Neuromorphic computing is a bio-inspired technology which aims at mimicking the brain working principles. It can be used for fast and energy-efficient applications through the implementation of networks of artificial neurons and synapses. Artificial synapses are implemented as electronic components called memristors. These are non-volatile memory devices whose resistance can take several intermediate values between minimum and maximum, the resistance varying monotonously as a function of the polarity of the current pulses flowing through the device. In the present study, a novel concept of spintronic memristor was proposed which takes advantage of the know-how acquired at SPINTEC in the development of magnetic memory (MRAM) and radiofrequency nano-oscillators.
This novel memristor consists of a magnetic tunnel junction as that used in MRAM cells. We remind that a magnetic tunnel junction is formed of two magnetic layers separated by a tunnel barrier. One of the layer called the reference layer has a fixed magnetization while the magnetization of the other (the storage layer) can be switched or rotated. When a current flows throughout this device, its resistance depends of the relative orientation of the magnetization in the two magnetic electrodes, a phenomenon named tunnel magnetoresistance. Two important modifications have been brought to the junction to change its functionality from MRAM cell to memristor. First, while MRAM encodes the information in a binary form (“0” and “1”) corresponding to two magnetic configurations (minimum resistance-parallel or maximum resistance-antiparallel), this memristor is designed to reach a large number of intermediate values of resistance. To achieve this, we exploited the cosine variation of tunnel conductance as a function of the angle between storage layer and reference layer magnetizations. A special storage layer based on ferromagnetic/antiferromagnetic coupled layers was developed exhibiting an isotropic coercivity thus allowing to stabilize the storage layer magnetization in any in-plane direction. Second, a necessary condition is to rotate the storage layer magnetization step by step clockwise or anticlockwise depending on the voltage pulse polarities while limiting the angular excursion between 0 and 180°. Based on our earlier work on spin transfer radiofrequency oscillators, we demonstrated that this can be achieved by adding to the junction an additional layer with out-of-plane magnetization and adjusting the spin-transfer influences from this additional layer and from the reference layer. The resulting device exhibits memristive functionality as illustrated in the Figure.
Funding: ERC MAGICAL n°669204
Further reading: Spintronic memristor for neuromorphic circuits based on the angular variation of tunnel magnetoresistance, M. Mansueto, A. Chavent, S. Auffret, I. Joumard, L. Vila, R.C. Sousa, L. D. Buda-Prejbeanu, I. L. Prejbeanu and B. Dieny, NANOSCALE 13, 11488 (2021).
Contact: Bernard DIENY