Spin Jahn-Teller effect in the antiferromagnet CoTi₂O₅

We have used a combination of neutron powder diffraction and muon spin rotation experiments, complemented with DFT calculations, to solve the magnetic structure of orthorhombic CoTi₂O₅, which we find adapts a long-range ordered, antiferromagnetic state below 26 K with a moment of 2.72(1)μ_B per Co²⁺ ion and propagation vector k=(±1/2,1/2,0) ^[1]. Interestingly, in the experimentally determined crystal structure all the magnetic exchange couplings are completely frustrated by the underlying symmetry. Therefore, we conclude that the magnetic transition must driven by a Spin Jahn-Teller effect, in which the large spin degeneracy is relieved by a distortion of the crystal structure and an associated lowering of the structural symmetry. We investigate this distortion using high resolution x-ray experiments and DFT calculations. Furthermore, we discuss recent experimental studies of FeTi₂O₅ and the observed similarities to CoTi₂O₅, which leads us to conlcude that spin-phonon coupling can induce magnetic order in lower symmetry systems than previously reported.

[1] arXiv:1808.01387v1