HELICITY-DEPENDENT ALL-OPTICAL SWITCHING IN HYBRID METAL- FERROMAGNET STRUCTURES FOR ULTRAFAST MAGNETIC DATA STORAGE

Transcription

1 HELICITY-DEPENDENT ALL-OPTICAL SWITCHING IN HYBRID METAL- FERROMAGNET STRUCTURES FOR ULTRAFAST MAGNETIC DATA STORAGE A Dissertation Presented By Feng Cheng to The department of Electrical and Computer Engineering in partial fulfillment of the requirements for the degree of Master of Science in the field of Electromagnetics, Plasma, and Optics Northeastern University Boston, Massachusetts November, 2016

2 1 Table of Contents Abstract Introduction Experimental system Magneto-optical Kerr microscopy setup Laser incident system Sample deposition Helicity-dependent all-optical switching results (HD-AOS) Helicity-dependent all-optical switching with hybrid CoPtAu sample Helicity-dependent all-optical switching with different repetition rates Helicity-dependent all-optical switching with different peak powers Switching with single pulses Helicity-dependent all-optical switching with plasmonic structures Multiphsyics modeling of the helicity-dependent all-optical switching effect The electron and phonon temperatures induced by laser dissipated power The magnetization induced by effective magnetic pulse Summary and Future Work Summary Future work Reference... 41

3 2 Abstract The emerging Big Data era demands the rapidly increasing need for speed and capacity of storing and processing information. Standalone magnetic recording devices, such as hard disk drives (HDDs), have always been playing a central role in modern data storage and continuously advancing. Recognizing the growing capacity gap between the demand and production, industry has pushed the bit areal density in HDDs to 900 Giga-bit/square-inch, a remarkable 450-million-fold increase since the invention of the first hard disk drive in However, the further development of HDD capacity is facing a pressing challenge, the so-called superparamagnetic effect, that leads to the loss of information when a single bit becomes too small to preserve the magnetization[1, 2]. This requires new magnetic recording technologies that can write more stable magnetic bits into hard magnetic materials. Recent research has shown that it is possible to use ultrafast laser pulses to switch the magnetization in certain types of magnetic thin films. Surprisingly, such a process does not require an externally applied magnetic field that always exists in conventional HDDs. Furthermore, the optically induced magnetization switching is extremely fast, up to subpicosecond (10 #$% s) level, while with traditional recording method the deterministic switching does not take place shorter than 20 ps [3]. It s worth noting that the direction of magnetization is related to the helicity of the incident laser pulses. Namely, the righthanded polarized laser pulses will generate magnetization pointing in one direction while left-handed polarized laser pulses generate magnetization pointing in the other direction.

4 3 This so-called helicity-dependent all-optical switching (HD-AOS) phenomenon can be potentially used in the next-generation of magnetic storage systems. In this thesis, I explore the HD-AOS phenomenon in hybrid metal-ferromagnet structures, which consist of gold and Co/Pt multilayers. The experiment results show that such CoPtAu hybrid structures have stable HD-AOS phenomenon over a wild range of repetition rates and peak powers. A macroscopic three-temperature model is developed to explain the experiment results. In order to reduce the magnetic bit size and power consumption to transform future magnetic data storage techniques, I further propose plasmonic-enhanced all-optical switching (PE-AOS) by utilizing the unique properties of the tight field confinement and strong local field enhancement that arise from the excitation of surface plasmons supported by judiciously designed metallic nanostructures. The preliminary results on PE-AOS are presented. Finally, I provide a discussion on the future work to explore the underline mechanism of the HD-AOS phenomenon in hybrid metalferromagnetic thin films. Different materials and plasmonic nanostructures are also proposed as further work.

5 4 1 Introduction Magnetization manipulation can be achieved by electric fields [4-6], spin-polarized currents [7-9] and laser pulses [10, 11], in addition to the well-known method by applying an external magnetic field. In particular, ultrafast optical manipulation of magnetization has emerged into a fascinating, multidisciplinary research topic, since the discovery of ultrafast demagnetization of a Ni film by a 60 femtosecond laser pulse [10]. Subsequent work not only confirmed the effect [12-14], but also demonstrated the possibility to optically generate coherent magnetic precession [15] and optically induce spin reorientation [16]. One of the most surprising results in ultrafast magnetization manipulation is the demonstration that circularly polarized laser pulses can directly and deterministically switch magnetic domains without applying external magnetic field. This is termed as helicity-dependent all-optical switching (HD-AOS) or simply all-optical switching (AOS) [11, 17]. Compared with the conventional method to record data bits via external applied magnetic field, which cannot obtain deterministic switching shorter than 20 ps [3], the HD-AOS method can reverse the magnetization at the sub-picosecond level. Therefore, this method potentially offers a pathway to record data bits times faster than the existing magnetic data storage techniques. The HD-AOS was initially found in ferrimagnetic systems involving rare earth and transition metal (RE-TM) elements [18-32]. Most studied samples are the Gd and Tb related amorphous ferrimagnetic alloy films [19-22, 24-27, 31]. Till now, though the fundamental mechanism remains unclear, researchers have utilized different approaches to modify magnetic material properties to obtain the HD-AOS feature. People have found that

6 5 for the magnetic samples with a low remanent magnetization and a nearly zero moment magnetization is critical for HD-AOS. Therefore, one method for the RE-TM samples to exhibit HD-AOS feature is to change the material concentration. Another method is to change the ambient temperature so that the sample magnetization can approach zero. It was also reported that heat plays an important role in the HD-AOS process, so that the substrates with high heat dissipation rate helps with the HD-AOS effect. Besides the ferrimagnetic systems involving rate earth and transition metal elements, people have found HD-AOS effect in more general magnetic materials, such as the ferromagnetic materials, namely, the CoPt multilayer thin films [33]. CoPt multilayer thin films are well-known for the perpendicular magnetic anisotropic (PMA) property. Due to the high areal density, CoPt multilayers thin films are among the candidates of magnetic materials that are suitable for industrial use. People have shown that CoPt multilayer thin films with certain Co, Pt layer thicknesses and repeats show HD-AOS properties, while other samples show thermal demagnetization. Figure 1 (a-b) Helicity-dependent all-optical switching (HD-AOS) in GdFeCo subjected to 40 fs pulses [19]. (a) Under a magneto-optical microscope, the initial magnetic state of the sample before laser exposure, in which white and black areas correspond to up (M+) and down (M ) magnetic domains, respectively. (b) Domain pattern obtained by sweeping circularly polarized (σ+ or σ-) or linear polarized (L) light across the surface of the sample. (c) Magneto-optical image of Co 0.4 nm /Pt 0.7 nm 4 multilayers in response to different laser polarizations [33].

