Torque magnetization measurements on YBa2Cu3Oy (YBCO) at doping y=6.67(p=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 Hirr(T=0)≈24 T. The differential susceptibility dM/dB, however, is more rapidly suppressed for B≳16 T than expected from the properties of the low field superconducting state, and saturates at a low value for fields B≳24 T. In addition, torque measurements on a p=0.11 YBCO crystal in pulsed field up to 65 T and temperatures down to 8 K show similar behaviour, with no additional features at higher fields. We discuss several candidate scenarios to explain these observations: (a) superconductivity survives but is heavily suppressed at high field by competition with 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; (c) the stronger 3D ordered CDW that sets in above 15 T may suppress the normal state spin susceptibility sufficiently to give an apparent diamagnetism of the magnitude observed.
@article{arxiv.1509.06206,
title = {Magnetization of underdoped YBa$_2$Cu$_3$O$_{y}$ above the irreversibility field},
author = {Jing Fei Yu and B. J. Ramshaw and I. Kokanović and K. A. Modic and N. Harrison and James Day and Ruixing Liang and W. N. Hardy and D. A. Bonn and A. McCollam and S. R. Julian and J. R. Cooper},
journal= {arXiv preprint arXiv:1509.06206},
year = {2016}
}