Eindhoven University of Technology

BACHELOR

Polarized Raman scattering in the van der Waals antiferromagnet NiPS3

Peeters, Lars A.J.G.

Award date:2019

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Eindhoven University of Technology

Bachelor End project

2nd quartile2018-2019

Polarized Raman Scattering in the van der Waals

Antiferromagnet NiPS3

Eindhoven, February 19, 2019

Name: Lars Peeters 0885416Supervisors: Dr. Marcos H. D. Guimaraes & Casper F. SchippersResponsible lecturer: Prof. dr. Bert Koopmans

Abstract

Recently, interest in two dimensional van der Waals materials has grown because they have inter-esting properties regarding the fabrication of heterostructures. Especially, those materials which havemagnetic properties promise to be a good candidate for applications in magnetic memory devices. Inthis thesis, we have investigated the antiferromagnetic crystal nickel thiophosphate NiPS3. Until now,the antiferromagnetic ordering below the Neel temperature has not been reported for very thin layers.Therefore, we have observed the magnetization dynamics for layers in the ultrathin regime. The NiPS3

samples are made by mechanically exfoliating the bulk crystal using the scotch tape technique. The layerthicknesses have been determined by making use of optical microscopy, atomic force microscopy andan optical contrast analysis. We have used polarized Raman scattering for studying the magnetizationdynamics. We have found that the antiferromagnetic ordering still holds for crystals of 7.4 nm in thickness,which corresponds with 10 layers. Furthermore, the crystallographic axes and the symmetries of NiPS3

have been determined by using angle-resolved polarized Raman scattering. We found that the crystal hasa tendency to cleave along zig-zag edges. These results are necessary for producing spin-orbit torques thatcan drive the most current-efficient type of magnetic reversal, which can be used in magnetic memorydevices.

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Contents

Abstract i

1 Introduction 1

2 Theory 32.1 Nickel thiophosphate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.1 Molecular structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.2 Group theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.3 Magnetic structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.1 Raman spectroscopy in Nickel thiophosphate . . . . . . . . . . . . . . . . . . . . . 62.2.2 Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.3 Magnon scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Experimental setup 93.1 The setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.1 Angle Resolved Polarized Raman Scattering . . . . . . . . . . . . . . . . . . . . . . 93.1.2 Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.2 Sample characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.3 Raman peak identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Results and discussion 144.1 Identifying the crystallographic axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.2 Magnetization dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.3 Polarization selection rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.4 Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5 Conclusion 24

6 Acknowledgement 25

References 26

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1 Introduction

Magnetism is already a long-practiced branch of physics. It can be found in a wide range of applications,such as medicine or memory devices. However, magnetism on the nanoscale is relatively new with lotsof opportunities, especially in the field of 2D materials. Magnetism on the nanoscale begins with themagnetic moment of single atoms. The magnetic moment consists of magnetic moments due to the orbitalmotion of the electrons around the nucleus and the intrinsic spin of the electrons. Ferromagnets arematerials in which all the spins are oriented in the same direction, generating a net magnetization. In anantiferromagnet the neighbouring spins are oriented in opposite direction, resulting in no magnetization.In figure 1.1 the spin configuration of an antiferromagnet is presented. While the resonance frequencies offerromagnets are in the GHz range, antiferromagnets have magnetic resonances in the THz range. Thismakes them interesting for future spintronic applications[1].

Figure 1.1: Example of a spin configuration in an antiferromagnet. Taken from: [2].

Spins that are close to each other, interact via an exchange interaction. So when a spin in a latticechanges by, for example, excitation with a laser, it changes the direction of the neighbouring spins andthereby generating a magnon. A magnon can be seen as a quantized spin wave. Different magnon modeshave different energies and are characteristic for a material.

In general, 2D-materials have very promising opportunities due to their unique properties and behaviour[3].For example, using a different number of stacking layers can lead to different electronic or optoelectronicproperties. 2D-materials are promising candidates to be used in magnetic memory devices, becausethey promise small and compact devices that can be faster and more energy efficient, while maintainingtheir properties. It has been reported that even for monolayers of graphene the physical properties aremaintained[4]. Monolayers are interesting because they can function as building blocks for van der Waalsheterostructures[5]. The stacking of different monolayers into heterostructures is a very broad emergingresearch area.

Nickelthiophosphate (NiPS3) is a 2D material, which has drawn the attention for being a possiblecandidate in magnetic memory devices. NiPS3 is an antiferromagnetic crystal with low symmetry with thelayers coupled by van der Waals forces. These forces are relatively weak and can therefore be mechanicallyexfoliated into atomically flat layers. In general, van der Waals materials are stable because they havestrong in-plane covalent bonds and the van der Waals forces keep the stack together[5]. Bulk crystalNiPS3 exhibits antiferromagnetic ordering below the Neel Temperature which is approximately 158 K[6].Furthermore, low symmetry layered materials have also been shown to allow for interesting spin-orbittorques which are usually forbidden in devices with higher symmetry[7]. These spin-orbit torques haveshown that they are able to manipulate the magnetization with a higher efficiency.

In this thesis, we want to study the magnetization dynamics upon cooling the sample below the Neeltemperature, as has been shown for FePS3[8]. Until now, this has not been reported for ultrathin layersof NiPS3

1. Therefore, we want to observe this for samples of different thickness and make an attemptto observe this for a monolayer. Furthermore, we want to identify the crystallographic axes of NiPS3.

1In the last traject of this thesis, another paper has been published that has similar results. According to the timeconstraint, we have not been able to take this into account. Reference: [9]

1

This is important because applying a current along different crystallographic axes is a way for controllingspin-orbit torques[7].

The technique we will use is polarized Raman scattering. Raman scattering is a non-invasive techniquewhich gives us access to the different vibrational and magnonic modes of the crystal. These modes willprovide information about the corresponding energies and their symmetries. Studying the evolution ofthe Raman spectra as we go below the Neel temperature will provide information of the antiferromagneticordering since we expect magnons to be observed, see section 4.2. The crystallographic axes can alsobe obtained with polarized Raman scattering. For this, the sample rotation angle will be varied withrespect to the incident polarization of the laser light, see section 4.1. The intensity of the vibrationalmodes is expected to change as a function of the sample rotation angle as has been shown for blackphosphorous[10].

In this thesis, we will first provide the theoretical aspects in chapter 2 for understanding the underlyingphysics about the crystal NiPS3, then about Raman scattering and finally about magnon scattering. Thenthe experimental setup will be explained along with some calibrations in chapter 3. Subsequently, theresults will be given along with a physical explanation in chapter 4. Finally, we will come to a conclusionand will give future research possibilities in chapter 5.

