Public Reporting Burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Manuscript published: Nature 439, 303-306 (2006) Report Title ABSTRACT This is a report of a publication supported by the research grant:"Artificial 'spin ice' in a geometrically frustrated lattice of nanoscale ferromagnetic islands", R.

We report Curie temperatures up to 150 K in annealed Ga 1-x Mn x As epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) show that the higher ferromagnetic transition temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material -suggesting that the free surface of G a 1-x Mn x As epilayers is important in determining their physical properties.(a) nsamarth@psu.edu

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 nearestneighbor correlations.