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The traditional method for exciting spin-wave dynamics in magnetic materials involves microwave magnetic fields generated by current injection into inductive antennas. However, there is a growing interest in non-inductive excitation methods. Magneto-acoustic effects present a viable alternative, where strains produced by applying voltages to a piezoelectric substrate can couple to spin-waves in a magnetic film. Recently, it has been proposed that surface acoustic waves (SAWs) can excite the gyrotropic mode of the vortex state in a magnetic disk. Here we report on experiments utilizing a magnetic resonance force microscope to investigate magnetization dynamics in CoFeB sub-micrometer disks in the vortex state, grown on a Z-cut LiNbO$_3$ substrate. The device design enables excitation of the gyrotropic mode either inductively, using an antenna on top of the disks, or acoustically via SAWs launched from an interdigital transducer. Our modelling indicates that the lattice rotation {\omega}xz generates a localized magneto-acoustic field that displaces the vortex core from the disk center, initiating the gyration motion. Tuning of the magneto-acoustic torque acting on the vortex structure is achieved by a perpendicular magnetic field. These results demonstrate the clear excitation of the vortex gyrotropic mode by magneto-acoustic excitation.

Synthetic antiferromagnets host spin waves that are highly tunable. It is of practical interest to analyze the symmetry of their coupling to surface acoustic waves with the design of hybrid devices in view. For this we calculate the layer-resolved susceptibility tensor of a synthetic antiferromagnet, the effective magneto-elastic and magneto-rotation fields associated to a travelling elastic wave, and the power irreversibly transferred by the elastic wave to the magnetic layers. We consider Rayleigh-type surface acoustic waves: (a) that travel in an elastically isotropic, non-piezoelectric substrate, or (b) that propagate along the X direction at the surface of a Z-cut LiNbO$_3$ substrate, or (c) that are guided in a thin Z-cut LiNbO$_3$ film grown on a sapphire substrate. In particular, we show that the complementary angular dependencies of the acoustic and optical spin wave modes in synthetic antiferromagnets makes it possible to excite spin waves for any relative orientation of magnetization and acoustic wavevector. In addition, we discuss the symmetries of the driving fields and of the energy transferred to the magnetic degree of freedom. We evidence new interaction channels coupling the magnetization eigenmodes when elastic anisotropy and piezoelectricity of the substrate are considered.

Controlling the uniaxial magnetic anisotropy is of practical interest to a wide variety of applications. We study Co$_{40}$Fe$_{40}$B$_{20}$ single films grown on various crystalline orientations of LiNbO$_3$ substrates and on oxidized silicon. We identify the annealing conditions that are appropriate to induce or suppress uniaxial anisotropy. Anisotropy fields can be increased by annealing up to 11 mT when using substrates with anisotropic surfaces. They can be decreased to below 1 mT when using isotropic surfaces. In the first case, the observed increase of the anisotropy originates from the biaxial strain in the film caused by the anisotropic thermal contraction of the substrate when back at room temperature after strain relaxation during annealing. In the second case, anisotropy is progressively removed by applying successive orthogonal fields that are assumed to progressively suppress any chemical ordering within the magnetic film. The method can be applied to CoFeB/Ru/CoFeB synthetic antiferromagnets but the tuning of the anisotropy comes with a decrease of the interlayer exchange coupling and a drastic change of the exchange stiffness.

This study conducts a comprehensive investigation into the reversal mechanism of magnetic vortex cores in a nanopillar system composed of two coupled ferromagnetic dots under zero magnetic field conditions. The research employs a combination of experimental and simulation methods to gain a deeper understanding of the dynamics of magnetic vortex cores. The findings reveal that by applying a constant direct current, the orientation of the vortex cores can be manipulated, resulting in a switch in one of the dots at a specific current value. The micromagnetic simulations provide evidence that this switch is a consequence of a deformation in the vortex profile caused by the increasing velocity of the vortex cores resulting from the constant amplitude of the trajectory as frequency increases. These findings offer valuable new insights into the coupled dynamics of magnetic vortex cores and demonstrate the feasibility of manipulating their orientation using direct currents under zero magnetic field conditions. The results of this study have potential implications for the development of vortex-based non-volatile memory technologies. \end{abstract}

