency of NbTiN superconducting resonators by combining magneto-optical imaging and high-frequency measurements. At temperatures below approximately half of the superconducting critical temperature, the development of magnetic flux avalanches manifests itself as a noisy response in the field-dependent resonance frequency. Magneto-optical imaging reveals different regimes and distinguishes the impact of avalanches in the ground plane and resonator. Our findings demonstrate that superconducting resonators represent a valuable tool to investigate magnetic flux dynamics. Moreover, the current blooming of niobium-based superconducting radio-frequency devices makes this report timely by unveiling the severe implications of magnetic flux dynamics.

Tunable domino effect of thermomagnetic instabilities in superconducting films with multiply-connected topological structures

Topology is a crucial ingredient for understanding the physical properties of superconductors. Magnetic field crowds to adopt the form of topologically-protected quantum flux lines which can lose this property when moving at high velocities. These extreme conditions can be realized when superconductors undergo a thermomagnetic instability for which the sample topology come also into play. In this work, utilizing the magneto-optical imaging technique, we experimentally study magnetic flux avalanches in superconducting films with multiply-connected geometries, including single and double rings. We observe a domino effect in which avalanches triggered at the outer ring, stimulate avalanches at the inner ring thus impairing the expected magnetic shielding resulting from the outer ring and gap. We implement numerical simulations in order to gain more insight into the underlying physical mechanism and demonstrate that such event is not caused by the heat conduction, but mainly attributed to the local current distribution variation near the preceding flux avalanche in the outer ring, which in turn has a ripple effect on the local magnetic field profile in the gap. Furthermore, we find that the domino effect of thermomagnetic instabilities can be switched on/off by the environmental temperature and the gap width between the concentric rings. These findings provide new insights on the thermomagnetic instability in superconducting devices with complex topological structures, such as the superconductor–insulator–superconductor multilayer structures of superconducting radio-frequency cavities.

Tunable magnetic flux avalanches triggered by a focalized laser spot

Magnetic flux avalanches caused by thermomagnetic instabilities are a common phenomenon occurring in type II superconducting films. The unpredictability of these catastrophic events threaten the application of superconducting thin film equipment, such as high-temperature superconducting magnets. In the present work, through the fast Fourier transform method, we numerically investigate artificially triggered flux avalanches in superconducting films by a focalized laser, unveiling new features beyond those associated to naturally occurring avalanches. The numerical modeling is validated by reproducing previous experimental results. We investigate the effects of laser irradiation on the nucleation and evolution of flux avalanches for different cases, namely varying the laser irradiation position, laser power, laser-spot size, ramping rate of applied magnetic field and working temperature. We find that the laser irradiation can control and guide the position of flux avalanches at applied magnetic fields with small ramping rate, while similar guidance effect cannot be observed at high ramping rates. We demonstrate that such phenomenon can be tuned by the environmental temperature, and the underlying physical mechanism can be ascribed to current crowding and local heating around the laser spot. Furthermore, by considering a pair of laser spots, we observe two possible scenarios depending on the laser power, (i) single flux avalanche triggered at one of the laser spots and (ii) double flux avalanches triggered at both laser spots.

Enhancement of critical current density and strong vortex pinning in high entropy alloy superconductor Ta[formula omitted]Nb[formula omitted]Hf[formula omitted]Zr[formula omitted]Ti[formula omitted] synthesized by spark plasma sintering

We synthesized the high entropy alloy (HEA) Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound by planetary ball milling and the spark plasma sintering (SPS) method. The sintered HEA sample had a BCC crystal structure with small amount of secondary phases. The superconducting phase transition is observed in the electrical resistivity (Tc∼ 7.8 K) and magnetic susceptibility (Tc∼ 7.6 K) measurements, and the Tc was similar to that of an arc-melted sample. The zero-temperature limit of the upper critical magnetic fields Hc2(0)= 10.5 T and coherence length ξ = 5.66 nm were slightly decreased or comparable to the arc-melted HEA sample (Hc2(0)= 12.05 T and ξ= 5.23 nm). The field-dependent isothermal magnetization hysteresis M(H) exhibited the typical type- 2 superconducting behavior with magnetic flux avalanches in the low magnetic field region. The calculated critical current densities were significantly increased by 286 % (Jc= 32,606 A cm−2 at 2 K) ∼ 687 % (Jc= 73,200 A cm−2 at 4 K) compared to the arc-melted HEA sample (Jc= 11,401 A cm−2 at T= 2 K). The enhanced Jc of the sintered HEA sample is caused by enhancement of the pinning force due to point pinning as well as surface pinning effects. The vortex relaxation measurements showed stable remanent magnetization for over 10,000 s after magnetic fields were turned off, which is unlike conventional sueprconducting vortex relaxation behavior. The strong pinning force in the SPS sintered Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound is beneficial for practical applications because of the significantly enhanced critical current density Jc with stable remanent magnetization Mrem(t).

