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.

Using a double axis vibrating sample magnetometer, we have made detailed magnetic measurements of the lower critical field Hc1 for fields parallel to the two crystallographic directions of MgB2 single crystals. Additionally, using a novel Hall probe magnetometer we have measured high precision magnetization loops, from which we directly determine the upper critical field Hc2 for both field orientations. Our results suggest that Hc1 is much larger than most previous estimates and that consequently the Ginzburg–Landau parameter κ is very low (less than 5). We find the anisotropy parameter γ ∼ 2, independent of temperature over the measured range.

A new method of utilizing a commercial silicon nitride membrane calorimeter to measure the latent heat at a first order phase transition is presented. The method is a direct measurement of the thermoelectric voltage jump induced by the latent heat, in a thermally isolated system ideally suited for single crystal and small microgram samples. We show that when combined with the ac calorimetry technique previously developed, the resultant thermal measurement capabilities are extremely powerful. We demonstrate the applicability of the combined method with measurements on a 100 microm size fragment of CoMnSi exhibiting a sizable magnetocaloric effect near room temperature, and obtain good agreement with previously reported values on bulk samples.

Here we examine the constituent components that make a magnetocaloric material attractive for application. The field-temperature phase diagram is studied and using calorimetry, the 1 st and 2 nd order components of the magnetic field driven magneto-structural phase transition in We identify 262K as a tricritical point and above this temperature T crit the transition shows only continuous, 2 nd order characteristics. Hall probe imaging that has a five micron pixel resolution is then used to study the striking differences in the spatial evolution of the transition above and belowT crit . We demonstrate that the hysteresis of the transition is linearly related to the magnitude of the latent heat; an observation that has important implications for the use of this and other 1 st order systems for application as magnetic refrigerants.

Here we study the calorimetric and magnetic behaviour of melt-spun LaFe 11.6 Si 1.4 , a potential magnetic refrigerant material system that exhibits the rare combination of a large entropy change and low thermal and magnetic field hysteresis. We are able to separate the calorimetric contribution from latent heat and changes in equilibrium heat capacity explicitly by using two separate calorimetric probes. The heat capacity of this sample exhibits significant changes of the order of 500-1000JK-1 kg-1 in response to magnetic field that results in large changes in entropy. The different contributions to entropy change from latent heat and heat capacity are shown to evolve as the material is field driven through its itinerant metamagnetic transition. We demonstrate explicitly that in the melt spun sample studied here, the majority of the total entropy change comes from the equilibrium change of heat capacity.

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We review recent measurements of upper (Hc2) and lower (Hc1) critical fields in clean single crystals of MgB2, and their anisotropies between the two principal crystallographic directions. Such crystals are far into the ‘clean limit’ of type II superconductivity, and indeed for fields applied in the c-direction, the Ginzburg–Landau parameter κ is only about 3, just large enough for type II behaviour. Because μ0 Hc2 is so low, about 3 T for fields in the c-direction, MgB2 has to be modified for it to become useful for high-current applications. It should be possible to increase Hc2 by the introduction of strong electron scattering (but because of the electronic structure and the double gap that results, the scatterers will have to be chosen carefully). In addition, pinning defects on a scale of a few nm will have to be engineered in order to enhance the critical current density at high fields.

We present a study of scanning Hall probe imaging on a bulk sample of the prototype giant magnetocaloric material Gd 5 Ge 4 across the magnetic field-induced antiferromagnetic to ferromagnetic and the reverse ferromagnetic to antiferromagnetic transitions at 35 K. We confirm that the sample presents phase coexistence of both antiferromagnetic and ferromagnetic phases through the crossover region in the field-temperature plane indicative of the first order nature of the phase transition. By studying the features of the local magnetizationfield loops corresponding to the pixel size of 30 m ϫ 30 m across the entire image region we show that the two magnetic transitions are not spatially symmetric and, therefore, are not governed entirely by static inhomogeneity. Our work adds further support to the idea that random strain fields play a considerable role in defining the magnetic behavior across the field driven transition.