Competing magnetic interactions in the extended Kagome system YBaCo4O7

YBaCo4O7 belongs to a new class of geometrically frustrated magnets like the
pyrochlores, in which Co-spins occupy corners of tetrahedra. The structure can
be viewed as an alternating stacking of Kagome and triangular layers. Exactly
half of the triangular units of the Kagome plane are capped by Co ions to form
columns running perpendicular to the Kagome sheets. Neutron powder diffraction
reveals a broad temperature range of diffuse magnetic scattering, followed by
long range magnetic ordering below 110K. A unique low-temperature magnetic
structure simultaneously satisfies an S=0 arrangement in the uncapped
triangular units and antiferromagnetic coupling along the columns. A spin
reorientation above 30K tracks the relative strengths of the in-plane and
out-of-plane interactions.