Spin ice
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Spin ice is a material where the behavior of the magnetic moments in the material are analogous to the behavior of the protons in water ice.
In 1935, Pauling noted that the structure of ice (i.e. the solid phase of water), exhibited degrees of freedom that would be expected to exist even at absolute zero. That is, even upon cooling to absolute zero, water ice is expected to have residual entropy (i.e. intrinsic randomness). This is a result of the fact that ice consists of Oxygen atoms with four neighboring Hydrogen atoms. For each Oxygen atom, two of the neighboring Hydrogen atoms are near (forming the traditional H20), and two are far (being the Hydrogen atoms of neighboring water molecules). What Pauling notes was that the number of configurations that conformed to this two-in two-out rule was non-trivial, and therefore the entropy was expected to be non-trivial. [1]
Although Pauling's findings were confirmed in a manner by experiments, pure crystals of water ice are particularly hard to create.
Spin ices are materials consisting of tetrahedra of spins, which must satisfy some two-in, two-out rule analogous to water ice. Spin ice materials therefore exhibit the same residual entropy properties of water ice. However, depending on the material used in a spin ice, it is generally much easier to create large single crystals of spin ice materials than the corresponding water ice materials. Additionally, the interaction of a magnetic field with the spins in a spin ice material make spin ice materials much nicer materials for examining residual entropy than water ice.
The only known spin ice materials are the pyrochlores Ho2Ti2O7, Dy2Ti2O7, and Ho2Sn2O7, although more discoveries are expected with further research.
Spin ice materials are characterized by disorder of magnetic ions even when said ions are at very low temperatures (which is usually characterized by no discernible "freezing" effects in C-T scans).
[edit] References
- ^ Pauling L. (1935), The Structure and Entropy of Ice and of Other Crystals with Some Randomness of Atomic Arrangement, Journal of the American Chemical Society, Vol. 57, p. 2680ff