Optimizing the cooperativity between two distinct particles is an important feature of quantum information processing. Of particular interest is the coupling between spin and phonon, which allows for integrated long range communication between gates operating at GHz frequency. Using local light scattering, we show that, in magnetic planar geometries, this attribute can be tuned by adjusting the orientation and strength of an external magnetic field. The coupling strength is enhanced by about a factor of 2 for the out-of-plane magnetized geometry where the Kittel mode is coupled to circularly polarized phonons, compared to the in-plane one where it couples to linearly polarized phonons. We also show that the overlap between magnon and phonon is maximized by matching the Kittel frequency with an acoustic resonance that satisfies the half-wave plate condition across the magnetic film thickness. Taking the frequency dependence of the damping into account, a maximum cooperativity of about 6 is reached in garnets for the normal configuration near 5.5 GHz., Comment: 11 pages, 10 figures

Current-induced self-sustained magnetization oscillations in spin-torque nano-oscillators (STNOs) are promising candidates for ultra-agile microwave sources or detectors. While usually STNOs behave as a monochrome source, we report here clear bimodal simultaneous emission of incommensurate microwave oscillations, where the two tones correspond to two parametrically coupled eigenmodes with tunable splitting. The emission range is crucially sensitive to the change in hybridization of the eigenmodes of free and fixed layers, for instance, through a slight tilt of the applied magnetic field from the normal of the nano-pillar. Our experimental findings are supported both analytically and by micromagnetic simulations, which ascribe the process to four-magnon scattering between a pair of radially symmetric magnon modes and a pair of magnon modes with opposite azimuthal index. Our findings open up new possibilities for cognitive telecommunications and neuromorphic systems that use frequency multiplexing to improve communication performance.

We experimentally demonstrate generation of coherent propagating magnons in ultra-thin magnetic-insulator films by spin-orbit torque induced by dc electric current. We show that this challenging task can be accomplished by utilizing magnetic-insulator films with large perpendicular magnetic anisotropy. We demonstrate simple and flexible spin-orbit torque devices, which can be used as highly efficient nanoscale sources of coherent propagating magnons for insulator-based spintronic applications.

We demonstrate a microscopic magnonic-crystal waveguide produced by nano-patterning of a 20 nm thick film of Yttrium Iron Garnet. By using the phase-resolved micro-focus Brillouin light scattering spectroscopy, we map the intensity and the phase of spin waves propagating in such a periodic magnetic structure. Based on these maps, we obtain the dispersion and the attenuation characteristics of spin waves providing detailed information about the physics of spin-wave propagation in the magnonic crystal. We show that, in contrast to the simplified physical picture, the maximum attenuation of spin waves is achieved close to the edge of the magnonic band gap, which is associated with non-trivial reflection characteristics of spin waves in non-uniform field potentials., Comment: 13 pages, 4 figures. Keywords: spin waves, magnonics, magnetic nanostructures

The mutual interaction between the different eigenmodes of a spin-torque oscillator can lead to a large variety of physical mechanisms from mode hopping to multi-mode generation, that usually reduce their performances as radio-frequency devices. To tackle this issue for the future applications, we investigate the properties of a model spin-torque oscillator that is composed of two coupled vortices with one vortex in each of the two magnetic layers of the oscillator. In such double-vortex system, the remarkable properties of energy transfer between the coupled modes, one being excited by spin transfer torque while the second one being damped, result into an alteration of the damping parameters. As a consequence, the oscillator nonlinear behavior is concomitantly drastically impacted. This efficient coupling mechanism, driven mainly by the dynamic dipolar field generated by the spin transfer torque induced motion of the vortices, gives rise to an unexpected dynamical regime of self-resonance excitation. These results show that mode coupling can be leveraged for controlling the synchronization process as well as the frequency tunability of spin-torque oscillators., Comment: 7 pages, 5 figures

