arXiv subjects

Pietro Gambardella

Publications and source records attributed to Pietro Gambardella.

At least 37 records · Page 2Linked to original sources

arXiv2025

Current-Controlled Magnon-Magnon Coupling in an On-Chip Cavity Resonator

Harnessing spin currents to control magnon dynamics enables new functionalities in magnonic devices. Here, we demonstrate current-controlled magnon-magnon coupling between cavity and boundary modes in an ultrathin film of Bi-doped yttrium iron garnet (BiYIG). Cavity modes emerge in a BiYIG region between two Pt nanostripes, where interfacial anisotropy modifies the magnon dispersion. These modes hybridize with boundary magnons confined within the Pt-capped BiYIG, resulting in an anticrossing gap. Modeling based on dipole-exchange spin-wave dispersion accurately reproduces the observed modes and their hybridization. Spin current injection via the spin Hall effect in a Pt nanostripe disrupts the cavity boundary conditions and suppresses both cavity modes and hybridization upon driving the system beyond the damping compensation threshold. Furthermore, tuning the microwave power applied to a microstrip antenna enables controlled detuning of the anticrossing gap. Our findings provide a platform for exploring spin current-magnon interactions and designing on-chip reconfigurable magnonic devices.

cond-mat.mes-hall↗

arXiv2025

Orbital Pumping in Ferrimagnetic Insulators

We report the detection of pure orbital currents generated by both coherent and thermal magnons in the magnetic insulator Bi-doped yttrium iron garnet (BiYIG). The pumping of orbital and spin currents is jointly investigated in nano-devices made of naturally oxidized Cu, pure Cu, Pt, and Cr. The absence of charge conduction in BiYIG and the negligible spin-to-charge conversion of oxidized Cu allows us to disambiguate the orbital current contribution. Comparative measurements on YIG and BiYIG show that the origin of the orbital pumping in BiYIG/oxidized Cu is the dynamics of the orbital magnetization in the magnetic insulator. In Cr, the pumping signal is dominated by the negative spin Hall effect rather than the positive orbital Hall effect, indicating that orbital currents represent a minority of the total angular momentum current pumped from the magnetic insulator. Our results also evidence that improving the interfacial transparency significantly enhances pumping efficiencies not only for spin, but also for orbital currents.

cond-mat.mes-hall↗

arXiv2024

Estimation of spin-orbit torques in the presence of current-induced magnon creation and annihilation

We present a comprehensive set of harmonic resistance measurements of the dampinglike (DL) and fieldlike (FL) torques in Pt/CoFeB, Pt/Co, W/CoFeB, W/Co, and YIG/Pt bilayers complemented by measurements of the DL torque using the magneto-optical Kerr effect and calibrated by nitrogen vacancy magnetometry on the same devices. The magnon creation-annihilation magnetoresistances depend strongly on temperature and on the magnetic and transport properties of each bilayer, affecting the estimate of both the DL and FL torque. The DL torque, the most important parameter for applications, is overestimated by a factor of 2 in W/CoFeB and by one order of magnitude in YIG/Pt when not accounting for the magnonic contribution to the planar Hall resistance. We further show that the magnonic contribution can be quantified by combining measurements of the nonlinear longitudinal and transverse magnetoresistances, thus providing a reliable method to measure the spin-orbit torques in different material systems.

cond-mat.mes-hall↗

arXiv2024

Nonlinear longitudinal and transverse magnetoresistances due to current-induced magnon creation-annihilation processes

Charge-spin conversion phenomena such as the spin Hall effect allow for the excitation of magnons in a magnetic layer by passing an electric current in an adjacent nonmagnetic conductor. We demonstrate that this current-induced modification of the magnon density generates an additional nonlinear longitudinal and transverse magnetoresistance for every magnetoresistance that depends on the magnetization. Using harmonic measurements, we evidence that these magnon creation-annihilation magnetoresistances dominate the second harmonic longitudinal and transverse resistance of thin Y$_{3}$Fe$_{5}$O$_{12}$/Pt bilayers. Our results apply to both insulating and metallic magnetic layers, elucidating the dependence of the magnetoresistance on applied current and magnetic field for a broad variety of systems excited by spin currents.

cond-mat.mes-hall↗

arXiv2024

Quantum magnetometry of transient signals with a time resolution of 1.1 nanoseconds