7 6 Figure 1 (a) and (b) present the pioneering work of the helicity-dependent all-optical switching of rare-earth transition-metal GdFeCo alloys [34]. Domains with magnetization up (M + ) and down (M - ) shows white and black contrast, respectively, under a magnetooptical microscope. One can see from Figure 1 (b) that each of the right-handed polarized ( σ 6 ) pulses reverse the magnetization in the black domain but does not affect the magnetization of the white domain. The opposite situation is observed for left-handed polarized (σ # ) pulses. These results convincingly demonstrated that all-optical magnetic switch can be achieved by single 40-femtosecond circularly polarized laser pulses without the aid of an external magnetic field. Disordered subdomains that minimize the dipole energy are generated at the end of scan lines [35]. Such subdomains do not depend on the light polarization and represent laser-induced thermal demagnetization. Most all-optical switching studies were focused on GdFeCo alloys [36, 37], while subsequent studies expanded to other rare earth-transition metals (RE-TM) [38], synthetic ferrimagnets [39], and very recently to ferromagnetic materials including magnetic thin films, multilayers and granular films used for high-capacity magnetic recording (Figure 1 (c)) [40]. The HD-AOS phenomenon can be phenomenologically explained by the inverse Faraday effect [41-43]. It is shown that circularly polarized light can produce an effective magnetic field, which is given by B 89 = βϵ = E E. Here β is the magneto-optical susceptibility, E(ω) is the electric field and E (ω) is the conjugate. For LCP and RCP light, the effective magnetic field is opposite in directions. Therefore, the magnetization can be controlled by simply switching the helicity of circularly polarized light. However, the fundamental mechanism of all-optical switching is still not completely understood. In addition to the inverse Faraday effect, alternative proposed mechanisms include the transfer of angular

8 7 momentum [44, 45], the formation of a transient ferromagnetic state [46], and laserinduced superdiffusive spin currents [47, 48]. In this thesis, I experimentally demonstrated the HD-AOS properties of hybrid metalferromagnet structures. By coating the CoPt multilayers with additional layer of Au, I am able to observe stable and robust HD-AOS phenomenon, which shows apparent improvement compared with the pure CoPt multilayers thin films. I have studied the HD- AOS phenomenon with different repetition rates and peak powers. I find that the hybrid CoPtAu hybrid structures show stable HD-AOS phenomenon with different repetition rates (10 khz~312 khz ) and peak powers (95 nj~285 nj, with repetition rate 200 khz ). Compared with HD-AOS effects of the pure CoPt multilayers, the results demonstrate that the hybrid CoPtAu structures obtain stable HD-AOS with good flexibility for the incident laser powers and repetition rates, which makes it more practical for industrial uses. Also, with plasmonic nanostructures integrated on the recording magnetic material, I have explored the HD-AOS feature of the CoPt multilayers with hole array structures. With the PE-AOS effect, the switching logical bit size is expected to be reduced to a few tens of nanometers with lower laser fluence. The thesis is organized in the following chapters. In Chapter 2, I will introduce the details of the experimental system and sample preparation procedures. In Chapter 3, the HD-AOS phenomenon for hybrid CoPtAu sample is demonstrated. I will also discuss the PE-AOS with hole arrays integrated on CoPt multilayers. A macroscopic three-temperature model for the HD-AOS phenomenon will be presented in the last section of Chapter 3. In Chapter 4, I will summarize the main achievements of the thesis and possible directions of the future work.

9 8 2 Experimental system The experimental system of this research work is based on two parts, the magneto-optical Kerr effect microscopy (MOKE) and the laser system. The MOKE system is used to image magnetic domains of the samples. The laser system provides circularly polarized laser pulses with specific peak power and repetition rate. The schematic view of the experimental system is shown in Figure 2. In the rest of this chapter I will discuss the MOKE system and laser incident system in more details. Figure 2 Experimental system for HD-AOS. The setup at is consisted of two parts, the MOKE imaging system (the bottom half) and the laser incident system (the upper half). (a) The schematic view. (b) The real experimental system. 2.1 Magneto-optical Kerr microscopy setup The MOKE imaging is a technique that used to study the domain structure of magnetic materials. The magnetic domains are regions of the magnetization that aligned together in

10 9 the same directions. Image the magnetic domains as miniature magnets within the thin films. Within the un-magnetized samples, the miniature magnets point in all possible directions, which overall results in a zero net-magnetization. When external magnetic field is applied to the sample, the miniature magnets will start to point in the direction of the applied field. This will result in the formation of magnetic domains. The magnetic domains grow larger with larger applied field and will eventually result in a magnetic thin film with the magnetization pointing in the same direction as the applied field. For the CoPt multilayer thin films with perpendicular magnetic anisotropy, magnetization either points upwards or downwards with respect to the surface normal direction of the sample. When the incident linearly polarized light is reflected by magnetic thin films with certain configuration, the polarization will rotate for an angle θ PQRR. This phenomenon is first reported by John Kerr in 1877 [49]. It s similar to the well-known Faraday effect discovered in 1845 by Faraday [50], in which linearly polarized light transmitted through a magnetic material will have a rotation angle θ STRTUTV in the polarization. MOKE can be further divided into three types regarding the direction of magnetization vector and the reflected light plane. They are the polar MOKE, longitudinal MOKE and the transverse MOKE. As shown Figure 3. Here I utilize the polar MOKE configuration to image the CoPt thin films because the samples show perpendicular magnetic anisotropy (PMA) properties. In polar MOKE, the magnetization directed perpendicular to the thin film sample plane. According to the coordinate in Figure 3, the dielectric tensor will be in the following form [51]: ε = ε = jκ 0 jκ ε = 0 Equation ε =

