Spin ices are exotic phases of matter characterized by frustrated spins obeying local “ice rules,” in analogy with the electric dipoles in water ice. Namely, on a tetrahedron with one spin or electric dipole moment sitting on each vertex, only the two-in-two-out configurations are allowed. Such a local constraint can lead to a macroscopic number of degenerate ground states or an extensive ground state entropy, which was first calculated by Pauling for water ice. In two dimensions, one can similarly define ice rules for in-plane Ising-like spins arranged on a kagome lattice, so that only one-in-two-out or two-in-one-out configurations are allowed. Such ice rules can result from either nearest neighbor ferromagnetic exchange coupling or long-range dipolar coupling between the spins, which can lead to different multi-stage ordering behavior under changing temperature. However, there have been very few materials systems that can be described as kagome spin ice.
In a recent Research Article published in Science [1], a multi-national team involving two CSU Physics faculty members Hua Chen and Kate Ross demonstrated that an intermetallic compound HoAgGe is a realization of the kagome ice state. Through experimental and theoretical approaches including magnetometry, thermodynamic measurements, neutron scattering, and Monte Carlo simulations, they have established that the low-temperature (<15K) spin states of the system can be reasonably well captured by a classical spin model hosting the kagome ice physics. The relatively high temperatures at which the spin ice behavior appears, in comparison with that in conventional 3D spin ices, are mainly due to the metallic nature of the system, which allows strong RKKY interaction between local Ho moments mediated by conduction electrons. The coexisting metallicity and spin ice phenomena may lead to other exotic physics such as interaction between electric currents and effective magnetic monopoles that deserves future study.
[1] Kan Zhao*, Hao Deng*, Hua Chen*, Kate A. Ross, Vaclav Petříček, Gerrit Günther, Margarita Russina, Vladimir Hutanu, and Philipp Gegenwart, “Realization of the kagome spin ice state in a frustrated intermetallic compound”, Science 367, 1218-1223 (2020).
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