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Geometric frustration of spin and orbital moments:

Spin-orbital liquid in FeSc2S4



Frustration characterizes the inability of a system to establish a long-range ordered ground state, despite strong interactions. Frustration and disorder provide the key concept to understand the spin-glass state in disordered magnets. But frustration also can govern fully ordered arrays of spins: Thus unusual ground states characterized, e. g., by resonating valence bonds or with spin order determined by Pauling's ice-rules can be expected. Already in 1956 P. W. Anderson pointed out that the octahedral sites in the spinel structure form a frustrated lattice in which it is possible to achieve perfect short range order while maintaining a finite entropy. Very early on, frustration was hold responsible for an attractive pair interaction between the hole spins in high-temperature superconductors.

frustration

Specific heat C(T)/T for MnSc2S4 (triangles up), FeSc2S4 (triangles down), and CdIn2S4 (circles). The solid line represents the calculated specific heat of the nonmagnetic reference compound CdIn2S4. The dashed line gives the estimated phonon contribution for ASc2S4 (A = Mn, Fe).

 

In transition-metal oxides the orbital moments of the d-electrons play an important role in determining the electronic and magnetic properties and it has been suggested by Y. Tokura and N. Nagaosa that electronic orbitals may play an essential role for a future correlated-electron technology. But orbital physics also became an important branch of basic solid-state physics determining the anisotropy of magnetic and electronic properties. Only recently it has been recognized that in strongly correlated electron systems geometric frustration also should govern the orbital sector. Theoretically it is expected that orbital frustration together with quantum fluctuations have the tendency to suppress any Jahn-Teller instability, yielding new and exotic low-temperature ground states like orbital liquids, orbital glasses or spin-orbital liquids.

In this communication we present structural, magnetic and heat capacity results of the normal cubic spinel FeSc2S4 where the Fe2+ ions, with a spin moment of S = 2, are located exclusively in the tetrahedral A-sites of the spinel structure and are JT active. Hence, long-range spin and orbital order is expected at low temperatures. Instead, we provide experimental evidence that geometric frustration in the spin, as well as in the orbital sector is strong, and the ground state of this thio-spinel has to be characterized as spin-orbital liquid. The results are compared with those obtained from MnSc2S4 with S = 5/2 and a half-filled d-shell which is JT inactive. In this case we find moderate spin frustration, but finally antiferromagnetic order is established at low temperatures. In the latter case residual magnetic interactions drive the magnetic order, while in the iron thio-spinel the strong dependence of the magnetic exchange on the orientation of the orbitals, which are random from site to site, enhances quantum fluctuations and suppresses magnetic and orbital order.

To learn more, see: V. Fritsch et al., Phys. Rev. Lett. 92, 116401 (2004), cond-mat/0402495.