Quantum magnetometers based on spin defects in solids enable sensitive imaging of various magnetic phenomena, such as ferro- and antiferromagnetism, superconductivity, and current-induced fields. Existing protocols primarily focus on static fields or narrow-band dynamical signals, and are optimized for high sensitivity rather than fast time resolution. Here, we report detection of fast signal transients, providing a perspective for investigating the rich dynamics of magnetic systems. We experimentally demonstrate our technique using a single nitrogen-vacancy (NV) center magnetometer at room temperature, reaching a best-effort time resolution of 1.1 ns, an instantaneous bandwidth of 0.9 GHz, and a time-of-flight precision better than 20 ps. The time resolution can be extended to the picosecond range by use of on-chip waveguides. At these speeds, NV quantum magnetometers will become competitive with time-resolved synchrotron X-ray techniques. Looking forward, adding fast temporal resolution to the spatial imaging capability further promotes single-spin probes as powerful research tools in spintronics, mesoscopic physics, and nanoscale device metrology.

quant-ph↗

arXiv2024

Observation of chiral domain walls in an octupole-ordered antiferromagnet

Spin chirality in antiferromagnets offers new opportunities for spintronics. The kagome antiferromagnet Mn3Sn is a paradigmatic material in which the antiferromagnetic order parameter can be detected and controlled by electrical means. However, direct investigation of the magnetic texture of Mn3Sn has been challenging because of the tiny moment hosted in its magnetic octupole, hindering further clarification of this unique material. Here, we address this issue by observing the stray magnetic field from Mn3Sn using a diamond quantum scanning magnetometer. The spatially-resolved intrinsic domains and domain walls in a high-quality single-crystalline Mn3Sn film quantitatively reveal the polarization angle of the magnetic octupole in the kagome plane, the domain's local magnetization, the domain wall's width and chirality, and the octupole order in domain walls. Our nanoscale investigation of Mn3Sn, a powerful complement to macroscopic measurements, paves the road for developing chiral antiferromagnetism and its potential for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Control of spin-orbit torque-driven domain nucleation through geometry in chirally coupled magnetic tracks

The interfacial Dzyaloshinskii-Moriya interaction (DMI) can be exploited in magnetic thin films to realize lateral chirally coupled systems, providing a way to couple different sections of a magnetic racetrack and realize interconnected networks of magnetic logic gates. Here, we systematically investigate the interplay between spin-orbit torques, chiral coupling and the device design in domain wall racetracks. We show that the current-induced domain nucleation process can be tuned between single-domain nucleation and repeated nucleation of alternate domains by changing the orientation of an in-plane patterned magnetic region within an out-of-plane magnetic racetrack. Furthermore, by combining experiments and micromagnetic simulations, we show that the combination of damping-like and field-like spin-orbit torques with DMI results in selective domain wall injection in one of two arms of a Y-shaped devices depending on the current density. Such an element constitutes the basis of domain wall based demultiplexer, which is essential for distributing a single input to any one of the multiple outputs in logic circuits. Our results provide input for the design of reliable and multifunctional domain-wall circuits based on chirally coupled interfaces.

cond-mat.mes-hall↗

arXiv2024

Competing Ordinary and Hanle Magnetoresistance in Pt and Ti Thin Films

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 analogue 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 thus 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, the crystalline samples shows a MR attributed to the ordinary MR, whereas we find a large Hanle MR for the samples without apparent structural order. We then provide a set of rules to distinguish between the ordinary and the Hanle MR. We conclude that in all materials with a spin or orbital Hall effect the Hanle MR and the ordinary MR coexist and the purity and crystallinity of the thin film determine the dominating effect.

cond-mat.mtrl-sci↗

arXiv2024

Pulse Shaping Strategies for Efficient Switching of Magnetic Tunnel Junctions by Spin-Orbit Torque

The writing energy for reversing the magnetization of the free layer in a magnetic tunnel junction (MTJ) is a key figure of merit for comparing the performances of magnetic random access memories with competing technologies. Magnetization switching of MTJs induced by spin torques typically relies on square voltage pulses. Here, we focus on the switching of perpendicular MTJs driven by spin-orbit torque (SOT), for which the magnetization reversal process consists of sequential domain nucleation and domain wall propagation. By performing a systematic study of the switching efficiency and speed as a function of pulse shape, we show that shaped pulses achieve up to 50% reduction of writing energy compared to square pulses without compromising the switching probability and speed. Time-resolved measurements of the tunneling magnetoresistance reveal how the switching times are strongly impacted by the pulse shape and temperature rise during the pulse. The optimal pulse shape consists of a preheating phase, a maximum amplitude to induce domain nucleation, and a lower amplitude phase to complete the reversal. Our experimental results, corroborated by micromagnetic simulations, provide diverse options to reduce the energy footprint of SOT devices in magnetic memory applications.

