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Enigmatic x-ray magnetic circular dichroism in greigite (Fe3S4)

Enigmatic x-ray magnetic circular dichroism in greigite (Fe3S4)
Enigmatic x-ray magnetic circular dichroism in greigite (Fe3S4)
Greigite (Fe3S4), a widely occurring iron thiospinel, was investigated using soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). XAS and XMCD spectra were recorded at the Fe L2,3 edges for pure synthetic and natural greigite samples. At the Fe L3 edge, the XAS spectra reveal two main absorption peaks at 707.2 and 708.6 eV, which are interpreted to originate from greigite and an oxidized surface layer on greigite crystals. The XMCD spectra, which are dominated by a greigite signal, contain three peaks at 705.3, 706.2, and 707.7 eV, all with the same sign. The expectation is that the spectrum would have two negative peaks representing Fe2+ and Fe3+ in octahedral coordination, and a positive peak representing Fe3+ in tetrahedral coordination, as found in stoichiometric magnetite (Fe3O4). A reasonable fit of the XMCD data can be achieved without the tetrahedral Fe component, which contradicts magnetic structural information provided by neutron diffraction analysis, and uses unreasonable parameters. The conundrum between theory and experimental data may be caused by the strong covalent effect in sulfides, which causes broadening of the hybridized XMCD peaks and also indicates that multiplet calculations cannot fully predict the properties of greigite. Our results indicate covalent 3d states in greigite, which can destroy the half-metallicity that is present in magnetite. Our measurements represent the best available XAS and XMCD spectra for greigite, but further experimental and modeling information are needed to explain the observed XMCD spectra and to understand what it represents in terms of electronic and magnetic structure. This is important because greigite widely contributes to the magnetic properties of sedimentary rocks.
greigite, XMCD, XAS, electronic structure, magnetism
0008-4476
667-674
Chang, L.
d6c90fb1-6a1c-4ac0-9b98-8556d2e80d4c
Pattrick, R.A.D.
e4424b08-56e2-4be6-a33b-26c17453299a
van der Laan, G.
e67ed3ec-3925-4180-826a-8ee3c8a24a36
Coker, V.S.
a6cab82c-a6d1-4ae1-a3a2-d72958822a6d
Roberts, A.P.
4497b436-ef02-428d-a46e-65a22094ba52
Chang, L.
d6c90fb1-6a1c-4ac0-9b98-8556d2e80d4c
Pattrick, R.A.D.
e4424b08-56e2-4be6-a33b-26c17453299a
van der Laan, G.
e67ed3ec-3925-4180-826a-8ee3c8a24a36
Coker, V.S.
a6cab82c-a6d1-4ae1-a3a2-d72958822a6d
Roberts, A.P.
4497b436-ef02-428d-a46e-65a22094ba52

Chang, L., Pattrick, R.A.D., van der Laan, G., Coker, V.S. and Roberts, A.P. (2012) Enigmatic x-ray magnetic circular dichroism in greigite (Fe3S4). The Canadian Mineralogist, 50 (3), 667-674. (doi:10.3749/canmin.50.3.667).

Record type: Article

Abstract

Greigite (Fe3S4), a widely occurring iron thiospinel, was investigated using soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). XAS and XMCD spectra were recorded at the Fe L2,3 edges for pure synthetic and natural greigite samples. At the Fe L3 edge, the XAS spectra reveal two main absorption peaks at 707.2 and 708.6 eV, which are interpreted to originate from greigite and an oxidized surface layer on greigite crystals. The XMCD spectra, which are dominated by a greigite signal, contain three peaks at 705.3, 706.2, and 707.7 eV, all with the same sign. The expectation is that the spectrum would have two negative peaks representing Fe2+ and Fe3+ in octahedral coordination, and a positive peak representing Fe3+ in tetrahedral coordination, as found in stoichiometric magnetite (Fe3O4). A reasonable fit of the XMCD data can be achieved without the tetrahedral Fe component, which contradicts magnetic structural information provided by neutron diffraction analysis, and uses unreasonable parameters. The conundrum between theory and experimental data may be caused by the strong covalent effect in sulfides, which causes broadening of the hybridized XMCD peaks and also indicates that multiplet calculations cannot fully predict the properties of greigite. Our results indicate covalent 3d states in greigite, which can destroy the half-metallicity that is present in magnetite. Our measurements represent the best available XAS and XMCD spectra for greigite, but further experimental and modeling information are needed to explain the observed XMCD spectra and to understand what it represents in terms of electronic and magnetic structure. This is important because greigite widely contributes to the magnetic properties of sedimentary rocks.

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More information

Published date: 2012
Keywords: greigite, XMCD, XAS, electronic structure, magnetism
Organisations: Geology & Geophysics

Identifiers

Local EPrints ID: 344888
URI: https://eprints.soton.ac.uk/id/eprint/344888
ISSN: 0008-4476
PURE UUID: 47283b5f-f311-4696-84ab-34635f9c11b2

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Date deposited: 05 Nov 2012 14:50
Last modified: 16 Jul 2019 21:50

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Contributors

Author: L. Chang
Author: R.A.D. Pattrick
Author: G. van der Laan
Author: V.S. Coker
Author: A.P. Roberts

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