The electronic and magnetic properties of thin film Fe₃Sn₂

This thesis is an exploration of the growth and properties of thin film Fe₃Sn₂.
Through precise stoichiometric deposition, this frustrated kagome ferromagnet
can be grown with minimal impurities and intergrowths from the other Fe-Sn
intermetallic alloys. The frustrated spin texture within the Fe₃Sn₂, and the reorientation these spins undergo with changes of field and temperature, have been predicted to produce the spin frustration needed to stabilise magnetic skyrmions.
These skyrmions in turn have potential applications in new architectures for
magnetic data storage and the development of neuromorphic computing. Beyond
this, the electronic band structure of Fe₃Sn₂ is predicted to have many novel prop-
erties such as flat bands and Dirac-points within tantalising reach of the Fermi
surface. The resulting contribution to the anomalous Hall effect as well as unique
magnetoresistance curves could also feature in future spintronic devices in order
to improve the energy efficiency of our already highly optimised computation
methods.

To begin, high quality 80 nm to 100 nm epitaxial films of Fe₃Sn₂ are fabricated
through sputter deposition on to heated single crystal sapphire substrates with
a seed layers of epitaxial Pt. Using X-ray diffraction these films were found to
be strongly orientated in the (001) direction with large 30 nm grain sizes. Scan-
ning transmission electron microscopy, combined with a custom phase matching
technique, allows for spatial maps of these thin films to be produced covering
100s of nm of the film’s cross section and with spatial resolution of 3 nm by 3 nm.
This large scale crystal characterisation confirms the ability to control the phase
content of the ferromagnetic Fe₃Sn₂ and the antiferromagnetic FeSn through
the growth process. The ratio of power fed to the Fe and Sn magnetron guns
alone, as opposed to temperature or growth rate, is found to be the determining
factor that changes the resulting film composition, due to the precise control of
growth rate that can achieved from each gun, that can then in turn lead to specific
stoichiometries in the resulting films.

The highly pure, with over 97% crystalline content, Fe₃Sn₂ films were found
to have saturation magnetisation of 777 emu/cm³ ± 9 emu/cm³ and very soft
coercivity of 1.5 mT ± 0.2 mT. Analysis of the magnetic properties of the resulting
films showed expected reduction in saturation magnetisation with increasing
FeSn intergrowth, but an unexpected minima behaviour in the coercivity of highly mixed Fe₃Sn₂ and FeSn film with changing temperature, indicating a strong
ferromagnetic-antiferromagnetic coupling. Fitting to the Bloch-3/2 law allows a spin wave stiffness of (4.1 ± 0.2) × 10⁻⁴⁰ J/m² to be extracted. Further high temperature measurements reveal that a temperature of 750 K results in the Fe₃Sn₂ irreversibly breaking down, which in turn causes the appearance of a hitherto unseen magnetic transition at 120 K in the ZFC/FC measurements.

The electronic properties of ultra-thin 5nm thick Fe₃Sn₂ films are also explored, with surprisingly no orbital contribution to the anomalous Hall effect observed despite being strongly suggested in the literature. Instead, a carrier type change-over is confirmed by the change of sign of the ordinary Hall effect at 75 K. This critical temperature was found to correspond closely to the point of spin reorientation and an observed change in the scaling of the anomalous Hall effect. Along with this, a linear negative magnetoresistance that is observed to have its gradient decrease with increasing temperature is observed, with currently no established theoretical explanation with potential applications of such consistent linear magnetoresistance being part of the detector system in a high field magnetic sensor.