Research Guides

Department of Chemistry University of Oxford

Dr Mark Senn

me 

Royal Commission for the Exhibition of 1851 Research Fellow

Inorganic Chemistry

mark.senn@chem.ox.ac.uk

Telephone: +44 (0) 1865 272610

Research Group Pages

 

Please visit http://www.senngroup.com for more details

 

I am a crystallographer with a strong interest in studying electronic and magnetic ordering phenomena in the solid state. The purpose of our work is to elucidate the microscopic mechanisms involved in these processes and correlate these with a material's observed physical properties.  Our work involves performing experiments at central facilities  to study subtle crystallographic distortions.  Modelling and interpreting the resulting complex structure is often the most challenging part of our work.  Symmetry analysis, correlation of local ordering parameters and electronic structure calculations are all used to aid us in this task.

Charge, Orbital and ‘Molecular-like’ Ordering in the Solid State


Our research in this area involves understanding the often complex crystal structures that arise as a result of phase transitions due to electronic ordering phenomena.  The complexity of the resulting structures is often due to the competition between local (ion centred) and global (lattice) degrees of freedom.
In magnetite Fe+3Fe+2.52O4 we have shown that charge ordering in the low temperature (Verwey) phase is only stabilised by a complex orbital ordering involving three sites (Figure 1) .[1,2]  This ‘Molecular-like’ ordering is also found to explain the unusual near integer charge ordering in Ru5.5+ of the Ru2O9 dimers that we have observed in Ba3NaRu2O9 (Figure 2).[3] 
Investigating how molecular like interactions may stabilise unusual forms of charge ordering remains an active area of research.

fig1

 

Magnetic Order

We study magnetic ordering in metal oxides by neutron powder diffraction and single crystal magnetic x-ray scattering.  The theoretical predictions of the structure of magnetically ordered phases remain at best unreliable. The reason for this is that the energy scales involved in magnetic exchange interactions are very small, and the ground state is often a result of a complex completion between many different exchange interactions.  Experimental determination of magnetic structures and rationalisation of the sign and relative magnitude of exchange interactions is hence vitally important to improve the development of reliable theoretical models.   
Understanding how the change in the electronic configuration of an ion may affect the sign (FM / AFM) of the magnetic exchange interaction (Figure 3), and the effect this has on the global magnetic structure is of particular interest to us. [4,5]

fig2

 

Multiferroics

As part of a 3 year RC1851 fellowship investigations into the “microscopic mechanisms in multiferroics materials” will be carried out.  Multiferroic materials which exhibit both magnetism and ferroelectricity are an intriguing subgroup of materials. Understanding how these two phenomena may be coupled together and used to “switch” each other is vitally important work if the technological implications of these materials are to be realised (Figure 4). 
By conducting in situ crystallographic studies at central facilities the switching mechanisms in these materials will be studied as a function of temperature, pressure and applied electric and magnetic fields.

fig3

 

References

[1] Charge order and three-site distortions in the Verwey structure of magnetite. MS Senn, JP Wright, JP Attfield, Nature 481 (2012), 173-176.
[2] Electronic orders in the Verwey structure of magnetite. MS Senn, I Loa, JP Wright, JP Attfield Physical Review B 85 (2012), 125119.
[3] Charge Order at the Frontier between the Molecular and Solid States in Ba3NaRu2O9, SAJ Kimber, MS Senn, S Fratini, H Wu, AH Hill, P Manuel, JP Attfield, Physical Review Letters 108 (2012), 217205.
[4] Spin orders and lattice distortions of geometrically frustrated 6H-perovskites Ba3B′Ru2O9 (B′= La3+, Nd3+, and Y3+), MS Senn, SAJ Kimber, AMA Lopez, AH Hill, JP Attfield, Physical Review B 87 (2013), 134402.
[5]  Nonmagnetic spin-singlet dimer formation and coupling to the lattice in the 6H perovskite Ba3CaRu2O9, MS Senn, AM Arevalo-Lopez, T Saito, Y Shimakawa, JP Attfield ,Journal of Physics: Condensed Matter 25 (2013), 496008.