Ultrafast single-pulse all-optical switching in synthetic ferrimagnetic Tb/Co/Gd multilayers

In this work, we investigate single-shot all-optical switching (AOS) in Tb/Co/Gd/Co/Tb multilayers in an attempt to establish AOS in synthetic ferrimagnets with high perpendicular magnetic anisotropy. In particular, we study the effect of varying Tb thicknesses to disentangle the role of the two rare-earth elements. Even though the role of magnetic compensation has been considered to be crucial, we find that the threshold fluence for switching is largely independent of the Tb content. Moreover, we identify the timescale for the magnetization to cross zero to be approximately within the first ps after laser excitation using time-resolved magneto-optic Kerr effect. We conclude that the switching is governed mostly by interactions between Co and Gd.</p

Ultrafast single-pulse all-optical switching in synthetic ferrimagnetic Tb/Co/Gd multilayers

In this work, we investigate single-shot all-optical switching (AOS) in Tb/Co/Gd/Co/Tb multilayers in an attempt to establish AOS in synthetic ferrimagnets with high magnetic anisotropy. In particular, we study the effect of varying Tb thicknesses to disentangle the role of the two rare earth elements. Even though the role of magnetic compensation has been considered to be crucial, we find that the threshold fluence for switching is largely independent of the Tb content. Moreover, we identify the timescale for the magnetization to cross zero to be within the first ps after laser excitation using time-resolved MOKE. We conclude that the switching is governed mostly by interactions between Co and Gd

Indirect excitation of ultrafast demagnetization

Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions

Transport dépendant du spin et couplage d'échange : de la jonction tunnel au capteur magnétique intégré

In a first part, the crystalline and magnetic properties of X/IrMn and X/IrMn/Y multilayers (X,Y= Co and/or Py) are studied. The magnetic properties are explained by the use of micromagnetic models. The exchange coupling difference for both interfaces X/IrMn and IrMn/Y is independent of the microstructure and is shown to be only linked to the stacking order. It is evidenced that depositing an antiferromagnetic layer on a ferromagnetic layer is completely different than the opposite case. In a second part, the IrMn/Co bilayer is used as a detection layer in a magnetic tunnel junction to build up a magnetic field sensor. The sensor output signal is linear and reversible in a field range +/-50 Oe. A clever choice of the junction parameters allowed us to make a temperature independent sensor sensitivity. On this basis, a demonstrator using this magnetic field sensor was realized with its complete electronic processing.Dans une première partie, les propriétés cristallographiques et magnétiques de couches minces X/IrMn et X/IrMn/Y (X,Y= Co et/ou Py) sont présentées. Le magnétisme est expliqué à l'aide de modèles micromagnétiques. La différence de couplage d'échange aux interfaces X/IrMn et IrMn/Y est indépendante de la microstructure et uniquement liée à l'ordre d'empilement des couches. Il est démontré que déposer une couche antiferromagnétique sur une couche ferromagnétique est totalement différent d'un point de vue magnétique de l'opération inverse. Dans une seconde partie, la bicouche IrMn/Co est utilisée comme couche de détection dans une jonction tunnel magnétique pour réaliser un capteur de champ magnétique linéaire et réversible dans la gamme –50 et 50 Oe. Un choix judicieux des paramètres de la jonction tunnel a permis de rendre la sensibilité du capteur indépendante de la température. Sur cette base, un démonstrateur de capteur magnétique avec son électronique de traitement est réalisé

Transport dépendant du spin et couplage d'échange (de la jonction tunnel au capteur magnétique intégré)

Dans une première partie, les propriétés cristallographiques et magnétiques de couches minces X/IrMn et X/IrMn/Y (X,Y= Co et/ou Py) sont présentées. Le magnétisme est expliqué à l'aide de modèles micromagnétiques. La différence de couplage d'échange aux interfaces X/IrMn et IrMn/Y est indépendante de la microstructure et uniquement liée à l'ordre d'empilement des couches. Il est démontrer que déposer une couche antiferromagnétique sur une couche ferromagnétique est totalement différent d'un point de vue magnétique de l'opération inverse. Dans une seconde partie, la bicouche IrMn/Co est utilisée comme couche de détection dans une jonction tunnel magnétique pour réaliser un capteur de champ magnétique linéaire et réversible dans la gamme 50 et 50 Oe. Un choix judicieux des paramètres de la jonction tunnel a permis de rendre la sensibilité du capteur indépendante de la température. Sur cette base, un démonstrateur de capteur magnétique avec son électronique de traitement est réalisé.In a first part, the crystalline and magnetic properties of X/IrMn and X/IrMn/Y multilayers (X,Y= Co and/or Py) are studied. The magnetic properties are explained by the use of micromagnetic models. The exchange coupling difference for both interfaces X/IrMn and IrMn/Y is independent of the microstructure and is shown to be only linked to the stacking order. It is evidenced that depositing an antiferromagnetic layer on a ferromagnetic layer is completely different than the opposite case. In a second part, the IrMn/Co bilayer is used as a detection layer in a magnetic tunnel junction to build up a magnetic field sensor. The sensor output signal is linear and reversible in a field range +/-50 Oe. A clever choice of the junction parameters allowed us to make a temperature independent sensor sensitivity. On this basis, a demonstrator using this magnetic field sensor was realized with its complete electronic processing.NANCY1-SCD Sciences & Techniques (545782101) / SudocSudocFranceF

