Magneto-orbital ordering in the divalent A -site quadruple perovskite manganites AMn7O12 ( A=<

Abstract

Through analysis of variable temperature neutron powder diffraction data, we present solutions for the magnetic structures of SrMn7O12, CdMn7O12, and PbMn7O12 in all long-range ordered phases. The three compounds were found to have magnetic structures analogous to that reported for CaMn7O12. They all feature a higher temperature lock-in phase with commensurate magneto-orbital coupling, and a delocked, multi-k magnetic ground state where incommensurate magneto-orbital coupling gives rise to a constant-moment magnetic helix with modulated spin helicity. CdMn7O12 represents a special case in which the orbital modulation is commensurate with the crystal lattice and involves stacking of fully and partially polarized orbital states. Our results provide a robust confirmation of the phenomenological model for magneto-orbital coupling previously presented for CaMn7O12. Furthermore, we show that the model is universal to the A 2+ quadruple perovskite manganites synthesised to date, and that it is tunable by selection of the A-site ionic radius.

Spin-induced multiferroicity in the binary perovskite manganite Mn2O3

“…Due to a complex character of the triclinic crystal structure with 20 symmetry independent magnetic sites per unit cell, solution of the magnetic structures is an extremely challenging problem. To impose some symmetry restrictions, we adapted an approach, which uses the higher-symmetry approximation for the crystal structure and is based on the knowledge of the magnetic structures in closely related A 2+ Mn 7 O 12 (A 2+  = Ca, Sr, Pb and Cd) quadruple manganites 27–29 . This approach is well justified, owing to the fact that the triclinic distortion is very small 11 and could not be resolved in our high-resolution neutron powder diffraction data.…”

“…The trigonal space group with a large unit cell and is an isotropy subgroup, associated with the k S  = (0,0,3/4) propagation vector 30,31 of the group, having a smaller unit cell size and . The latter structural model was used as the parent structure for the magnetic structure solutions and Landau free-energy decomposition in the A 2+ Mn 7 O 12 manganites 27–29 . We also used the (, ) symmetry in our analysis, which allowed us to drastically reduce the number of symmetry unrelated sites and resulted in successful determination of the spin structures in both magnetic phases of Mn 2 O 3 (Fig.…”

“…Apart from k 0 , a set of additional magnetic reflections is observed, which can be indexed using propagation vectors from the series k n ±  =  k 0 ± n k S ( n   =  0, 1, 2…), similar to the multi- k ground state of A 2+ Mn 7 O 12 manganites 28,29 . This observation strongly supports the presence of 3:1 (Mn 3+ :Mn 4+ ) charge order and orbital density wave in the system and reveals a crucial role of the magneto-orbital coupling 28,29,32 . The diffraction patterns provide evidence for the four distinct magnetic components, with k 0  = (0,0,1.2439(3)), k 1 + = (0,0,1.0060(3)), k 1−  = (0,0,0.4940(3)) and k 2+  =  k 2−  (0,0,0.2560(3)) at T  = 1.5 K, which were combined to get a constant-moment phase-modulated cycloid (Fig.…”

“…The magnetic point group is triclinic 1′ and sets no restrictions on the polarization direction. In addition, the symmetry allows an admixture of a helical component similar to the ground state of A 2+ Mn 7 O 12 manganites 28,29 . A tilting of the plane of the spin rotation by 20(2) degrees from the c -axis further improves the refinement quality, implying that the magnetic structure is very general and combines both the major cycloidal and the minor helical components (see Supplementary Table 2 for details).…”

“…Factors that stabilize the complex magnetic structures in the binary perovskite Mn 2 O 3 are probably common for the whole series of A 2+ Mn 7 O 12 manganites 28,29,32 . A competing exchange interactions of the Mn ions occupying both the A-site and B-site positions in the perovskite structure and modulated by the orbital density wave, play a central role in the stabilization of these magnetic structures.…”