11 10 With the incident light along z direction, the x and y components of the electric field is coupled. Therefore, when linearly p-polarized (s-polarized) light is reflected from the sample, the reflected beam presents a small s-polarized (p-polarized) component. The nondiagonal Fresnel coefficients induces the polarization conversion. The reflectivity matrix will be: r = r \\ r \] κ r ]\ κ r ]] Equation 2 The reflectivity gives the rotation angle as θ + iφ = R ab c R aa. Figure 3 Three types of the magneto-optical Kerr effect microscopy. (a) The polar MOKE, (b) longitudinal MOKE and (c) transverse MOKE. As shown in Figure 2, a pair of polarizers are required to detect the rotated polarized light. In our system, two Glan-Taylor polarizers are utilized. One linear polarizer is installed right after the illumination light source, generating linearly polarized light illumination. The reflected light is collected by the objective on the microscope. Another polarizer is installed in front of the camera. The two polarizers are installed with the polarization-axis perpendicular to each other, thus blocking all other imaging light expect the Kerr rotated light. Due to the rotation directions with magnetization pointing downwards and upwards,

12 11 the magnetic domains will be imaged as black and white contrasts on the camera. Figure 4 shows one MOKE image of the CoPt multilayer samples. The magnetization pointing downwards and upwards can be easily distinguished. Figure 4 MOKE image for CoPt multilayer thin films. The magnetization pointing upwards and downwards are presented with white and black contrast correspondingly. Scale bar 20um. 2.2 Laser incident system The laser pulses are regeneratively amplified pulses from a Ti:sapphire laser with a central wavelength λ = 800 nm. The pulse duration is 200 fs and the repetition rate can be varied from 10 khz to 312 khz. As shown in Figure 2, the laser pulses are circularly polarized after passing through a polarizer and a quarter-waveplate, and then focused by a lens with focal length f f = 100 mm. Consider that each pulse has the Gaussian intensity profile, the focused radius is estimated to be r = 25.4 um. The ND filter is used to control the intensity of incident pulses, and the electric shutter is used as an on/off switch for the pulses.

13 Sample deposition The thin-film samples are grown by DC magnetron sputtering from elemental sources onto glass substrates at room-temperature. The sputtering is performed with the AJA sputtering system from Prof. Nian Sun s group at Northeastern University, as shown in Figure 5. Figure 5 The AJA sputtering system. In this master thesis, I focus on HD-AOS in the CoPt ferromagnetic multilayers thin films. Based on a recent published research work [33], a series of samples have been made. The recipe of the CoPt multilayer thin films is Ta 3 Pt 0.7 Co 0.4 /Pt Ta 3. The numbers in parentheses present layer thickness in nanometers. The top and bottom Ta layers work as seeding and capping layer correspondingly. For the hybrid CoPtAu sample, additional 30nm of Au is deposited in the last deposition process. Vibrating sample magnetometer (VSM) measurements are performed to investigate the magnetic property of the deposited CoPt samples and hybrid CoPtAu samples. The VSM

14 13 measurement is performed with the sample vibrating along one direction within a uniform magnetic field. Due to the variation of magnetization detected within the pick-up coils, an electric signal proportional to the magnetization is obtained. As a result, the relationship between the applied magnetic field and the induced magnetization within the sample is recorded. The VSM results are shown in Figure 6. Clear hysteresis loop is obtained for both CoPt multilayer and the hybrid CoPtAu samples in the perpendicular direction. Both samples show apparent perpendicular magnetic anisotropy. Also, I observed that the hybrid CoPtAu sample has smaller saturation magnetization compared with the CoPt multilayer sample. Figure 6 VSM measurement results for the pure CoPt thin films and hybrid CoPtAu thin films. The saturation magnetization M ] and coercivity field H l of hybrid CoPtAu thin films is smaller than the pure CoPt thin films.

15 14 3 Helicity-dependent all-optical switching results (HD-AOS) The HD-AOS experiment results are presented and discussed in this chapter. Firstly, the thin film samples are placed under the MOKE microscope, which can distinguish magnetization pointing upwards and downwards by white and black contrasts. The laser pulses with a certain repetition rate and peak power are incident perpendicular to the sample surface. After passing through a polarizer and a quarter-waveplate, depending on the angle between the linear polarized direction and the fast axis of the quarter-waveplate θ, circularly polarized light with opposite directions can be generated. Namely, θ = 45 will generate right-handed circularly polarized light (σ 6 ) and θ = 135 will generate lefthanded circularly polarized light σ #. After the laser pulses are circularly polarized, an optically-induced magnetic field along the Poynting vector direction k will be generated through the inverse Faraday effect (IFE) [52, 53]. The direction of such an opto-magnetic field depends on the helicity. More details about IFE will be discussed later in this chapter. The incident laser beam is then focused down to a spot with radius r = 25.4 um and the optically-induced magnetic field changes the initial magnetization of the thin film samples. The changes are observed by the MOKE microscopy. In each experiment, right-handed polarized light and left-handed polarized light scan over magnetization pointing downwards (black contrast) and upwards (white contrast). If the switching effect is helicity-dependent, the scan results show difference for the σ 6 and σ # laser beams on magnetization pointing downwards and upwards correspondingly. Otherwise, the

