Master Thesis. The structural, magnetic and electrical behavior of (110) oriented LSMO.

Transcription

1 Master Thesis The structural, magnetic and electrical behavior of (110) oriented LSMO. November 12, 2009 Author: Jaap Kautz Supervisors: Prof. dr. ing. Dave H.A. Blank Dr. ing. Guus Rijnders Dr. ir. Alexander Brinkman Ir. Hans Boschker

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3 Abstract This thesis investigates the suitability of La 0.7 Sr 0.3 MnO 3 grown on SrTiO 3 (110) as electrode material for magnetic tunnel junctions. Samples with thicknesses from 1 to 25 nanometers were fabricated using pulsed laser deposition. We observe monocrystalline epitaxial growth. XRD measurements showed out of plane compressive strain in the LSMO combined with a tilt of the (001) plane. VSM measurements were done to analyze the magnetic behavior and were compared to results for LSMO grown on STO(001). The results show a bulk like saturation magnetization of 3.7 µ B and no indication of a dead layer as observed for the (001) orientation. For electrical characterization van der Pauw measurements were done. For layers thicker than 2.7 nanometers electrical conduction is observed in which all layers seem to take part. For thinner layers there is no conduction and a not yet understood ferromagnetic behavior is observed above the Curie temperature.

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5 Contents 1 Introduction 1 2 Theory LSMO Magnetism and Conductivity Magnetic Tunnel Junctions Causes for Low TMR-Ratio Fabrication Pulsed Laser Deposition Patterning Van der Pauw measurements Transport anisotropy measurements Discussion Crystal structure XRD Measurements Anisotropy Simulations Magnetic anisotropy measurements Transport anisotropy measurements Discussion Interface influences Results Discussion Conclusions 37 iii

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7 Chapter 1 Introduction Soon after the invention of the computer a distinction was made between small capacity quick volatile internal memory and large capacity slower nonvolatile external memory. An ideal memory would combine the best of both worlds; it should be quick, non-volatile and have a large capacity. Magnetic tunnel junctions could in theory form the base for such an ideal memory. The theory behind magnetic tunnel junctions is explained in chapter 2. Magnetic tunnel junctions consist of two ferromagnetic electrodes separated by a thin insulating layer. A good candidate for the electrode material is La 0.7 Sr 0.3 MnO 3 (LSMO), because of its 100% spin polarization. However results from measurements done on TMR junctions using LSMO do not live up to expectations. In section several theories explaining this reduced performance are discussed. One possible explanation is that the diminished results are caused by the polar discontinuity at the interface between LSMO and SrTiO 3 (001), which is used as insulator. If this theory is correct, no such effect should occur for a LSMO grown on STO with a different crystal orientation. The goal of this research project is to investigate if LSMO grown on STO(110) is suited to be used as electrode material for TMR devices. To do so, the structural, magnetic and electrical behavior of LSMO on STO(110) was examined and compared to results obtained in earlier studies for LSMO grown on STO(001). For the TMR effect to work we need well defined interfaces, monocrystalline growth and a controlled doping level. Pulsed laser deposition fulfills all these requirements and furthermore gives excellent control over growth speed and layer thickness. To verify that we do indeed obtain monocrystalline growth the samples were analyzed using atomic force microscopy(afm) and x-ray diffraction(xrd) measurements. The fabrication process is described in chapter 3. The XRD results were also used to analyze the structural effects of the STO substrate. Since the large scale magnetic and electrical behavior is determined by small scale interactions, changes in crystal structure can drastically alter the material properties. In chapter 4 the results are shown. 1

8 CHAPTER 1. INTRODUCTION Used in memory devices the magnetization direction of the electrodes has to be switched frequently. Therefore good understanding of the magnetic behavior of LSMO is essential. The magnetization switching was analyzed extensively using a vibrating sample magnetometer and compared to predictions from theory. Earlier problems with TMR junctions using LSMO were subscribed to a dead layer caused by polar discontinuity. If polar discontinuity is the sole cause for the dead layer in LSMO on STO(001) there should be no dead layer in LSMO grown on STO(110). To investigate the interface interactions for the (110) orientation, samples of varying thickness were grown and analyzed. The results can be found in chapter 6. Finally in chapter 7 we discuss the obtained results and draw conclusions on the suitability of LSMO grown on STO(110) as electrode material in magnetic tunnel junctions. 2

9 Chapter 2 Theory 2.1 LSMO Lanthanum strontium manganite is an oxide with an perovskite crystal structure. The chemical composition of LSMO is La 1-x Sr x MnO 3, where x is a variable indicating the doping level. One unit cell of LSMO is given in Fig 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 Sr 2+. The manganese atoms are either ionized to Mn 3+ or to Mn 4+ depending on the doping level. This results in the 3d shell of the manganese atoms being filled with either 4 or 3 electrons. The electrical and magnetic behavior is largely determined by the electrons in the 3d shell of the manganese atoms[1]. The orbitals of the d shell are given in Fig For an atom in free space these states are degenerate. However the oxygen octahedron surrounding the manganese atoms lifts this degeneracy. This can be seen in Fig This figure also shows the spin polarization caused by the Hund s interaction energy. Due to a strong electron electron interaction it is energetically favorable for the electrons in the 3d shell to align their spins. This Hund s interaction energy is larger than the energy gap between the t 2g and e g states. As a result a fourth electron in the 3d shell will align its spin and occupy one of the e g states rather than assuming an antiparallel configuration in one of the t 2g states. This effect causes the electrons of one manganese atom to align their spins Magnetism and Conductivity The overall magnetic and electrical behavior depends on the interactions between the different manganese atoms. Four counteracting processes play a role in the behavior of the 3d electrons. These four processes are: double exchange interaction, super exchange interaction, Jahn Teller effect and charge ordering. 3