Recent advances in the studies of pure spin currents - flows of angular momentum (spin) not accompanied by the electric currents - have opened new horizons for the emerging technologies based on the electron's spin degree of freedom, such as spintronics and magnonics. The main advantage of pure spin current, as compared to the spin-polarized electric current, is the possibility to exert spin transfer torque on the magnetization in thin magnetic films without electrical current flow through the material. In addition to minimizing Joule heating and electromigration effects, this characteristic enables the implementation of spin torque devices based on the low-loss insulating magnetic materials, and offers an unprecedented geometric flexibility. Here we review the recent experimental achievements in investigations of magnetization oscillations excited by pure spin currents in different magnetic nanosystems based on metallic and insulating magnetic materials. We discuss the spectral properties of spin-current nano-oscillators, and relate them to the spatial characteristics of the excited dynamic magnetic modes determined by the spatially-resolved measurements. We also show that these systems support locking of the oscillations to external microwave signals, as well as their mutual synchronization, and can be used as efficient nanoscale sources of propagating spin waves.

Excitation of magnetization dynamics by pure spin currents has been recently recognized as an enabling mechanism for spintronics and magnonics, which allows implementation of spin-torque devices based on low-damping insulating magnetic materials. Here we report the first spatially-resolved study of the dynamic modes excited by pure spin current in nanometer-thick microscopic insulating Yttrium Iron Garnet disks. We show that these modes exhibit nonlinear self-broadening preventing the formation of the self-localized magnetic bullet, which plays a crucial role in the stabilization of the single-mode magnetization oscillations in all-metallic systems. This peculiarity associated with the efficient nonlinear mode coupling in low-damping materials can be among the main factors governing the interaction of pure spin currents with the dynamic magnetization in high-quality magnetic insulators.

We demonstrate ultra-fast (down to 400 ps) bipolar magnetization switching of a three-terminal perpendicular Ta/FeCoB/MgO/FeCoB magnetic tunnel junction. The critical current density rises significantly as the current pulse shortens below 10 ns, which translates into a minimum in the write energy in the ns range. Our results show that SOT-MRAM allows fast and low power write operations, which renders it promising for non-volatile cache memory applications., Comment: 15 pages, 3 figures

Due to their nonlinear properties, spin transfer nano-oscillators can easily adapt their frequency to external stimuli. This makes them interesting model systems to study the effects of synchronization and brings some opportunities to improve their microwave characteristics in view of their applications in information and communication technologies and to design innovative computing architectures. So far, mutual synchronization of spin transfer nano-oscillators through propagating spin-waves and exchange coupling in a common magnetic layer has been demonstrated. Here we show that the dipolar interaction is also an efficient mechanism to synchronize neighbouring oscillators. We experimentally study a pair of vortex-based spin-transfer nano-oscillators, in which mutual synchronization can be achieved despite a significant frequency mismatch between oscillators. Importantly, the coupling efficiency is controlled by the magnetic configuration of the vortices, as confirmed by an analytical model highlighting the physics at play in the synchronization process as well as by micromagnetic simulations.

We report on a theoretical study about the magneto-dipolar coupling and synchronization between two vortex-based spin-torque nano-oscillators. In this work we study the dependence of the coupling efficiency on the relative magnetization parameters of the vortices in the system. For that purpose, we combine micromagnetic simulations, Thiele equation approach, and analytical macro-dipole approximation model to identify the optimized configuration for achieving phase-locking between neighboring oscillators. Notably, we compare vortices configurations with parallel (P) polarities and with opposite (AP) polarities. We demonstrate that the AP core configuration exhibits a coupling strength about three times larger than in the P core configuration., Comment: 8 pages, 11 figures

Spin-polarised radio-frequency currents, whose frequency is equal to that of the gyrotropic mode, will cause an excitation of the core of a magnetic vortex confined in a magnetic tunnel junction. When the excitation radius of the vortex core is greater than that of the junction radius, vortex core expulsion is observed, leading to a large change in resistance, as the layer enters a predominantly uniform magnetisation state. Unlike the conventional spin-torque diode effect, this highly tunable resonant effect will generate a voltage which does not decrease as a function of rf power, and has the potential to form the basis of a new generation of tunable nanoscale radio-frequency detectors.

