the structural, magnetic and electrical behavior of (110
TRANSCRIPT
Master Thesis
The structural, magnetic and electricalbehavior of (110) oriented LSMO.
November 12, 2009
Author:
Jaap Kautz
Supervisors:
Prof. dr. ing. Dave H.A. BlankDr. ing. Guus Rijnders
Dr. ir. Alexander BrinkmanIr. Hans Boschker
Abstract
This thesis investigates the suitability of La0.7Sr0.3MnO3 grown on SrTiO3(110)as electrode material for magnetic tunnel junctions. Samples with thicknessesfrom 1 to 25 nanometers were fabricated using pulsed laser deposition. Weobserve monocrystalline epitaxial growth. XRD measurements showed outof plane compressive strain in the LSMO combined with a tilt of the (001)plane. VSM measurements were done to analyze the magnetic behavior andwere compared to results for LSMO grown on STO(001). The results showa bulk like saturation magnetization of 3.7µB and no indication of a deadlayer as observed for the (001) orientation. For electrical characterization vander Pauw measurements were done. For layers thicker than 2.7 nanometerselectrical conduction is observed in which all layers seem to take part. Forthinner layers there is no conduction and a not yet understood ferromagneticbehavior is observed above the Curie temperature.
Contents
1 Introduction 1
2 Theory 32.1 LSMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Magnetism and Conductivity . . . . . . . . . . . . . . 32.2 Magnetic Tunnel Junctions . . . . . . . . . . . . . . . . . . . . 8
2.2.1 Causes for Low TMR-Ratio . . . . . . . . . . . . . . . 9
3 Fabrication 113.1 Pulsed Laser Deposition . . . . . . . . . . . . . . . . . . . . . 113.2 Patterning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2.1 Van der Pauw measurements . . . . . . . . . . . . . . . 143.2.2 Transport anisotropy measurements . . . . . . . . . . . 16
3.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4 Crystal structure 194.1 XRD Measurements . . . . . . . . . . . . . . . . . . . . . . . 19
5 Anisotropy 235.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.2 Magnetic anisotropy measurements . . . . . . . . . . . . . . . 265.3 Transport anisotropy measurements . . . . . . . . . . . . . . . 285.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
6 Interface influences 316.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
7 Conclusions 37
iii
Chapter 1
Introduction
Soon after the invention of the computer a distinction was made betweensmall capacity quick volatile internal memory and large capacity slower non-volatile external memory. An ideal memory would combine the best of bothworlds; it should be quick, non-volatile and have a large capacity. Mag-netic tunnel junctions could in theory form the base for such an ideal mem-ory. The theory behind magnetic tunnel junctions is explained in chapter 2.Magnetic tunnel junctions consist of two ferromagnetic electrodes separatedby a thin insulating layer. A good candidate for the electrode material isLa0.7Sr0.3MnO3(LSMO), because of its 100% spin polarization. However re-sults from measurements done on TMR junctions using LSMO do not liveup to expectations. In section 2.2.1 several theories explaining this reducedperformance are discussed. One possible explanation is that the diminishedresults are caused by the polar discontinuity at the interface between LSMOand SrTiO3(001), which is used as insulator. If this theory is correct, no sucheffect should occur for a LSMO grown on STO with a different crystal orien-tation. The goal of this research project is to investigate if LSMO grown onSTO(110) is suited to be used as electrode material for TMR devices. To doso, the structural, magnetic and electrical behavior of LSMO on STO(110)was examined and compared to results obtained in earlier studies for LSMOgrown on STO(001).
For the TMR effect to work we need well defined interfaces, monocrys-talline growth and a controlled doping level. Pulsed laser deposition fulfillsall these requirements and furthermore gives excellent control over growthspeed and layer thickness. To verify that we do indeed obtain monocrystallinegrowth the samples were analyzed using atomic force microscopy(AFM) andx-ray diffraction(XRD) measurements. The fabrication process is describedin chapter 3. The XRD results were also used to analyze the structural effectsof the STO substrate. Since the large scale magnetic and electrical behavioris determined by small scale interactions, changes in crystal structure candrastically alter the material properties. In chapter 4 the results are shown.
1
CHAPTER 1. INTRODUCTION
Used in memory devices the magnetization direction of the electrodes hasto be switched frequently. Therefore good understanding of the magneticbehavior of LSMO is essential. The magnetization switching was analyzedextensively using a vibrating sample magnetometer and compared to predic-tions from theory. Earlier problems with TMR junctions using LSMO weresubscribed to a dead layer caused by polar discontinuity. If polar discontinu-ity is the sole cause for the dead layer in LSMO on STO(001) there should beno dead layer in LSMO grown on STO(110). To investigate the interface in-teractions for the (110) orientation, samples of varying thickness were grownand analyzed. The results can be found in chapter 6. Finally in chapter 7 wediscuss the obtained results and draw conclusions on the suitability of LSMOgrown on STO(110) as electrode material in magnetic tunnel junctions.
2
Chapter 2
Theory
2.1 LSMO
Lanthanum strontium manganite is an oxide with an perovskite crystal struc-ture. The chemical composition of LSMO is La1-xSrxMnO3, where x is a vari-able indicating the doping level. One unit cell of LSMO is given in Fig. 2.1.The manganese atoms are surrounded by an octahedron of oxygen atoms.The oxygen atoms are ionized to O
2-, lanthanum to La
3+, and strontium
to Sr2+
. The manganese atoms are either ionized to Mn3+
or to Mn4+
de-pending on the doping level. This results in the 3d shell of the manganeseatoms being filled with either 4 or 3 electrons. The electrical and magneticbehavior is largely determined by the electrons in the 3d shell of the man-ganese atoms[1]. The orbitals of the d shell are given in Fig. 2.2. For anatom in free space these states are degenerate. However the oxygen octa-hedron surrounding the manganese atoms lifts this degeneracy. This can beseen in Fig. 2.3. This figure also shows the spin polarization caused by theHund’s interaction energy. Due to a strong electron electron interaction it isenergetically favorable for the electrons in the 3d shell to align their spins.This Hund’s interaction energy is larger than the energy gap between thet2g and eg states. As a result a fourth electron in the 3d shell will align itsspin and occupy one of the eg states rather than assuming an antiparallelconfiguration in one of the t2g states. This effect causes the electrons of onemanganese atom to align their spins.
2.1.1 Magnetism and Conductivity
The overall magnetic and electrical behavior depends on the interactions be-tween the different manganese atoms. Four counteracting processes play arole in the behavior of the 3d electrons. These four processes are: double ex-change interaction, super exchange interaction, Jahn Teller effect and chargeordering.
3
CHAPTER 2. THEORY
Figure 2.1: Unit cell of LSMO.The manganese atom(green) issurrounded by an oxygen octahe-dron(red). The atoms at the yel-low sites can be either lanthanumor strontium.
eg
t2g
xy
2 23z -r
2 2x -y
xz yz
Figure 2.2: 3d orbitals of the manganeseatom[2]. The otherwise degenerate statessplit up due to the presence of the nega-tively charged oxygen octahedron.
