M. S. Lund

Artificial ‘spin ice’ in a geometrically frustrated lattice of nanoscale ferromagnetic islands

Frustration, defined as a competition between interactions such that not all of them can be satisfied, is important in systems ranging from neural networks to structural glasses. Geometrical frustration, which arises from the topology of a well-ordered structure rather than from disorder, has recently become a topic of considerable interest. In particular, geometrical frustration among spins in magnetic materials can lead to exotic low-temperature states, including ‘spin ice’, in which the local moments mimic the frustration of hydrogen ion positions in frozen water. Here we report an artificial geometrically frustrated magnet based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipole interactions create a two-dimensional analogue to spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short-range correlations and an absence of long-range correlations, behaviour which is strikingly similar to the low-temperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide an uncharted arena in which the physics of frustration can be directly visualized.

Large area nanolithographic templates by selective etching of chemically stained block copolymer thin films

Magnetic nanodot arrays were prepared from thin film templates of cylinder-forming polystyrene-polylactide diblock copolymers by first annealing the films to obtain PLA cylinders oriented perpendicular to the surface, then selectively staining the PS matrix with RuO4 vapors, followed by etching the cylinders by O2-reactive ion etching. Metal was then deposited by molecular beam deposition and finally the polymer mask was lifted off.

Temperature-dependent magnetic interface location in interdiffused exchange biased bilayers

Antiferromagnetic (AF) binary alloys are attractive choices for exchange pinning of ferromagnets (F) in applications. Unfortunately, inducing AF ordering in these alloys often requires annealing which leads to interdiffusion at the AF∕F interface and a subsequent, and poorly understood, reduction in exchange bias. We report a study of the effects of interdiffusion in epitaxial NiMn∕Ni bilayers. Using polarized neutron reflectometry, we deduce that the competition between AF and F interactions in the interdiffused region leads to a temperature-dependent magnetic interface location, “glassy” behavior, memory effects, and low-temperature training. The results have important implications for the understanding of the temperature dependence of the exchange bias in these materials.Antiferromagnetic (AF) binary alloys are attractive choices for exchange pinning of ferromagnets (F) in applications. Unfortunately, inducing AF ordering in these alloys often requires annealing which leads to interdiffusion at the AF∕F interface and a subsequent, and poorly understood, reduction in exchange bias. We report a study of the effects of interdiffusion in epitaxial NiMn∕Ni bilayers. Using polarized neutron reflectometry, we deduce that the competition between AF and F interactions in the interdiffused region leads to a temperature-dependent magnetic interface location, “glassy” behavior, memory effects, and low-temperature training. The results have important implications for the understanding of the temperature dependence of the exchange bias in these materials.