Metamorphosis of discontinuity lines and rectification of magnetic flux avalanches in the presence of noncentrosymmetric pinning forces

Considering a noncentrosymmetric pinning texture composed of a square array of triangular holes, the magnetic flux penetration and expulsion are investigated experimentally and theoretically. A direct visualization of the magnetic landscape obtained using a magneto-optical technique on a Nb film is complemented by a multiscale numerical modeling. This combined approach allows the magnetic flux dynamics to be identified from the single flux quantum limit up to the macroscopic electromagnetic response. Within the theoretical framework provided by time-dependent Ginzburg-Landau simulations, an estimation of the in-plane current anisotropy is obtained and its dependence with the radius of the curvature of hole vertices is addressed. These simulations show that current crowding plays an important role in channeling the flux motion, favoring hole-to-hole flux hopping rather than promoting interstitial flux displacement in between the holes. The resulting anisotropy of the critical current density gives rise to a distinct pattern of discontinuity lines for increasing and decreasing applied magnetic fields, in sharp contrast to the invariable patterns reported for centrosymmetric pinning potentials. This observation is partially accounted for by the rectification effect, as demonstrated by finite-element modeling. At low temperatures, where magnetic field penetration is dominated by thermomagnetic instabilities, highly directional magnetic flux avalanches with a fingerlike shape are observed to propagate along the easy axis of the pinning potential. This morphology is reproduced by numerical simulations. Our findings demonstrate that anisotropic pinning landscapes and, in particular, ratchet potentials produce subtle modifications to the critical state field profile that are reflected in the distribution of discontinuity lines.

Selective triggering of magnetic flux avalanches by an edge indentation

We numerically investigate the effect of an edge indentation on the threshold field of thermomagnetic instabilities in superconducting films subjected to a ramping magnetic field, applied perpendicular to the plane of the film. In particular, we are able to address the question on whether edge indentations promote magnetic flux avalanches. For the magnetic field-independent critical current density model, the triggering of the first magnetic flux avalanche systematically occurs at the edge indentation. In contrast to that, for the more realistic field-dependent critical current density model the first flux avalanche can take place either at or away from the indentation. This selective triggering of magnetic flux avalanches is shown to result from two effects. Namely, (i) the variation of the threshold magnetic field for the first flux avalanche triggered at the indentation and (ii) the reduction of the critical current density by large local magnetic fields at the tip of the indentation which translates in a lower power density dissipated near the tip. We demonstrate that this interplay can be tuned by varying the indentation size, ramp rate of applied field ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{H}}_{a}$, and working temperature ${T}_{0}$. We build up a phase diagram in the ${\ensuremath{\mu}}_{0}{\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{H}}_{a}\ensuremath{-}{T}_{0}$ plane with well-defined boundaries separating three distinct regimes of thermomagnetic instability.

Magnetic flux avalanches in Nb/NbN thin films

Technological applications of NbN thin films may be threatened by the development of magnetic flux avalanches of thermomagnetic origin appearing in a large portion of the superconducting phase. In this work, we describe an approach to substantially suppress the magnetic flux avalanche regime, without compromising the upper critical field. This procedure consists of depositing a thin Nb layer before the reactive deposition of NbN, thus forming a bi-layered system. We use ac susceptibility and dc magnetometry to characterize both the single-layer films, Nb and NbN, and the bi-layered specimen, as well as calibrated magneto-optical imaging to map the instability regime of the studied samples. Magnetic flux imaging reveals interesting features of the dendritic flux avalanches in the bi-layer system, including halo-like patterns and crossing avalanches.

Numerical simulation on the flux avalanche behaviors of microstructured superconducting thin films

Controlling and suppressing the propagation of magnetic flux avalanches is an important issue for the application of type-II superconductors. The effects of engineered pinning centers (antidots) on the guidance of flux avalanche propagation paths in type-II superconducting thin films are numerically investigated by solving the coupled nonlinear Maxwell's equations and the thermal diffusion equations. The field dependence of critical current density is considered in the simulation in this paper. Dynamic propagations of the thermomagnetic avalanches within the superconducting films patterned with different arrangements of antidots (like random, periodic square, and conformal mapping arrays) are presented. We reveal that presence of the antidots significantly modifies the propagation paths of the avalanches. The flux avalanche patterns of the superconducting films change with the variation of the arrangements of antidots. The patterned antidots in the form of conformal mapping arrays within the superconducting film exhibit strong guidance to the thermomagnetic avalanches. In addition, introducing the antidots in the form of conformal mapping arrays into the superconducting film can effectively lower the magnetic flux jump sizes.