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2 Theory

In this chapter, a theoretical background of the sample, techniques and the physical concepts of thisproject will be provided. First, a detailed description of the molecular structure of the sample willbe given following the magnetic structure of the sample. Then, the physics behind polarized Ramanscattering in general. At last, the theoretical aspects of Raman scattering applied on our sample will bediscussed.

2.1 Nickel thiophosphate

2.1.1 Molecular structure

Nickel thiophosphate (NiPS3) belongs to the transition metal thiophosphates (MPS3), where M is atransition metal, e.g. Ni, Fe, Mn, Co or Zn. As mentioned in the introduction, NiPS3 is a crystal whichbehaves like an antiferromagnet below the transition temperature, which is approximately TN = 155 K[6].The unit cell is build up of two molecular formula units Ni2P2S6. It has a hexagonal lattice made of theNickel atoms which is bonded by three sulphur atoms. These sulphur atoms are then again bonded viathe phosphor atoms. A three-dimensional view of the multiple unit cell is depicted in figure 2.1. Thecrystal belongs to the family of the van der Waals crystals, meaning that the layers are bonded via therelatively weak van der Waals forces. This makes it possible for the fabrication of atomically flat, singlecrystals via the scotch tape technique. This is well-known technique for mechanically exfoliating 2Dmaterials[11]. This technique is capable of peeling the bulk crystal into thinner layers by pulling aparttwo pieces of scotch tape in which the crystal in enclosed.

(a) (b)

Figure 2.1: A three dimensional representation of a multiple unit cell of the sample. With in (a) a viewfrom the side and in (b) a view from the top. Here, the green points represent the Nickel atoms, the

yellow points represent the sulphur atoms and the orange points the phosphor atoms. Furthermore, thesymmetry point/axes are also embedded in the figure. The i is the point of inversion symmetry, the C2

represents the two-fold rotation axis and the σh is a mirror plane. Adapted from: [12]

In the case we simplify the crystal to a hexagonal lattice containing only the Nickel atoms, two distinctcrystallographic directions can be defined, the armchair direction and the zigzag direction which areshown in figure 2.2. The in-plane cleavage behaviour is often, although not necessarily, related to thesetwo directions[13]. Combined with the C3 symmetry of the simplified hexagonal lattice, we have 6 possiblecrystal orientations. These directions are important for determining the crystallographic axes of NiPS3.

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Figure 2.2: A schematic representation of the two configurations in which left the side edge structures arecalled armchair and to the right are called zigzag directions. Taken from: [14]

2.1.2 Group theory

A physical system can be labeled by its symmetries according to specific symmetry groups. Severalproperties of the physical system can be described using only symmetry arguments. These groups are setsthat are equipped with an operation(addition, multiplication or composition) such that it satisfies certainbasic properties[15]. It is an area of abstract algebra that is useful to study symmetries. According togroup theory, all crystals can be subdivided into space groups according to their symmetries. There are atotal of 230 space groups with increasing symmetries. By definition: The point group and translationsymmetry operations which carry the crystal into itself form a group called the space group[16]. Thedifference between a point group and a space group is that in a point group at least one point in spaceis fixed. A point group includes all symmetry operations with a fixed origin under which the object isinvariant.NiPS3 belongs to the low-symmetry space group #12 (point group C2h), which means it has a monoclinicalstructure with a two-fold rotation symmetry a mirror symmetry and an inversion symmetry. All possiblesymmetry operations of a point group can be listed in an array which is referred to as the character table.The character table for the C2h point group is given in table 1. The phonon modes can be grouped, bytheir symmetry, according to the irreducible representations of the point group which are given in thefirst column. A and B are the Mulliken symbols for the one-dimensional case representing symmetric oranti-symmetric with respect to the principal rotational axis, respectively. In case of inversion symmetry,the g and u subscripts mean symmetric or anti-symmetric with respect to the center of the inversionsymmetry. So for example, Au means that we have a one dimensional representation which is symmetricwith respect to the principal rotational axis and anti-symmetric w.r.t the center of inversion. Bg meanswe have a one dimensional representation which is antisymmetric w.r.t. the principal rotational axisand is symmetric w.r.t. the center of inversion. The first element of the column is one of the symmetryoperations with E the identity, C2(z) the two-fold rotation, i the inversion symmetry and σh the mirrorsymmetry. The characters in the table show how each irreducible representation transforms with eachoperation.

2.1.3 Magnetic structure

NiPS3 is a quasi-two-dimensional material where the Nickel atoms lie in a honeycomb lattice. The Nickelatoms carry a localized magnetic moment due to their 2+ electronic ionization state. The magneticstructure of NiPS3 has been determined by magnetometry and several neutron diffraction techniques[6].

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E C2(z) i σh linear, rotations quadraticAg 1 1 1 1 Rz x2, y2, z2, xyBg 1 -1 1 -1 Rx, Ry xz, yzAu 1 1 -1 -1 zBu 1 -1 -1 1 x, y

Table 1: The character table of the point group C2h. Taken from [17]

They have shown that below the Neel temperature TN = 158 K, the compound adopts a ~k = [010]antiferromagnetic structure which means that the spins are antiferromagnetically coupled in the crystaldirection b. Moreover, the compound forms ferromagnetic zig-zag chains along the ~k = [100] (a axis).

The structure is also coupled ferromagnetically in the ~k = [001] (c) axis[18]. The magnetic structure isdepicted in figure 2.3. For antiferromagnets, an often used convention is to arrange the neighbouring spinson different sublattices where the spins are pointing in different directions. Moreover, neutron elasticscattering has been used on NiPS3 powdered samples to obtain the strengths of the magnetic exchangeinteractions[18]. From this data the exchange parameters between different nearest neighbours has beenquantified. This is important for calculating the magnonic dispersion from which the density of statescan be derived.

Figure 2.3: The magnetic structure of the top view of NiPS3. Nickel- A represents the sublattice A wherethe spins are oriented down (in the paper) and Nickel-B represents the sublattice B where the spins are

oriented up (out of the paper).

2.2 Raman spectroscopy

One of the most commonly used technique for fingerprinting materials is Raman scattering[19]. Thismeans that this technique is able to identify compounds because the obtained Raman spectrum is unique,like DNA is unique to humans. The purpose of Raman scattering is identifying the vibrational modesof a system. The principle behind Raman scattering can be explained using two different pictures, thequantum pictures and a semi-classical picture. The quantum picture states that light of a laser interactswith phonons in the system. When the photons of the light interact with the matter of the system, theycan either be absorbed or scattered. They produce a virtual state with a very short lifetime before theyare scattered, see figure 2.5a. The scattering can happen elastically or inelastically. In the dominantcase, when they are elastically scattered and there is no energy change between the incident and reflectedlight, this is called Rayleigh scattering. When they are inelastic scattered, the scattered photons comeout with a lower (or higher) energy. This energy difference can be due, for example, the creation (orannihilation) of vibrational modes of the system. In the former case, it created a phonon, which is calledthe anti-Stokes process and in the latter case it destroyed a phonon, which is called the Stokes process. Avisual explanation is provided in figure 2.4.

