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Olivier Klein

Publications and source records attributed to Olivier Klein.

34 records · Page 2Linked to original sources
arXiv2016

Room temperature chiral magnetic skyrmion in ultrathin magnetic nanostructures

Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometer size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetisation at the nanoscale. To date, chiral skyrmion structures have been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films and under external magnetic field or at low temperature. Here, we report on the observation of stable skyrmions in sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero applied magnetic field. We use high lateral resolution X-ray magnetic circular dichroism microscopy to image their chiral Néel internal structure which we explain as due to the large strength of the Dzyaloshinskii-Moriya interaction as revealed by spin wave spectroscopy measurements. Our results are substantiated by micromagnetic simulations and numerical models, which allow the identification of the physical mechanisms governing the size and stability of the skyrmions.

cond-mat.mtrl-sci↗
arXiv2015

Generation of coherent spin-wave modes in Yttrium Iron Garnet microdiscs by spin-orbit torque

Spin-orbit effects [1-4] have the potential of radically changing the field of spintronics by allowing transfer of spin angular momentum to a whole new class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed that by depositing Platinum (Pt, a normal metal) on top of a 1.3 $μ$m thick Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer spin angular momentum through the interface between these two different materials. The outstanding feature was the detection of auto-oscillation of the YIG when enough dc current was passed in the Pt. This finding has created a great excitement in the community for two reasons: first, one could control electronically the damping of insulators, which can offer improved properties compared to metals, and here YIG has the lowest damping known in nature; second, the damping compensation could be achieved on very large objects, a particularly relevant point for the field of magnonics [6,7] whose aim is to use spin-waves as carriers of information. However, the degree of coherence of the observed auto-oscillations has not been addressed in ref. [5]. In this work, we emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current. This requires to reduce both the thickness and lateral size in order to reach full damping compensation [8] , and we show clear evidence of coherent spin-orbit torque induced auto-oscillation in micron-sized YIG discs of thickness 20 nm.

cond-mat.mtrl-sci↗
arXiv2015

Improved spectral stability in spin transfer nano-oscillators: single vortex versus coupled vortices dynamics

We perform a comparative study of spin transfer induced excitation of the gyrotropic motion of a vortex core with either uniform or vortex spin polarizers. The microwave output voltage associated with the vortex dynamics, detected in both cases, displays a strong reduction of phase fluctuations in the case of the vortex polarizer, with a decrease of the peak linewidth by one order of magnitude down to 200kHz at zero field. A thorough study of rf emission features for the different accessible vortex configurations shows that this improvement is related to the excitation of coupled vortex dynamics by spin transfer torques.

cond-mat.mes-hall↗
arXiv2014

Mechanical magnetometry of Cobalt nanospheres deposited by focused electron beam at the tip of ultra-soft cantilevers

Using focused-electron-beam-induced deposition, Cobalt magnetic nanospheres with diameter ranging between 100 nm and 300 nm are grown at the tip of ultra-soft cantilevers. By monitoring the mechanical resonance frequency of the cantilever as a function of the applied magnetic field, the hysteresis curve of these individual nanospheres are measured. This enables to evaluate their saturation magnetization, found to be around 430 emu/cm^3 independently of the size of the particle, and to infer that the magnetic vortex state is the equilibrium configuration of these nanospheres at remanence.

cond-mat.mtrl-sci↗
arXiv2014

Measurement of the intrinsic damping constant in individual nanodisks of YIG and YIG{\textbar}Pt

We report on an experimental study on the spin-waves relaxation rate in two series of nanodisks of diameter $ϕ=$300, 500 and 700~nm, patterned out of two systems: a 20~nm thick yttrium iron garnet (YIG) film grown by pulsed laser deposition either bare or covered by 13~nm of Pt. Using a magnetic resonance force microscope, we measure precisely the ferromagnetic resonance linewidth of each individual YIG and YIG{\textbar}Pt nanodisks. We find that the linewidth in the nanostructure is sensibly smaller than the one measured in the extended film. Analysis of the frequency dependence of the spectral linewidth indicates that the improvement is principally due to the suppression of the inhomogeneous part of the broadening due to geometrical confinement, suggesting that only the homogeneous broadening contributes to the linewidth of the nanostructure. For the bare YIG nano-disks, the broadening is associated to a damping constant $α= 4 \cdot 10^{-4}$. A 3 fold increase of the linewidth is observed for the series with Pt cap layer, attributed to the spin pumping effect. The measured enhancement allows to extract the spin mixing conductance found to be $G_{\uparrow \downarrow}= 1.55 \cdot 10^{14}~ Ω^{-1}\text{m}^{-2}$ for our YIG(20nm){\textbar}Pt interface, thus opening large opportunities for the design of YIG based nanostructures with optimized magnetic losses.

cond-mat.mtrl-sci↗
arXiv2013

Perfect and robust phase-locking of a spin transfer vortex nano-oscillator to an external microwave source

We study the synchronization of the auto-oscillation signal generated by the spin transfer driven dynamics of two coupled vortices in a spin-valve nanopillar to an external source. Phase-locking to the microwave field hrf occurs in a range larger than 10% of the oscillator frequency for drive amplitudes of only a few Oersteds. Using synchronization at the double frequency, the generation linewidth is found to decrease by more than five orders of magnitude in the phase-locked regime (down to 1 Hz, limited by the resolution bandwidth of the spectrum analyzer) in comparison to the free running regime (140 kHz). This perfect phase-locking holds for frequency detuning as large as 2 MHz, which proves its robustness. We also analyze how the free running spectral linewidth impacts the main characteristics of the synchronization regime.