Modeling the Effect of Thermal Field in Formation of Magnetic Flux Avalanches in Hard Superconductors

Magnetothermal instabilities are one of the uncharacteristic phenomena of interest in conventional type-II as well as in high-T c superconductors. Historically, some authors undertake to analyze the nature and origin of the magnetothermal instabilities of the critical state and flux jumps phenomena in superconductors in the light of theoretical and experimental results (Wipf, Phys. Rev. 161:404, 1967; Wipf, Cryogenics 31:936, 1992; Lee at al., J. Appl. Phys. 107:013902, 2010) [1, 2]. In contrast to previous reported works which had studied the magnetic flux avalanches in hard superconductors postulating an instability criterion, in this paper, we report the development of a theoretical model for describing the magnetization curves of type-II superconductors considering the fact that the jumps should arise naturally, i.e., without the need to impose a priori conditions of instability. The model developed is applied to make numerical calculations in order to get magnetization curves of MgB2 as well as to calculate and analyze the impact of changes in the thermal field and functions that describe the critical current. We investigate the effect of initial magnetic state on generation of magnetic flux avalanches. The initial state determines the evolution of the magnetization curves and their jumps. The results obtained with the application of this model allow us to reproduce the experimental conditions with greater accuracy than that achieved with models based on unstable conditions; further, the model predicts the impact of initial nonzero fields in the formation of avalanches.

Thermo-magnetic stability of superconducting films controlled by nano-morphology

Appearance of dendritic magnetic flux avalanches in superconducting films, which are associated with thermo-magnetic instability (TMI), very often indicates serious limitations for the ultimate performance of superconducting devices made of type-II superconducting thin films. We demonstrate that the stability can be controlled by a thorough adjustment of samples morphology at nano-scale, which affects internal material parameters. By this, the metal coating, commonly used as for stabilization, becomes redundant. Most importantly, we directly show by the mean of magneto-optical imaging that introduction of nano-scaled disorder dramatically changes the mode of magnetic flux propagation in the superconductors, from uniform motion of individual vortices to correlated jumps of relatively large vortex bundles, revealing the triggering mechanism of TMI.

The avalanche process in gold covered MgB2 films

Below a certain threshold temperature, the critical state of superconducting thin films can become unstable, which results in large magnetic flux jumps, called flux avalanches. The phenomenon of magnetic flux avalanches is strongly connected to the thermal and electrical conductivity in the material. Enhancing both by metallic cover layers on the superconducting film can suppress the magnetic avalanches. It was found that it is feasible to subdivide the avalanche process into a formation and a propagation step. In this work we want to address the question regarding the stage at which the metallic cover layer influences the avalanche mechanism. The investigations are carried out on thin MgB2 films with an inhomogeneous current density distribution, where both the formation and propagation of magnetic flux avalanches are highly supported. Evaporating a gold cover layer on top of the inhomogeneous MgB2 films leads to a suppression of the instabilities in this case. Magnetization measurements and magneto-optical imaging are used to observe the instabilities in pure MgB2 films and MgB2 films covered with gold layers. We investigate how the two steps of the avalanche process, nucleation and propagation, are influenced by the additional gold layer. We find a particular influence on the initial phase of the avalanche formation.

On the role of pre-existing defects and magnetic flux avalanches in the degradation of YBa2Cu3O7–x coated conductors by quenching

YBa2Cu3O7–x (YBCO) coated conductors are emerging as an important option for magnets for energy systems and experimental science. One of the remaining challenges for YBCO superconducting magnets is quench protection, i.e. ensuring that the YBCO is not damaged due to a fault condition. One key issue is understanding the underlying causes of degradation during a quench. Here, the microstructure of a quenched, degraded sampled is compared to that of an unquenched control sample. To facilitate microstructural analysis of the YBCO surface, the Cu stabilizer and Ag cap layer were removed by etching. Reactions between the Cu etchant and YBCO proved to be a signature of Ag/YBCO delamination. Two types of pre-existing defects were identified as initiation points of degradation. Defects on the conductor edge resulting in delaminated Ag lead to dendritic flux avalanches and high local heating, which cause further Ag delamination. This self-propagating effect results in dendritic Ag delamination, which is seen through etchant–YBCO reactions. Defects within the YBCO layer result in breaches in the protective Ag layer such that Cu etchant penetrates and reacts with the YBCO. Energy-dispersive X-ray spectroscopy analysis showed similar reactions as in the edge degradation but also showed pure Ag particles, which indicates that the local temperature was sufficient to cause localized Ag melting.

