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.

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.

Altermagnets are compensated magnets belonging to spin symmetry groups that allow alternating spin polarizations both in the coordinate space of the crystal and in the momentum space of the electronic structure. In these materials the anisotropic local crystal environment of the different sublattices lowers the symmetry of the system so that the opposite-spin sublattices are connected only by rotations, which results in an unconventional spin-polarized band structure in the momentum space. This low symmetry of the crystal structure is expected to be reflected in the anisotropy of the anomalous Hall effect. In this work, we study the anisotropy of the anomalous Hall effect in epitaxial thin films of Mn$_5$Si$_3$, an altermagnetic candidate material. We first demonstrate a change in the relative N\'eel vector orientation when rotating the external field orientation through systematic changes in both the anomalous Hall effect and the anisotropic longitudinal magnetoresistance. We then show that the anomalous Hall effect in this material is anisotropic with the N\'eel vector orientation relative to the crystal structure and that this anisotropy requires high crystal quality and unlikely correlates with the magnetocrystalline anisotropy. Our results provide further systematic support to the case for considering epitaxial thin films of Mn$_5$Si$_3$ as an altermagnetic candidate material.

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.

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.

Magnetic skyrmions are localized chiral spin textures, which offer great promise to store and process information at the nanoscale. In the presence of asymmetric exchange interactions, their chirality, which governs their dynamics, is generally considered as an intrinsic parameter set during the sample deposition. In this work, we experimentally demonstrate that a gate voltage can control this key parameter. We probe the chirality of skyrmions and chiral domain walls by observing the direction of their current-induced motion and show that a gate voltage can reverse it. This local and dynamical reversal of the chirality is due to a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we attribute to ionic migration of oxygen under gate voltage. Micromagnetic simulations show that the chirality reversal is a continuous transformation, in which the skyrmion is conserved. This control of chirality with 2-3 V gate voltage can be used for skyrmion-based logic devices, yielding new functionalities.

Spin-Orbit Torque (SOT) Magnetic Random-Access Memories (MRAM) have shown promising results toward the realization of fast, non-volatile memory systems. Oxidation of the heavy-metal (HM) layer of the SOT-MRAM has been proposed as a method to increase its energy efficiency. But the results are widely divergent due to the difficulty in controlling the HM oxidation because of its low enthalpy of formation. Here, these differences are reconciled by performing a gradual oxidation procedure, which allows correlating the chemical structure to the physical properties of the stack. As an HM layer, Pt is chosen because of the strong SOT and the low enthalpy of formation of its oxides. The evidence of an oxide inversion layer at the ferromagnet (FM)/HM interface is found: the oxygen is drawn into the FM, while the HM remains metallic near the interface. Moreover, the oxygen migrates in the volume of the FM layer rather than being concentrated at the interface. Consequently, it is found that the intrinsic magnitude of the SOT is unchanged compared to the fully metallic structure. The previously reported apparent increase of SOTs is not intrinsic to platinum oxide and instead arises from systemic changes produced by oxidation.

Friction plays an essential role in most physical processes that we experience in our everyday life. Examples range from our ability to walk or swim, to setting boundaries of speed and fuel efficiency of moving vehicles. In magnetic systems, the displacement of chiral domain walls (DW) and skyrmions (SK) by Spin Orbit Torques (SOT), is also prone to friction. Chiral damping (αc), the dissipative counterpart of the Dzyaloshinskii Moriya Interaction (DMI), plays a central role in these dynamics. Despite experimental observation, and numerous theoretical studies confirming its existence, the influence of chiral damping on DW and SK dynamics has remained elusive due to the difficulty of discriminating from DMI. Here we unveil the effect that αc has on the flow motion of DWs and SKs driven by current and magnetic field. We use a static in-plane field to lift the chiral degeneracy. As the in-plane field is increased, the chiral asymmetry changes sign. When considered separately, neither DMI nor αc can explain the sign reversal of the asymmetry, which we prove to be the result of their competing effects. Finally, numerical modelling unveils the non-linear nature of chiral dissipation and its critical role for the stabilization of moving SKs.

Optical detection techniques are among the most powerful methods used to characterize spintronic phenomena. The spin orientation can affect the light polarization, which, by the reciprocal mechanism, can modify the spin density. Numerous recent experiments, report local changes in the spin density induced by a circularly polarized focused laser beam. These effects are typically probed electrically, by detecting the variations of the photoresistance or photocurrent associated to the reversal of the light helicity. Here we show that in general, when the light helicity is modified, the beam profile is slightly altered, and the barycenter of the laser spot is displaced. Consequently, the temperature gradients produced by the laser heating will be modulated, producing thermo-electric signals that alternate in phase with the light polarization. These unintended signals, having no connection with the electron spin, appear under the same experimental conditions and can be easily misinterpreted. We show how this contribution can be experimentally assessed and removed from the measured data. We find that even when the beam profile is optimized, this effect is large, and completely overshadows the spin related signals in all the materials and experimental conditions that we have tested.