16 15 switching effect is not related to helicity of the light beams. In the following I will discuss our observations for varies samples. 3.1 Helicity-dependent all-optical switching with hybrid CoPtAu sample In this section, I will demonstrate the HD-AOS results for hybrid CoPtAu sample. The sample recipe is Ta 3 Pt 0.7 Co 0.4 /Pt Ta 3 Au(30). The MOKE image and corresponding laser scan results are shown in Figure 7. The laser beams have a repetition rate f = 200 khz and the average power P = mw. Figure 7 (a) shows the initial MOKE image for hybrid CoPtAu samples. The magnetic domains are manually positioned with a magnet. The black contrasts show magnetic domains pointing downwards, while the white contrasts show magnetic domains pointing upwards. Figure 7 The HD-AOS experiment results for hybrid CoPtAu samples. (a) Initial MOKE image for hybrid CoPtAu samples. Black and white contrast show magnetization pointing downwards (M # ) and upwards

17 16 (M 6 ) correspondingly. (b) Right-handed polarized σ 6 laser beams scan from magnetic domains initially pointing downwards to magnetic initially domains pointing upwards; left-handed polarized σ # laser beams scan from magnetic domains initially pointing upwards to magnetic domains initially pointing downwards. (c) Left-handed polarized laser beam erase the line right-handed polarized light scanned in (b). (d) Right-handed polarized laser beam erase the line left-handed polarized light scanned in (b). Scale bar 20um. Figure 7 (b) shows the experiment results of σ 6 laser beams scan from magnetic domains initially pointing downwards to magnetic domains initially pointing upwards. And σ # laser beams scan from magnetic domains initially pointing upwards to magnetic domains initially pointing upwards. Clear HD-AOS phenomenon is demonstrated in Figure 7. The σ 6 laser beams can generate magnetic domains pointing downwards on the hybrid CoPtAu magnetic thin films, which is regardless of the initial magnetization directions. Correspondingly, the σ # laser beams show opposite trends for both initial magnetization directions. The stable helicity-dependent phenomenon is evident in Figure 7 (c) and (d). The σ # laser beams clearly erase the line the σ 6 laser beams scanned in Figure 7 (b), and the σ # laser beams clearly erases the line that σ # laser beams scanned in Figure 7 (b). The hybrid CoPtAu sample shows apparent HD-AOS features that allow us to write and erase an arbitrary pattern. Such an interesting result potentially could be used as active magnetic template to assemble magnetic colloidal particles. In Figure 8 (a), the label NU, stands for Northeastern University, is written with σ 6 and σ # laser beams. The character N is written with σ # on magnetization initially pointing downwards, while the character U is written with σ # on magnetization initially pointing upwards. Figure 8 (b) and (c) show the results that σ 6 and σ # clearly erase the character N and U. This results

18 17 demonstrate that the switching phenomenon can be very well controlled by the helicity of the incident laser beams, which can be potentially used in magnetic storage systems. Figure 8 Write and erase NU label on the hybrid CoPtAu thin films. (a) The character N is written with σ # laser beam on the magnetization initially pointing downwards, and the character U is written with σ 6 laser beam on magnetization initially pointing upwards. (b) The character N is erased by σ 6 laser beam. (c) The character U is erased by σ # laser beam. Scale bar 20um Helicity-dependent all-optical switching with different repetition rates To demonstrate the pronounced HD-AOS effects of the hybrid CoPtAu thin films, I have conducted systematic optical experiments to explore the dependence of HD-AOS on different repetition rates and peak powers. In each experiment set, the sample magnetization is firstly saturated in one direction, namely, black contrast or white contrast, then with both σ 6 and σ # laser beams (with certain peak power and repetition rate) scan a line of 50 um on the sample with a speed of 100 um/s. The average power and repetition rate are kept constant for each experiment set, so that the switching effect will only be effected by the polarization of laser beams. To demonstrate the switching abilities of light beams with different polarizations, I define a parameter

19 18 s. c. = rstqu # ]wsxlyqu z{ utl} Tl}~Rz{U#rstQu # ]wsxlyqu z{ wysxq Tl}~Rz{U rstqu # ]wsxlyqu z{ utl} Tl}~Rz{U6rstQu # ]wsxlyqu z{ wysxq Tl}~Rz{U Equation 3 Thus, the amplitude and sigh of s. c. means the ability of the laser beams with different polarizations to switch magnetizations on the thin film samples. Larger amplitude presents better switching ability. Positive sign means switching effect on magnetization pointing downwards while negative sign means switching effect on magnetization pointing upwards. Each experiment set is repeated 5 times at random locations on the sample. In this section, I will study the switching behavior for laser beams with different repetition rates. The repetition rate varies from 10 khz to 312 khz. And the peak power of each single pulse is kept constant at P = = 7 10 W. This is calculated with P = 1.4 mw, f = 10 khz and σ = 200 fs. Then the peak power P = = r = ƒ 7 10 W. For better contrast, the experimental images are subtracted by the original images and processed with Matlab 2-D median filtering tools. A typical experiment results with f = 60 khz are shown in Figure 9. I notice that there is an ending switching point for each scan. This is due to the thermal accumulation at each ending point. This thermally induced switching is not related to helicity and should be the same for both σ 6 and σ # scans and won t contribute to the switching capability parameter. Therefore, I can directly count the number of pixels switched in each scan figures. For example, the experiment set in Figure 9, the number of pixels switched for σ 6 on black and white background is 1702 and 21917; while the number of pixels switched for σ # on black and white background is and By definition, I will get the switching capability for σ 6 and σ # at 60khz are s. c. ƒ = and s. c. ƒ =

