Upper Limit for the Critical Field in Hard Superconductors

An examination is presented of some recent reports dealing with the extremely high critical fields in certain hard superconductors. Observations of nuclear magnetic resonance in V/sub 3/Si and V/sub 3/Ga are shown to set a severe upper limit on the attainable critical field because of a reduction in the spin susceptibility of these materials in the superconducting state. Equations are derived for the maximum critical field using this limiting criterion. A table of maximum and estimated critical fields is presented for V/sub 2.95/ Ga, Nb/sub 3/ Sn, V/sub 3/Si, and V/sub 1.95/ Ga. It is concluded that the critical fields for these BETA -wolfran compounds are so high that they may be effectively limited at low temperatures by the normal-state paramagnetism. If this is the case, critical fields higher than about 300 kilogauss will not be realized unless materials can be discovered with high transition temperatures. (H.D.R.)