Diabetes is a major global health threat. Both academics and industry are striving to develop effective treatments for this disease. In this work, we present a new approach to induce insulin release from β-islet pancreatic cells (INS-1E) by mechanical stimulation. Two types of experiments were carried out. First, a local stimulation was performed by dispersing anisotropic magnetic particles within the cell medium, which settled down almost immediately on cell plasma membranes. Application of a low frequency magnetic field (up to 40 Hz) generated by a custom-made magnetic device resulted in oscillations of these particles, which then exerted a mechanical constraint on the cell plasma membranes. The second type of experiment consisted of a global stimulation, where cells were grown on magneto-elastic membranes composed of a biocompatible polymer with embedded magnetic particles. Upon application of a rotating magnetic field, magnetic particles within the membrane were attracted towards the field source, resulting in the membrane's vibrations being transmitted to the cells grown on it. In both experiments, the cell response to these mechanical stimulations caused by application of the variable magnetic field was quantified via the measurement of insulin release in the growth medium. We demonstrated that the mechanical action induced by the motion of magnetic particles or by membrane vibrations was an efficient stimulus for insulin granule secretion from β-cells. This opens a wide range of possible applications including the design of a system which triggers insulin secretion by β-islet pancreatic cells on demand.

Thanks to their unique combination of properties: non-volatility, speed, density and write endurance, spintronic memory called spin transfer-torque Magnetic Random Access Memory (STT-MRAM) is expected to play a major role in the future development of the Internet of Things (IoT) and more generally in information and communication technologies. This type of spintronic device is usually made of materials, some of which can be classified as critical. Recent studies have evaluated critical materials contained in magnetic random access memory (Ku, 2018; European Commission, 2020 [1,2]). However, in those cases the type of memory analyzed belongs to the first generation of MRAMs developed in the early 2000s. Nowadays, the memory devices are magnetized perpendicularly to the plane of the layers and contain a synthetic antiferromagnet (SAF) that provides a high coercivity to the STT-MRAM reference layer with reduced stray field. This SAF is typically made of cobalt (Co) and platinum (Pt) multilayers antiferromagnetically coupled across a thin ruthenium (Ru) layer. Due to the high-embodied energy of platinum group metals (PGMs), a common concern when evaluating these materials is the environmental risk associated with their production. An evaluation of the environmental and economic risks of using such multilayers is first reported here, followed by a discussion of its supply risk. Substitution of Co/Pt multilayers by Co/Ni multilayers can lead to a reduction by 3–4 orders of magnitude in terms of energy requirements or global warming potential (GWP) associated with the use of these multilayers. An alternative concept based on perpendicular shape anisotropy (PSA) can also yield a reduction by 1–2 orders of magnitude in these quantities. However, for the case of STT-MRAM, tiny quantities of PGM layers are used in comparison with the mass of the silicon wafers on which these type of devices are grown. Therefore, the environmental and economic impact of the silicon wafer fabrication is found to be much higher than that of the PGM materials incorporated in the STT-MRAM stacks. Nonetheless, the high supply risk associated with PGMs remains a reason for awareness. One explored possibility is a SAF structure based on Co/Ni multilayers which can have similar performance. A second more challenging alternative is also proposed based on the aforementioned PSA concept. Finally, we address the case of several other metals identified by the European Commission as critical and used in MRAM such as W or Ta, both recently included in the EU's Conflict Minerals Regulation released in January 2021 (https://ec.europa.eu/trade/policy/in-focus/conflict-minerals-regulation/regulation-explained/, 2020 [3]).

Magnetic skyrmions are topological spin textures holding great potential as nanoscale information carriers. Recently, skyrmions have been predicted in antiferromagnets, with key advantages in terms of stability, size and dynamical properties over their ferromagnetic analogs. However, their experimental demonstration is lacking. Here we show that skyrmions can be stabilized at zero field and room temperature at the interface of sputtered IrMn thin films exchange-coupled to a ferromagnetic layer. This was realised by replicating the skyrmionic spin texture of the ferromagnet in the antiferromagnet, via annealing above the blocking temperature of the ferromagnet/antiferromagnet bilayer. Using the high-spatial-resolution magnetic microscopy technique XMCD-PEEM, we observe the skyrmions within the IrMn interfacial layer from the XMCD signal of the uncompensated Mn spins at the interface. This result opens up a path for logic and memory devices based on skyrmion manipulation in antiferromagnets.

