Symmetry-breaking 60°-spin order in the A-site-ordered perovskite $\mathrm{LaM}{\mathrm{n}}_3{\mathrm{V}}_4{\mathrm{O}}_{12}$

The magnetism of the A-site-ordered perovskite $\mathrm{LaM}{\mathrm{n}}_3{\mathrm{V}}_4{\mathrm{O}}_{12}$ is studied comprehensively by means of neutron powder diffraction experiments and theoretical calculations. Magnetic neutron diffraction results show that a rhombohedral 60° spin structure emerges on the cubic lattice below a 44-K Néel transition. Ab initio electronic structure calculations confirm that high-spin ${\mathrm{Mn}}^{2+}$ moments are localized while V $3d$-band states are itinerant, and that the noncollinear 60° spin structure is more stable than collinear ferromagnetic or G-type antiferromagnetic alternatives. Effective Heisenberg model calculations reveal that the appearance of such a nontrivial spin structure can be attributed to significant next-nearest-neighbor and third-nearest-neighbor magnetic interactions.

Figure 1

Temperature dependence of the magnetic susceptibility of $\mathrm{LaM}{\mathrm{n}}_3{\mathrm{V}}_4{\mathrm{O}}_{12}$ measured in a magnetic field of 1 T. The crystal structure of an A-site-ordered perovskite $A{A}_3^{\prime }{B}_4{\mathrm{O}}_{12}$ is shown in the inset.

Figure 2

Rietveld plots of the neutron powder diffraction patterns at (a) 300 K and (b) 2 K. The observed (+), calculated (solid line), and difference (bottom) patterns are shown. The ticks indicate the allowed Bragg reflections for nuclear (above) and magnetic (below) phases.

Figure 3

Neutron powder diffraction patterns of $\mathrm{LaM}{\mathrm{n}}_3{\mathrm{V}}_4{\mathrm{O}}_{12}$ at 300 and 2 K. The ticks indicate the allowed Bragg reflections for the (lower) cubic nuclear lattice and (upper) rhombohedral magnetic lattice. The magnetic Bragg reflections are shaded red. A single reflection from the vanadium container (*) is observed at around $2\theta ={40}^{\circ }$.

Figure 4

Two views of the spin arrangement of $\mathrm{LaM}{\mathrm{n}}_3{\mathrm{V}}_4{\mathrm{O}}_{12}$ at 2 K. The rhombohedral magnetic unit cell is shown in a hexagonal setting, $a_{\mathrm{r}}\times b_{\mathrm{r}}\times 2c_{\mathrm{r}}$. The cubic cell is also drawn in the left figure. The different $z$ heights of the spins are shown by different colors: red ($z=0$), green ($z=1/6$), blue ($z=1/3$), black ($z=1/2$), sky blue ($z=2/3$), and purple ($z=5/6$).

Figure 5

(a) Density of states (DOS) of $\mathrm{LaM}{\mathrm{n}}_3{\mathrm{V}}_4{\mathrm{O}}_{12}$. Partial DOS (PDOS) of each constituent atom is also shown in the bottom panel. (b) Mn-Mn spin interactions in $\mathrm{LaM}{\mathrm{n}}_3{\mathrm{V}}_4{\mathrm{O}}_{12}$. $J$₁, $J$₂, and $J$₃ are the NN, NNN, and third-NN interactions, respectively. Spin arrangements considered for the magnetic energy calculation: (c) ferromagnetic (FM) arrangement, (d) G-type antiferromagnetic (AFM) arrangement, (e) noncollinear rhombohedral arrangement with the nearest-neighboring spins making an angle of 60° (NR-AFM1), and (f) noncollinear rhombohedral arrangement with nearest-neighboring spins making an angle of 120° (NR-AFM2). (g) Magnetic phase diagrams stabilized by different $J$₁, $J$₂, and $J$₃ magnetic interactions. Cases for $J_1>0$ and $J_1<0$ are shown.