10 CHAPTER 2. THEORY 2 2 x-y 2 2 3z -r e g t 2g xy xz yz Figure 2.1: Unit cell of LSMO. The manganese atom(green) is surrounded by an oxygen octahedron(red). The atoms at the yellow sites can be either lanthanum or strontium. Figure 2.2: 3d orbitals of the manganese atom[2]. The otherwise degenerate states split up due to the presence of the negatively charged oxygen octahedron. E 2 2 x-y e g 2 2 3z -r 3d e g yz,zx t 2g xy t 2g e g 2 2 x-y 2 2 3z -r E F xy t 2g yz,zx 3d band in free space 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. Figure 2.3: Schematic representation of the bands formed by the 3d orbitals of the manganese atoms in LSMO. Except for the last graph all Fermi levels are drawn for the non doped situation. 4

11 2.1. LSMO Double Exchange Interaction Double exchange interaction is a process in which two neighboring manganese atoms and their connecting oxygen atom play a role. The process is depicted in Fig One of the manganese atoms has one of its e g states occupied, the other has an empty e g shell. One electron tunnels from the oxygen atom to the manganese atom without e g electrons. The e g electron from the other manganese 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. Since electrons retain their spin while tunneling, tunneling is only possible between states with parallel spins. This is why double exchange interaction only occurs between manganese atoms whose t 2g electrons have their spins aligned. Because double exchange interaction increases the freedom of electrons, it lowers their energy. This makes it energetically favorable for electrons of neighboring manganese atoms to align their spins, inducing ferromagnetism. Since double exchange interaction requires filled e g states as well as empty ones(holes) it is highly doping dependent. Figure 2.4: Double exchange interaction. 5

12 CHAPTER 2. THEORY Super Exchange Interaction Super exchange interaction occurs due to the slightly overlapping t 2g and e g orbitals of two neighboring manganese atoms. Electrons in the these shells can tunnel to the neighboring manganese atom. When this neighboring shell is already half filled, tunneling is only possible to the antiparallel spin states. Due to the Hund s interaction energy, these antiparallel states have a higher energy level, making the electrons tunnel back directly to their original manganese atom. However this tunneling possibility increases the freedom of the electrons, decreasing their energy. Since electrons retain their spin while tunneling, super exchange interaction is only possible between manganese atoms whose electrons have antiparallel spins. This makes it energetically favorable for electrons of neighboring manganese atoms to assume antiparallel spins. This induces antiferromagnetism. 2 2 x-y 2 2 3z -r e g 2 2 3z -r 2 2 x-y xy yz,zx t 2g yz,zx xy Figure 2.5: Jahn Teller effect: Deformation of the oxygen octahedron lifts the degeneracy of the e g and t 2g bands. Figure 2.6: Lattice distortion due to Jahn Teller effect. The manganese atoms retain their positions, while the oxygen octahedrons are deformed. Jahn Teller Effect The Jahn Teller effect is an interaction effect between the crystal lattice and the manganese 3d electrons. Deformation of the oxygen octahedron causes energy differences within the otherwise degenerate t 2g and e g electron states. This shown in Fig Orbitals which are compressed by the deformation gain energy, while orbitals which are elongated by the deformation loose energy. When the e g shell is half filled it is energetically favorable to create a deformation, because one of the orbitals will loose energy. The electrons in 6

13 2.1. LSMO Figure 2.7: Charge ordering: For doping of x=0.5 e g electrons spread over the manganese atoms due to coulomb repulsion. Due to this electron distribution the lattice gets deformed. Figure 2.8: Magnetic/electronic phase diagrams of La 1-x Sr x MnO 3 (created by Urushibara et al.[3]). PI: paramagnetic insulator, PM: paramagnetic metal, CNI: canted antiferromagnetic insulator, FI: ferromagnetic insulator, FM: ferromagnetic metal, AFM: antiferromagnetic metal. the e g shell will then occupy this lower energy state. Creating a deformation of the oxygen octahedrons without altering the overall crystal lattice leads to the typical Jahn Teller distortions shown in Fig The Jahn Teller distortions are only favorable when a lot of manganese atoms have a half filled e g shell. This makes this effect doping dependent. The degeneracy within the t 2g and e g states can also be lifted by deformations due to externally imposed strain. Charge Ordering Charge ordering is caused by Coulomb repulsion between e g electrons at different manganese atoms. The electrons order themselves in a pattern shown in Fig This configuration is so stable that electrons retain their positions preventing electrical conduction. This stability has two causes. The first one is the aforementioned Coulomb repulsion. The second one is electron phonon coupling. The electrons locally deform the oxygen octahedrons. To break the electron configuration the lattice would have to be reformed as well. Electrical ordering is only favorable for certain electron hole ratios, making it doping dependent. 7

