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Modeling of the ultrafast magnetization dynamics
To explain the ultrafast magnetization dynamics, many theoretical models focus mainly on the thermalization of spins via the absorption of the laser pulse by the electronic system [20] and highlight the major role of laser heating in the demagnetization process [21, 22]. In the following we will discuss the different models proposed in the literature.
Phenomenological three-temperature model
The ultrafast demagnetization initially demonstrated in Ni films was first attributed to the energy transfer from the laser-excited electrons to the spin system [15]. To elucidate the de-magnetization process, a simple model was proposed by Beaurepaire et al. [15] called the “three-temperature model” (3TM). This phenomenological model is based on the interactions between three different systems, namely the electrons, the lattice and the spins. Each system is a reservoir for energy. By taking into account the spin system, the 3TM is an extension of the two-temperature model (2TM) describing the energy transferred from the laser-excited electrons only to the lattice [23].
As illustrated in figure 1.2, the absorption of a laser pulse by electrons leads to a heat transfer between three-coupled reservoirs. Each of these coupled reservoirs has then its own temper-ature and heat capacity. Taking into account the energy balance, the temporal evolution of the temperature in each reservoir can be expressed by the following coupled equations: C (T ) dTe =−G (T e −T)− G (T e − T ) + P(t) (1.1).
Experimental demonstration of all-optical switching in GdFeCo alloy films
A fascinating outcome of the experimental investigations on the ultrafast magnetization dy-namics in magnetic systems is the demonstration of the all-optical switching (AOS) of mag-netization using femtosecond laser pulses. Indeed, Stanciu et al. showed in 2007 that the magnetization in a ferrimagnetic Gd22Fe74.6Co3.4 alloy film could be deterministically reversed using circularly polarized laser pulses and without any external applied field [12]. The direction of magnetization is given by the right- or left-circular polarization of the pulses. Later, it was demonstrated by Radu et al. [32] that the magnetization in Gd25Fe65.6Co9.4 alloy film could be also reversed using a single linearly polarized fs laser pulse. In this section we will introduce the pioneering experimental observations of the all-optical switching in GdFeCo alloy films.
Multiple-pulse and single-pulse helicity-dependent switching
In 2007, the first observation of AOS using circularly polarized light was demonstrated on a 30 nm-thick Gd22Fe74.6Co3.4 alloy film with perpendicular magnetic anisotropy (PMA). The sample is first placed under the Faraday microscope. As shown in figure 1.4(a), the domains with magnetization “up” and “down” could be observed as white and black regions, respectively. The sample is then excited using a Ti: sapphire laser with a repetition rate of 1 kHz, a wave-length of 800 nm and a pulse duration of 40 fs. The Gaussian fs laser beam illuminates the sample perpendicularly to the film plane, and is swept at a high speed across the sample so that each pulse landed at a different spot. Figure 1.4(b) shows that σ+ (resp. σ- pulses switch the magnetization in the black (resp. white) domain, without affecting the magnetization of the white (resp. black) domain. This reversal with circularly polarized light is called all-optical helicity-dependent switching (AO-HDS). The region exposed to linearly polarized light is turned into small domains randomly oriented up or down. Moreover, figure 1.4(b) shows that a demagnetized area is located to the right of the scanned region, at the point where the laser beam was turned off.
Temperature influence on the AO-HDS of ferrimagnets
It was mentioned in section 1.2 that the fs-pulsed laser heating plays an important role in the demagnetization process [21, 22], which was qualitatively explained by the three-temperature model [15]. Hence, the temperature of the interacting reservoirs is also expected to play a crucial role in the AOS process. Different approaches to investigate the temperature influence on AOS in ferrimagnets have been explored in previous studies [44, 57, 58].
