  Interfaces between materials with differently ordered phases present unique
opportunities to study fundamental problems in physics. One example is the
interface between a singlet superconductor and a half-metallic ferromagnet,
where Cooper pairing occurs between electrons with opposite spin on one side,
while the other displays 100% spin polarisation. The recent surprising
observation of a supercurrent through half-metallic CrO_2 therefore requires a
mechanism for conversion between unpolarised and completely spin polarised
supercurrents. Here we suggest a conversion mechanism based on electron spin
precession together with triplet pair rotation at interfaces with broken
spin-rotation symmetry. In the diffusive limit the triplet supercurrent is
dominated by inter-related odd-frequency s-wave and even-frequency p-wave
pairs. In the crossover to the ballistic limit additional symmetry components
become relevant. The interface region exhibits a superconducting state of
mixed-spin pairs with highly unusual symmetry properties that opens up new
perspectives for exotic Josephson devices.
