Effect of annealing temperature on some physical properties of MgB2 by using the Hall probe ac-susceptibility method

A commercially available powder of MgB2 is used as starting material for the examination of the influence of the annealing temperature on the properties of this intermediate-Tc superconductor. We performed scanning electron microscopy (SEM) and Hall ac-susceptibility measurements as a function of temperature and ac-field amplitude on samples annealed at 650, 750, 850 and 950 °C. The imaginary part of ac-susceptibility measurements is used to calculate both the inter-granular critical current density, Jc(Tp) and density of pinning force, αj(0). It was observed that all Tc, Jc(Tp) and αj(0) exhibit a non-monotonic behavior on the annealing temperature range studied in this work. Tc is measured to be 39.85±0.02 K and Jc(Tp) is estimated to be as high as 60 A/cm2 at 39.2 K for the sample annealed at 850 °C. The peak temperature, Tp, in the imaginary part of the ac-susceptibility curves shifts to lower temperatures with both decreasing the annealing temperature and increasing the amplitude of the ac-magnetic fields. A comparison of the experimental ac-susceptibility data with theoretical critical-state models that are currently available is performed. SEM investigations showed that the grain size increases, and the grain connectivity improves when the annealing temperature increases up to 850 °C. The possible reasons for the observed changes in transport, microstructure and magnetic properties due to annealing temperature are discussed.