Magnetic order in three-dimensional topological insulators
Keywords: X-rays
Neutrons
Magnetism
Topological Insulators

Topological insulators, a type of quantum material, are of intense interest to researchers due to their ability to house time reversal symmetry protected, gapless, linearly dispersed surface states, offering dissipationless conductivity. Through the introduction of magnetic dopants or proximity coupling which induces long-range ferromagnetic order, time reversal symmetry can be broken, introducing a band gap in the surface state. This has led to the experimental observation of exotic quantum effects such as the quantum anomalous Hall (QAH) effect which does not require an externally applied magnetic eld in order to be realised and is the anomalous counter part to the quantum Hall (QH) effect. However, the QAH effect has only previously been observed in magnetic topological insulators (MTI) at sub mK temperatures, limiting the application potential of these extremely promising materials.
This thesis presents an investigation into the structural and magnetic properties of 3D MTI thin films grown by molecular beam epitaxy (MBE) using reflection high energy electron diffraction (RHEED), X-ray reflectometry (XRR), X-ray diffraction (XRD), vibrating sample magnetometry (VSM), X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and polarised neutron reflectometry (PNR). The element specific magnetometry of XMCD and magnetic depth profiling capabilities of PNR allows for an in-depth investigation of the different magnetic coupling scenarios which occur in magnetically doped samples as well as magnetic enhancements caused by proximity coupling and interface effects in MTI heterostructure.
Firstly, an investigation of single-layered TI thin films and the effects of using different magnetic dopants in the form of the transition metal Cr and the rare earth Dy ions is presented. Using Cr as a dopant in Sb2Te3 is shown to produce samples of a high structural quality with long-range ferromagnetic order which persists up to a TC of 120 K. XRD, XAS and XMCD measurements demonstrate that Cr substitutionally replaces Sb during growth. PNR measurements demonstrate an equal distribution of Cr throughout the sample with no surface enhancement. XMCD measurements demonstrate that the long-range ferromagnetic order is caused by a carrier-mediated coupling mechanism where the exchange interaction is mediated by polarised Te valence holes, leading to the formation of Cr 3d impurity bands near the Fermi level, which is detrimental to the observation of the QAH effect.
Using a Dy dopant with Bi2Te3 leads once again to a sample of high structural quality. VSM and XMCD measurements demonstrate that there is no long-range ferromagnetic order induced. PNR measurements combined with Muon spin relaxation measurements show that the Dy dopant leads to a system which displays short-range ferromagnetic order within patches of the material where a large internal magnetic field can be induced at moderate applied fields. Unlike in the case of Cr-doped Sb2Te3, the exchange interaction in the sample is not mediated by the conduction band, as demonstrated by XMCD. In an attempt to enhance the magnetic properties of an MTI system, two transition metal dopants Cr and V are used to co-dope Sb2Te3. This leads to the formation of Cr2Te3 within the sample which is caused by the V acting as a surfactant which prevents the Cr from substitutionally replacing Sb within the MTI structure as demonstrated by XRD, XAS and XMCD measurements.
The previous studies conducted on the single layer MTI samples are then used to inform the fabrication of MTI heterostructures in an attempt to investigate proximity coupling and interface effects in multilayer structures. Growing a ferromagnetic Co layer on top of Cr:Sb2Te3 demonstrates the strength of the long-range ferromagnetic order, where Co is unable to polarise the Cr moments contained within the MTI to a significant degree, as demonstrated by Arrott plots which show that the single layer Cr:Sb2Te3 with a TC of 86.7 K is only minimally enhanced to 92.7 K after the deposition of ferromagnetic Co.
Producing high quality heterostructures consisting of multiple bilayers of Cr:Sb2Te3/Dy:Bi2Te3 with well dened interfaces leads to the enhancement of the magnetic properties of the Dy:Bi2Te3 layers, where XMCD measurements show that they demonstrate ferromagnetic behaviour up to a temperature of 17 K as determined by Arrott plots. XMCD measurements demonstrate that despite single layer Cr:Sb2Te3 and Dy:Bi2Te3 having an easy axis of magnetisation perpendicular to one another, the average Cr and Dy moments contained within the structure are parallel out-of-plane, demonstrating that the dopants magnetically couple.
This research demonstrates the possibility of enhancing MTI systems through the use of magnetic heterostructures. It also shows how XMCD and PNR are powerful techniques for gaining insight into the magnetic properties of such systems in order understand how these materials can be both utilised and enhanced to further the eld of MTIs in the pursuit of a QAH effect realised in a device friendly scenario.