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(a) (b)

Figure 2.4: The Feynman diagrams of the two processes in Raman scattering involving the Stokes in (a)and anti-Stokes in (b). The sinusoidal lines are photons and the triangular wave-shape lines represent the

phonons. Adapted from: [20]

The semi-classical picture of Raman scattering considers a light wave as a propagating oscillatingdipole[19]. This dipole polarizes the electrons around the nuclei. This causes the molecule to vibratewith a characteristic frequency. When performing a Raman measurement, the energy of the incident lightcan either absorb the energy of the vibration (anti-Stokes process) or cede the energy to the vibration(Stokes process). The intensity of the scattered light will be measured for a range of wavelengths. Hereby,a Raman spectrum is obtained. If the excitation wavelength of the used light is known, this wavelengthcan be translated into an energy. The schematical representation of the quantum picture of energy statesin Raman scattering is given in 2.5a. Here, the excitations to the virtual states are visible, in whichthe energy difference of the inelastically scattered light are represented with arrows of different lengthscompared with the ones of the excitation. Furthermore, the Rayleigh scattering is depicted via twodifferent transitions, indicating that the elastic scattering is more probable. In figure 2.5b, a schematicalrepresentation of an example of a Raman spectrum is provided. The ∆νi represent the difference inwavelengtj of the incident and scattered light of different phonon modes which is a measure of the energydifference. Measurements usually analyze the Stokes component since the anti-Stokes component is lessintense. This occurs because only molecules that are vibrationally excited prior to irradiation can giverise to the anti-Stokes line[21].

2.2.1 Raman spectroscopy in Nickel thiophosphate

Bulk NiPS3 belongs to the point group C2h. However, monolayers of NiPS3 have D3h symmetry inthe virtual structure, where we have ideal stacking. A general Raman spectrum in the backscatteringconfiguration only encompasses phonon modes at the Brillouin zone center (Γ-point)[23]. The zone-centermodes for a monolayer can then be classified according to the irreducible representation of the D3h pointgroup[24], which is given by:

Γ = 3A1g + 2A2g +A1u + 4A2u + 5Eg + 5Eu. (2.1)

Here, the E modes are Mulliken symbols for the two-dimensional representations. The actual stacking ofthe layers breaks the degeneracy of the Eg-modes and causes this mode to split into Ag and Bg modes,giving the C2h point group[24]. Since the interlayer coupling is very weak, this splitting is difficult toobserve. The irreducible representation for the zone-center phonon modes of the point group C2h is thengiven by:[24]

Γ = 8Ag + 6Au + 7Bg + 9Bu. (2.2)

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(a) (b)

Figure 2.5: A schematical representation of the quantum picture of Raman scattering in (a), here thearrows up mean an excitation to a higher state and the arrows down stand for falling back into a lowerenergy state. You can see that Rayleigh scattering can happen in more than one way. A schematicalrepresentation of the semi-classical picture of Raman scattering in (b). Here, the ∆νi stand for the

wavelengths difference between the incident and the scattered light for the different vibrational modes.Taken from: [19] and [22], respectively.

There are 8 Raman-active phonon modes predicted to be observed, the (3Ag and the nearly degenerated5Ag, Bg)[24]. A mode is Raman active (allowed) if the vibration changes the polarizability of themolecule[19].The symmetry of the modes observed is determined by the Raman tensors corresponding to the pointgroup of the crystal[25]. Raman tensors are 3x3 matrices that interrelate the electric field vector of theexciting radiation with the electric field vector of the Raman scattered radiation[26]. The Raman tensorsfor the irreducible representations of the C2h point group are described by[27]:

R(Ag) =

a 0 d0 b 0d 0 c

R(Bg) =

0 e 0e 0 f0 f 0

(2.3)

where, the letters a-f indicate the structure of the Raman tensor. For example, notice that the non-diagonalcomponents are symmetric. The intensity of a given Raman mode can be calculated using the Ramantensors via:[28].

I ∝| ~es ·R · ~ei |2, (2.4)

where ~ei and ~es are the vectors of the electric field component of the incident and scattered lightrespectively. Furthermore, a-f are not necessarily constants, we expect them to be depend on the samplerotation angle φ, making the intensity depend on the angle φ as well. The φ-dependence has also beenshown for black phosphorous[10]. We will elaborate on this in more detail in chapter 3.

2.2.2 Polarization

Because equation (2.4) tells us that the intensity is dependent of the polarization of the incident andscattered light, we can use polarized Raman scattering to obtain the crystallographic axes. ”Polarized”means that the incident light will be polarized in a certain direction and an analyzer is used in the same oran other direction. This results in either a parallel or a crossed polarization configuration. This techniquewill provide useful information about the crystal orientation using the symmetry of the vibrations. Acommon used notation is the Porto’s notation. This notation follows the convention:

A(BC)D, (2.5)

where A means the propagation direction of the incident light and B means the polarization directionof the incident light. C means the polarization direction of the scattered light and D stands for the

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direction of propagation of the scattered light. A negative direction can be notated with −A or A. Sincea backscattering geometry will be used, there are four distinct polarization configurations: Z(XX)Z,Z(XY )Z, Z(Y X)Z and Z(Y Y )Z. This also means that the z-components of the Raman tensors usedin the previous section are forbidden in the backscattering setup. Therefore, the tensors in 2.3 can bereduced to two dimensional tensors, which is described in:

R(Ag) =

[a 00 b

]R(Bg) =

[0 ee 0

]. (2.6)

These tensors give information about which modes are present in which configuration. the Ag modes arepresent in the parallel polarization configurations Z(XX)Z, Z(Y Y )Z and are therefore forbidden in thecrossed polarization while the Bg modes are present in the crossed polarization configurations Z(XY )Z,Z(Y X)Z while forbidden in the parallel configuration.

