  In hole-doped cuprates, Nernst experiments imply that the superconducting
state is destroyed by spontaneous creation of vortices which destroy phase
coherence. Using torque magnetometry on $\rm Bi_2Sr_2CaCu_2O_{8+\delta}$, we
uncover a field-enhanced diamagnetic signal $M$ above the transition
temperature $T_c$ that increases with applied field to 32 Tesla and scales just
like the Nernst signal. The magnetization results above $T_c$ distinguish $M$
from conventional amplitude fluctuations, and strongly support the vortex
scenario for the loss of phase coherence at $T_c$.
