Spin ice

Figure 1. The arrangement of hydrogen atoms (black circles) about oxygen atoms (open circles) in ice. Two hydrogen atoms (bottom ones) are close to the central oxygen atom while two of them (top ones) are far and closer to the two other (top left and top right) oxygen atoms.

A spin ice is a magnetic substance that does not have a single minimal-energy state. It has magnetic moments (i.e. "spin") as elementary degrees of freedom which are subject to frustrated interactions. By their nature, these interactions prevent the moments from exhibiting a periodic pattern in their orientation down to a temperature much below the energy scale set by the said interactions. Spin ices show low-temperature properties, residual entropy in particular, closely related to those of common crystalline water ice.[1] The most prominent compounds with such properties are dysprosium titanate (Dy2Ti2O7) and holmium titanate (Ho2Ti2O7). The orientation of the magnetic moments in spin ice resembles the positional organization of hydrogen atoms (more accurately, ionized hydrogen, or protons) in conventional water ice (see figure 1).

Experiments have found evidence for the existence of deconfined magnetic monopoles in these materials,[2][3][4] with properties resembling those of the hypothetical magnetic monopoles postulated to exist in vacuum.

  1. ^ Bramwell, S. T.; Gingras, M. J. P. (2001). "Spin Ice State in Frustrated Magnetic Pyrochlore Materials". Science. 294 (5546): 1495–1501. arXiv:cond-mat/0201427. Bibcode:2001Sci...294.1495B. doi:10.1126/science.1064761. PMID 11711667. S2CID 9402061.
  2. ^ Castelnovo, C.; Moessner, R.; Sondhi, S. L. (2008-01-03). "Magnetic monopoles in spin ice". Nature. 451 (7174): 42–45. arXiv:0710.5515. Bibcode:2008Natur.451...42C. doi:10.1038/nature06433. ISSN 0028-0836. PMID 18172493. S2CID 2399316.
  3. ^ Tchernyshyov, Oleg (2008-01-03). "Magnetism: Freedom for the poles". Nature. 451 (7174): 22–23. Bibcode:2008Natur.451...22T. doi:10.1038/451022b. ISSN 0028-0836. PMID 18172484. S2CID 30259694.
  4. ^ Gingras, M.J.P. (2009). "Observing Monopoles in a Magnetic Analog of Ice". Science. 326 (5951): 375–376. arXiv:1005.3557. doi:10.1126/science.1181510. PMID 19833948. S2CID 31038263.