Switching and imaging of magnetic structures in iron oxides
Keywords: XMCD
spin dynamics
XMLD
antiferromagnetics
topological structures
multiferroics

Spintronics is a multidisciplinary science aiming to develop the next-generation electronic device at the age when Moore’s Law has met its limit. Magnetic oxides, as a traditional material widely applied in CMOS and memory devices, have attracted tremendous attention in spintronics because of many important compounds such as multiferroics and antiferromagnets; both are dominant platforms for studying strong correlation physics. In this thesis, I look at a new type of multiferroic single crystal Ba0.5Sr1.5Mg2Fe12O22 and a time-honoured antiferromagnet α-Fe2O3 thin film. I investigate the magnetic domains and the coupling between different magnetic orders for multiferroic candidates. As for the antiferromagnets, I developed a state-of-art imaging technique for probing antiferromagnetic domains, and a series of topological structures were discovered to profit from this technique.
I study the fundamental multiferroic properties of a novel Y-type hexaferrite Ba0.5Sr1.5Mg2Fe12O22 by utilising soft X-ray resonant magnetic diffraction and transport techniques. The investigation focuses on the coupling between magnetic, ferroelectric, and structural order, especially on domains. The puzzling physical properties arising from switching the multiferroic domains are elucidated from the perspective of the magnetic structure. The multiferroic phase accompanies an in- commensurate phase while switching the polarised magnetic domains. Furthermore, the imaging of the multiferroic phase is performed on the two-fold fan structure by utilising the X-ray magnetic circular dichroism effect. The manipulation of the magnetic domain is tried by static electric voltage and THz electric pulses; the experiment results show that the multiferroicity in Y-type hexaferrite can be switched in domain scale by an external electric field, even likely at a transient time scale of picoseconds. The result provides promising evidence for applying type-II multiferroics for ultrafast spintronic materials.
I also study the domains and topological structures in α-Fe2O3, a room-temperature antiferromagnet, by developing an advanced imaging technique based on an X-ray photoemission electron microscope (X-PEEM). Using the XMLD-PEEM method based on XAS spectroscopy for antiferromagnetism, a family of exotic topological structures are identified at room temperatures, such as meron, anti-meron and bimeron structures. This technique highlights that the in-plane Neel vectors can be mapped for the in-plane domains at room temperature above the Morin transition and the in-plane domain walls at low temperatures below the Morin transition. The intermediate nucleation process from OOP to the IP phase can also be depicted by constructing such a vector map.
Not confined to the experiment, more in-depth studies are taken on high- performance computing clusters, where many simulations on the dynamical be- haviour of the topological structures found in experiments are performed by the atomic micromagnetic tool called VAMPIRE. The complex superexchange interactions in α-Fe2O3 are constructed with atomistic resolution. The Landau- Lifshitz-Gilbert dynamics reveal the generation and annihilation of the topological meron and bimeron structures. Furthermore, the spin wave emission by non- trivial topological structure is also demonstrated in an ultrafast time scale that the experiment cannot observed so far.