Introduction

(LEED), Auger electron spectropy (AES), magneto-optical Kerr effect(MOKE), and medium energy electron diffraction (MEED). Some general background knowledge involves thin film growth, crystalline structure and magnetism will be introduced in chapter two. Detail experiments and results on Fe/Mn/fcc-like Fe/Cu₃Au(001) trilayer will be described in chapter three. Two most important results are coerciv-ity enhancement and spin-reorientation transition due to the fcc-like Fe buffer layer effect. For the exchange coupling between ferromagnetic (FM)/ antiferromagnetic (AFM) bilayers, the fcc-like iron buffer layer with perpendicular magnetization may enhance the exchange coupling between Fe overlayer and Mn layer indirectly and in the meantime, the critical thickness of spin reorientation transition is shifted to higher coverage.

Chapter 2 Apparatus

2.1 UHV System in NSRRC

Figure 2.1: Ultrahigh vaccum system combined with MEED, LEED, AES, and MOKE at 05B2 beamline in NSRRC

Due to the large ratio of surface to bulk atoms in ultrathin films and nanoparti-3

2.2. Low Energy Electron Diffraction (LEED) 4 cles, the surface contamination is crucial to the crystalline structure and magnetic properties. Therefor all the experiments included in the thesis are performed in an ultrahigh vacuum (UHV, base pressure∼2×10⁻¹⁰). Basically the experiments are carried out in a multi-functional chambers, as shown in Fig. 2.1. All the experimen-tal apparatuses are combined in it so that every process can be in situ performed . The pumping system consists of an ion pump and a turbo pump combined with two mechanical pump as its fore-pumping. After baking the chamber to about 110-130

∘𝐶 for more than 24 hours, the ultrahigh vacuum (UHV) can be reached. A 4 or 5 dimensional manipulator with the capabilities of cooling and heating samples is also equipped in the UHV system. Medium energy electron diffraction (MEED) is used to monitor the growth condition and to calibrate the thickness of thin films. Auger electron microscopy (AES) is used to check the surface element composition and also helps to calibrate the film thickness. Low energy electron diffraction (LEED) and LEED-I/V are used to characterized the lateral and vertical crystalline structure.

Besides, there are also MOKE, sputter gun, residual gas analyzer (RGA), evapo-ration guns equipped in the chamber for the measurements of magnetic properties, sample cleaning, leak test, and evaporation etc.

2.2 Low Energy Electron Diffraction (LEED)

2.2.1 Theory

Low-Energy Electron Diffraction (LEED) is a technique that is used to investi-gate the surface crystalline order. Since the electrons do not penetrate deeply into material, the diffraction pattern that only arises from the atoms at the surface. In order to understand LEED images we need to deal with two-dimensional Bravais Lattices. In general, a crystal is a periodic arrangement of atoms or molecules that can be represented by a unit cell. A real vector 𝑅 and a reciprocal lattice vector 𝐾 can be respectively written as

2.2. Low Energy Electron Diffraction (LEED) 5

𝑅 = 𝑟⃗ ₁𝑎⃗₁+ 𝑟₂𝑎⃗₂+ 𝑟₃𝑎⃗₃ 𝐾 = 𝑘⃗ ₁𝑏⃗₁ + 𝑘₂𝑏⃗₂+ 𝑘₃𝑏⃗₃

The relation between real and reciprocal bases is

𝑏⃗1 = 2𝜋 𝑎⃗2⊗ ⃗𝑎3

Next we need to know what happens when a wave is scattered from a crystal. If the incident waves are in phase then the scattered waves will also be in phase, and we will get constructive interference when the scattered waves have a mutual phase shift of a integral number of wavelengths. The path length that the lower wave has traveled longer than the upper wave is given by 2𝑑 sin 𝜙 and the infterference condition or Braggs law (Fig. 2.2) is

Figure 2.2: Illustration of Bragg’s Law

2𝑑 sin 𝜙 = 𝑛𝜆, 𝑛 ∈ 𝑍

This means that if we measure the scattering angles we can obtain information about perpendicular distance in the crystal. We take a look at two scattering centers with mutual distance 𝑑 that are hit by an incident plane wave in direction ⃗𝑛 with wave vector 𝑘 (Fig. 2.3). The wavelength of the wave vector is then given by 𝑘 = 2𝜋/𝜆.

