  Magnetization, specific heat, and neutron scattering measurements were
performed to study a magnetic transition in jarosite, a spin-5/2 kagome lattice
antiferromagnet. When a magnetic field is applied perpendicular to the kagome
plane, magnetizations in the ordered state show a sudden increase at a critical
field H_c, indicative of the transition from antiferromagnetic to ferromagnetic
states. This sudden increase arises as the spins on alternate kagome planes
rotate 180 degrees to ferromagnetically align the canted moments along the
field direction. The canted moment on a single kagome plane is a result of the
Dzyaloshinskii-Moriya interaction. For H < H_c, the weak ferromagnetic
interlayer coupling forces the spins to align in such an arrangement that the
canted components on any two adjacent layers are equal and opposite, yielding a
zero net magnetic moment. For H > H_c, the Zeeman energy overcomes the
interlayer coupling causing the spins on the alternate layers to rotate,
aligning the canted moments along the field direction. Neutron scattering
measurements provide the first direct evidence of this 180-degree spin rotation
at the transition.
