  In this paper we compute the mean field phase diagram of a doped
antiferromagnet, in a magnetic field and with anisotropic exchange. We show
that at zero temperature there is a metamagnetic transition from the
antiferromagnetic configuration along the z direction to a spin-flop state. In
the spin flop phase the system prefers a commensurate magnetic order, at low
doping, whereas at larger doping the incommensurate phase is favorable.
Contrary to the pure Heisenberg case, the spin flop region does not span an
infinite area in the ('Delta',h) plane, where 'Delta' is the exchange
anisotropy and h is the external magnetic field. We characterize the magnetic
and charge-transport properties of the spin-flop phase, computing the magnetic
susceptibility and the Drude weight. This latter quantity presents a sudden
variation as the spin-flop to paramagnet phase transition line is crossed. This
effect could be used as a possible source of large magneto-resistance. Our
findings may have some relevance for doped La_{2-\delta}Sr_{\delta}CuO_4 in a
magnetic field.
