Spiral magnetism, spin flop, and pressure-induced ferromagnetism in the negative charge-transfer-gap insulator ${\mathrm{Sr}}_{2}{\mathrm{FeO}}_{4}$

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Spiral magnetism, spin flop, and pressure-induced ferromagnetism in the negative charge-transfer-gap insulator ${Sr}_{2} {FeO}_{4}$

Peter Adler, Manfred Reehuis, Norbert Stüßer, Sergey A. Medvedev, Michael Nicklas, Darren C. Peets, Joel Bertinshaw, Christian Kolle Christensen, Martin Etter, Andreas Hoser, Liane Schröder, Patrick Merz, Walter Schnelle, Armin Schulz, Qingge Mu, Dimitrios Bessas, Aleksandr Chumakov, Martin Jansen, and Claudia Felser

Phys. Rev. B 105, 054417 – Published 17 February 2022

Abstract

Iron(IV) oxides are strongly correlated materials with negative charge-transfer energy (negative Δ), and exhibit peculiar electronic and magnetic properties such as topological helical spin structures in the metallic cubic perovskite ${SrFeO}_{3}$ . Here, the spin structure of the layered negative-Δ insulator ${Sr}_{2} {FeO}_{4}$ was studied by powder neutron diffraction in zero field and magnetic fields up to 6.5 T. Below $T_{N} = 56 K$ , ${Sr}_{2} {FeO}_{4}$ adopts an elliptical cycloidal spin structure with modulated magnetic moments between 1.9 and 3.5 $μ_{B}$ and a propagation vector $k = (τ, τ, 0)$ with $τ = 0.137$ . With increasing magnetic field the spin structure undergoes a spin-flop transition near 5 T. Synchrotron ${}^{57}{Fe}$ -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 $K_{2} {NiF}_{4}$ -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.

Received 1 September 2021
Revised 3 February 2022
Accepted 3 February 2022

DOI:https://doi.org/10.1103/PhysRevB.105.054417

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Magnetic order Pressure effects Structural properties

Noncollinear magnets

Mössbauer spectroscopy Neutron diffraction Raman spectroscopy X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Peter Adler ^1,*, Manfred Reehuis², Norbert Stüßer², Sergey A. Medvedev¹, Michael Nicklas ¹, Darren C. Peets ³, Joel Bertinshaw⁴, Christian Kolle Christensen⁵, Martin Etter ⁵, Andreas Hoser ², Liane Schröder¹, Patrick Merz¹, Walter Schnelle¹, Armin Schulz ⁵, Qingge Mu¹, Dimitrios Bessas ⁶, Aleksandr Chumakov ⁶, Martin Jansen ^1,4,†, and Claudia Felser¹

¹Max-Planck-Institut für Chemische Physik fester Stoffe, 01187 Dresden, Germany
²Helmholtz-Zentrum für Materialien und Energie, 14109 Berlin, Germany
³Institut für Festkörper- und Materialphysik, Technische Universität Dresden, 01069 Dresden, Germany
⁴Max-Planck-Institut für Festkörperforschung, 70569 Stuttgart, Germany
⁵Deutsches Elektronen-Synchrotron (DESY), 22607 Hamburg, Germany
⁶ESRF-The European Synchrotron, CS 40220, 38043 Grenoble Cedex 9, France

^*adler@cpfs.mpg.de
^†M.Jansen@fkf.mpg.de

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Vol. 105, Iss. 5 — 1 February 2022

