Magnetic Multi-layer Crystals for Spintronic Physics and Devices
Supervisors: Dr. Gavin Bell and Dr. Tom Hase
Background
Combining magnetic and semiconducting materials in new
artificial structures opens up a plethora of potential
applications and allows us to explore fundamental physics
with unprecedented control. So far, most devices which
utilise the electron spin as a control parameter have been
based on pure metallic or metallic/insulator systems. The
first generation of such spintronics devices to process and
store information is already with us in the form of read
heads in modern computer hard disks. The resistance of
thin metallic “sandwiches” known as spin-valves which are
composed of magnetic and non-magnetic layers of
materials (fig. 1) changes depending on the relative
orientation of the magnetisation in the layers. The large
resistance changes (>1000%) shown to date could yet be
surpassed through the use of novel materials with a
controlled magnetic structure. Furthermore, by including
so-called half-metallic materials in the layer structures,
completely new functionality can be explored, including
potential application to quantum computing.
Fig. 1: A schematic of a spin valve structure. The
bottom magnetic layer is fixed through interaction
with the pinning layer. An external magnet can
rotate the free layer with respect to this pinned
layer resulting in a change in the resistance of the
sample. We will grow similar structures but with
several important innovations: our layers are fully
crystalline, and we will incorporate half-metals. We
will focus on transition metal antimonides.
Basic outline
This project is principally experimental, although you will also be encouraged to work on modelling of
the electronic structure of the devices using density functional theory (particularly via the CASTEP and
Munich SPR-KKR packages). The main tasks will be (1) to grow the layer structures, and (2) to characterise
their physical, electronic and magnetic structures. Growth will be done using the molecular beam epitaxy
(MBE) system in the Surface, Interface & Thin Film Group. This lets you grow layers with atomic-level
thickness control, and examine them in situ with techniques such as scanning tunnelling microscopy
(STM). Characterisation will rely on both lab-based and central facility-based techniques. These include
X-ray diffraction (XRD), angle-resolved photoemission spectroscopy (ARPES) and polarised neutron
reflectivity (PNR). We have an excellent track record of winning facility time and funding to support this
work, and the Ph.D. project will involve regular travel to neutron and synchrotron radiation facilities in
Europe, Japan and the USA, and our “local” facilities in Oxfordshire (Diamond Light Source and ISIS).
In more detail
The project will make use of the unique growth and characterisation expertise in the department to
explore the correlation between spin-dependent transport phenomena and structure in novel transition
metal pnictide magnetic heterostructures. Initial work will concentrate on the binary materials MnSb,
NiSb and CrSb which show ferromagnetic, paramagnetic and anti-ferromagnetic behaviour in the bulk
before moving onto investigations of the ternary systems. Effects such as composition, strain and crystal
structure are known to modify the band structure at the Fermi energy resulting in range of magnetic
states in these weakly metallic materials. We will also explore the half-metallic cubic polymorph of MnSb
(we are the first group to grow this polymorph successfully). Half-metals have 100% spin polarisation at
the Fermi level and enable maximally efficient spintronic devices as well as opening up exciting new
physics. They have not yet been fully exploited because of the subtleties of their delicate electronic
structure which depend sensitively on local strain and interface structures. We are able to grow bulk
crystals of the most widely studied half-metallic alloy, NiMnSb, which allows comparison to cubic MnSb
which has far superior predicted properties.
Your project
The overall goals are to (1) control half-metallicity in MnSbbased polymorph structures and (2) evaluate inter-layer
magnetic interactions and exchange bias in MnSb-NiSbCrSb-InSb multilayers.
You will be in charge of running a dedicated MBE system in
Warwick where the thin film samples will be grown. These
will be characterised by surface science methods, XRD,
magnetometry, X-ray magnetic circular dichroism (XMCD)
for element-specific magnetism, ARPES (for mapping the
band structure – we plan to perform spin-resolved ARPES
at Synchrotron SOLEIL in Paris and other facilities), PNR (to
study magnetic coupling between layers) and electron /
scanning probe microscopy. You will not need to become
an expert in all these techniques (!) but can focus on the
key structural and magnetic methods plus the MBE growth
itself.
Fig. 2: some recent results from
polymorphic MnSb films and NiMnSb
bulk crystals. (a) High resolution
transmission
electron
micrograph
showing atomic planes at the sharp
interface between cubic c-MnSb and its
ordinary
n-MnSb
structure.
(b)
Reciprocal space map obtained at
Brookhaven National Laboratory in the
USA of a multi-polymorph MnSb thin
film. (c) Theoretical results comparing
the spin polarisation of c-MnSb and
NiMnSb (inset – NiMnSb bulk crystal
Laue diffraction pattern). The low spin
gap of NiMnSb causes the spin
polarisation to collapse far below 300K.
Vacuum science techniques, MBE, low temperature
measurements and X-ray/neutron facilities are all highly
valuable skills for a future career in science. You will have
the opportunity to collaborate with central facility
specialists and electronic structure theorists as well as the
Warwick Microscopy Group. Our MnSb-based structures
are of interest for spintronic devices presently being
developed by Toshiba Research Europe Ltd. (TREL – based
in Cambridge) and with several UK academic partners
(Cambridge, Southampton, York). We plan to supply
material to TREL for fabrication into devices; a placement
of 2-3 months with an industrial collaborator could be
arranged and would further enhance your PhD experience.
Further reading
Cubic MnSb: epitaxial growth of a predicted room-temperature halfmetal, J.D. Aldous, C.W. Burrows, I. Maskery, M. dos Santos Dias, M.K.
Bradley, A.M. Sanchez, R. Beanland, J.B. Staunton and G.R. Bell,
Physical Review B Rapid Communications (2012), vol. 85, 060403(R)
Depth-dependent magnetism in epitaxial MnSb thin films: effects of
surface passivation and cleaning, J.D. Aldous, C.W. Burrows, I. Maskery,
M. S. Brewer, T.P.A. Hase, J.A. Duffy, M.R. Lees, C. Sánchez-Hanke, T.
Decoster, W. Theis, A. Quesada, A.K. Schmid and G.R. Bell, Journal of
Physics: Condensed Matter (2012), vol. 24, 146002
Growth and characterisation of NiSb(0001)/GaAs(111)B epitaxial films
J.D. Aldous, C.W. Burrows, I. Maskery, M. Brewer, D. Pickup, M. Walker,
J. Mudd, T.P.A. Hase, J.A. Duffy, S. Wilkins, C. Sánchez-Hanke
and G.R. Bell,, Journal of Crystal Growth (2012), vol. 357, p. 1