Spin ice state

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 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)...

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. 

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)...

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 ... 

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. ... 

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. 

The fact that the population difference between spin states is greater for electrons than for nuclei means that EPR spectroscopy is much more sensitive than NMR. Thus, while modern NMR methods (Section 3.3) still require sample concentrations of at least 0.01 Af (moles per liter), EPR signals can be detected fi om radicals in as low a concentration as 10" M at room temperature (ICE mol in a sample volume of 0.1-0.2 mL). 

The second form of cancellation takes place because, for a spin-V2 nucleus, the two cones corresponding to the two allowed spin states (a and P) oppose each other (the orientations of the two cones is opposite—don t try this with pins and an actual ice cream cone or we rvill have pins everywhere on the floor ). The Boltzmann equation dictates that the number of spins (or pins) in the two cones is very nearly equal under normal experimental conditions. At 20°C (293 K), perhaps only 1 in about 25,000 hydrogen nuclei will... 

StiU, Tikhonov and Volkov repeatedly observed the above shift in the ratio of water spin isomers after contacting water vapor with the polymeric adsorbent both under dynamic (Fig. 10.19) and static conditions [30]. Activated carbons, zeolites, and sifica gels generated similar effects. The adsorbed water was strongly para-enriched. Interestingly, water samples enriched in the ortho- or para-isomers were found to be stable in the form of ice, but equilibrated to the ratio of 3 1 in the hquid state at room temperature within 20—30 min. As the separation takes place at the... 

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