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Figure 1
Magnetic properties of ${Sr}_{2} {FeO}_{4}$ . (a) Top: dc magnetic susceptibility measured after zero-field cooling in different magnetic fields. S and P correspond to measurements using a SQUID and a PPMS-based magnetometer, respectively. Bottom: Real part of the ac susceptibility measured in zero field at the indicated frequencies; the inset highlights a weak feature near 10 K. (b) Isothermal magnetization measured at the indicated temperatures. Inset: Heat capacity $C_{p} / T$ in zero field as a function of temperature.
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Figure 2
Powder neutron diffraction patterns of ${Sr}_{2} {FeO}_{4}$ . Data were collected on diffractometer E2 $(λ = 2.380 Å)$ at 80 and 1.8 K (top and middle panel) at zero field. The observed magnetic reflections at 1.8 K are generated with the rule ${(h k l)}_{M} = {(h k l)}_{N} \pm k$ , where the incommensurate propagation vector is found to be $k = (τ, τ, 0)$ with $τ = 0.1366 (2)$ . In an applied vertical magnetic field of 6.5 T magnetic satellites with higher $l$ like ${(002)}^{\pm}$ , ${(004)}^{\pm}$ , ${(103)}^{-}$ and ${(103)}^{+}$ show a decrease in intensity (bottom panel). The results from Rietveld refinements are only shown for the data sets collected in zero field, where for the magnetic structure at 1.8 K we used Model II. The calculated patterns (red solid line) are compared with the observations (black-filled circles). The positions of the generated magnetic reflections (black bars) as well as the difference patterns ( $I_{obs} - I_{cal}$ ) (blue solid line) are also shown. In the case of the data set collected at 6.5 T only the observed intensities are shown and the solid black line is a guide-to-the-eye.
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Figure 3
Powder neutron diffraction study on ${Sr}_{2} {FeO}_{4}$ . (a) Magnetic Bragg reflections at 0 and 5.5 T (red and green) obtained by subtraction of the pattern at 80 K from the ones at 1.8 K. The intensity change of magnetic intensities between 0 and 5.5 T (black) is shown at the bottom. The magnetic satellites ${(h k l)}^{\pm}$ are generated with the propagation vector $k = (0.137, 0.137, 0)$ . (b) Temperature dependence of magnetic moments $μ_{\min}$ and $μ_{maj}$ and the vector component $k_{x} = τ$ . (c) Field dependence of the ${(002)}^{\pm}$ satellite intensity compared with the magnetization obtained from PPMS measurements. (d) Illustration of the magnetic structure of ${Sr}_{2} {FeO}_{4}$ in zero field. The magnetic moments of the Fe atoms form ferromagnetic layers perpendicular to the propagation vector $k$ pointing along the [110] direction. At the bottom the magnetic structure is projected onto the $a b$ plane. The moments are tilted away from the $c$ axis by an angle β ∼ 70°.
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Figure 4
(a) Raman spectra of ${Sr}_{2} {FeO}_{4}$ at the indicated temperatures. The star marks an anomalous Raman band referred to in the text. (b) Synchrotron powder x-ray diffraction patterns measured at 100 and 500 K. The calculated patterns for space group $I 4$ /mmm (red solid lines) are compared with the observations (black-filled circles). The difference patterns ( $I_{obs} - I_{cal}$ ) (blue solid lines) are also shown.
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Figure 5
${}^{57}{Fe}$ Mössbauer spectra of ${Sr}_{2} {FeO}_{4}$ (a) Mössbauer spectra at the indicated temperatures. The sample was enriched with ${}^{57}{Fe}$ to 20%. Dots correspond to experimental data, solid lines to calculated spectra. The spectrum at 6 K was fitted by using a superposition of eight hyperfine components (colored lines). (b) Hyperfine fields $B_{hf}$ and corresponding quadrupole shifts 2ε obtained from this fit model. (c) Fit of the spectrum at 6 K obtained by assuming a linear correlation between $B_{hf}$ and 2ε and extracting a $B_{hf}$ distribution which is shown in (d). (e) $B_{hf}$ distribution calculated for an incommensurate elliptical spin structure according to Eq. (1).
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Figure 6
Synchrotron Mössbauer spectra of ${Sr}_{2} {FeO}_{4}$ at 5 and 8 GPa in zero field and at 8 GPa in a longitudinal field of 2 T. The vertical lines (green) highlight the outer line positions at 8 GPa and 0 T. The samples used here were enriched with ${}^{57}{Fe}$ to 20% (8-GPa spectra) or 10% (5-GPa spectrum).
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Physical Review B

covering condensed matter and materials physics

Spiral magnetism, spin flop, and pressure-induced ferromagnetism in the negative charge-transfer-gap insulator ${Sr}_{2} {FeO}_{4}$

Phys. Rev. B 105, 054417 – Published 17 February 2022

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Physical Review B

covering condensed matter and materials physics

Spiral magnetism, spin flop, and pressure-induced ferromagnetism in the negative charge-transfer-gap insulator Sr2FeO4

Phys. Rev. B 105, 054417 – Published 17 February 2022

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Spiral magnetism, spin flop, and pressure-induced ferromagnetism in the negative charge-transfer-gap insulator ${Sr}_{2} {FeO}_{4}$