The field of cryoelectronics holds promise for high-performance computing applications, particularly within the realm of quantum computing. Recent advances in quantum computing systems have spurred the investigation of various technologies suitable for cryogenic environments. Among these technologies, spintronic memory elements have received significant attention due to their potential for minimal energy consumption. Specifically, these elements achieve the lowest writing energy at time scales around 1 nanosecond pulse widths, thereby significantly reducing total energy dissipation. In cryogenic settings, the thermal stability requirements are substantially relaxed, a reduction of more than 100 times compared to room temperature, shifting the primary challenge to optimizing the energy barrier of the memory cell. For spin transfer torque (STT) written magnetic memories, this optimization requires precise control of magnetic anisotropy at the specified operating temperature.
Room temperature effective anisotropy and model extrapolation of room temperature results to cryogenic temperature environment.
Our research paper focuses on the control of the perpendicular interface anisotropy for cryogenic operation, illustrated in a perpendicular magnetic tunnel junction (pMTJ) cell written by spin transfer torque (STT-MRAM). The novelty of this work lies in the experimental method we have developed to extrapolate thin film magnetic properties measured at room temperature to the electrical device properties at cryogenic temperatures. The method employs magneto-optical Kerr effect (MOKE) magnetometry to calculate the effective anisotropy as a function of the storage layer thickness using wedge depositions, achieving sub-nanometer layer resolutions. Key findings of our study include: confirmation that Bloch and Callen-Callen laws correctly describe the evolution of effective anisotropy in fabricated devices, that these laws can be used to extrapolate low temperature measurements from values obtained at higher temperature, and that the proposed methodology is applicable to any magnetic stack relying on perpendicular interface anisotropy.
The figure presents the effective anisotropy map as a function of the FeCoB layer thickness for different oxidized MgO thicknesses oxidized under identical conditions. As the free layer thickness increases, it becomes possible to reduce the effective anisotropy, transitioning from a perpendicular easy axis (out-of-plane, OOP) to a reorientation (spin reorientation transition, SRI) into an in-plane (IP) anisotropy region. Our measurements demonstrate that the switching voltage amplitude decreases with increasing FeCoB thickness, i.e. the decrease in effective anisotropy. This result underscores the potential of our method to select, from room-temperature measurements, regions of the layer stack space that are suitable for cryogenic memory applications, either by controlling the ferromagnetic layer thickness or the tunnel barrier oxidation state.
Team: MRAM
Collaboration: Institut Neel (Grenoble)
Funding: ANR Crymco (ANR-20-CE24-0009), PEPR project PRESQUILE (ANR-22-PETQ-0002).
Further reading: Optimizing Effective Anisotropy in Magnetic Tunnel Junctions for Operation at Cryogenic Temperatures, S. Martín Rio, K. S. Senapati, L. Farcis, L. Soumah, P. B. Veiga, K. Garello, R. C. Sousa. Phys. Rev. Appl. 2025, 24 (2), 024016. Open access: hal-05217689
Contact: Ricardo Sousa