E
3d
eg
t2g
eg
t2g
eg
t2g
yz,zx
xy
2 23z -r
2 2x -y
yz,zx
xy
2 23z -r
2 2x -y
Splitting of 3d band due to oxygen octahedron.
Spin polarization due to Hunds coupling.
Splitting of bands due to Jahn Teller distortions.
Lowering of Fermi level by hole doping.
EF
3d band in free space
Figure 2.3: Schematic representation of the bands formed by the 3d orbitals of themanganese atoms in LSMO. Except for the last graph all Fermi levels are drawnfor the non doped situation.
4
2.1. LSMO
Double Exchange Interaction
Double exchange interaction is a process in which two neighboring manganeseatoms and their connecting oxygen atom play a role. The process is depictedin Fig. 2.4. One of the manganese atoms has one of its eg states occupied,the other has an empty eg shell. One electron tunnels from the oxygen atomto the manganese atom without eg electrons. The eg electron from the othermanganese atom then tunnels to the freed 2p position on the oxygen atom.The net result is one electron moving from one manganese atom to the next.This how double exchange interaction induces electrical conductivity. Sinceelectrons retain their spin while tunneling, tunneling is only possible betweenstates with parallel spins. This is why double exchange interaction onlyoccurs between manganese atoms whose t2g electrons have their spins aligned.Because double exchange interaction increases the freedom of electrons, itlowers their energy. This makes it energetically favorable for electrons ofneighboring manganese atoms to align their spins, inducing ferromagnetism.Since double exchange interaction requires filled eg states as well as emptyones(holes) it is highly doping dependent.
Figure 2.4: Double exchange interaction.
5
CHAPTER 2. THEORY
Super Exchange Interaction
Super exchange interaction occurs due to the slightly overlapping t2g andeg orbitals of two neighboring manganese atoms. Electrons in the theseshells can tunnel to the neighboring manganese atom. When this neighboringshell is already half filled, tunneling is only possible to the antiparallel spinstates. Due to the Hund’s interaction energy, these antiparallel states have ahigher energy level, making the electrons tunnel back directly to their originalmanganese atom. However this tunneling possibility increases the freedom ofthe electrons, decreasing their energy. Since electrons retain their spin whiletunneling, super exchange interaction is only possible between manganeseatoms whose electrons have antiparallel spins. This makes it energeticallyfavorable for electrons of neighboring manganese atoms to assume antiparallelspins. This induces antiferromagnetism.
eg
t2g
yz,zx
xy
2 23z -r
2 2x -y
yz,zx
xy
2 23z -r2 2x -y
Figure 2.5: Jahn Teller effect:Deformation of the oxygen octa-hedron lifts the degeneracy of theeg and t2g bands.
Figure 2.6: Lattice distortion due toJahn Teller effect. The manganese atomsretain their positions, while the oxygenoctahedrons are deformed.
Jahn Teller Effect
The Jahn Teller effect is an interaction effect between the crystal lattice andthe manganese 3d electrons. Deformation of the oxygen octahedron causesenergy differences within the otherwise degenerate t2g and eg electron states.This shown in Fig. 2.5. Orbitals which are compressed by the deformationgain energy, while orbitals which are elongated by the deformation looseenergy. When the eg shell is half filled it is energetically favorable to createa deformation, because one of the orbitals will loose energy. The electrons in
6
2.1. LSMO
Figure 2.7: Charge ordering:For doping of x=0.5 eg elec-trons spread over the manganeseatoms due to coulomb repulsion.Due to this electron distributionthe lattice gets deformed.
Figure 2.8: Magnetic/electronic phasediagrams of La1-xSrxMnO3 (created byUrushibara et al.[3]). PI: paramag-netic insulator, PM: paramagnetic metal,CNI: canted antiferromagnetic insulator,FI: ferromagnetic insulator, FM: ferro-magnetic metal, AFM: antiferromagneticmetal.
the eg shell will then occupy this lower energy state. Creating a deformationof the oxygen octahedrons without altering the overall crystal lattice leadsto the typical Jahn Teller distortions shown in Fig. 2.6. The Jahn Tellerdistortions are only favorable when a lot of manganese atoms have a half filledeg shell. This makes this effect doping dependent. The degeneracy withinthe t2g and eg states can also be lifted by deformations due to externallyimposed strain.
Charge Ordering
Charge ordering is caused by Coulomb repulsion between eg electrons atdifferent manganese atoms. The electrons order themselves in a patternshown in Fig. 2.7. This configuration is so stable that electrons retain theirpositions preventing electrical conduction. This stability has two causes. Thefirst one is the aforementioned Coulomb repulsion. The second one is electronphonon coupling. The electrons locally deform the oxygen octahedrons. Tobreak the electron configuration the lattice would have to be reformed aswell. Electrical ordering is only favorable for certain electron hole ratios,making it doping dependent.
7
CHAPTER 2. THEORY
Phase diagram
The four described mechanisms all contribute to the electric and magneticproperties of LSMO. The temperature, doping, strain and shape of the sam-ple determine which mechanism is dominant. This is clearly visible in aphase diagram as shown in Fig. 2.8. For this project the LSMO needs to beferromagnetic and have a high Curie temperature. This is why the dopinglevel x is set at 0.33. In the rest of this report all references to LSMO willmean La0.67Sr0.33MnO3 unless specified otherwise.
2.2 Magnetic Tunnel Junctions
In magnetic tunnel junctions the tunneling probability between two materialsis influenced by the magnetization of the materials. The effect was first ob-served by Wyatt[4] and later extensively researched[5][6]. The setup consistsof two ferromagnetic electrodes separated by a thin insulating layer. Sinceelectrons retain their spin while tunneling, tunneling is only possible betweensites which have their spins aligned. Only energy states near the Fermi levelplay a role in the tunneling process. In a ferromagnetic material the numberof electrons at the Fermi level with spin up is unequal to the number of elec-trons with spin down. Ferromagnetic materials are therefore said to to bespin polarized. As a result of this spin polarization the tunneling probabilityis higher when the majority spins of the two electrodes are aligned. The mainfigure of merit for magnetic tunnel devices is the TMR-ratio. It is defined as
TMR =Rap −Rp
Rp
, (2.1)
where Rp is the resistance for the situation where the two electrodes havetheir spins aligned and Rap is the resistance for the anti-parallel situation.There are to two requirements for a successful TMR device: spin polarizedconducting electrodes and an insulator which is insulating, but thin enoughto tunnel through.
