The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (... more The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain wall motion is driven by the spin transfer torque mechanism. A current density gradient is found either to stabilize domains with walls perpendicular to current lines or to produce finger-like patterns, depending on the domain wall motion direction. The instability mechanism is shown to result from the non-adiabatic contribution of the spin transfer torque mechanism.
Spintronic devices based on domain wall (DW) motion driven by spin-orbit torque (SOT) offer excit... more Spintronic devices based on domain wall (DW) motion driven by spin-orbit torque (SOT) offer exciting perspectives for non-volatile memory and logic applications. One of the main issues to overcome is DW pinning at intrinsic material defects. Pinning leads to thermally activated creep and depinning regimes [1], reducing the efficiency of DW motion.<br/><br/>W-CoFeB-MgO based structures are archetype materials for SOT-driven DW motion. To get high PMA, crystallization at <i>T </i>> 350°C is usually needed, which induces structural inhomogeneities. Here, we investigate DW motion in perpendicularly magnetized W-CoFeB-MgO films crystallized by either annealing at <i>T</i> = 400°C, or by He<sup>+</sup> irradiation at reduced temperatures [2]. As we have shown recently, He<sup>+</sup> irradiation is a powerful tool to engineer magnetic materials at the atomic scale [3-6].<br/><br/>Combining a variety of experimental...
Magnetic field driven domain wall velocities in [Co/Ni] based multilayers thin films have been me... more Magnetic field driven domain wall velocities in [Co/Ni] based multilayers thin films have been measured using polar magneto-optic Kerr effect microscopy. The low field results are shown to be consistent with the universal creep regime of domain wall motion, characterized by a stretched exponential growth of the velocity with the inverse of the applied field. Approaching the depinning field from below results in an unexpected excess velocity with respect to the creep law. We analyze these results using scaling theory to show that this speeding up of domain wall motion can be interpreted as due to the increase of the size of the deterministic relaxation close to the depinning transition. We propose a phenomenological model which allows to accurately fit the observed excess velocity and to obtain characteristic values for the depinning field H_d, the depinning temperature T_d, and the characteristic velocity scale v_0 for each sample.
Magnetic domain wall motion is at the heart of new magneto-electronic technologies and hence the ... more Magnetic domain wall motion is at the heart of new magneto-electronic technologies and hence the need for a deeper understanding of domain wall dynamics in magnetic systems. In this context, numerical simulations using simple models can capture the main ingredients responsible for the complex observed domain wall behavior. We present a scalar-field model for the magnetization dynamics of quasi-two-dimensional systems with a perpendicular easy axis of magnetization which allows a direct comparison with typical experimental protocols, used in polar magneto-optical Kerr effect microscopy experiments. We show that the thermally activated creep and depinning regimes of domain wall motion can be reached, and the effect of different quenched disorder implementations can be assessed with the model. In particular, we show that the depinning field increases with the mean grain size of a Voronoi tessellation model for the disorder.
Magnetic field driven domain wall dynamics in a ferrimagnetic GdFeCo thin film with perpendicular... more Magnetic field driven domain wall dynamics in a ferrimagnetic GdFeCo thin film with perpendicular magnetic anisotropy is studied using low temperature magneto-optical Kerr microscopy. Measurements performed in a practically athermal condition allow for the direct experimental determination of the velocity (β = 0.30 ± 0.03) and correlation length (ν = 1.3 ± 0.3) exponents of the depinning transition. The whole family of exponents characterizing the transition is deduced, providing evidence that the depinning of magnetic domain walls is better described by the quenched Edwards-Wilkinson universality class.
For measuring accurately the energy of relativistic high energy charged particles, we have constr... more For measuring accurately the energy of relativistic high energy charged particles, we have constructed a novel transition radiation detector (TRD) utilizing superheated superconducting microspheres of tin with diameters in the range of 20-40 μm In order to enhance transition radiation for higher sensitivity, the TRD is in a special dilution refrigerator operated at~100 mK. Test has been conducted in a high energy electron beam facility at the CERN PS in the energy range of 1-10 GeV showing an energy dependence of the TR X-ray photon produced and hence the value γ=E/mc^2 of the charged particle.