physics.app-ph↗

arXiv2024

Mitigation of Gilbert Damping in the CoFe/CuOx Orbital Torque System

Charge-spin interconversion processes underpin the generation of spin-orbit torques in magnetic/nonmagnetic bilayers. However, efficient sources of spin currents such as 5d metals are also efficient spin sinks, resulting in a large increase of magnetic damping. Here we show that a partially-oxidized 3d metal can generate a strong orbital torque without a significant increase in damping. Measurements of the torque efficiency ξ and Gilbert damping α in CoFe/CuOx and CoFe/Pt indicate that ξ is comparable. The increase in damping relative to a single CoFe layer is Δα<0.002 in CoFe/CuOx and Δα ~ 0.005 - 0.02 in CoFe/Pt, depending on CoFe thickness. We ascribe the nonreciprocal relationship between Δα and ξ in CoFe/CuOx to the small orbital-to-spin current ratio generated by magnetic resonance in CoFe and the lack of an efficient spin sink in CuOx. Our findings provide new perspectives on the efficient excitation of magnetization dynamics via the orbital torque.

cond-mat.mtrl-sci↗

arXiv2024

Orbital Torque in Rare-Earth Transition-Metal Ferrimagnets

Orbital currents have recently emerged as a promising tool to achieve electrical control of the magnetization in thin-film ferromagnets. Efficient orbital-to-spin conversion is required in order to torque the magnetization. Here we show that the injection of an orbital current in a ferrimagnetic GdyCo100-y alloy generates strong orbital torques whose sign and magnitude can be tuned by changing the Gd content and temperature. The effective spin-orbital Hall angle reaches up to -0.25 in a GdyCo100-y/CuOx bilayer compared to +0.03 in Co/CuOx and +0.13 in GdyCo100-y/Pt. This behavior is attributed to the local orbital-to-spin conversion taking place at the Gd sites, which is about five times stronger and of the opposite sign relative to Co. Furthermore, we observe a manyfold increase in the net orbital torque at low temperature, which we attribute to the improved conversion efficiency following the magnetic ordering of the Gd and Co sublattices.

cond-mat.mtrl-sci↗

arXiv2024

Spin torque driven electron paramagnetic resonance of a single spin in a pentacene molecule

Control over quantum systems is typically achieved by time-dependent electric or magnetic fields. Alternatively, electronic spins can be controlled by spin-polarized currents. Here we demonstrate coherent driving of a single spin by a radiofrequency spin-polarized current injected from the tip of a scanning tunneling microscope into an organic molecule. With the excitation of electron paramagnetic resonance, we established dynamic control of single spins by spin torque using a local electric current. In addition our work highlights the dissipative action of the spin-transfer torque, in contrast to the nondissipative action of the magnetic field, which allows for the manipulation of individual spins based on controlled decoherence.

cond-mat.mes-hall↗

arXiv2024

Deterministic and stochastic aspects of current-induced magnetization reversal in perpendicular nanomagnets

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.

cond-mat.mes-hall↗

arXiv2024

Orbital Hanle Magnetoresistance in a 3d Transition Metal

The Hanle magnetoresistance is a telltale signature of spin precession in nonmagnetic conductors, in which strong spin-orbit coupling generates edge spin accumulation via the spin Hall effect. Here, we report the existence of a large Hanle magnetoresistance in single layers of Mn with weak spin-orbit coupling, which we attribute to the orbital Hall effect. The simultaneous observation of a sizable Hanle magnetoresistance and vanishing small spin Hall magnetoresistance in BiYIG/Mn bilayers corroborates the orbital origin of both effects. We estimate an orbital Hall angle of 0.016, an orbital relaxation time of 2 ps and diffusion length of the order of 2 nm in disordered Mn. Our findings indicate that current-induced orbital moments are responsible for magnetoresistance effects comparable to or even larger than those determined by spin moments, and provide a tool to investigate nonequilibrium orbital transport phenomena.

cond-mat.mes-hall↗

arXiv2023

Spin-orbit torques and magnetization switching in Gd/Fe multilayers generated by current injection in NiCu alloys

