We report on isofield magnetic relaxation data on a single crystal of Ba 1−x K x Fe 2 As 2 with superconducting transition temperature T c = 32.7 K which exhibit the so-called fish-tail effect. A surface map of the superconducting transition temperature shows that the superconducting properties are close to homogeneous across the sample. Magnetic relaxation data, M͑t͒, was used to obtain the activation energy U͑M͒ in order to study different vortex-dynamics regimes. Results of this analysis along with time-dependent measurements as a function of field and temperature extended to the reversible region of some M͑H͒ curves demonstrate that the irreversibility as well the second magnetization peak position, H p ͑T͒, are time dependent and controlled by plastic motion of the vortex state. In the region delimited by a characteristic field Hon

Electromagnetic metamaterials 1 are a class of materials which have been artificially structured on a subwavelength scale. They are currently the focus of a great deal of interest because they allow access to previously unrealisable properties like a negative refractive index 2 . Most metamaterial designs have so far been based on resonant elements, like split rings 3 , and research has concentrated on microwave frequencies and above. In this work, we present the first experimental realisation of a non-resonant metamaterial designed to operate at zero frequency. Our samples are based on a recently-proposed template 4 for an anisotropic magnetic metamaterial consisting of an array of superconducting plates. Magnetometry experiments show a strong, adjustable diamagnetic response when a field is applied perpendicular to the plates. We have calculated the corresponding effective permeability, which agrees well with theoretical predictions. Applications for this metamaterial may include non-intrusive screening of weak DC magnetic fields. The first metamaterials 3,5 were designed to operate at microwave frequencies. Since then, while there has been some research on radio-frequency metamaterials 6 , most of the research effort has been focused on higher frequencies: technologically-important microwaves or visible light 7 . The low-frequency end of the spectrum has remained relatively unexplored. In addition, the majority of metamaterials devised to date consist of an arrangement of resonant components. There is a good reason for this: the response of a resonator varies greatly as a function of the frequency at which it is being driven. Close to the resonant frequency, the amplitude of the response can be very large, while the phase changes. The range of available values of the response function, or susceptibility, is therefore very wide. One of the crowning achievements of (and driving forces behind) metamaterials research is the realization of a negative refractive index 2,8 , and a simple argument shows that this cannot be achieved without relying on resonant structures. However, the price of working close to the resonant frequency is that losses and frequency dispersion are greatest here. When a negative response is not required then a non-resonant structure is advantageous. Another recent development means that there is new demand for metamaterials with nonnegative anisotropic parameters. Transformation optics 9 is a design paradigm that allows a new level of control over electromagnetic fields. For a given design, it provides a recipe for the material parameters as a function of position. The parameters generated in this way are always non-negative and anisotropic. A spectacular demonstration of the technique was provided by the construction of an electromagnetic cloak 10 using metamaterials. The interior of the cloak is shielded from microwaves with minimal disruption to the exterior fields.

Citation: MOORE, J.D. ... et al, 2009. Reducing extrinsic hysteresis in firstorder la (Fe,Co,Si)13 magnetocaloric systems. Applied Physics Letters, 95 (25)

Here we use global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygendeficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around individual particles, and second phase neodymium oxide which may be responsible for the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 10 3 A/cm 2 . From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 microns it is clear that on smaller scales, there is better current connectivity with a critical current density of the order of 5 x 10 4 A/cm 2 . We find enhanced flux creep around the second peak anomaly in the magnetisation curve and an irreversibility line significantly below H c2 (T) as determined by ac calorimetry.

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