2.2. Low Energy Electron Diffraction (LEED) 6 The plane wave is scattered into the direction ⃗𝑛^′ with wave vector 𝑘^′. Scince the scattering is elastic, e.g. 𝑘 = 𝑘 = 2𝜋/𝜆, so the wavelength of the wave vector is invariable except its direction. Constructive interference occurs when the difference in path length must be an integer number of wavelength.

Figure 2.3: Illustration of two scattering waves

𝑑 cos 𝜙 + 𝑑 cos 𝜙^′ = ⃗𝑑 ⋅ (⃗𝑛 − ⃗𝑛^′) = 𝑚𝜆 where m is and integer mutiply with 2𝜋/𝜆

𝑑 ⋅ (⃗⃗ 𝑛 − ⃗𝑛^′) = 2𝜋𝑚 for integer m

We now look at a row of atoms with mutual spacing R in a Bravais lattice, 𝑅 ⋅ (⃗⃗ 𝑛 − ⃗𝑛^′) = 2𝜋𝑚 for integer m and all Bravais lattice vector R

This is consistent with the condition that the scattering vector ⃗𝐾 = (⃗𝑘 − ⃗𝑘^′) must be a reciprocal lattice vector, e.g.

𝐾 = 𝑘⃗ ₁𝑏⃗₁+ 𝑘₂𝑏⃗₂+ 𝑘₃𝑏⃗₃

All above lead us the Laue condition: The scattering vector ⃗𝑘^′ − ⃗𝑘 must be a reciprocal lattice vector ⃗Δ𝐺. We will have constructive interference if this condition is fulfilled.

𝐺 = ⃗⃗ 𝑘^′ − ⃗𝑘

The LEED pattern is an image of the reciprocal lattice. Those points in reciprocal space that fulfill the two-dimensional Laue condition result in intensity maxima.

2.2. Low Energy Electron Diffraction (LEED) 7 These can be explained in a graphical way (Fig. 2.4 and Fig. 2.5) by means of the Ewald construction. The sphere radius represents the wave vector ⃗𝑘 of the

Figure 2.4: Illustration of Ewald sphere

incident electron beam, and diffracted beams with wave vector ⃗𝑘^′ appear wherever a reciprocal lattice rod intersects the Ewald sphere. The diffracted pattern thus reflects the symmetry of the surface unit cell, and the separation between the beams is inversely proportional to the interatomic distance. If the circle (sphere) intersects a reciprocal lattice point then fulfilling the Laue condition. We therefore get diffraction in all direction where ⃗𝑘^′ ends on a reciprocal lattice vector. From the Ewald sphere construction it is easy to show geometrically that the Laue condition are equivalent to Braggs law. If the change of the wave vector by scattering on atoms at ⃗𝑑𝑖 and 𝑑𝑗 is given by ⃗⃗ 𝐺 = ⃗𝑘^′ − ⃗𝑘, then the phase shift between the two scattered waves is given by ⃗𝐺 ⋅ ( ⃗𝑑𝑖 − ⃗𝑑𝑗). The total amplitude scattered from all atoms is then a sum of the individual scattering events with appropriate inclusions of the phase differences.

If we have n atoms in the unit cell, each with a scattering strength then the total amplitude 𝑆_𝑘 scattered from the unit cell is

𝑆_𝑘 =

𝑛

∑

𝑗=1

𝑓_𝑗(𝑘) exp^{𝑖 ⃗𝐺⋅ ⃗𝑑𝑗}

This is called the structure factor or structure amplitude. In this case the scattered intensity is equal to ∣𝑆_𝑘∣².

2.2. Low Energy Electron Diffraction (LEED) 8

Figure 2.5: Relationship between LEED and Ewald sphere

Figure 2.6: Diagram of LEED apparatus

2.2.2 Basic components of the standard LEED apparatus

The common LEED apparatus is composed of a hemispherical phosphorescent screen with a fixed electron gun aligned along the central axis of the screen. The crystal sample is positioned at the centre of the hemispherical screen. With this arrangement, diffraction beams leave from the surface of the crystal travel towards the screen. The incident electron beam is emitted from the electron gun behind the hemispherical fluorescent glass screen and strikes the sample through a hole in the screen. Before the electrons impact the screen they must pass through a retarding field energy analyzer. Standard modern LEED optics are illustrated schematically in

2.2. Low Energy Electron Diffraction (LEED) 9 Fig. 2.6. The screen is metallic, coated in a phosphorescent material and biased to 4 − 6 𝑘𝑉 . Therefore electrons are accelerated directly on to the surface coating and those which impinge on the screen lead a spot proportional to the beam intensity.