14 CHAPTER 2. THEORY Phase diagram The four described mechanisms all contribute to the electric and magnetic properties of LSMO. The temperature, doping, strain and shape of the sample determine which mechanism is dominant. This is clearly visible in a phase diagram as shown in Fig For this project the LSMO needs to be ferromagnetic and have a high Curie temperature. This is why the doping level x is set at In the rest of this report all references to LSMO will mean La 0.67 Sr 0.33 MnO 3 unless specified otherwise. 2.2 Magnetic Tunnel Junctions In magnetic tunnel junctions the tunneling probability between two materials is influenced by the magnetization of the materials. The effect was first observed by Wyatt[4] and later extensively researched[5][6]. The setup consists of two ferromagnetic electrodes separated by a thin insulating layer. Since electrons retain their spin while tunneling, tunneling is only possible between sites which have their spins aligned. Only energy states near the Fermi level play a role in the tunneling process. In a ferromagnetic material the number of electrons at the Fermi level with spin up is unequal to the number of electrons with spin down. Ferromagnetic materials are therefore said to to be spin polarized. As a result of this spin polarization the tunneling probability is higher when the majority spins of the two electrodes are aligned. The main figure of merit for magnetic tunnel devices is the TMR-ratio. It is defined as T MR = R ap R p R p, (2.1) where R p is the resistance for the situation where the two electrodes have their spins aligned and R ap is the resistance for the anti-parallel situation. There are to two requirements for a successful TMR device: spin polarized conducting electrodes and an insulator which is insulating, but thin enough to tunnel through. Magnetic tunnel junctions have successfully been used in commercially available devices. In these devices the used electrode materials are mostly cobalt 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 alternative would be LSMO. Due to the 100% spin polarization at the fermi level theoretically very high TMR-ratios could be obtained. However experiments done with LSMO tunnel junctions[8][9][10] do not obtain such high TMR values. Several theories try to explain this result. They will be discussed in the following section. 8

15 2.2. MAGNETIC TUNNEL JUNCTIONS 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 either in a leaky insulator or in the electrodes not being as spin polarized as expected. The properties of the insulating STO layer were studied extensively [11][12][13]. The main cause for leakage are oxygen vacancies. These vacancies have a twofold influence on the insulating properties. The STO gets electron doped by the absence of Oxygen acceptors. Furthermore the vacancies themselves can travel through the STO and provide hole-like charge transport. The other possible cause for a low TMR ratio is the electrodes not really being as spin polarized as expected. Although bulk LSMO has been shown to be truly half metallic[14], various studies[15][16] suggest LSMO loses its half metallicity at the interface due to interactions with the STO. Observation of a so called dead layer, which appears not to be magnetic nor electrically conducting[17][18][19], supports this theory. One possible explanation for this dead layer is found in the polar discontinuity model[20][21]. This model describes (001) oriented LSMO as layers of La 0.67 Sr 0.33 O (0.67)+ and MnO 2 (0.67)- and STO as layers of SrO and TiO 2. This is shown in Fig In LSMO the layers are charged while in STO there is no polarization of the layers. At the interface of these materials a polar discontinuity occurs leading to electronic redistribution. This electronic redistribution causes a reduced or enhanced doping depending on the type of interface as can be seen in Fig and The altered doping at the interface causes the e g states to be either under- or overpopulated, in both cases preventing the double exchange mechanism needed for conduction and ferromagnetism. Interface engineering has been suggested as an solution for this problem[22][23]. In interface engineering the doping of the first unit cell layer of LSMO is altered to counter the effect of electronic redistribution and maintain a constant population of the e g -states throughout the LSMO. This research tries to clarify if this dead layer can indeed be explained by the polar discontinuity model. To do so an alternative setup is chosen in which polar discontinuity does not play a role. By using (110) oriented STO as a substrate, the layers parallel to the surface have a different constitution. STO is now a stack of SrTiO 4+ and O LSMO (110) has layers of La 0.67 Sr 0.33 MnO 4+ and O The interface is shown in Fig It is clear that there is no polar discontinuity. If the polar discontinuity model is indeed the sole cause for the dead layer formation, there should be no dead layer in this configuration. 9

16 CHAPTER 2. THEORY 0.67 (La 0.67 Sr0.33O) (MnO 2) 0.67 (La0.67Sr0.33O) 0 (SrO) 0 (TiO 2) 0 (SrO) (MnO 2) 0.67 (La0.67Sr0.33O) (MnO 2) 0 (SrO) 0 (TiO 2) 0 (SrO) Figure 2.9: LSMO as a stack of La 0.67 Sr 0.33 O (0.67)+ and (0.67)- MnO 2 and STO as a stack of SrO and TiO 2. Figure 2.10: Due to polar discontinuity the e g states of the first manganite layer get underpopulated (La0.67Sr0.33O) -1 (MnO 2) 0.67 (La0.67Sr0.33O) 0 (TiO 2) 0 (SrO) 0 (TiO 2) -4 (O 2) (La0.67Sr0.33MnO) -4 (O 2) (La0.67Sr0.33MnO) -4 (O 2) 4 (SrTiO) -4 (O 2) 4 (SrTiO) -4 (O 2) 4 4 Figure 2.11: Due to polar discontinuity the e g states of the first manganite layer get overpopulated. Figure 2.12: Due to the (110) crystal orientation, there is no polar discontinuity. 10

