  Josephson junctions made with conventional s-wave superconductors and
containing multiple layers of ferromagnetic materials can carry spin-triplet
supercurrent in the presence of certain types of magnetic inhomogeneity. In
junctions containing three ferromagnetic layers, the triplet supercurrent is
predicted to be maximal when the magnetizations of adjacent layers are
orthogonal, and zero when the magnetizations of any two adjacent layers are
parallel. Here we demonstrate on-off control of the spin-triplet supercurrent
in such junctions, achieved by rotating the magnetization direction of one of
the three layers by 90$^{\circ}$. We obtain "on-off" ratios of 5, 7, and 19 for
the supercurrent in the three samples studied so far. These observations
directly confirm one of the most salient predictions of the theory, and pave
the way for applications of spin-triplet Josephson junctions in the nascent
area of "superconducting spintronics."
