Magnetický tunelový spoj (MTJ) je spintronická součástka komerčně používaná ve vysoce citlivých čtecích hlavách pevných disků. Počínaje rokem 2007 přispěla k udržení exponenciálního nárůstu hustoty magnetického zápisu. Kromě toho se také stala stavebním kamenem rychlé, odolné, úsporné a nevolatilní magnetické paměti s přímým přístupem (MRAM). Tento nový typ polovodičové paměti, stejně jako je tomu u čtecích hlav disků, využívá tunelové spoje založené na krystalickém oxidu hořečnatém (MgO) spolu s 3d kovovými magnetickými prvky (Fe a Co). Pro zmenšení MTJ a současné udržení dlouhodobé stability paměti proti tepelným fluktuacím je zapotřebí silná magnetická anizotropie ve směru kolmém na rozhraní kov|MgO. V této práci proto nejdříve provedeme analýzu magnetokrystalické anizotropie (MCA) kubického prostorově centrovaného Fe, Co a Ni na MgO pomocí ab initio simulací. Dále bude vyvinut program pro výpočet tvarové anizotropie, která je kromě MCA velmi podstatná, neboť v součtu dávají efektivní anizotropii. Na závěr implementujeme program pro výpočet MCA na základě poruchové teorie druhého řádu. To nám umožní dát pozorované anizotropní vlastnosti do souvislosti přímo s elektronickou strukturou systému (pásovou strukturou a hustotou stavů).

Spin-torque transfer (STT) effects have brought spintronics ever closer to practical electronic applications, such as MRAM and active broadband microwave spin-torque oscillator (STO), and have emerged as an increasingly attractive field of research in spin dynamics. Utilizing materials with perpendicular magnetic anisotropy (PMA) in such applications offers several great advantages such as low-current, low-field operation combined with high thermal stability. The exchange coupling that a PMA thin film exerts on an adjacent in-plane magnetic anisotropy (IMA) layer can tilt the IMA magnetization direction out of plane, thus creating a stack with an effective tilted magnetic anisotropy. The tilt angle can be engineered via both intrinsic material parameters, such as the PMA and the saturation magnetization, and extrinsic parameters, such as the layer thicknesses.       STOs can be fabricated in one of a number of forms—as a nanocontact opening on a mesa from a deposited pseudospin-valve (PSV) structure, or as a nanopillar etching from magnetic tunneling junction (MTJ)—composed of highly reproducible PMA or predetermined tilted magnetic anisotropy layers.       All-perpendicular CoFeB MTJ STOs showed high-frequency microwave generation with extremely high current tunability, all achieved at low applied biases. Spin-torque ferromagnetic resonance (ST-FMR) measurements and analysis revealed the bias dependence of spin-torque components, thus promise great potential for direct gate-voltage controlled STOs.       In all-perpendicular PSV STOs, magnetic droplets were observed underneath the nanocontact area at a low drive current and low applied field. Furthermore, preliminary results for microwave auto-oscillation and droplet solitons were obtained from tilted-polarizer PSV STOs. These are promising and would be worth investigating in further studies of STT driven spin dynamics. / Effekter av spinnvridmoment (STT) har fört spinntroniken allt närmare praktiska elektroniska tillämpningar, såsom MRAM och den spinntroniska mikrovågsoscillatorn (STO), och har blivit ett allt mer attraktivt forskningsområde inom spinndynamik. Användning av material med vinkelrät magnetisk anisotropi (PMA) i sådana tillämpningar erbjuder flera stora fördelar, såsom låg strömförbrukning och funktion vid låga