cond-mat.mes-hall↗
arXiv2013

Detection of the microwave spin pumping using the inverse spin Hall effect

We report electrical detection of the dynamical part of the spin pumping current emitted during ferromagnetic resonance (FMR) using the inverse Spin Hall Effect (ISHE). The experiment is performed on a YIG$|$Pt bilayer. The choice of YIG, a magnetic insulator, ensures that no charge current flows between the two layers and only pure spin current produced by the magnetization dynamics are transferred into the adjacent strong spin-orbit Pt layer via spin pumping. To avoid measuring the parasitic eddy currents induced at the frequency of the microwave source, a resonance at half the frequency is induced using parametric excitation in the parallel geometry. Triggering this nonlinear effect allows to directly detect on a spectrum analyzer the microwave component of the ISHE voltage. Signals as large as 30 $μ$V are measured for precession angles of a couple of degrees. This direct detection provides a novel efficient means to study magnetization dynamics on a very wide frequency range with great sensitivity.

cond-mat.mes-hall↗
arXiv2013

Conduction of spin currents through insulating oxides

Spintronics is a field of electronics based on using the electron spin instead of its charge. The recent advance in the manipulation of pure spin currents, i.e. angular momentum transfer not associated to conventional charge currents, has opened new opportunities to build spin based devices with low energy consumption. It has also allowed to integrate ferromagnetic insulators in spintronic devices, either as spin sources or spin conductors using their magnetic excitations to propagate a spin signal. Antiferromagnetic insulators belong to another class of materials that can also sustain magnetic excitations, even with a higher group velocity. Hence, they have potential as angular momentum conductors, possibly making faster spin devices. At the opposite end, angular momentum insulators are also required in spintronic circuits. The present letter underlines some essential features relevant for spin current conduction, based on measurements of angular momentum transmission in antiferromagnetic NiO and in the non-magnetic light element insulator SiO$_2$.

cond-mat.mtrl-sci↗
arXiv2013

Origin of spectral purity and tuning sensitivity in a vortex-based spin transfer nano-oscillator

We investigate the microwave characteristics of a spin transfer nano-oscillator (STNO) based on coupled vortices as a function of the perpendicular magnetic field $H_\perp$. While the generation linewidth displays strong variations on $H_\perp$ (from 40 kHz to 1 MHz), the frequency tunability in current remains almost constant (~7 MHz/mA). We demonstrate that our vortex-based oscillator is quasi-isochronous independently of $H_\perp$, so that the severe nonlinear broadening usually observed in STNOs does not exist. Interestingly, this does not imply a loss of frequency tunability, which is here governed by the current induced Oersted field. Nevertheless this is not sufficient to achieve the highest spectral purity in the full range of $H_\perp$ either: we show that the observed linewidth broadenings are due to the excited mode interacting with a lower energy overdamped mode, which occurs at the successive crossings between harmonics of these two modes. These findings open new possibilities for the design of STNOs and the optimization of their performance.

cond-mat.mtrl-sci↗
arXiv2013

Comparative Measurements of Inverse Spin Hall and Magnetoresistance in YIG|Pt and YIG|Ta

We report on a comparative study of spin Hall related effects and magnetoresistance in YIG|Pt and YIG|Ta bilayers. These combined measurements allow to estimate the characteristic transport parameters of both Pt and Ta layers juxtaposed to YIG: the spin mixing conductance $G_{\uparrow \downarrow}$ at the YIG$|$normal metal interface, the spin Hall angle $Θ_{SH}$, and the spin diffusion length $λ_{sd}$ in the normal metal. The inverse spin Hall voltages generated in Pt and Ta by the pure spin current pumped from YIG excited at resonance confirm the opposite signs of spin Hall angles in these two materials. Moreover, from the dependence of the inverse spin Hall voltage on the Ta thickness, we extract the spin diffusion length in Ta, found to be $λ_{sd}^\text{Ta}=1.8\pm0.7$ nm. Both the YIG|Pt and YIG|Ta systems display a similar variation of resistance upon magnetic field orientation, which can be explained in the recently developed framework of spin Hall magnetoresistance.

cond-mat.mes-hall↗
arXiv2012

Measurement of the dynamical dipolar coupling in a pair of magnetic nano-disks using a Ferromagnetic Resonance Force Microscope

We perform an extensive experimental spectroscopic study of the collective spin-wave dynamics occurring in a pair of magnetic nano-disks coupled by the magneto-dipolar interaction. For this, we take advantage of the stray field gradient produced by the magnetic tip of a ferromagnetic resonance force microscope (f-MRFM) to continuously tune and detune the relative resonance frequencies between two adjacent nano-objects. This reveals the anti-crossing and hybridization of the spin-wave modes in the pair of disks. At the exact tuning, the measured frequency splitting between the binding and anti-binding modes precisely corresponds to the strength of the dynamical dipolar coupling $Ω$. This accurate f-MRFM determination of $Ω$ is measured as a function of the separation between the nano-disks. It agrees quantitatively with calculations of the expected dynamical magneto-dipolar interaction in our sample.

cond-mat.mtrl-sci↗
arXiv2008

Ferromagnetic resonance force spectroscopy of individual sub-micron size samples

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 $0.5 μ$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.

cond-mat.mtrl-sci↗
arXiv2007

Magnetic resonance studies of the fundamental spin-wave modes in individual submicron Cu/NiFe/Cu perpendicularly magnetized disks

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.

cond-mat.mtrl-sci↗
arXiv2007

Spectroscopy of the parametric magnons excited by 4-wave process

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.

cond-mat.mtrl-sci↗
arXiv2006

Magnetic resonance spectroscopy of perpendicularly magnetized permalloy multilayer disks

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.

cond-mat.mtrl-sci↗