Hexapod Hall scanner for high-resolution large area magnetic imaging.

We demonstrate a six-axis scanning imaging apparatus using piezo bending actuators with a large scan range. The six axes of motion of the bending actuators together with the coupling mechanism to the translation stage allow complete control of the sensor position and orientation over the scanning surface, which is ideal for the use of planar sensors such as Hall devices. In particular, the design allows for in situ correction of the probe tilt angle so that the sensor distance to sample surface can be minimized. We investigate the impact of this alignment on the quality of the measured data using an InSb Hall sensor and a magnetic sample. We also demonstrate a synchronous commutation setup that can greatly enhance the magnetic image by reducing the Hall signal offset.

First order magnetic transition in doped CeFe2 alloys: phase coexistence and metastability.

First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped CeFe2 alloys is studied with the micro-Hall probe technique. Clear visual evidence of magnetic phase coexistence on micrometer scales and the evolution of this phase coexistence as a function of temperature, magnetic field, and time across the first order FM-AFM transition is presented. Such phase coexistence and metastability arise as a natural consequence of an intrinsic disorder-influenced first order transition. The generality of these phenomena involving other classes of materials is discussed.

Enhancement of the high-magnetic-field critical current density of superconducting MgB2 by proton irradiation.

Magnesium diboride, MgB2, has a relatively high superconducting transition temperature, placing it between the families of low- and high-temperature (copper oxide based) superconductors. Supercurrent flow in MgB2 is unhindered by grain boundaries, making it potentially attractive for technological applications in the temperature range 20-30 K. But in the bulk material, the critical current density (Jc) drops rapidly with increasing magnetic field strength. The magnitude and field dependence of the critical current are related to the presence of structural defects that can 'pin' the quantized magnetic vortices that permeate the material, and a lack of natural defects in MgB2 may be responsible for the rapid decline of Jc with increasing field strength. Here we show that modest levels of atomic disorder induced by proton irradiation enhance the pinning of vortices, thereby significantly increasing Jc at high field strengths. We anticipate that either chemical doping or mechanical processing should generate similar levels of disorder, and so achieve performance that is technologically attractive in an economically viable way.

Vortex dynamics in superconducting MgB2 and prospects for applications.

The recently discovered superconductor magnesium diboride, MgB2, has a transition temperature, Tc, approaching 40 K, placing it intermediate between the families of low- and high-temperature superconductors. In practical applications, superconductors are permeated by quantized vortices of magnetic flux. When a supercurrent flows, there is dissipation of energy unless these vortices are 'pinned' in some way, and so inhibited from moving under the influence of the Lorentz force. Such vortex motion ultimately determines the critical current density, Jc, which the superconductor can support. Vortex behaviour has proved to be more complicated in high-temperature superconductors than in low-temperature superconductors and, although this has stimulated extensive theoretical and experimental research, it has also impeded applications. Here we describe the vortex behaviour in MgB2, as reflected in Jc and in the vortex creep rate, S, the latter being a measure of how fast the 'persistent' supercurrents decay. Our results show that naturally occurring grain boundaries are highly transparent to supercurrents, a desirable property which contrasts with the behaviour of the high-temperature superconductors. On the other hand, we observe a steep, practically deleterious decline in Jc with increasing magnetic field, which is likely to reflect the high degree of crystalline perfection in our samples, and hence a low vortex pinning energy.