2.2.3 Magnon scattering

Because of the interaction between spins in a lattice, magnons can propagate through the crystal. Magnonsare characterized by a wavevector ~k. All magnons are generated by letting ~k take on every values forall N unit cells in the sample of values which are evenly spaced in the Brillouin zone[29]. For NiPS3

the wavevector is ~k = (0, 1, 0) since the ferromagnetic chains are antiferromagnetically coupled in theb-direction of the crystal[18]. Systems where the magnetic order parameter is unstable in two-dimensionalmaterials can be described by the XY Hamiltonian[30]. This is important for determining how a groundstate in a physical system emerges. To describe the system by the XY model, the generic form of themagnetic Hamiltonian is given by:

H = −∑<i,j>

(JxSxi S

xj + JyS

yi S

yj + JzS

zi S

zj ), (2.7)

where Jx,y,z are the exchange parameters between the nearest neighbours[18]. Sxi , Sy

i and Szi is the

component of the total spin at site i, where i and j run over all lattice sites and all nearest neighbours.For the XY-model, the nearest neighbour exchange parameters are of the form (Jx = Jy, Jz)In Raman spectroscopy, the scattering of the light when a laser excites a magnon can be observed.We are considering two cases, first-order (one-magnon) light scattering and second-order (two-magnon)scattering[29]. First-order scattering is considered by two processes, one involving a direct magnetic-dipolecoupling and the other involving an indirect electric-dipole coupling which proceeds through a spin-orbitinteraction. Their experiments have shown that the latter one is more important. The second-ordermagnon scattering is based on an excited-state exchange interaction between opposite sublattices. Ramanspectra of NiPS3 reveal a very broad two-magnon band[31].The one-magnon and two-magnon excitations can be distinguished experimentally quite easily because oftheir Raman activity[32]. The one-magnon scattering originates from the spin-orbit interactions whichare much weaker than the typical phonon bands. This would mean that the Raman intensity of theone-magnon peak would be lower than the phonon peaks. Second, one magnon scattering involvesonly Γ-point excitations, these can be characterized by very narrow lines in the spectrum. Since thetwo-magnon is characterized as a very broad feature, the present Raman activity is due to the scatteringby a two-magnon excitation.However, the exact reason for the broadness of the two-magnon is not entirely clear. The two-magnonfeature in the Raman spectra of NiPS3 (spin S =1) appears to be of comparable relative width to thatof La2CuO4 (spin S =1/2) and other cuprates[31]. Beforehand, one thought that the width of thetwo-magnon in the Raman spectra of the cuprates is believed to rely on quantum fluctuations intrinsic tothe S =1/2 system. It is observed that the two-magnon of NiPS3 interferes with a nearby phonon, whichrefutes the former established theory[31]. They propose that the anomaly is due instead to magnon decayinvolving strong magnetostrictive coupling to phonons.

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3 Experimental setup

In this section, we will first provide an explanation of the Raman spectrometer and the settings. Next,a description of the measurement during cooling will be given. Finally, the methods for characterizingthe sample will be explained. This will include optical microscopy, Atomic Force Microscopy (AFM)and a contrast analysis. Also the different phonon modes in the Raman spectrum of NiPS3 will beidentified.

3.1 The setup

For the measuring the Raman spectra the Renishaw inVia Raman microscope has been used. The layoutof the Raman setup is shown in figure 3.1. The system consists of a 100 mW solid-state laser with anexcitation wavelength of 514 nm. The laser beam will be directed into a box which aligns and focuses thebeam in a desired way. Then it will be led through the microscope onto the sample and get back. Theelastically scattered light then reflects on a dichroic mirror, while the inelastically scattered light falls ona grating with 1800 lines/mm. Then the light will fall onto a CCD camera which acquires the data andconverts it into a spectrum using the software WiRE. This software program is designed for the dataacquisition and analysis for Raman spectroscopy and is running on an external PC which is connected tothe Raman system. For obtaining a maximum resolution, the grating lines are aligned with the CCD chip.Important for our measurements are the λ/2-plates which are indicated by the arrows in figure 3.1b. Thealready polarized laser beam fall on the first (PC controlled) λ/2-plate which rotates the polarizationby π. The second (manually controlled) λ/2-plate, then rotates the outgoing polarization to match theoptimal polarization for the grating, which is aligned to the fixed direction of the analyzer. λ/2-plate B iscontrolled manually and analyses the scattered light with respect to the λ/2-plate A. This apparatus willbe used for every measurements, but with different settings or additional equipment.

(a) (b)

Figure 3.1: The layout in (a) and the schematics of the optics in the Raman system in (b). Courtesy of:C.A.A. van Helvoirt.

3.1.1 Angle Resolved Polarized Raman Scattering

In the previous chapter we mentioned that the intensity of a peak in the Raman spectrum is expectedto be dependent of the sample rotation angle φ. The configuration with the sample rotation angle isschematically presented in figure 3.2. In this figure the top view is visualized indicating the rotationangle and the polarization angle with respect to a restframe in Cartesian coordinates. To transform the

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intensity between the two coordinate systems (x, y) and (x′, y′) a transformation matrix M is needed.The intensity of equation (3.1) now becomes:

I ∝| ~es ·M ·R ·MT · ~ei |2 with M =

[cos(φ) sin(φ)− sin(φ) cos(φ)

][33]. (3.1)

In order to rotate the sample a special rotation stage has been custom designed to fit under the Ramanmicroscope. This stage is able to rotate the sample 360°.

(a) (b)

Figure 3.2: In (a), the Raman setup configuration with backscattering geometry with a rotational samplestage. Also the polarization of the light is indicated, where the analyzer can be either parallel or

orthogonal to the laser polarization. In (b) a topview of the sample is given in which the red arrow is thelaser polarization and the the blue arrow the direction of the analyzer. Adapted from: [33].

3.1.2 Cooling

For studying the magnetization dynamics, additional equipment was needed for cooling down the sample.We use the THMS600 system which consists of a temperature controller and a cooling pump with acryogenic storage dewar and a microscope stage. These components are also displayed in figure 3.3.Here, the cryogenic storage dewar will be filled with liquid nitrogen which has the boiling point at 77K. The temperature and also the rate of change in temperature can be set using the controller. Thisapparatus communicates with the cooling pump that pumps the liquid nitrogen into the microscopestage causing a temperature decrease. The sample is loaded into the stage and is then placed underthe Raman microscope in order to measure Raman spectra at very low temperature. As you can seein 3.3b a lot of connectors are attached to the stage. The most important ones are the coolant pipeswhich serve as the inlet or outlet of the liquid nitrogen. We also have the gas port valves, these aremeant for blowing out the air before cooling such that no ice formation takes place inside the stagethat could possibly ruin our samples. However, we found out that it is not ideal, so ice formation waspresent on our sample upon cooling. Therefore, we compared two spectra at room temperature before andafter the cooling procedure to visualize the damage. Fortunately, the sample proved to be quite resilientagainst the ice formation, meaning that the results are reliable. Furthermore, during the measurementsat low temperature, condensation of the water from the surroundings happened on the lid window of themicroscope stage. Therefore, a small tube was placed next to the window which blows air to prevent thecondensation on the window.

