Giacomo Sala

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Sala

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Giacomo

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0000-0001-5696-5627

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Publications 1 - 10 of 18

Magnetization fluctuations and magnetic after-effect probed via the anomalous Hall effect
Nabben, Nadine; Sala, Giacomo; Nowak, Ulrich; et al. (2024)
Physical Review Research
Taking advantage of the anomalous Hall effect, we electrically probe low-frequency magnetization fluctuations at room temperature in a thin ferromagnetic Pt/Co/AlOx layer stack with perpendicular magnetic anisotropy. We observe a strong enhancement of the Hall voltage fluctuations within the hysteretic region of the magnetization loop. Analyzing both the temporal evolution of the anomalous Hall voltage and its frequency-dependent noise power density, we identify two types of magnetic noise: abrupt changes in the magnetic domain configuration, evident as Barkhausen-like steps in the Hall voltage time trace, yield a noise power density spectrum scaling with frequency as 1/fβ with β≈2.0. In contrast, quasistationary magnetization configurations are connected with a magnetic noise power density with an exponent β≈1.0. The observation of Barkhausen steps and relaxation effects shows that the magnetic system is in a nonstationary state in the hysteresis region, such that the fluctuation-dissipation theorem cannot be expected to hold. However, the time-dependent change in the Hall voltage for constant magnetic field strength resembles the integrated noise power.
Real-time Hall-effect detection of current-induced magnetization dynamics in ferrimagnets
Sala, Giacomo; Krizakova, Viola; Grimaldi, Eva; et al. (2021)
Nature Communications
Measurements of the transverse Hall resistance are widely used to investigate electron transport, magnetization phenomena, and topological quantum states. Owing to the difficulty of probing transient changes of the transverse resistance, the vast majority of Hall effect experiments are carried out in stationary conditions using either dc or ac. Here we present an approach to perform time-resolved measurements of the transient Hall resistance during current-pulse injection with sub-nanosecond temporal resolution. We apply this technique to investigate in real-time the magnetization reversal caused by spin-orbit torques in ferrimagnetic GdFeCo dots. Single-shot Hall effect measurements show that the current-induced switching of GdFeCo is widely distributed in time and characterized by significant activation delays, which limit the total switching speed despite the high domain-wall velocity typical of ferrimagnets. Our method applies to a broad range of current-induced phenomena and can be combined with non-electrical excitations to perform pump-probe Hall effect measurements.
A two-terminal spin valve device controlled by spin–orbit torques with enhanced giant magnetoresistance
Avci, Can O.; Lambert, Charles-Henri; Sala, Giacomo; et al. (2021)
Applied Physics Letters
We report on the combination of current-induced spin–orbit torques and giant magnetoresistance in a single device to achieve all-electrical write and readout of the magnetization. The device consists of perpendicularly magnetized TbCo and Co layers separated by a Pt or Cu spacer. Current injection through such layers exerts spin–orbit torques and switches the magnetization of the Co layer, while the TbCo magnetization remains fixed. Subsequent current injection of lower amplitude senses the relative orientation of the magnetization of the Co and TbCo layers, which results in two distinct resistance levels for parallel and antiparallel alignment due to the current-in-plane giant magnetoresistance effect. We further show that the giant magnetoresistance of devices including a single TbCo/spacer/Co trilayer can be improved from 0.02% to 6% by using a Cu spacer instead of Pt. This type of devices offers an alternative route to a two-terminal spintronic memory that can be fabricated with a moderate effort.
Interplay of Voltage Control of Magnetic Anisotropy, Spin-Transfer Torque, and Heat in the Spin-Orbit-Torque Switching of Three-Terminal Magnetic Tunnel Junctions
Krizakova, Viola; Grimaldi, Eva; Garello, Kevin; et al. (2021)
Physical Review Applied