fält i kombination med hög termisk stabilitet. Den utbyteskoppling (”exchange bias”) en PMA-tunnfilm utövar på ett intilliggande skikt med magnetisk anisotropi i planet (IMA) kan få IMA-magnetiseringsriktningen att vridas ut ur planet, vilket ger en materialstack med en effektivt sett lutande magnetisk anisotropi. Lutningsvinkeln kan manipuleras med både inre materialparametrar, såsom PMA och mättningsmagnetisering, och yttre parametrar, såsom skikttjocklekarna. STO:er kan tillverkas som flera olika typer - som en nanokontaktsöppning på en s.k. mesa av en deponerad pseudospinnventilstruktur (PSV) eller som en nanotråd etsad ur en magnetisk tunnlingsövergång (MTJ) –och bestå av mycket reproducerbar PMA eller av skikt med på förhand bestämt lutning av dess magnetiska anisotropi. MTJ-STO:er av CoFeB med helt vinkelrät anisotropi visar högfrekvent mikrovågsgenerering med extremt stort frekvensomfång hos strömstyrningen, detta vid låg biasering. Mätning och analys av spinnvridmoments-ferromagnetisk resonans (ST-FMR) avslöjade ett biasberoende hos spinnvridmomentskomponenter, vilket indikerar en stor potential för direkt gate-spänningsstyrda STO:er. I helt vinkelräta PSV-STO:er observerades magnetiska droppar under nanokontaktområdet vid låg drivström och lågt pålagt fält. Dessutom erhölls preliminära resultat av mikrovågssjälvsvängning och av s.k. ”droplet solitons” hos PSV-STO:er med lutande polarisator. Dessa är lovande och skulle vara värda att undersökas i ytterligare studier av STT-driven spinndynamik. / <p>QC 20160829</p>

Chemically ordered FePt is one of the most promising materials to reach the ultimate limitations in storage density of future magnetic recording devices due to its high uniaxial magnetocrystalline anisotropy and a corrosion resistance superior to rare-earth based magnets. In this study, FePt and FePt/Cu bilayers have been sputter deposited at room temperature onto thermally oxidized silicon wafers, glass substrates and self-assembled arrays of spherical SiO2 particles with diameters down to 10 nm. Millisecond flash lamp annealing, as well as conventional rapid thermal annealing was employed to induce the phase transformation from the chemically disordered A1 phase into the chemically ordered L10 phase. The influence of the annealing temperature, annealing time and the film thickness on the ordering transformation and (001) texture evolution of FePt films with near equiatomic composition was studied. Whereas flash lamp annealed FePt films exhibit a polycrystalline morphology with high chemical L10 order, rapid thermal annealing can lead to the formation of chemically ordered FePt fifilms with (001) texture on amorphous SiO2/Si substrates. The resultant high perpendicular magnetic anisotropy and large coercivities up to 40 kOe are demonstrated. Simultaneuosly to the ordering transformation, rapid thermal annealing to temperatures exceeding 600 °C leads to a break up of the continuous FePt film into separated islands. This dewetting behavior was utilized to create regular arrays of FePt nanostructures on SiO2 particle templates with periods down to 50 nm. The addition of Cu improves the (001) texture formation and chemcial ordering for annealing temperatures T < 600 °C. In addition, the magnetic anisotropy and the coercivity of the ternary FePtCu alloy can be effectively tailored by adjusting the Cu content. The prospects of FePtCu based exchange spring media, as well as the magnetic properties of FePtCu nanostructures fabricated using e-beam and nanoimprint lithography have been investigated.