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(a) (b)

Figure 3.3: All components are displayed in (a), which are the temperature programmer, the coolingpump, cryogenic storage dewar and the microscope stage. The samples will be loaded into the

microscope stage which is displayed more extensively in (b). Adapted from: [34]

3.2 Sample characterization

In this section, we characterize the sample using three different methods/techniques. First we willidentify the samples using optical microscopy, then atomic force microscopy (AFM) is used for heightdetermination of the flakes. Moreover, an optical contrast analysis has been performed which will becombined with the results of the AFM to determine the number of layers of the flakes.Sample fabrication has been performed by using the scotch tape technique, as is explained in chapter 2,inside the glove box. The glove box has an inert (nitrogen) atmosphere where the O2 and H2O levels arebelow 0.5 ppm. The samples were intensively searched for thin flakes by using an optical microscope.The results presented in the sections 4.2 - 4.3 originate from two different flakes on two distinct samples.Coordinates have been assigned to flakes in order to navigate on the sample. From now on, we refer to asflake A for the sample 1: D2-4-1 and flake B for sample 2: A5-5-2. Microscopical pictures of these flakeswere also taken and are displayed in figure 3.4.

Atomic force microscopy (AFM) has been performed on the flakes A and B, which are included infigure 3.4c and 3.4d. With AFM, the height of the flakes can be determined. In figure 3.4c tiny whitestripes are visible. This indicates that the surface of the layers is rippled a little bit. The results of theAFM conclude that the height of the regions on which the measurement are performed is 7.4 nm for flakeA and 13.7 nm for flake B. These results can be combined with the results of the optical contrast analysisin order to assign the number of layers to a specific thickness.Finally, an optical contrast analysis has been performed on many other flakes with different thicknesses.Thick flakes appear as yellow or green areas. But the thin layer exhibit discrete levels of optical contrastwith respect to the substrate, allowing us for unambiguous discrimination of between regions with adifferent number of layers[23]. The optical image can be decomposed in the red, green and blue intensitychannels. The optical contrast between the flake of n layers and the substrate can then be calculated foreach color channel. The optical contrast of a flake with n layers CnL can then be calculated via:[23]

CnL = (InL − Isubs)/Isubs (3.2)

where InL is the color channel intensity of the n-layer flake and Isubs is the intensity of the same channelof the substrate. In figure 3.5, the optical contrast has been calculated for many different flakes. Theoptical contrast of flakes A and b are indicated by A and B respectively. There are two data sets forB meaning that this has been calculated twice, but then for images which were taken under a differentobjective. The data sets are ranked from low to high optical contrast in the blue in order to keep a better

11

(a) (b)

(c) (d)

Figure 3.4: Microscopical images of the flakes A and B in (a) and (b) respectively. The pictures weretaken with the 20x and 100x objective, respectively. The controur in red encloses the region of interest.The AFM images of the NiPS3 sheets of flake A and B, in (c) and (d) respectively. Courtesy of: Marcos

H.D. Guimaraes.

overview in the graph. Furthermore, we would expect the optical contrast in blue to be a measure ofthickness, like has been shown in [23]. However, according to the figure 3.5, flake B should be thinnerthan flake A in that case. This is not true, because the AFM results state otherwise. Perhaps, our flakesare not thin enough to allow for an unambiguous discrimination of the number of layers by the opticalcontrast. Probably, a better job could have been done if more AFM results were acquired.Nevertheless, we know that a monolayer has an apparent height of approximately 1.5 nm with respect tothe substrate[23]. The interlayer spacing is thinner and is linear with the number of layers. The interlayerspacing between consecutive layers is approximately 0.6 nm[23]. Then the thickness D can be describedby D = 1.5 + 0.6 ∗ n where D is in nanometers. This allows us to conclude that flake A consists of 10layers and flake B consists of 20 layers.

3.3 Raman peak identification

To provide an explanation of a Raman spectrum of NiPS3, the spectra are plotted for a parallel and acrossed polarization in figure 3.6. Also the corresponding irreducible representations of the peaks havebeen included. Here, the nearly degenerated Ag, Bg have been indicated as a ∆. The spectra appear to

12

Figure 3.5: An optical intensity contrast analysis for different flakes of different thicknesses. Theintensity contrast is subdivided in the red, green and blue color channels, with reference to the substrate.

The data sets for the optical contrast of flakes A and B are also indicated.

have a background intensity with superimposed narrow peaks. These peaks are representing the phononmodes inside the crystal. These phonon modes are grouped by symmetry, according to the irreduciblerepresentation of the point group. The spectrum in the parallel configuration more peaks are presentbecause only Ag can exist in this configuration and the possible Raman modes are Ag, or the Ag modesthat originate from the splitting of the Eg mode. Subsequently, in the crossed polarization, only the Bg

modes originated from the Eg mode are visible. Furthermore, in both spectra the silicon peak at 520cm−1 is visible. However, as described in the theory, 8 modes should be Raman-active for NiPS3[24]. Inour measured spectra, the peak at 236 cm−1 corresponding with the Ag, Bg mode, is missing with noapparent reason. Also present in the spectrum is the 2Ag mode in the spectral region 700-850 nm, thiscontains numerous peaks that have been interpreted as second-order processes[23].Another important feature present in both spectra is a very broad band with low intensity, this representsthe two-magnon peak. When lowering the temperature, this band is expected to have an increase inintensity. However, since these Raman spectra are taken at room temperature, the antiferromagnetictransition has not taken place. Although, localized antiferromagnetic regions are present above TN whichbecome larger upon cooling and the sample eventually becomes totally antiferromagnetic at TN [35].In the next chapter we will study the evolution of Raman spectra as a function of the sample rotation angleφ and as a function of temperature, using the experimental setup as described in this chapter.

13

(a) (b)

Figure 3.6: Raman spectrum of exfoliated NiPS3, acquired with an excitation wavelength of 514 nm. In(a) detected with the parallel polarization and in (b) detected in the crossed polarization. Also the

nomenclature of the different phonon modes are indicated. The nearly degenerated Ag, Bg modes havebeen indicated as a ∆ in order to keep an overview. Moreover, the peak at 520 cm−1 originates from the

silicon substrate.

4 Results and discussion

In this chapter, the results of four topics will come to its attention. First, the crystallographic axeswill be determined accompanied with the symmetries of the particular flakes discussed. Second, themagnetization dynamics will be studied and a full analysis of the magnon will be done. Subsequently,the Raman spectra of the different polarization configurations will be presented and compared with theliterature. Finally, the degradation of NiPS3 will be investigated.

