PGI Colloquium:

Prof. Dr. J. Paul Attfield,
University of Edinburgh, Edinburgh, UK

 

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31 Jan 2020 11:00

Copyright: Prof Dr. Attfield

Early concepts of magnetism emerged from studies of magnetic minerals, notably magnetite (Fe₃O₄). Today we know of many types of magnetism and magnetic materials, but transition metal oxides remain important as they are based on abundant, non-toxic elements and can offer large magnetisations at room temperature. They have also been investigated intensively for coupling of magnetism to other phenomena, for example, to electronic conductivity for spintronic materials; to ferroelectricity in multiferroics; and to lattice thermodynamics in magnetocalorics. This talk will present new chemical and physical aspects of spintronic oxides.

‘Manganites’ are manganese oxides such as La_0.7Sr_0.3MnO₃ where mixing of large La and Sr cations at the A sites of the ABO₃ perovskite structure induces ferromagnetism and electronic conductivity. High pressure has recently been used to synthesise new ‘A-site manganites’ with Mn²⁺ cations at the A-sites,^1,2 such as Mn₂FeReO₆ which has a high Curie temperature of 520 K and similar ferrimagnetic and spin-polarised conducting properties to the much-studied magnetoresistive double perovskite Sr₂FeMoO₆, but also shows a novel switch from negative to large positive magnetoresistances at low temperatures driven by Mn²⁺ spin ordering. Investigation of possible rare earth (R) analogues has led to discovery of a new ‘double double perovskite’ type MnRMnSbO₆ (R = La, Pr, Nd, Sm) with simultaneous 1:1 cation order at both A and B sites.

Magnetite (Fe₃O₄) is the original magnetic material and undergoes the complex Verwey structural distortion below 125 K. The nature of the ground state was unclear for over 70 years until determination of the full superstructure showed that Fe²⁺/Fe3+ charge ordering occurs with a pronounced orbital ordering of Fe2+ states, but an unexpected localization of electrons in linear, three-Fe ‘trimeron’ units was also discovered. Electronic phase separation driven by trimeron formation has recently been discovered in CaFe₃O₅. Finally, some recent results revealing the origin of the Verwey transition will be presented.

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[iii] E. Solana-Madruga, Á. M. Arévalo-López, A. J. Dos Santos-García, E. Urones-Garrote, D. Ávila-Brande, R. Sáez-Puche, J. P. Attfield. Angew. Chem. 2016, 55, 9340.

[iv] M.S. Senn, J.P. Wright, J.P. Attfield, Nature 2012, 481, 173.

[v] K. H. Hong, A. M. Arevalo-Lopez, J. Cumby, C. Ritter; J. P. Attfield Nature Comm. 2018, 9, 2975.

[vi] G. Perversi, E. Pachoud, J. Cumby, J. M. Hudspeth, J. P. Wright, S. A. J. Kimber; J. P. Attfield, Nature Comm. 2019.