Imaging of domain walls in small shape anisotropy

dominated magnetic structures

Michael Beljaars

May 9, 2005

Summary

In the development of fast magnetic memories (MRAMs) the ordering ofmagnetic materials in domains is of great importance. The switching ofthe magnetization of a memory element by means of the displacement of adomain wall is the most important motivation for the research. To preventthe existence of multiple domain walls within the element it is importantto create a structure with a preference for a single domain configuration.When applying a rotating magnetic field to such a structure, a domain wallcan be induced. The final step is to propagate this domain wall using con-trolled current pulses. Because of the extensive dimension of this researchonly the search for a single domain configuration is subject to this internship.

In this experiment Magnetic Force Microscopy is used to examine the do-main configurations in magnetic structures. The results from MFM highlydepend on the characteristics of the device itself and the magnetic probe,the tip, used to scan the sample. Therefore an extensive amount of scansare made to study the behavior of the MFM. Two kind of tips are examined,MFM tips bought at Nanoworld [1] and contact tips bought at NT-MDT[2], which are coated with a magnetic layer by means of sputtering. Withthe Nanoworld tips, the whole system (tip, cantilever, cantilever holder) iscoated with a magnetic layer, whereas with the sputtered tips only the tipis partially provided with a magnetic layer. This difference leads to a highersensitivity for the sputtered tips as can be seen in the scans. Furthermorean important conclusion that can be drawn from this research is that stillfew is known about the tip and tip-sample interaction. More insight in thissystem makes it possible to draw more conclusions out of MFM scans.

The first step towards such a memory element is to create a structure smallenough to display single domain formation. Therefore a sample with smallshape anisotropy dominated CoFe structures is created using e-beam litho-graphy. The structures of the first sample are approximately 1500 nm wide.MFM scans of these structures reveal a multi domain configuration. Non theless these scans are valuable as they confirm the possibility of the accuratelymapping of domain structures.

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A second sample is created in the same way as the first, containing smallerversions of the same structures. Also some alternative structures with elec-trical contacts are made which may be suitable for the next step of theresearch: the actual manipulation of a domain wall. MFM scans of one ofthe last mentioned structures indicates single domain formation in a part ofthe structure that is less than 1200 nm wide.

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Contents

1 Introduction 4

2 Theory 62.1 Scanning Force Microscopy . . . . . . . . . . . . . . . . . . . 6

2.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.2 Measure cantilever deflection . . . . . . . . . . . . . . 72.1.3 Related forces . . . . . . . . . . . . . . . . . . . . . . . 82.1.4 Different modes . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Magnetic materials . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Experimental setup 193.1 AFM Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.2 Magnetic tips . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.3 Artifacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.3.1 Probe artifacts . . . . . . . . . . . . . . . . . . . . . . 213.3.2 Scanner artifacts . . . . . . . . . . . . . . . . . . . . . 233.3.3 Vibrations . . . . . . . . . . . . . . . . . . . . . . . . . 253.3.4 Interference . . . . . . . . . . . . . . . . . . . . . . . . 253.3.5 MFM artifacts . . . . . . . . . . . . . . . . . . . . . . 26

4 Measure results 284.1 Exploring the setup . . . . . . . . . . . . . . . . . . . . . . . 284.2 Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2.1 Nanoworld tips . . . . . . . . . . . . . . . . . . . . . . 294.2.2 Sputtered tips . . . . . . . . . . . . . . . . . . . . . . 31

4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.3.1 E-beam sample I . . . . . . . . . . . . . . . . . . . . . 344.3.2 E-beam sample II . . . . . . . . . . . . . . . . . . . . 38

5 Conclusion and discussion 445.1 Interpreting MFM images . . . . . . . . . . . . . . . . . . . . 445.2 Coating tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.3 Magnetic Domains . . . . . . . . . . . . . . . . . . . . . . . . 45

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Chapter 1

Introduction

In the development of new high density storage media the dynamics of mag-netization play a great role. With the development of research techniquesas Magnetic Force Microscopy (MFM) new possibilities to explore the or-dering of magnetic structures on microscopic level have become available.Although MFM provides high resolution images of static magnetization con-figurations, it can not image the dynamics itself. By means of a discreet fieldsweep and making scans at the different fields one can acquire insight in thedynamic processes of magnetization.

One of the key features of magnetic materials that can be useful for appli-cations is the ordering in magnetic domains, in particular the domain wallsthat separate the domains. Above a certain characteristic length scale ofthe magnetic structure, typically 1 µm, the formation of domains is a en-ergetically favorable situation. Structures below that limit favor a uniformmagnetization, unless the form of the structure is chosen in a smart way.Because of shape anisotropy such a structure can develop two domains sep-arated by a single domain wall if an external field is applied in the rightdirection.

Domain walls can be manipulated by an external field, but also by applyingcurrent pulses perpendicular to the domain wall. This effect is called thespin torque effect. It is due to the transfer of spin torque from the electronsto the domain wall. A controlled shifting of the domain wall is possible giventhe right length and intensity of the current pulses. Combining domain wallmovement with a Tunneling Magneto Resistance (TMR), one creates a elec-trically switchable Magnetic Tunnel Junction (MTJ). This structure has allthe features necessary to create the ideal memory (Magnetic Random Ac-cess Memory or MRAM): fast access time, fast read / write time, persistent

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without power and no wear out.

The first step towards the development of these MRAMs is to creature shapeanisotropy dominated structures that are small enough to naturally displaysingle domain formation. Therefore special shaped magnetic structures arecreated with e-beam lithography combined with sputtering and subsequentlyscanned with the MFM. Then, by applying an external field in the rightdirection, a domain wall should be created. A scan with the MFM willprovide the proof for the presence of a domain wall.

In chapter two a overview of the theory on the AFM / MFM setup is givenfollowed by a short introduction on magnetic domains and the relevant mech-anisms that play a role in the formation of these domains. Chapter threedescribes the actual setup that is used and subsequently the possible artifactsthat must be accounted for. In chapter four the results of the experiment arepublished, beginning with some example scans made to explore the setup. Inthe following subsection the behavior and resolution of the used tips (bothspatial as magnetic) is examined. The chapter ends with the presentationof the results on two samples, E-beam sample I and II, specially created toinvestigate the single domain formation. The report is concluded in chapterfive with a discussion on the results and a overall conclusion.