本論文分為兩部分，第一部分探討使用OOMMF磁性材料模擬軟體來模擬奈米線波導中的自旋波特性，除了以往文獻所熱門的水平異向性薄膜合金中的自旋波，還模擬了垂直異向性的材料，我們模擬了在奈米線一端施加0 ~ 100 GHz外加磁場的自旋波響應。在模擬的結果中，我們發現了水平異向性與垂直異向性的重要差別，垂直異向性比水平異向性波導在頻率小於10 GHz時，少了複雜的自旋波傳遞。而在改變線寬的條件中，我們發現了垂直異向性波導在線寬夠大時，會因退磁場的效應，使得磁矩翻轉，形成許多磁壁，而水平異向性材料則不會，從水平異向性波導大於120 nm線寬的波型中，則會發現自旋波在波導中產生破碎的相位改變。而模擬具有水平寬度變化與垂直厚度變化的週期性邊界，則發現兩者所具有的濾波效果非常相似，而濾波的三個頻段，則是水平寬度變化所截止的頻段，寬於垂直厚度變化的頻段。 第二部分為 ，使用離子濺鍍製成總厚度200 nm，改變交錯層數的(Co/Pt)×N、(Co/Pd) ×N與(Pt/Pd) ×N，三種多層膜的磁性電性分析。Co/Pt與Co/Pd多層膜在Co厚度小於1 nm時為熱門垂直異向性材料，而本實驗專注於Co厚度大於1 nm時介面的特性以及兩種材料的差別。在磁阻的量測上面，得到不同於一般異向性磁阻的規律，一般的異向性磁阻的現象為，平行於電流施加磁場比垂直電流施加磁場所量測的電阻，前者電阻較大(ρ_(H∥I)>ρ_(H⊥I))，但是同為垂直於電流的平行於膜面磁場的電阻(ρ_(H⊥I,in-plane H))與垂直膜面磁場(ρ_(H⊥I,H perpendicular to plane))則呈現了不一樣的行為，尤其為垂直加場的部分，在某些條件的多層膜，會有明顯的垂直方向的異向性磁阻，是為介面所造成額外的垂直方向異向性磁阻，稱作”異向性介面磁阻”(Anisotropic Interface Magnetoresistance)。異向性磁阻與異性向介面磁阻都具有高電阻軸與垂直此軸的低電阻平面，而兩者差別在於異向性磁阻為電流方向軸，而異向性介面磁阻為膜面法向量軸，對於本實驗的量測方法來說，兩軸相差90度角，也因此可辨析兩者不同現象間的差異，並且在我們的分析之中發現，異向性介面磁阻在Co厚度為7 nm以下，才會明顯的顯現。 / The thesis is divided into two main parts. The first part discusses the properties of spin waves propagation in magnetic nanowire waveguide by micromagnetic simulation software OOMMF. In addition to in-plane magnetic anisotropy (IMA) in the thin film alloys, we simulate the perpendicular magnetic anisotropy (PMA) of the material. A transverse magnetic field is applied at one end of the waveguide wire and the frequency range is from 0 to 100 GHz. When frequency is less than 10 GHz, we observed that complex modes were generated in the IMA waveguide but there is no spin wave propagates in the PMA waveguide. We also studied the spin wave propagations in wires with different width. Irregular domain wall was generated by demagnetizing field in wider PMA waveguide but IMA waveguide does not have this behavior. In width-modulated and thickness-modulated waveguide spin wave simulations, these two filters have similar results with three band gaps from 0 to 100 GHz and the band gaps in width-modulated wire is wider than in thickness-modulated one. The second part is experimental measurements of the electrical and magnetic properties of (Co/Pt)×N, (Co/Pd) ×N, and (Pt/Pd) ×N multilayers, which are deposited by sputtering and the total thickness is 200nm. Co/Pt and Co/Pd were popular PMA materials when Co thickness is less than 1 nm. We focused on the multilayers with Co thicker than 1nm and the difference between these multilayers. In magnetoresistance measurement, the R-H curve is different from normal anisotropic magnetoresistance (AMR). AMR effect has different resistivity when H∥I or H⊥I, but the measurement results show that ρ_(H⊥I,in-plane H) and ρ_(H⊥I,H perpendicular to plane) also have different MR ratio in specific multilayer configuration. The effect is caused by the interface so it is anisotropic interface magnetoresistance (AIMR) as discussed in the literature. AMR and AIMR have both high resistivity axis and low resistivity plane which is perpendicular to the axis. The difference of two MRs is that the high resistivity axis is parallel to current in AMR and perpendicular to plane in AIMR. In the analysis, the AIMR effect is observed in multilayer with Co thickness less than 7 nm.

