The authors would like to thank Eric Tkaczyk and J.A.X. Alexander for providing an algorithm on which the critical field fitting routine was based. They would also like to thank Eliot Dresselhaus for installing the software that produced the figures in this paper. We would like to thank G. Doll, J. Speck, Y. Iye, T. Enoki, and R. Al-Jishi for their advice on this project. The authors A.C., M.S.D., and G.D. gratefully acknowledge support by AFOSR contract #F49620--83--C--0011 for the early stages of this work and NSF grant #DMR 83-10482 for support during the last year. The Francis Bitter National Magnet Laboratory is supported by the NSF.