20 19 Figure 9 The scan results for f=60khz experiment set. (a) The σ 6 laser beams scan with initial magnetization saturated downwards. (b) The σ 6 laser beams scan with initial magnetization saturated upwards. (c) The σ # laser beams scan with intial magnetization saturated downwards. (d) The σ # laser beams scan with initial magnetization saturated upwards. The images are post-procsessed with Matlab 2-D median filtering tools to suppress white noise. Follow the same image processing strategy, the switching capability of the hybrid CoPtAu thin film sample with repetition rates vary from 10 khz to 312 khz is measured. The experiment results for f = 60 khz, 100 khz, 200 khz are shown in Figure 10. The results are subtracted with the initial figure before laser scanning for better contrast. Figure 10 (a), (c) and (e) show the scanning results with initial magnetization pointing downwards (black background). Figure 10 (b), (d) and (f) show scanning results with initial magnetization pointing upwards (white background). From the results, I observe apparent HD-AOS

21 20 features for different repetition rates. The switching capability for each experiment can be calculated with equation 3. Figure 10 HD-AOS results with different repetition rates. (a) and (b) f = 60 khz, P = 8.4 mw. (c) and (d) f = 100 khz, P = 14.0 mw. (e) and (f) f = 200 khz, P = 28.0 mw. Apparent HD-AOS observed for different configurations. Scale bar 20 um. The switching capability with different repetition rates are demonstrated in Figure 11. Notice that each repetition rate is measured at 5 random locations on the sample to demonstrate the stability of the HD-AOS effect. The results show statistic median switching capability value and error bars. From Figure 11 I notice that the switching capability value increases from 10 khz to 50 khz and decreases from 50 khz to 125 khz, and finally becomes stable from 125 khz to 312 khz. This is because when the repletion rate of incident laser increase from 10 khz to 50 khz, the sample temperature is increased to a suitable temperature for the HD-AOS to happen [54]. Stable HD-AOS features from

22 21 50 khz to 312 khz demonstrate great flexibility over a large window of parameters that the hybrid CoPtAu thin film provides. Figure 11 Switching capability of the hybrid CoPtAu thin films with repetition rate varies from 10 khz to 312 khz. The value of switching capability increases from 10 khz to 50 khz, decreases from 50 khz and finally tends to be stable from 50 khz to 312 khz. Form 10 khz to 50 khz, the sample temperature is increased to a suitable range, thus facilitate the HD-AOS phenomenon Helicity-dependent all-optical switching with different peak powers In this section, I will focus on HD-AOS effect for the hybrid CoPtAu samples with different peak powers. I keep the repetition rate constant as 200 khz and change the peak powers. The average power varies from 19.0 mw to 57.0 mw. Figure 12 shows the switching results for P = 25.0 mw, 39.0 mw and 57.0 mw. The results are subtracted from the initial background image for better visualization. Figure 12 (a), (c) and (e) show the results for σ 6 and σ # with initially magnetization saturated downwards (black background), and Figure 12 (b), (d) and (f) show the results for σ 6 and σ # with initially magnetization

23 22 saturated upwards (white background). The shadows line (or gray dark) region for σ 6 scan in (a) and σ # scan in (b) demonstrate the thermal damage on the sample, due to the high intensity of laser beams. However, the switching results are still strongly related to helicity at such high power (thermal damage even happens). In this case, the number of pixels switched for σ 6 on black and white background is 1501 and 13675; while the number of pixels switched for σ # on black and white background is and By definition, I will get the switching capability for σ 6 and σ # at 60khz are s. c. ƒ = and s. c. ƒ = Figure 12 Switching results different peak powers with the same repetition rate f = 200 khz. (a) and (b) P = 25.0 mw. (c) and (d) P = 39.0mw. (e) and (f) P = 57.0 mw. Apparent HD-AOS features are observed. Notice that clear HD-AOS effects exist even with thermal damage of sample surface with P = 57.0 mw. Scale bar 20um. Following similar image process strategy, I can calculate the switching capability for different peak powers as shown in Figure 13. One can clearly see that that the hybrid

24 23 CoPtAu thin film obtains a large window of average power that exist stable HD-AOS phenomenon. The results are shown in Figure 13. The hybrid CoPtAu thin film sample obtains stable HD-AOS phenomenon over a wide range of peak powers. This is a significant increase compared with the previous reported results in GdFeCo alloy films, which demonstrate HD-AOS only for a narrow window of the threshold fluence (estimated to be 1.5% [25]). The hybrid CoPt thin film sample exhibits robust HD-AOS effects even with high power that can cause thermal damage on the sample surface, which is drastically different from previously published work [25, 55]. Figure 13 Switching capability of the hybrid CoPtAu thin films with average power varies from 19.0 mw to 57.0 mw at 200 khz. The hybrid CoPtAu thin film shows stable HD-AOS phenomenon over a larger range of average powers. Stable HD-AOS effects exist even within thermal damage region Switching with single pulses In this section I will describe the switching results with single pulses to reveal the underlying mechanism of HD-AOS in the hybrid CoPtAu thin films structures. The repetition rate of the laser pulses is reduced from 10 khz to 100 hz with a chopper. The

25 24 chopper setup is illustrated in Figure 14. Figure 14 (a) shows the optical setup for the single pulse experiment. Beam shrinking is necessary because the slot s width is smaller than the laser diameter, which is realized by a pair of lenses. The photodiode is used to detect the repetition rate of the laser pulses modulated by the chopper. The chopper is MC1F60 from Thorlabs, with 60 slot blades. To reduce the repetition rate of the laser pulses, I need to manually block 59 slots, only leave 1 slot open for the laser beam to pass through (Figure 14 (b)). With synchronization between the laser pulses and the chopper motor, the repetition rates of laser pulses can be reduced from 10 khz to 100 hz, as the chopper motor MC2000 runs at 100 hz itself. By moving the sample stage at the velocity of v = 7 mm/s, each single pulse shoot onto the sample with different locations. Figure 14 Chopper setup for single pulse experiment. (a) The optical setup for single pulse experiment. Beam sink is realized with a pair of lenses. (b) To reduce the repetition rate of the laser pulses from 10 khz to 100 hz, MC1F60 chopper blades are manually modified with only 1 slot open. The single pulse experiment results are demonstrated in Figure 15. The average power of the 100 hz laser pulses is uw, and hence the peak power is P = = r ƒ = W. The switching results for σ 6 and σ # pulses with initial magnetization pointing downwards