The metallic nature of the screen allows the incident electrons to be conducted away and avoiding the undesirable charging. The electron gun is made of a LaB6 filament placed inside a metallic cylinder called wehnelt. Electrons emitted from the filament must be accelerated to an anode which is at a positive potential with respect to the filament. Electrons near the filament have low kinetic energies compared to the total potential drop and so will tend to follow field lines and also can be adjusted by the wehnelt voltage. However, as the electrons are continuously accelerated towards the anode, their kinetic energy may become so much greater through the anode aperture. Because the anode is virtually positive respect to the cathode, strong deviation of the field lines only occurs quite near the anode in a region where electrons are approaching their maximum kinetic energy. Following are a system of electrostatic lenses (A,B,C,D). If we choose the poper potential of the wehnelt, the incident electron beam is possible to be focused by the electrostatic lenses.

The LEED Optics typically consist of four hemispherical grids concentric with the luminescent screen, each containing a central hole through which the electron gun is inserted. The first grid is on ground potential and therefore around the sample is a field free region. The next two grids are set to the retarding voltage which is slightly lower than the kinetic energy of the electrons emitted by the electron gun.

It repels almost all the inelastically scattered electrons and the elastically scattered electrons pass the next grid which is set to ground voltage. Then they accelerated towards the luminescent screen (set to a high positive voltage). Behind the screen there is a view port in the UHV system so that the LEED pattern can be observed directly or recorded with a CCD camera.

2.2.3 LEED-I/V

In real experiments, since the incident direction of electron beam is nearly normal to the substrate, by consideration of electron mean free path, the low- energy electrons will penetrate several atomic layers and then be scattered. In this situation,

2.2. Low Energy Electron Diffraction (LEED) 10 the interference and scattering within the different layers will also contribute to the intensity of diffraction spots. The setup of LEED-I/V (Fig. 2.7) is used for measuring the interlayer distance by recording the (0, 0) beam intensity varying with the incident energy of the electron beam. From the Bragg condition and the de-Brogile relation, we have:

2𝑑 cos 𝜃 = 𝑛𝜆 = 𝑛 ℎ

√2𝑚(𝐸_𝑘− 𝑉 )

with the vertical interlayer distance 𝑑, the kinetic energy of the incident electron beam 𝐸_𝑘, and the potential cost for electrons to escape from the atoms. Therefore

𝐸_𝑘 = 𝑛² ℎ²

8𝑚𝑑²cos²𝜃 + 𝑉

If 𝐸_𝑘 and 𝑛² are taken as the Y and X axis respectively, we can fit the peaks of LEED-IV curve, as shown in Fig. 2.7(c). What has to be reminded of is that not all the peaks come from the Bragger condition, some of them are due to multi-diffraction. Thus we have to pick out the most appropriate peak that can fit the equation.

Figure 2.7: (a) Illustration of Bragg diffraction. (b) LEED-I/V curve of bulk Cu₃Au(100) measured at 100 K. (c) The LEED-I/V curve of bulk Cu₃Au(100) fitting

2.3. Auger Electron Spectropy (AES) 11

2.3 Auger Electron Spectropy (AES)

2.3.1 Theory

The principle of the Auger process is illustrated in schematically in Fig. 2.8. It is an atomic non-radiative emission process, mediated by electrostatic interaction.

When an atom is irradiated either by high energy photons or electrons, with sub-sequent core hole formation, it rearranges its electronic structure such that deep initial hole in the core level is filled by an electron from one of the outer shells. This transition may be accompanied by the emission of a characteristic X-ray photon or a Auger transition. If an electron beam of energy 𝐸_𝑝 of the order of 𝑘𝑒𝑉 𝑠 incidents

Figure 2.8: Energy level diagram of AES process

on the surface of a solid, a continuous spectrum of electron energies ranging from 0 𝑒𝑉 to 𝐸_𝑝 can be detected. Peaks of variou mechanisms will be seen in the spectrum.