Magnetic tunnel junctions have successfully been used in commerciallyavailable devices. In these devices the used electrode materials are mostlycobalt or iron cobalt[7]. The advantage of these materials is their simplicity,but the obtained TMR-ratio of around 400% is relatively low. A good alter-native would be LSMO. Due to the 100% spin polarization at the fermi leveltheoretically very high TMR-ratios could be obtained. However experimentsdone with LSMO tunnel junctions[8][9][10] do not obtain such high TMRvalues. Several theories try to explain this result. They will be discussed inthe following section.
8
2.2. MAGNETIC TUNNEL JUNCTIONS
2.2.1 Causes for Low TMR-Ratio
As discussed before there are two requirements for a successful TMR device.If the measured TMR ratio is below expectation the cause must lie eitherin a leaky insulator or in the electrodes not being as spin polarized as ex-pected. The properties of the insulating STO layer were studied extensively[11][12][13]. The main cause for leakage are oxygen vacancies. These va-cancies have a twofold influence on the insulating properties. The STO getselectron doped by the absence of Oxygen acceptors. Furthermore the va-cancies themselves can travel through the STO and provide hole-like chargetransport.
The other possible cause for a low TMR ratio is the electrodes not reallybeing as spin polarized as expected. Although bulk LSMO has been shown tobe truly half metallic[14], various studies[15][16] suggest LSMO loses its halfmetallicity at the interface due to interactions with the STO. Observationof a so called ’dead layer’, which appears not to be magnetic nor electricallyconducting[17][18][19], supports this theory.
One possible explanation for this dead layer is found in the polar discon-tinuity model[20][21]. This model describes (001) oriented LSMO as layers of
La0.67Sr0.33O(0.67)+
and MnO2(0.67)-
and STO as layers of SrO and TiO2. Thisis shown in Fig. 2.9. In LSMO the layers are charged while in STO thereis no polarization of the layers. At the interface of these materials a polardiscontinuity occurs leading to electronic redistribution. This electronic re-distribution causes a reduced or enhanced doping depending on the type ofinterface as can be seen in Fig. 2.10 and 2.11. The altered doping at theinterface causes the eg states to be either under- or overpopulated, in bothcases preventing the double exchange mechanism needed for conduction andferromagnetism. Interface engineering has been suggested as an solution forthis problem[22][23]. In interface engineering the doping of the first unit celllayer of LSMO is altered to counter the effect of electronic redistribution andmaintain a constant population of the eg-states throughout the LSMO.
This research tries to clarify if this dead layer can indeed be explainedby the polar discontinuity model. To do so an alternative setup is chosenin which polar discontinuity does not play a role. By using (110) orientedSTO as a substrate, the layers parallel to the surface have a different consti-tution. STO is now a stack of SrTiO
4+and O2
4-. LSMO (110) has layers of
La0.67Sr0.33MnO4+
and O24-
. The interface is shown in Fig. 2.12. It is clearthat there is no polar discontinuity. If the polar discontinuity model is indeedthe sole cause for the dead layer formation, there should be no dead layer inthis configuration.
9
CHAPTER 2. THEORY
(La0.67
0.67Sr O)0.33
-0.67(MnO )2
0.67(La Sr O)0.67 0.33
0(SrO)
0(TiO )2
0(SrO)
Figure 2.9: LSMO as a stackof La0.67Sr0.33O(0.67)+ andMnO2
(0.67)- and STO as a stackof SrO and TiO2.
0.67(La Sr O)0.67 0.33
-0.33(MnO )2
0(SrO)
0(TiO )2
-0.67(MnO )2
0(SrO)
Figure 2.10: Due to polar discontinuitythe eg states of the first manganite layerget underpopulated.
0.67(La Sr O)0.67 0.33
-1(MnO )2
0.67(La Sr O)0.67 0.33
0(SrO)
0(TiO )2
0(TiO )2
Figure 2.11: Due to polar dis-continuity the eg states of thefirst manganite layer get over-populated.
4(La Sr MnO)0.67 0.33
-4(O )2
4(La Sr MnO)0.67 0.33
-4(O )2
-4(O )24(SrTiO)
4(SrTiO)
-4(O )2
-4(O )2
Figure 2.12: Due to the (110) crystal ori-entation, there is no polar discontinuity.
10
Chapter 3
Fabrication
3.1 Pulsed Laser Deposition
The LSMO layers were fabricated using pulsed laser deposition(PLD). Thesetup is shown in Fig. 3.1. A target of polycrystalline La0.7Sr0.3MnO3 islocally heated very rapidly using a focussed laser pulse causing instantaneousevaporation. The vapor is further heated by the pulse forming a high pressureplasma of ionized LSMO atoms. The plasma expands rapidly away from thetarget due to the high pressure gradient. The substrate is placed in the pathof this expanding plasma. The shape of the plasma plume leads to an evendistribution over the surface. After reaching the substrate, the LSMO atomsuse their remaining kinetic energy to diffuse over the surface. This process isshown in Fig. 3.2. For the atoms to diffuse into an atomically smooth layerthey need sufficient kinetic energy. The kinetic energy of the atoms reachingthe substrate can be influenced by altering the process parameters. Thesurface area and intensity of the focussed laser beam influences the amountof atoms as well as their energy. Increasing the pressure in the chambercauses the expansion of the plasma to slow down and in doing so reducesthe kinetic energy of the arriving atoms. Finally the energy of the arrivingatoms can be adjusted by varying the target substrate distance. All thesefactors influence the quality and uniformity of the deposited layer. Earlierstudies[19] have led to an optimization of the parameters for the depositionon LSMO on STO. The used parameters are shown in table 3.1. The usedlaser is an KrF excimer laser. A rectangular mask is placed in the laser beamto select that part of the beam with the most homogeneous intensity. Thebeam is then focussed, creating an image of the mask on the target.
Before deposition the (110) oriented STO substrates were cleaned in anultrasonic bath: First 10 minutes in acetone, followed by a 10 minute bathof ethanol. Lens paper moistened with ethanol is used to wipe any residuesfrom the sample. Then the substrate is annealed for one hour at 950 degreesCelsius under an oxygen atmosphere of 1 bar. For the fabrication of the
11
CHAPTER 3. FABRICATION
Figure 3.1: Setup for pulsed laser deposition.
Figure 3.2: Model for growth duringpulsed laser deposition. After adsorptionunit cells diffuse over the surface untilthey are incorporated in the lattice.
Figure 3.3: Typical rheed pat-tern. The intensity of the centerreflection spot is used to monitorthe growth.
12
3.1. PULSED LASER DEPOSITION
Parameter Value UnitO2 pressure 0.27 mbarTarget substrate distance 50 mmSubstrate temperature 830 C
Laser fluency 200 J/mm2
Spot size 2.07 mm2
repetition rate 1 HzAnneal pressure 100 mbar
Optical setupWavelength 248 nmPulse duration 25 ns
Mask area 56.5 mm2
Mask lens distance 3264 mmLens target distance 526 mmLens focal distance 453 mmOptical losses ≈ 10 %
Table 3.1: Parameters used in fabrication process.
capped layers, the deposition of the LSMO layer is directly followed by adeposition of STO using the same parameters. After deposition the sampleis cooled down under an oxygen atmosphere of 100 mbar.
