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

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Table of Contents

Abstract ............................................................................................................................... 2

1 Introduction ...................................................................................................................... 4

2 Experimental system ........................................................................................................ 8

2.1 Magneto-optical Kerr microscopy setup ................................................................... 8

2.2 Laser incident system .............................................................................................. 11

2.3 Sample deposition ................................................................................................... 123 Helicity-dependent all-optical switching results (HD-AOS) ......................................... 14

3.1 Helicity-dependent all-optical switching with hybrid CoPtAu sample ................... 15

3.1.1 Helicity-dependent all-optical switching with different repetition rates .......... 17

3.1.2 Helicity-dependent all-optical switching with different peak powers .............. 21

3.1.3 Switching with single pulses ............................................................................ 23

3.2 Helicity-dependent all-optical switching with plasmonic structures ...................... 25

3.2 Multiphsyics modeling of the helicity-dependent all-optical switching effect ....... 31

3.3.1 The electron and phonon temperatures induced by laser dissipated power ..... 343.3.2 The magnetization induced by effective magnetic pulse .................................. 36

4 Summary and Future Work ............................................................................................ 38

4.1 Summary ................................................................................................................. 38

4.2 Future work ............................................................................................................. 39

Reference .......................................................................................................................... 41

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

1956. 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 sub-

picosecond (10#$%𝑠) level, while with traditional recording method the deterministic

switching does not take place shorter than 20𝑝𝑠 [3]. It’s worth noting that the direction of

magnetization is related to the helicity of the incident laser pulses. Namely, the right-

handed polarized laser pulses will generate magnetization pointing in one direction while

left-handed polarized laser pulses generate magnetization pointing in the other direction.

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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 metal-

ferromagnetic thin films. Different materials and plasmonic nanostructures are also

proposed as further work.

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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

20𝑝𝑠 [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 100-1000 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

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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 0.4𝑛𝑚 /𝑃𝑡 0.7𝑛𝑚 4multilayers in response to

different laser polarizations [33].

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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 magneto-

optical microscope. One can see from Figure 1 (b) that each of the right-handed polarized

(𝜎6 ) 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 B89 = 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

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momentum [44, 45], the formation of a transient ferromagnetic state [46], and laser-

induced superdiffusive spin currents [47, 48].

In this thesis, I experimentally demonstrated the HD-AOS properties of hybrid metal-

ferromagnet 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

(10𝑘ℎ𝑧~312𝑘ℎ𝑧 ) and peak powers (95𝑛𝐽~285𝑛𝐽 , with repetition rate 200𝑘ℎ𝑧 ).

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.

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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

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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 𝜃PQRR . 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 𝜃STRTUTV 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= 00 0 𝜖=

Equation 1

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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 non-

diagonal Fresnel coefficients induces the polarization conversion. The reflectivity matrix

will be:

𝑟 =𝑟\\ 𝑟\] 𝜅𝑟]\ 𝜅 𝑟]]

Equation 2

The reflectivity gives the rotation angle as 𝜃 + 𝑖𝜙 = Rab cRaa

.

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,

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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 = 800𝑛𝑚 . The pulse duration is 200𝑓𝑠 and the repetition rate can be

varied from 10𝑘ℎ𝑧 to 312𝑘ℎ𝑧. 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 𝑓f = 100𝑚𝑚. Consider that each pulse has the Gaussian intensity

profile, the focused radius is estimated to be 𝑟 = 25.4𝑢𝑚. 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.

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2.3 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 0.7 4𝑇𝑎 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

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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 𝐻l of hybrid CoPtAu thin films is smaller than the pure CoPt thin films.

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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 (𝜎6) and 𝜃 = 135° will generate left-

handed 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 = 25.4𝑢𝑚 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 𝜎6 and 𝜎# laser beams on

magnetization pointing downwards and upwards correspondingly. Otherwise, the

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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 0.7 4𝑇𝑎 3 𝐴𝑢(30). The MOKE image and

corresponding laser scan results are shown in Figure 7. The laser beams have a repetition

rate 𝑓 = 200𝑘ℎ𝑧 and the average power 𝑃 = 15.06𝑚𝑤. 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

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(𝑀6) correspondingly. (b) Right-handed polarized 𝜎6 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 𝜎6 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

𝜎6 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 𝜎6 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 𝜎6 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 𝜎6 and 𝜎# clearly erase the character ‘N’ and ‘U’. This results

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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 𝜎6

laser beam on magnetization initially pointing upwards. (b) The character ‘N’ is erased by 𝜎6 laser beam.

