Magnetization Measurements on Single Crystals of Ba_0.6K_0.4BiO_3

Extensive measurements of the magnetization of several superconducting single crystal samples of Ba_0.6K_0.4BiO₃ have been made using SQUID and force magnetometry at temperatures ranging from 1.5 to 35 K and in magnetic fields from near 0 to 27 T. The superconducting magnetic phase diagram over all fields in which the samples are superconducting is determined. We find that the reversibility field and H_c2 come together at a critical point near 12.5 K and 8 T. From complete hysteresis curves the thermodynamic critical field is determined. Our results along with specific heat measurements point to there being a phase transition of thermodynamic order greater than 2 at temperatures above 13 K.