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 a ected 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.
