PhD defense: Dennis discovers magnetism when non-magnetic materials are put together

Monday 16 Oct 17

Exploring Magnetic and Electronic Properties in γ-Al2O3/SrTiO3

Main supervisor: Nini Pryds

Co-supervisors: Yunzhong Chen and Anders Smith

When two materials are combined with atomic precision, completely new phenomena may arise at their interface – phenomena which do not exist in either material, but only at the interface. On 12 October 2017 Dennis Valbjørn Christensen defended his PhD thesis in which he has investigated such systems.

Composite materials often have properties which resemble the properties of their constituents: Two metals put together still behave as a metal, and two electrically insulating materials placed on top of each other do not suddenly begin to conduct a current. However, if you can grow the composite material with atomic precision, layer by layer, unexpected things may happen. One example is that the interface between two insulating ceramic materials can become electrically conducting. In his PhD study Dennis has carefully made such systems using pulsed laser deposition, a proces in which atoms are ablated from a substrate using a powerful laser. When the atoms reach the target, an extremely thin film is created, atomic layer by atomic layer. Afterwards, Dennis has studied the properties of the interface between the two materials.

Dennis Christensen at his PhD defense 12 October 2017In his thesis "Exploring Magnetic and Electronic Properties in γ-Al2O3/SrTiO3" he investigates the many new properties arising when the two oxides alumina (γ-Al2O3) and strontium titanate (SrTiO3) are put together. Individually, these oxides are electrically insulating and non-magnetic, but their interface becomes electrically conducting and exhibits magnetic properties. "It is very fascinating, not least because you can twist the properties in a number of ways, e.g., by applying an electrical potential, by putting the sample inside a magnetic field, or by deforming the sample," Dennis Christensen explains. He has shown that the conductivity of the interface can be increased by a factor of 10,000 by applying a potential and that the resistivity changes drastically in a magnetic field.

"Using a piece of equipment called a SQUID (Superconducting Quantum Interference Device) I discovered that a magnetic order is created when I put alumina together with strontium titanate. The intensity of the magnetic signal can be controlled by applying a slight mechanical pressure to the sample surface", Dennis continues.

The long term goal of Dennis' investigations is to be able to exploit the many fascinating properties of oxide systems in electronic components. An important step is the increase of the conductivity of the interface. But challenges still remain because many of the novel properties studied by Dennis only occur at very low temperatures. If a way can be found to stabilize the properties at temperatures closer to room temperature, oxides can suddenly become relevant for many applications, including fast transistors and components taking advantage of effects such as ferromagnetism, ferroelasticity, and ferroelectricity.

"We have still some way to go for that to happen. But the research area is also relatively young," Dennis Christensen adds. He joined DTU after having studied nanoscience at the University of Copenhagen. His BSc project was on magnetic refrigeration at DTU Energy. After that, he stayed around due to the many exciting projects in the department. DTU Energy is strong on materials science, and that suited Dennis well.

"It is exciting to learn new things all the time. My experience is that the learning curve is steep at first and then levels off after a year or so. You simply learn most at the beginning. That is why I try to have several projects running at the same time – I want to learn as much as possible while I can", he says.

Dennis is going to continue his research on the fascinating properties of oxide interfaces. He has been hired as a postdoc at DTU Energy on a project which involves a collaboration with some of the leading research institutions in the field, including MIT and Imperial College, London.