Fully understanding domain wall motion in ferromagnetic systems is considered to be essential for... more Fully understanding domain wall motion in ferromagnetic systems is considered to be essential for the design of new magneto-electronic devices. Along with experiments, numerical simulations are therefore a key to gaining insight into the underlying mechanisms. However, simulating model systems at time and length scales comparable with those of experiments still represents a great challenge. Here, we present a simplified micromagnetic model --halfway between full micromagnetism and Ginzburg-Landau theory-- to study the dynamics of domain walls in quasi two-dimensional ferromagnetic systems with perpendicular magnetic anisotropy (PMA). Our approach relies on the local parametrization of the in-plane magnetization in terms of its out-of-plane component. We show that our model quantitatively reproduces previous experimental velocity-field data in the archetypal PMA Pt/Co/Pt ultra-thin films in the three dynamical regimes of domain wall motion (creep, depinning and flow). In addition, we...
Journal of Statistical Mechanics: Theory and Experiment, 2021
Along with experiments, numerical simulations are key to gaining insight into the underlying mech... more Along with experiments, numerical simulations are key to gaining insight into the underlying mechanisms governing domain wall motion in thin ferromagnetic systems. However, a direct comparison between numerical simulation of model systems and experimental results still represents a great challenge. Here, we present a tuned Ginzburg–Landau model to quantitatively study the dynamics of domain walls in quasi two-dimensional ferromagnetic systems with perpendicular magnetic anisotropy. This model incorporates material and experimental parameters and the micromagnetic prescription for thermal fluctuations, allowing us to perform material-specific simulations and at the same time recover universal features. We show that our model quantitatively reproduces previous experimental velocity-field data in the archetypal perpendicular magnetic anisotropy Pt/Co/Pt ultra-thin films in the three dynamical regimes of domain wall motion (creep, depinning and flow). In addition, we present a statistic...
Domain wall dynamics and spatial fluctuations are closely related to each other and to universal ... more Domain wall dynamics and spatial fluctuations are closely related to each other and to universal features of disordered systems. Experimentally measured roughness exponents characterizing spatial fluctuations have been reported for magnetic thin films, with values generally different from those predicted by the equilibrium, depinning and thermal reference states. Here, we study the roughness of domain walls in GdFeCo thin films over a large range of magnetic field and temperature. Our analysis is performed in the framework of a model considering length-scale crossovers between the reference states, which is shown to bridge the differences between experimental results and theoretical predictions. We also quantify for the first time the size of the depinning avalanches below the depinning field at finite temperatures.
Magnetic domain wall motion is at the heart of new magneto-electronic technologies and hence the ... more Magnetic domain wall motion is at the heart of new magneto-electronic technologies and hence the need for a deeper understanding of domain wall dynamics in magnetic systems. In this context, numerical simulations using simple models can capture the main ingredients responsible for the complex observed domain wall behavior. We present a scalar-field model for the magnetization dynamics of quasi-two-dimensional systems with a perpendicular easy axis of magnetization which allows a direct comparison with typical experimental protocols, used in polar magneto-optical Kerr effect microscopy experiments. We show that the thermally activated creep and depinning regimes of domain wall motion can be reached, and the effect of different quenched disorder implementations can be assessed with the model. In particular, we show that the depinning field increases with the mean grain size of a Voronoi tessellation model for the disorder.