Heat-induced reestablishment of all-optical switching
A first approach consists in heating the sample with a resistor or by tuning the pulse repetition rate to reestablish the AOS ability in ferrimagnetic films, as demonstrated by Hassdenteufel et al. [44]. Indeed, they investigated the optical response of a 16 nm-thick Tb34Fe66 alloy film to the action of 100 fs-laser beam. The temperature influence is studied either by increasing the repetition rate υ from 10 kHz to 250 kHz, or by externally increasing the sample temperature by a heatable sample holder via Joule heating. Indeed, a pure thermal demagnetization is observed for υ = 10 kHz at room temperature, as depicted in figures 1.17(d)-1.17(f). However, AO-HDS is reestablished by increasing the repetition rate from 10 kHz to 250 kHz at RT, as shown in figures 1.17(a)-1.17(c). Moreover, AO-HDS is also reestablished gradually for υ = 10 kHz by increasing the sample temperature up to 370 K. In both cases, the reestablishment of AOS was attributed to the heat-induced reduction of the remanence and the coercivity [44].
DC sputtering technique
The sputtering technique was used in this thesis to elaborate samples. Sputtering is a physical vapor deposition (PVD) whereby atoms are ejected from a pure element called “target”, and are subsequently deposited on the “substrate”. The bombardment of the target requires the formation of gaseous plasma, which is typically made of inert gas ions such as Argon ions Ar+. Indeed, Ar atoms are first introduced into a vacuum chamber at a pressure ranging from 1 to 10 mTorr. A DC voltage is then placed between the target and the substrate, enabling to ionize the Ar atoms and to create the plasma. These Ar+ ions are accelerated to the cathode target, leading to the ejection of its atoms in the form of neutral particles into the vacuum space. These neutral particles can be either individual atoms, clusters of atoms or molecules. They can then ballistically travel from the target in straight lines until reaching the substrate. These ejected atoms start to bind to each other at the molecular level, thus forming a tightly bound atomic layer. Moreover, the electrons released during the ionization of Ar atoms are also accelerated to the anode substrate. These electrons collide subsequently with the addi-tional Ar atoms, leading to more Ar ions and free electrons and thus continuing the cycle.
In order to enhance the sputtering process, sputtering systems often use magnetrons as for the Alliance system used in this thesis. Indeed, this so-called magnetron sputtering consists in using two permanent magnets of opposite polarity located below the target, as illustrated in figure 2.1. These magnets create a strong magnetic field parallel to the target surface and orthogonal to the electric field used to ionize the Ar atoms. Moreover, these two fields create field lines that trap the electrons released during the ionization process. These electrons follow then helical paths around the magnetic field lines due to the Lorentz force, leading to more ionizing collisions with the gaseous neutral atoms near the target surface. Therefore, the magnetron generates stable plasma with high density of ions, which is confined to an area near the target. This technique increases the efficiency of the sputtering process and improves the quality of coatings.
Light-ion irradiation
The manipulation of magnetic anisotropy triggered a huge interest due to the strong contribu-tion of the anisotropy in the recording stability in spintronic devices [1]. Therefore, Chappert et al. introduced in 1998 the light-ion irradiation as an innovative technique enabling to tailor the magnetic anisotropy in Co/Pt multilayers with PMA [2]. Indeed, this technique enables to modify both structural and magnetic properties in thin films; such modifications depend essentially on the energy density of the ion beam transferred to the atoms [3]. The ion irra-diation differs from the ion implantation. Indeed, the latter enables to modify the physical properties of a material not by changing its structure, but via the direct effect of adding the atoms in the material.
In order to perform the ion irradiation, He+ ions are mainly used Chappert98x,Devolder00x since they do not lead to extended collision cascades with energies going from 5 to 150 keV. The structural modifications are located in the vicinity of the ion path in the metal, and consist in recoils limited to one or two atomic distances [4]. The schematic representation of the He+-irradiation setup is illustrated in figure 2.2. This device enables to ionize the Helium source by applying a high voltage, and thus creates He+ ion beam with controlled energy and dose. As depicted in figure 2.2, the electrode allows extracting the He+ ion beam from the source, which is then deflected by the magnet. The Faraday cage enables to set the voltage and the intensity, while the Einzel lens focuses the ion beam. Finally, the latter goes through a diaphragm before irradiating the sample. Our samples were irradiated with He+ ions at irradiation doses ranging from 5 1014 to 5 1015 He+/cm2 and at a constant energy of 15 keV.