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Chapter 2

Theory

2.1 Scanning Force Microscopy

2.1.1 Overview

Scanning Force Microscopy (SFM) is undoubtedly the most important break-through in the imaging of surfaces of the 20th century. Using the rightequipment and settings it is possible to even make scans with atomic reso-lution.

The general idea of this technique is to scan a surface, using a probe. Thisprobe, normally referred to as the ‘tip’, is mounted on a holder, called the‘cantilever’. The tip can interact on various ways with the surface. It canbe used for example to measure the interaction due to the Coulomb forceor the Van der Waals forces. This gives an image of the topography of thesample. But also the imaging of local electric or magnetic forces is possible,given the right probe. The relevancy of the different forces for scanningprobe microscopy is described in the subsection Related forces.

The interaction of the tip with the sample causes a change in the orientationof the cantilever, which can be measured in several ways. These are discussedin the subsection Measure cantilever deflection.

Also there are a few different manners to probe the sample. Depending onwhich interaction is to be measured and what kind of tip is used, certainmodes are suitable, where others aren’t. In the subsection Different modesa variety of modes and their appliances is discussed.

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Because the extensive area of possibilities only the relevant part of SFM isdiscussed. Mainly Atomic Force Microscopy (AFM) in semi-contact modeand AC Magnetic Force Microscopy (AC MFM) are relevant for the exper-iments and are therefore are dealt with more extensively. For a full outlineon SFM it is advised to read Scanning Tunneling Microscopy II [5] and theinformation on SFM on the website of NT-MDT [2].

2.1.2 Measure cantilever deflection

The deflection of the cantilever is due to the interaction of the tip with thesurface. The relation between the interacting force F and the deflection δzis given by The Hooke’s Law

F = c · δz (2.1)

where c is the force constant of the cantilever. The magnitude of the forceconstant depends on the dimensions of the cantilever as well as it’s materialand the temperature. The latter isn’t relevant because the measuring occursat constant (room) temperature.

To measure the deflection several techniques, like tunneling current method,optical interference, capacitance method and laser beam deflection can beused. The last mentioned is used in the experiments. The functioning ofthis method is displayed schematically in figure 2.1.2.

Because of the deflection of the cantilever, the reflection of a laser beam onthe rear side of the cantilever changes in orientation. A position-sensitivedetector (PSD) senses the location of the reflected beam. Trough calibra-tion the signal of the PSD can be used to ascribe a absolute value to thedeflection. In comparison to other techniques the Laser Beam Deflectionmethod is very basic and easy to apply. It’s influence on the cantilever isnegligible and gives a reasonably good resolution. Furthermore it does notneed a very clean (vacuum) environment like the tunneling current method.

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Figure 2.1: Laser Beam Deflection

2.1.3 Related forces

Van der Waals forces

The attractive Van der Waals Forces are due to the interaction of electricdipoles and exist between all atoms or molecules. The interaction of theseforces is used to make topographic scans of surface There are basically threetypes of such an interaction:

• permanent dipoles interacting by a dipole-dipole interaction

• the induction of a dipole in a non-polar molecule by a permanentdipole, therefore creating a interaction between the permanent andthe induced dipole

• spontaneous dipole creation due to fluctuations in charge distribution,with the possibility to induce another dipole in a non-polar molecule,which can interact with the permanent dipole.

The Van der Waals Forces are relevant for distances between a few and a fewhundred Angstroms. As shown in figure 2.1.3 at short distances the overlapof electron orbits causes a repulsive interaction, when at large distances thedipole interaction causes a attractive force.

Capillary forces

Because of the small curvature of the tip a meniscus of water can be formedaround it. Typically a tip of 1000 A or less is a nucleus of condensation,

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Figure 2.2: Van der Waals potential curve

providing that the environment contains vapor. The formation of this wa-ter layer causes an additional force between the surface and the tip. Anapproximation of this force is given by the equation

F =4πRγ cos Θ

(1 + s/(R(1− cos φ))(2.2)

where γ is the surface tension, R the radius of curvature, Θ the contact angle,s the distance between tip and sample and φ the angle of the meniscus asshown in figure 2.1.3.

Figure 2.3: Schematic drawing of the tip

Given this equation a value for the maximum force

Fmax = 4πRγ cos Θ (2.3)

can be derived. For a tip with a curvature of 1000 A a maximum forceFmax = 9.3 · 10−8N is found. This relatively large in comparison to thetypical operating forces, which are of the order of 10−7) to 10−8 N.

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Magnetic forces

Providing the AFM with a magnetic tip it becomes possible to image mag-netic structures on surfaces. Because of the long range of magnetic forcesscans can be performed at several hundreds of Angstroms distance from thesurface. If the distance to the surface is maintained constant, the Van derWaals forces do not change as function of the place above the sample. Infigure 2.1.3 and figure 2.1.3 these forces are visualized. Figure 2.1.3 showsthe distance dependance of the topographic forces. The lighter the color,the weaker the force is. As appears in the figure, the force is constant for acertain fixed distance to the surface.

Figure 2.4: Distance dependence of the Van der Waals forces

The magnetic forces however still vary as function of the place over the sam-ple due to the difference in magnetization at the surface of the sample (seefigure 2.1.3). The colors assigned to the different magnetization direction arearbitrarily chosen. Furthermore for simplification only four possible mag-netization directions are used. The colors representing magnetization aredrawn lighter as the distance to the sample increases, indicating that alsothe strength of magnetic forces diminishes with distance. The differencein magnetization depending on the position over the sample neverthelessremains.

Figure 2.5: Distance dependance of the magnetic forces

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Electrostatic forces

Similar to the imaging of magnetic forces it is possible to scan a surface mea-suring the electrostatic interactions between a charged tip and the sample.This scanning method is mainly useful for measurement at insulators. Prac-tice has proven it is possible to measure individual electrons or currents downto 10−19A. In this experiment a possible influence of electrostatic forces isnot desirable, so the device is provided with an earth connection. This rulesout the possibility of images features caused by electrostatic forces.

2.1.4 Different modes

The possibilities and modes of Scanning Force Microscopy are almost infi-nite, therefore only the techniques that are used, are described more exten-sively. Depending on the used mode an other region of the surface potentialis used for scanning as shown in figure 2.1.3.