A sharp peak at the energy 𝐸𝑝 and a broad one between 0 and 200 𝑒𝑉 correspond to respectively the elastic scattering and the emission of the secondary electrons as the result of a cascade inelastic scattering inside the solid. Weak peaks can be seen are associated with Auger processes between them described in the following. For an incident electron with the energy 𝐸𝑝 collides a single atom, the inner shells involved in the processes resulting in the emission of an Auger electron are denoted A, B, and C (Fig. 2.8). As the consequence of the collision, an electron from the inner shell A is expelled from the atom, leaving a single ionized atom whose electronic configuration is far from its ground state. The hole left behind is illustrated by an open circle. Levels, B and C, are all shifted downward equivalently. To minimize the energy, an electron from the level B, experiences a decay into the empty state in the

2.3. Auger Electron Spectropy (AES) 12 shell A. Following this process, a photon with the corresponding energy is ejected and may be absorbed by an electron from the other shallow level. As the hole moves upward, the shell B is set at the initial energy, since it affected by the fully screened nucleus charge. Then if the energy emitted following the B to A transition will be absorbed by an electron in the C shell, it will leave the atom and the energy gain overcomes the vacuum level. The electron emitted into vacuum is called the Auger electron, whose energy depends on the binding energies of the levels involved in the transitions. It is obvious that the Auger electron collects the whole information of the inner electronic configuration which is determined by the atomic number. The Auger energy of the transition ABC of the element determined by the difference of the total energy before and after the transition.

2.3.2 Basic components of the standard AES apparatus

Figure 2.9: Diagram of AES apparatus

The standard equipments for AES (Fig. 2.9) consist of electron gun, energy analyzer and data processing electronics. The electron gun produces the primary electron beam with a typical energy of 1 to 5 𝑘𝑒𝑉 . In the Auger electron spectra, Auger peaks are superimposed on a rather lager continuous background. This back-ground can be removed by differentiating the energy distribution function 𝑁 (𝐸).

The current 𝐼(𝑉 ) collected by the semispherical collector (screen) with the

retard-2.4. Magneto-Optical Kerr Effect (MOKE) 13 ing potential 𝑉_𝑟 is related to the distribution of Auger number 𝑁 (𝐸) as

𝐼(𝑉_𝑟) ∝

If a modulating voltage 𝑉_𝑚sin 𝑤𝑡 is applied to the retarded voltage, then 𝐼(𝑉_𝑟) becomes𝐼(𝑉_𝑟+ 𝑉_𝑚sin 𝑤𝑡). In Taylor expansion:

𝐼(𝑉_𝑟+ 𝑉_𝑚sin 𝑤𝑡) = 𝐼(𝑉_𝑟) + 𝐼^′(𝑉_𝑟)𝑉_𝑚sin 𝑤𝑡 +𝐼^′′(𝑉_𝑟)

2! (𝑉_𝑚²sin²𝑤𝑡)²+ . . .

= 𝐼(𝑉𝑟) + [𝐼^′(𝑉𝑟)𝑉𝑚+ . . .] sin 𝑤𝑡 − [𝐼^′′(𝑉_𝑟)

4 𝑉_𝑚² + . . .] cos 2𝑤𝑡 + . . . because 𝑉_𝑚𝑟, the higher order terms for 𝑉_𝑚can be neglected. From equation of (3.2), the amplitude for the collected current component with characteristic frequencies 𝑢 and 2𝑢 are proportional to 𝑁 (𝐸) and 𝑁^′(𝐸).

2.4 Magneto-Optical Kerr Effect (MOKE)

If a linear polarized light is incident into a ferromagnetic sample, since of the different reflection coefficients of right and left circular polarization components, the reflected beam will become elliptical polarized. This phenomenon is so called magneto-optical Kerr effect. The angle between the primary axis of the elliptical polarization and the linear polarization is called Kerr rotation, and the ellipticity of the elliptical polarization is called Kerr elliptical, as shown in Fig. 2.10. Let

Figure 2.10: Schematic display of DC MOKE

2.4. Magneto-Optical Kerr Effect (MOKE) 14 𝑟₊𝑒^𝑖𝜃+ and 𝑟−𝑒^𝑖𝜃− stand for the reflection coefficients of right and left circular po-larization, respectively. The Kerr rotation and Kerr ellipticity can be illustrated as 𝜑_𝐾 = −^𝜃+−𝜃₂ ⁻and 𝜀_𝐾 = _𝑎^𝑏 = ^𝑟_𝑟^+−𝑟−

++𝑟−, respectively. Both of them are proven to be proportional to the magnetization of sample. thus by measuring 𝜑_𝐾 and 𝜀_𝐾 with cyclic applied magnetic field, we can get the hysteresis loop. In general, there are three types of MOKE measurement. Each of them has different geometry of the magnetization and the light path, as shown in Fig. 2.11. In the polar Kerr effect, the magnetization lies in the plane of incidence and is perpendicular to the surface.

