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.

Block copolymer thin films are ideal templates for a wide range of technologies where large area patterns of nanoscale features are desired. One of the main challenges in using lamellae-and cylinderforming block copolymers for this purpose is to induce the block copolymer domains to orient perpendicular to the film surface. We here show that perpendicular domain orientation can be easily achieved with polystyrene-b-polylactide (PS-PLA) thin films. Cylinder-forming PS-PLA films were prepared by spincoating on a variety of substrates followed by thermal annealing. The molecular weight, film thickness, annealing temperature, and annealing time were varied. When the film thickness was larger than the repeat spacing of the bulk morphology, the domains oriented perpendicular to the surface independent of the substrate/film interface. The films were then used to prepare nanoporous templates by a combination of hydrolytic PLA degradation and oxygen reactive ion etching (O 2-RIE). The template pattern was then transferred to the substrate using CF4-RIE to form an array of nanoscale pits.

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The dependence of exchange bias on antiferromagnet thickness has been measured in FeF 2 /Fe and MnF 2 /Fe bilayers. The two fluoride systems have identical crystal structures, similar lattice constants, but anisotropy fields that differ by a factor of 20. Hence, by comparing the antiferromagnetic layer thickness dependence of the exchange bias in the two systems we are able to directly establish the effect of the antiferromagnet anisotropy. We find that the critical antiferromagnet thickness for the onset of exchange biasing is an order of magnitude smaller for the more anisotropic fluoride, confirming the often-used assumption that the anisotropy dictates the critical thickness. By measuring the temperature dependence of the exchange bias and the structural morphology of the layers we are able to prove that the effects we observe are not due to the blocking-temperature thickness dependence or the onset of discontinuity in thin antiferromagnet layers.

We present an investigation of the effect of ferromagnetic layer thickness on the exchange bias and coercivity enhancement in antiferromagnet/ferromagnet bilayers. At low temperatures both the exchange bias and coercivity closely follow an inverse thickness relationship, contrary to several recent theoretical predictions. Furthermore, the temperature dependence of the coercivity as a function of the ferromagnet thickness provides clear evidence for the existence of two distinct regimes. These regimes were probed with conventional magnetometry, anisotropic magnetoresistance, and polarized neutron reflectometry. At low thickness the coercivity exhibits a monotonic temperature dependence, whereas at higher thickness a broad maximum occurs in the vicinity of the Néel temperature. These regimes are delineated by a particular ratio of the ferromagnet to antiferromagnet thickness. We propose that the ratio of the anisotropy energies in the two layers determines whether the coercivity is dominated by the ferromagnetic layer itself or the interaction of the ferromagnetic layer with the antiferromagnet.