4.1 Identifying the crystallographic axes

Angle-resolved polarized Raman scattering has been executed for two different flakes because we want tosee how the Raman spectrum evolves as a function of the sample rotation angle φ. Therefore, the Ramanspectrum has been measured for steps of 10° between 0° and 90° and in steps of 20° between 90° and 180°.The spectra were measured with parallel polarization under a 1% of the total laser intensity with a 50xobjective from the microscope. In order to reduce the noise an exposure time of 10 s was taken with threeaccumulations. The results are presented in figure 4.1. The left column of the figure belongs to flake Aand the right column belongs to flake B. The phonon modes in the Raman spectra showed a periodicvariation in the intensity as expected.To visualize this properly a colormap has been made for which the intensity has been interpolated betweenthe data points. In these colormaps, we have three different variables. On the x-axis we have the Ramanshift, just as in the Raman spectra. Now, these spectra are measured for every sample rotation angle,which is on the y-axis. The third variable is the intensity, which is expressed on a color scale. Here, highintensity has a bright color which correspond with the phonon modes in our spectra. The most intensepeak is the silicon peak at 520 cm−1 showing a π/2 periodicity, corresponding to the cubic symmetry ofthe Si crystal. The peaks that showed the highest intensity variation, which are the peaks at 175 cm−1

and 384 cm−1, have been examined in more detail. The Ag, Bg mode at a shift of 175 cm−1 showed alarge intensity variation for both flakes. Another interesting feature of this phonon mode is that thecenter of the phonon mode shows a fluctuation in the Raman shift. This means that this mode has achange in the energy of the vibration due to a change in frequency of the vibration. In 4.1c and 4.1d theintensity has been plotted as a function of rotation angle, showing a π periodicity. Furthermore, in 4.1f

14

and 4.1e polar plots are also made which are fitted with the function for an ellipse:

I = As ∗ (sin(x− x0))2 +Ac ∗ (cos(x− x0))2 + y0. (4.1)

Here, As and Ac are amplitudes of the sine and cosine respectively x0 the angle offset in radians and y0 theintensity offset. The polar plots show that there is a difference between the magnitude of the major andminor axis. This suggests that the crystal is anisotropic because the ratio between Raman intensities in alldirections should be equal to 1 for isotropic materials. This is also confirmed for SnS-flakes, which indicatethat the ratio in Raman intensities between the zigzag- and armchair direction should be proportional tothe anisotropy[36]. If we calculate this for the polar plot in 4.1e then we get a value of 0.79. This ratio isalso described as the depolarization ratio ρ which can be approximated by: [37]

ρ =3β2

45α2 + 4β2(4.2)

where, α and β can be calculated from the diagonal and non-diagonal components of the polarizabilitytensor, respectively. Since the vibration mode at 175 cm−1 is degenerate, α should be zero and we shouldtherefore yield a value of 3/4[37]. This is quite consistent with the value of 0.79, confirming the anisotropyof flake A. Furthermore, the anisotropy should also be dependent of the sample thickness[36]. The polarplot in 4.1f has less difference in Raman intensity between the major and minor axes, indicating thatthe anisotropy is different than of flake A. Since flake B is thicker than flake A, we can conclude thatthe anisotropy increases for thinner flakes. Although more data from samples with different thicknessesshould be required to validate this conclusion.Despite a π-periodicity is observed in flake B, the results are causing doubt because it is not reallyconvincing due to the large fluctuations and intensity difference with flake A. Therefore, we will continuewith flake A for determining the crystallographic directions.We know that the in-plane cleavage behavior for FePS3, which has a similar crystal structure as NiPS3,have a strong tendency to cleave along the 〈1 0 0〉 or 〈1 1 0〉, which form angles of 60°[13]. These edgesare along the zigzag directions of the crystal[13]. Moreover, we know that there is an armchair directionperpendicular to the zigzag direction. The minimum and maximum of the intensity of the phonon modeare also separated by 90°. We have the initial condition that for the angle φ =10°, a zigzag edge wasalong the horizontal, which is parallel to the polarization direction of the incident light. Then everyk ∗ 60°, there is another zigzag and every k ∗ 60° + 30° there is an armchair direction for k = 0, 1, 2, 3, 4, 5.From this there can be concluded that one of the armchair edges has minimum Raman intensity and thezigzag edges perpendicular to this armchair edge have a maximum Raman intensity. For the symmetryoperations of the crystal, which is shown in 2.1, we can derive that the C2(z) rotation axis is along one ofthe three armchair directions. The σh mirror plane is perpendicular to the C2(z) rotation axis. In otherwords, the normal vector of the mirror plane is parallel to the C2(z) rotation axis.

15

(a) (b)

(c) (d)

(e) (f)

Figure 4.1: In (a) and (b) are the interpolated colormaps displayed of the Raman spectra for all differentangles in steps of 10 or 20 degrees for the two different flakes. (c) and (d) represent the intensity

variation of the Ag mode at 175 cm−1 as a function of the sample rotation angle φ. In figures (e) and (f),is shown the same as in (c) and (d), but then using polar coordinates, accompanied with a suitable fit.

16

Figure 4.2: The Raman spectrum of flake A at 233 K in the parallel polarization configuration. Thelorentzians that make up the spectral region at 750-850 cm−1, confirming the existence of at least two

lorentzians.

4.2 Magnetization dynamics

To study the magnetization behavior, we have cooled down the sample to the boiling point of liquidnitrogen (T = 77 K). The measurements have been performed on flake A and flake B. A 5% of the laserintensity was used this time, because the intensity of the laser inside the microscope stage was much lower.Again, 3 accumulations of 10 seconds exposure with the 50x objective. for flake A the measurementshave been performed on the bulk and on the thin layer region. For each region the measurements wereperformed in the parallel and crossed polarization configurations. The evolution of the Raman spectra ofboth the bulk and the thin layer are presented in figure 4.3.

As we observe, the intensity of the magnon band gradually increases upon lowering the temperature. In4.1a, a colormap has been made of flake A in the crossed polarization for the thin layer region, for whichhas been interpolated between the temperature steps of 20 K. A similar procedure has been executedfor only the thin region of flake B in 4.3d. As predicted for low temperature, an intense broad peak isvisible in green/yellow which represents the two-magnon peak. Also the phonon modes are visible andsome of them tend to have a temperature dependence as well. For the Ag mode at a Raman shift of175 cm−1, the intensity of the mode increases as the temperature decreases, while the spectral region at819 cm−1 show an opposite relation. The broadness of the phonon spectral lines also varies for differenttemperatures. The temperature dependence of some of the phonon modes in flake B is examined and theresults are presented in figure 4.4.A reason for this could be that phonons in real life are not totally harmonic because they interact withother phonons which causes a broadening of phonon spectral lines in general[38]. The broadening andintensity change in Raman phonon modes could be explained by Brillouin zone folding. This is a processthat due to the antiferromagnetic ordering of spins, the unit cell adopts a different form. This has asconsequence that phonon modes at the edge of the Brillouin zone become Raman active, this has alsobeen reported for FePS3[8]. For NiPS3 this would mean that these peaks would be built out of moreLorentzians. Apparently, when fitting the 2Ag mode at 819 cm−1 with, it confirms the existence of twoLorentzians: a small peak centered at 809 cm−1 and a larger peak at 819 cm−1. These Lorentzians areplotted along with the Raman spectrum in figure 4.2.

