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The magnetism of the A-site-ordered perovskite LaMm3V4O12 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 Mn2+ 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. [ABSTRACT FROM AUTHOR]

Crystal structure and magnetic ordering in YMn3Al4O12 are determined by neutron powder diffraction measurement, indicating that S=2 spins of the A'-site Mn3+ ions are ordered in G-type antiferromagnetic (AFM) arrangement in the Im3 cubic crystal structure. Using the first-principles electronic structure calculations, it is found that the AFM spin structure is stabilized by the nearest-neighbour (NN) exchange interaction. The underlying mechanism of the NN exchange is revealed to be the antiferromagnetic Mn-O-Mn superexchange interaction from the calculations of electronic transfer integrals. [ABSTRACT FROM AUTHOR]