Moving spins: Novel methods and materials for ultrafast spintronics

December 22, 2022

Tom Lichtenberg defended his PhD thesis at the department of Applied Physics on December 21st.

Electrons have many properties, including spin, whereby electrons can be seen as tiny magnets that can point ‘up’ or ‘down’ like small magnets. More than 10 years ago, scientists discovered that spin currents can be generated by firing a very short (less than one millionth of a millionth of a second), but very strong laser pulse at a thin magnet. The physical process behind this is still debated to this day, mainly because it is very difficult to observe the spin currents. For his PhD research, Tom Lichtenberg approached the problem from a new angle and used a two-layer system to generate and store a spin current.

Electric currents play an important role in our daily lives. They are used to power our equipment, as well as to move and process information. To achieve this, electrons move through wires, where the charge of the electron is used to store energy and data.

Electrons have another property that is very interesting for electrical applications – spin. In the world of spins, electrons can be viewed as small magnets where the north pole can assume one of two directions, either “up” or “down”. If the number of up and down spins are not balanced, this leads to a spin or magnetic current, which can be used to influence magnets in an efficient way. This property is already used today for data storage applications.

A new angle

For his PhD research, Tom Lichtenberg and colleagues in the Physics of Nanostructures group at the department of Applied Physics approached this problem from a new angle.

Lichtenberg employed a two-layer system. In the first layer, a spin current is generated with a short laser pulse, which is then absorbed in the second magnetic layer. This situation can be compared to throwing a stone into a pool of water: When the stone hits the water, it instantly loses energy, which is then transferred to the water. This causes the water to ripple.

Likewise, the magnetization (which indicates the magnitude and direction of magnetism) of the second layer gets a kick from the spin current, and leads to the generation of so-called spin waves. By studying these spin waves in detail, Lichtenberg sought to unravel several important properties of spin current.

It was previously thought that the main mechanism responsible for spin current generation is related to the laser heating up electrons, which makes them very mobile, leading to a current flowing out of the magnet.

However, the experiments and theoretical model of Lichtenberg and his co-workers show that this is not necessarily the case; a spin current can also be generated through local dissipation. His findings open up the discussion about laser-induced spin-current generation, and take the community at large one step closer to fully understanding this process.

Title of PhD thesis: Moving spins: Novel methods and materials for ultrafast spintronics. Supervisors: Bert Koopmans, Rembert Duine, and Marcos Guimarães.