26 25 and upwards are presented in Figure 15 (a) and (b) correspondingly. The results show a demagnetized spot regardless of helicity of the incident laser pulses, thus no apparent HD- AOS phenomenon is observed. This results agree with a recent research work claiming that the HD-AOS switching mechanism in RE-TM systems and CoPt multilayers are different. For the CoPt multilayers, a certain number of pulses is required to achieve HD-AOS phenomenon [55]. Our results indicate that this observation is also true for the hybrid CoPtAu thin films. This phenomenon is new and has not been fully understood at the current stage. Figure 15 Single pulse experiment results for hybrid CoPtAu sample. (a) The switching results with one σ 6 pulse on magnetization pointing downwards and upwards. (b) The switching results with one σ # pulse on initial magnetization pointing upwards and downwards. The switching results indicate no helicity dependence, which proves that a certain number of pulses is required to achieve HD-AOS for the hybrid CoPtAu thin films. Scale bar 20um. 3.2 Helicity-dependent all-optical switching with plasmonic structures Ever since 2007, significant progresses have been achieved in HD-AOS experiments with magnetic materials with new materials and structures. However, there are still some issues

27 26 for HD-AOS to be commercialized. Firstly, in current research the required laser intensity is quite high considering practical applications. Secondly, the laser beam diameter, thus the size of the data bits, cannot be reduced to less than one nanometers because of the fundamental diffraction limit of classical electromagnetics. Take the laser beams diameter as 50 um (which is the focused laser spot size in our experiment setup) for estimation, the data density will be bit/inch %. Compared with the heat-assistant magnetic recording (HAMR) technology, which provide the data density as ~375 Tb/m % [56], the data density of current HD-AOS experiment results is too small for industrial use. To reduce the bit size of HD-AOS recording, we integrate plasmonic structures on the magnetic materials, resulting the plasmonic-enhanced all-optical switching (PE-AOS) effect. Taking advantage of the pronounced plasmonic effect of metallic nanostructures, the power consumption can be decreased to folds, and the data density can be increased to Tb/m % level. This PE-AOS effect provides transformative technology for next-generation, high-density magnetic storage systems. Plasmonics research has become an extremely vibrant and successfully subarea in optics. Photons can excite and couple with the collective electron oscillations, producing surface plasmon polaritons (SPPs). At a semi-infinite metal-dielectric interface, SPPs behave as surface waves that propagate along the interface while exponentially decaying into both the dielectric and metal, as shown in Figure 16 (a). The wavelength of SPPs is always smaller than that of the propagating light in the dielectric medium at the same frequency. By further reducing the geometric dimensions (such as metallic nanoparticles, demonstrated in Figure 16 (b)), SPPs can be confined into three-dimensional space beyond the diffraction limit and the local intensity ( E % ) can be enhanced up to times

28 27 compared to the incidence intensity. The unique properties of SPPs, i.e., sub-diffractionlimited confinement and strong field enhancement, have led breakthroughs in superresolution imaging [57, 58] and lithography [59], biomedical sensing [60, 61], novel optical devices [62-64] and energy harvesting[65, 66]. For instance, the plasmonic Luneburg lenses are designed to focus SPPs [67], giant photobleaching suppression and fluorescence emission induced by plasmonic nanocativies [68], and a tunable optical antenna to radiate directional light [69]. As a result, combining the plasmonic-enhancement effect and HD- AOS phenomenon, the data density will show over two orders of magnitude improvement compared with previous reports. Furthermore, the local field intensity can be drastically enhanced to over one order of magnitude thanks to the field confinement. These two unique properties of surface plasmons manifest the possibility to solve the areal density and power consumption in HD-AOS process. Figure 16 (a) Illustration of SPPs arising from the collective electron oscillation. (b) Schematic of surface plasmons supported by a metallic nanoparticle showing strong electric field enhancement factor compared with the incidence. In this project, I explore the PE-AOS phenomenon by integrating the hole arrays on CoPt multilayer thin films. The plasmonic enhancement effect of metallic hole arrays was reported a long time ago [70, 71]. In these patterned nanostructures, due to the coupling between light and plasmons (the electronic excitations) on the surface of the periodically

29 28 patterned metallic film, unusual zero-order transmission spectra at the wavelengths larger than the array period will occur, with sharp peaks in the transmission spectrum. The hole array samples are expected to provide plasmonic-enhancement phenomenon at ~800 nm. As shown in Figure 17. In Figure 17 (a), the circularly polarized light in incident on the sample, which then induces the magnetization vectors pointing either upwards or downwards depending on helicity. Due to the local resonance at the interface between gold and magnetic materials, the diameter of the spot can be as small as tens of nanometers. Furthermore, as simulated in Figure 17 (b), the electric field is confined to the interface between the two media, and the field amplitude shows an enhancement of over fifteen times. This indicates that the required power can be reduced at least one hundred times. Figure 17 (c) shows the scanning electron microscope (SEM) image for hole-array nanostructures fabricated. Figure 17 Sample for the PE-AOS effect. (a) The schematic view of the sample, with hole arrays integrated on the top of the CoPt multilayers. Circularly polarized light can be tightly focused by the nano holes with diameter of about 50 nm milled through the gold film. The magnetization of the underneath magnetic materials can be flipped. (b) Simulated electric field amplitude within a single hole, showing the plasmonicenhancement effect. (c) The SEM image showing the hole arrays fabricated via electrical beam lithography (EBL). Scale bar 1um.