The interfacial Dzyaloshinskii-Moriya Interaction (DMI) plays a crucial role in chiral domain wal... more The interfacial Dzyaloshinskii-Moriya Interaction (DMI) plays a crucial role in chiral domain wall (DW) motion, favoring fast DW velocities. We explore the effect of interface disorder on DMI and DW dynamics in perpendicular magnetized Ta/CoFeB/MgO thin films. Light He + irradiation has been used to gently engineer interface intermixing on a scale of 0.1 nm. We demonstrate that a slight modification of the Ta/CoFeB interface leads to an increase of the DMI value accompanied by an enhancement of DW velocity in the flow regime. Using micromagnetic simulations based on granular structures, we show that the enhancement of DW velocity is mainly related to an increase in the distribution of magnetic parameters related to the interface. We further infer that the DMI modulation is related to the asymmetric disorder induced by irradiation leading to alloying with the Ta buffer layer. Understanding the role of disorder is therefore crucial for the design of future devices where post-growth interface alloying can be used to finely tune the DMI.
Chiral domain walls in ultrathin perpendicularly magnetised layers have a Néel structure stabilis... more Chiral domain walls in ultrathin perpendicularly magnetised layers have a Néel structure stabilised by a Dzyaloshinskii-Moriya interaction (DMI) that is generated at the interface between the ferromagnet and a heavy metal. Different interface materials or properties are required above and below a ferromagnetic film in order to generate the structural inversion asymmetry needed to ensure that the DMI arising at the two interfaces does not cancel. Here we report on the magnetic properties of epitaxial Pt/Co/AuxPt1−x trilayers grown by sputtering onto sapphire substrates with 0.6 nm thick Co. As x rises from 0 to 1 a structural inversion asymmetry is progressively generated. We characterise the epilayer structure with x-ray diffraction and cross-sectional transmission electron microscopy, revealing (111) stacking. The saturation magnetization falls as the proximity magnetisation in Pt is reduced, whilst the perpendicular magnetic anisotropy Ku rises. The micromagnetic DMI strength D was determined using the bubble expansion technique and also rises from a negligible value when x = 0 to ∼ 1 mJ/m 2 for x = 1. The depinning field at which field-driven domain wall motion crosses from the creep to the depinning regime rises from ∼ 40 to ∼ 70 mT, attributed to greater spatial fluctuations of the domain wall energy with increasing Au concentration. Meanwhile, the increase in DMI causes the Walker field to rise from ∼ 10 to ∼ 280 mT, meaning that only in the x = 1 sample is the steady flow regime accessible. The full dependence of domain wall velocity on driving field bears little resemblance to the prediction of a simple one-dimensional model, but can be described very well using micromagnetic simulations with a realistic model of disorder. These reveal a rise in Gilbert damping as x increases.
We study the ultraslow domain-wall motion in ferromagnetic thin films driven by a weak magnetic f... more We study the ultraslow domain-wall motion in ferromagnetic thin films driven by a weak magnetic field. Using time-resolved magneto-optical Kerr effect microscopy, we access to the statistics of the intermittent thermally activated domain-wall jumps between deep metastable states. Our observations are consistent with the existence of creep avalanches: roughly independent clusters with broad size and ignition waiting-time distributions, each one composed by a large number of spatiotemporally correlated thermally activated elementary events. Moreover, we evidence that the large-scale geometry of domain walls is better described by depinning rather than equilibrium universal exponents.
Normal and superconducting state domains in a 10 μm thick indium slab are observed by the magneto... more Normal and superconducting state domains in a 10 μm thick indium slab are observed by the magneto‐optical imaging technique. The sample is subjected to a magnetic field H with different sweeping rates H. The domain patterns are found to depend on the penetration regime of the magnetic flux. For the lowest sweeping rate (H=1 mT s−1), the domains are randomly oriented. For the highest sweeping rate (H =230 mT s−1) and sufficiently high H‐values, the domains consist of a nearly straight laminar structure with a well‐defined period, oriented perpendicularly to the edge of the sample. These first results open new perspectives for studying the dynamics of formation of magnetic domains.
Like many other systems (magnetic fluids, Langmuir polarized layers) type-I superconducting (SC) ... more Like many other systems (magnetic fluids, Langmuir polarized layers) type-I superconducting (SC) films exhibit a phase modulation known as the intermediate state (IS). It consists of coexisting domains of the normal state (NS) and SC phases. The striking similarity of the domain patterns for various systems has stimulated the development of general models [1] based on the competition between the
The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (... more The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain wall motion is driven by the spin transfer torque mechanism. A current density gradient is found either to stabilize domains with walls perpendicular to current lines or to produce finger-like patterns, depending on the domain wall motion direction. The instability mechanism is shown to result from the non-adiabatic contribution of the spin transfer torque mechanism.