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 Ni$_{1-x}$Cu$_{x}$ alloys in perpendicularly magnetized ferrimagnetic Gd/Fe multilayers. We show that the spin-orbit torque efficiency of Ni$_{1-x}$Cu$_{x}$ increases with the Ni/Cu atomic ratio, reaching values comparable to those of Pt for Ni$_{55}$Cu$_{45}$. 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 Ni$_{1-x}$Cu$_{x}$$-$based magnetic heterostructures allow for efficient control of the magnetization by electric currents.

cond-mat.mtrl-sci↗

arXiv2023

Spin-orbit torques and spin Hall magnetoresistance generated by twin-free and amorphous Bi0.9Sb0.1 topological insulator films

Topological insulators have attracted great interest as generators of spin-orbit torques (SOTs) in spintronic devices. Bi\textsubscript{1-x}Sb\textsubscript{x} is a prominent topological insulator that has a high charge-to-spin conversion efficiency. However, the origin and magnitude of the SOTs induced by current-injection in Bi\textsubscript{1-x}Sb\textsubscript{x} remain controversial. Here we report the investigation of the SOTs and spin Hall magnetoresistance resulting from charge-to-spin conversion in twin-free epitaxial layers of Bi\textsubscript{0.9}Sb\textsubscript{0.1}(0001) coupled to FeCo, and compare it with that of amorphous Bi\textsubscript{0.9}Sb\textsubscript{0.1}. We find a large charge-to-spin conversion efficiency of 1 in the first case and less than 0.1 in the second, confirming crystalline Bi\textsubscript{0.9}Sb\textsubscript{0.1} as a strong spin injector material. The SOTs and spin Hall magnetoresistance are independent of the direction of the electric current, indicating that charge-to-spin conversion in single-crystal Bi\textsubscript{0.9}Sb\textsubscript{0.1}(0001) is isotropic despite the strong anisotropy of the topological surface states. Further, we find that the damping-like SOT has a non-monotonic temperature dependence with a minimum at 20~K. By correlating the SOT with resistivity and weak antilocalization measurements, we conclude that charge-spin conversion occurs via thermally-excited holes from the bulk states above 20~K, and conduction through the isotropic surface states with increasing spin polarization due to decreasing electron-electron scattering below 20~K.

cond-mat.mtrl-sci↗

arXiv2023

Field-Free Spin-Orbit Torque driven Switching of Perpendicular Magnetic Tunnel Junction through Bending Current

Current-induced spin-orbit torques (SOTs) enable fast and efficient manipulation of the magnetic state of magnetic tunnel junctions (MTJs), making it attractive for memory, in-memory computing, and logic applications. However, the requirement of the external magnetic field to achieve deterministic switching in perpendicular magnetized SOT-MTJs limits its implementation for practical applications. Here, we introduce a field-free switching (FFS) solution for the SOT-MTJ device by shaping the SOT channel to create a "bend" in the SOT current. The resulting bend in the charge current creates a spatially non-uniform spin current, which translates into inhomogeneous SOT on an adjacent magnetic free layer enabling deterministic switching. We demonstrate FFS experimentally on scaled SOT-MTJs at nanosecond time scales. This proposed scheme is scalable, material-agnostic, and readily compatible with wafer-scale manufacturing, thus creating a pathway for developing purely current-driven SOT systems.

cond-mat.mes-hall↗

arXiv2023

Electrically programmable magnetic coupling in an Ising network exploiting solid-state ionic gating

Two-dimensional arrays of magnetically coupled nanomagnets provide a mesoscopic platform for exploring collective phenomena as well as realizing a broad range of spintronic devices. In particular, the magnetic coupling plays a critical role in determining the nature of the cooperative behaviour and providing new functionalities in nanomagnet-based devices. Here, we create coupled Ising-like nanomagnets in which the coupling between adjacent nanomagnetic regions can be reversibly converted between parallel and antiparallel through solid-state ionic gating. This is achieved with the voltage-control of magnetic anisotropies in a nanosized region where the symmetric exchange interaction favours parallel alignment and the antisymmetric exchange interaction, namely the Dzyaloshinskii-Moriya interaction, favours antiparallel alignment. Applying this concept to a two-dimensional lattice, we demonstrate a voltage-controlled phase transition in artificial spin ices. Furthermore, we achieve an addressable control of the individual couplings and realize an electrically programmable Ising network, which opens up new avenues to design nanomagnet-based logic devices and neuromorphic computers

cond-mat.mes-hall↗