Successful PhD defense of Ke Gu

Congratulations to Ke Gu!

Title: “Investigation of novel freestanding heterostructures"

Abstract

Freestanding thin films exhibit many intriguing properties and can be readily integrated with other types of materials and structures to create novel properties, going beyond normal epitaxial thin films and facilitating applications in next-generation devices and the investigation of emergent phenomena. In this thesis, we utilize a water membrane based lift-off and transfer technique to fabricate several different types of freestanding heterostructures and systematically investigate their functionalities and device performance.

The fabrication of three-dimensional (3D) nanostructures is crucial for nanoelectronic devices with a small device footprint. One of the most promising future memory devices is magnetic racetrack memory (RTM), in which data is encoded in magnetic nanoobjects, e.g. magnetic domain walls (DWs), which are moved along nanowires by current pulses. RTM is promising due to its high packing density, low energy consumption and high speed. However, so far it has only been explored in two dimensions. Here, we show that freestanding racetracks formed from heavy metal/ferromagnetic (HM/FM) heterostructures have comparable device performance to those fabricated from conventional as-deposited thin films. Having demonstrated that the structures and magnetic properties of freestanding HM/FM heterostructures are largely preserved throughout the entire process, we then fabricate, for the first time, 3D racetracks by covering protrusions patterned on a sapphire substrate with freestanding magnetic membranes. We show that the currentinduced DW motion (CIDWM) in 3D racetracks formed from HM/FM heterostructures can be modulated by the local geometry. We further demonstrate efficient CIDWM with a velocity of up to 600 m s⁻¹ in 3D racetracks formed from freestanding synthetic antiferromagnetic heterostructures transferred onto ∼900 nm high protrusions. In addition, we fabricate freestanding HM/FM racetracks without any buffer layer and reduce the thickness of the HM layer down to 25 Å . We show that they have almost identical performance to the devices formed from conventional HM/FM heterostructures. Freestanding magnetic heterostructures, as demonstrated here, may enable future DW logic and 3D spintronic devices with high data capacity.

Another application of the water membrane based freestanding technique is to form “Twisted structures”. Due to the rich interactions at twisted interfaces, twistronics has attracted much attention and sparked a wealth of new physics. However, up till now, twisted correlated oxides, which can stimulate the discovery of novel phenomena, have been little studied. Through our technique, we create twisted La_2/3Sr_1/3MnO₃ bilayers with a controllable twist angle that show anomalous diamagnetic behavior. This diamagnetism is attributed to the appearance of antiferromagnetic coupling at the twisted interface.

Our results demonstrate the robustness and versatility of the water membrane based freestanding technique and show the great potential of freestanding heterostructures not only in applications such as memory devices, but also in fundamental research.