We use three-terminal magnetic tunnel junctions (MTJs) designed for field-free switching by spin-orbit torques (SOTs) to systematically study the impact of dual voltage pulses on the switching performance. We show that the concurrent action of an SOT pulse and an MTJ bias pulse allows for reducing the critical switching energy below the level typical of spin-transfer-torque while preserving the ability to switch the MTJ on the subnanosecond time scale. By performing dc and real-time electrical measurements, we discriminate and quantify three effects arising from the MTJ bias: the voltage-controlled change of the perpendicular magnetic anisotropy, current-induced heating, and the spin-transfer torque. The experimental results are supported by micromagnetic modeling. We observe that, depending on the pulse duration and the MTJ diameter, different effects take a lead in assisting the SOTs in the magnetization-reversal process. Finally, we present a compact model that allows for evaluating the impact of each effect due to the MTJ bias on the critical switching parameters. Our results provide input to optimize the switching of three-terminal devices as a function of time, size, and material parameters.
Deterministic and stochastic aspects of current-induced magnetization reversal in perpendicular nanomagnets
Sala, Giacomo; Meyer, Joana; Flechsig, Anne; et al. (2023)
Physical Review B
We study the incubation and transition times that characterize the magnetization switching induced by spin-orbit torques in nanomagnets with perpendicular anisotropy. We present a phenomenological model to interpret the dependence of the incubation time on the amplitude of the voltage pulse and assisting magnetic field and estimate the volume of the seed domain that triggers the switching. Our measurements evidence a correlation between the incubation and transition times that is mediated by the temperature variation during the electric pulse. In addition, we discuss the stochastic distributions of the two times in terms of the energy barriers opposing the nucleation and expansion of the seed domain. We propose two models based on the log-normal and gamma functions to account for the different origin of the variability of the incubation and transition times, which are associated with a single nucleation barrier and multiple pinning sites, respectively.
Competing ordinary and Hanle magnetoresistance in Pt and Ti thin films
Sailler, Sebastian; Sala, Giacomo; Reustlen, Denise; et al. (2025)
Physical Review B
One of the key elements in spintronics research is the spin Hall effect, allowing to generate spin currents from charge currents. A large spin Hall effect is observed in materials with strong spin-orbit coupling, e.g., Pt. Recent research suggests the existence of an orbital Hall effect, the orbital analog to the spin Hall effect, which also arises in weakly spin-orbit-coupled materials like Ti, Mn, or Cr. In Pt both effects are predicted to coexist. In any of these materials, a magnetic field perpendicular to the spin or orbital accumulation leads to additional Hanle dephasing and thereby the Hanle magnetoresistance (MR). To reveal the MR behavior of a material with both spin and orbital Hall effect, we first study the MR of Pt thin films over a wide range of thicknesses. Careful evaluation shows that the MR of our textured samples is dominated by the ordinary MR rather than by the Hanle effect. We analyze the intrinsic properties of Pt films deposited by different groups and show that next to the resistivity also the structural properties of the film influence which MR dominates. We further show that this correlation can be found in both spin Hall active materials like Pt and orbital Hall active materials, like Ti. For both materials, we find a large Hanle MR for the samples without apparent structural order, whereas the ordinary MR dominates in the crystalline samples. We then provide a set of rules to distinguish between the ordinary and the Hanle MR. We suggest that in all materials with a spin or orbital Hall effect the Hanle MR and the ordinary MR coexist and the purity, crystallinity, and electronic structure of the thin film determine the dominating effect.
Generation, transmission, and conversion of orbital torque by an antiferromagnetic insulator
Ding, Shilei; Noël, Paul; Krishnaswamy , Gunasheel Kauwtilyaa; et al. (2025)
Nature Communications
Electrical control of magnetization in nanoscale devices can be significantly improved through the efficient generation of orbital currents and their conversion into spin currents. In nonmagnetic/ferromagnetic bilayers, this conversion produces a torque on the magnetization, enabling magnetization switching and dynamic manipulation. While previous studies focus on metallic ferromagnets, we demonstrate a large orbital torque and enhanced orbital-to-spin conversion by an antiferromagnetic insulating CoO layer. Measurements in CuOx/CoO/Co trilayers show that inserting CoO reverses the torque’s sign and triples its magnitude compared to CuOx/Co. This behaviour stems from the inverted oxygen gradient at the CuOx/CoO interface and CoO’s high orbital multiplicity, which favours the transmission of orbital momenta and efficient orbital-to-spin conversion. At low temperatures, the onset of antiferromagnetic order induces a further many-fold increase of the torque, which we attribute to the efficient excitation and propagation of spin-orbit excitons induced by magnetic coupling. Comparative measurements of CuOx/NiO/Co and CuOx/MnO/Co trilayers show that the torque efficiency scales with the orbital momentum of the Co²⁺, Ni²⁺, and Mn²⁺ ions in the antiferromagnet. These results reveal that antiferromagnetic insulators like CoO provide highly effective orbital-to-spin transduction, combining orbital torque and exchange bias functionalities to improve the performance of spintronic devices.
