The Oxford MBE Group: Growing Tailored Structures to Probe Quantum Effects

 

We focus on the growth of ferromagnets, topological insulators and magnetic insulators through Molecular Beam Epitaxy (MBE), Sputtering and Chemical Vapour Deposition (CVD). Topological insulators are a cutting edge class of materials where only the surface states are conducting, but perfectly so, and have transformative potential for the electronics industry. We seek to probe the resilience and character of his topological state through high-precision growth techniques that allow us to minutely tune properties, such as through doping with ferromagnetic ions, bilayering or cleaving. Our characterisation methods include X-Ray Diffraction and Reflectometry, Muon Spin Rotation, Ferromagnetic Resonance and ARPES. Our fortunate position in close proximity to world lead facilities gives us access to a diverse community of scientific knowledge as well as second-to-none experiments through which we study our samples and push at the boundaries of our current understanding. Experimental work is carried out at large scale research facilities including DLS, ISIS, ALS, Bessy, PSI and FRMII

MBE Growth of TIs

Topological insulators have attracted much interest recently from Condensed Matter Physics as well as the wider scientific community. They are characterized by having bulk electronic states like that of a standard band gap insulator but with spin-momentum locked conduction channels only at the surface of the material. This spin-polarised surface conduction channel opens up a wide range of exciting technological applications in the future including spintronics and quantum computation. By extension of this effect to so-called topological superconductors it may be possible to observe in the system a Majorana Fermion state. For a well-written review of topological insulators so far please try (M. Z. Hasan , C. L. Kane Rev. Mod. Phys. (82), 3045-3067)

Molecular Beam Epitaxy (MBE) allows for growth of high-quality single crystal films by deposition onto a substrate material of molecular beams produced from ultra-high purity elemental sources. MBE growth, a stalwart of the semiconductor industry, is used by our group to grow thin films of Bi2Se3 and Bi2Te3 and related compounds. These films are then used in a wide range of experiments with our collaborators at ISIS and Diamond as well as further afield to try and probe the fundamental nature of the topological surface state. It is hoped that high-quality MBE growth of films may overcome some of the materials problems that have so far held back development of TI-based technologies.

 

Skyrmions

Skyrmions are topologically stable, vortex-like magnetization states that form periodic, three-fold symmetric lattices. They were observed in non-centrosymmetric crystals, such as B20 systems, in which the Dzyaloshinskii-Moriya interaction plays a role, using small angle neutron scattering and magnetotransport measurements (topological Hall effect), and in real space using Lorentz transmission electron microscopy. Since each Skyrmion can carry one bit of (binary) information, the crystal itself can be regarded as a high-density, non-volatile information matrix. Most interestingly, the Skyrmion state can be simply manipulated with current densities that are 5-6 orders of magnitude smaller than the ones needed for spin transfer torque (STT)-based schemes. Moreover, direct logic communication can be achieved by introducing the interaction and propagation of vortex/anti-vortex pairs. The value of the Skyrmionics devices lie in the fast and efficient evaluation of suitable materials for STT-MRAM scaling beyond the 65-nm- node, as well as novel emerging memory and logic applications which could become possible by making use of these intriguing physical properties.

 

Ferromagnetic Resonance

FMR allows us to study the magnetic relaxation of ferromagnetic films and doped topological insulators, giving information on interlayer coupling, internal damping mechanisms and spin pumping. This sophisticated technique uses a Vector Network Analyser to pass microwaves in the GHz range across our samples and measure absorption. When the microwaves are in resonance with the precession of magnetic moments within the sample they are absorbed more strongly. This absorption is affected by factors such as magnetisation of the sample or the orientation of magnetocrysalline easy axes, and we access such information by fitting functions to the data.

 

Recent Publications

A. Kohn, N. Tal, A. Elkayam, A. Kovacs, D. Li, S. Wang, S. Ghannadzadeh, T. Hesjedal, R.C.C. Ward (2013)
“Structure of epitaxial L10-FePt/MgO perpendicular magnetic tunnel junctions”
Applied Physics Letters 102, 030307.

SG Wang, RCC Ward, T Hesjedal, XG Zhang, C Wang, A Kohn, QL Ma, J Zhang, HF Liu, XF Han (2012)
“Interface Characterization of Epitaxial Fe/MgO/Fe Magnetic Tunnel Junctions”
Journal of Nanoscience and Nanotechnology 12, 1006-1023.

GBG Stenning, GJ Bowden, SA Gregory, PAJ de Groot, G van der Laan, LR Shelford, P Bencok, P Steadman, AN Dobrynin, T Hesjedal (2012)
“Transverse magnetic exchange springs in a DyFe2/YFe2 superlattice”
Physical Review B: Condensed Matter and Materials Physics 86, 174420.

GBG Stenning, GJ Bowden, SA Gregory, J-ML Beaujour, PAJ de Groot, G van der Laan, LR Shelford, P Bencok, P Steadman, AN Dobrynin, T Hesjedal (2012)
“Magnetic reversal in a YFe2 dominated DyFe2/YFe2 multilayer film”
Applied Physics Letters 101, 072412.

LR Shelford, T Hesjedal, L Collins-McIntyre, SS Dhesi, F Maccherozzi, G van der Laan (2012)
“Electronic structure of Fe and Co magnetic adatoms on Bi2Te3 surfaces”
Physical Review B - Condensed Matter and Materials Physics 86, 081304(R).

T Hesjedal, U Kretzer, A Ney (2012)
“Magnetic susceptibility of n-type GaAs”
Semiconductor Science and Technology 27, 055018.