New magnetic structures for spintronics
Tom Hase and Gavin Bell
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. To
date 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 spinvalves which are composed of magnetic and nonmagnetic layers of materials (fig. 1) changes
depending on the relative orientation of the
magnetisation in the layers. The large effects
(>1000%) that have been observed to date could yet
be surpassed through the use of novel materials with
a controlled magnetic structure.
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.
You will make use of the unique facilities in the department to continue our recent developments in the
molecular beam epitaxial growth of thin films of the transition metal pnictides on GaAs. Initial work will
concentrate on the binary materials MnSb, NiSb and CrSb which respectively show ferromagnetic,
paramagnetic and anti-ferromagnetic behaviour in the bulk. The key point is that these materials can be
growth epitaxially, i.e. as a series of aligned single crystal layers. Effects such as composition, strain and
crystal structure are known to modify the band structure at the Fermi energy in these pnictide materials
resulting in range of magnetic states in these semi-metallic materials. Several theoretical models predict a
half-metallic phase of MnSb if it can be stabilised in the metastable cubic structure. These routes to
magnetic control motivate our research programme – we will probe the magnetic structures of our
materials and correlate them with a detailed and systematic structural study.
Detailed structural studies will be undertaken using a range of both surface and bulk techniques. In-situ
monitoring during growth will be undertaken using electron diffraction (RHEED and LEED). Post-growth
analysis will be performed using x-ray and microscopy techniques. High resolution x-ray characterisation
(XRD, X-ray reflectivity) will be combined with TEM, SEM and AFM. You will also undertake work at central
facilities such as the Diamond Light Source, European Synchrotron Radiation Facility (Grenoble) and the
National Synchrotron Light Source, USA. Element specific magnetometry data will be extracted using the
XMCD technique and compared to bulk magnetometry recorded in the department using the SQUID,
magneto-transport apparatus and a new MOKE set-up. All the experimental data will be correlated with
ongoing theoretical studies.
This project will be jointly supervised by Gavin Bell (growth) and Tom Hase (characterisation) and will give
you the opportunity to explore a range of condensed matter physics and experimental techniques. You
will develop skills in thin film growth and a wide range of characterisation techniques. The work will be
collaborative and you will work closely with theorists, synchrotron scientists and microscopists.