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.

This study investigates how device size influences the behavior of Tb/Co-based magnetic tunnel junctions subjected to femtosecond laser pulses driving an all-optical precessional switching reversal of the magnetization. Complete magnetization switching is achieved in 80-nanometer pillars, whereas larger pillars exhibit multiple intermediate resistance states. This difference arises from the formation of magnetic domains in larger pillars, which prevents uniform switching under a single laser pulse. Moreover, to enhance stability at high annealing temperatures and comply with CMOS process requirements, a trilayer $\text{FeCoB} / \text{MgO} / \text{FeCoB}$ capping stack is implemented. This capping enhances the interfacial perpendicular magnetic anisotropy and enables thermal annealing up to 350 °C. The corresponding device shows a full switching at 300 °C annealing temperature with a resistance area of $7.5 \Omega \mu \mathrm{m}^{2}$.

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.

In ultrathin ferromagnetic films sandwiched between two distinct heavy metal layers or between a heavy metal and an oxide layer, the Dzyaloshinskii-Moriya interaction (DMI) is of interfacial origin. Its chirality and strength are determined by the properties of the adjacent heavy metals and the degree of oxidation at the interfaces. Here, we demonstrate that the DMI chirality can change solely with variations in the thickness of the ferromagnetic layer - an effect that has not been experimentally studied in details or explained until now. Our experimental observation in the trilayer system Ta/FeCoB/TaOx is supported by ab initio calculations: they reveal that variations in orbital filling and inter-atomic distances at the interface, driven by the structural relaxations in the ultrathin regime, lead to an inversion of DMI chirality. We hence propose a new degree of freedom to tune DMI chirality and the associated chiral spin textures by tailoring crystal structure e.g. using strain or surface acoustic waves.

This study introduces a novel vortex-based magnetic tunnel junction (MTJ) sensor designed for high-performance detection of out-ofplane magnetic fields. Unlike conventional in-plane vortex sensors, this architecture employs nanoscale diameters (60-150 nm) and higher aspect ratios (t/D ∼ 0.4−1), enabling the vortex core to contract or expand in response to perpendicular fields, resulting in a highly linear and low-noise tunneling magnetoresistance (TMR) response. Experimental and micromagnetic simulations demonstrate a wide dynamic range exceeding 200 mT, enhanced detectivity, and reduced Barkhausen noise, outperforming previous MTJ-based vortex sensors. Noise characterization reveals dominant 1/f noise with low Hooge parameters. The nanoscale footprint and array scalability make this design ideal for next-generation magnetic sensing applications requiring high resolution, low power, and compact integration.

Correction for 'Optical response of magnetically actuated biocompatible membranes' by H. Joisten et al., Nanoscale, 2019, 11, 10667-10683, https://doi.org/10.1039/C9NR00585D.

We demonstrate a good agreement between mean dwell times measured in 50 nm diameter, perpendicularly magnetized superparamagnetic tunnel junctions (SMTJ), and theoretical predictions based on Langer's theory. Due to a large entropic contribution, the theory yields Arrhenius prefactors in the femtosecond range for the measured junctions, in stark contrast to the typically assumed value of 1 ns. Thanks to the low prefactors, and fine-tuning of the perpendicular magnetic anisotropy, we report measured mean dwell times as low as 2.7 ns under an in-plane applied field at negligible bias voltage. Under a perpendicular applied field, we predict a Meyer-Neldel compensation phenomenon, whereby the prefactor scales like an exponential of the activation energy, in line with the exponential dependence of the measured dwell time on the field. We further predict the occurrence of (sub)nanosecond dwell times as a function of effective anisotropy and junction diameter at zero bias voltage. These findings pave the way towards the development of ultrafast, low-power, unconventional computing schemes operating by leveraging thermal noise in perpendicular SMTJs, which can be scaled down below 20 nm.

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.

