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Matt Butchers

Supervisors

Dr. Jon Duffy

Prof. Malcolm Cooper

Research interests:


I am a fourth year PhD student under the supervision of Dr. Duffy. I graduated from Exeter University in 2008 working in the magnetic materials group using the magneto optical Kerr effect (MOKE) to image and characterise magnetic surfaces and interfaces. My project at Warwick in the magnetic x-ray scattering group is called 'spin polarisation in magnetic materials'.

We probe magnetic materials using spin polarised x-ray scattering experiments. We look at electron momentum distributions of magnetic systems mainly using the magnetic Compton scattering technique. Materials of current interest to the group are actinide magnetism is superconductors, low dimensional systems. This in conjunction with ab initio calculations can reveal interesting and useful properties, orbital ocupancies, spin states, spin and orbital moments, spin polarisation are all examples.

These experiments require high energy elliptically polarised x-ray photons so they are done at synchotrons in France (ESRF, Grenoble) and Japan (SPring-8, Harima). Samples are grown and characterised (VSM, SQUID, Laue cameras) here at Warwick with help and support from other groups in the department.

My current area of interest is using molecular orbital calculation to produce momentum wavefunctions that can be directly compared to our experimental data (Fig.1) . The electron orbital distribution in real space can be accurately modelled using quantum chemistry codes, these are directly related in momentum space via a Fourier transform. This method allows for the determination of orbital occupations to be made, very useful for resolving spin states in complex oxides, further comments on electron hybridisation effects can also be made.

spin_density

Fig. 1. Reconstructed momentum density of a magnetic oxide from fitting molecular orbitals to magnetic Compton profiles of 3 crystallogrphic axes.

Also of interest is the magnetic structure of uranium superconductors, as the phenomena of superconductivity and ferromagnetism have been long thought to be mutually exclusive, the discovery of the coexisitence in certain uranium alloys is very exciting. Accurate estimates of the spin and orbital moments allow us to compare to theoretical models and refine our understanding of these intriguing systems.

Courses taught

  • PX110: First year laboratory. Demonstrator

X-ray scattering and crystallography - script

  • PX140: First year electronic workshop. Demonstrator

Conferences attended


Date Where? Event
November '08 Nottingham IOP Low Temperature Techniques Conference
December '08 London IOP Magnetism Group Conference (CRIM)
July '09 Munich

Poster at Hands on KKR Course

'Momentum density studies at Warwick'

August '09 Santa Fe

Poster at Sagamore Conference + oral contribution

'High field magentic Compton experiments at the ESRF'

December '09 Warwick

Poster at CMMP 2009

'Spin moments in metamagnetic materials'

June '10 Diamond (Oxford)

Contributed talk and poster at XRMS10

'Spin density in UCoGe'

December '10 Warwick

Poster at CMMP 2010

'Determination of spin density and orbital occupation in the spin chain Ca3Co2O6'

September '11 Durham

Poster at IOP Magnetism Group Conference (CRIM)

'Determination of spin density and orbital occupation in the spin chain Ca3Co2O6'


Experiments Undertaken


Date

What? Experiment Where?

October '08

Magnetic Compton scattering Spin density in bilayer ruthenate Sr3Ru2O7 ID15 ESRF
February '09 XMCD Correlations at magentic / superconducting interface BL025 SPring-8
April '09 Magnetic Compton scattering Spin and orbital moments in spin chain Ca3Co2O6 ID15 ESRF
June '09 Charge Compton scattering 3-dimensionality of Fermi surface in iron pnictide superconductor BL08 SPring-8
June '09 Magnetic Compton scattering Spin moments in quantum critical NbFe2 BL08 SPring-8
July '09 Magnetic Compton scattering Itinerancy of 4f electrons in cerium compounds ID15 ESRF
September '09 Magnetic Compton scattering Spin density in UCoGe ID15 ESRF
June '10 Magnetic Compton scattering High temperature MCS on invar alloys BL08 SPring-8
June '10 Magnetic Compton scattering   BL08 SPring-8
July '10 Magnetic Compton scattering
  ID15 ESRF
Feb '11 Magnetic Comtpon scattering Spin and orbital magnetism in uranium superconductors
ID15 EDRF
April '11 XMCD   I06 Diamond
June '11 Magnetic Compton scattering High temperature Compton study on nickel ID15 ESRF