30 29 The hole array samples have a period of 450 nm and the diameter 175 nm with 50 nm gold thickness. The measurement results of the hole array samples are shown in Figure 18. Figure 18 (a) shows an overview of the experimental system. The sample is mounted on a 3D translate stage, with illuminate light from both sides. Flip mirror1 is install to switch between transmission and reflection measurements. Flip mirror2 is installed to switch between the spectrometer and the camera. Figure 18 (b) shows the transmission measurement of the hole array samples. The results are normalized with 50 nm pure Au films. From the transmission spectrum, I can observe strong resonance features at 800 nm, which corresponds to the laser incident wavelength. Figure 18 The experimental system for hole array spectrum measurement. (a) Experimental system for transmission measurement. Flip mirror1 enables the system to switch between transmission and reflection measurements. Flip mirror2 switches between the spectrometer and camera. (b) Measured transmission spectrum of the hole array samples. The results are normalized with transmission spectrum from 50 nm Au films. The experiment results the CoPt multilayer thin films integrated with hole array samples are shown in Figure 19. Figure 19 (a) demonstrates the MOKE images for the CoPt multilayers integrated with hole array samples. Notice that in this experiment the

31 30 magnetization direction is opposite to the previous results for better contrast in the MOKE images, namely, with black and white contrasts present magnetizations pointing upwards and downwards correspondingly. The PE-AOS experiment is performed with σ 6 and σ # laser beams scan across the border between the pattern and unpatterned areas average power 1.51 mw and repetition rate 10 khz at 800 nm wavelength. The results with initial magnetization pointing upwards and downwards are presented in Figure 19 (b) and (c) accordingly. As expected, the laser power is not strong enough to flip the magnetization in the unpatterned area, while apparent flipping results are observed in the flipped areas. This phenomenon is strong indication of the plasmonic-enhancement in such an all-optical switching effect. From Figure 19 (b) with initial magnetization pointing upwards, σ 6 laser pulses can generate magnetization pointing downwards, while σ # laser beams cause certain kinds of thermally damage results on hole array samples. For Figure 19 (c) the trend is opposite with initial magnetization pointing downwards. These results demonstrate that the plasmonic-enhanced switching is also related to helicity of the incident laser beams. Due to the diffraction limit, the nanoscale HD-AOS in the hole areas cannot be resolved under the MOKE microscope. In the future, I plan to employ other characterization techniques, such as magnetic force microscopy, magnetic circular and magnetic linear dichroism in the x-ray domain (XMCD and XMLD, respectively), that provide nanometer spatial resolution.

32 31 Figure 19 Experiment results for plasmonic-enhanced all-optical switching (PE-AOS) effects, with the laser scan across the border between patterned and unpatterned areas. (a) MOKE image of the CoPt multilayer integrated with hole array samples, with black and white contrasts present magnetization pointing upwards and downwards accordingly. (b-c) The laser scan across the border between unpatterned and patterned areas for σ 6 and σ # laser beams with initial magnetization pointing upwards and downwards. For the unpatterned area, no magnetization reversal is observed, while for the patterned area, helicity dependent switching is observed. The results demonstrate clear plasmonic-enhanced helicity-dependent switching effect. Scale bar 20um. 3.2 Multiphsyics modeling of the helicitydependent all-optical switching effect In this section, I will present a Multiphysics modeling scheme to numerically simulate HD- AOS to better understand the underlying mechanism. Up to now the underline mechanism for HD-AOS is still under debate [19, 20, 23, 25-28, 32, 33, 54, 55, 72-82]. In this thesis, I utilize a macroscopic three-temperature model to describe the HD-AOS effect observed in hybrid CoPtAu thin film systems [22, 27, 72, 80, 83]. To model the effect of a laser pulse

33 32 on a magnetic medium, assume a two-fold impact on the medium. Firstly, the laser pulse with certain fluence will dramatically increase the sample temperature via a threetemperature mechanism [72, 80], the laser power dissipated into the medium can be calculated from the absorbed electromagnetic field [84-86]; secondly, due to the inverse Faraday effect, the circularly polarized laser pulse will generate an effective magnetic field B Œ along the transport direction [53, 87, 88]. In the following sections, I will introduce the modelling results for opto-thermal and opto-magnetic effects of a laser pulse on the sample. I will also discuss the HD-AOS results simulated from the model and compare it with the experiment results. The three-temperature model consists of three thermalized reservoirs that can exchange energy, namely, the electron system with temperature T Q, the phonon system with temperature T \ and the spin system with temperature T ]. Through derivation, the spin system temperature leads to the magnetization m = M/M ] of the system [72, 80]. Therefore, the model gives a set of coupled differential equations for the electron temperature T Q and phonon temperature T \ and the magnetization m: C Q Ž x = g Q\ T \ T Q + Q Q κ T Q T T C \ Ž a x = g Q\ T Q T \ = Ž a m + B x Ž Q 1 m coth Ž Ž m + B Q Equation 4 where the electron specific heat capacity C Q increases linearly with the electron temperature T Q, that is, C Q = γt Q, g Q\ presents the coupling constant between the electronphonon systems, P is the input power by heat dissipation from the laser pulse, κ is the heat diffusion constant with the surrounding environment, in our model the diffusion effect with

