A study was made of the upper critical field, HC2, the irreversibility field, Hirr, and the lower critical field, HC1, of a dense polycrystalline MgB2 superconductor which was prepared by spark plasma sintering. The isothermal magnetizations M(H) of MgB2 sample was measured between 5 and 50K in fields of up to 8T. The HC2(T) data were well fitted to the BCS-type function HC2(T) = HC2(0)[1−(T/TC)α]β, with HC2(0) = 8.5T, α = 1.60 and β = 1.15. The Hirr (melting line) could be described by the power-law relationship, Hirr(T) = Hirr(0)[1−(T/TC)]n with n = 1.10 and Hirr(0) = 8.1T. The deduced values of HC1 approximately followed a linear temperature dependence, rather than following the parabolic dependence of HC1(T) = HC1(0)[1−(T/TC)2].

Superconducting Properties of Polycrystalline MgB2 Superconductors Fabricated by Spark Plasma Sintering. K.J.Song, C.Park, S.W.Kim, R.K.Ko, H.S.Ha, H.S.Kim, S.S.Oh, Y.K.Kwon, S.H.Moon, S.I.Yoo: Physica C, 2005, 426-431[1], 588-93