Elastic and anelastic relaxations accompanying magnetic ordering and spin-flop transitions in hematite, Fe 2 O 3

Oravova, Lucie and Zhang, Zhiying and Church, Nathan S. and Harrison, Richard J. and Howard, Christopher J. and Carpenter, Michael A. (2013) Elastic and anelastic relaxations accompanying magnetic ordering and spin-flop transitions in hematite, Fe 2 O 3. Journal of Physics: Condensed Matter, 25 (11). p. 116006. ISSN 0953-8984, 1361-648X DOI 10.1088/0953-8984/25/11/116006

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Abstract

Hematite, Fe2O3, provides in principle a model system for multiferroic (ferromagnetic/ferroelastic) behavior at low levels of strain coupling. The elastic and anelastic behavior associated with magnetic phase transitions in a natural polycrystalline sample have therefore been studied by resonant ultrasound spectroscopy (RUS) in the temperature range from 11 to 1072 K. Small changes in softening and attenuation are interpreted in terms of weak but significant coupling of symmetry-breaking and non-symmetry-breaking strains with magnetic order parameters in the structural sequence $R\overline{3}c{1}^{\prime}\rightarrow C 2/c\rightarrow R\overline{3}c$. The $R\overline{3}c{1}^{\prime}\rightarrow C 2/c$ transition at TN = 946 ± 1 K is an example of a multiferroic transition which has both ferromagnetic (from canting of antiferromagnetically ordered spin moments) and ferroelastic (rhombohedral → monoclinic) character. By analogy with the improper ferroelastic transition in Pb3(PO4)2, W and W' ferroelastic twin walls which are also 60° and 120° magnetic domain walls should develop. These have been tentatively identified from microstructures reported in the literature. The very low attenuation in the stability field of the C2/c structure in the polycrystalline sample used in the present study, in comparison with the strong acoustic dissipation reported for single crystal samples, implies, however, that the individual grains each consist of a single ferroelastic domain or that the twin walls are strongly pinned by grain boundaries. This absence of attenuation allows an intrinsic loss mechanism associated with the transition point to be seen and interpreted in terms of local coupling of shear strains with fluctuations which have relaxation times in the vicinity of ~10−8 s. The first order $C 2/c\rightarrow R\overline{3}c$ (Morin) transition occurs through a temperature interval of coexisting phases but the absence of an acoustic loss peak suggests that the relaxation time for interface motion is short in comparison with the time scale of the applied stress (at ~0.1–1 MHz). Below the Morin transition a pattern of attenuation which resembles that seen below ferroelastic transitions has been found, even though the ideal low temperature structure cannot contain ferroelastic twins. This loss behavior is tentatively ascribed to the presence of local ferromagnetically ordered defect regions which are coupled locally to shear strains.

Item Type:	Article
Additional Information:	See related URL link for corrigendum
Uncontrolled Keywords:	2013AREP; IA65;
Subjects:	03 - Mineral Sciences
Divisions:	03 - Mineral Sciences
Journal or Publication Title:	Journal of Physics: Condensed Matter
Volume:	25
Page Range:	p. 116006
Identification Number:	10.1088/0953-8984/25/11/116006
Depositing User:	Sarah Humbert
Date Deposited:	25 Feb 2013 13:48
Last Modified:	23 Jul 2013 10:05
URI:	http://eprints.esc.cam.ac.uk/id/eprint/2682

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