Various means of doping MgB with carbon (e.g. using 10% malic acid) can improve the upper critical field and the maximum current density
(also with polyvinyl acetate).
5% doping with carbon can raise H from 16 to 36 T while lowering T only from 39 K to 34 K. The maximum critical current (J) is reduced, but doping with TiB can reduce the decrease. (Doping MgB with Ti is patented.)
The maximum critical current (J) in magnetic field is enhanced greatly (approx double at 4.2 K) by doping with ZrB.
Even small amounts of doping lead both bands into the type II regime and so no semi-Meissner state may be expected.