Dans le domaine des couches minces (épaisseur~Å) associant un métal magnétique (Fe, Co, Ni) et un élément non magnétique (essentiellement métallique ou isolant), de remarquables propriétés physiques (aimantation, transport) nécessitent des caractérisations structurales fines. En particulier, citons le cas de jonctions tunnel (métal/isolant/métal) à aimantation perpendiculaire qui sont en cours d'étude au laboratoire Spintec (UMR8191 (CEA/CNRS/UJF). Ces nanomatériaux sont déposés par voie physique (pulvérisation cathodique) au sein de ce laboratoire. Ces nanostructures sont également sondées par diffraction aux rayons X au sein de l'Institut Néel (UPR 2940) via une collaboration entre Spintec et une équipe de cet Institut (Surface, interfaces et nanostructures du Département MCMF, Matière Condensée, Matériaux, et Fonctions). Ces mesures de réflectivité X constituent la sonde privilégiée de choix dans la cadre de cette thèse. D'autres voies sont également exploitées: à l'aide des moyens de rayonnement synchrotron tels que la spectroscopie d'absorption de rayons X : EXAFS, XANES et XMCD. La thèse aura pour but d'étudier expérimentalement ces phénomènes en couches continues sur ces empilements à jonction tunnel avec aimantation perpendiculaire. Plus précisément, le travail de thèse permettra de comprendre les mesures magnétiques (effectuées à l'institut Néel notamment par magnétométrie SQUID et HALL à basse température) grâce à une batterie de mesures structurales (diffraction aux rayons X, rasant, figures de pôles, réflectivité, absorption X …). Notamment, l'influence des paramètres de dépôt (types de couches, épaisseurs, recuits) du matériau sont étudiées via la collaboration entre les différents groupes de recherche précédemment cités. Cette thématique s'inscrit d'une part dans le cadre de travaux menés à Spintec et dédiés à la recherche de nouveaux matériaux à forte valeur ajoutée industrielle (sur le stockage d'information à ultrahaute densité sur media discrets par exemple). Elle s'inscrit d'autre part dans le renforcement de liens entre recherches fondamentales (laboratoire propre du CNRS comme l'institut Néel) et appliquées (CEA), avec un recours aux solides compétences en caractérisations structurales et magnétiques de l'Institut Néel. / In the domain of thin film (thickness ~ Å) combining a magnetic metal (Fe, Co, Ni) and a non-magnetic (largely metal or insulator), remarkable physical properties (magnetization, transport) require fine structural characterization. In particular, include the case of tunnel junctions (metal / insulator / metal) with perpendicular magnetization which are being studied in the laboratory Spintec (UMR8191 (CEA / CNRS / UJF). These nanomaterials are deposited by physical (sputtering) in this laboratory. These nanostructures are probed by X-ray diffraction in the Neel Institute (UPR 2940) via collaboration between Spintec and a team of the Institute (Surface, Interfaces and Nanostructures Department MCFP, Condensed Matter Materials and Functions). These reflectivity measurements X are the preferred sensor of choice in the context of this thesis. Other routes are also used: using means such as synchrotron radiation absorption spectroscopy X-ray: EXAFS, XANES and XMCD. The thesis will aim to study these phenomena experimentally in continuous layers on the tunnel junction stacks with perpendicular magnetization. Specifically, the thesis will include the magnetic measurements (performed at the Institut Néel SQUID magnetometry including HALL and low temperature) through a battery of structural. This theme is part of a share in the context of work carried Spintec and dedicated to research of new materials with high added value industries (information storage on ultra-high density of discrete media for example). It registers on the other hand in strengthening links between basic research (CNRS own laboratories as Neel Institute) and applied (ECA), with strong skills in use of structural and magnetic characterization of the Institute Neel.

