A tunnel junction is a device in which an insulating layer isolates two conducting layers. Typically, the resistance for driving current to pass through an insulating layer is infinite, but if the insulating layer is thin (~ 1 to 2 nm), charge carriers might manage to tunnel via the insulating layer, as a result of their quantum nature.
If the conducting layers are magnetic by nature, the result is a magnetic tunnel junction (MTJ), the resistance of which relies on the magnetic configurations. Existing MTJs include only two resistance states as they enable either parallel or anti-parallel magnetic configurations of the two magnetic layers.
The two-state MTJ has been vital to the field of spintronics—a branch of electronics that makes use of the magnetic moment related to the spin of the electron apart from the electron charge utilized in conventional electronics. Therefore, the two-state MTJ is the essential foundation for the magnetic random-access memory (MRAM).
Currently, scientists from the Department of Physics and Institute of Nanotechnology and Advanced Materials of the Bar-Ilan University collaborated with a team from Instituto Superior Tecnico (IST), Universidade de Lisboa, and INESC Microsystems and Nanotechnologies to develop a new kind of MTJ with four resistance states.
The researchers successfully illustrated switching between the states with the help of spin currents. They realized a high number of states by substituting one of the magnetic layers with a structure that resembled two crossing ellipses.
As it has recently been shown that structures in the form of N crossing ellipses can support two to the power of 2N states, the current results may pave the way to MTJs with much larger number of resistance states.
Lior Klein, Professor and Chairman, Department of Physics, Bar-Ilan University
Klein added, “Such MTJs may enable novel spintronics devices, e.g., multi-level MRAM which stores data much more densely, or neuromorphic memory that meets artificial intelligence challenges in performing cognitive tasks.”
Klein headed the Bar-Ilan group, which included Dr Shubhankar Das, Ariel Zaig, and Dr Moty Schultz. The group from Instituto Superior Tecnico (IST), Universidade de Lisboa, and INESC Microsystems and Nanotechnologies was led by Prof. Susana Cardoso, along with Dr Diana C. Leitao.
Das, S., et al. (2020) A four-state magnetic tunnel junction switchable with spin-orbit torques. Applied Physics Letters. doi.org/10.1063/5.0014771.