  We report the observation of large exchange bias in Ni50-xCoxMn38Sb12 Heusler
alloys with x=0, 2, 3, 4, 5, which is attributed to the coexistence of
ferromagnetic and antiferromagnetic phases in the martensitic phase. The phase
coexistence is possibly due to the supercooling of the high temperature
ferromagnetic phase and the predominant antiferromagnetic component in the
martensitic phase. The presence of exchange bias is well supported by the
observation of training effect. The exchange bias field increases with Co
concentration. The maximum value of 480 Oe at T=3K is observed in x=5 after
field cooling in 50 kOe, which is almost double the highest value reported so
far in any Heusler alloy system. Increase in the antiferromagnetic coupling
after Co substitution is found to be responsible for the increase in the
exchange bias.