Contact techniques

The highest spatial resolution is achieved in contact mode. Depending onthe environment and the size of the tip a atomic resolution can be achieved.As the name suggests in this mode the tip is in constant contact with thesurface of the sample, which means that the interaction takes place in therepulsive region of the Van der Waals force. One important condition forthis mode to work properly is a relatively hard sample, otherwise the tipjust scratches the atoms or molecules of the substrate. Biochemical samplesfor example are mostly to soft to be scanned in contact mode.

Non-Contact techniques

The non-contact mode is most suitable to scan soft samples. Because theworking region is the attractive part of the potential (figure 2.1.3), the tipdoesn’t touch the surface. In this way damaging of the surface is prevented.Unfortunately the non-contact technique has a lower spatial resolution andsensitivity compared to contact or semi-contact techniques.

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Semi-contact techniques

Semi-contact mode or Tapping mode means briefly tapping the surface whilescanning. Briefly tapping means jumping from the attractive to the repulsiveregion of the surface potential (see figure 2.1.3). In this way a numberof point measurements is combined to form a image of the surface. Theadvantage of this mode in comparison to the contact mode is that non orless damage is done to the sample and the spatial resolution is better than innon-contact mode. However the optimal resolution is still below the atomicresolution of the contact mode. Semi-contact techniques can be divided intwo methods, which are discussed in the next two paragraphs.

Semi-contact mode In semi-contact mode the cantilever is driven in itsresonance frequency. By means of a feedback signal the amplitude of thecantilever is maintained constant. As a result the feedback signal containsinformation about the topography of the sample.

Phase Imaging mode Using a different method also other data of thesurface of the sample can be acquired. Due to the force working on the tipthere is a change in frequency, which causes a difference in phase betweenthe resonance frequency, in which the cantilever is driven, and the actualfrequency in which the cantilever oscillates. By looking at this phase shiftinformation about local forces on the surface can be obtained. Still keepingthe amplitude at a constant level, the surface is scanned. Depending on theinteraction of adhesive forces a different phase shift is to be found, givinginformation about the homogeneity and composition of the surface.

Many-pass techniques

When scanning the long range interactions of a sample, like magnetic orelectric interactions, it is important to keep a certain constant distance be-tween the tip and the surface of the sample. Maintaining a fixed distancethe Van der Waals force is constant. The only way of doing this properly isexactly knowing the topography of the surface. Therefore in the first pass,the topography is scanned and subsequently, in the second pass, the tip israised en maintained at a certain height, while scanning the same line again.

AC Magnetic Force Microscopy A sensitive way to measure magneticinteractions is in vibrating, AC MFM, mode, using a magnetic coated tip.

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In stead of detecting the interacting force, the gradient of the stray field ismapped, which makes it possible to detect much smaller interactions thanin non vibrating DC MFM mode.

For the second pass the feedback is turned off leaving the cantilever freelyvibrating in the driven (resonance) frequency ω0. For small amplitudesthe cantilever can be approximated as a harmonic oscillator. In case ofno additional interacting force the force F on the end of the cantilever isproportional to the amplitude z and is given by

F = −k0z (2.4)

where k0 is the spring constant. The resonance frequency ω0 of the systemin terms of the spring constant k0 and the mass m is then given by

ω0 =

√k0

m. (2.5)

When a magnetic force acts on the tip, the spring constant is altered inaccordance to

k′ = k0 +dFm

dz. (2.6)

Substituting this in the equation for the resonance frequency, the shift ofthe resonance frequency is found.

ω′ = ω0

√1 +

1k0

dFm

dz(2.7)

For small gradients of the force the square root can be approximated by

√1 +

1k0

dFm

dz≈ 1 +

12k0

dFm

dz(2.8)

resulting in a shift of the resonance frequency given by

∆ω = ω′ − ω0 =1

2k0

dFm

dzω0. (2.9)

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To determine this shift the new resonance frequency has to be redeterminedfor every point of measurement. That is why this method is a time con-suming way of scanning a magnetic surface. Therefore normally not thefrequency shift, but the shift in phase is used as a measure for the gradient,which is given by

∆φ =Q

k0

dFm

dz(2.10)

The magnitude of the phase shift clearly depends on the quality factor Q,which is a measure to what degree the system retains its energy. For a highsensitivity a good quality factor is required. Figure 2.6 shows the phase shiftfor a relatively high and a relatively low quality factor.

Figure 2.6:

The magnetic force is caused by the interaction of the stray field H of asample with the magnetization M of the tip and is given by the equation

F = ∇(M ·H). (2.11)

Approximating the tip as a magnetic dipole, there are two cases to con-sider: (1) the perpendicular Mz and (2) the parallel alignment Mxy of themagnetization of the tip to the sample surface. The first case relates the zderivative of the force to the z component of the stray field by

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dFm

dz= Mz

d2Hz

dz2(2.12)

whereas the second case gives the equation

dFm

dz= cos γMxy

d2Hxy

z2(2.13)

relating the z derivative of the force and the in plane component of the strayfield Hxy, where γ is the angle between the magnetization of the tip Mxy

and the stray field Hxy. In both cases the z derivative of the force is relatedto the second derivative of the stray field.

Because in this mode the MFM is only sensitive to the gradient the bestcontrast is seen where there’s a lot of divergence in the stray field. Becausethe divergence of Hxy is generally small compared to Hz most contributionsof the signal will be due to the z component of the stray field. Thereforebest contrast may be expected near transitions in magnetization, where thestray field comes out or goes into the sample.

2.2 Magnetic materials

Many books are written about the extensive subject of magnetism and theorigins of domains that it would be too much to discuss all theory here.Therefore only a qualitative treatment of the subject is given here, which isbasically enough to provide a logical explanation of the results. For moreinformation it is recommended to read [4].

The magnetic properties of materials originate from the magnetic moment ofthe electrons in the materials. This moment consists of a magnetic momentdue to the angular motion of the electron around the nucleus and of anintrinsic magnetic moment which can be associated with the spin of theelectron.

Depending on the filling of the atomic shells and the density of states atthe Fermi level, a material can have a certain magnetic character like dia-magnetic, paramagnetic or ferromagnetic. At room temperature the CoFeused in the experiment belongs to the latter category. Above the so calledCurie temperature ferromagnetic materials become paramagnetic. For Co

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and Fe this temperature is much higher than room temperature. Ferromag-netic materials have a net magnetic moment and align parallel to an externalfield.

