Frustrated Magnetism

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Science  28 Mar 2014:
Vol. 343, Issue 6178, pp. 1405
DOI: 10.1126/science.343.6178.1405-c

In magnetic materials, ions positioned on the lattice sites attempt to minimize their energy by aligning their spins; in some cases, however, the configuration of the lattice makes it impossible to minimize the local energy on all sites at the same time—a situation known as geometrical frustration. Besides spin, charge and orbital degrees of freedom can also be associated with frustration. In the compound CuIr2S4, which has Ir ions of mixed valence, a charge order sets in below 230 K with alternating octamers of Ir3+ and Ir4+, and a spin-singlet ground state has been predicted to occur. Kojima et al. use a technique sensitive to magnetism, muon spin rotation spectroscopy, to detect a glass-like disordered paramagnetic state below ∼100 K instead of the expected spin singlet. The authors find that the magnetism is associated with Ir4+ ions in specific locations and that even a minute substitution of Cu by Zn (which also notably reduces the charge order) leads to its disappearance. As this small substitution, equivalent to a single hole per 12 Ir octamers, influences the electronic state in such a dramatic way, the authors hypothesize the existence of interoctamer correlations, which may stabilize the glass-like ground state.

Phys. Rev. Lett. 112, 087203 (2014).

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