(c) The character ‘U’ is erased by 𝜎# laser beam. Scale bar 20um.

3.1.1 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 𝜎6 and 𝜎# laser beams (with certain peak power and repetition rate) scan a

line of 50𝑢𝑚 on the sample with a speed of 100𝑢𝑚/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

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𝑠. 𝑐. = rstQu#]wsxlyQUz{|uTl}|Tl}~Rz�{U#rstQu#]wsxlyQUz{wysxQ|Tl}~Rz�{UrstQu#]wsxlyQUz{|uTl}|Tl}~Rz�{U6rstQu#]wsxlyQUz{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 10𝑘ℎ𝑧 to 312𝑘ℎz. And the peak power of each

single pulse is kept constant at 𝑃= = 7×10�𝑊. This is calculated with 𝑃 = 1.4𝑚𝑤, 𝑓 =

10𝑘ℎ𝑧 and 𝜎 = 200𝑓𝑠. Then the peak power 𝑃= =r��= 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 = 60𝑘ℎ𝑧 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 𝜎6 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 𝜎6 on black and white background is 1702 and 21917; while the

number of pixels switched for 𝜎# on black and white background is 19306 and 1346. By

definition, I will get the switching capability for 𝜎6 and 𝜎# at 60khz are 𝑠. 𝑐.�� =

−0.8559 and 𝑠. 𝑐.�� = 0.8696.

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Figure 9 The scan results for f=60khz experiment set. (a) The 𝜎6 laser beams scan with initial magnetization

saturated downwards. (b) The 𝜎6 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 10𝑘ℎ𝑧 to 312𝑘ℎ𝑧 is measured. The

experiment results for 𝑓 = 60𝑘ℎ𝑧, 100𝑘ℎ𝑧, 200𝑘ℎ𝑧 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

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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 = 60𝑘ℎ𝑧, 𝑃 = 8.4𝑚𝑤. (c) and (d)

𝑓 = 100𝑘ℎ𝑧, 𝑃 = 14.0𝑚𝑤 . (e) and (f) 𝑓 = 200𝑘ℎ𝑧, 𝑃 = 28.0𝑚𝑤 . Apparent HD-AOS observed for

different configurations. Scale bar 20𝑢𝑚.

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 10𝑘ℎ𝑧 to 50𝑘ℎ𝑧 and decreases from 50𝑘ℎ𝑧 to 125𝑘ℎz,

and finally becomes stable from 125𝑘ℎ𝑧 to 312𝑘ℎz. This is because when the repletion

rate of incident laser increase from 10𝑘ℎ𝑧 to 50𝑘ℎ𝑧 , the sample temperature is increased

to a suitable temperature for the HD-AOS to happen [54]. Stable HD-AOS features from

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50𝑘ℎ𝑧 to 312𝑘ℎ𝑧 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 10𝑘ℎ𝑧 to

312𝑘ℎ𝑧. The value of switching capability increases from 10𝑘ℎ𝑧 to 50𝑘ℎ𝑧, decreases from 50𝑘ℎ𝑧 and

finally tends to be stable from 50𝑘ℎ𝑧 to 312𝑘ℎ𝑧 . Form 10𝑘ℎ𝑧 to 50𝑘ℎ𝑧 , the sample temperature is

increased to a suitable range, thus facilitate the HD-AOS phenomenon.

3.1.2 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 200𝑘ℎ𝑧 and change the peak powers.

The average power varies from 19.0𝑚𝑤 to 57.0𝑚𝑤 . Figure 12 shows the switching

results for 𝑃 = 25.0𝑚𝑤, 39.0𝑚𝑤𝑎𝑛𝑑57.0𝑚𝑤 . The results are subtracted from the

initial background image for better visualization. Figure 12 (a), (c) and (e) show the results

for 𝜎6 and 𝜎# with initially magnetization saturated downwards (black background), and

Figure 12 (b), (d) and (f) show the results for 𝜎6 and 𝜎# with initially magnetization

22

saturated upwards (white background). The shadows line (or gray dark) region for 𝜎6 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 𝜎6 on black and white background is 1501 and 13675; while the number of

pixels switched for 𝜎# on black and white background is 26895 and 4924. By definition, I

will get the switching capability for 𝜎6 and 𝜎# at 60khz are 𝑠. 𝑐.�� = −0.8022 and

𝑠. 𝑐.�� = 0.6905.