17

(a)

(b) (c)

(d) (e)

Figure 4.3: Evolution of the Raman spectra upon cooling. In (a) and (d) the colormaps of the Ramanspectra at different temperatures is shown for flakes A and B. In (b) and (c) the Raman spectra

evolution is shown for the bulk and the thin layer region of flake A, respectively. The similar has beendone in (e) for flake B.

18

Figure 4.4: Temperature dependence of four phonon modes in the Raman spectrum of flake B. The Neeltemperature is also included with the vertical line.

Another interesting feature emerging in the low temperature regime is the superimposition of the phononmodes with the two-magnon peak. They cause lineshape deformations and antiresonance in the Ramanspectra[39]. This is due to Fano-type interference between the two-magnon continuum and the phononmode at 558 cm−1, which has already been reported for bulk NiPS3[31]. Fano-resonance is based on theinterference between a background resonance (two-magnon) and a discrete excitation (phonon), theircombination results in the asymmetric lineshape[40]. The lineshape of the Fano-resonance is describedby:[30]

I(ω) = I0(1 + 2(ω − ω0)/(qΓ))2

1 + 4(ω − ω0)2/Γ2, (4.3)

where, q is the asymmetry parameter where| 1/q | corresponds with the coupling strength of the Fanoresonance, Γ is the linewidth and ω0 the bare phonon frequency. The strength of the coupling constant| 1/q | for Fano-resonance is an indication for the development of antiferromagnetic ordering[30]. For thethin layer regime of flake A below the Neel temperature, the Fano resonance is still observed, indicatingthe magnon-phonon coupling. The bulk crystal and the thin layer region are illustrated in figures 4.3band 4.3c, respectively. Here, the shift of the two-magnon peak center can be seen, indicating a changeof energy of the two-magnon peak due to the ordering of spins. In figure 4.3d, a similar colormap hasbeen made of the Raman spectra obtained from flake B. In figure 4.3e, the Raman spectra at differenttemperatures have been plotted. The experimental conditions were slightly different, because a 1% of thelaser’s total intensity was used and it was taken in the Z(XX)Z configuration. Because of this, onlythe Ag phonon modes are present in this spectrum. Fortunately, the results for both flakes appear to beconsistent with each other.Subsequently, an in-depth analysis of the peak parameters of the two-magnon peak has been made forthe bulk and the thin region of flake A. The peak parameters were obtained by fitting Lorentzians.We analyzed the temperature dependence of the peak center, the peak height, the Full-Width at Half-Maximum (FWHM) and the area under the magnon peak. The results of this analysis are presentedin figure 4.5. These peak parameters give information about the lineshape of the two-magnon peak.

19

(a) (b)

(c) (d)

Figure 4.5: Analysis of the two-magnon peak in flake A. Providing the peak center in (a), the normalizedheight in (b), the area under the magnon in (c) and the FWHM in (d). This is done for both a bulk

region of the sample and a thin region, which is approximately 6 nm in thickness.

The evolution of the lineshape of the two-magnon peak as we go below the Neel temperature providesinformation about the development of the antiferromagnetic ordering, like the energy distribution ofthe excited magnons. In general, direct measurements of the magnetic properties in atomically thinmaterials is difficult, especially for antiferromagnets due to the lack of stray fields. Therefore, the changesin the Raman spectrum as a function of temperature is a good alternative for studying the magnetizationdynamics[39].From the magnon analysis can be concluded that there is an equivalent relationship for both the bulk andthe thin layers. The center of the two-magnon peak is shifting linear towards a larger Raman shift withdecreasing temperature. This is a redshift which corresponds with a energy change of the two-magnonpeak. However, the lineshape of the peak center of the two-magnon does not change near the Neeltemperature indicating that there is no correlation with the antiferromagnetic ordering at TN .

20

In figure 4.5b the intensity of the magnon has been normalized towards the intensity of the magnonat T =77 K. A peak height analysis has shown that the intensity of the magnon peak also increases,when decreasing the temperature. This indicates that there are more scattered photons at that frequencyand therefore more magnons are present for a given Raman shift. The antiferromagnetic transition isclearly visible here because below the transition temperature TN , there is a decreasing relation towardsTN indicating that the magnons are distributed over a larger energy range. Above the Neel temperature,the intensity of the magnon seems to change very little, meaning that the spin configuration does almostnot respond to thermal changes. This is expected because above the Neel temperature, NiPS3 has aparamagnetic structure. These findings correspond with the relation found in a similar experiment onFePS3, for which this has been done for various layer thicknesses including a monolayer[8].In figure 4.5c the area under the magnon in the bulk and thin layers are depicted. The area should beproportional to the volume that is accompanied with the excitation. However, the thin layer has a largerarea than the bulk crystal. Presumably, this is due to constructive interference enhancement effect whichonly applies for very thin layers on a Silicon substrate with a SiO2 layer[41]. The integrated area underthe magnon seems, as expected, to remain at constant value because the same volume is considered whentaking the Raman spectra upon cooling. The results for the FWHM in figure 4.5d show a transition nearthe Neel temperature because the FHWM increases relatively a lot. The fact that the area is constantwhile the FHWM is decreasing with decreasing temperature indicates that the peak becomes higher butmore narrow. This suggests that the energy range of the magnons becomes smaller which leads to moremagnons adopting the same energy.Overall, the analogy between the results for bulk NiPS3 and the thin layers, especially for the peakposition, intensity and FWHM tells us that there is very little difference in the layer number dependence.The evolution over decreasing temperature of the two-magnon peak in the Raman spectra for both the thinlayer region as the bulk crystal show similar properties. Thus, we can conclude that the antiferromagneticordering still holds for layers down to 7.4 nm.