Spintronic devices based on domain wall (DW) motion driven by spin-orbit torque (SOT) offer excit... more Spintronic devices based on domain wall (DW) motion driven by spin-orbit torque (SOT) offer exciting perspectives for non-volatile memory and logic applications. One of the main issues to overcome is DW pinning at intrinsic material defects. Pinning leads to thermally activated creep and depinning regimes [1], reducing the efficiency of DW motion.<br/><br/>W-CoFeB-MgO based structures are archetype materials for SOT-driven DW motion. To get high PMA, crystallization at <i>T </i>> 350°C is usually needed, which induces structural inhomogeneities. Here, we investigate DW motion in perpendicularly magnetized W-CoFeB-MgO films crystallized by either annealing at <i>T</i> = 400°C, or by He<sup>+</sup> irradiation at reduced temperatures [2]. As we have shown recently, He<sup>+</sup> irradiation is a powerful tool to engineer magnetic materials at the atomic scale [3-6].<br/><br/>Combining a variety of experimental...
Magnetic field driven domain wall velocities in [Co/Ni] based multilayers thin films have been me... more Magnetic field driven domain wall velocities in [Co/Ni] based multilayers thin films have been measured using polar magneto-optic Kerr effect microscopy. The low field results are shown to be consistent with the universal creep regime of domain wall motion, characterized by a stretched exponential growth of the velocity with the inverse of the applied field. Approaching the depinning field from below results in an unexpected excess velocity with respect to the creep law. We analyze these results using scaling theory to show that this speeding up of domain wall motion can be interpreted as due to the increase of the size of the deterministic relaxation close to the depinning transition. We propose a phenomenological model which allows to accurately fit the observed excess velocity and to obtain characteristic values for the depinning field H_d, the depinning temperature T_d, and the characteristic velocity scale v_0 for each sample.
Magnetic domain wall motion is at the heart of new magneto-electronic technologies and hence the ... more Magnetic domain wall motion is at the heart of new magneto-electronic technologies and hence the need for a deeper understanding of domain wall dynamics in magnetic systems. In this context, numerical simulations using simple models can capture the main ingredients responsible for the complex observed domain wall behavior. We present a scalar-field model for the magnetization dynamics of quasi-two-dimensional systems with a perpendicular easy axis of magnetization which allows a direct comparison with typical experimental protocols, used in polar magneto-optical Kerr effect microscopy experiments. We show that the thermally activated creep and depinning regimes of domain wall motion can be reached, and the effect of different quenched disorder implementations can be assessed with the model. In particular, we show that the depinning field increases with the mean grain size of a Voronoi tessellation model for the disorder.
Magnetic field driven domain wall dynamics in a ferrimagnetic GdFeCo thin film with perpendicular... more Magnetic field driven domain wall dynamics in a ferrimagnetic GdFeCo thin film with perpendicular magnetic anisotropy is studied using low temperature magneto-optical Kerr microscopy. Measurements performed in a practically athermal condition allow for the direct experimental determination of the velocity (β = 0.30 ± 0.03) and correlation length (ν = 1.3 ± 0.3) exponents of the depinning transition. The whole family of exponents characterizing the transition is deduced, providing evidence that the depinning of magnetic domain walls is better described by the quenched Edwards-Wilkinson universality class.
For measuring accurately the energy of relativistic high energy charged particles, we have constr... more For measuring accurately the energy of relativistic high energy charged particles, we have constructed a novel transition radiation detector (TRD) utilizing superheated superconducting microspheres of tin with diameters in the range of 20-40 μm In order to enhance transition radiation for higher sensitivity, the TRD is in a special dilution refrigerator operated at~100 mK. Test has been conducted in a high energy electron beam facility at the CERN PS in the energy range of 1-10 GeV showing an energy dependence of the TR X-ray photon produced and hence the value γ=E/mc^2 of the charged particle.