Magnetostrictive materials belong to an important class of smart magnetic materials which have potential applications as ultrasonic transducers, sensors, actuators, delay lines, energy harvesting devices etc. Although, magnetostrictive property is exhibited by almost all ferro and ferrimagnetic materials, the R-Fe type (R represents rare earth elements) intermetallic compounds display maximum promise owing to the large magnetostriction exhibited by them at ambient temperature. Among the several R-Fe type compounds, Tb-Fe and Sm-Fe alloys are found to exhibit maximum room temperature positive and negative magnetostriction respectively. Recently, Fe-Ga based alloys have gained significant interest as newly emerging magnetostrictive material due to a good combination of magnetic and mechanical properties. These magnetostrictive materials in thin film form are of interests for several researchers both from fundamental and applied perspectives. Currently, many researchers are exploring the possibility of using magnetostrictive thin films in micro- and nano-electromechanical systems (MEMS and NEMS). Three material systems viz. Fe-Ga, Tb-Fe and Sm-Fe in thin film form have been chosen for our investigations. DC magnetron sputtering and e-beam evaporation techniques were used for deposition of these thin films on Si (100) substrates. Several aspects such as evolution of microstructure, film surface morphology, structure and change in film composition with different processing conditions were investigated in detail, as the existing literature could not provide a clear insight. Further, detailed magnetic characterizations of these films were carried out and established a process-structure-property correlation. The thesis is divided into seven chapters. The first chapter presents a brief introduction of magnetostrictive phenomena and the physics behind its origin. A brief history of evolution of magnetostrictive materials with superior properties is also brought out. Introduction to the material systems considered for the present study has also been presented. Discussions on various aspects like crystal structures, magnetic properties, and phase diagrams of these material systems are also included in this chapter. Magnetostriction in thin films and its importance in current technological applications are discussed in short. Further, a summary of existing literature on thin films of these materials has been narrated to highlight the perspective of the work done in subsequent chapters. In addition to this, a clear picture of the grey area for further investigations has been provided. Formulation of detailed scope of work for this study is also provided in this chapter. Details of different experimental techniques used in this study for deposition and characterization of these films are given in chapter 2. In the third chapter of the thesis a detailed study on the structural, microstructural and magnetic properties of Fe-Ga films deposited using dc magnetron sputtering technique are presented. The effect of sputtering parameters such as (i) Ar pressure, (ii) sputtering power, (iii) substrate temperature and (iv) deposition time/film thickness on the magnetic properties of the films are discussed in detail. All the films are found to be polycrystalline in nature with A2 type structure as evidenced from grazing incidence X-ray diffraction (GIXRD) and transmission electron microscope (TEM) studies. Surface morphology of the films are found to be affected with processing conditions considerably. Thermomagnetic behaviour of the films studied using a Superconducting Quantum Interference Device (SQUID) magnetometer under zero field cooled (ZFC) and field cooled (FC) conditions are also presented. The sputtering parameters are also found to influence the magnetic properties of the films through modifications in microstructure, surface morphology and film compositions. Irrespective of the sputtering parameters, room temperature (RT) deposited Fe-Ga films are found to exhibit large magnetic coercively and large saturation magnetic field as compared to the bulk alloy of similar compositions which are not desirable for micromagnetic device applications. A significant improvement in the magnetic properties of the films was obtained in the films deposited at higher substrate temperatures and is correlated with modifications in grain size and film surface roughness. These films are also found to exhibit better magnetostriction than the RT deposited films. Further, the magnetic properties of Fe-Ga films as a function of film thickness in the range 2 – 480 nm are also presented. The nature of variation of coercively with film thickness was correlated with grain size effect and explained successfully with the help of random anisotropy model. In the fourth chapter, studies on the microstructural and magnetic properties of Tb-Fe films were presented. It was reported earlier that TbxFe100-x films exhibit in-plane magnetic anisotropy for the films with x > 42 at.