Session X23: Magnetic Field Effects: Experimental II

Investigating the field evolution of four-fold anisotropy in the basal plane of TmNi$_2$B$_2$C

The superconductor TmNi$_2$B$_2$C possesses a significant 4-fold in-plane anisotropy originating from the Fermi surface and possibly also from the superconducting pairing. However, unlike other members of the borocarbide superconductors, the anisotropy appears to decrease with increasing field, attributed to strong Pauli paramagnetic effects (PPE) and a vortex core expansion close to $H_{c2}$. We have investigated the field evolution of the four-fold anisotropy by small-angle neutron scattering (SANS) of the vortex lattice (VL), measuring several higher order Bragg peaks which allow a real space reconstruction of the VL field modulation. The measurements are possible due to the PPE which lead to a large field modulation (e.g. $\sim$~65 \% of the applied field at $0.2$~T). Our results present the first {\em direct} demonstration of the decreasing anisotropy and furthermore, allows this to be measured quantitatively. This provides an explanation for the reentrant square VL phase observed in TmNi$_2$B$_2$C.   

Vortex Lattice Anisotropy in Sr$_2$RuO$_4$ with $H \parallel a$

We have studied the vortex lattice (VL) in superconducting Sr$_2$RuO$_4$ using spin-polarized small-angle neutron scattering (SP-SANS) and with magnetic fields ($H$) close to, but not perfectly aligned with, the crystalline basal plane. In this configuration the VL possesses a large transverse field component, due to the large anisotropy of Sr$_2$RuO$_4$, which greatly increases the spin-flip SANS scattered intensity. Bragg reflections indicative of a highly distorted hexagonal VL were observed, and an analysis of the magnitude of the scattering vector yielded an anisotropy of $50 - 60$, roughly two times the $H_{c2}$ anisotropy. We discuss implications of this result in relation to theoretical predictions of Pauli paramagnetic effects in Sr$_2$RuO$_4$ with $H \perp c$ [K. Machida and M. Ichioka, Phys. Rev. B {\bf 77}, 184515 (2008)].   

Influence of intrinsic superconducting parameters on the vortex dynamics and critical currents of different superconductor materials

Vortex physics became a major field in condensed matter and statistical physics since the discovery of the oxide high temperature superconductors (HTS). Although the rich HTS vortex phenomenology arises from the large influence of thermal fluctuations, there is no hard boundary between HTS and conventional low temperature superconductors. The discovery of the iron-based superconductors provided a chance to study vortex matter in a new family of materials with broad ranges of superconductor transition temperature and crystalline anisotropy, where the small coherence length in some of them results in large fluctuation effects similar to those found in the oxide HTS. In this presentation, we will discuss the vortex dynamics of iron arsenide and oxide HTS by performing magnetization measurements of the critical current density ($J_{c})$ and flux creep rate ($S)$. We will analyze the different creep regimes and crossovers that appear in the different samples and their relations to intrinsic superconducting parameters.   

Electrostatic manipulation of magnetic flux quanta at the nanoscale

The electrostatic tuning of physical properties in materials offers significant potential in a large variety of systems. For example, the application of an electric field allows depressing or enhancing superconductivity in certain oxides. Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O7-x), we demonstrate here the nanoscale modulation of the superconducting condensate via ferroelectric field effects [1]. The ability to design the ferroelectric domain structure at will enables us to create nanoscale ``patterns'' of normal regions within the superconductor, in a reversible and modifiable way. This produces an energy landscape for magnetic flux quanta and, in turn, couples the local polarization in the ferroelectric to the local magnetic induction in the superconductor. This new form of magnetoelectric coupling allows the electrostatic manipulation of magnetic flux quanta. \\[4pt] [1] A. Crassous et al., Phys. Rev. Lett. in press (2011)   

Strong periodic flux pinning in oxygen-ion-irradiated high-T$_{C}$ superconductors

We used oxygen ion irradiation to transfer into high-T$_{C}$ superconducting thin films the nanoscale pattern of different types of masks (alumina [1] and [2] PMMA templates with ordered arrays of holes). This causes a nanoscale spatial modulation of superconductivity, and strongly affects the magneto-transport in the mixed-state. By tuning the irradiation dose and the array parameters, it is possible to engineer vortex energy landscapes sufficiently strong to govern flux dynamics. This is evidenced by a periodic series of strong magneto-resistance oscillations, the well-known fingerprint of periodic flux pinning. Interestingly, this irradiation technique allows tuning the \textit{geometry} and the \textit{strength} of the pinning potential wells at the nanoscale. This allows the observation of unusually strong matching effects at relatively high fields (up to several kOe). We show that the amplitude of the magneto-resistance oscillations is intimately connected with vortex channeling effects. [1] J.E. Villegas \textit{et al.} Nanotechnology\textbf{ 22}~075302~(2011). [2] I. Swiecicki \textit{et al.} submitted   

Single vortex manipulation in superconducting NdFeAsO$_{1-x}$F$_x$

Vortex pinning challenges have severely hampered attempts to incorporate cuprate high $T_c$ superconductors into technology. Understanding and improving the pinning of quantized magnetic vortices in high $T_c$ superconductors remains an important challenge. We use a homebuilt low temperature magnetic force microscope to image and manipulate individual vortices in single crystal NdFeAsO$_{1-x}$F$_x$ ($T_c$=50K). By exerting a large force on a single vortex we can deliberately depin it from its original position and permanently move it to a different, predetermined position. We can also drag the top of a single vortex for a short distance without permanently depinning its full length. By dragging individual vortices along different directions, we observe a 4-fold anisotropy of the dragging distance, with the easy-drag direction along the Fe-Fe axis. Our results shed light on the questions of anisotropy and pinning mechanisms in iron pnictides.   

