Liliana D. Buda-Prejbeanu

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.

This work investigates the influence of bias voltage on the optical switching behavior of a Tb/Co-based Magnetic Tunnel Junction (MTJ) in an 80-nm diameter pillar device. The experimental results demonstrate enhanced switching performance at high bias voltage, with a maximum switching probability achieved at bias voltage values of 200 and 400 mV. At high bias, Joule heating effect reduces the effective magnetic anisotropy of the pillar device, thereby favoring in-plane magnetization precession, lowering the switching threshold, and expanding the toggle switching window. The optical switching exhibits a pronounced asymmetry, showing a favorable switching under negative voltage polarity. Under this polarity, tunneling electrons are injected toward the top electrode, causing the decrease of magnetic anisotropy in the Tb/Co-based free layer and, therefore, facilitating the magnetization reversal at a laser pulse. The thermal analysis of All-Optical Switching (AOS)-MTJ devices under opposite bias polarities shows a temperature difference of 5–10 K, depending on the pillar diameter. These findings demonstrate voltage-selective control for multiple AOS devices integrated on the same chips, enabling an application for CMOS-compatible magneto-photonic memory.

This study investigates a vortex sensor based on a nanoscale (sub-100 nm) magnetic tunnel junction (MTJ) with a strong shape anisotropy, designed for sensitivity to the out-of-plane magnetic field component (H_z). The sensor comprises a free layer with a vortex configuration and a perpendicularly magnetized reference layer, which provides a reproducible and linear response when excited by a perpendicular magnetic field. Experimental measurements and micromagnetic simulations were combined to systematically assess the influence of structural parameters, specifically aspect ratio and defect landscape, on key sensor performance metrics, including dynamic range, sensitivity, and detectivity. The out-of-plane vortex sensor demonstrates a significantly improved dynamic range exceeding 200 mT, compared to the 40-80 mT typical of conventional in-plane vortex sensors. Frequency-dependent noise measurements reveal that the sensor exhibits low intrinsic noise, along with improved detectivity and resolution. This performance is ascribed to the field-dependent expansion and contraction of the vortex core, which reduces Barkhausen-type noise caused by defect-induced pinning potentials. Moreover, the sub-100 nm lateral dimensions of the sensor enable scalable array integration, providing further enhancements in noise and detectivity through collective averaging. These results underscore the potential of this sensor architecture for advanced magnetic field sensing applications requiring a wide dynamic range and high measurement accuracy at the same time.

The atomistic Landau-Lifshitz-Gilbert equation is challenged when modeling spintronic devices where Joule heating is significant, due to its core assumption of a constant magnetization magnitude. Based on a statistical framework that treats the magnetization magnitude as a dynamic variable coupled to a thermal bath, we derive a dynamic Landau-Lifshitz-Bloch-Slonczewski set of equations for torques, that captures the transient, heating-induced demagnetization that occurs during high-current operation. Integrating these dynamic equations and comparing them to their stochastic equivalents reveals that both the energy landscape and switching dynamics in high-anisotropy systems are similarly modified. This approach yields accurate and accelerated predictions of critical currents and switching times.

This study investigates a vortex sensor based on a nanoscale (sub-100 nm) magnetic tunnel junction (MTJ) with a strong shape anisotropy, designed for sensitivity to the out-of-plane magnetic field component ($H_z$). The sensor comprises a free layer with a vortex configuration and a perpendicularly magnetized reference layer, which provides a reproducible and linear response when excited by a perpendicular magnetic field. Experimental measurements and micromagnetic simulations were combined to systematically assess the influence of structural parameters, specifically aspect ratio and defect landscape, on key sensor performance metrics, including dynamic range, sensitivity, and detectivity. The out-of-plane vortex sensor demonstrates a significantly improved dynamic range exceeding 200 mT, compared to the 40-80 mT typical of conventional in-plane vortex sensors. Frequency-dependent noise measurements reveal that the sensor exhibits low intrinsic noise, along with improved detectivity and resolution. This performance is ascribed to the field-dependent expansion and contraction of the vortex core, which reduces Barkhausen-type noise caused by defect-induced pinning potentials. Moreover, the sub-100\,nm lateral dimensions of the sensor enable scalable array integration, providing further enhancements in noise and detectivity through collective averaging. These results underscore the potential of this sensor architecture for advanced magnetic field sensing applications requiring a wide dynamic range and high measurement accuracy at the same time.

The laser excitation of magnetization dynamics without applying external fields is a topic of high interest for its potential applications (e.g., magnetic memories, oscillators, and THz emitters). We explore the use of an ultrashort infrared laser pulse for triggering transient changes in a magnetic vortex, probing the induced dynamics by time-resolved magnetic helicoidal dichroism (MHD) in resonant extreme ultraviolet scattering. MHD describes the optical response of a magnetic sample upon sign reversal of either the light orbital angular momentum or the magnetization and was demonstrated to be sensitive to the sample spin texture. Here, we show that, in addition to the well-known ultrafast demagnetization and remagnetization laser-induced processes, the analysis of the MHD signal, supported by micromagnetic simulations, provides direct evidence of important transient reorganizations of the spin texture. In particular, we find that an ultrafast laser pulse of sufficient intensity can induce a surface transient magnetic texture where the vortex curling direction is reversed with respect to the bulk. This result provides insight into the preparation of metastable complex spin states in magnetic films by optical methods without applying an external field, which is of relevance for novel applications in data storage and manipulation.

Magnetic skyrmions are topological magnetic textures that hold great promise as nanoscale bits of information in memory and logic devices. Although room-temperature ferromagnetic skyrmions and their current-induced manipulation have been demonstrated, their velocity has been limited to about 100 meters per second. In addition, their dynamics are perturbed by the skyrmion Hall effect, a motion transverse to the current direction caused by the skyrmion topological charge. Here, we show that skyrmions in compensated synthetic antiferromagnets can be moved by current along the current direction at velocities of up to 900 meters per second. This can be explained by the cancellation of the net topological charge leading to a vanishing skyrmion Hall effect. Our results open an important path toward the realization of logic and memory devices based on the fast manipulation of skyrmions in tracks.