The way in which the density of states is filled determines several charac-teristics. One of these characteristics that is of importance to evaluate themutual influence of magnetic systems is coercivity. The coercivity is relatedto the field necessary to reduce the materials natural magnetization to zero.A material with large / small coercivity is often referred to as a hard / softmagnetic material. Thus a soft magnetic material is easily influenced by aexternal magnetic field, whereas a hard magnetic material is persistent inits magnetic configuration.

As all system encountered in physics until today, a magnetic system evolvestowards a minimum energy given its conditions. For a magnetic materialenergy contributions from several effects are important to consider.

The preference to align with structural axes of the sample is called anisotropy.The energy associated with this effect depends on the direction of the mag-netization relative to the specific axis and has its minimum if the magne-tization aligns with this axis. The most important anisotropous effects arecrystal anisotropy (the preference to align with crystal axis), surface andinterface anisotropy and exchange anisotropy (the preference to align withneighboring spins).

Ferromagnetic structures tend evolve to a equilibrium with a constant mag-netization direction. In this case all magnetic moments are aligned. Anydeviation of this ideal case will cause a increase in so called exchange energy.This energy is due to the interaction of neighboring spins.

The magnetic field energy of a magnetic sample can be divided in two parts.The first is the applied field energy, also called the Zeeman energy, which iscaused by the interaction of the magnetization vector field with an externalfield. The second part is the stray field energy. This is the field generatedby the magnetic structure itself. An effective way to minimize this field isflux closure. This means that the magnetization aligns in such a way thatmost of the magnetic flux is kept within the sample. This effect makes theorigination of domains a energetically favorable situation.

Magnetic domains are regions of uniform magnetization and appear evenin unstructured magnetic samples. At the border of the domains, so calleddomain walls are formed. In these domain walls, which are typically severalhundreds of lattice constant in width, the magnetization gradually changesdirection. Although it takes some energy to form domain walls, in manycases the formation of domains (and thus domain walls) reduces the stray

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field energy more than the system gains on domain wall energy. The figures2.7 to 2.9 show some examples of possible domains.

Figure 2.7: Magnetic structures of permalloy (left: topography, right: mag-netic image)

Figure 2.8: Magnetic structures of permalloy (zoom) (left: topography,right: magnetic image)

Figure 2.9: Magnetic domains of garnet film

Below a certain spatial limit the formation of domains is not energetically

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favorable any more. A structures with one or more dimensions below thislimit will have a uniform magnetization direction. However domains can beforced into these structures if their shape anisotropy is large enough to allowsfor this. Therefore if the shape and dimensions of a structure are smartlychosen, an external field applied in the right direction should induce a singledomain wall in the structure.

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Chapter 3

Experimental setup

3.1 AFM Setup

For the experiments the Solver P47H Pro of the Russian company NT-MDTis used. To minimize distortions due to floor vibrations the device is placedon a actively dimmed table which itself is placed on a heavy stone table.

(a) (b)

Figure 3.1: a)Overview of the setup and b) a zoom in on the actual AFMsetup.

The base of the Solver P47H Pro contains a sample holder driven by a stepengine, which automatically brings the sample to the tip until a certain valueof the feedback signal, the setpoint, is reached. On top of the base, abovethe sample holder, the actual scan head of the AFM (see figure 3.2) is found.

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Figure 3.2: The AFM head

The setup contains a camera which images the sample trough a mirror inthe AFM head. The image of the camera is displayed on a monitor. Thisimage is very useful for placing the laser spot correctly on the cantilever andto search the right area of the sample.

The sample holder can be equipped with a electromagnet. This electromag-net consists of a pair of poles, creating a field parallel to the surface. Themaximum field that can be applied is approximately 500 Gauss.

A Hall probe is used to determine the strength and direction of the appliedfield.The AFM is electronically connected to a computer to collect, storeand analyze the data.

3.2 Magnetic tips

Two different types of magnetic tips are used in the experiment. One typeof the magnetic tips is bought at Nanoworld [1]. These tips are coated withCoFe, which is a relatively hard magnetic material. The silicon cantileverof these tips is also covered with a CoFe layer.

The other type is created from normal non-magnetic tips from NT-MDT [2]are provided with a magnetic layer. For deposition of the magnetic layer asputter mask is used. The mask can contain 4 cantilevers with tips. Thecantilevers are mounted vertical in the mask in such a way that the frontside of the tip is aligned towards the opening in the mask (see figure 3.3).The mask with cantilevers is placed in a sputter machine. By aiming highenergy particles on a target sample material is sputtered. Mostly an Argon

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plasm is used for this purpose, because Argon is an inert gas. Therefore itdoesn’t attach to the sample surface. Part of this material damp evaporatedby the Argon floats trough the mask and condenses on the tip, forming alayer of the desired material.

Figure 3.3: Deposition of a (magnetic) layer on a tip by sputtering.

3.3 Artifacts

It’s important to realize that the images created with AFM / MFM arenot always representing real structures or magnetic properties. To drawconclusions from these pictures a critical look is therefore necessary. Thereare a lot of different artifacts that can occur. These are discussed in thefollowing sections.

3.3.1 Probe artifacts

The shape of the tip has a large impact on the final image that is obtained.Depending on the characteristics of the tip, a surface object can look bigger,less deep, deformed or appear multiple times.

Figure 3.4 gives an example of how a structure looks bigger than it actuallyis. When the curvature of the tip is of the order of the size of the structure,the image of the structure is magnified. If the curvature is one or more

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order of magnitude smaller than the structure being imaged, this effect canbe neglected.

Figure 3.4: Depending on the tip curvature a object can look bigger.

The width of the tip is of importance when it comes to measuring featuresthat are below the surface (see figure 3.5). In this case the tip gets stuckbecause of it’s width before it reaches the bottom of the pit.

Figure 3.5: Tip artifact affecting the measured depth.

Furthermore performing a scan correlates the tip with the surface, creatingstrange fake features that originate from the shape of the tip as shown infigure 3.6.

Figure 3.6: Tip artifact affecting the shape of features.

