Spin ices

A spinning molecule on a copper surface and a soccer-ball molecule tethered to a protein may seem no more useful than a spinning ice-skater or a tetherball. Nonetheless, advocates of nanotechnology cite a wealth of potential applications for this new field, including tailored synthetic membranes that can collect specific toxins from industrial waste and computers that process data much faster than today s best models. The list of possible benefits from nanotechnology is limited only by our imaginations. 

Spin Glasses, Spin Liquids, and Spin Ices Macroscopic Degeneracy. Another ground state commonly found in GFM materials is the spin glass (SG) state, although its occurrence... 

It is remarkable that the nature of the ground state in these pyrochlores can be modified so strongly from a mixed long-range/short-range order (Gd) to a completely dynamic spin liquid (Tb) to a spin ice (Dy,Ho) simply by substituting one rare earth ion for another. 

Dy2Ti207 and H02Ti2O7 are widely regarded as examples of spin ice ground-state materials and a very nice summary of this phenomenon is available. The spin ice state depends on two conditions one, that the magnetic moments be constrained by anisotropy to lie parallel to the (111) directions and two. 

Figure 61 The connection between spin ice and water ice. (a) The two short, two long 0-H bonds about a single O atom in water ice. (b) The two in, two out configuration (obeying the ice rules ) in any tetrahedron for a F Ising model, (c) One possible ground state of an infinitely degenerate set for a spin ice . The open circles are spin in and the solid circles, spin out. (Reprinted with permission from S.T. Bramwell, M.J.P. Gingras, Science, 2001, 294, 1495. 2001 American Association for the Advancement of Science, www.sciencemag.org)...

Figure 62 Heat capacity and entropy for Dy2Ti207 showing the excess entropy, experimental evidence for the spin ice stated (Reprinted by permission from Nature Ref 124, 1999 Macmillan Publishers Ltd. www.nature.com)...

Saha, S., Singh, S., Dkhil, B Dhar, S Suryanarayanan, R Dhalenne, G Revcolevschi, A., and Sood, A.K. (2008) Temperature-dependent Raman and X-ray studies of the spin-ice pyrochlore Dy2Ti207 and nonmagnetic pyrochlore Lu2Ti207. Phys. Rev. B, 78 (21),... 

The spin ices undergo a type of spin flop transition in applied magnetic fields in which the 2 in 2 out ground state is transformed to a 3 in 1 out ... 

Figure 2.42 Heat capacity (a) and entropy (b) for spin ice Dy2Ti207 showing the agreement with the Pauling prediction of S = R(lii2 - l/2ln3/2).t Reprinted with permission from Ramirez et al., 1999 [22]. Copyright (1999) Macmillan Publishers Ltd...

State at rather modest fields of 1 T applied along <111>. Interestingly, application of pressure in the GPa range to spin ices does not induce long range magnetic order unlike the case for the spin liquid, Tb2Ti207. ... 

At this stage the so-called stuffed spin ice materials are relevant. These are pyrochlores with excess A ions substituted on the B sites, such as Ho2+xTi2-xOy These phases can be regarded as intermediates... 

Et2Ti207 is one of two easy plane titanates and with a large negative 9, it is not a spin ice candidate. Very early heat capacity data indicated LRO at A comprehensive study by Champion et... 

The A = Pr phase, which does not exist for the B = Ti series has been assigned a spin ice ground state from studies using a variety of probes such as a.c. susceptibility, neutron scattering and heat capacity. Note the easy axis anisotropy and slightly positive Oq which match with the canonical spin ices for the heavy rare earths, A = Dy and Ho. However, some unusual very low temperature spin dynamics surest unique properties for this new spin ice. 

Both Dy2Sn207 and Ho2Sn20y appear to be spin ice materials in parallel with their titanate counterparts, although only the A = Ho material has been studied in detail. 

Sni2M02O7 has been also been suggested as an AHE material and shows a giant magneto resistance.More recently, it has been proposed as a candidate metallic spin ice. ° ... 

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