Abstract
Using small angle neutron scattering, the anisotropy of the magnetic vortex lattice (VL), in the
heavily hole-doped pnictide superconductor, KFe2As2, was studied. Well-ordered VL scattering
patterns were measured with elds applied in directions between B k c and the basal plane,
rotating either towards [100] or [110]. Slightly distorted hexagonal patterns were observed when
B k c. However, the scattering pattern distorted strongly as the eld was rotated away from the c-
axis. At low eld, the arrangement of vortices is strongly aected by the anisotropy of penetration
depth in the plane perpendicular to the eld. By tting the distortion with the anisotropic London
model, we obtained an estimate of 3:4 for the anisotropy factor,
, between the in-plane and
c-axis penetration depths at the lowest temperature studied. The results further reveal VL phase
transitions as a function of eld direction. We discuss these transitions using the "Hairy Ball"
theorem.