Abstract
Iron(IV) oxides are strongly correlated materials with negative charge-transfer energy (negative \ensuremath{\Delta}), and exhibit peculiar electronic and magnetic properties such as topological helical spin structures in the metallic cubic perovskite ${\mathrm{SrFeO}}_{3}$. Here, the spin structure of the layered negative-\ensuremath{\Delta} insulator ${\mathrm{Sr}}_{2}{\mathrm{FeO}}_{4}$ was studied by powder neutron diffraction in zero field and magnetic fields up to 6.5 T. Below ${T}_{\mathrm{N}}=56\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, ${\mathrm{Sr}}_{2}{\mathrm{FeO}}_{4}$ adopts an elliptical cycloidal spin structure with modulated magnetic moments between 1.9 and 3.5 ${\ensuremath{\mu}}_{\mathrm{B}}$ and a propagation vector $\mathbit{k}=(\ensuremath{\tau},\ensuremath{\tau},0)$ with $\ensuremath{\tau}=0.137$. With increasing magnetic field the spin structure undergoes a spin-flop transition near 5 T. Synchrotron $^{57}\mathrm{Fe}$-M\"ossbauer spectroscopy reveals that the spin spiral transforms to a ferromagnetic structure at pressures between 5 and 8 GPa, just in the pressure range where a Raman-active phonon nonintrinsic to the ${\mathrm{K}}_{2}{\mathrm{NiF}}_{4}$-type crystal structure vanishes. These results indicate an insulating ground state which is stabilized by a hidden structural distortion and differs from the charge disproportionation in other Fe(IV) oxides.
Highlights
The interplay between charge, orbital, spin, and lattice degrees of freedom in compounds with strong electron correlations leads to a large, complex array of structural and physical properties [1]
Synchrotron 57Fe-Mössbauer spectroscopy reveals that the spin spiral transforms to a ferromagnetic structure at pressures between 5 and 8 GPa, just in the pressure range where a Raman-active phonon nonintrinsic to the K2NiF4-type crystal structure vanishes. These results indicate an insulating ground state which is stabilized by a hidden structural distortion and differs from the charge disproportionation in other Fe(IV) oxides
In this work we report a powder neutron diffraction study which reveals that Sr2FeO4 forms an elliptical cycloidal spin spiral undergoing a spin-flop transition in a magnetic field of about 5 T
Summary
The interplay between charge, orbital, spin, and lattice degrees of freedom in compounds with strong electron correlations leads to a large, complex array of structural and physical properties [1]. Examples of negative- oxides include NaCuO2 [4] and several rare-earth nickelates LnNiO3 the insulating ground states of which feature a charge disproportion (CD) transition of the nominal Ni3+ ions accompanied by a breathing-type structural distortion [5,6]. CaFeO3 [14,15] or Sr3Fe2O7 [16,17,18,19] are known to exhibit a comparable CD, whereas metallic SrFeO3 [20,21] as well as the cubic phase of BaFeO3 [22] do not show any CD These ferrates were shown to be negativematerials [23] and the CD state was formulated as 2d5L−1 (t2g3eg1 ) → d5 (t2g3eg2 ) + d5L−2(t2g3eg0 ), where, similar to the nickelates, singly occupied σ -type eg∗ orbitals occur, being coupled to more-localized π -type t2g. Our results shed new light on the nature of insulating states and emerging spin structures in negative- materials which may be of relevance for understanding topological spin textures in this class of compounds
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