Fully understanding domain wall motion in ferromagnetic systems is considered to be essential for... more Fully understanding domain wall motion in ferromagnetic systems is considered to be essential for the design of new magneto-electronic devices. Along with experiments, numerical simulations are therefore a key to gaining insight into the underlying mechanisms. However, simulating model systems at time and length scales comparable with those of experiments still represents a great challenge. Here, we present a simplified micromagnetic model --halfway between full micromagnetism and Ginzburg-Landau theory-- to study the dynamics of domain walls in quasi two-dimensional ferromagnetic systems with perpendicular magnetic anisotropy (PMA). Our approach relies on the local parametrization of the in-plane magnetization in terms of its out-of-plane component. We show that our model quantitatively reproduces previous experimental velocity-field data in the archetypal PMA Pt/Co/Pt ultra-thin films in the three dynamical regimes of domain wall motion (creep, depinning and flow). In addition, we...
Journal of Statistical Mechanics: Theory and Experiment, 2021
Along with experiments, numerical simulations are key to gaining insight into the underlying mech... more Along with experiments, numerical simulations are key to gaining insight into the underlying mechanisms governing domain wall motion in thin ferromagnetic systems. However, a direct comparison between numerical simulation of model systems and experimental results still represents a great challenge. Here, we present a tuned Ginzburg–Landau model to quantitatively study the dynamics of domain walls in quasi two-dimensional ferromagnetic systems with perpendicular magnetic anisotropy. This model incorporates material and experimental parameters and the micromagnetic prescription for thermal fluctuations, allowing us to perform material-specific simulations and at the same time recover universal features. We show that our model quantitatively reproduces previous experimental velocity-field data in the archetypal perpendicular magnetic anisotropy Pt/Co/Pt ultra-thin films in the three dynamical regimes of domain wall motion (creep, depinning and flow). In addition, we present a statistic...
Domain wall dynamics and spatial fluctuations are closely related to each other and to universal ... more Domain wall dynamics and spatial fluctuations are closely related to each other and to universal features of disordered systems. Experimentally measured roughness exponents characterizing spatial fluctuations have been reported for magnetic thin films, with values generally different from those predicted by the equilibrium, depinning and thermal reference states. Here, we study the roughness of domain walls in GdFeCo thin films over a large range of magnetic field and temperature. Our analysis is performed in the framework of a model considering length-scale crossovers between the reference states, which is shown to bridge the differences between experimental results and theoretical predictions. We also quantify for the first time the size of the depinning avalanches below the depinning field at finite temperatures.
Magnetic domain wall motion is at the heart of new magneto-electronic technologies and hence the ... more Magnetic domain wall motion is at the heart of new magneto-electronic technologies and hence the need for a deeper understanding of domain wall dynamics in magnetic systems. In this context, numerical simulations using simple models can capture the main ingredients responsible for the complex observed domain wall behavior. We present a scalar-field model for the magnetization dynamics of quasi-two-dimensional systems with a perpendicular easy axis of magnetization which allows a direct comparison with typical experimental protocols, used in polar magneto-optical Kerr effect microscopy experiments. We show that the thermally activated creep and depinning regimes of domain wall motion can be reached, and the effect of different quenched disorder implementations can be assessed with the model. In particular, we show that the depinning field increases with the mean grain size of a Voronoi tessellation model for the disorder.
The interfacial Dzyaloshinskii-Moriya Interaction (DMI) plays a crucial role in chiral domain wal... more The interfacial Dzyaloshinskii-Moriya Interaction (DMI) plays a crucial role in chiral domain wall (DW) motion, favoring fast DW velocities. We explore the effect of interface disorder on DMI and DW dynamics in perpendicular magnetized Ta/CoFeB/MgO thin films. Light He + irradiation has been used to gently engineer interface intermixing on a scale of 0.1 nm. We demonstrate that a slight modification of the Ta/CoFeB interface leads to an increase of the DMI value accompanied by an enhancement of DW velocity in the flow regime. Using micromagnetic simulations based on granular structures, we show that the enhancement of DW velocity is mainly related to an increase in the distribution of magnetic parameters related to the interface. We further infer that the DMI modulation is related to the asymmetric disorder induced by irradiation leading to alloying with the Ta buffer layer. Understanding the role of disorder is therefore crucial for the design of future devices where post-growth interface alloying can be used to finely tune the DMI.
