  Low-temperature magnetization (M) measurements down to 0.1 K have been
performed in magnetic fields up to 14.5 T for a single piece of a tiny
single-crystalline sample (0.2 mg weight) of the spin-gap system YbAl3C3. At
the base temperature of 0.1 K, several metamagnetic transitions were clearly
observed for H // c in the range 6 T < H < 9 T whereas only two transitions
were observed, one at 4.8 T and the other at 6.6 T, for H // a. At fields above
9 T, the magnetization becomes almost saturated for both H // a and H // c. The
present results indicate that a singlet-triplet crossover occurs in a
relatively narrow field range, suggesting a rather weak interdimer interaction
in spite of the nearly triangular lattice of Yb ions.