In the longitudinal Kerr effect, the magnetization lies in the plane of incidence and is parallel to the surface. In the transverse geometry, the magnetization is perpen-dicular to the plane of incidence and on the surface. In magnetic ultrathin film, the

Figure 2.11: Different geometry for MOKE measurement

Kerr signal is so small that the noise may result in significant effect. Therefore, in our experiments, a modulator is added between the polarizer and the sample such that the modulated signal can be taken by lock-in technique with a larger ratio of signal to noise. The schematic illustration is shown in Fig. 2.12. This method is called AC MOKE. Fig. 2.13 shows the in situ experimental apparatus of polar and longitudinal MOKE. The intensity of the reflection beam is taken by a photodiode, and then amplified by the preamplifier. By the preamplifier, the signal of current from the photodiode is expanded and transformed to voltage signal. Next the volt-age signal is sent to the lock-in amplifier. The oscilloscope is used to check the modulation of the laser beam.

2.5. Medium Energy Electron Diffraction (MEED) 15

Figure 2.12: Schematic display of AC MOKE

Figure 2.13: Diagram of MOKE apparatus

2.5 Medium Energy Electron Diffraction (MEED)

Medium Energy Electron Diffraction (MEED) is a surface sensitive technique, which allow us to measure properties of the sample surface during the growth pro-cess. As the names of MEED , suggest the most different characteristics between them are the incident energy and incident angle. In MEED, incident electron beam

2.5. Medium Energy Electron Diffraction (MEED) 16 strikes a single crystal surface at a glancing angle, forming a diffraction pattern on a screen as shown in Fig. 2.14. The electrons with 3-5 𝑘𝑒𝑉 order energy for MEED

Figure 2.14: Schematic display of the MEED measurement

are focused with glancing angle 3^∘− 5^∘ for MEED. Then, the electrons are scattered by the periodic potential of the crystal surface, which results in a characteristic diffraction pattern on the screen. The combination of glancing incidence and strong electron-substrate interactions reduces the penetration depth of incident electrons to a few monolayers. Compared to LEED, 3-100 𝑘𝑒𝑉 electrons are used in MEED.

This results in a mean free path in the 1-10 𝑛𝑚 range, which is substantially greater than that used in LEED. However, surface sensitivity is maintained via the glancing incidence geometry which ensures that the normal component of the incident elec-tron wave vector is small. Therefore, the penetration depth will be small. As shown in Fig 2.14. the diffracted intensity is displayed directly on a screen to be detected instantly. Furthermore, MEED arrangement in UHV chamber allows it to be used conveniently for in-situ and real-time observation of MBE thin film growth process.

By monitoring the specular spot ((0,0) beam) intensity during deposition, growth information can be obtained in real time for feed back control. In a good layer-by-layer growth system, the MEED intensity reveals regular oscillation behavior at the condition of constant deposition rate. This effect qualitatively correlates the beam intensity and the surface roughness. In general, the decay of MEED intensity is

2.5. Medium Energy Electron Diffraction (MEED) 17 attributed to the roughness of the sample surface. However, in other growth modes such as island growth and step flow-growth, MEED oscillation do not reveal the obvious oscillation.

Chapter 3 Background

3.1 Two-dimensional lattices and superlattices

In principle, the surface region of a crystal is a three-dimensional entity; relax-ations and reconstructions usually extend into the crystal by more than one atomic layer. Moreover, the experimental probes in surface experiments, even slow elec-trons, usually have a non-negligible penetration depth but compared to subsurface layers, the topmost atomic layer is always predominant in any technique based on the use of electrons or atoms. As a result, each layer of atoms in the surface is in-trinsically inequivalent to other layers and the only symmetry properties which the surface posses are those which operate in a plane parallel to the surface. Although the surface region is three-dimensional, all symmetry properties are two-dimensional.

The basic surface lattice can be described by a set of two-dimensional translational vectors: 𝑅_𝑚𝑛 = 𝑚𝑎₁+ 𝑛𝑎₂ where (m,n) denotes a pair of integers numbers, and the 𝑎_𝑖 are the two unit mesh vectors. As seen before, the topmost atomic layer could be rescostructed with different periodicity compared to the one of the bulk; in this

The basic surface lattice can be described by a set of two-dimensional translational vectors: 𝑅_𝑚𝑛 = 𝑚𝑎₁+ 𝑛𝑎₂ where (m,n) denotes a pair of integers numbers, and the 𝑎_𝑖 are the two unit mesh vectors. As seen before, the topmost atomic layer could be rescostructed with different periodicity compared to the one of the bulk; in this