In this study, a new type of compact magnetic memristor is demonstrated. It is based on the variation of the conductivity of a nano-sized magnetic tunnel junction as a function of the angle between the in-plane reference layer magnetization and a free layer exhibiting an isotropic in-plane coercivity. The free layer magnetization is rotated by two spin transfer torque contributions: one originating from the in-plane magnetized reference layer and the other one from an additional perpendicular polarizer integrated in the stack. Thanks to a proper tuning of the relative influence of these two torques, the magnetization of the free layer can be rotated step by step clockwise or anticlockwise in a range of angle between 0° (parallel configuration) and 180° (anti-parallel configuration) by sending pulses of current through the stack, of one or opposite polarity. The amplitude of the rotation steps and therefore of the conductance variations depends on the pulse amplitude and duration. In this way, we achieve monotonous variations of the resistance with the voltage polarity through the application of pulses in the ns range. We also retrieve the analytical expression of critical current density which is found to be in good agreement with the experimental results. The thermal stability of the intermediate resistance levels and the role of Joule heating are also discussed.

This paper reports the first experimental demonstration of a new concept of double magnetic tunnel junctions comprising a magnetically switchable assistance layer. These double junctions are used as memory cells in spin transfer torque magnetic random access memory (STT-MRAM) devices. Their working principle, fabrication and electrical characterization are described and their performances are compared to those of reference devices without an assistance layer. We show that thanks to the assistance layer, the figure of merit of STT-MRAM cells can be increased by a factor of 4 as compared to that of STT-MRAM based on conventional stacks without the assistance layer. A detailed discussion of the results is given supported by numerical simulations. The simulations also provide guidelines on how to optimize the properties of the assistance layer to get the full benefit from this concept.

Suppression of superconductivity due to the proximity effect between a superconductor and a ferromagnet can be partially alleviated when a Cooper pair simultaneously samples different directions of the short-range exchange field. The superconductor’s critical temperature, TC, is therefore expected to partially recover when the ferromagnet is in a multi-domain state, as opposed to a single-domain state. Here, we discuss series of experiments performed with ferromagnet(Pt/Co)/spacer(IrMn and Pt)/superconductor(NbN) heterostructures. By tuning the various parameters in play, e.g., superconducting coherence length-to-thicknesses ratio, and domain sizes, we obtained up to 10% recovery of the superconducting critical temperature ΔTC/TC. This large-scale recovery made novel investigations possible. In particular, from the spacer thickness-dependence of ΔTC/TC, it was possible to deduce the characteristic length for Cooper pair penetration in an IrMn antiferromagnet. This information is crucial for electronic transport, and up to now has been difficult to access experimentally for antiferromagnets.

Biocompatible suspended magneto-elastic membranes were prepared. They consist of PDMS (polydimethylsiloxane) films, with embedded arrays of micrometric magnetic pillars made with lithography techniques. For visible light wavelengths, our membranes constitute magnetically tunable optical diffraction gratings, in transmission and reflection. The optical response has been quantitatively correlated with membrane structure and deformation, through optical and magneto-mechanical models. In contrast to the case of planar membranes, the diffraction patterns measured in reflection and transmission vary very differently upon magnetic field application. Indeed, the reflected beam is largely affected by the membrane bending, whereas the transmitted beam remains almost unchanged. In reflection, even weak membrane deformation can produce significant changes of the diffraction patterns. This field-controlled optical response may be used in adaptive optical applications, photonic devices, and for biological applications.

State-of-the-art spin-transfer-torque magnetic random access memory (STT MRAM) is based on out-of-plane magnetized magnetic tunnel junctions (pMTJ), which commonly comprise a composite storage layer (CSL) of the form Fe72Co8B20/nonmagnetic spacer (e.g., Ta, W, Mo)/Fe72Co8B20. Because STT MRAM has to operate over a wide range of temperature (e.g., -40 degrees C to 150 degrees C for automotive applications), it is desirable to minimize the temperature variation of the magnetic and transport properties of pMTJs. In this context, we report an alternative design of a dual-W CSL of the form "Fe72Co8B20/W/ferromagnet/W/Fe72Co8B20", which brings additional degrees of freedom, to further improve the performance of the storage layer. In particular, this design reduces the thermal variation of magneto-transport properties of dual-W CSL. A detailed investigation of the thermal variation of magnetic and electrical properties of such dual-W CSLs for various ferromagnet insertions, as characterized by different exchange stiffnesses, is reported. In particular, cobalt insertion between two W laminations significantly enhances the effective perpendicular anisotropy (K-eff) at room temperature, relative to that of the conventional composite storage layer (Fe72Co8B20/W/Fe72Co8B20), and reduces the relative thermal variation of Keff in the range 300-400 K. Moreover, STT MRAM cells with diameters of 80 nm comprising Co inserted into dual-W CSL exhibit, on average, 15% and 38% higher thermal stability factors, as well as 26% and 15% higher tunneling magnetoresistance ratios at room temperature and 110 degrees C, respectively, than those of conventional CSLs. These observations are interpreted in terms of magnetic stiffening of the storage layer achieved thanks to the very high Curie temperature (1388 K) of cobalt.