Self-Assembly of Magnetic Spheres in Strong Homogeneous Magnetic Field
  The self-assembly in two dimensions of spherical magnets in strong magnetic field is addressed theoretically. %% It is shown that the attraction and assembly of parallel magnetic chains is the result of a delicate interplay of dipole-dipole interactions and short ranged excluded volume correlations. %% Minimal energy structures are obtained by numerical optimization procedure as well as analytical considerations. For a small number of constitutive magnets $N_{\rm tot}\leq26$, a straight chain is found to be stable. In the regime of larger $N_{\rm tot}\geq27$, the magnets form \textit{two touching} chains with equally long tails at both ends. We succeed to identify the transition from \textit{two} to \textit{three} touching chains at $N_{\rm tot}=129$. 
Ren\'e Messina and Igor Stankovi\'c
cond-mat.stat-mech math-ph math.MP