Bruno M. S. Teixeira

This study investigates the enhancement of double magnetic tunnel junction (DMTJ) performance through the integration of an assistance layer with perpendicular magnetic anisotropy. The diameter of the device and the thickness of the storage layer are identified as the key parameters that define thermal stability and switching speed. Both macrospin-model simulations and experimental measurements confirm that an increase in effective anisotropy produces faster switching. Previous reports confirmed that dual-spin torque mechanisms can be achieved with a hard polarizer or by introducing a thinner assistance layer alternative, but this can reduce tunneling magnetoresistance. In this work, by tuning the capping magnesium oxide, higher tunneling-magneto resistance values are achieved. Quasi-static measurements reveal that the highest spin-transfer torque efficiency is obtained when the effective anisotropy is maximized. Write-error-rate measurements, performed as a function of pulse width down to 7 ns, are analyzed with a new unified model introduced in this work, which combines ballistic and thermally activated regimes. One main result of this work shows that reducing the thicknesses of storage and assistance layers decreases the characteristic switching time τsw and that τsw further exhibits a clear dependence on device diameter, consistent with domain-wall-mediated switching dynamics, thereby indicating enhanced switching speed in the ballistic regime. The unified model introduces a characteristic switching time associated with minimum reversal energy, enabling the definition of a new figure of merit based on this minimum energy. The analysis highlights a trade-off where enhancing the ratios of thermal stability to current leads to longer characteristic switching times and higher write energy. These findings provide a comprehensive understanding of the factors that influence DMTJ performance and offer strategies to optimize their design for low-energy magnetic memory applications.

Spintronic devices have recently attracted a lot of attention in the field of unconventional computing due to their non-volatility for short- and long-term memory, nonlinear fast response, and relatively small footprint. Here we demonstrate experimentally how voltage driven magnetization dynamics of dual free layer perpendicular magnetic tunnel junctions can emulate spiking neurons in hardware. The output spiking rate was controlled by varying the dc bias voltage across the device. The field-free operation of this two-terminal device and its robustness against an externally applied magnetic field make it a suitable candidate to mimic the neuron response in a dense neural network. The small energy consumption of the device (4-16 pJ/spike) and its scalability are important benefits for embedded applications. This compact perpendicular magnetic tunnel junction structure could finally bring spiking neural networks to sub-100 nm size elements.

In situ ion-beam-induced luminescence measurements reveal a strong enhancement of the Cr³⁺ emission yield in electrically conductive chromium doped β-Ga₂O₃ single crystals upon proton irradiation. The observed effect can be explained based on the Fermi-level pinning caused by radiation defects. This pinning of the Fermi level activates deep carrier traps that can act as sensitizers of the Cr³⁺ emission. In agreement with this model, in semi-insulating samples, where the Fermi level lies deep in the bandgap, the Cr³⁺ emission is present already in as-grown samples, and no enhancement of its intensity is observed upon proton irradiation. The boost of the Cr³⁺ emission yield by irradiation, observed in conductive samples, is reversed by thermal annealing in argon at temperatures above 550 °C for 30 s. The results reveal a high potential of Cr-doped β-Ga₂O₃ for in situ and ex situ optical radiation detection and dosimetry.

This work reports experimental confirmation of the working principles of a double magnetic tunnel junction (DMTJ) to achieve highly efficient spin-transfer-torque (STT) switching. This concept uses a magnetically switchable assistance layer (ASL) acting as a top perpendicular spin polarizer. The STT-induced switching dynamics were described by macrospin simulations, while the magnetic and electrical properties of the devices were investigated in switching experiments. The reversal dynamics of the storage layer/ASL coupled system were validated by time-resolved measurements of the device resistance during write operation, confirming that the storage layer is subjected to additive STT contributions both from the reference layer and the ASL throughout its parallel-to-antiparallel and antiparallel-to-parallel transitions. The STT efficiency of the ASL-DMTJ was compared to that of single MTJ stacks comprising the same storage layer and no assistance layer. The figure of merit ∆/Ic (stability/critical current) was found to nearly double in devices of 80 and 100 nm diameter, with a smaller 30% increase obtained for 50 nm diameter cells.

The impact of 400 keV Ar ⁺ irradiation on the magnetic and electrical properties of in-plane magnetized magnetic tunnel junction (MTJ) stacks was investigated by ferromagnetic resonance, vibrating sample magnetometry and current-in-plane tunneling techniques. The ion fluences ranged from 10 12  c m − 2 to   5     ×     10 15  c m − 2 . Below   10 14  c m − 2 , the anisotropy of the Ta-capped FeCoB free layer was weakly modulated, following a decrease in the saturation magnetization. The tunnel magnetoresistance (TMR), along with the exchange-bias and the interlayer exchange coupling providing a stable magnetic configuration to the reference layer, decreased continuously. Above 10 14  c m − 2 , a strong decrease in the saturation magnetization was accompanied by a loss of the magnetic coupling and of the TMR. We show there is an ion-fluence window where the modulation of magnetic anisotropy can occur while preserving a large TMR and stable magnetic configuration of the MTJ, and demonstrate that the layers surrounding the free layer play a decisive role in determining the trend of the magnetic anisotropy modulation resulting from the irradiation. Our results provide guidance for the tailoring of MTJ parameters via ion irradiation, which we propose as a potentially suitable technique for setting the magnetic easy-cone state in MTJ for attaining field-free, fast, and non-stochastic magnetization switching.

We present a ferromagnetic resonance study of the dynamic properties of a set of amorphous Fe-B films deposited on Corning Glass® and MgO (001) substrates, either with or without capping. We show that the in plane anisotropy of the MgO grown films contains both uniaxial and biaxial components whereas it is just uniaxial for those grown on glass. The angular dependence of the linewidth strongly differs in terms of symmetry and magnitude depending on the substrate and capping. We discuss the role of the interfaces on the magnetization damping and the generation of the anisotropy. We obtained values of the intrinsic damping parameters comparable to the lowest ones reported for amorphous films of similar compositions.