Skyrmions are magnetic bubbles with nontrivial topology envisioned as data bits for ultrafast and power-efficient spintronic memory and logic devices. They may be stabilized in heavymetal/ferromagnetic/oxide trilayer systems. The skyrmion chirality is then determined by the sign of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Nevertheless, for apparently identical systems, there is some controversy about the DMI sign. Here, we show that the degree of oxidation of the top interface and the thickness of the ferromagnetic layer play a major role. Using Brillouin light-scattering measurements in Ta/Fe-Co-B/TaOX trilayers, we demonstrate a sign change of the DMI with the degree of oxidation of the Fe-Co-B/TaOX interface. Using polar magneto-optical Kerr effect microscopy, we consistently observe a reversal of the direction of current-induced motion of skyrmions with the oxidation level of TaOX; this is attributed to their chirality reversal. In addition, a second chirality reversal is observed when changing the Fe-Co-B thickness, probably due to the proximity of the two Fe-Co-B interfaces in the ultrathin case. By properly tuning the chirality of the skyrmion, spin-transfer and spin-orbit torques combine constructively to enhance the skyrmion velocity. These observations thus allow us to envision an optimization of the material parameters to produce highly mobile skyrmions. Moreover, this chirality control enables a versatile manipulation of skyrmions and paves the way towards multidirectional devices.

The possibility of higher electrical efficiency in computing by operating at low temperatures raises the need for non-volatile memory cells optimized for cryogenic operation. We report a study on low temperature spin transfer torque switching of magnetic tunnel junctions with 20 to 100 nm in diameter with thermal stability adapted to low temperature operation. The evolution of magnetic and electrical properties are characterized for four different stacks from 300 to 10 K comprising insertions of Mg, Ru and permalloy (Py) in the storage layer to reduce its effective anisotropy. Two figures of merit are used to compare different devices and stacks, Δ/Ic and Δ/Esw, normalizing the thermal stability Δ by the critical current or switching energy. Devices with a Py insertion layer show a higher FOM (3.78 kBTop/μA) and switching energy Esw below 655 fJ for 100 ns pulses at Top = 10 K. A procedure to optimize the reference layer stray field was also implemented to achieve full compensation using a synthetic antiferromagnetic layer for 20 nm diameter devices.

We identified through numerical simulations the optimal condition to have a deterministic switching regime assisted by voltage-controlled magnetic anisotropy (VCMA). To minimize the write energy required to reach this regime, we measure the VCMA coefficient $\xi $ on perpendicular magnetic tunnel junctions (pMTJ) with high resistance area (RA) product and varying thicknesses of the FeCoB storage layer and the naturally oxidized tunnel barrier. The VCMA coefficient is higher as the effective anisotropy decreases, which is the case for larger Mg and FeCoB thicknesses. The temperature dependence of $\xi $ was shown to increase from room temperature to 5 K, showing values up to 35 fJ/Vm at 10 K.

Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. In particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards THz frequencies. This work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of Tb/Co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. Toggling of the magnetization of the Tb/Co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm2, for Co-rich compositions of the stack either in isolation or coupled to a CoFeB-electrode/MgO-barrier tunnel-junction stack. Successful switching of the CoFeB-[Tb/Co] electrodes was obtained even after annealing at 250 °C. After integration of the [Tb/Co]-based electrodes within perpendicular magnetic tunnel junctions yielded a maximum tunneling magnetoresistance signal of 41% and RxA value of 150 Ωμm2 with current-in-plane measurements and ratios between 28% and 38% in nanopatterned pillars. These results represent a breakthrough for the development of perpendicular magnetic tunnel junctions controllable using single laser pulses, and offer a technologically-viable path towards the realization of hybrid spintronic-photonic systems featuring THz switching speeds.

Skyrmions are magnetic bubbles with nontrivial topology envisioned as data bits for ultrafast and power-efficient spintronic memory and logic devices. They may be stabilized in heavy-metal/ferromagnetic/oxide trilayer systems. The skyrmion chirality is then determined by the sign of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Nevertheless, for apparently identical systems, there is some controversy about the DMI sign. Here, we show that the degree of oxidation of the top interface and the thickness of the ferromagnetic layer play a major role. Using Brillouin light-scattering measurements in Ta/Fe-Co-B/TaOx trilayers, we demonstrate a sign change of the DMI with the degree of oxidation of the Fe-Co-B/TaOx interface. Using polar magneto-optical Kerr effect microscopy, we consistently observe a reversal of the direction of current-induced motion of skyrmions with the oxidation level of TaOx; this is attributed to their chirality reversal. In addition, a second chirality reversal is observed when changing the Fe-Co-B thickness, probably due to the proximity of the two Fe-Co-B interfaces in the ultrathin case. By properly tuning the chirality of the skyrmion, spin-transfer and spin-orbit torques combine constructively to enhance the skyrmion velocity. These observations thus allow us to envision an optimization of the material parameters to produce highly mobile skyrmions. Moreover, this chirality control enables a versatile manipulation of skyrmions and paves the way towards multidirectional devices.

