Spin ice

In spin ice are materials in which the magnetic moments in the material behave analogously to the protons in water ice.

1935 introduced Linus Pauling found that the structure of ice ( that is, the solid phase of water) has degrees of freedom that should exist at absolute zero. This means that even when cooled to absolute zero, a residual entropy ( ie an intrinsic disorder ) is retained. This is a consequence of the fact that the ice contains four oxygen atoms with adjacent hydrogen atoms. For each oxygen atom per two hydrogen atoms are closer (these are the traditional H2O molecule ) and two further away ( these correspond to hydrogen atoms more distant molecules). Pauling found that the configuration that corresponds to this "two- close - two- remote - control " is not trivial and therefore attracts a nontrivial entropy by itself. This is an example of geometrical frustration.

Pauling considerations were verified experimentally, although pure water ice are difficult to produce.

In spin ice are tetrahedra of ions, all of which have a non-vanishing spin. These must, due to the interactions between neighboring ions satisfy analogous to the above-discussed case of the ice is also a "two - close - two- remote - control ". Therefore spin ice shows the same residual entropy as water ice. Depending on the materials used for the spin- ice is large, single crystals in this case, however, easier to manufacture as pure water ice. In addition, the interaction of the ions with a magnetic spin ensures that these materials are more appropriate for this to investigate the residual entropies.

While Philip Anderson in 1956 the relationship between the frustrated Ising antiferromagnets recognized on a tetrahedral lattice of pyrochlore and the Pauling Wassereisproblem, real spin ice materials were not discovered until 1997. The first identified as a spin- ice materials were the pyrochlore Ho2Ti2O7, Dy2Ti2O7 and Ho2Sn2O7. Furthermore, were also published strong evidence that Dy2Sn2O7 is also a spin ice.

Spin ice is characterized by a disorder of magnetic ions even at very low temperatures. Measurements of the dynamic magnetic susceptibility provide an indication of a dynamic freezing of magnetic moments below temperatures at which the specific heat is at a maximum.

Spin ice materials are frustrated magnetic systems. While frustration is usually associated with triangular or tetrahedral arrangements of magnetic moments, which are coupled by antiferromagnetic exchange interactions, the conditions in spin ice materials are complicated: It is frustrated ferromagnets. The locally acting, strong crystal field forces the magnetic moments in either the tetrahedral or out of the tetrahedron out to show what has become an antiferromagnetically interacting frustrated " exchange " system equivalent. In reality, however, is antiferromagnetic interaction did not front, but responsible for the long-range magnetic dipolar interactions are the frustration, not the exchange interaction of nearest neighbors. From the frustration of the "two -rein - two- out - spin orientation " results, and thus the spin ice state.

In spin ice in 2008 were first detected and measured magnetic quasi- monopolies. They are sources of magnetization, but not of the magnetic flux; this is still divergence-free.