The deposition is monitored using reflective high energy electron diffrac-tion(rheed). An electron beam is aimed at the sample under a grazing angle.Interference between the electrons reflected from different atoms causes adistinctive angle dependence of the intensity of the diffracted electron beam.A phosphor screen is used to visualize the diffraction pattern. Such a diffrac-tion pattern is shown in Fig. 3.3. The bottom spot is caused by that partof the electron beam that misses the sample and can be ignored. The topcenter spot is the main out of plane reflection spot. The two side spots haveout of plane as well as in plane components. The intensity of the reflection ishighly influenced by the surface morphology of the sample. This makes thereflection intensity a good indicator of the smoothness of the sample surface.The intensity of the center spot was measured during deposition. The resultsare shown in Fig. 3.4. Each oscillation corresponds to the completion of oneunit cell layer. A complete unit cell layer is very smooth and results in a highintensity. Further deposition first reduces the smoothness until a new unitcell layer is almost complete and the intensity increases again. Zooming inon the rheed signal shows the effect of the individual pulses. The reflectiondeteriorates directly after each pulse but is then restored due to diffusion.The oscillations show that the first two unit cell layers of LSMO have a longerdeposition time than the other unit cell layers. The increase in deposition
13
CHAPTER 3. FABRICATION
150 200 250 300 350 400 450 500 550
10
20
30
40
50
60
70
80
90
Time (s)
Inte
nsity (
A.U
.)
160 165 170 175 180 18510
15
20
25
Time (s)
Inte
nsity (
A.U
.)
Figure 3.4: Intensity of the central reflection spot of the rheed signal duringdeposition of the 8 layer sample with capping. The first set of 8 oscillations areduring the deposition of LSMO. The second set of 6 oscillations are during thedeposition of the capping layer. The average deposition speed is 19 seconds perlayer for LSMO and 9 second per layer for STO. The inset shows how the individualpulses first reduce the intensity after which the intensity increases again.
time is on average 40 percent for the first unit cell layer and 4 percent for thesecond unit cell layer. No reduction of the deposition speed was observed forthe first layers of the STO capping on LSMO.
The sample surfaces were analyzed before and after deposition using anatomic force microscope(AFM). The results of one of the samples are shownin Fig. 3.5 and 3.6. The clean substrate clearly showed the atomic stepscaused by the miscut of the substrate. The height of the steps correspondsto the lattice constant of STO divided by
√2 due to the (110) orientation.
The image after the deposition did no longer show the atomic steps but didshow the sample is still atomically smooth.
3.2 Patterning
3.2.1 Van der Pauw measurements
For the van der Pauw measurements gold contact pads were created on thecorners of the samples using plasma sputtering. A contact mask was usedto obtain the desired pattern. The pattern is shown in Fig. 3.7 A. Using asputter power of 150W for four minutes, a gold layer was grown.
14
3.2. PATTERNING
Figure 3.5: AFM image of sub-strate. The terrace steps areclearly visible.
Figure 3.6: AFM image afterLSMO deposition. Terrace stepsare no longer clearly visible, butsurface is still atomically smooth.
A B
4 mm
C
5 mm 5 mm
Figure 3.7: Masks used for transport measurements. A: Mask used for golddeposition to create contact points for van der Pauw measurements. Mask B isused to etch tracks of LSMO for transport anisotropy measurements. The twotrack widths are 10µm and 100µm. The length of the tracks is 400µm. Mask Cis used to create gold contact pads for the transport anisotropy measurements.
15
CHAPTER 3. FABRICATION
A
B3 Distance(10 nm)
He
igh
t (n
m)
39.1 10 nm
Figure 3.8: A: AFM image of one of the small tracks of the transport anisotropymeasurement. The track consists of lsmo while the background is formed by theSTO substrate. B: A cross section of the tracks shows the width is 9µm.
3.2.2 Transport anisotropy measurements
After deposition of the LSMO layer, the samples used for transport anisotropymeasurements were patterned using photolithography and argon ion etching.Fig.3.7 B shows the mask used for the lithography. Using positive photoresistall LSMO but the dark areas on the mask was etched away. Gold contactpads were applied using a lift off process: Photolithography with the maskfrom Fig.3.7 C provided a layer of positive photo resist with openings at thelight areas of the mask. Plasma sputtering of gold followed by solution ofthe resist in acetone resulted in gold contact pads at the location of the lightareas of the mask. The fabrication of the track was verified using AFM. Theresults are shown in Fig 3.8.
3.3 Discussion
The reduced growth speed for the first two layers is remarkable. Using themodel from Fig. 3.2, a reduced deposition rate is caused by an reduced dif-ference between adsorption and desorption. This could be a consequence ofa lower adsorption of LSMO on STO compared to LSMO on LSMO. An-other possible explanation is found in the substrate being almost perfectly
16
3.3. DISCUSSION
smooth. This reduces the probability of incorporation and thus increasesthe desorption rate. As growth continues step edges get more disorderedand incorporation probability increases, leading to less desorption and fastergrowth. Further research is needed to investigate this behavior. The diffu-sion behavior could be analyzed by comparing the recovery after each pulsefor the first and later unit cell layers. Another option is to stop depositionduring growth of the first layer and use AFM to analyze the process.
The measured channel width for the anisotropy measurements is 1 mi-crometer smaller than the the used mask pattern. This is probably due toillumination of the area under the mask at the edges of the pattern. The re-sistance of the channel will therefore be higher then could be expected fromthe mask dimensions.
17
CHAPTER 3. FABRICATION
18
Chapter 4
Crystal structure
Bulk LSMO has a rhombohedral crystal structure at room temperature[24].Depositing LSMO on a substrate with a different lattice parameters leadsto straining of the deposited layer. LSMO was grown epitaxially on anSTO(110) substrate. There is only a slight mismatch between the latticeparameters of LSMO(0.388 nm) and STO(0.3905 nm). Therefore we assumethe LSMO layer will adapt an (110) orientation as well. Since the latticeconstant of STO is larger than that of LSMO, the in plain features of theLSMO layer are elongated. In the out of plain direction, the LSMO hasno imposed dimensions. Assuming the Mn-Mn distances in [100] and [010]direction in the strained LSMO will be the same as in unstrained LSMO,the out of plane lattice parameter will be reduced in comparison to naturalLSMO. The expected out of plane lattice parameter is
d =
√a2
LSMO −a2
STO
2=
√0.388 nm2 − 0.3905 nm2
2= 0.2726 nm. (4.1)
4.1 XRD Measurements
The crystal structure was investigated using X-ray diffraction measurements.Measurements were done on samples with a thickness up to 80 unit cell layers.The results indicated a monocrystalline growth. Fig. 4.1 shows a scan of theout of plane direction of the reciprocal lattice obtained by performing a Θ2Θ-scan. The large peaks belong to the STO substrate, while the side peaks arecaused by the LSMO layer. The out of plane lattice constant was obtainedfrom the distance between the peaks:
d =2π
∆Q(4.2)
For the STO substrate this gives an out of plane lattice constant of0.2761 nm. The LSMO out of plane lattice constant is 0.2724 nm. The
19
CHAPTER 4. CRYSTAL STRUCTURE
Figure 4.1: Θ2Θ-scan of 80 ml LSMOon an STO substrate. The large peakscan be subscribed to the STO substrateand correspond to an out of plane latticeparameter of 0.2761 nm. The side peakscorrespond to an out plane lattice param-eter of 0.2724 nm, indicating the LSMOhas a slightly smaller out of plane latticeparameter as the STO substrate.