Small features that are scanned width a relatively large tip reflect the geom-etry of the tip, rather than their own. In figure 3.7 a example of a so calleddouble tip is shown. This defect causes an object to appear double in thescan image, first scanned by the front tip , second by the tip on the back.

The basic rule for artifacts caused by the geometry of the tip is when thetip curvature is small compared to the structures that are to be imaged, theinfluence of these artifacts is small.

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Figure 3.7: Double tip causing multiple appearances of the same object.

3.3.2 Scanner artifacts

When evaluating the results of a AFM / MFM setup one should consider thewhole system because also the characteristics of the scanner will be visiblein the images.

One of the most common scanner artifacts is due to a too high scanningspeed. In this case the system is to slow to keep track of the surface. De-pending on the scan direction, structures in the image will appear sharp onone side but blurred on the other side because of a artificial slope in thesignal as shown in figure 3.8. The extra bump in the signal is called an edgeovershoot and is a very common AFM artifact.

Figure 3.8: With the scan made in the positive x direction, the slope on theright side is much steeper than on the left side of the structure.

An other kind of scanner artifact can occur when the angle between thecantilever and the sample is too large. The side of the tip hits the structure,instead of the point of the tip, which leads to a distorted images as shownin figure 3.9.

Figure 3.9: Because of a too large angle, the tip hits the structure at thebeginning with its side.

To avoid deformed images (see figure 3.10) it is necessary to be able to count

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on the linearity of the movements of the scanner. Although one can correctfor possible non-linearities in the piezoelectric elements, other artifacts mayarise from the non-linearity that can not be accounted for. Therefore itis preferable to work with a (near) linear system. To be able to relatedimensions to scanned structures the movements of the X and Y piezoelectricelements have to be properly calibrated.

(a) (b)

Figure 3.10: Providing a test grid with squares, a linear scanner (a) and ascanner with a non-linearity in the X direction (b).

In the same way the Z direction should be calibrated and act linear toguarantee non-deformed images representing real heights.

Because of the curved way the piezoelectric elements move the cantileverover the surface during a scan, there is a certain background bow to be seenwhen scanning a large area (see figure 3.12). This effect is unavoidable butcan be compensated by subtracting the background from the image. Nonthe less, for the most reliable results it is better to keep the scan area limited,thereby keeping the scan movements almost linear.

Figure 3.11: Curved motion of the cantilever, creating a background bow.

A second possible cause for an edge overshoot is hysteresis in the piezo-electric element that controls the Z position of the cantilever. Edges ofstructured samples appear higher on one side and lower on the other side(see figure 3.12). This effect makes the image look better, because the edgesappear sharper. Non the less it is not a real feature of the sample.

Logically hysteresis can also occur in the X and Y piezoelectric elements.This artifact also often happens at the beginning of a new scan, when the

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Figure 3.12: Edge overshoot / undershoot because of hysteresis.

new area is just set. The tip has to be moved into the right position and ittakes some time to stabilize (see figure 3.13).

Figure 3.13: Hysteresis in the X and Y elements cause drift at the begin ofa new scan.

Due to changes in temperature or thermal drift in the piezoelectric elementsthe scan area can be shifted over time. If the temperature during a scancan be supposed constant, this effect is mostly seen between two successivescans.

3.3.3 Vibrations

Because of the scale of measurement a AFM is very sensitive to both floorand acoustic vibrations. To avoid influence from floor vibrations the AFMsetup is placed on an actively dimmed table, which itself is placed on aheavy stone table. Non the less one should not underestimate the effectof acoustic noise. Something little as a cough can dramatically distort thesignal. Therefore it is advisable to cover the AFM with a protective capwhile measuring.

3.3.4 Interference

When using laser beam deflection it is inevitable that some of the laser lightreflects on the surface. Unfortunately it can happen that the light is reflected

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on the photodiode and interferes with the reflected light of the cantilever.This interference creates a image of a seemingly ordered structure on thesurface, but is nothing real. To solve this problem it mostly is enough tochange the angle between the sample and the AFM head, thereby directingthe reflection away from the photodiode.

Figure 3.14: Interference from the reflection of the surface with the reflectedlight of the cantilever.

3.3.5 MFM artifacts

Compared to AFM artifacts many MFM artifacts are difficult to recognize.The first difficulty is to determine wether or not the signal that is detected isreally magnetic. Scanning high above the surface(> 3000A) makes it likelyto only detect magnetic signal, but mostly gives a poor signal. Scanning closeto the surface however results in a clearer signal, but makes it questionableif the signal is purely magnetic. At these distances it is possible that thetip slightly touches the surface. In this way not a pure magnetic signal isimaged, but the image will contain some topographic features. Sometimeshitting the surface gives such a distortion on the phase signal that it iseasy recognizable as an artifact. However most of the time it is not easy tosay if the magnetic scan is indeed purely magnetic or partially (or entirely)topographic.

In explaining the behavior of the tip-sample magnetic system two proper-ties are relevant, which are the coercivity and strength of the stray field.The meaning of coercivity is already explained in de chapter on MagneticMaterials. The strength of the stray field is the external field induced by amagnetic material. It is because of these stray fields that a technique likeMFM works.

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Measuring a magnetic sample with a magnetized tip, one must take in ac-count the possible mutual influence of the tip and the sample. As followsout of the theory on coercivity and strength of the stray field, the most de-sirable situation is to work with a magnetically hard tip with a small strayfield. Magnetically hard means in this case that the tip has a coercive fieldhigher than the stray field of the scanned sample. The stray field may becalled small as long as it is lower than the coercive field of the sample. Withsuch a tip it is possible to scan relatively soft magnetic samples with a largestray field without significantly influencing the tip or the sample.

In practice too little is know about the effective stray field of both tipand sample, even as their effective coercivity. Different experiments canbe thought of to gain more insight in these properties, however this is notthe goal of this internship. Therefore, until there are methods to character-ize the tip, the only way to determine influences of the tip on the sampleor the other way around is by looking at the scan image. Certain influencesare indeed easily to recognize out of a scan image. A common influenceartifact when scanning with a relatively soft magnetic tip is the switching ofthe magnetization of the tip. It inverts the image from the scan line wherethe switching took place. It’s very ease to recognize as an artifact becauseof the inversion.

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Chapter 4

Measure results

The AFM scans are made in semi-contact mode, while the MFM scans wereperformed in AC MFM.

