Non-magnetic defects in the classical Kagome bilayer antiferromagnet

We consider non-magnetic substitutions in the quasi two-dimensional Kagome bilayer lattice (relevant to experiments on SCGO~\footnote{L.~Limot {\it et. al}, Phys. Rev. B 65, 14447(2002).}) which consists of both triangular and tetrahedral units. Correlated defects, where impurities substitute for all but one spin on a ``defective" triangle or tetrahedron, are particularly interesting in this context. The lone ``orphan spin"~\footnote{P.~Schiffer and R.~Daruka, Phys. Rev. B 56, 13712 (1997), R.~Moessner and A.~J.~Berlinsky, Phys. Rev. Lett. 83, 3293 (1999)} on the simplex acts like a paramagnetic spin as $T \rightarrow 0$ and induces a long-ranged spin texture around it~\footnote{C.~L.~Henley, Can. J. Phys. 79, 1307 (2001).} at zero temperature. We study such defects at low temperatures and magnetic fields by performing classical Monte-Carlo simulations. We also use a simple effective field theory to reasonably capture the features of the induced texture.