[110]LSM
[100]LSM
[110]LSM
[110]STO
[100]STO
[110]STO
dSTO
dLSMO
Figure 4.2: Schematic representa-tion of the straining of the LSMOlayer. Seen from [001] direction.
measured value corresponds within the measurement error to the theoret-ical value which was based on the assumption of a constant Mn-Mn distance.A schematic representation of the strained LSMO layer is given in Fig. 4.2.To obtain more information on the crystal structure of the LSMO layer, re-ciprocal maps were made of several nodes of the crystal lattice. These areshown in Fig. 4.3. The resulting lattice parameters are given in table 4.1.
For the STO substrate all lengths are 3.905 nm and the angles are 90 degrees.The in plane lattice parameters of the LSMO film match those of the sub-strate, confirming the fact that the film assumes the in plane feature sizesof the substrate. As already observed in the Θ2Θ-scan the reciprocal out ofplane values of the film exceed those of the substrate indicating a smaller outof plane lattice parameter for the film. Furthermore the 331 and 331 filmpeaks show different out of plane values. This indicates that the (001) planeis tilted by an angle of 0.6 degrees. This is shown in Fig. 4.4. All latticeparameters obtained from the reciprocal maps are given in table 4.1. Thetilting angle only influences the lattice parameters angles αLSMO and βLSMO.They approach their natural value of 89.7 degrees.
20
4.1. XRD MEASUREMENTS(
mQ
1
/)
OF
L
AE
OU
T
PN
Q (1/m) IN PLANE
Q (1/m) IN PLANEQ (1/m) IN PLANE
Q (1/m) IN PLANE
(m
Q
1/
)O
F
LA
EO
UT
P
NQ
(m
1/
)O
UO
F P
LA
NE
T
(Q
1
/m)
OF
LE
OU
T
PA
N
Q (1/m) IN PLANE
(m
Q
1/
)O
F
LA
EO
UT
P
N
Q (
1/m
)O
UT
OF
PL
AN
E
Q (1/m) IN PLANE
(420) (240)
(310) (130)
(331) (331)
Figure 4.3: Reciprocal mapping of several reflection peaks. The substrate peaksshow the cubic crystal lattice of STO. The different QOut of plane values for the 331and 331 peaks indicate a tilt of the (001) plane.
Parameter length Parameter anglenm
aSTO = bSTO = cSTO 0.3905 αSTO = βSTO = γSTO 90.0dSTO 0.2761
|aSTO − bSTO| =√
2 · aSTO(in plane) 0.5523 ∠(cSTO, (aSTO + bSTO)) 90.0
|aSTO + bSTO| =√
2 · aSTO 0.5523 ∠(cSTO, (aSTO + bSTO)) 90.0aLSMO 0.388 α 89.6bLSMO 0.388 β 89.6cLSMO = cSTO (in plane) 0.3905 γ 89.2dLSMO 0.2724|aLSMO − bLSMO| = |aSTO − bSTO|(in plane) 0.5523 ∠(cSTO, (aSTO + bSTO)) 89.4|aLSMO + bLSMO| 0.545 ∠(cSTO, (aSTO − bSTO)) 90.0
Table 4.1: Lattice parameters obtained from reciprocal maps of STO substrateand LSMO film. The parameters a, b and c are the unit cell lengths in [100],[010]and [001] direction respectively. α, β and γ are the angles facing these lengths.
21
CHAPTER 4. CRYSTAL STRUCTURE
[110]STO
[001]STO
[110]LSMO
[001]LSMO
Figure 4.4: Schematic represen-tation of the tilting of the (001)plane. Seen from [110] direction
22
Chapter 5
Anisotropy
For the use as a magnetic memory device the magnetic properties of LSMOare of great importance. Research had already been done on the magneticproperties of bulk LSMO and LSMO grown on STO(001). In this projectthe magnetic properties of LSMO on STO(110) were investigated.
In order to explain the magnetic behavior of LSMO each manganese atomis modeled as a magnetic dipole. The Hund’s coupling is assumed to be sostrong that all 3d electrons of one manganese atom have their spins aligned.The total magnetic moment of one manganese atom, comprising spin as wellas orbital momentum, is assumed to be constant. Only the direction of themagnetic moment can be varied. The direction of the magnetic moment foreach manganese atom is the result of an energy equilibrium. There are fourtypes of energy which play a role in this equilibrium:
Double exchange interaction energy
The energy gain due the double exchange interaction depends on the mag-netic moment alignment of neighboring manganese atoms. The energy of oneatom due to interaction with a neighboring atom is[25]:
EDE,ij = −JS2cos (φij) , (5.1)
where S is the spin quantum number, J is the exchange integral and φij isthe expectation value of the angle between the two spin vectors. By summingthis energy over all neighboring atoms and summing again over all atoms,the total exchange energy can be calculated.
Zeeman energy
When the sample is placed in a magnetic field, the magnetization tends toalign with the magnetic field to gain zeeman energy. The zeeman energy isgiven by
EZeeman = −KZeemancosχ, (5.2)
23
CHAPTER 5. ANISOTROPY
where χ is the angle of the magnetization with the magnetic field. KZeeman
is given by:
KZeeman = msat ·B, (5.3)
where msat is the magnetic dipole moment per volume and B is the mag-netic flux density.
Crystal anisotropy
Crystal anisotropy is caused by spin-orbit coupling[25]. While an eg electronorbits around the manganese nucleus it experiences an alternating electricfield from the positively charged nucleus and thus a magnetic field. Themagnetic momentum of this orbital field influences the spin of the electron.It is energetically favorable for the electron to align its spin with the orbitalmagnetic moment. When the crystal structure of LSMO deviates from thecubic shape, the degeneracy between the eg orbitals is lifted. As one of theorbitals is preferred, the LSMO will obtain a preferred spin direction as welldue to spin orbit coupling. The preferred spin direction will be in line withthe magnetic moment of the orbital with the lowest energy. This causesmagnetic anisotropy. On average, the orbitals with the lowest energies lie inthe direction in which the crystal is mostly elongated. For our case all in planedirections are equally elongated. The out of plane direction is compressed,making the out of plane direction unfavorable. However, due to the tiltof the crystal, the elongation can be increased by assuming an out of planecomponent. Taking only uniaxial contributions into account, the crystal easyaxis will lie in the (110) plane between the [001] and [111] direction. Theenergy it costs to deviate from the easy axis is:
Ecrystal = −Kcrystal (cosζ)2 , (5.4)
where φ is the angle of the magnetization with the easy axis.
