Abstract
Orthogonal spin ordering is rarely observed in magnetic oxides because nearest-neighbor symmetric Heisenberg superexchange interactions usually dominate. We have discovered that in the quadruple perovskite $\mathrm{Ca}{\mathrm{Fe}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$, where only the $S=2$ ${\mathrm{Fe}}^{2+}$ ion is magnetic, long-range magnetic order consisting of an unusual arrangement of three interpenetrating orthogonal sublattices is stabilized. Each magnetic sublattice corresponds to a set of $\mathrm{Fe}{\mathrm{O}}_{4}$ square planes sharing a common orientation. This multi-$k$ magnetic spin ordering is the result of fourth-neighbor spin couplings with a strong easy-axis anisotropy. In an applied magnetic field, each sublattice tends towards ferromagnetic alignment, but remains polarized by internal magnetic fields generated by the others, thus stabilizing in a noncollinear canted ferromagnetic structure. $\mathrm{Ca}{\mathrm{Fe}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ provides a rare example of how nontrivial long-range spin order can arise when near-neighbor Heisenberg superexchange is quenched.
Highlights
Nontrivial spin orderings often underlie exotic physical phenomena such as high-TC superconductivity, magnetoresistance,ferroelectricity, and topological insulating behavior [1,2,3]
Neutron powder-diffraction data collected from CaFe3Ti4O12 at 300 K confirm the A-site ordered quadruple perovskite structure [Fig. 1(a), Figs
A second small doublet corresponds to the signal of the octahedrally coordinated Fe2+ centers in FeTiO3, which is detected in neutron-diffraction data [approximately 4.6(1) wt. %]
Summary
Nontrivial spin orderings often underlie exotic physical phenomena such as high-TC superconductivity, magnetoresistance, (multi)ferroelectricity, and topological insulating behavior [1,2,3]. In the majority of complex magnetic transition-metal oxides, magnetic properties are dominated by symmetric (Heisenberg) exchange interactions which are typically orders of magnitude stronger than other interactions between magnetic centers. This tends to result in comparatively simple (anti)ferromagnetic structures [5,6,7]. The material CaFe3Ti4O12 is the only known quadruple perovskite where the A sites are exclusively occupied by Fe2+ centers with S = 2, yet initial studies did not find any long-range magnetic order down to 4.2 K [15,16]. Heisenberg superexchange between first- and secondneighbor spins, allowing further-neighbor interactions to stabilize an orthogonal long-range spin arrangement
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