  A helix has a wavevector along the z axis with the magnetic moments
ferromagnetically-aligned within xy planes with a turn angle kd between the
moments in adjacent planes in transverse field Hx = 0. The magnetic structure
and x-axis average magnetization per spin of this system in a classical XY
anisotropy field HA is studied versus kd, HA, and large Hx at zero temperature.
For values of HA below a kd-dependent maximum value, the xy helix phase
transitions with increasing Hx into a spin-flop (SF) phase where the ordered
moments have x, y, and z components. The moments in the SF phase are taken to
be distributed on either one or two xyz spherical ellipses. The minor axes of
the ellipses are oriented along the z axis and the major axes along the y axis
where the ellipses are flattened along the z axis due to the presence of the XY
anisotropy. From energy minimization of the SF spherical ellipse parameters for
given values of kd, HA and Hx, four kd-dependent SF phases are found: either
one or two xyz spherical ellipses and either one or two xy fans, in addition to
the xy helix phase and the paramagnetic (PM) phase with all moments aligned
along Hx. The PM phase occurs via second-order transitions from the xy fan and
SF phases with increasing Hx. Phase diagrams in the Hx-HA plane are constructed
by energy minimization with respect to the SF phases, the xy helix phase, and
the xy fan phase for four kd values. One of these four phase diagrams is
compared with the magnetic properties found experimentally for the model
helical Heisenberg antiferromagnet EuCo2P2 and semiquantitative agreement is
found.
