  The magnetization processes of the spin-3/2 antiferromagnet LiInCr4O8
comprising a "breathing" pyrochlore lattice, which is an alternating array of
small and large tetrahedra, are studied under ultrahigh magnetic fields of up
to 130 T using state-of-the-art pulsed magnets. A half magnetization plateau is
observed above 90 T to 130 T, suggesting that LiInCr4O8 has a strong
spin-lattice coupling, similar to conventional chromium spinel oxides. The
magnetization of LiGa0.125In0.875Cr4O8, in which the structural and magnetic
transitions at low temperatures have been completely suppressed, shows a sudden
increase above 13 T, indicating that a spin gap of 2.2 meV exists between a
tetramer singlet ground state and an excited state with total spin 1, with the
latter being stabilized by the application of a magnetic field. The breathing
pyrochlore antiferromagnet is found to be a unique frustrated system with
strong spin-lattice coupling and bond alternation.
