  Magnetic field induced antiferromagnetic phase of the underdoped cuprates is
studied within the t-t'-J model. A magnetic field suppresses the pairing
amplitude, which in turn may induce antiferromagnetism. We apply our theory to
interpret the recently reported quantum oscillations in high magnetic field in
ortho-II YBa2Cu3O6.5 and propose that the total hole density abstracted from
the oscillation period is reduced by 50% due to the antiferromagnetism.
