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Magnetization studies were undertaken to determine the change in free energy of the superconducting state of YiBa2Cu307 with changes in magnetic field and temperature. Close to the transition temperature T, magnetic Aux moves easily and reversibly in these materials so that magnetization is a thermodynamically reversible variable. Hence, the free-energy surfaces can be measured and quantities such as entropy and specific heat can be derived. Magnetization is found to be linear in (T, -T)~n ear T,. The free-energy surface shows substantial changes with magnetic field all the way up to 92 K, well outside the usual 0,2 vs T plots derived from resistivity or ac susceptibility data.

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