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Licensed Unlicensed Requires Authentication Published by De Gruyter September 20, 2020

Morin-type transition in 5C pyrrhotite

Charles R.S. Haines ORCID logo, Giulio I. Lampronti, Wim T. Klooster, Simon J. Coles, Sian E. Dutton and Michael A. Carpenter
From the journal American Mineralogist

Abstract

We report the discovery of a low-temperature spin-flop transition in 5C pyrrhotite at ~155 K that is similar to those seen in hematite at 260 K and FeS (troilite) at 440 K. The 5C crystal was produced by annealing a 4C pyrrhotite crystal at 875 K to produce a change in the vacancy-ordering scheme that developed during cooling. The 5C structure is confirmed by single-crystal X‑ray diffraction and the stoichiometry and homogeneity by electron microprobe and SEM BSE mapping. Resonant ultrasound spectroscopy (RUS), heat capacity, and magnetization measurements from room temperature down to 2 K are reported. The transition is marked by a steep change in elastic properties at the transition temperature, a peak in the heat capacity, and weak anomalies in measurements of magnetization. Magnetic hysteresis loops and comparison with the magnetic properties of 4C pyrrhotite suggest that the transition involves a change in orientation of moments between two different antiferromagnetic structures, perpendicular to the crystallographic c-axis at high temperatures and parallel to the crystallographic c-axis at low temperatures. The proposed structures are consistent with a group theoretical treatment that also predicts a first-order transition between the magnetic structures.

  1. Funding

    The authors acknowledge funding from the Leverhulme Foundation, grant number RPG2016-298. Heat capacity and DC magnetic measurements were carried out using the Advanced Materials Characterization Suite, funded by EPSRC Strategic Equipment Grant EP/M000524/1. RUS facilities in Cambridge were funded by grants to MAC from the Natural Environment Research Council of Great Britain (grant nos. NE/B505738/1 and NE/F17081/1) and from the Engineering and Physical Sciences Research Council (grant no. EP/I036079/1).

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Received: 2019-08-20
Accepted: 2020-02-26
Published Online: 2020-09-20
Published in Print: 2020-09-25

© 2020 Walter de Gruyter GmbH, Berlin/Boston

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