  Large Hall resistance jumps are observed in microdevices patterned from
epitaxial (Ga,Mn)As layers when subjected to a swept, in-plane magnetic field.
This giant planar Hall effect is four orders of magnitude greater than
previously observed in metallic ferromagnets. This enables extremely sensitive
measurements of the angle-dependent magnetic properties of (Ga,Mn)As. The
magnetic anisotropy fields deduced from these measurements are compared with
theoretical predictions.