Demagnetization energy
The energy of a magnetic field depends on the local magnetic permeability.The magnetic permeability of LSMO is orders of magnitude larger than thatof the surrounding air. It is energetically favorable to minimize the magneticfield lines outside the sample. This causes two effects: First, the magneti-zation tends to align with the direction in which the sample has the largestdimensions. Second, the formation of magnetic domains.
For thin films the in plane dimensions are much larger than the out ofplane dimensions leading to a tendency for in plane magnetization. Theenergy due to this shape anisotropy is[26]
24
5.1. SIMULATIONS
φ
Θ
−200 −100 0 100 200
0
50
100
150
Figure 5.1: Simulation for no fieldapplied field. The [111] direction isused as the crystal easy axis. Kcrystal
is set to 0.67µ0m2sat.
φ
Θ
−200 −100 0 100 200
0
50
100
150
Figure 5.2: Simulation for situationjust before switching. The anglesfor the magnetic field direction areΘfield = 90 and φfield = 45 Themagnetic field strength is 0.7msat.
Eshape = −Kshape (sinΘ)2 , (5.5)
where Θ is the angle of the magnetization with the surface normal andKshape is given by
Kshape =1
2µ0m
2sat, (5.6)
where msat is the saturation magnetization per unit volume.
5.1 Simulations
The manganese atoms align their magnetic moments in such a way as tominimize their energy. This was simulated by numerically finding energyminima for given combinations of crystal and shape anisotropy and appliedfield. Without any applied field there are two stable magnetization direc-tions. This is shown in Fig. 5.1. The stable magnetization directions in thissituation are determined by the crystal anisotropy and the shape anisotropy.By applying a magnetic field, the energy minima are rotated in the directionof the magnetization direction. The minimum closest to the applied fielddirection gets deeper while the other minimum looses depth until it vanishesand only one minimum remains. In Fig. 5.2 the situation just before thevanishing of the second minimum is shown. If at this point the state of thesystem was in this second minimum, it will quickly rotate its magnetizationto the remaining minimum. The magnetization parallel to the applied fieldwas simulated for a field along the in plane easy and hard axis. This is shownin Fig. 5.3.
25
CHAPTER 5. ANISOTROPY
5.2 Magnetic anisotropy measurements
A sample with 80 unit cell layers of LSMO was analyzed using a vibrat-ing sample magnetometer(VSM) with an in plane rotatable magnetic fieldsource. Magnetic sweeps were made under various in plane angles to analyzethe anisotropic behavior of the LSMO. Fig. 5.4 shows the magnetization par-allel to the applied field. The characteristic behavior for an easy and hardaxis is clearly visible. To gain more information on the processes duringthese hysteresis loops the magnetization angle and total in plane magne-tization are plotted in Fig. 5.5 and 5.6. When reducing the applied fieldthe magnetization first rotates back towards the easy axis. At zero appliedfield the magnetization is aligned with the easy axis. When further reducingthe magnetic field to negative values the magnetization turns away from theeasy axis again, until switching occurs. During switching the magnetizationquickly switches towards the other stable direction. This switching happensstep by step as can be seen from the total magnetization. This step by stepswitching can be explained by domain formation. The steps represent theswitching of individual domains. When we plot the remanent field and co-ercivity, again the uniaxial anisotropy is clearly visible. The coercivity datawas fitted with a model proposed by Suponev et al.[27]:
Hc(ξ) = Hc(0)Acosξ
sin2ξ + Acos2ξ(5.7)
Here A = NA+Nx
Nzis the ratio of demagnetization factors. The remanent
field is assumed to always lie in the direction of the easy axis. The expectationvalue of the remanent field is therefore:
Mrem = Msat · cosξ, (5.8)
where ξ is the angle between the easy axis and the measurement direction.Be aware that since the easy axis has an out of plane component, the angle ξdiffers from the in plane angle with the easy axis φ. The maximum remanentmagnetization is around 0.75 MSat. Taking the inverse sine of this numberindicates the easy axis makes an angle of 50 degrees with the surface nor-mal. To measure this out of plane magnetization, magnetic force microscopy(MFM) was done. The results are shown in Fig. 5.9.
26
5.2. MAGNETIC ANISOTROPY MEASUREMENTS
-5 2.50-2.5 5
Figure 5.3: Simulation of the magneti-zation parallel to field for varying fieldstrengths for easy and hard axis.
Figure 5.4: Measured magneti-zation parallel to field for varyingfield strengths for easy and hardaxis.
Figure 5.5: In plane angle ofthe magnetization direction with thesample for varying field strengthsfor applied fields along easy andhard axis. When reducing the fieldstrength the angle rotates from thefield angle to the easy axis. Uponfurther reducing the field the anglerotates further until switching occursand the magnetization switches tothe other stable direction
Figure 5.6: Total magnitude ofthe magnetization for varying fieldstrengths for applied fields along easyand hard axis. Upon reducing thefield strength the total magnetizationis reduced due to out of plane ro-tation of the magnetization. Whenswitching occurs the total magneti-zation is further reduced due to mag-netic domains inverting their direc-tion. At zero magnetization exactlyhalf of material has switched.
27
CHAPTER 5. ANISOTROPY
[001]
[110] [110]
-90 -40 10 60 110
Figure 5.7: Remanent magnetiza-tion parallel to the applied field forvarying in plane field angle. The bot-tom axis gives the in plane field anglewhile the top axis gives the total an-gle of the magnetic field with the easyaxis. The fit corresponds to an easyaxis making an in plane angle of 2degrees with the sample edge and anangle of 50 degrees with the surfacenormal.
[001]
[110] [110]
-90 -40 10 60 110
88.5 52.9 41.5 68.9 106.7
Figure 5.8: Coercive field for vary-ing in plane field angle. The bot-tom axis gives the in plane field an-gle while the top axis gives the totalangle of the magnetic field with theeasy axis. The fit corresponds to ananisotropy parameter A = 193 andHc(0) =0.363 kA/m.
5.3 Transport anisotropy measurements
The transport anisotropy was measured using the wheel structure introducedin section 3.2. The resulting sheet resistances are shown in Fig. 5.10. Thesheet resistance was calculated using
Rsheet =R(
LW
)channel
+(
LW
)contacts
, (5.9)
where(
LW
)contacts
, the unknown aspect ratio of the LSMO leading to thetracks, was obtained by fitting. The results were fitted using
Rsheet(α) = Rsheet,0 +Rsheet,C · cos(2(α− α0)) (5.10)
The following fit parameters were obtained:Rsheet,0 = 714Ω, Rsheet,C =83Ω, α0 = −19 and
(LW
)contacts
= 2.