Ab initio study of electronic temperature effects on magnetic materials properties

International audienceWe present ab initio calculations of the electronic temperature dependence of the magnetization, electronic energy, and specific heat of FePt L1 0 , fcc-Ni, hcp-Co, and bcc-Fe. We find that atomic magnetic moments in each compound disappear at very high temperature, ranging from 3100 to 8100 K. However, for some compounds, the consequences of this phenomenon are noticeable on the electronic energy and specific heat even at low electronic temperature. Consequently, large deviations from the Sommerfeld approximation and from some previous work that did not take into account explicitly the dependence of the electronic structure on the electronic temperature are shown. Our results are of interest in the field of laser-induced ultrafast magnetization dynamics, since they provide a more precise estimate of the electronic specific heat that enters in the three-temperature model

Ab initio theory of magnetization induced by light absorption in ferromagnets

International audienceWe use density functional theory to quantitatively compute the effect of light absorption in ferromagnetic materials. We show that, in the presence of spin-orbit coupling, optically induced transitions do not conserve the magnetization and that a systematic induced demagnetization, whose magnitude depends on both the helicity of the light and the direction of the magnetization, is observed. Very differently from the inverse Faraday effect, this mechanism is due to the absorption of light and depends on the magnetic state of each atom, and therefore cannot be described by an effective optomagnetic field. Then, based on these results, we derive a set of parameters which can be used in micromagnetic simulations in order to account for light transition effects on the magnetization dynamics. To face the continuing demand for large density and energy efficient data storage devices, the possibilities of magnetiza-tion manipulation without using a magnetic field are widely being investigated [1]. One of the promising candidates is the all-optical helicity-dependent switching (AO-HDS), as it allows the control of the magnetization state by only using circularly left (σ +) or right (σ −) polarized light pulses of a few tens of femtoseconds. AO-HDS has been observed in a wide range of materials such as GdFeCo ferrimagnetic alloys [2], rare earth-transition metal alloys and multilayers [3,4], and FePt L1 0 granular media [5] which is considered promising for ultrahigh-density storage devices. This outstanding diversity of materials suggests a common underlying mechanism, although it remains debated. To explain the AO-HDS, the two theoretical explanations usually invoked in the literature are the inverse Faraday effect (IFE) [2,4-6] and a difference of absorption induced by the magnetic circular dichroism (MCD) [7,8]. While the IFE was first introduced to describe the influence of the presence of a circularly polarized light on the magnetic state of transparent media [9], Battiato et al. showed that, without any assumption on the nature of the material, light generates a static contribution at the second-order perturbation in the density matrix [10,11], which they hold responsible for the IFE in lossy media. However, in this approach [10] the repopulation at the origin of the IFE does not grow linearly with time, as it would be the case for an absorption-related phenomenon, and it fades away after the perturbation has been switched off. This fact leaves a gap between the IFE and the mechanism involved in the irreversible change of magnetization leading to the AO-HDS phenomenon [12]. Then, using this formulation and density functional theory, Berritta et al. [13] computed the value of this contribution for different types of materials. Conversely , the second effect, due to MCD, relies on a difference of absorption inducing a different temperature depending on * [email protected] the relative orientation of the magnetization and the helicity of the light. Through this thermal effect, the switching probability depends on the magnetization orientation, as shown by several parametrized models [7,8]. Moreover, its probabilistic and absorption-based nature is in agreement with the fact that the AO-HDS phenomenon is cumulative, i.e., it requires multiple pulses [4], as well as a large absorptivity of the compound

Hot-electron transport and ultrafast magnetization dynamics in magnetic multilayers and nanostructures following femtosecond laser pulse excitation

Understanding and controlling the magnetization dynamics on the femtosecond timescale is becoming indispensable both at the fundamental level and to develop future technological applications. While direct laser excitation of a ferromagnetic layer was commonly used during the past twenty years, laser induced hot-electrons femtosecond pulses and subsequent transport in magnetic multilayers has attracted a lot of attention. Indeed, replacing photons by hot-electrons offers complementary information to improve our understanding of ultrafast magnetization dynamics and to provide new possibilities for manipulating the magnetization in a thin layer on the femtosecond timescale. In this review, we report on experiments of hot-electrons induced ultrafast magnetic phenomena. We discuss the role of hot-electrons transport in the ultrafast loss of magnetization in magnetic single and multilayers and how it is exploited to trigger magnetization dynamics in magnetic multilayers