% of Tb and out-of-plane anisotropy for the composition 28 < x < 42. Presence of these anisotropies is technologically important for different applications. We have studied the magnetic properties of Tb-Fe films in these two composition range. TbxFe100-x films with 54  x  59 were prepared using dc magnetron sputtering technique under varying Ar pressure and sputtering power and the details about microstructural and magnetic properties are presented in this chapter. All the films are found to be amorphous in nature. While the composition of the film is found to remain constant with sputtering power, the Fe concentration in the film is found to be depleted with increase in Ar pressure. Magnetic properties are found to change from superparamagnetic to ferromagnetic behaviour with increase in sputtering power. Curie temperature of the films are found to be low (below RT) and is explained based on sperimagnetic ordering of magnetic sub-lattices. The perpendicular magnetic anisotropy (PMA) or out-of-plane anisotropy behaviour of Tb-Fe films were not studied in detail as a function of film thickness. We have successfully prepared TbxFe100-x films with 29  x  40 using e-beam evaporation technique using alloy target composition of TbFe in order to study the PMA behaviour as a function of film thickness. The thickness of the films was varied from 50 to 800 nm. All the films are found to be amorphous and columnar growth structure with fine channels of voids are observed from the TEM studies. Detailed magnetization and thermomagnetic measurements were carried out using SQUID magnetometer at different temperatures. The out-of-plane magnetic coercivity of the films was found to increase with film thickness and then decreases with further increase in thickness. Maximum coercivity of ~ 20 kOe has been obtained for the 400 nm thick film. Magnetic domain patterns were studied using magnetic force microscopy (MFM) technique and the observed magnetic properties are correlated with domain pattern and microstructures. Although there are several reports on device applications of Sm-Fe thin films which exhibit negative magnetostriction, a comprehensive study on the effect of different process parameters on the magnetic properties and its correlation with structure and microstructure is still elusive. Hence, Sm-Fe films were deposited on Si (100) substrate using dc magnetron sputtering technique under varying Ar pressure and sputtering power. Effect of these parameters on the microstructural and magnetic properties of the films was studied in detail and is presented in chapter 5. The curie temperature of the films was found to increase with increase in sputtering power and Ar pressure. This was attributed to increase in film thickness and size of islands (atomic clusters). Coercivity as low as 30 Oe has been achieved in the film deposited at 15 mTorr Ar pressure. The Curie temperature for the films deposited at higher Ar pressure (10 and 15 mTorr) are found to be above RT. Maximum saturation magnetostriction of ~ - 390 -strains has been achieved in the film deposited at 15 mTorr Ar pressure. Rapid thermal processing (RTP) experiments were also carried out to increase the magnetic ordering in the films deposited at low Ar pressure (5 mTorr) by imparting structural ordering. Large improvement in magnetization and Curie temperature of the film was observed after RTA. However, this could be attributed to the formation of nano-crystalline Fe phase as evidenced from the TEM studies and thermomagnetic measurements. An overall summary of the experimental results has been presented in chapter 6. The scope of work for further study in future has also been highlighted in chapter 7.

Chemically ordered FePt is one of the most promising materials to reach the ultimate limitations in storage density of future magnetic recording devices due to its high uniaxial magnetocrystalline anisotropy and a corrosion resistance superior to rare-earth based magnets. In this study, FePt and FePt/Cu bilayers have been sputter deposited at room temperature onto thermally oxidized silicon wafers, glass substrates and self-assembled arrays of spherical SiO2 particles with diameters down to 10 nm. Millisecond flash lamp annealing, as well as conventional rapid thermal annealing was employed to induce the phase transformation from the chemically disordered A1 phase into the chemically ordered L10 phase. The influence of the annealing temperature, annealing time and the film thickness on the ordering transformation and (001) texture evolution of FePt films with near equiatomic composition was studied. Whereas flash lamp annealed FePt films exhibit a polycrystalline morphology with high chemical L10 order, rapid thermal annealing can lead to the formation of chemically ordered FePt fifilms with (001) texture on amorphous SiO2/Si substrates. The resultant high perpendicular magnetic anisotropy and large coercivities up to 40 kOe are demonstrated. Simultaneuosly to the ordering transformation, rapid thermal annealing to temperatures exceeding 600 °C leads to a break up of the continuous FePt film into separated islands. This dewetting behavior was utilized to create regular arrays of FePt nanostructures on SiO2 particle templates with periods down to 50 nm. The addition of Cu improves the (001) texture formation and chemcial ordering for annealing temperatures T < 600 °C. In addition, the magnetic anisotropy and the coercivity of the ternary FePtCu alloy can be effectively tailored by adjusting the Cu content. The prospects of FePtCu based exchange spring media, as well as the magnetic properties of FePtCu nanostructures fabricated using e-beam and nanoimprint lithography have been investigated.