28
5.4. DISCUSSION
Figure 5.9: Magnetic force mi-croscopy image of the sample. Theimage shows that the magnetizationdoes have an out of plane compo-nent, but gives no quantitative infor-mation.
0 100 200 300 400500
600
700
800
900
1000
Angle (deg)
Rsh
eet (Ω
)
Fit100 µm tracks
10 µm tracks
Figure 5.10: Transport anisotropymeasurement results at room tem-perature. The easy axis for electri-cal transport appears be 19 degreesrotated from the magnetic easy axis.
5.4 Discussion
The magnetic anisotropy of LSMO was shown to be very well described bythe used model. The results showed that the easy axis makes an in planeangle of 2 degrees with the sample edge. This is well within the alignmenterror margin of the used VSM. We therefore assume the in plane componentof the easy axis is aligned with the [001] direction as expected from the crystalstructure. The out of plane angle of the easy axis was measured to be 50degrees with the surface normal.
The transport measurements showed a transport easy axis of 19 degreeswhich seems to have no relation to the crystal structure. Step edges at theinterface might be able to explain the direction of this anisotropy. How-ever, the miscut direction of this sample is not known, so further research isnecessary before any conclusive statements can be made.
29
CHAPTER 5. ANISOTROPY
30
Chapter 6
Interface influences
As discussed in chapter 2, a possible cause for magnetic tunnel devices notobtaining the predicted TMR ratio are interface effects. To determine what ishappening at the STO-LSMO interface samples with varying layer thicknesswere fabricated. To rule out the influence of the LSMO-air interface, sampleswere made with and without an STO capping layer. The magnetic propertieswere analyzed using the VSM function of a Quantum Design PPMS. Thetransport properties were analyzed using van der Pauw measurements. Theresults are compared to results obtained for LSMO grown on STO (001).Three types of (001) oriented samples are used for the comparison: cappedsamples, uncapped samples and uncapped samples with interface engineering.
6.1 Results
The results for the saturation magnetization are shown in Fig. 6.1. It is clearthat the results for LSMO grown on STO (110) are significantly better thanfor LSMO grown on STO (001). There is no indication of a magnetic deadlayer. The results for capped samples and samples without a capping layerdo not differ significantly. Be aware that the units on the x-axis are unit celllayers. The unit cell layers in the (110) oriented crystal are about a factor√
2 thinner than in the (001) oriented crystal. Comparing samples withan equal thickness in nanometers would result in an even larger difference.However for comparing dead layer behavior, a scale in unit cell layers is moreconvenient.
The thermal dependence of the saturation magnetization is shown inFig. 6.2 A. The saturation magnetisation decreases with increasing temper-ature. Above the Curie temperature the sample is no longer ferromagnetic.Similar behavior is observed for all other samples of 10 unit cell layers andthicker. Samples thinner than 10 unit cell layers exhibit a different behavior.Hysteresis loops for a thin sample at varying temperatures were plotted inFig. 6.4. Above a certain temperature the hysteresis loops remain constant.
31
CHAPTER 6. INTERFACE INFLUENCES
Figure 6.1: Saturation magnetization per surface unit cell. The bulk magnetiza-tion value of 3.7µB/Mn is depicted in black. There is no sign of a magnetic deadlayer in LSMO(011).
This temperature was used as the TC value in our results. The remainingferromagnetism has an almost constant saturation magnetization value be-tween 1 and 1.2µB/Mn for all samples. The remanent magnetization is givenin table 6.1. No temperature influences were observed up to 350 K. For sam-ples of 10 unit cell layers or thicker no ferromagnetism is observed above TC.In Fig. 6.3 the Curie Temperature of LSMO (110) with and without cap-ping was compared to results obtained for LSMO(001).The results showedan increased Curie temperature for LSMO (110).
To analyze the interface influence on the transport behavior of the LSMOfilms, van der Pauw measurements were used. The sheet conductance was
Sample Msat Mremanent(unit cell layers) (µB/Mn) (µB/Mn)9 ML 1 ±0.2 0.1 ±0.058 ML capped −∗ −∗5 ML 1 ±0.2 0.3 ±0.15 ML capped 1 ±0.2 0.3 ±0.13 ML 0 ±0.01 0 ±0.0013 ML capped 1 ±0.1 0.3 ±0.1∗For the capped 8 ML sample the remaining ferromagnetism was observed,but not quantitatively measured.
Table 6.1: Saturation and remanent magnetization for thin LSMO(110) samplesabove TC.
32
6.1. RESULTS
A
B
C
Figure 6.2: The magnetic andelectrical behavior of the 40 unitcell uncapped sample was shownfor varying temperatures. Thephase transition at the Curietemperature is visible in themagnetic as well as the electricalmeasurements.
0 0.2 0.4 0.6 0.8 1 1.2
x 10−8
0
50
100
150
200
250
300
350
400
Thickness (m)
TC (
K)
001001 capIE110 cap110
Figure 6.3: Curie temperature for vary-ing layer thickness. The LSMO(110)samples have a higher Curie temperaturethan LSMO(001) samples of the samethickness.
−400 −300 −200 −100 0 100 200 300 400−4
−3
−2
−1
0
1
2
3
4
Applied Field (kA/m)
M (
µ B/M
n)
10 K30 K50 K70 K90 K110 K130 K150 K170 K190 K350 K
Figure 6.4: Hysteresis loops for varying temperatures for the 9 unit cell layersample without capping. Above the Curie temperature there is still ferromagneticbehavior.
33
CHAPTER 6. INTERFACE INFLUENCES
Figure 6.5: Sheet conductance for varying layer thickness. Lines were fitted tononzero values of conductance. The LSMO(001) samples show a dead layer, whilein the LSMO(110) samples there seems to be either conduction in all layers or innone.
measured and compared to results obtained for LSMO(001). The nonzeroconductance values were fitted with a linear fit. The LSMO(001) sampleswithout interface engineering showed an electrical dead layer of 8 unit celllayers. For the LSMO(110) samples there seems to be a different behavior.For thicker samples all layers seem to take part in the transport, while forthin samples there is no conductivity at all. The results for LSMO(110)should be taken with caution, since the fit is based on very little data points.For the LSMO(001) samples with interface engineering no fit was made dueto lack of data points for thicker samples. However for thin samples theconductivity seems to be better than for the (001) oriented samples withoutInterface engineering.
The magnetoresistance of the samples was determined. The relation be-tween the magnetic field and the magnetoresistance is approximately quadratic.The second order fit parameter is a good indicator of the strength of themagnetoresistive effect. The temperature dependence of the second order fitparameter was plotted in Fig. 6.2 together with the temperature dependenceof the sheet resistance.