17 Chapter 3 Fabrication 3.1 Pulsed Laser Deposition The LSMO layers were fabricated using pulsed laser deposition(pld). The setup is shown in Fig A target of polycrystalline La 0.7 Sr 0.3 MnO 3 is locally heated very rapidly using a focussed laser pulse causing instantaneous evaporation. The vapor is further heated by the pulse forming a high pressure plasma of ionized LSMO atoms. The plasma expands rapidly away from the target due to the high pressure gradient. The substrate is placed in the path of this expanding plasma. The shape of the plasma plume leads to an even distribution over the surface. After reaching the substrate, the LSMO atoms use their remaining kinetic energy to diffuse over the surface. This process is shown in Fig For the atoms to diffuse into an atomically smooth layer they need sufficient kinetic energy. The kinetic energy of the atoms reaching the substrate can be influenced by altering the process parameters. The surface area and intensity of the focussed laser beam influences the amount of atoms as well as their energy. Increasing the pressure in the chamber causes the expansion of the plasma to slow down and in doing so reduces the kinetic energy of the arriving atoms. Finally the energy of the arriving atoms can be adjusted by varying the target substrate distance. All these factors influence the quality and uniformity of the deposited layer. Earlier studies[19] have led to an optimization of the parameters for the deposition on LSMO on STO. The used parameters are shown in table 3.1. The used laser is an KrF excimer laser. A rectangular mask is placed in the laser beam to select that part of the beam with the most homogeneous intensity. The beam is then focussed, creating an image of the mask on the target. Before deposition the (110) oriented STO substrates were cleaned in an ultrasonic bath: First 10 minutes in acetone, followed by a 10 minute bath of ethanol. Lens paper moistened with ethanol is used to wipe any residues from the sample. Then the substrate is annealed for one hour at 950 degrees Celsius under an oxygen atmosphere of 1 bar. For the fabrication of the 11

18 CHAPTER 3. FABRICATION Figure 3.1: Setup for pulsed laser deposition. Figure 3.2: Model for growth during pulsed laser deposition. After adsorption unit cells diffuse over the surface until they are incorporated in the lattice. Figure 3.3: Typical rheed pattern. The intensity of the center reflection spot is used to monitor the growth. 12

19 3.1. PULSED LASER DEPOSITION Parameter Value Unit O 2 pressure 0.27 mbar Target substrate distance 50 mm Substrate temperature 830 C Laser fluency 200 J/mm 2 Spot size 2.07 mm 2 repetition rate 1 Hz Anneal pressure 100 mbar Optical setup Wavelength 248 nm Pulse duration 25 ns Mask area 56.5 mm 2 Mask lens distance 3264 mm Lens target distance 526 mm Lens focal distance 453 mm Optical losses 10 % Table 3.1: Parameters used in fabrication process. capped layers, the deposition of the LSMO layer is directly followed by a deposition of STO using the same parameters. After deposition the sample is cooled down under an oxygen atmosphere of 100 mbar. The deposition is monitored using reflective high energy electron diffraction(rheed). An electron beam is aimed at the sample under a grazing angle. Interference between the electrons reflected from different atoms causes a distinctive angle dependence of the intensity of the diffracted electron beam. A phosphor screen is used to visualize the diffraction pattern. Such a diffraction pattern is shown in Fig The bottom spot is caused by that part of the electron beam that misses the sample and can be ignored. The top center spot is the main out of plane reflection spot. The two side spots have out of plane as well as in plane components. The intensity of the reflection is highly influenced by the surface morphology of the sample. This makes the reflection intensity a good indicator of the smoothness of the sample surface. The intensity of the center spot was measured during deposition. The results are shown in Fig Each oscillation corresponds to the completion of one unit cell layer. A complete unit cell layer is very smooth and results in a high intensity. Further deposition first reduces the smoothness until a new unit cell layer is almost complete and the intensity increases again. Zooming in on the rheed signal shows the effect of the individual pulses. The reflection deteriorates 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 longer deposition time than the other unit cell layers. The increase in deposition 13

20 CHAPTER 3. FABRICATION Intensity (A.U.) Intensity (A.U.) Time (s) Time (s) Figure 3.4: Intensity of the central reflection spot of the rheed signal during deposition of the 8 layer sample with capping. The first set of 8 oscillations are during the deposition of LSMO. The second set of 6 oscillations are during the deposition of the capping layer. The average deposition speed is 19 seconds per layer for LSMO and 9 second per layer for STO. The inset shows how the individual pulses 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 the second unit cell layer. No reduction of the deposition speed was observed for the first layers of the STO capping on LSMO. The sample surfaces were analyzed before and after deposition using an atomic force microscope(afm). The results of one of the samples are shown in Fig. 3.5 and 3.6. The clean substrate clearly showed the atomic steps caused by the miscut of the substrate. The height of the steps corresponds to 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 did show the sample is still atomically smooth. 3.2 Patterning Van der Pauw measurements For the van der Pauw measurements gold contact pads were created on the corners of the samples using plasma sputtering. A contact mask was used to obtain the desired pattern. The pattern is shown in Fig. 3.7 A. Using a sputter power of 150W for four minutes, a gold layer was grown. 14

21 3.2. PATTERNING Figure 3.5: AFM image of substrate. The terrace steps are clearly visible. Figure 3.6: AFM image after LSMO deposition. Terrace steps are no longer clearly visible, but surface is still atomically smooth. 4 mm 5 mm 5 mm A B C Figure 3.7: Masks used for transport measurements. A: Mask used for gold deposition to create contact points for van der Pauw measurements. Mask B is used to etch tracks of LSMO for transport anisotropy measurements. The two track widths are 10 µm and 100 µm. The length of the tracks is 400 µm. Mask C is used to create gold contact pads for the transport anisotropy measurements. 15