It is demonstrated that the decay time of spin-wave modes existing in a magnetic insulator can be reduced or enhanced by injecting an in-plane dc current, $I_\text{dc}$, in an adjacent normal metal with strong spin-orbit interaction. The demonstration rests upon the measurement of the ferromagnetic resonance linewidth as a function of $I_\text{dc}$ in a 5~$\mu$m diameter YIG(20nm){\textbar}Pt(7nm) disk using a magnetic resonance force microscope (MRFM). Complete compensation of the damping of the fundamental mode is obtained for a current density of $\sim 3 \cdot 10^{11}\text{A.m}^{-2}$, in agreement with theoretical predictions. At this critical threshold the MRFM detects a small change of static magnetization, a behavior consistent with the onset of an auto-oscillation regime., Comment: 6 pages 4 figures

High quality nanometer-thick (20 nm, 7 nm and 4 nm) epitaxial YIG films have been grown on GGG substrates using pulsed laser deposition. The Gilbert damping coefficient for the 20 nm thick films is 2.3 x 10-4 which is the lowest value reported for sub-micrometric thick films. We demonstrate Inverse spin Hall effect (ISHE) detection of propagating spin waves using Pt. The amplitude and the lineshape of the ISHE voltage correlate well to the increase of the Gilbert damping when decreasing thickness of YIG. Spin Hall effect based loss-compensation experiments have been conducted but no change in the magnetization dynamics could be detected.

The anharmonicity of the potential well confining the position of the magnetic vortex core is measured dynamically with a Magnetic Resonance Force Microscope (MRFM). The stray field of the MRFM tip is used to displace the core position away from the well minimum. Anharmonicity is then inferred from the relative frequency shift induced on the eigen-frequency of the vortex core translational mode. Traces of these shifts are recorded while scanning the tip above an isolated nanodot, patterned out of a single crystal FeV film. An analytical framework is proposed to analyze the data. It results in a quantitative measurement of the anharmonic coefficient found to be positive and 50% of the parabolic contribution. This calibrates the tunability of the gyrotropic mode by external magnetic fields. In our sample, we observe a variation of the eigen-frequency as high as +10% for a displacement of the vortex core to about one third of the nanodot radius., Comment: 5 pages, 4 figures

We investigate the dynamics of two coupled vortices driven by spin transfer. We are able to independently control with current and perpendicular field, and to detect, the respective chiralities and polarities of the two vortices. For current densities above $J=5.7*10^7 A/cm^2$, a highly coherent signal (linewidth down to 46 kHz) can be observed, with a strong dependence on the relative polarities of the vortices. It demonstrates the interest of using coupled dynamics in order to increase the coherence of the microwave signal. Emissions exhibit a linear frequency evolution with perpendicular field, with coherence conserved even at zero magnetic field.

Spin-valve based nanojunctions incorporating Co|Ni multilayers with perpendicular anisotropy were used to study spin-torque driven ferromagnetic resonance (ST-FMR) in a nonlinear regime. Perpendicular field swept resonance lines were measured under a large amplitude microwave current excitation, which produces a large angle precession of the Co|Ni layer magnetization. With increasing rf power the resonance lines broaden and become asymmetric, with their peak shifting to lower applied field. A nonhysteretic step jump in ST-FMR voltage signal was also observed at high powers. The results are analyzed in in terms of the foldover effect of a forced nonlinear oscillator and compared to macrospin simulations. The ST-FMR nonhysteretic step response may have applications in frequency and amplitude tunable nanoscale field sensors., Comment: 4 pages, 3 figures, to appear in Applied Physics Letters

We review how a magnetic resonance force microscope (MRFM) can be applied to perform ferromagnetic resonance (FMR) spectroscopy of \emph{individual} sub-micron size samples. We restrict our attention to a thorough study of the spin-wave eigen-modes excited in permalloy (Py) disks patterned out of the same 43.3 nm thin film. The disks have a diameter of either 1.0 or $0.5 \mu$m and are quasi-saturated by a perpendicularly applied magnetic field. It is shown that \emph{quantitative} spectroscopic information can be extracted from the MRFM measurements. In particular, the data are extensively compared with complementary approximate models of the dynamical susceptibility: i) a 2D analytical model, which assumes an homogeneous magnetization dynamics along the thickness and ii) a full 3D micromagnetic simulation, which assumes an homogeneous magnetization dynamics below a characteristic length scale $c$ and which approximates the cylindrical sample volume by a discretized representation with regular cubic mesh of lateral size $c=3.9$ nm. In our analysis, the distortions due to a breaking of the axial symmetry are taken into account, both models incorporating the possibility of a small misalignment between the applied field and the normal of the disks.

