Mark’s research activities are centred on the understanding and development of functional materials, in particular thermoelectrics, magnetostrictive and multiferroic systems, and superconductivity. He frequently investigates these materials using state-of-the-art scattering techniques available at large facilities worldwide, and contributes to the development of further powerful techniques. He serves on a number of committees and advisory panels at international facilities, one of these being the European Spallation Source, planned for construction in Lund.
Examples of specific research projects are outlined below:
Terfenol-D Tb1-xDyxFe2 exhibits a giant magnetostriction ~2000 ppm and is currently the leading material used by industry for magnetomechanical applications. Mark and scientists at the University of Maryland explored the morphotropic phase boundary (MPB) in this ferromagnetic material using both synchrotron X-ray and neutron diffraction. The results are highly topical in the light of progress – and controversy – surrounding the nature of the MPBs in ferroelectric materials, including the much-studied lead piezoelectrics. This project formed the bulk of the PhD thesis of Richard Bergstrom. After his thesis, Richard moved to California to work for SGB Labs,an optics company producing holographic displays.
Iron-gallium alloys also exhibit a large magnetostriction ~400 ppm. While this is smaller than for Terfernol-D, iron-gallium possesses more favourable mechanical and magnetic properties, giving breath to new applications such as microactuators and artificial cilia. However the mechanism underpinning magnetostriction is open to controversy. Together with scientists at the University of Maryland, Mark demonstrated the existence of heterogeneities in iron-gallium that are distinctly magnetic. The heterogeneities are further found to couple to the magnetostriction, responding to both magnetic and strain fields. This project formed a large part of the PhD thesis of Chaitanya Mudivarthi. After obtaining his PhD in 2010, Chaitanya moved to Arizona to work for Intel.
- Magnetostriction and magnetic heterogeneities in iron-gallium
M. Laver, C. Mudivarthi, J. R. Cullen, A. B. Flatau, W.-C. Chen, S. M. Watson, M. Wuttig
Physical Review Letters 105, 027202 (2010)
- Magnetic domain observations in Fe–Ga alloys
C. Mudivarthi, S.-M. Na, R. Schaefer, M. Laver, M. Wuttig, A. B. Flatau
Journal of Magnetism and Magnetic Materials 322, 2023–2026 (2010)
- Origin of magnetostriction in Fe–Ga
C. Mudivarthi, M. Laver, J. Cullen, A. B. Flatau, M. Wuttig
Journal of Applied Physics 107, 09A957 (2010)
Nanostructuring of thermoelectrics
The performance of thermoelectric materials is quantified by a dimensionless figure of merit ZT = Normal σS2T/k where Normal 0 σ is the electrical conductivity, S the Seebeck coefficient, T the temperature and k the thermal conductivity. A new paradigm in materials design has emerged following the realization that k may be strongly reduced – and ZT increased – through nanostructuring. In work on (Bi,Sb2)Te3 as part of an international collaboration, we observed that nanocrystalline domains generated by melt-spinning can scatter phonons,reducing k and leading to significant improvement of ZT.
- The microstructure network and thermoelectric properties of bulk (Bi,Sb2)Te3
W. Xie, D. A. Hitchcock, H. J. Kang, J. He, X. Tang, M. Laver, B. Hammouda
Applied Physics Letters 101, 113902 (2012)
- Identifying the specific nanostructures responsible for the high thermoelectric performance of (Bi,Sb2)Te3 nanocomposites
W. Xie, J. He, H. J. Kang, X. Tang, S. Zhu, M. Laver, S. Wang, J. R. D. Copley, C. M. Brown, Q. Zhang, T. M. Tritt
Nano Letters 10, 3283-3289 (2010)
The multiferroic material BiFeO3 exhibits coupled ferroelectric and ferromagnetic order parameters at room temperature. BiFeO3 and related compounds augur well for applications from data storage to spintronics. However the observed ferromagnetism is weak. In collaboration with a team from Rutgers University and the NIST Center for Neutron Research, we resolved a long-standing question as to the nature of the magnetic structure in bulk BiFeO3, where spins arrange antiferromagnetically but with a long-wavelength cycloid modulation superimposed. Our study using polarised neutrons unveils a small but crucial spin-density wave component to this modulation, representing a local ferromagnetic coupling. This represents an important step in our understanding of BiFeO3 and of related multiferroic compounds wherethe magnetic modulation is suppressed.
- Local weak ferromagnetism in single-crystalline ferroelectric BiFeO3
M. Ramazanoglu, M. Laver, W. Ratcliff II, S. M. Watson, W. C. Chen, A. Jackson, K. Kothapalli, Seongsu Lee, S. W. Cheong, V. Kiryukhin
Physical Review Letters 107, 207206 (2011)
Flux lines in superconductors
Flux lines form within a Type-II superconductor in response to an applied magnetic field. Mark’s interest in this area evolved during his PhD with Prof. Ted Forgan in the School of Physics and Astronomy. On the one hand, the flux line system provides a test-bed for theories of structural order in solids and glasses. One the other hand, the orientation and shape of a lattice of flux lines provide insights into the properties of the underlying superconducting state.
In one project, Ted and Mark examined the impact of a well-known mathematical theorem – the hairy ball theorem – upon the formation of flux line lattices in superconductors. The resulting topological effects are general to all Type-II superconductors, and help explain why unconventional flux line lattice shapes may manifest in conventional superconductors. Our study lays the foundation for angle-resolved studies of flux line lattice morphology to aid in the resolution of uncertainties in the physics of unconventional superconductors.
In another ongoing project, Mark is unravelling what happens to flux line correlations in the presence of disorder. Mark and scientists from the Paul Scherrer Institut have mapped out the interplay between magnetism and superconductivity in the La2-xSrxCuO4 family of superconductors, using chemical composition and applied field to tune the emergence of magnetism. Before magnetism completely suppresses superconductivity, it is initially found to cause a disordering in the arrangement of vortices .
Mark has also developed a numerical refinement technique to explore in detail the positional correlations between flux lines from neutron scattering data. This enables, for the first time, a direct test of the theories predicting what happens to flux line correlations in the presence of disorder. As part of his PhD project at DTU Risø, Rasmus Toft-Petersen has been extending Mark’s method with data on superconducting vanadium. After obtaining his doctorate, Rasmus moved to Berlin to work as an instrument scientist on the FLEXX triple-axis spectrometer at the Helmholtz Zentrum Berlin (HZB).
- Spin density wave induced disordering of the vortex lattice in superconducting La2-xSrxCuO4
J. Chang, J. S. White, M. Laver, C. J. Bowell, S. P. Brown, A. T. Holmes, L. Maechler, S. Strässle, R. Gilardi, S. Gerber, T. Kurosawa, N. Momono, M. Oda, M. Ido, O. J. Lipscombe, S. M. Hayden, C. D. Dewhurst, R. Vavrin, J. Gavilano, J. Kohlbrecher, E. M. Forgan, J. Mesot
Physical Review B 85, 134520 (2012)
- Uncovering flux line correlations in superconductors by reverse Monte Carlo refinement of neutron scattering data
M. Laver, E. M. Forgan, A. B. Abrahamsen, C. Bowell, Th. Geue, R. Cubitt
Physical Review Letters 100, 107001 (2008)
- FLEXX instrument at the HZB
Other research interests
- Magnetic nanoparticles
- Correlated electrons and superconductivity
- Frustrated magnetism