22 CHAPTER 3. FABRICATION A nm Height (nm) 3 Distance(10 nm) B Figure 3.8: A: AFM image of one of the small tracks of the transport anisotropy measurement. The track consists of lsmo while the background is formed by the STO substrate. B: A cross section of the tracks shows the width is 9 µm Transport anisotropy measurements After deposition of the LSMO layer, the samples used for transport anisotropy measurements were patterned using photolithography and argon ion etching. Fig.3.7 B shows the mask used for the lithography. Using positive photoresist all LSMO but the dark areas on the mask was etched away. Gold contact pads were applied using a lift off process: Photolithography with the mask from Fig.3.7 C provided a layer of positive photo resist with openings at the light areas of the mask. Plasma sputtering of gold followed by solution of the resist in acetone resulted in gold contact pads at the location of the light areas of the mask. The fabrication of the track was verified using AFM. The results are shown in Fig Discussion The reduced growth speed for the first two layers is remarkable. Using the model from Fig. 3.2, a reduced deposition rate is caused by an reduced difference between adsorption and desorption. This could be a consequence of a lower adsorption of LSMO on STO compared to LSMO on LSMO. Another possible explanation is found in the substrate being almost perfectly 16

23 3.3. DISCUSSION smooth. This reduces the probability of incorporation and thus increases the desorption rate. As growth continues step edges get more disordered and incorporation probability increases, leading to less desorption and faster growth. Further research is needed to investigate this behavior. The diffusion behavior could be analyzed by comparing the recovery after each pulse for the first and later unit cell layers. Another option is to stop deposition during growth of the first layer and use AFM to analyze the process. The measured channel width for the anisotropy measurements is 1 micrometer smaller than the the used mask pattern. This is probably due to illumination of the area under the mask at the edges of the pattern. The resistance of the channel will therefore be higher then could be expected from the mask dimensions. 17

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25 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 leads to straining of the deposited layer. LSMO was grown epitaxially on an STO(110) substrate. There is only a slight mismatch between the lattice parameters of LSMO(0.388 nm) and STO( nm). Therefore we assume the LSMO layer will adapt an (110) orientation as well. Since the lattice constant of STO is larger than that of LSMO, the in plain features of the LSMO layer are elongated. In the out of plain direction, the LSMO has no 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 natural LSMO. The expected out of plane lattice parameter is d = a 2 LSMO a2 ST O 2 = nm nm2 2 = 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 the out of plane direction of the reciprocal lattice obtained by performing a Θ2Θscan. The large peaks belong to the STO substrate, while the side peaks are caused by the LSMO layer. The out of plane lattice constant was obtained from the distance between the peaks: d = 2π (4.2) Q For the STO substrate this gives an out of plane lattice constant of nm. The LSMO out of plane lattice constant is nm. The 19

26 CHAPTER 4. CRYSTAL STRUCTURE d LSMO [110] LSM [100] LSM [110] LSM dsto [110] STO [100] STO [110] STO Figure 4.1: Θ2Θ-scan of 80 ml LSMO on an STO substrate. The large peaks can be subscribed to the STO substrate and correspond to an out of plane lattice parameter of nm. The side peaks correspond to an out plane lattice parameter of nm, indicating the LSMO has a slightly smaller out of plane lattice parameter as the STO substrate. Figure 4.2: Schematic representation of the straining of the LSMO layer. Seen from [001] direction. measured value corresponds within the measurement error to the theoretical 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 To obtain more information on the crystal structure of the LSMO layer, reciprocal maps were made of several nodes of the crystal lattice. These are shown in Fig The resulting lattice parameters are given in table 4.1. For the STO substrate all lengths are nm and the angles are 90 degrees. The in plane lattice parameters of the LSMO film match those of the substrate, confirming the fact that the film assumes the in plane feature sizes of the substrate. As already observed in the Θ2Θ-scan the reciprocal out of plane values of the film exceed those of the substrate indicating a smaller out of plane lattice parameter for the film. Furthermore the 331 and 331 film peaks show different out of plane values. This indicates that the (001) plane is tilted by an angle of 0.6 degrees. This is shown in Fig All lattice parameters obtained from the reciprocal maps are given in table 4.1. The tilting angle only influences the lattice parameters angles α LSMO and β LSMO. They approach their natural value of 89.7 degrees. 20

27 4.1. XRD MEASUREMENTS QOUT OF PLANE (1/m) Q IN PLANE (1/m) Q IN PLANE (1/m) QOUT OF PLANE (1/m) QOUT OF PLANE (1/m) QOUT OF PLANE (1/m) QOUT OF PLANE (1/m) QOUT OF PLANE (1/m) Q IN PLANE (1/m) Q IN PLANE (1/m) (331) (331) (240) (420) Q IN PLANE (1/m) (310) Q IN PLANE (1/m) (130) Figure 4.3: Reciprocal mapping of several reflection peaks. The substrate peaks show the cubic crystal lattice of STO. The different QOut of plane values for the 331 and 331 peaks indicate a tilt of the (001) plane. Parameter ast O = bst O = cst O dst O asto bsto = 2 ast O (in plane) asto + bsto = 2 ast O alsm O blsm O clsm O = cst O (in plane) dlsm O alsmo blsmo = asto bsto (in plane) alsmo + blsmo length nm Parameter αst O = βst O = γst O 90.0 (csto, (asto + bsto )) (csto, (asto + bsto )) α β γ (csto, (asto + bsto )) (csto, (asto bsto )) Table 4.1: Lattice parameters obtained from reciprocal maps of STO substrate and 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 angle

