Spin Ice - Technical Description

Technical Description

In 1935, Linus Pauling noted that the structure of water ice exhibited degrees of freedom that would be expected to remain disordered even at absolute zero. That is, even upon cooling to zero temperature, water ice is expected to have residual entropy (i.e., intrinsic randomness). This is a result of the fact that the structure of ice contains oxygen atoms with four neighboring hydrogen atoms. For each oxygen atom, two of the neighboring hydrogen atoms are near (forming the traditional H₂O molecule), and two are further away (being the hydrogen atoms of neighboring water molecules). Pauling noted that the number of configurations conforming to this "two-in two-out" rule grows exponentially with the system size, and therefore that the zero-temperature entropy of ice was expected to be extensive. Pauling's findings were confirmed by specific heat measurements, though pure crystals of water ice are particularly hard to create.

Spin ices are materials consisting of tetrahedra of ions, each of which has a non-zero spin, which must satisfy some two-in, two-out rule analogous to water ice because of the interactions between neighbouring ions. Spin ice materials therefore exhibit the same residual entropy properties as 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 better materials for examining residual entropy than water ice.

While Philip Anderson had already noted in 1956 the connection between the problem of the frustrated Ising antiferromagnet on a (pyrochlore) lattice of corner-shared tetrahedra and Pauling's water ice problem, real spin ice materials were only discovered quite recently. The first materials identified as spin ices were the pyrochlores Dy₂Ti₂O₇ (dysprosium titanate), Ho₂Ti₂O₇ (holmium titanate) and Ho₂Sn₂O₇ (holmium stannate). Very recently, compelling evidence has been reported that Dy₂Sn₂O₇ (dysprosium stannate) is also a spin ice.

Spin ice materials are characterized by disorder of magnetic ions even when said ions are at very low temperatures. AC magnetic susceptibility measurements find evidence for a dynamical freezing of the magnetic moments as the temperature is lowered somewhat below the temperature at which the specific heat displays a maximum.