Evolving spin periodicity and lock-in transition in the frustrated ordered ilmenite-type $\beta \text{−}{\mathrm{Mn}}_2{\mathrm{InSbO}}_6$

Polar magnets are promising materials for applications such as multiferroics or in spintronics. In double-corundum-related oxides, the cation ordering imposes a polar structure and the use of high pressure facilitates the insertion of magnetic cations into the compounds. Here we present the high-pressure synthesis of a polar and ferrimagnetic corundum derivative of ${\mathrm{Mn}}_2{\mathrm{InSbO}}_6$, which adopts the ordered-ilmenite-type structure. Neutron powder diffraction reveals that the high-temperature nearly collinear ferrimagnetic phase evolves to an incommensurate helical structure with $k_{\delta }=\left[00k_z\right]$ propagation vector, which then locks to the commensurate value of $k_z=1/8$. This complex magnetic behavior is likely to be related to magnetic frustration and the polar nature of the ordered double-corundum structure.

Figure 1

(a) Crystal structure of ${\left(\mathrm{M}{\mathrm{n}}_{0.5}\mathrm{I}{\mathrm{n}}_{0.5}\right)}_2\mathrm{MnSb}{\mathrm{O}}_6$ along the [110] direction. Gray and blue spheres represent the ordered Sb and Mn sites, respectively. Blue-yellow spheres show the mixed Mn-In sites. (b) Thermal evolution of the direct and inverse magnetic susceptibilities measured at 0.5 and 0.01 T in FC and ZFC modes. Dashed line represents the Curie-Weiss fit.

Figure 2

(a) 2D contour plot showing the temperature evolution of the (003) and (101) magnetic diffraction maxima. The green lines at 38, 17, and 8 K mark the transitions between the different magnetic structures. The panel on the right shows the change in the $z$ component of the propagation vector $k_{\delta }=\left[00k_z\right]$. (b) Magnetic structure fits to the difference between 20 and 40 K (top) and 2 and 40 K (bottom) WISH NPD profiles of ${\left(\mathrm{M}{\mathrm{n}}_{0.5}\mathrm{I}{\mathrm{n}}_{0.5}\right)}_2\mathrm{MnSb}{\mathrm{O}}_6$. The vertical ticks indicate the positions of the Bragg reflections for $k_1=\left[000\right]$ (20 K) and $k_2=\left[001/8\right]$ (2 K) (lower row) models. The panel on the right shows the thermal evolution of the cell parameters, revealing magnetostriction induced by the evolving magnetic structure.

Figure 3

[001] and [110] projections of the magnetic structures of ${\left(\mathrm{M}{\mathrm{n}}_{0.5}\mathrm{I}{\mathrm{n}}_{0.5}\right)}_2\mathrm{MnSb}{\mathrm{O}}_6$. Yellow, red, and blue arrows represent the ${\mathrm{Mn}}^{2+}$ spins at $M1$, $M2$, and $M3$ sites, respectively (Y, R, B). (a) $k_1=\left[000\right]$ at 20 K. (b) Half cell of the $k_2=\left[001/8\right]$ structure showing the frustration relieved by B rotation; there is a ${15}^{\circ }$ rotation between Y-R-B spins slabs. (c) Thermal evolution of the magnetic moments and phase fraction. This reveals the progressive transition of the $k_1$ model into the $k_2$ model below 17 K, both coexisting at 2 K.

Figure 4

Thermal evolution of the dielectric response of ${\left(\mathrm{M}{\mathrm{n}}_{0.5}\mathrm{I}{\mathrm{n}}_{0.5}\right)}_2\mathrm{MnSb}{\mathrm{O}}_6$ (top) showing a transition in the dielectric permittivity below 8 K. The variation in the polarization (middle), determined from the experimental atomic displacements using point-charge model calculations, evidence the onset of a weak magnetoelectric coupling at $T_{N1}=38\mathrm{K}$ which becomes progressively stronger upon cooling down to $T_{N2}=8\mathrm{K}$. Thermal behaviour of average $\langle M-\mathrm{O}\rangle$ bonds (bottom).