Natural, Artificial, and Stuffed Spin Ice

P. Schiffer Department of Physics and Materials Research Institute, Pennsylvania State University, University Park, PA 16802 USA

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 including measurable "ground state entropy." We have performed extensive studies of spin ice materials, examining how the spins freeze into a low temperature disordered state which differs in important ways from that of a spin glass. We also have examined ''stuffed'' spin ice materials -- a chemical alteration of Ho₂Ti₂O₇ spin ice in which extra Ho magnetic moments are placed on normally non-magnetic Ti sites to create the series Ho₂(Ti_2-xHo_x)O_7-x/2. Surprisingly, the measured ground state entropy per spin appears unchanged by these excess spins. Additionally, we examine ''artificial'' spin ice formed from lithographically fabricated regular arrays of single-domain ferromagnetic islands arranged such that the dipolar interactions are analogous to those in spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. The results demonstrate that artificial frustrated magnets can provide a rich new arena in which to study the physics of frustration.

References: Nature 413, 48 (2001); Physical Review Letters 91, 107201 (2003); Physical Review B 69, 064414 (2004); Physical Review B 70, 184431 (2004); Nature Physics 2, 249 (2006); Nature 439, 303 (2006).

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