High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

The discovery of superconductivity at 39 K in magnesium diboride(1) offers the possibility of a new class of low-cost, high-performance superconductin g materials for magnets and electronic applications. This compound has twic e the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, a nd the vital prerequisite of strongly linked current flow has already been demonstrated(2-5). One possible drawback, however, is that the magnetic fie ld at which superconductivity is destroyed is modest. Furthermore, the fiel d which limits the range of practical applications-the irreversibility fiel d H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), signific antly lower than about 10 T for Nb-Ti (ref. 6) and similar to 20 T for Nb3S n (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In add ition, very high critical current densities at 4.2 K are achieved: 1 MA cm( -2) at 1 T and 10(5) A cm(-2) at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.