4.1 Exploring the setup

To explore the AFM and it’s possibilities and restrictions, at first a series ofscans on test samples is performed. The same is done for MFM with piecesof harddisk. In the figures 4.1 and 4.2 some examples of scans are showed.

(a) 3.5 x 3.5 µm topographic image (b) 8 x 8 µm topographic image

Figure 4.1: The step structure of Aluminum Oxide (a) and Aluminum Oxidewith gold particles (b).

Figure 4.1a shows how Aluminum Oxide (AlOx) grows in a order manner

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depending on the crystal lattice. Furthermore the image shows a strangedeveloped canyon within the AlOx, probably due to a contamination duringthe grow of the sample. In figure 4.1b a scan of a AlOx sample covered withgold particles is showed. The gold particles appear sharp at the left side andrather blurred at the right side. This is not a real feature as discussed in theArtifacts part of the Experimental Setup, but is caused by the apparentlytoo high scanning speed.

(a) 28 x 28 µm topographic image (b) 28 x 28 µm magnetic image

Figure 4.2: MFM scan of a piece of hard disk.

Because of the movement of the hard disk head on the disk scratches appearon the surface as is to see in figure 4.2a. The bit pattern aligns with thesescratches (figure 4.2b).

4.2 Tips

The tips used for AFM / MFM form a very important part of the setup,because the characteristics of the tip are reflected in the scan image asdiscussed in the chapter on probe artifacts. To develop more understandingin the behavior of the magnetic tips both the tips bought at Nanoworld asthe sputtered tips are tested.

4.2.1 Nanoworld tips

Figure 4.2 shows a scan of a piece of harddisk. This scan is made using aNanoworld tip. Evaluating the width of the smallest bit the spatial resolu-tion is of approximately 200 nm.

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Because of the pour results with one of the Nanoworld tips, an Yttrium IronGarnet (YIG) sample is used to test the tip. YIG formes very nice magneticpatterns. As was to be expected no magnetic contrast can be seen in thescan. Remagnetizing the tip does not affect the magnetic interaction withthe surface. Combining this with the fact that the tip has crashed into asample a few times leads to the conclusion that most likely a part of the tiphas been damaged. This can cause a dramatically reduction of the effectivemagnetic moment of the tip, resulting in almost no magnetic interactionwith the sample.

With a new Nanoworld tip the YIG sample is scanned again, obtainingthe images shown in figure 4.3. Although there are clearly some artifactsin the image, it is clear that image 4.3(b) shows the magnetic influence ofthe sample on the tip. The apparently frayed magnetic structure could bethe result of the magnetic influence of the tip on the sample. But afterreducing the scanning speed, the artifact disappears. The effect is clearly aconsequence of a too high scanning speed. The system basically can’t keepup with the changing stray field.

(a) 23 x 23 µm topographic image (b) 23 x 23 µm magnetic image

Figure 4.3: YIG sample scan with working Nanoworld tip.

Figure 4.4 shows how the magnetic signal of the defective tip is affected byan applied field. This image is obtained by varying the applied external fieldduring a single scan of the YIG sample. At 0 Gauss there is only a weakmagnetic signal and when applying an increasing external field the signaleventually entirely disappears. The pour signal at 0 Gauss implies a smalltip magnetization perpendicular to the sample surface. As the field parallelto the sample surface is increased, the tip magnetization is canted moreparallel to the sample surface. Eventually this results in a zero perpendicularcomponent of the magnetization (see figure 4.5). The damaging of the tip

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makes it likely for this process to happen because of breaking the symmetryof the tip and changing its shape anisotropy.

Figure 4.4: 16 x 16 µm magnetic scan showing the field dependance of theMFM signal. From left to right the vertical stripes belong to an appliedfield of 0, 500, 0, 100, 50, 25, 10 and 0 Gauss.

The defective tip is measured in a SQUID to look for anomalies in themagnetization curve which may explain the strange behavior. The result ofthe measurement is shown in figure 4.6. The jumps that are visible in figure4.6(a) are caused by the measurement sequence. The shifted parts (C1 toC4) of the diagram are measured after the other parts. Probably the tip hasshifted a little bit between these measurements.

A zoom of the relevant part of the figure is displayed in figure 4.6(b). Itdoesn’t show any certain jumps, in fact it is what was to be expected: themagnetization curve of the total tip and cantilever. To get information aboutthe magnetic behavior of the tip alone a more local measure technique isnecessary. The Magneto Optical Kerr Effect (MOKE) could be used forthis, providing that it is possible to focus only on the tip.

4.2.2 Sputtered tips

Normal AFM tips are coated with a magnetic layer using a mask and thesputter machine. The advantage of this technique is that only the tip willbe coated with a magnetic layer.

In a first attempt to create MFM tips non-contact tips are coated with alayer of CoFe. For good adhesion first a thin layer of Magnesium is sputtered.

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Figure 4.5: Rotation of the tip magnetization as consequence of the increas-ing applied field.

To avoid oxidation of the CoFe the layer is inclosed in Aluminum. In table4.1 an overview of the different sputtered layers is showed.

The tips are tested on a piece of harddisk to characterize their response tomagnetism. Unfortunately it turns out that it is practically impossible toretain a reasonable magnetic signal with these tips. The noise is of the sameorder as the actual signal. In a second attempt the same coating sequenceis used on contact tips, with more success as shows figure 4.7. This scan ismade with the first tip that was to be tested.

Both the topographic as the magnetic image show one strange feature,namely a horizontal band. The probable cause for this artifact is a additionalparticle that got stuck on the tip. This explains why the topographic imageshows a sudden change in height and why the magnetic image is blurred atthe location of the band. Because of the particle the distance of the tip tothe sample is raised with a certain amount. This increase in distance alsocauses a lower spatial magnetic resolution (the blur).

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(a) Total measurement range

(b) Enlargement of the rectangular area (A1 andA2)

Figure 4.6: SQUID measurement on the defect Nanoworld tip.

After this successful scan, the e-beam sample I (see next section) is scannedwith the same tip, resulting in the image shown in figure 4.8. This imagealso shows a resolution improvement in comparison to the NanoWorld tips.

