Spin singlet formation in MgTi${}_2$O${}_4$: evidence of a helical dimerization pattern

Abstract

The transition metal spinel MgTi${}_2$O${}_4$ undergoes a metal-insulator transition on cooling below $T_{M-I}=260$ K. A sharp reduction of the magnetic susceptibility below $T_{M-I}$ suggests the onset of a magnetic singlet state. Using high-resolution synchrotron and neutron powder diffraction, we have solved the low-temperature crystal structure of MgTi${}_2$O${}_4$, which is found to contain dimers with short Ti-Ti distances (the locations of the spin singlets) alternating with long bonds to form helices. Band structure calculations based on hybrid exchange density functional theory show that, at low temperatures, MgTi${}_2$O${}_4$ is an orbitally ordered band insulator.