28 CHAPTER 4. CRYSTAL STRUCTURE [110] LSMO [001] LSMO [110] STO [001] STO Figure 4.4: Schematic representation of the tilting of the (001) plane. Seen from [110] direction 22

29 Chapter 5 Anisotropy For the use as a magnetic memory device the magnetic properties of LSMO are of great importance. Research had already been done on the magnetic properties of bulk LSMO and LSMO grown on STO(001). In this project the magnetic properties of LSMO on STO(110) were investigated. In order to explain the magnetic behavior of LSMO each manganese atom is modeled as a magnetic dipole. The Hund s coupling is assumed to be so strong that all 3d electrons of one manganese atom have their spins aligned. The total magnetic moment of one manganese atom, comprising spin as well as orbital momentum, is assumed to be constant. Only the direction of the magnetic moment can be varied. The direction of the magnetic moment for each manganese atom is the result of an energy equilibrium. There are four types of energy which play a role in this equilibrium: Double exchange interaction energy The energy gain due the double exchange interaction depends on the magnetic moment alignment of neighboring manganese atoms. The energy of one atom due to interaction with a neighboring atom is[25]: E DE,ij = JS 2 cos (φ ij ), (5.1) where S is the spin quantum number, J is the exchange integral and φ ij is the expectation value of the angle between the two spin vectors. By summing this 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 to align with the magnetic field to gain zeeman energy. The zeeman energy is given by E Zeeman = K Zeeman cosχ, (5.2) 23

30 CHAPTER 5. ANISOTROPY where χ is the angle of the magnetization with the magnetic field. K Zeeman is given by: K Zeeman = m sat B, (5.3) where m sat is the magnetic dipole moment per volume and B is the magnetic flux density. Crystal anisotropy Crystal anisotropy is caused by spin-orbit coupling[25]. While an e g electron orbits around the manganese nucleus it experiences an alternating electric field from the positively charged nucleus and thus a magnetic field. The magnetic momentum of this orbital field influences the spin of the electron. It is energetically favorable for the electron to align its spin with the orbital magnetic moment. When the crystal structure of LSMO deviates from the cubic shape, the degeneracy between the e g orbitals is lifted. As one of the orbitals is preferred, the LSMO will obtain a preferred spin direction as well due to spin orbit coupling. The preferred spin direction will be in line with the magnetic moment of the orbital with the lowest energy. This causes magnetic anisotropy. On average, the orbitals with the lowest energies lie in the direction in which the crystal is mostly elongated. For our case all in plane directions are equally elongated. The out of plane direction is compressed, making the out of plane direction unfavorable. However, due to the tilt of the crystal, the elongation can be increased by assuming an out of plane component. Taking only uniaxial contributions into account, the crystal easy axis will lie in the (110) plane between the [001] and [111] direction. The energy it costs to deviate from the easy axis is: E crystal = K crystal (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 that of the surrounding air. It is energetically favorable to minimize the magnetic field lines outside the sample. This causes two effects: First, the magnetization tends to align with the direction in which the sample has the largest dimensions. Second, the formation of magnetic domains. For thin films the in plane dimensions are much larger than the out of plane dimensions leading to a tendency for in plane magnetization. The energy due to this shape anisotropy is[26] 24

31 5.1. SIMULATIONS Θ Θ φ Figure 5.1: Simulation for no field applied field. The [111] direction is used as the crystal easy axis. K crystal is set to 0.67µ 0 m 2 sat φ Figure 5.2: Simulation for situation just before switching. The angles for the magnetic field direction are Θ field = 90 and φ field = 45 The magnetic field strength is 0.7m sat. E shape = K shape (sinθ) 2, (5.5) where Θ is the angle of the magnetization with the surface normal and K shape is given by K shape = 1 2 µ 0m 2 sat, (5.6) where m sat is the saturation magnetization per unit volume. 5.1 Simulations The manganese atoms align their magnetic moments in such a way as to minimize their energy. This was simulated by numerically finding energy minima for given combinations of crystal and shape anisotropy and applied field. Without any applied field there are two stable magnetization directions. This is shown in Fig The stable magnetization directions in this situation are determined by the crystal anisotropy and the shape anisotropy. By applying a magnetic field, the energy minima are rotated in the direction of the magnetization direction. The minimum closest to the applied field direction gets deeper while the other minimum looses depth until it vanishes and only one minimum remains. In Fig. 5.2 the situation just before the vanishing of the second minimum is shown. If at this point the state of the system was in this second minimum, it will quickly rotate its magnetization to the remaining minimum. The magnetization parallel to the applied field was simulated for a field along the in plane easy and hard axis. This is shown in Fig