A second scan sequence with the first tip astonishingly hardly gives a visiblemagnetic signal, whereas the third try again results in a very clear imagewith very good contrast. The other three tips appear to be useless afterseveral attempts to optimize the magnetic signal by adjusting the distanceto the sample and the amplitude. To understand why these tips behave sounpredictable and why some of them don’t even work, more research has tobe done on the coating of these tips. Most probably some Scanning ElectronMicroscope (SEM) scans and SQUID measurements will tell more about thesputtered magnetic layer.

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Table 4.1: Sputtered layers on the tips

Material Rate (A/s) Time(s) Layer thickness (A)

Mg 1,2 300 50,5Al 0,5 300 21,0

CoFe 0,6 3600 300Al 0,5 600 42,1

(a) 28 x 28 µm topographic image (b) 28 x 28 µm magnetic image

Figure 4.7: Harddisk scan with self sputtered tip.

4.3 Results

4.3.1 E-beam sample I

On a GaAs substrate 50 nm high structures are created using e-beam litho-graphy (see figure 4.9). The design of the sample is shown in figure 4.10.

The structures [3] are created in eight sets of nine, numbered as shown infigure 4.10. Each set consists of three structures of 10 µm width, three of 5µm width and three of 1 µm width. The length of the legs of the structuresscales with the width, going from approximately 150 µm for the 10 µmwidth to 15 µm for the 1 µm width.

The structures consist of a 90 ◦ curve with on one end a diamond shapedpad and on the other end a sharp needle. The diamond at one side of the

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(a) 4 x 8 µm topo-graphic image

(b) 4 x 8 µm mag-netic image

Figure 4.8: E-beam sample I scan with self sputtered tip.

structure reduces the field necessary to form a domain wall. The other sideis sharpened to make it easy for domain walls to leave the structure.

At first a topographic scan is made to check if the structures are really thereand if they have sharp borders as they should. Figure 4.11 shows this scan.

The image shows a nicely sharp edged structure without serious pollution.As will appear in following images, due to the exposition to normal air, overtime dust particles are attracted to the sides of the structure.

Second a MFM scan is performed on one of the 1 µm structures. This scanis shown in figure 4.12.

Clearly visible is the appearance of a multi domain structure in the cornerof the structure as well as in the diamond shape. Evidently even the 1µm structures are not small enough to display single domain. The 5 µmand 10 µm structures are therefore refrained from scanning. Accuratelyremeasuring the width of the supposedly 1 µm structure reveals an actualwidth of approximately 1 µm for the straight parts but more than 1500 nmin the corner of the structure. Further the structures appear to be 80 nmheight in stead of the expected 50 nm.

To research the effect of an external field on the domain structure severalscans are made at different field strengths. Figure 4.13 gives an overview ofthe results.

The first scan at 0 Gauss is a little bit shifted, but it is clearly visible that the

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Figure 4.9: This figure explains in six steps how e-beam lithography works.First (a) the GaAs substrate is equipped with two different layers of photoresist, PMMA950 and PMMA450. Second (b) a focused electron beamis used to write the desired structures in the photo resist. Because thePMMA450 is more sensitive to the electron beam more material will beaffected as shown in the figure. The next step is to develop the photoresist and etch the exposed material away, leaving the structure as shown in(c). Then with sputtering the sample is provided with a layer of CoFe (d).Because of the sputtering there will not only be CoFe on top of the sample,but also in the etched holes. After this step the other photo resist is etchedaway, leaving only the substrate with the CoFe structures and a separatelayer of CoFe, which can be removed (e). The final step (f) is to sputter aprotective coating of AlOx to prevent the CoFe from oxidation.

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(a) (b)

Figure 4.10: Layout of the first with e-beam lithography created sample.

Figure 4.11: 17 x 17 µm topographic scan of one of the L structures.

structure contains domains. When in creasing the field the image doesn’tchange dramatically. The only effect visible is the shrinking of some of thedomains between -50 and -75 Gauss. When the field is raised from -150 to -160 Gauss, suddenly al domains are gone and there is apparently one uniformmagnetization. It is true that at relatively high field, one would expect auniform magnetization and no domains, but this transition is expected tobe gradually by growing of some domains (the ones that are aligned withthe field) and shrinking of others. Eventually this process leads to a uniformmagnetization. That this is not the process happening here, becomes clearwhen again a scan is performed at 0 Gauss. The domain structures presentin the first scan at 0 Gauss stay out. Assuming that the domains are stillpresent in the structure (which is very likely, at least at 0 Gauss), onemust conclude that apparently the tip is mall functioning. One possibleexplanation is the partially reversal of the effective magnetization of the

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(a) 25 x 25 µm topographic image (b) 25 x 25 µm magnetic image

Figure 4.12: First MFM scan of one of the smallest structures of the E-beamsample I

tip, leading to a state in which the tip has (almost) zero net magnetization.Another explanation for the sudden reduction of the magnetization of thetip could be due to some damage to the tip. Because of the applied field,the tip is more drawn to the surface, which in several cases has lead to acrash of the tip in the sample, possibly causing damage to the tip.

As is visible in 4.13 E-beam sample I still shows multi domain formation.This means that the dimensions of the structures are to large to favor asingle domain state. Therefore a sample with smaller structures is createdas will be discussed in the next section.

4.3.2 E-beam sample II

Even though the very beautiful domain structures that appeared in E-beamsample II were a useful to test the spatial resolution of the MFM, it is nota desirable situation for single domain wall manipulation. To be certain tohave a single domain wall in the structure, it has to be down scaled. Inearlier publications structures of typically 200 nm are created to observesingle domain walls. Therefore a second sample is created with e-beamlithography. This sample contains 6 arrays of 5 by 4 L shaped structuresof different sizes and exposure times. Each array is split by a special shapestructure connected to two contacts. In figure 4.14(a) the layout of thetotal sample is drawn. Figure 4.14(b) shows a close up of one of the arrays.The square in the middle can contain one of the structures shown in figure

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(a) 0 Gauss (b) -50 Gauss

(c) -75 Gauss (d) -100 Gauss

(e) -150 Gauss (f) -160 Gauss

(g) -170 Gauss (h) 0 Gauss

Figure 4.13: 5.5 x 5.5 µm magnetic image at different strengths of theapplied field.

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4.14(c), depending on the location on the sample as appears from figure4.14(a).