List of Publications



Phys. Rev. Lett. 109, 137005 (2012)

Author(s): S. R. Giblin, J. W. Taylor, J. A. Duffy, M. W. Butchers, C. Utfeld, S. B. Dugdale, T, Nakamura, C. Visani and J. Santamaria

Abstract: The existence of coherence magnetic correlations in the normal phase of cuprate high-temperature superconductors has proven difficult to measure directly. Here we report on a study of ferromagnetic-superconductor bilayers La2/3Ca1/3MnO3/YBa2Cu3O7 (LCMO/YBCO) with varying YBCO layer thicknesses. Using X-ray magnetic circular dichroism, we demonstrate that th ferromagnetic layer induces a Cu magnetic moment in the adjacent high-temperature superconductor. For thin samples, this moment exists at all temperature below the Curie temperature of the LCMO layer. However, for a YBCO layer thicker than 12 unit cells, the Cu moment is suppressed for temperatures above the superconducting transition, suggesting this to be a direct measurement of magnetic coherence in the normal state of a superconducting oxide.

Phys. Rev. B. 85, 115137 (2012)

Author(s): T. D. Haynes, I. Maskery, M. W. Butchers, J. A. Duffy, J. W. Taylor, S. R. Giblin, C. Utfeld, J. Laverock, S. B. Dugdale, Y. Sakurai, M. Itou, C. Pfleiderer, M. Hirschberger, A. Neubauer, W. Duncan, and F. M. Grosche

Abstract: We report a study of the spin moment in single crystal Nb1-yFe2+y with y=0.015 using spin-dependent Compton scattering in conjunction with ab-initio electronic structure calculations. From the experiment, we find that the total spin moment is 0.245 ± 0.004 μB. Comparison of the measured spin density with theoretical results from LMTO calculations determines there to be a ferrimagnetic arrangement of Fe moments, with the 2a Fe site aligned antiparallel to the bulk moment. There is a small moment on the Nb site, also antiparallel to the net moment. The orbital moment has been determined to be 0.02 ± 0.02μB.


Phys. Rev. B. 81, 134424 (2010)


Author(s): J. A. Duffy, J. W. Taylor, S. B. Dugdale, C. Shenton-Taylor, M. W. Butchers, S. R. Giblin, M. J. Cooper, Y. Sakurai and M. Itou


Abstract: We report a study of the spin moment in Fe3O4 as a function of temperature, using spin-dependent Compton scattering. Data were obtained for applied magnetic fields of 2.5 T and 7 T. We find that there is a spin moment of 4 μB for all temperatures and crystallographic directions. The orbital moment, determined by comparison with bulk magnetometry is less than 0.1 μB. The magnetic Compton profiles have the characteristic shape for Fe 3d electrons as all temperatures. No evidence for any anomalies in the spin moment at the Verwey transition was observed. Our data are consistent with the highly spin polarized electronic struture expected for bulk Fe3O4.

Phys. Rev. B. 101, 064509 (2010)

Author(s): C. Utfeld, J. Laverock, T. D. Haynes, S. B. Dugdale, J. A. Duffy, M. W. Butchers, J. W. Taylor, S. R. Giblin, J. G. Analytis, J. Chu, I. R. Fisher, M. Itou, and Y. Sakurai


Abstract: We report high-resolution, bulk Compton scattering measurements unveiling the Fermi surface of an optimally-doped iron-arsenide superconductor, Ba(Fe0.93Co0.07)2As2. Our measurements are in agreement with first-principles calculations of the electronic structure, revealing both the X-centered electron pockets and the Γ-centered hole pockets. Moreover, our data are consistent with the strong three-dimensionality of one of these sheets that has been predicted by electronic structure calculations at the LDA-minimum As position. Complementary calculations of the non-interacting susceptibility, χ0(q, ω), suggest that the broad peak that develops due to interband Fermi surface nesting, and which has motivated several theories of superconductivity in this class of material, survives the measured three-dimensionality of the Fermi surface in this family.

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Write to:

Department of Physics, University of Warwick, Coventry, CV4 7AL

Contact Details:

Office: P461
Telephone: +44 (0)2476151911

E-mail: M.W.Butchers@warwick.ac.uk

Links:

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