  The d.c. magnetization and magnetic relaxation studies of the calcium doped
samples, Y0.95Ca0.05BaCo2O5.5 and YBa0.95Ca0.05Co2O5.5, show the existence of a
magnetic glass like behaviour in the family of cobaltites for the first time.
Our investigations reveal glass-like arrest of kinetics at low temperature
which prevents the system from reaching its magnetic ground state. We show that
the low temperature state of these calcium doped phases, which consists of
coexisting antiferromagnetic and ferro (or ferri) magnetic phase fractions, can
be tuned in a number of ways. Our observations establish that the low
temperature state of this oxide is not in thermal equilibrium. The glassy state
is formed with the assistance of an external magnetic field, which makes it
distinctly different from the more well known metastable state, the spin glass
state. The cooling field can tune the fractions of the coexisting phases, and
the glass-like state formed at low temperature can also be devitrified by
warming the sample. The role of Ca doping in the appearance of these phenomena
is discussed in terms of phase separation, involving Co3+ disproportionation
into Co4+ ferromagnetic clusters and Co2+ antiferromagnetic clusters.