(a) overview layout

(b) closeup array (c) special structures

Figure 4.14: Structure of the E-beam sample II

The widths of the L like structures are, from left to right, 0.5 µm, 0.1 µm,0.2 µm and 1 µm. Because of the limited focusing of the used e-beamlithography, writing structures smaller than 1 µm is very likely to fail. Toenlarge the change on success al the four sizes are created in five fold usingdifferent intensities of the e-beam. In figure 4.14(b) this is indicated bydifferent shades of gray: the lighter the color, the longer the exposure.

The sample is viewed under a microscope with a maximal enlargement of100 times. As was expected most of the L structures smaller than 1 µmare completely gone. However some of the smaller structures had someluck and are (partially) still in tact. Figure 4.15 shows some pictures takenwith the digital camera attached to the microscope. Because of the low

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resolution of the image, it is difficult to accurately determine the width ofthese structures. The best estimation that can be made ranges from 7 to 9µm.

(a) 23 x 23 µm photograph (b) 27 x 27 µm photograph

Figure 4.15: Microscopic photographs of two of the remained structures.

Also of the special structures photos are taken. These are shown in figure4.16. As the photos show, all structures are present and without artifactsexcept structure 2. Somehow some relatively large pieces material are stuckto this structure. Furthermore it becomes clear that the structure is dupli-cated. This most clearly can be seen in the middle of the photo, where thestructure consists of two parts. Also some of the L structures are doubleprojected on the substrate. Probably this is due to some error during thewriting of the structures.

Because of the few time that was left, only a couple test scans were per-formed. Figure 4.17 shows nicely the domain configuration of the ellipseformed structure (5). Because of the high scanning speed and the use of anold MFM tip the image is somewhat unsharpen and contains some speedartifacts.

The ellipse structure itself displays multi domain formation, but the con-necting stripe on the right side of the image appears to be single domain.This is the first indication of succeeding in creating a single domain pre-ferred structure. The width of the connecting strip is approximately 1200nm, whereas the smallest structures of E-beam sample I were larger than1500 nm. Considering this result it is very likely to find single domains in thestructures of figure 4.15, because the width of the structures is substantiallyless than 1200 nm.

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(a) Special structure 1 (no photo taken;same as structure6)

(b) Special structure 2; 56 x 56 µm

(c) Special structure 3; 65 x 65 µm (d) Special structure 4; 54 x 54 µm

(e) Special structure 5; 53 x 53 µm (f) Special structure 6; 47 x 47 µm

Figure 4.16: Photographs of the 6 special structures.

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(a) 11 x 11 µm topographic image (b) 11 x 11 µm magnetic image

Figure 4.17: Test scan of the 5 structure.

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Chapter 5

Conclusion and discussion

One can safely say that this report tells more about the properties of AFMand especially MFM than it does about domains. Of course this is notvery surprising considering the magnitude of the project, especially whenbeginning from scratch. Non the less some major progress has been made.

5.1 Interpreting MFM images

One of the most important conclusions to draw from this research is that stilla lot has to be done to gain more insight in the behavior of the MFM. A greatstep forward would be to be able to characterize the tip. By determiningits effective stray field and switching field(s), more can be said about itsinfluence on the sample. A small conducting loop on a substrate could beused to deduce all these properties. Provided that the diameter of the loop isknown, it is possible to accurately calculate its magnetic field that it induceswhen a certain current is send trough it. By knowing the magnetic field ofthe sample, its conceivable, not only to calibrate the tip, but also to examinethe tips behavior as function of an external field.

During the experiments it turned out that the achieved signal due to mag-netic interaction depends greatly on the different parameters of the AFM.Until now it is just a case of monitoring the signal while varying the pa-rameters to optimize the signal. In this way the height is more or lessoptimized for a certain amplitude, when in fact both should be optimized.To gain more insight in the relations between the height, amplitude andsample magnetization and the signal, also the current loop as mentionedabove could be used.

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The characterization of tips and the determination of the relationships men-tioned above makes the development of tips towards the ’ideal’ tip possible:high coercivity, low stray field, single domain.

5.2 Coating tips

Coating tips to use them for MFM scans is a very delicate activity. Onlyone of the in total eight sputter-coated tips appears to work more or less.Unfortunately even that one seems not reliable, sometimes for no apparentreason giving no magnetic signal at all. But apart from these difficultiesthe scans show the advantage of these tips above the totally coated tipsfrom Nanoworld. Because a smaller region of interaction and a smallertip magnetization, the spatial as well as the magnetic resolution is higherthan with the Nanoworld tips. It is definitely worth to do more researchoptimizing these tips. The coating of the tips has to be controlled withhigher precision. Now a mask together with sputtering is used to equip thetips with a magnetic coating. This system is very sensitive to the correctplacing of the tip. When the tip is not correctly in front of the mask,there might be no magnetic layer on the tip at all. And even if the tipis somewhat coated with magnetic material, the cover layer that preventsthe magnetic material from oxidizing might not cover the whole layer. Themagnetic moment of this tip will over time disappear as a consequence of theoxidation. However it pays to solve these difficulties because higher contrastand spatial resolution can be achieved with these tips.

5.3 Magnetic Domains

The imaging of domains in the L shaped structures has been successful. Theimages give a good estimation of the spatial resolution of the MFM. Also thedependance of domain structures on an external field have been examinedby means of a discreet field sweep. This remains rather difficult because thetip is influenced greatly by the external field. Hopefully this can be solvedby using the sputtered tips. The width of at least 1500 nm was still to largefor single domain formation.

Also the first breakthrough in the imaging of a single domain has been made.The connecting stripes to the special structures of the E-beam sample II aresmall enough (<1200nm) to display single domain formation. This makes itplausible to also find single domain formation in the other small L structures.Therefore continuation of the research is valuable.

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Bibliography

[1] Nanoworld website. http://www.nanoworld.com.

[2] Nt-mdt website. http://www.ntmdt.com.

[3] T. Ono A. Yamaguchi and S. Nasu. Real-space observation of current-driven domain wall motion in submicron magnetic wires. PRL,92(7):077205, 2004.

[4] Rudolf Schaffer Alex Hubert. Magnetic Domains. Springer, 1998.

[5] R. Wiesendanger and H.J. Guntherodt. Scanning Tunneling MicroscopyII. 1992.

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