Magnetization Of Underdoped Yba2cu3oy Above The Irreversibility Field

Torque magnetization measurements on YBa2Cu3Oy (YBCO) at doping y = 6.67 (rho = 0.12), in dc fields (B) up to 33 T and temperatures down to 4.5 K, show that weak diamagnetism persists above the extrapolated irreversibility field H-irr(T = 0) approximate to 24 T. The differential susceptibility dM/dB, however, is more rapidly suppressed for B greater than or similar to 16 T than expected from the properties of the low field superconducting state, and saturates at a low value for fields B greater than or similar to 24 T. In addition, torque measurements on a rho = 0.11 YBCO crystal in pulsed field up to 65 T and temperatures down to 8 K show similar behavior, with no additional features at higher fields. We offer two candidate scenarios to explain these observations: (a) superconductivity survives but is heavily suppressed at high field by competition with charge-density-wave (CDW) order; (b) static superconductivity disappears near 24 T and is followed by a region of fluctuating superconductivity, which causes dM/dB to saturate at high field. The diamagnetic signal observed above 50 T for the rho = 0.11 crystal at 40 K and below may be caused by changes in the normal state susceptibility rather than bulk or fluctuating superconductivity. There will be orbital (Landau) diamagnetism from electron pockets and possibly a reduction in spin susceptibility caused by the stronger three-dimensional ordered CDW.