Figure 12 Switching results different peak powers with the same repetition rate 𝑓 = 200𝑘ℎ𝑧. (a) and (b)

𝑃 = 25.0𝑚𝑤. (c) and (d) 𝑃 = 39.0𝑚𝑤. (e) and (f) 𝑃 = 57.0𝑚𝑤. Apparent HD-AOS features are observed.

Notice that clear HD-AOS effects exist even with thermal damage of sample surface with 𝑃 = 57.0𝑚𝑤.

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

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 19.0𝑚𝑤 to

57.0𝑚𝑤 at 200𝑘ℎ𝑧. 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.

3.1.3 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 10𝑘ℎ𝑧 to 100ℎ𝑧 with a chopper. The

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 10𝑘ℎ𝑧 to 100ℎ𝑧, as the chopper motor

MC2000 runs at 100ℎ𝑧 itself. By moving the sample stage at the velocity of 𝑣 = 7𝑚𝑚/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 10𝑘ℎ𝑧 to

100ℎ𝑧, 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 100ℎ𝑧 laser pulses is 16.58𝑢𝑤, and hence the peak power is 𝑃= =r��= 8.29×10�𝑊.

The switching results for 𝜎6 and 𝜎# pulses with initial magnetization pointing downwards

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 𝜎6

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

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 50𝑢𝑚 (which is the focused laser spot size in our experiment setup) for estimation, the

data density will be 3.29×10�𝑏𝑖𝑡/𝑖𝑛𝑐ℎ% . Compared with the heat-assistant magnetic

recording (HAMR) technology, which provide the data density as ~375𝑇𝑏/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 10 − 100 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 100 − 1000 times

27

compared to the incidence intensity. The unique properties of SPPs, i.e., sub-diffraction-

limited confinement and strong field enhancement, have led breakthroughs in super-

resolution 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

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 ~800𝑛𝑚.

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 50𝑛𝑚 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 plasmonic-

enhancement effect. (c) The SEM image showing the hole arrays fabricated via electrical beam lithography

(EBL). Scale bar 1um.

29

The hole array samples have a period of 450𝑛𝑚 and the diameter 175𝑛𝑚 with 50𝑛𝑚

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 50𝑛𝑚 pure Au

films. From the transmission spectrum, I can observe strong resonance features at 800𝑛𝑚,

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 50𝑛𝑚 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

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 𝜎6 and 𝜎#

laser beams scan across the border between the pattern and unpatterned areas average

power 1.51𝑚𝑤 and repetition rate 10𝑘ℎ𝑧 at 800𝑛𝑚 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, 𝜎6 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.

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 𝜎6 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 helicity-

dependent 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

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 three-

temperature 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�� 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 𝑇Q , 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 𝑇Q and phonon temperature 𝑇\ and the magnetization 𝑚:

𝐶Q����x= 𝑔Q\ 𝑇\ − 𝑇Q + 𝑄Q − 𝜅 𝑇Q − 𝑇T�|

𝐶\��a�x= 𝑔Q\ 𝑇Q − 𝑇\

���x= ��a

��𝑚 + 𝐵Q�� 1 − 𝑚 𝑐𝑜𝑡ℎ ��

��𝑚 + 𝐵Q��

Equation 4

where the electron specific heat capacity 𝐶Q increases linearly with the electron

temperature 𝑇Q, that is, 𝐶Q = 𝛾𝑇Q, 𝑔Q\ presents the coupling constant between the electron-

phonon 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

33

environment is neglected. The demagnetization rate 𝑅 can be calculated as 𝑅 = ��b����~�a

}�����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 𝑄Q term in the first equation. The second is the effective magnetic field