4.3 Polarization selection rules

In this section, we want to observe the Raman spectra for different polarization configurations in thebackscattering geometry. We want to validate the polarization selection rules for the magnonic behaviour.We expect to have two-magnon peaks present in both the parallel and crossed polarization configurations.Therefore, we measure the Raman spectra for the four possible configurations which are described in thefirst chapter. In the previous section we have observed that a broad two-magnon peak emerges uponlowering the temperature. We have seen that this is present in both polarization configurations, althoughwith different intensity. In figure 4.6, the Raman spectra for the possible polarization configurations aregiven according to equation (2.6). This allows us to conclude that this magnon exists due to a mixture ofthe Ag and Bg symmetries. This suggests the presence of a two-magnon band in the spectrum. A similarstudy of these polarization selection rules for the magnon band is done for the antiferromagnet coppermetaborate (CuB2O4)[32]. This material has a completely different structure, but the presence of themagnon peak is defined by its Raman tensors, as is NiPS3. They showed that the magnonic behaviour wasin agreement with the effective Raman Hamiltonian for the different configurations. Although the parallelpolarization configurations show optimal consistency, the orthogonal configurations do not. We wouldhave expected that the Z(XY )Z and Z(Y X)Z have similar Raman spectra. The Z(Y X)Z configurationis showing a three times higher intensity than the Z(XY )Z configuration. The reason for this is ratherunclear.

21

Figure 4.6: The Raman spectra of NiPS3 for different polarization configurations measured at T = 77 K.

4.4 Degradation

The compound NiPS3 is unknown to be air-stable. Therefore, there has been kept track of the degradationof NiPS3 in air. Therefore, two samples have been made by first using the scotch tape technique. Thenone of the samples has been spin-coated with PMMA (poly(methylmethacrylate)). This will makes surethat the sample is protected against the air (water). Changes in the Raman spectra are better observedbecause structural changes have more impact on the spectrum. To study the degradation, the evolutionof the Raman spectra have been observed over a time span of 43 days after exfoliation. The sample wasmaintained at ambient conditions. We expect that if there is degradation that some of the NiPS3 willchange its crystal structure. This will lead to a smaller intensity of certain phonon modes, since there areless vibrations possible in that mode. In this case, however, it is also possible that new phonon modesemerge due to new structures.In figure 4.7a the Raman spectra of a flake at both samples are presented for the first and last measurement.The spectra were taken in the parallel polarization configuration. From the first graph can be concludedthat no other peaks have emerged or vanished. The intensity for the sample with PMMA show lessintensity in comparison with the sample that is not coated. So, the intensity of two peaks on bothsamples have been analyzed in more detail. The intensities of the Ag, Bg mode at 176 cm−1 and the Ag

mode at 384 cm−1 have been determined for the Raman spectra for every measurement. The data isplotted in figure 4.7b, showing almost no changes in intensity of the modes over time. However, bothflakes probably have different thicknesses, leading to different intensity which makes a conclusion lessreliable. Nevertheless, the difference in initial intensity for the same mode on both flakes, minus theoffset between the spectra with and without PMMA, partially make up for this. Also a surprisinglylow intensity is visible for the flake without PMMA which is 21 days after sample fabrication. This isprobably a measurement error because the next measurement showed a much higher intensity.The main conclusion that can be drawn is that the sample barely ages over a time span of 43 days. Thedifference between the samples for a given mode is minimal, although the sample that is coated withPMMA shows a more constant behaviour.

22

(a) (b)

Figure 4.7: Degradation of NiPS3, comparing the Raman spectrum of a sample with only NiPS3 with asample of NiPS3 spin-coated with PMMA. In (a) four spectra are given for a flake with PMMA on day 2

and day 43 and two equivalent spectra are given for a flake without PMMA. The first spectrum wasmeasured on day 2. In (b) the intensity of certain peaks of both samples for increasing time. The spectra

were taken in parallel configuration by room temperature.

23

5 Conclusion

The intention of this thesis is to study the magnetization dynamics in NiPS3 below the Neel temperaturein the thin layer regime. A sample with a thickness of 7.4 nm which corresponds with 10 layers, shows usthat antiferromagnetic ordering is happening, below TN . Thus, the main goal has been achieved.

However, observation of the magnetization dynamics in a few layers or a monolayer of NiPS3 hasnot been succeeded. Despite this state of affairs, the 10 layers show no difference in the evolutionof the two-magnon peak in the Raman spectra upon cooling, even at 77 K. This indicates that theantiferromagnetic ordering is therefore independent down to 10 layers. During the writing of thisreport, a paper has been published reporting Raman measurements in NiPS3 down to a monolayer thick-ness. They show that the antiferromagnetic ordering persists down to the bilayer and stops for a monolayer.

We have also identified the crystallographic axes and the symmetries within the crystal. Rotationmeasurement with polarized Raman scattering showed us that zig-zag directions are formed along theedges of the crystal. These edges make up angles of 60°, in which are armchair directions in between. Thecrystal has a two-fold rotation axis C2(z) which is along an armchair direction. Moreover, the crystal hasa σh mirror plane which is perpendicular to this C2(z) axis.

Furthermore, we have investigated the consistency of the polarization selection rules with the observedmagnonic behaviour. This has been accomplished by observing the Raman spectra was measured at77 K in different polarization configurations. The lineshapes of both Raman spectra in the parallelconfigurations prove to be fully consistent with each other, while the lineshapes of the spectra in thecrossed polarization configuration show discrepancies, which is rather unclear. Nonetheless, these resultsallows us to conclude that these are due to a mixture of the Ag and Bg symmetries.

Finally, NiPS3 has been investigated for its stability under ambient conditions. Therefore, two samplehave been prepared from which one is spin-coated with PMMA. The samples have been exposed toambient conditions for 43 days. After 43 days, there is almost no evidence of degradation.

The research presented with this thesis can be seen as an identification of the molecular and mag-netic structure of the crystal. This research can be used as a framework for further research onspin-source/ferromagnet bilayers which are used for magnetic Random Access Memory (mRAM) devices.Here, NiPS3 can be used as the spin-source material, which allows components of the spin-orbit torque thatare usually forbidden in higher symmetry crystals[7]. This is interesting for manipulating perpendicularmagnetic materials with a higher efficiency.

The sample fabrication and/or observations with the optical microscope could have been executedmore carefully. Then we would probably have found even thinner flakes, and maybe even a monolayer.Measuring the temperature dependence of the Raman spectra of these samples would have given moreinsight concerning the antiferromagnetic ordering in a monolayer.

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6 Acknowledgement

In this chapter I would like to thank some people for the support during this project. I have really enjoyedthis Bachelor End Project.

I would really like to thank Marcos and Casper for being my daily supervisors. They have helpedme a lot during this project, theoretical and experimental. Throughout this project, they have been veryenthusiastic which made me enjoy the last few months. They have introduced me to research, therebyteaching me the skills inside the lab and to write scientifically. Their feedback during the writing process,has improved my scientific writing a lot. Despite the absence of Marcos the last part of the project, hestill provided me the useful feedback for the report. Furthermore, their accessibility and availability madeit very easy for me to ask questions when I got stuck.

Furthermore, I would like to thank ing. Cristian van Helvoirt for the instructions on Raman spec-troscopy and facilitating the setup for cooling. I would also like to thank the FNA group in general forfacilitating the project.

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