Multifunctional hardware technologies for neuromorphic computing are essential for replicating the complexity of biological neural systems, thereby improving the performance of artificial synapses and neurons. Integrating ionic and spintronic technologies offers new degrees of freedom to modulate synaptic potentiation and depression, introducing novel magnetic functionalities alongside the established ionic analogue behavior. We demonstrate that magneto-ionic devices can perform as synaptic elements with dynamically tunable depression linearity controlled by an external magnetic field, a functionality reminiscent of neuromodulation in biological systems. By applying magnetic fields we significantly reduce the nonlinearity of synaptic depression, transitioning from an exponential dependence to a linear response at higher fields. Neural network simulations reveal that this magnetically induced linearity enhancement improves learning accuracy across a wide range of learning rates, which is retained after the magnetic field is removed. These findings highlight the versatility and promise of magneto-ionic devices for developing tunable synaptic elements for neuromorphic hardware.
Magnetic tunnel junction (MTJ) used currently for data storage are characterized by perpendicular magnetic anisotropy, which is beneficial in terms of low current density required for switching and the thermal stability of the free magnetic layer. The other ferromagnet of MTJ, namely the reference layer is fixed using so-called synthetic antiferromagnetic (SAF) structure, which typically involves less-abundant material, such as Pt or Pd. We present an alternative stack structure, with the SAF based on Ni-Co superlattices, which is Pt-free. The reference layer of MTJ is characterized by the switching field above 250 mT. In MTJ nanopillars of diameter down to 80 nm, we show a robust switching with voltage pulses between 1 ms and 5 ns, tunneling magnetoresistance up to 140%, high thermal stability and switching current density of 2.6 MA/cm2. Our result show a promising route towards design of MTJs made of abundant materials.
Mobile magnetic textures hold great potential for applications in magnetic memories and neuromorphic computing. However, energy consumption remains an issue as magnetic textures are manipulated using electrical currents. We demonstrate using magneto-ionic gating that a spin reorientation transition (SRT) between two magnetic anisotropy states can be nucleated and propagated across a magnetic track, like magnetic domain walls or skyrmions. The SRT dynamics can be controlled exclusively using gate voltage pulses down to the submillisecond range and monitored electrically via the anomalous Hall effect, with an estimated energy consumption of 64 pJ for a full switching of the device. Intermediate nonvolatile states allow for analogue operation, which could provide a synaptic potentiation/depression functionality with a significantly reduced energy cost per weight-update operation compared to current-based approaches. These results add a voltage-driven magnetic information carrier to the spintronics tool kit, offering a new avenue for the design of low-power spintronics multistate memories.
AbstractThe Ta/CoFeB/Pt/MgO/HfO2 system is investigated, whose magnetic anisotropy can be controlled through magneto‐ionic gating, using both ionic liquid and solid state gating, via a non‐oxidative mechanism combining reversible and irreversible gating effects. Analysis of X‐ray absorption spectroscopy at the Co and Fe edges reveals no indications of oxidation after gating, while a reversible change at the oxygen K edge suggests the involvement of oxygen species in the magneto‐ionic process. In addition, X‐ray diffraction measurements reveal that gating can irreversibly increase the crystalline volume of MgO, through an increase in the MgO/Mg(OH)2 ratio. This is in line with measurements in solid state devices showing that in a series of 150 gating cycles a reversible effect combines with a progressive increase in the strength of the perpendicular magnetic anisotropy contribution that saturates after extensive cycling. Consequently, the observed gate‐induced changes in magnetic anisotropy can be attributed to the combined effects of Mg(OH)2 dehydration into MgO (irreversible) and most likely a gentle reordering of oxygen species at the CoFeB interface (reversible) leading to a non‐oxidative magneto‐ionic mechanism. This study provides valuable insights into the underlying mechanisms governing the complex magneto‐ionic phenomena, including the coexistence of both reversible and irreversible effects, and a pathway to voltage‐control of crystalline order in spintronics materials.
Spin orbit torque (SOT) provides an efficient way to induce magnetization dynamics and switching of nanomagnets using electrical currents. In this study we present a SOT-induced magnetization switching in a-W-based magnetic tunnel junction (MTJ) with an effective in-plane magnetic anisotropy. 6-nm thick W is characterized by a relatively low resistivity and the spin-Hall angle of 0.1. In the MTJ nanopillars with an easy-axis oriented perpendicular to the current direction we observed a magnetization reversal with the current density approximately 7 x 10(11) A/m(2)
