Magnetic memory relies on using the magnetization direction of individual ferromagnetic cells to store binary information. Nowadays, the most efficient way to switch the magnetization direction, i.e. to write or delete a bit of information, is by using so-called spin torques. These torques arise in certain, typically, thin multilayer films when an electric current is passed through them, and the larger the torque, the less current is required for the switching, thus led energy is dissipated in the device. It is known that the magnitude of the torques varies when altering the systems constituents, for example, changing the materials or the layer thicknesses. In this case, however, the torques remain fixed once the multilayer is fabricated. In the light of developing a new generation of devices with simplified architectures and decreased power consumption, of key importance is to be able to control the spin torques “on the fly”. Our results recently published in Physical Review Letters demonstrate for the first time experimentally that the low-power control of the spin torques can be achieved by means of piezoelectric strain, which we explain by advanced theoretical calculations carried out in the collaboration with Jan-Philipp Hanke and Yuriy Mokrousov within the TopDyn – Dynamics and Topology Research Centre.
We generate and control the piezoelectric strain by low-power electric fields, which is an energy-efficient approach as it avoids using power-hungry electric currents and the associated energy losses due to heating. We find that the tensile strain enhances the amplitude of the torque in a thin perpendicularly magnetized multilayer, while the compressive strain leads to its decrease. This means that we can not only dynamically tune the torques by electrically controlled but also reach even higher torques than possible for a given system at zero strain. Using theoretical calculations, we uncover the microscopic origin of the observed behavior of the torques and reveal which phenomena are at the core and need to be considered when engineering the torques.
Our results are remarkable because they show that two energy-efficient approaches of magnetization manipulation, the electric field-induced strain and the spin torque magnetization switching, can be combined to enable novel device concepts.
Additional information can be found in the press release on the JGU website: https://www.uni-mainz.de/presse/aktuell/11684_ENG_HTML.php
The full text of the publication in Physical Review Letters is accessible at:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.217701
