K. Chen, J. Li, C. Nisoli, P. E. Lammert, W. McConville, R. Wang, V. H. Crespi and P. Schiffer, "Energy Minimization and ac Demagnetization in a Nanomagnet Array," Phys. Rev. Lett. 101, 037205 (2009)

We study ac demagnetization in frustrated arrays of single-domain ferromagnetic islands, exhaustively resolving every (Ising-like) magnetic degree of freedom in the systems. Although the net moment of the arrays is brought near zero by a protocol with sufficiently small step size, the final magnetostatic energy of the demagnetized array continues to decrease for finer-stepped protocols and does not extrapolate to the ground-state energy. The resulting complex disordered magnetic state can be described by a maximum-entropy ensemble constrained to satisfy just nearest-neighbor correlations.