Table of contents :
1 All-optical magnetization switching
1.1 Introduction
1.2 Phenomenology of laser-induced ultrafast magnetization dynamics
1.2.1 Observation of ultrafast laser-induced demagnetization
1.2.2 Modeling of the ultrafast magnetization dynamics
1.3 All-optical switching in GdFeCo thin films
1.3.1 Experimental demonstration of all-optical switching in GdFeCo alloy films
1.3.2 Unification of helicity-dependent and heat-only switching
1.4 Novel materials for all-optical helicity-dependent switching
1.4.1 Ferrimagnetic materials
1.4.2 Ferromagnetic materials
1.5 Any general mechanism for all-optical switching ?
1.5.1 Temperature influence on the AO-HDS of ferrimagnets
1.5.2 Possible origins of the symmetry breaking for AO-HDS
1.5.3 Orientation conservation
1.6 Outlines of the thesis
2 Experimental tools and samples
2.1 Introduction
2.2 Samples elaboration and characterization
2.2.1 DC sputtering technique
2.2.2 Light-ion irradiation
2.2.3 Magnetic characterization
2.3 Femtosecond pulsed laser
2.3.1 Femtosecond laser pumping
2.3.2 Femtosecond laser amplification
2.4 Magneto-optical Faraday microscope
2.4.1 Magneto-optical Kerr effect
2.4.2 Experimental setup
3 Domain size criterion for the observation of all-optical switching
3.1 Introduction
3.2 Magnetic interactions and domains
3.2.1 Magnetic interactions
3.2.2 Magnetic domains and domain walls
3.2.3 Magnetization reversal in thin films
3.3 Thickness-dependence of all-optical switching in ferrimagnets and ferromagnets
3.3.1 Studied ferri- and ferro-magnets
3.3.2 Thickness-dependence of the all-optical switching in ferrimagnets
3.3.3 Thickness-dependence of the all-optical switching in ferromagnets
3.4 Domain size criterion for the observation of all-optical switching
3.4.1 Magnetic parameters for studied materials
3.4.2 Domain size calculation for ferro- and ferrimagnets
3.4.3 Conclusions
3.5 Magnetic anisotropy dependence of the all-optical switching
3.5.1 Tailoring magnetic anisotropy by light-ion irradiation
3.5.2 All-optical control of magnetization in irradiated Co/Pt multilayers .
3.6 Summary
4 Distinction between two types of all-optical switching mechanisms via magnetotransport measurements
4.1 Introduction
4.2 Electrical characterization of AO-HDS in ferromagnetic Pt/Co/Pt Hall crosses
4.2.1 Introduction
4.2.2 Sweeping beam measurement
4.2.3 Static beam measurement with an off-centered beam
4.2.4 Static beam measurement with a centered beam
4.2.5 Summary
4.3 Electrical characterization of AOS in ferromagnetic CoFeB/MgO Hall crosses
4.3.1 Sweeping beam measurement
4.3.2 Static beam measurement with a centered beam
4.3.3 Summary
4.4 Electrical characterization of AO-HDS in ferrimagnetic TbCo Hall crosses
4.4.1 Sweeping beam measurement
4.4.2 Static beam measurement with an off-centered beam
4.4.3 Static beam measurement with a centered beam
4.4.4 Summary
4.5 Two types of all-optical switching mechanism distinguished via the anomalous Hall effect
4.5.1 Introduction
4.5.2 Single-pulse switching of GdFeCo based Hall crosses and continuous films
4.5.3 Single-pulse induced demagnetization of [Co/Pt]N and TbCo continuous film
4.5.4 Multiple-shot switching of Co/Pt and TbCo based Hall crosses
4.5.5 Discussion: Microscopic origin of multiple-pulse helicity- dependent switching
4.5.6 Summary
5 General conclusions and perspectives
