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The influence of crystallinity on magnetic ordering in natural ferrihydrites

Published online by Cambridge University Press:  09 July 2018

E. Murad ,
L. H. Bowen ,
G. J. Long  and
T. G. Quin
E. Murad
Affiliation:
Lehrstuhl für Bodenkunde, Technische Universität München, D-8050 Freising-Weihenstephan, Federal Republic of Germany
L. H. Bowen
Affiliation:
Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA
G. J. Long
Affiliation:
Department of Chemistry, University of Missouri-Rolla, Rolla, MO 65401-0249, USA, and Department of Physics, University of Liverpool, Liverpool L69 3BX, UK
T. G. Quin
Affiliation:
Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, UK
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Abstract

Mössbauer spectra of three natural ferrihydrites of different crystallinities were collected between 295 and 4·2 K. Optimal fits of the room-temperature spectra were obtained using distributions of quadrupole-split doublets; half-widths of the distributions (0·71–0·86 mm/s) increased, and maxima (at 0·62–0·78 mm/s) shifted to higher quadrupole splittings with decreasing crystallinity. Rather poorly-defined magnetically-ordered spectral components appeared and gradually replaced the superparamagnetic doublets as temperatures were reduced. Blocking temperatures between 115 and 28 K, derived from the relative areas under the doublets and magnetic sextets, indicated average particle sizes between ∼ 5 and 3 nm. The sextets were relatively broad and could be fitted with distributions of hyperfine fields. These distributions narrowed and their maxima shifted to higher fields as temperatures decreased; the influence of superparamagnetism was no longer apparent at 4·2 K.

Type
Research Article
Information
Clay Minerals , Volume 23 , Issue 2 , June 1988 , pp. 161 - 173
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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