Fine Structure of Thermal Runaway Process in the V3Si Single–crystal Superconductor as a Result of Pinning Center Response

Abstract For the first time, time-dependent fine structure of a huge magnetic flux avalanches, Φ ( t ), going out from good crystalline quality superconducting disc as a result of thermomagnetic avalanche, was observed. It is shown that some of thermomagnetic avalanches consist of a large number of very short (∼10 μs) microavalanches and that each of the microavalanche has its own structure. The role of microstructure of pinning centers is discussed assuming that microavalanches are connected with jumps of flux bundles. Methodically, it may be a new facility for pinning centers investigation. The role of cubic-to-tetragonal transformation of a crystal structure of the investigated sample in appearance of pinning centers is considered.

Stability of the current-carrying state in nonhomogeneous MgB2films

Superconducting thin films can get unstable against magnetic flux jumping. At low temperatures, the probability of an unstable critical state increases since a high critical current density leads to large gradients in the flux density distribution. This phenomenon is often referred to as magnetic flux avalanches. The formation and propagation of these avalanches can be extensively described using a thermomagnetic model. The description of the avalanche process gets more complex when the superconducting film is not homogeneous. Quantitative magneto-optical imaging at tailored MgB${}_{2}$ films revealed that the stability of the current-carrying state is substantially reduced in inhomogeneous superconductors. This is closely related to flux focusing and increased electric fields.

The formation and propagation of flux avalanches in tailored MgB2 films

The applications of superconducting magnesium diboride are substantially limited by the presence of magnetic flux avalanches at low temperatures. Here, quickly moving magnetic vortices create large amounts of heat and magnetic noise. Such avalanches can be suppressed by evaporating metal layers to the surface of the superconductor, which acts both as a heat sink and as an electromagnetic drag by induced eddy currents. We show that it is necessary to distinguish between the mechanisms that are responsible for the formation and the propagation of avalanches. A high critical current favors avalanche formation but avalanche propagation is suppressed. The diverse consequences for creation and propagation explain the preference of avalanches for inhomogeneous superconductors.

Statistics of avalanches in the self-organized criticality state of a Josephson junction

Magnetic flux avalanches in Josephson junctions that include superconductor-insulator-superconductor (SIS) tunnel junctions and are magnetized at temperatures lower than approximately 5 K have been studied in detail. Avalanches are of stochastic character and appear when the magnetic field penetration depth λ into a junction becomes equal to the length a of the Josephson junction with a decrease in the temperature. The statistical properties of such avalanches are presented. The size distribution of the avalanches is a power law with a negative noninteger exponent about unity, indicating the self-organized criticality state. The self-organized criticality state is not observed in Josephson junctions with a superconductor-normal metal-superconductor (SNS) junction.

Chapter 7 Flux avalanches and magnetic asymmetry in Josephson arrays

Dependence of the magnetic moment of square arrays of Josephson junctions on a dc magnetic field and temperature has been studied experimentally. The arrays had junctions of SIS and SNS type. In SIS arrays, magnetic flux avalanches were observed at low temperatures. Statistical size distribution of magnetic avalanches is a power function with a crossover (the exponent changes from −0.7 for small sizes of avalanches to −6 for large sizes). The spectrum of avalanches at a linear development of the field is a power function of frequency. Such a behavior of avalanches is interpreted as a manifestation of the self-organized criticality. In SNS arrays, no avalanches are developed because of the high dissipation in junctions, but a considerable asymmetry of magnetic dynamics was found.

Magnetic moment of square SIS Josephson arrays: Self-organized criticality

The temperature-and magnetic-field dependences of the magnetic moment of square Josephson arrays with SIS-type junctions are studied experimentally. Two temperature regions are observed with different types of magnetization curves. Magnetic flux avalanches are detected in the low-temperature region. Statistical analysis of avalanche amplitudes A shows that their size distribution varies in accordance with the power law P∝An with crossover, when exponent n varies from n=−0.7 for small avalanches to n=−6 for large avalanches, while the frequency spectrum varies in accordance with the law 1/fα. Such behavior is interpreted as a manifestation of self-organized criticality.

Universal phenomenon of self-organized criticality in magnetic flux dynamics

Magnetization curves of square arrays of Josephson junctions of two basic types were investigated: superconductor–insulator–superconductor (SIS) and superconductor–normal metal–superconductor (SNS). Magnetic flux avalanches were observed in SIS arrays. A statistical analysis of flux avalanches showed that their size distribution can be described by a power law with a crossover where the exponent n varies from −1.2 for small avalanches to −3.5 for the large ones. Such a behavior of avalanches is interpreted as the self-organized criticality (SOC) manifestation. In SNS arrays, the flux avalanches were not observed, but a considerable asymmetry of a hysteresis curve was revealed.