  This paper has been withdrawn by the authors. Measurements at many different
angles spanning a broad range of electron densities indicate that the observed
doubling of the period of Shubnikov-de Haas oscillations, which we had
attributed to a transition to a new spin-ordered phase, is also consistent with
an alternative explanation. A full interpretation of our data will be posted at
a later date.

Strongly Correlated Electrons;Mesoscale and Nanoscale Physics