  The family of hole-doped Pr-based perovskite cobaltites,
Pr$_{0.5}$Ca$_{0.5}$CoO$_{3}$ and
(Pr$_{1-y}$RE$_{y}$)$_{0.3}$Ca$_{0.7}$CoO$_{3}$ (where RE is rare earth) has
recently been found to exhibit simultaneous metal-insulator, spin-state, and
valence transitions. We have investigated magnetic-field-induced phase
transitions of (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$ by means of
magnetization measurements at 4.2$-$100 K up to an ultrahigh magnetic field of
140 T with the chemical pressure varied by $y$ = 0.0625, 0.075, 0.1. The
observed magnetic-field-induced transitions were found to occur simultaneously
with the metal-insulator transitions up to 100 T. The obtained magnetic
field-temperature ($B$-$T$) phase diagram and magnetization curves are well
analyzed by a spin-crossover model of a single ion with interion interactions.
On the other hand, the chemical pressure dependence of the experimentally
obtained magnetization change during the phase transition disagrees with the
single ion model when approaching low temperatures. The significant $y$
dependence of the magnetization change at low temperatures may arise from the
itinerant magnetism of Co$^{3+}$ in the paramagnetic metallic phase, where the
chemical pressure enhances the exchange splitting by promoting the
double-exchange interaction. The observed $B$-$T$ phase diagrams of
(Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$ are quite contrary to that of
LaCoO$_{3}$, indicating that in (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$
the high-field phase possesses higher entropy than the low-field phase, whereas
it is the other way around in LaCoO$_{3}$.
