Kharkiv Quantum Seminar: May 13, 2025

Magnetization Dynamics in Artificial Spin Ice Based on Magnetic Tunnel Junctions

Sara Majetich (Carnegie Mellon University, Pittsburgh, PA, USA)

Magnetic frustration significantly alters the dynamics of magnetostatically coupled nanomagnets. Artificial Spin Ice (ASI), simultaneous frustration of multiple nanomagnets leads to complex and long-range behavior [1, 2], including nonequilibrium excitations known as Emergent Magnetic Monopoles (EMMs). Here the nanomagnets are free layers of magnetic tunnel junctions (MTJs) with in-plane magnetization and a pinned synthetic antiferromagnet (SAF) fixed layer. This enables faster dynamics and has potential for electrical rather than magnetic field control. The focus in this work is on square lattice ASI made from circular 60 nm diameter MTJs with a 30 nm spacing (Figure 1a). Tunnel magnetoresistance (TMR) is measured through individual devices using direct contact to a conductive atomic force microscopy (C-AFM) tip that is biased relative to the substrate (Figure 1b). The spontaneous magnetization directions of the MTJs follow a pattern similar to that of square lattice ASI made from larger elongated nanomagnets [1] except that the moment directions vary on a millisecond time scale. The symmetry is broken by a small (8 Oe) coupling field between the free layer nanomagnets and unpatterned fixed layer. This symmetry breaking also means that half of the nanomagnets have moments parallel (0°) or antiparallel (180°) to an easy axis, and show typical two level telegraphing, while the other half have moments more or less aligned along the hard axis. The hard axis nanomagnets have time-dependent canting, with an average angle of ± 90° but with significant deviations that depend on the local field due to neighboring nanomagnets. Analysis of the distribution of dwell time reveals the importance of the spin canting in long-range coupling of nanomagnets that leads to short-term memory of the ensemble. Strategies for using MTJ-ASI in passive neural networks are discussed.