𝐵Q�� 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

𝑔Q\,�zrx e-p coupling constant for CoPt 2.5×10$�𝑊/ 𝑚4𝐾

𝑔Q\,¡� e-p coupling constant for Au 4×10$¢𝑊/ 𝑚4𝐾

𝑘Q,�zrx Electron conductivity in CoPt 61.5𝑊/ 𝑚𝐾

𝑘Q,¡� Electron conductivity in Au 304.1𝑊/ 𝑚𝐾

𝜎=,�zrx CoPt conductivity 0.91823×10£𝑆/𝑚

𝜎=,¡� Au conductivity 4.54545×10£𝑆/𝑚

𝑘\,�zrx CoPt thermal conductivity from Wiedemann-Franz law 5𝑊/ 𝑚𝐾

𝑘\,¡� Au thermal conductivity from Wiedemann-Franz law 9.91𝑊/ 𝑚𝐾

𝑘\,¥s Thermal conductivity for fused silica substrate 5𝑊/ 𝑚𝐾

𝜌¥s Density of the fused silica substrate 2.203𝑔/𝑐𝑚4

𝐶\,�zrx Phonon heat capacity in CoPt 3×10¢𝐽/ 𝑘𝑔𝐾

𝐶\,¡� Phonon heat capacity in Au 2.49×10�𝐽/ 𝑘𝑔𝐾

𝐶\,¥s Phonon heat capacity in fused silica 753.94𝐽/ 𝑘𝑔𝐾

𝛾Q\,�zrx CoPt electron thermal capacity coefficient 655𝐽/ 𝑚4𝐾%

34

3.3.1 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 = 800𝑛𝑚, the radius of the laser beam 𝑊= =

1𝑚𝑚, the focus length is 𝑓 = 100𝑚𝑚. The laser beam size can be estimated from the

following equation:

𝑊=f ≅ ¨

©ª«𝑓 = �=={�

©×$��×100𝑚𝑚 = 25.5𝑢𝑚 Equation 5

The electric field calculated from the laser beam parameters will be:

𝐸= =%r

¬«l�� ©R� Equation 6

where𝜖= = 8.854𝐹/𝑚 is the vacuum permittivity, and 𝑐 = 3×10�𝑚/𝑠 is the speed of

light. The repetition rate of the laser pulses 𝑓 = 200𝑘ℎ𝑧, the average power 𝑃 = 12.5𝑚𝑤

and the laser duration is 𝜎 = 200𝑓𝑠. The parameters are from the experiment with hybrid

CoPtAu thin films. Take those numbers into the equation, I will get 𝐸= = 3.40×10�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¡� = 0.1886 + 4.7053𝑖; for the CoPt layer,

I have the thickness 𝑑�zrx = 10𝑛𝑚 . For the refractive index, I assume the weighted

𝛾Q\,¡� Au electron thermal capacity coefficient 67.6𝐽/ 𝑚4𝐾%

𝑅 Demagnetization rate 9.4×10$%𝑠#$

𝑇l Curie temperature 550𝐾

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�z = 2.4880 + 4.8024𝑖 and 𝑛rx =

2.8582 + 4.9617𝑖 , so that 𝑛�zrx =¯$$𝑛�z +

£$$𝑛rx = 2.7235 + 4.9038𝑖 . The substrate

refractive index 𝑛]�| = 1.4533.

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 𝑄Q= =

$%𝑅𝑒 𝐽U𝐸∗ 𝐸=% , where the displacement current 𝐽U = 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:

𝑄Q = 𝑄Q= 𝑒𝑥𝑝 − x�

%�� 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 𝑄Q. Then the electron

and phonon systems couple with each other through the term 𝑔Q\ and finally get a balanced

state, as plotted in Figure 20 (b).

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 𝑇Q rises after the thermal pulse

incident. Electron system reaches a maximum temperature 𝑇Qu∗ within 1𝑝𝑠. 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.

3.3.2 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�� = 𝛽𝜖= 𝐸×𝐸∗ Equation 8

where the 𝛽 is the magneto-optical susceptibility and can be calculated as 𝛽 = ¬³´�«

. 𝜖tV is

the off diagonal parameter that contributes to the Faraday or Kerr rotation. 𝜖tV for Co

𝜖tV = 1.3 + 0.06𝑖 [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

37

of the opto-magnetic field 𝜏UQlTV = 400𝑓𝑠 [27]. Therefore, the opto-magnetic field can

be written as:

𝐵 =𝐵=𝑒𝑥𝑝 − x�

%¶�, 𝑡 < 𝜏

𝐵= 𝑒𝑥𝑝 − x#¶ �

% ¶6¶¸��¹´� , 𝑡 > 𝜏

Equation 9

Figure 21 presents the opto-magnetic field and corresponding magnetizations with 𝜎6, 𝜎#

and 𝜋 beams. Figure 21 (a) shows the experiment results for the hybrid CoPtAu thin film

sample. The laser repetition rate is 𝑓 = 200𝑘ℎ𝑧 and the average power is 𝑃 = 12.5𝑚𝑤.