Chiral domain walls in ultrathin perpendicularly magnetised layers have a Néel structure stabilis... more Chiral domain walls in ultrathin perpendicularly magnetised layers have a Néel structure stabilised by a Dzyaloshinskii-Moriya interaction (DMI) that is generated at the interface between the ferromagnet and a heavy metal. Different interface materials or properties are required above and below a ferromagnetic film in order to generate the structural inversion asymmetry needed to ensure that the DMI arising at the two interfaces does not cancel. Here we report on the magnetic properties of epitaxial Pt/Co/AuxPt1−x trilayers grown by sputtering onto sapphire substrates with 0.6 nm thick Co. As x rises from 0 to 1 a structural inversion asymmetry is progressively generated. We characterise the epilayer structure with x-ray diffraction and cross-sectional transmission electron microscopy, revealing (111) stacking. The saturation magnetization falls as the proximity magnetisation in Pt is reduced, whilst the perpendicular magnetic anisotropy Ku rises. The micromagnetic DMI strength D was determined using the bubble expansion technique and also rises from a negligible value when x = 0 to ∼ 1 mJ/m 2 for x = 1. The depinning field at which field-driven domain wall motion crosses from the creep to the depinning regime rises from ∼ 40 to ∼ 70 mT, attributed to greater spatial fluctuations of the domain wall energy with increasing Au concentration. Meanwhile, the increase in DMI causes the Walker field to rise from ∼ 10 to ∼ 280 mT, meaning that only in the x = 1 sample is the steady flow regime accessible. The full dependence of domain wall velocity on driving field bears little resemblance to the prediction of a simple one-dimensional model, but can be described very well using micromagnetic simulations with a realistic model of disorder. These reveal a rise in Gilbert damping as x increases.
We study the ultraslow domain-wall motion in ferromagnetic thin films driven by a weak magnetic f... more We study the ultraslow domain-wall motion in ferromagnetic thin films driven by a weak magnetic field. Using time-resolved magneto-optical Kerr effect microscopy, we access to the statistics of the intermittent thermally activated domain-wall jumps between deep metastable states. Our observations are consistent with the existence of creep avalanches: roughly independent clusters with broad size and ignition waiting-time distributions, each one composed by a large number of spatiotemporally correlated thermally activated elementary events. Moreover, we evidence that the large-scale geometry of domain walls is better described by depinning rather than equilibrium universal exponents.
Normal and superconducting state domains in a 10 μm thick indium slab are observed by the magneto... more Normal and superconducting state domains in a 10 μm thick indium slab are observed by the magneto‐optical imaging technique. The sample is subjected to a magnetic field H with different sweeping rates H. The domain patterns are found to depend on the penetration regime of the magnetic flux. For the lowest sweeping rate (H=1 mT s−1), the domains are randomly oriented. For the highest sweeping rate (H =230 mT s−1) and sufficiently high H‐values, the domains consist of a nearly straight laminar structure with a well‐defined period, oriented perpendicularly to the edge of the sample. These first results open new perspectives for studying the dynamics of formation of magnetic domains.
Like many other systems (magnetic fluids, Langmuir polarized layers) type-I superconducting (SC) ... more Like many other systems (magnetic fluids, Langmuir polarized layers) type-I superconducting (SC) films exhibit a phase modulation known as the intermediate state (IS). It consists of coexisting domains of the normal state (NS) and SC phases. The striking similarity of the domain patterns for various systems has stimulated the development of general models [1] based on the competition between the
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Papers by Vincent Jeudy