Anisotropy and Vortex Pinning of Heavy Ion irradiated SmFeAsO$_{0.8}$F$_{0.15}$ and BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ Crystals

We report specific heat and magnetization measurements on SmFeAsO$_{0.8}$F$_{0.15}$ and BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ single crystals irradiated with high energy heavy ions of 1.4GeV Pb to dose matching fields up to 4 Tesla. We find a nearly one half reduction in the superconducting anisotropy and doubling of the irreversibility field in SmFeAsO$_{0.8}$F$_{0.15 }$after irradiation and virtually no change in the zero-field superconducting transition temperature. In both SmFeAsO$_{0.8}$F$_{0.15}$ and BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ crystals, we find a substantial increase in the critical current determined from SQUID and micro-Hall probe magnetization measurements. Pinning force analysis on proton and heavy ion irradiated pristine overdoped BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ crystals indicates presence of induced $\Delta $T$_{c}$-type pinning defects in these samples.   

High Dose Heavy-Ion Irradiation Effects on the Multiband Superconductor (Ba$_{0.6}$K$_{0.4})$Fe$_{2}$As$_{2}$

Optimal doped crystals of (Ba$_{0.6}$K$_{0.4})$Fe$_{2}$As$_{2 }$were irradiated with 1.4 GeV Pb ions to dose-matching fields ranging from 4 Tesla to 21 Tesla. Plan-view transmission electron microscopy shows creation of defects with diameters of 2 $\sim $ 5 nm. Post-irradiation characterization shows that the superconducting anisotropy is reduced to near unity, probably due to the increase in intra-band scattering. In addition, the critical current density $J_{C}$ determined from magnetization measurements shows systematic enhancement up to $\sim $5 MA/cm$^{2}$ at T=5K. We show that the decay of the critical current with magnetic field can be greatly mitigated with dense defects with approximately 20nm spacing produced by a dose matching field irradiation of 21T. Remarkably, the superconducting transition temperature remain unchanged for all matching field irradiation, suggesting that inter-band scattering due to non-magnetic impurity does not play a dominant role in pair-breaking.   

Angular dependence of magneto-transport properties in Ba$_{1-x}$K$_{x}$(FeAs)$_{2}$ and Ba (Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$single crystals

We find unexpectedly sharp minima in the angular dependence of the flux-flow resistance of a series of Ba$_{1-x}$K$_{x}$(FeAs)$_{2}$ and Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$crystals when the magnetic field is applied parallel to the FeAs-planes. These minima are too sharp to be accounted for by effective mass anisotropy. Furthermore, since the c-axis coherence length is substantially larger than the FeAs layer spacing, intrinsic pinning mechanisms are ruled out. However, high-resolution cross-sectional SEM reveal the presence of plate-like inclusions of 50 - 100 nm thickness, which in a natural way can account for the enhanced pinning for H $\vert \vert $ ab. On crystals that have been irradiated with heavy ions along the c-axis we observe additional resistance minima for H $\vert \vert $ c, indicating the correlated nature of the irradiation induced defects.   

Combination of strong natural and artificial pinning centers in Co-doped BaFe$_{2}$As$_{2}$ films

Studying the angular dependence of the critical current density (J$_{c})$ as a function of temperature in superconductors with complex pinning landscapes is very important from technical and fundamental points of view. The low anisotropy found in the BaFe$_{2}$As$_{2 }$(Ba122) family together with strong naturally grown pinning make Ba122 films very attractive. We present results on Ba122 superconducting films with naturally grown correlated defects with the addition of different amounts of random defects produced by consecutive irradiations with 3MeV protons. We analyze the changes of J$_{c}$, H$_{irr}$ and H$_{c2}$ as a function of field, angle and temperature. Irradiations produce a small decrease in critical temperature (T$_{c})$ of 0.5 K per 1x10$^{16}$ cm$^{-2 }$dose. After the irradiation, the pinning near the c-axis remains almost unchanged (except for the effects from the decrease of T$_{c})$. On the other hand, an increase of J$_{c}$ is observed for other field orientations indicating and stronger random pinning contribution, particularly at high fields.   