Figure 21 (b) presents the simulated results for the magnetization reversal with three

different polarizations. The results indicate that 𝜎6 and 𝜋 laser pulses cannot trigger

magnetization reversal, while 𝜎# laser pulse clear switches the magnetization, after about

0.12𝑝𝑠. 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: 𝜎6 and 𝜎# beams scanning with initial

magnetization pointing downwards. The laser repetition rate 𝑓 = 200𝑘ℎ𝑧 and the average power 𝑃 =

12.5𝑚𝑤. The image is subtracted with original image for better contrast. (b) The simulated results for the

effect of three polarizations (𝜎6, 𝜎# and 𝜋) on magnetizations. The results show clear HD-AOS phenomenon

38

with 𝜎6 and 𝜋 magnetization reversal not observed, while with 𝜎# magnetization reversal observed. Scale

bar 20um.

4 Summary and Future Work

4.1 Summary

In this master thesis work, I have built a customized system for HD-AOS experiment. The

system is consisted of a polar MOKE microscope and a femtosecond laser incident system.

The polar MOKE microscopy is capable of imaging magnetization pointing upwards and

downwards with white and black contrasts. The incident laser system provides circularly

polarized laser pulses at a central wavelength 𝜆 = 800𝑛𝑚. The repetition rate and laser

peak power of the pulses can be controlled.

In this thesis, a hybrid CoPtAu thin film structure for the HD-AOS phenomenon is

proposed and experimentally demonstrated for the first time. This structure has good HD-

AOS switching capability (𝑠. 𝑐.) with a wide range of repetition rate 𝑓 = 10𝑘ℎ𝑧~312𝑘ℎ𝑧

and shows robust HD-AOS switching capability with average power 𝑃 =

19.0𝑚𝑤~57.0𝑚𝑤 at 𝑓 = 200𝑘ℎ𝑧. The all-optical switching phenomenon is apparent

even at high power that induces thermal damage to the sample surface. I perceive that such

a stable HD-AOS switching capability is suitable for future magnetic data storage. Single

pulse experiment is also performed with the hybrid CoPtAu samples, which indicates a

different all-optical switching mechanism with the well-studied RE-TM samples [55]. I

explore the PE-AOS phenomenon by integrating patterned hole arrays on the CoPt sample.

Preliminary plasmonic-enhanced results are presented in this thesis.

39

Moreover, the HD-AOS switching phenomenon is studied by a Multiphysics model with

COMSOL. The model is based on the electromagnetic model, heat transfer in electron

systems, heat transfer in phonon systems and magnetization partial differential equations.

I demonstrate good agreement of the simulated results with the experiment results.

4.2 Future work

The hybrid CoPtAu thin film structure is proposed for the first time and has robust HD-

AOS switching capability for industrial use. However, the underlining mechanisms for the

additional Au layer is not clear. One potential explanations is the domain size criterion [90].

A recent work shows that HD-AOS happens when the laser spot size is smaller than the

magnetization domain size. The additional gold layer hybrid CoPtAu samples may help

increase larger domain size. Thus, the hybrid CoPtAu samples show robust HD-AOS

phenomenon. Another research shows that a low-saturated magnetization is necessary for

HD-AOS effect [54]. From the VSM measurements in Figure 6, the hybrid CoPtAu

samples do have smaller saturated magnetization compared with the pure CoPt samples,

which provides indication of the additional gold layer’s contribution. Heat dissipation may

be another potential reason. The HD-AOS phenomenon is closely related to the

temperatures in the electron, phonon and spin systems. If the temperature is too high, the

magnetization quenches, resulting thermal demagnetization state. The additional Au layer

may help dissipate local heat into the surrounding mediums [32]. However, further

experiment verifications are required for the above explanations.

Also, pump-probe experiment is necessary to reveal the underline mechanism and

switching dynamics of the HD-AOS phenomenon with hybrid metal-ferromagnetic thin

40

films. As the data bits are too small (~100𝑛𝑚) for observation with the current imaging

setup, magnetic-force microscopy (MFM) or better imaging technologies are necessary.

As for the sample, different magnetic thin films with PMA property and coating metal

materials can be explored. Other plasmonic nanostructures can also be studied, such as the

cross-dipole antenna [91-94], Au nanorods [95, 96] and the annealed Au islands [97-101].

The research work in this thesis provides the pathway to the high-density, lower power-

consumption PE-AOS technology. With more researches on those samples, significant PE-

AOS can be achieved in near future.

41

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