32 CHAPTER 5. ANISOTROPY 5.2 Magnetic anisotropy measurements A sample with 80 unit cell layers of LSMO was analyzed using a vibrating sample magnetometer(vsm) with an in plane rotatable magnetic field source. Magnetic sweeps were made under various in plane angles to analyze the anisotropic behavior of the LSMO. Fig. 5.4 shows the magnetization parallel to the applied field. The characteristic behavior for an easy and hard axis is clearly visible. To gain more information on the processes during these hysteresis loops the magnetization angle and total in plane magnetization are plotted in Fig. 5.5 and 5.6. When reducing the applied field the magnetization first rotates back towards the easy axis. At zero applied field the magnetization is aligned with the easy axis. When further reducing the magnetic field to negative values the magnetization turns away from the easy axis again, until switching occurs. During switching the magnetization quickly switches towards the other stable direction. This switching happens step by step as can be seen from the total magnetization. This step by step switching can be explained by domain formation. The steps represent the switching of individual domains. When we plot the remanent field and coercivity, again the uniaxial anisotropy is clearly visible. The coercivity data was fitted with a model proposed by Suponev et al.[27]: Acosξ H c (ξ) = H c (0) sin 2 ξ + Acos 2 ξ (5.7) Here A = N A+N x N z is the ratio of demagnetization factors. The remanent field is assumed to always lie in the direction of the easy axis. The expectation value of the remanent field is therefore: M rem = M sat 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 ξ differs from the in plane angle with the easy axis φ. The maximum remanent magnetization is around 0.75 M Sat. Taking the inverse sine of this number indicates the easy axis makes an angle of 50 degrees with the surface normal. To measure this out of plane magnetization, magnetic force microscopy (MFM) was done. The results are shown in Fig

33 5.2. MAGNETIC ANISOTROPY MEASUREMENTS Figure 5.3: Simulation of the magnetization parallel to field for varying field strengths for easy and hard axis. Figure 5.4: Measured magnetization parallel to field for varying field strengths for easy and hard axis. Figure 5.5: In plane angle of the magnetization direction with the sample for varying field strengths for applied fields along easy and hard axis. When reducing the field strength the angle rotates from the field angle to the easy axis. Upon further reducing the field the angle rotates further until switching occurs and the magnetization switches to the other stable direction Figure 5.6: Total magnitude of the magnetization for varying field strengths for applied fields along easy and hard axis. Upon reducing the field strength the total magnetization is reduced due to out of plane rotation of the magnetization. When switching occurs the total magnetization is further reduced due to magnetic domains inverting their direction. At zero magnetization exactly half of material has switched. 27

34 CHAPTER 5. ANISOTROPY [110] [110] [001] Figure 5.7: Remanent magnetization parallel to the applied field for varying in plane field angle. The bottom axis gives the in plane field angle while the top axis gives the total angle of the magnetic field with the easy axis. The fit corresponds to an easy axis making an in plane angle of 2 degrees with the sample edge and an angle of 50 degrees with the surface normal. [001] [110] [110] Figure 5.8: Coercive field for varying in plane field angle. The bottom axis gives the in plane field angle while the top axis gives the total angle of the magnetic field with the easy axis. The fit corresponds to an anisotropy parameter A = 193 and H c (0) =0.363 ka/m. 5.3 Transport anisotropy measurements The transport anisotropy was measured using the wheel structure introduced in section 3.2. The resulting sheet resistances are shown in Fig The sheet resistance was calculated using R R sheet = ( L ) + ( ), (5.9) L W channel W contacts where ( ) L, the unknown aspect ratio of the LSMO leading to the W contacts tracks, was obtained by fitting. The results were fitted using R sheet (α) = R sheet,0 + R sheet,c cos(2(α α 0 )) (5.10) The following fit parameters were obtained:r sheet,0 = 714Ω, R sheet,c = 83Ω, α 0 = 19 and ( ) L = 2. W contacts 28

35 5.4. DISCUSSION Fit 100 µm tracks 10 µm tracks R sheet (Ω) Angle (deg) Figure 5.9: Magnetic force microscopy image of the sample. The image shows that the magnetization does have an out of plane component, but gives no quantitative information. Figure 5.10: Transport anisotropy measurement results at room temperature. The easy axis for electrical transport appears be 19 degrees rotated from the magnetic easy axis. 5.4 Discussion The magnetic anisotropy of LSMO was shown to be very well described by the used model. The results showed that the easy axis makes an in plane angle of 2 degrees with the sample edge. This is well within the alignment error margin of the used VSM. We therefore assume the in plane component of the easy axis is aligned with the [001] direction as expected from the crystal structure. The out of plane angle of the easy axis was measured to be 50 degrees with the surface normal. The transport measurements showed a transport easy axis of 19 degrees which seems to have no relation to the crystal structure. Step edges at the interface might be able to explain the direction of this anisotropy. However, the miscut direction of this sample is not known, so further research is necessary before any conclusive statements can be made. 29

36 CHAPTER 5. ANISOTROPY 30

37 Chapter 6 Interface influences As discussed in chapter 2, a possible cause for magnetic tunnel devices not obtaining the predicted TMR ratio are interface effects. To determine what is happening at the STO-LSMO interface samples with varying layer thickness were fabricated. To rule out the influence of the LSMO-air interface, samples were made with and without an STO capping layer. The magnetic properties were analyzed using the VSM function of a Quantum Design PPMS. The transport properties were analyzed using van der Pauw measurements. The results are compared to results obtained for LSMO grown on STO (001). Three types of (001) oriented samples are used for the comparison: capped samples, uncapped samples and uncapped samples with interface engineering. 6.1 Results The results for the saturation magnetization are shown in Fig It is clear that the results for LSMO grown on STO (110) are significantly better than for LSMO grown on STO (001). There is no indication of a magnetic dead layer. The results for capped samples and samples without a capping layer do not differ significantly. Be aware that the units on the x-axis are unit cell layers. The unit cell layers in the (110) oriented crystal are about a factor 2 thinner than in the (001) oriented crystal. Comparing samples with an equal thickness in nanometers would result in an even larger difference. However for comparing dead layer behavior, a scale in unit cell layers is more convenient. The thermal dependence of the saturation magnetization is shown in Fig. 6.2 A. The saturation magnetisation decreases with increasing temperature. Above the Curie temperature the sample is no longer ferromagnetic. Similar behavior is observed for all other samples of 10 unit cell layers and thicker. Samples thinner than 10 unit cell layers exhibit a different behavior. Hysteresis loops for a thin sample at varying temperatures were plotted in Fig Above a certain temperature the hysteresis loops remain constant. 31

