  We report the effect of Ga substitution on magnetization, magnetoresistance,
and magnetostriction in polycrystalline Pr0.6Sr0.4Co1-yGayO3 (y=0.0-0.3)
samples. Upon substitution of the non-magnetic Ga3+ cation for magnetic Co3+,
the low-temperature ground state transforms from ferromagnetic metallic for y =
0 to cluster glass semiconductor for y = 0.2. Magnetoresistance at 7T is
negative and its magnitude increases from 2 percentage for y = 0 to 30
percentage for y = 0.3 at 10 K. On the other hand, magnetostriction at 10 K is
positive and its value decreases with increasing y. Interestingly, the
field-dependent magnetization, magnetoresistance, and magnetostriction for y
greater than or equal to 0.2 and at T less than or equal to 3 K show reversible
abrupt steps for both positive and negative magnetic field whereas all these
quantities vary smoothly with the magnetic field above 4 K. Such steps in all
three distinct physical quantities were never reported earlier in perovskite
cobaltites and they differ from observations made in manganites and
intermetallic alloys. It is suggested that field-induced avalanche flipping of
ferromagnetic clusters could be the origin of observed steps in all these three
quantities.