34
6.2. DISCUSSION
6.2 Discussion
As explained in chapter 2, ferromagnetism is caused by the alignment ofspins due to the double exchange interaction energy. Thermal energy onthe other hand causes disorder. This leads to a decrease of magnetizationwith increasing temperatures. At the Curie temperature a phase transitionoccurs from a ferromagnetic phase to a paramagnetic phase. Both ferromag-netism and electrical transport are governed by the same process: doubleexchange interaction. This relation becomes visible when magnetic and elec-trical properties are plotted as function of temperature as done in Fig. 6.2.At the Curie temperature, when thermal energy gains influence over dou-ble exchange interaction energy, three things happen: The sample changesfrom ferromagnetic to paramagnetic, there is an increased rise in sheet re-sistance and the magnetoresistance has a minimum. This shows that indeedone process is responsible for magnetic as well as electrical behavior.
The results obtained for LSMO grown on STO(110) were compared withLSMO grown on STO(001) with and without interface engineering. The sat-uration magnetization of LSMO(110) assumed the bulk value of 3.7 µB/Mnregardless of layer thickness. There is no indication of a dead layer. Bothinterface engineering as well as using an polar continuous orientation (110)showed an improvement of the magnetic and electrical behavior of LSMO.The effect of capping was also investigated. No significant influence of cap-ping on the magnetization was observed. For samples thinner than 10 unitcell layers the behavior changes drastically. The sheet conductivity goes tozero and there remains ferromagnetism above TC. The scaling of the strengthof the remaining ferromagnetism with sample thickness and the absence offerromagnetism above TC for thicker layers makes it improbable that theeffect is caused by contaminations.
Although with the current information it is impossible to tell what is re-ally happening in the extremely thin layers, several properties of the effectcan be given. We have shown that for thick layers double exchange interac-tion governs ferromagnetic as well as transport behavior. For the thin layersthere is no electrical transport but there is ferromagnetism. This might bean indication that the remaining ferromagnetism is not induced by the dou-ble exchange interaction but by another effect. Another possibility is thatdouble exchange interaction does take place, but only locally. Looking at thephase diagram of LSMO there is also a ferromagnetic insulating phase for lowdoping levels. This phase is characterized by Jahn Teller deformations. Dueto the Jahn Teller deformations two neighboring Mn atoms have their ener-getically favorable eg shells perpendicular to each other. This makes doubleexchange interaction with neighboring Mn atoms unfavorable and thus pre-vents electrical conduction. The possibility of tunneling can however increasethe kinetic energy of electrons and still induce ferromagnetism. Structural
35
CHAPTER 6. INTERFACE INFLUENCES
deformations can also lift the degeneracy of the eg and t2g shell. Possiblystructural deformation can induce Jahn-Teller-like behavior and induce anferromagnetic insulating phase. This leaves the problem why the effect onlyoccurs in thin layers and does not lead to a dead layer.
A solution might lie in the discontinuities at the interface. Although thereis no polar discontinuity at the interface, there still is a structural disconti-nuity. The crystal parameters of the STO differ from those of the strainedLSMO. We have assumed this change in crystal parameters happens abruptlyat the interface. This leads to awkward bond angles for the oxygen atoms atthe interface. Furthermore there is a buckling of the oxygen octahedrons inLSMO which is absent in STO. It is possible that for thicker layers there isindeed an almost abrupt change in crystal parameters. The energy loss fromthe awkward interface bonds would be compensated by the energy gain of theoxygen buckling in LSMO. The unit cells close to the surface would be forcedto buckle along with the core of the layer. For thinner layers the core of theLSMO layer gets thinner. The gain in energy by buckling is reduced and thesurface energy gains influence leading to unbuckled oxygen octahedrons inthe LSMO layer, thereby drastically changing its properties.
Another possibility is that going from STO to LSMO there is a smoothtransition from a non-buckling square lattice to a buckling compressed andtilted lattice. Depending on the LSMO layer thickness, this transition wouldhappen largely in the STO substrate or largely in the LSMO layer. Furtherresearch and especially an investigation of the positions of the oxygen atomscould help clarify this behavior.
36
Chapter 7
Conclusions
The goal of this project was to investigate if LSMO grown on STO(110) issuited as an electrode material for TMR junctions. We have grown LSMOlayers of varying thickness using pulsed laser deposition. Rheed monitor-ing, AFM and XRD measurements all showed an epitaxial monocrystallinegrowth. During deposition, we observed a reduced growth speed for the firsttwo unit cell layers of LSMO. This has no apparent consequences for thematerial properties. The XRD results showed that the crystal structure ofthe strained LSMO uses its remaining degrees of freedom to assume latticeparameters similar to bulk values. This results in an out of plane compres-sive strain and a tilt of the (001) plane. From the crystal structure theexpected easy axis for magnetization was extracted. The measured in planecomponent of the easy axis matches the predictions. The magnetic angulardependent behavior of an 80 unit cell layer LSMO sample was extensivelyanalyzed. The magnetic behavior was shown to be modeled very well by acombination of coherent rotation and domain wall shifting.
For magnetic tunnel junctions the relative orientation of the magnetiza-tion of the electrodes is of great importance. Control over the direction ofmagnetization is essential for creating functional devices. The influence ofthe strain imposed by the substrate on the easy axis of the LSMO layer is aconvenient tool to direct the magnetization and so control the magnetizationdirection.
The results obtained for LSMO grown on STO(110) were compared withLSMO grown on STO(001) with and without interface engineering. The sat-uration magnetization of LSMO(110) assumes the bulk value of 3.7 µB/Mnregardless of layer thickness. There is no indication of a dead layer. Bothinterface engineering as well as using an polar continuous orientation (110)show an improvement of the magnetic behavior of LSMO. This is a strongindication that polar discontinuities do indeed play a role in the reduced mag-netic behavior of LSMO grown on STO(001). This is very promising for theuse of LSMO(110) as electrode material. The effect of capping with STO was
37
CHAPTER 7. CONCLUSIONS
also investigated. No significant influence of capping on the magnetizationwas observed.
For samples thinner than 10 unit cell layers the magnetic as well as theelectrical behavior changes drastically. Where in thicker layers all layers seemto take part in conduction, for thin layers there is no conduction at all. Themagnetization exhibits noteworthy behavior as well. The Curie temperatureis reduced with layer thickness as expected. However, when the thicknessdecreases below 10 unit cell layers, the magnetization no longer goes to zeroabove TC. Instead an apparently temperature independent ferromagneticbehavior is observed. The strength of the magnetization above TC scaleswith the sample thickness. Several possible ideas which could explain thiseffect or lead to an better comprehension were posed.
Overall we have shown that the magnetic and electrical properties ofLSMO on STO(011) are significantly better than LSMO grown on STO(001).Magnetization reaches the bulk value independent of layer thickness and thereis no indication of a dead layer. This makes the 110 orientation a very goodcandidate for TMR junctions. The next step would be to analyze the tunnelbehavior. This could be done directly by fabricating tunnel junctions.
38
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