34 33 environment is neglected. The demagnetization rate R can be calculated as R = b Ž š ~ a } œ Ž ž b. The simulation parameters are listed in Table 1 [80]. From the macroscopic three-temperature model, I notice that there are two sources in the model. The first one is the power dissipated from the incident laser into the system, presented as the Q Q term in the first equation. The second is the effective magnetic field B Q in the third equation, which can be derived from the inverse Faraday effect. As will be discussed in later sections. Table 1 Parameters used in the macroscopic three-temperature model. Most parameters are obtained from [80]. Parameters Description Value g Q\,Ÿzrx e-p coupling constant for CoPt $ W/ m 4 K g Q\, e-p coupling constant for Au 4 10 $ W/ m 4 K k Q,Ÿzrx Electron conductivity in CoPt 61.5 W/ mk k Q, Electron conductivity in Au W/ mk σ =,Ÿzrx CoPt conductivity S/m σ =, Au conductivity S/m k \,Ÿzrx CoPt thermal conductivity from Wiedemann-Franz law 5 W/ mk k \, Au thermal conductivity from Wiedemann-Franz law 9.91 W/ mk k \, s Thermal conductivity for fused silica substrate 5 W/ mk ρ s Density of the fused silica substrate g/cm 4 C \,Ÿzrx Phonon heat capacity in CoPt 3 10 J/ kgk C \, Phonon heat capacity in Au J/ kgk C \, s Phonon heat capacity in fused silica J/ kgk γ Q\,Ÿzrx CoPt electron thermal capacity coefficient 655 J/ m 4 K %

35 34 γ Q\, Au electron thermal capacity coefficient 67.6 J/ m 4 K % R Demagnetization rate $% s #$ T l Curie temperature 550 K The electron and phonon temperatures induced by laser dissipated power The value of the dissipated power from laser can be calculated through the coupling between the electromagnetic model and heat transfer model in COMSOL Multiphysics. The laser has a central wavelength of λ = 800 nm, the radius of the laser beam W = = 1 mm, the focus length is f = 100 mm. The laser beam size can be estimated from the following equation: W = f == { f = 100 mm = 25.5 um Equation 5 ª «$ The electric field calculated from the laser beam parameters will be: E = = %r «l ƒ R Equation 6 whereε = = F/m is the vacuum permittivity, and c = 3 10 m/s is the speed of light. The repetition rate of the laser pulses f = 200 khz, the average power P = 12.5 mw and the laser duration is σ = 200 fs. The parameters are from the experiment with hybrid CoPtAu thin films. Take those numbers into the equation, I will get E = = V/m. This value is quite high compared with conventional continuous lasers because the pulsed femtosecond laser is used here. For the hybrid CoPtAu thin films, I have the Au layer thickness d = 30nm and refractive index n = i; for the CoPt layer, I have the thickness d Ÿzrx = 10nm. For the refractive index, I assume the weighted

36 35 average for both Co and Pt in the thin film structures with 4/11 Co and 7/11 Pt. The refractive indexes of Co and Pt at 800nm are n Ÿz = i and n rx = i, so that n Ÿzrx = $$ n Ÿz + refractive index n ] = $$ n rx = i. The substrate In the electromagnetic wave model, the incident light is modeled as a circularly polarized light with amplitude unit amplitude, namely, E = $ x + s y, to prevent potential % % convergence issue in COMSOL. Because only linear responses are considered in this case, the incident power term E = is treated in the heat transfer model with a scale factor of E = %, so that the dissipated power is calculated as the Joule heating in the medium with Q Q= = $ Re J % UE E % =, where the displacement current J U = σe [85, 86]. This Joule heating works as heat source for the electron temperature, and it follows a Gaussian temporal trend (same as the laser pulse). This thermal pulse induced by incident laser can be presented with the following equation: Q Q = Q Q= exp x %ƒ Equation 7 The thermal pulse temporal and corresponding electron and phonon temperatures distribution is plotted in Figure 20. In Figure 20 (a) the Gaussian temporal profile is presented. From three-temperature model equations, I know that after the thermal pulse incidence, the electron system will firstly be heated up due to the term Q Q. Then the electron and phonon systems couple with each other through the term g Q\ and finally get a balanced state, as plotted in Figure 20 (b).

37 36 Figure 20 The thermal pulse induced by laser dissipated power and the corresponding electron and phonon temperatures. (a) The normalized laser pulse shows a temporal Gaussian profile. (b) The electron and phonon temperatures induced by the thermal pulse. The electron temperature T Q rises after the thermal pulse incident. Electron system reaches a maximum temperature T Qu within 1 ps. Then due to the coupling with the phonon system, the electron temperature gradually decreases while the phonon temperature increases. The two systems finally come to a balanced state The magnetization induced by effective magnetic pulse The opto-magnetic field can be calculated through the inverse Faraday effect [25, 27] using the following equation: B Œ = βε = E E Equation 8 where the β is the magneto-optical susceptibility and can be calculated as β = ³ Œ «. ε tv is the off diagonal parameter that contributes to the Faraday or Kerr rotation. ε tv for Co ε tv = i [89]; M = is the magnetization of the magnetic medium, and for Co is M = = 5 10 A/m. In some existing three-temperature modeling of the HD-AOS effect, people have assumed a potentially existed decay of the opto-magnetic field with respect to the laser pulse [24, 27] in the HD-AOS phenomenon. In our simulation, I assume this decay

38 37 of the opto-magnetic field τ UQlTV = 400 fs [27]. Therefore, the opto-magnetic field can be written as: B = B = exp x, t < τ % x# B = exp, t > τ % 6 ¹ Equation 9 Figure 21 presents the opto-magnetic field and corresponding magnetizations with σ 6, σ # and π beams. Figure 21 (a) shows the experiment results for the hybrid CoPtAu thin film sample. The laser repetition rate is f = 200 khz and the average power is P = 12.5 mw. Figure 21 (b) presents the simulated results for the magnetization reversal with three different polarizations. The results indicate that σ 6 and π laser pulses cannot trigger magnetization reversal, while σ # laser pulse clear switches the magnetization, after about 0.12 ps. The three-temperature model shows good agreement with the experiment results. Figure 21 Experiment and simulated results for the HD-AOS phenomenon of the hybrid CoPtAu thin films. (a) The experiment results of the hybrid CoPtAu thin films: σ 6 and σ # beams scanning with initial magnetization pointing downwards. The laser repetition rate f = 200 khz and the average power P = 12.5 mw. The image is subtracted with original image for better contrast. (b) The simulated results for the effect of three polarizations (σ 6, σ # and π) on magnetizations. The results show clear HD-AOS phenomenon