Switching of magnetic tunnel junction using femto-second laser enables a possible path for THz frequency memory operation, which means writing speeds 2 orders of magnitude faster than alternative electrical approaches based on spin transfer or spin orbit torque. In this work we demonstrate successful field-free 50fs single laser pulse driven magnetization reversal of [Tb/Co] based storage layer in a perpendicular magnetic tunnel junction. The nanofabricated magnetic tunnel junction devices have an optimized bottom reference electrode and show Tunnel Magnetoresistance Ratio values (TMR) up to 74\% after patterning down to sub-100nm lateral dimensions. Experiments on continuous films reveal peculiar reversal patterns of concentric rings with opposite magnetic directions, above certain threshold fluence. These rings have been correlated to patterned device switching probability as a function of the applied laser fluence. Moreover, the magnetization reversal is independent on the duration of the laser pulse. According to our macrospin model, the underlying magnetization reversal mechanism can be attributed to an in-plane reorientation of the magnetization due to a fast reduction of the out-of-plane uniaxial anisotropy. These aspects are of great interest both for the physical understanding of the switching phenomenon and their consequences for all-optical-switching memory devices, since they allow for a large fluence operation window with high resilience to pulse length variability.

Spin-orbit torques (SOTs) have opened a novel way to manipulate the magnetization using in-plane current, with a great potential for the development of fast and low power information technologies. It has been recently shown that two-dimensional electron gases (2DEGs) appearing at oxide interfaces provide a highly efficient spin-to-charge current interconversion. The ability to manipulate 2DEGs using gate voltages could offer a degree of freedom lacking in the classical ferromagnetic/spin Hall effect bilayers for spin-orbitronics, in which the sign and amplitude of SOTs at a given current are fixed by the stack structure. Here, we report the non-volatile electric-field control of SOTs in an oxide-based Rashba-Edelstein 2DEG. We demonstrate that the 2DEG is controlled using a back-gate electric-field, providing two remanent and switchable states, with a large resistance contrast of 1064%. The SOTs can then be controlled electrically in a non-volatile way, both in amplitude and in sign. This achievement in a 2DEG-CoFeB/MgO heterostructures with large perpendicular magnetization further validates the compatibility of oxide 2DEGs for magnetic tunnel junction integration, paving the way to the advent of electrically reconfigurable SOT MRAMS circuits, SOT oscillators, skyrmion and domain-wall-based devices, and magnonic circuits.

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.

Magnetic tunnel junctions with perpendicular magnetic anisotropy for Magnetic Random Access Memory need to combine high speed and low critical switching current. Higher spin transfer torque (STT) write efficiency is required. This can be achieved introducing a switchable assistance layer, which can be designed to maximize the STT efficiency independently of the switching direction. At the same time, the assistance layer also increases the retention in standby. The reversal process was confirmed with time-resolved measurements. The outlook for scaling to the sub-20 nm diameter range will also be reviewed looking at STT driven switching in perpendicular shape anisotropy cells.

In this work, we used the soft X-ray resonant magnetic reflectivity to study the depth-resolved out-of-plane (oop) magnetization profile of a CoFeB/MgO sample with W/Ta cap layer after annealing at 400°C. It is a powerful technique to probe buried magnetic interfaces of ultra-thin films by combining the depth-resolved information of X-ray reflectivity with the species selectivity of X-ray magnetic circular dichroism. It allowed us to resolve the oop magnetization within a 1.36 nm thick CoFeB layer by the measurement of angle-dependent specular reflectivity at large scattering angles (up to 80°). We determined a graded magnetic distribution for both Fe and Co with a 20% increase at the interface with MgO, decreasing slightly over a thickness of 0.7 nm from MgO before it rapidly decreases to 50% at the interface with W. After applying a non-saturating magnetic field in the plane of the sample, we also quantified a similar magnetization profile with an inclined moment configuration. This indicates that the magnetization gradient is a robust property of the CoFeB layer in the studied sample.