Ferrimagnetic Dynamics Induced by Spin-Orbit Torques
Sala, Giacomo; Gambardella, Pietro (2022)
Advanced Materials Interfaces
Ferrimagnets are the magnetic materials with the fastest current-induced dynamics reported so far. Among them, rare-earth transition-metal (RE-TM) alloys offer a fertile playground for studying the behavior of multi-sublattice systems with tunable composition and magnetic interactions. This review provides a survey of the magnetic dynamics excited by current-induced spin-orbit torques (SOTs) in RE-TM ferrimagnets coupled to heavy-metal layers. It summarizes the magnetic properties of RE-TM alloys and discusses how interfacial SOTs result in efficient magnetization switching and fast domain-wall motion close to the magnetization and angular momentum compensation points. Recent work shows that the switching is a multiphase process affected by significant stochastic fluctuations. However, strong SOTs results in fast and deterministic sub-ns switching with minimal energy dissipation. In addition, the RE and TM magnetizations can respond asynchronously to SOTs during the reversal. This asynchronous dynamics pinpoints the different strength of the SOTs acting on the two sublattices and challenges the usual assumption of rigid inter-sublattice antiferromagnetic coupling. Overall, the ability to tailor the timescale and reversal mode of RE-TM alloys allows for optimizing the speed of ferrimagnetic spintronic devices and provides insight into the current-induced transfer of angular momentum in systems with synergistic ferromagnetic and antiferromagnetic interactions.
Giant orbital Hall effect and orbital-to-spin conversion in 3d, 5d, and 4f metallic heterostructures
Sala, Giacomo; Gambardella, Pietro (2022)
Physical Review Research
The orbital Hall effect provides an alternative means to the spin Hall effect to convert a charge current into a flow of angular momentum. Recently, compelling signatures of orbital Hall effects have been identified in 3d transition metals. Here, we report a systematic study of the generation, transmission, and conversion of orbital currents in heterostructures comprising 3d, 5d, and 4f metals. We show that the orbital Hall conductivity of Cr reaches giant values of the order of 5 × 10⁵ [ℏ over bar 2e] Ω^(−1) m ^(−1) and that Pt presents a strong orbital Hall effect in addition to the spin Hall effect. Measurements performed as a function of thickness of nonmagnetic Cr, Mn, and Pt layers and ferromagnetic Co and Ni layers reveal how the orbital and spin currents compete or assist each other in determining the spin-orbit torques acting on the magnetic layer. We further show how this interplay can be drastically modulated by introducing 4f spacers between the nonmagnetic and magnetic layers. Gd and Tb act as very efficient orbital-to-spin current converters, boosting the spin-orbit torques generated by Cr by a factor of 4 and reversing the sign of the torques generated by Pt. To interpret our results, we present a generalized drift-diffusion model that includes both spin and orbital Hall effects and describes their interconversion mediated by spin-orbit coupling.
Spin-orbit torques and magnetization switching in Gd/Fe multilayers generated by current injection in NiCu alloys
Nasr, Federica; Binda, Federico; Lambert, Charles-Henri; et al. (2023)
Applied Physics Letters
Light transition metals have recently emerged as a sustainable material class for efficient spin–charge interconversion. We report measurements of current-induced spin–orbit torques generated by Ni₁-ₓCuₓ alloys in perpendicularly magnetized ferrimagnetic Gd/Fe multilayers. We show that the spin–orbit torque efficiency of Ni₁-ₓCuₓ increases with the Ni/Cu atomic ratio, reaching values comparable to those of Pt for Ni₅₅Cu₄₅. Furthermore, we demonstrate magnetization switching of a 20-nm-thick Gd/Fe multilayer with a threshold current that decreases with increasing Ni concentration, similar to the spin–orbit torque efficiency. Our findings show that Ni₁-ₓ Cuₓ- based magnetic heterostructures allow for efficient control of the magnetization by electric currents.

Publications 1 - 10 of 18

Giacomo Sala

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