A sensor that integrates high sensitivity micro-Hall effect magnetometry and high-frequency electron paramagnetic resonance spectroscopy capabilities on a single semiconductor chip is presented. The Hall-effect magnetometer was fabricated from a two dimensional electron gas GaAs/AlGaAs heterostructure in the form of a cross, with a 50x50 um2 sensing area. A high-frequency microstrip resonator is coupled with two small gaps to a transmission line with a 50 Ohms impedance. Different resonator lengths are used to obtain quasi-TEM fundamental resonant modes in the frequency range 10-30 GHz. The resonator is positioned on top of the active area of the Hall-effect magnetometer, where the magnetic field of the fundamental mode is largest, thus optimizing the conversion of microwave power into magnetic field at the sample position. The two gaps coupling the resonator and transmission lines are engineered differently. The gap to the microwave source is designed to optimize the loaded quality factor of the resonator (Q = 150) while the gap for the transmitted signal is larger. This latter gap minimizes losses and prevents distortion of the resonance while enabling measurement of the transmitted signal. The large filling factor of the resonator permits sensitivities comparable to that of high-quality factor resonant cavities. The integrated sensor enables measurement of the magnetization response of micron scale samples upon application of microwave fields. In particular, the combined measurement of the magnetization change and the microwave power under cw microwave irradiation of single crystal of molecular magnets is used to determine of the energy relaxation time of the molecular spin states. In addition, real time measurements of the magnetization dynamics upon application of fast microwave pulses are demonstrated, Comment: Submitted to Review of Scientific Instruments

Spin relaxation between the two lowest-lying spin-states has been studied in the S=4 single molecule magnet Ni$_4$ under steady state conditions of low amplitude and continuous microwave irradiation. The relaxation rate was determined as a function of temperature at two frequencies, 10 and 27.8 GHz, by simultaneously measuring the magnetization and the absorbed microwave power. A strong temperature dependence is observed below 1.5 K, which is not consistent with a direct single-spin-phonon relaxation process. The data instead suggest that the spin relaxation is dominated by a phonon bottleneck at low temperatures and occurs by an Orbach mechanism involving excited spin-levels at higher temperatures. Experimental results are compared with detailed calculations of the relaxation rate using the universal density matrix equation., Comment: 6 pages, 4 figures, submitted to Europhys. Lett

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study magnetic excitations in Co/Ni synthetic layers confined in nanojunctions. Field swept ST-FMR measurements were conducted with a magnetic field applied perpendicular to the layer surface. The resonance lines were measured under low amplitude excitation in a linear response regime. The resulting resonance fields were compared with those obtained using conventional rf field driven FMR on extended films with the same Co/Ni layer structure. A lower resonance field is found in confined structures. The effect of both dipolar fields acting on the Co/Ni layer emanating from other magnetic layers in the device and finite size effects on the spin wave spectrum are discussed., Comment: 3 pages, 3 figures

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study thin Co/Ni synthetic layers with perpendicular anisotropy confined in spin-valve based nanojunctions. Field swept ST-FMR measurements were conducted with a magnetic field applied perpendicular to the layer surface. The resonance lines were measured under low amplitude rf excitation, from 1 to 20 GHz. These results are compared with those obtained using conventional rf field driven FMR on extended films with the same Co/Ni layer structure. The layers confined in spin valves have a lower resonance field, a narrower resonance linewidth and approximately the same linewidth vs frequency slope, implying the same damping parameter. The critical current for magnetic excitations is determined from measurements of the resonance linewidth vs dc current and is in accord with the one determined from I-V measurements., Comment: 3 pages, 3 figures

