Magnetoelastic coupling associated with vacancy ordering and ferrimagnetism in natural pyrrhotite, Fe7S8

C R S Haines,G I Lampronti,M A Carpenter

doi:10.1088/1361-648x/ab9053

C R S Haines, G I Lampronti + Show 1 more

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https://doi.org/10.1088/1361-648x/ab9053

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Abstract

Magnetoelastic coupling associated with the hexagonal—monoclinic transition in a natural sample of the mineral pyrrhotite, Fe7S8, has been analysed in terms of separate coupling of spontaneous strains with two discrete order parameters, qv for Fe/vacancy ordering and qm for magnetic ordering. Coupling of the two order parameters separately with strain gives rise to two terms for coupling between them, λ and λ, and a pattern of evolution in which qv varies continuously and qm discontinuously through a single transition point. The transition is ferrimagnetic and ferroelastic but the relatively slow relaxation rate for Fe/vacancy ordering, in comparison with magnetic ordering, results in elastic and anelastic properties which are quite different from those observed in other ferroic or multiferroic materials with two instabilities. Instead of classical elastic softening, there is stiffening of the elastic constants which scales with and . Instead of the normal pattern of acoustic loss associated with the mobility and subsequent freezing for ferroelastic twin walls, the loss is consistently low throughout the temperature range 300 K–875 K.

Highlights

A notable feature of the mineral pyrrhotite, Fe1−xS, is that it displays wide variations in magnetic and thermodynamic properties over a narrow range of compositions corresponding to 0 > x > 0.125 [1,2,3,4,5,6]
Magnetoelastic coupling associated with the hexagonal—monoclinic transition in a natural sample of the mineral pyrrhotite, Fe7S8, has been analysed in terms of separate coupling of spontaneous strains with two discrete order parameters, qv for Fe/vacancy ordering and qm for magnetic ordering
The transition is ferrimagnetic and ferroelastic but the relatively slow relaxation rate for Fe/vacancy ordering, in comparison with magnetic ordering, results in elastic and anelastic properties which are quite different from those observed in other ferroic or multiferroic materials with two instabilities

Summary

Introduction

A notable feature of the mineral pyrrhotite, Fe1−xS, is that it displays wide variations in magnetic and thermodynamic properties over a narrow range of compositions corresponding to 0 > x > 0.125 [1,2,3,4,5,6]. These have resulted in it having signi cance in contexts as disparate as palaeomagnetism on Earth [7] and Mars [8, 9], mineral exploration [10] and phase change magnetic memories [11]. Changes in the hard and soft directions for magnetic ordering as functions of composition and temperature lead to a further set of magnetic transitions below the primary Néel point, as summarised, for example, by Schwarz and Vaughan (1972) [2]

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