Increases in the irreversibility field and the upper critical field of bulk MgB2 by ZrB2 addition

Abstract

In a study of the influence of ZrB2 additions on the irreversibility field, μ0Hirr and the upper critical field Bc2, bulk samples with 7.5at% ZrB2 additions were made by a powder milling and compaction technique. These samples were then heated to 700–900°C for 0.5h. Resistive transitions were measured at 4.2K and μ0Hirr and Bc2 values were determined. An increase in Bc2 from 20.5Tto28.6T and enhancement of μ0Hirr from 16Tto24T were observed in the ZrB2 doped sample as compared to the binary sample at 4.2K. Critical field increases similar to those found with SiC doping were seen at 4.2K. At higher temperatures, increases in μ0Hirr were also determined by M-H loop extrapolation and closure. Values of μ0Hirr which were enhanced with ZrB2 doping (as compared to the binary) were seen at temperatures up to 34K, with μ0Hirr values larger than those for SiC doped samples at higher temperatures. The transition temperature, Tc, was then measured using dc susceptibility and a 2.5K drop of the midpoint of Tc was observed. The critical current density was determined using magnetic measurements and was found to increase at all temperatures between 4.2K and 35K with ZrB2 doping.