Spin effects in the magnetodrag between double quantum wells

We report on the selectivity to spin in a drag measurement. This selectivity to spin causes deep minima in the magnetodrag at odd filling factors for matched electron densities at magnetic fields and temperatures at which the bare spin energy is only one tenth of the temperature. For mismatched densities the selectivity causes a $1/B$ periodic oscillation, such that negative minima in the drag are observed whenever the majority spins at the Fermi energies of the two-dimensional electron gases are antiparallel, and positive maxima whenever the majority spins at the Fermi energies are parallel.