  Double layer two-dimensional electron systems at high perpendicular magnetic
field are used to realize magnetic tunnel junctions in which the electrons at
the Fermi level in the two layers have either parallel or anti-parallel spin
magnetizations. In the anti-parallel case the tunnel junction, at low
temperatures, behaves as a nearly ideal spin diode. At elevated temperatures
the diode character degrades as long-wavelength spin waves are thermally
excited. These tunnel junctions provide a demonstration that the spin
polarization of the electrons in the $N=1$ Landau level at filling factors $\nu
= 5/2$ and 7/2 is essentially complete, and, with the aid of an in-plane
magnetic field component, that Landau level mixing at these filling factors is
weak in the samples studied.