Dilute frozen solutions of the single molecule magnet Ni$_4$ (S=4) have been studied using high frequency D-band (130 GHz) EPR. Despite the random orientation of the molecules, well defined EPR absorption peaks are observed, due to the strong variation of the splittings between the different spin-states on magnetic field. Temperature dependent studies above 4 K and comparison with simulations enable identification of the spin transitions and determination of the Hamiltonian parameters. The latter are found to be close to those of Ni$_4$ single crystals. No echo was detected from Ni$_4$ in pulsed experiments, which sets an upper bound of about 50 ns on the spin coherence time., Comment: 4 pages, 3 figures, accepted for publication in Journal of Applied Physics (52nd MMM conference proceedings)

Using a Magnetic Resonace Force Microscope, we have performed ferromagnetic resonance (FMR) spectroscopy on parametric magnons created by 4-wave process. This is achieved by measuring the differential response to a small source modulation superimposed to a constant excitation power that drives the dynamics in the saturation regime of the transverse component. By sweeping the applied field, we observe abrupt readjustement of the total number of magnons each time the excitation coincides with a parametric mode. This gives rise to ultra-narrow peaks whose linewith is lower than $5~10^{-6}$ of the applied field., Comment: 4 pages

Integrated magnetic sensors that allow simultaneous EPR and magnetization measurements have been developed to study single molecule magnets. A high frequency microstrip resonator has been integrated with a micro-Hall effect magnetometer. EPR spectroscopy is used to determine the energy splitting between the low lying spin-states of a Ni$_4$ single crystal, with an S=4 ground state, as a function of applied fields, both longitudinal and transverse to the easy axis at 0.4 K. Concurrent magnetization measurements show changes in spin-population associated with microwave absorption. Such studies enable determination of the energy relaxation time of the spin system., Comment: 4 pages, 4 figures, accepted for publication (Proceedings of the 10th Joint MMM/Intermag Conference, which will be published as special issues of the Journal of Applied Physics)

Using a Magnetic Resonance Force Microscope, we compare the ferromagnetic resonance spectra of individual micron-size disks with identical diameter, 1 $m$m, but different layer structures. For a disk composed of a single 43.3 nm thick permalloy (Py) layer, the lowest energy mode in the perpendicular configuration is the uniform precession. The higher energy modes are standing spin-waves confined along the diameter of the disk. For a Cu(30)/Py(100)/Cu(30) nm multilayer structure, it has been interpreted that the lowest energy mode becomes a precession localized at the Cu/Py interfaces. When the multilayer is changed to Py(100)/Cu(10)/Py(10) nm, this localized mode of the thick layer is coupled to the precession of the thin layer.

Spin wave spectra of perpendicularly magnetized disks with trilayers consisting of a 100 nm permalloy (Py) layer sandwiched by two Cu layers of 30 nm, are measured individually with a Magnetic Resonance Force Microscope (MRFM). It is demonstrated by 3D micromagnetic simulations that in disks having sub-micron size diameters, the lowest energy spin wave mode of the saturated state is not spatially uniform but rather is localized at the center of the Py/Cu interface in the region of a minimum demagnetizing field.

This letter reports the first detailed measurement of $M_z$, the component parallel to the effective field direction, when ferromagnets are excited by microwave fields at high power levels. It is found that $M_z$ drops dramatically at the saturation of the main resonance. Simultaneous measurements of $M_z$ and absorption power show that this drop corresponds to a diminution of the spin-lattice relaxation rate. These changes are interpreted as reflecting the properties of longitudinal spinwaves excited above Suhl's instability. Similar behavior should be expected in spinwave emission by currents., Comment: 10 pages, 4 figures

We report the observation in the direct space of the transport of a few thousand charges submitted to a tunable electric field along the surface of a silicon oxide layer. Charges are both deposited and observed using the same Electrostatic Force Microscope. During the time range accessible to our measurements (i.e. $t=1\sim1000\un{s}$), the transport of electrons is mediated by traps in the oxide. We measure the mobility of electrons in the "surface" states of the silicon oxide layer and show the dispersive nature of their motion. It is also demonstrated that the saturation of deep oxide traps strongly enhance the transport of electrons under lateral electric field., Comment: submitted to EPL