38 CHAPTER 6. INTERFACE INFLUENCES Figure 6.1: Saturation magnetization per surface unit cell. The bulk magnetization value of 3.7µ B /Mn is depicted in black. There is no sign of a magnetic dead layer in LSMO(011). This temperature was used as the T C value in our results. The remaining ferromagnetism has an almost constant saturation magnetization value between 1 and 1.2µ B /Mn for all samples. The remanent magnetization is given in table 6.1. No temperature influences were observed up to 350 K. For samples of 10 unit cell layers or thicker no ferromagnetism is observed above T C. In Fig. 6.3 the Curie Temperature of LSMO (110) with and without capping was compared to results obtained for LSMO(001).The results showed an increased Curie temperature for LSMO (110). To analyze the interface influence on the transport behavior of the LSMO films, van der Pauw measurements were used. The sheet conductance was Sample M sat Mremanent (unit cell layers) (µ B /Mn) (µ B /Mn) 9 ML 1 ± ± ML capped 5 ML 1 ± ±0.1 5 ML capped 1 ± ±0.1 3 ML 0 ± ± ML capped 1 ± ±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) samples above T C. 32

39 6.1. RESULTS A B C T C (K) cap IE 110 cap Figure 6.2: The magnetic and electrical behavior of the 40 unit cell uncapped sample was shown for varying temperatures. The phase transition at the Curie temperature is visible in the magnetic as well as the electrical measurements Thickness (m) x 10 8 Figure 6.3: Curie temperature for varying layer thickness. The LSMO(110) samples have a higher Curie temperature than LSMO(001) samples of the same thickness. M (µ B /Mn) K 30 K 50 K 70 K 90 K 110 K 130 K 150 K 170 K 190 K 350 K Applied Field (ka/m) Figure 6.4: Hysteresis loops for varying temperatures for the 9 unit cell layer sample without capping. Above the Curie temperature there is still ferromagnetic behavior. 33

40 CHAPTER 6. INTERFACE INFLUENCES Figure 6.5: Sheet conductance for varying layer thickness. Lines were fitted to nonzero values of conductance. The LSMO(001) samples show a dead layer, while in the LSMO(110) samples there seems to be either conduction in all layers or in none. measured and compared to results obtained for LSMO(001). The nonzero conductance values were fitted with a linear fit. The LSMO(001) samples without interface engineering showed an electrical dead layer of 8 unit cell layers. 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 for thin 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 due to lack of data points for thicker samples. However for thin samples the conductivity seems to be better than for the (001) oriented samples without Interface engineering. The magnetoresistance of the samples was determined. The relation between the magnetic field and the magnetoresistance is approximately quadratic. The second order fit parameter is a good indicator of the strength of the magnetoresistive effect. The temperature dependence of the second order fit parameter was plotted in Fig. 6.2 together with the temperature dependence of the sheet resistance. 34

41 6.2. DISCUSSION 6.2 Discussion As explained in chapter 2, ferromagnetism is caused by the alignment of spins due to the double exchange interaction energy. Thermal energy on the other hand causes disorder. This leads to a decrease of magnetization with increasing temperatures. At the Curie temperature a phase transition occurs from a ferromagnetic phase to a paramagnetic phase. Both ferromagnetism and electrical transport are governed by the same process: double exchange interaction. This relation becomes visible when magnetic and electrical properties are plotted as function of temperature as done in Fig At the Curie temperature, when thermal energy gains influence over double exchange interaction energy, three things happen: The sample changes from ferromagnetic to paramagnetic, there is an increased rise in sheet resistance and the magnetoresistance has a minimum. This shows that indeed one process is responsible for magnetic as well as electrical behavior. The results obtained for LSMO grown on STO(110) were compared with LSMO grown on STO(001) with and without interface engineering. The saturation magnetization of LSMO(110) assumed the bulk value of 3.7 µ B /Mn regardless of layer thickness. There is no indication of a dead layer. Both interface 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 capping on the magnetization was observed. For samples thinner than 10 unit cell layers the behavior changes drastically. The sheet conductivity goes to zero and there remains ferromagnetism above T C. The scaling of the strength of the remaining ferromagnetism with sample thickness and the absence of ferromagnetism above T C for thicker layers makes it improbable that the effect is caused by contaminations. Although with the current information it is impossible to tell what is really happening in the extremely thin layers, several properties of the effect can be given. We have shown that for thick layers double exchange interaction governs ferromagnetic as well as transport behavior. For the thin layers there is no electrical transport but there is ferromagnetism. This might be an indication that the remaining ferromagnetism is not induced by the double exchange interaction but by another effect. Another possibility is that double exchange interaction does take place, but only locally. Looking at the phase diagram of LSMO there is also a ferromagnetic insulating phase for low doping levels. This phase is characterized by Jahn Teller deformations. Due to the Jahn Teller deformations two neighboring Mn atoms have their energetically favorable e g shells perpendicular to each other. This makes double exchange interaction with neighboring Mn atoms unfavorable and thus prevents electrical conduction. The possibility of tunneling can however increase the kinetic energy of electrons and still induce ferromagnetism. Structural 35