Double-dosing induces magnetism, strengthens electron quantum oscillations in topological insulator

The resulting crystals show clear ferromagnetic ordering, a large bulk band gap, high electronic mobility, and the opening of a gap of surface state, making this system a good candidate to achieve QAHE at the higher temperatures necessary for viable, sustainable future low-energy electronics.

“The combination of electronic and magnetic properties in topological systems is the keystone of novel topological devices, and one of the core projects in FLEET,” says project leader Prof Xiaolin Wang (UOW). “We have proposed and successfully realised a new way to magnetise a novel electronic material – a topological insulator – by adding two different magnetic ions.”

Each one of various different magnetic elements used in magnetising a topological insulator possesses its own advantages and disadvantages. However, while previous studies employed only one element, the UOW-Monash University-RMIT team found that combining two elements also combined the advantages of each.

“The dual doping strategy is thus proved viable for the growth of extremely high-quality topological insulators with both magnetism and excellent electron mobility, which are vital for low-energy electronic devices,” says the study’s lead author, Dr Weiyao Zhao.

The two key elements in the quantum anomalous Hall effect (QAHE) that ‘drives’ desirable properties in topological insulators and all related electronic technology are ferromagnetism and the topological electronic insulating property.

The collaborative FLEET study, combining expertise from UOW, Monash and RMIT pioneered a ‘dual element’ doping strategy to introduce magnetism in a topological insulator, thus improving both key elements at once.

Combining the advantages of two different doping elements, iron and samarium, results in large crystal growth, with a large surface band-gap, and huge quantum transport effect.

Previous approaches to realise quantum anomalous Hall effect (QAHE) in a topological insulator employed doping with a single transition-metal, such as iron, to create ferromagnetism.

However, while this doping technique was successful in creating the desired magnetic ordering, the in-lattice transition metal jeopardises the desired high-mobility of the topological insulator, which in the case of low-energy electronics, defeats the purpose of using topological insulators at all!

Thus, QAHE has been realised via a transition metal doping strategy only at extremely low temperatures, which would require energy-intensive cooling. Again, this reduces the viability of such materials for future low-energy electronics.

To increase the operating temperature of QAHE, stronger magnetic interaction and higher mobility are desired.

After considering the successful elements of doping using transition metals such as iron, the research team decided to further introduce a stronger magnet, the rare earth element samarium.

The doping elements iron and samarium create the necessary ferromagnetic ordering in the crystals, which can open a massive gap at the Dirac cone of surface state. This is an essential element of achieving QAHE.

Further, the team proved that the electron mobility remains very high in the dual magnetic doped crystals. The mobility of topological insulators such as bismuth-selenide is several times faster than in classical semiconductors, such as silicon.

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