Highly Mobile In-plane Vortex Arrangement in SmFeAs(O,F)

We observed a strong enhancement of flux flow dissipation for current flowing along the inter-planar direction in single crystals of the iron pnictide high-T$_c$ superconductor SmFeAs(O,F) (T$_c \sim$ 50K) in high magnetic fields precisely aligned with the FeAs planes. The dissipation reaches significant fractions of the resistance in the normal state at all temperatures and fields, far below H$_{c2}||ab$, estimated to be well above 100T at low temperatures. Even slightest field misalignments from the FeAs planes ($<$0.1deg) restore the dissipation free state characterized by very high critical current densities ($\sim10^6 A/cm^2$) at low temperatures. We attribute this feature to vortices arranging themselves between the FeAs layers, accompanied by a reduced effectiveness of pinning. The qualitative features are reminiscent of the well-known lock-in effect in the cuprates, yet there are clear differences evident: The angular range of enhanced dissipation is reduced upon cooling in SmFeAs(O,F), whereas in the cuprates it significantly broadens as H$_{c1}$ increases at lower temperatures. Furthermore, the lock-in effect is most pronounced in strongly anisotropic materials, while SmFeAs(O,F) is moderately anisotropic ($\gamma \sim$ 6-8), becoming more isotropic at low temperature.   

Meissner holes in iron-based superconductors

Magnetic flux penetrates into a superconductor in the form of quantized vortices. This process is usually described by the Bean model, and the flux front forms a regular pattern reflecting the shape of the sample. However, a novel form of flux penetration accompanying wiggling fronts between vortices and antivortices has been observed in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ upon remagnetization [1]. Such a phenomenon is ascribed to the presence of special arrangements of vortices at the front accompanying flux free regions and excess current around it. The flux free region is called as `Mesissner hole'. We have performed extensive magneto-optical imagings of iron-based superconductor single crystals and found similar anomalous features for the first time in superconductors other than 123-type cuprates [2]. Implications of this finding will be discussed with possible origins of the anomalous vortex arrangements. [1] V. K. Vlasko-Vlasov \textit{et al}., Phys. Rev. B \textbf{56}, 5622 (1997). [2] S. Mohan, Y. Tsuchiya, Y. Nakajima, and T. Tamegai, Phys. Rev. B \textbf{84}, 18050X (2011).   

Thermo-magnetic stability of NbN films with controlled nano-granularity

The critical state in superconductors (SC) is metastable and can be destructed either by flux creep or by abrupt massive flux avalanches. The avalanches are associated with thermo-magnetic instability (TMI) of superconductors, which appears when the heat generated by the moving vortices is greater than the heat released into an environment. In that sense NbN films are known to be unstable, i.e. even the smallest increase of the external field may trigger a massive flux avalanche. Most theories developed to formulate the criteria of TMI operate in terms of intrinsic parameters of SC, such as heat capacity, critical current etc., and disregard the origins and the nature of the triggering mechanisms of the avalanches. We will present the most recent results of magneto-optical visualizations of flux dynamics in a series of NbN films with nano-scale disorder (ND) introduced in a well controlled fashion. We will demonstrate that not only do ND stipulate increase of the critical current but also promote correlated motion of large vortex bundles - micro jumps, which in turn trigger the macro avalanches.   

Quasi-1D intermittent flux dynamics in superconducting films

The stability of pinned vortex systems is constantly challenged in superconductors. In this work, magneto-optical imaging was used to reveal a new type of intermittent flux behavior in films of YBa$_{2}$Cu$_{3}$O$_{x}$. Films were grown on tilted NdGaO$_{3}$ substrates, where the terrace structure creates a high density of planar defects. The flux penetration along the terrace steps consists of numerous 1-dimensional avalanches, some starting at the film edge, some fully internal. In spite the vivid dynamics the flux front advances in accordance with the critical state model. Analysing more than 10000 avalanche events, we find a power-law size distribution and finite-size-scaling with the depth of the flux front as crossover length. The intermittent behaviour shows no threshold value in the applied field. These new characteristics largely contrast those of the thermo-magnetic avalanches observed in many superconducting films, and suggest that a different mechanism is responsible for the 1-dimensional avalanches.   

Investigation of the magnetic field angle dependence of resistance, irreversibility field, upper critical field and critical current density in DC sputtered Bi-2223 thin film

We measured resistivity and transport critical current density as a function of DC magnetic field and the angle between the surface of the film and the magnetic field on ex-situ annealed, c-axis oriented Bi-2223 thin films fabricated by DC sputtering method. Irreversibility field and upper critical field were determined from the resistivity vs. the applied magnetic field graph. It is observed that the superconducting properties of the films strongly depend on the direction and strength of the field. Penetration depths and coherence lengths were also determined from the irreversibility field and upper critical field versus temperature graphs, respectively. The anisotropic J$_{c}$ behavior of the film is found to be intrinsic. We provided a theoretical analysis of the obtained results in the framework of intrinsic pinning theory of superconductors. Microstructural properties of the produced films were investigated by XRD and SEM measurements. XRD patterns indicate that the films are c-axis oriented based on the prominent (00l) peaks. SEM images show needle-like grain structures dominate the surface morphology of the films.