The present thesis is about the investigation of ferromagnetic thin film systems with respect to exchange coupling, magnetization reversal behavior and effects appearing in magnetic heterostructures, namely the exchange bias and the giant magnetoresistance effect. For this purpose, DC magnetron sputtered thin films and multilayers with perpendicular magnetic anisotropy were prepared on single crystalline and rigid as well as flexible amorphous substrates. The first part concentrates on magnetic data storage applications based on the combination of the concept of bit patterned media and three dimensional magnetic memory, consisting of at least two exchange decoupled ferromagnetic storage layers. Here, [Co/Pt] multilayers, revealing different magnetic anisotropies, have been applied as storage layers and as spacer material Pt and Ru was employed. By the characterization of the magnetization reversal behavior the exchange coupling in dependence of the spacer layer thickness was studied. Furthermore, with regard to the concept of bit patterned media, the layers were also grown on self-assembled silica particles, leading to an exchange decoupled single-domain magnetic dot array, which was studied by magnetic force microscope imaging and angular dependent magneto-optic Kerr effect magnetometry to evaluate the reversal mechanism and its dependence on the array dimensions, mainly the diameter of the silica particles and layer thicknesses. To complete the study, micromagnetic simulations were performed to access smaller dimensions and to investigate the dependence of intralayer as well as interlayer coupling on the magnetization reversal of the dot array with multiple storage layers. The second part focuses on the investigation of the giant magnetoresistance effect in systems with perpendicular magnetic anisotropy, where L10 -chemically ordered FePt alloys and [Co/Pt] as well as [Co/Pd] multilayers were utilized. In case of FePt, where high temperatures during the deposition are necessary to induce the chemical ordering, diffusion and alloying of the spacer material often prevent a sufficient exchange decoupling of the ferromagnetic layers. However, with Ru as spacer material a giant magnetoresistance effect could be achieved. Large improvements of the magnetoresistive behavior of such trilayer structures are presented for [Co/Pt] and [Co/Pd] multilayers, which can be deposited at room temperature not limiting the choice of spacer as well as substrate material. Furthermore, in systems consisting of one ferromagnet with perpendicular magnetic anisotropy and one ferromagnet with in-plane magnetic easy axis, a linear and almost hysteresis-free field dependence of the electrical resistance was observed and the behavior for various thickness series has been intensively studied. Finally, the corrosion resistance in dependence of the capping layer material as well as the magnetoresistance of a strained flexible pseudo-spin-valve structure is presented. In addition, in chapter 2.5.2 an experimental study of an improved crystal growth of FePt at comparable low temperatures by molecular beam epitaxy and further promoted by a surfactant mediated growth using Sb is shown. Auger electron spectroscopy as well as Rutherford backscattering spectrometry were carried out to confirm the surface segregation of Sb and magnetic characterization revealed an increase of magnetic anisotropy in comparison to reference layers without Sb. / Die vorliegende Dissertation beschäftigt sich mit der Untersuchung ferromagnetischer Dünnschichtsysteme im Hinblick auf die Austauchkopplung, das Ummagnetisierungsverhalten und Effekte wie z.B. den Exchange Bias Effekt oder den Riesenmagnetwiderstandseffekt (GMR), welche in derartigen Heterostrukturen auftreten können. Die Probenpräparation erfolgte mittels DC Magnetronsputtern, wobei auf einkristallinen aber auch flexiblen sowie starren amorphen Substraten abgeschieden wurde. Im ersten Teil der Arbeit werden Untersuchungen mit dem Hintergrund einer Anwendung als magnetischer Datenträger vorgestellt. Konkret werden hier die Konzepte Bit Patterned Media (BPM) und 3D Speicher miteinander kombiniert. Letzteres Konzept basiert auf der Verwendung wenigstens zweier austauschentkoppelter ferromagnetischer Schichten, für welche [Co/Pt] Multilagen mit unterschiedlicher magnetischer Anisotropie verwendet wurden. Als Zwischenschichtmaterial diente Pt und Ru. Durch die Charakterisierung des Ummagnetisierungsverhaltens wurde die Austauschkopplung in Abhängigkeit der Zwischenschichtdicke untersucht. Darüber hinaus wurden jene Schichtstapel zur Realisierung des BPM-Konzeptes auf selbstangeordnete SiO2 Partikel mit unterschiedlichen Durchmessern aufgebracht, durch welche sich lateral austauschentkoppelte, eindomänige magnetische Nanostrukturen erzeugen lassen. Zur Untersuchung des Ummagnetisierungsverhaltens und der jeweiligen Größenabhängigkeiten (maßgeblich Durchmesser und Schichtdicke) wurden diese mittels Magnetkraftmikroskopie sowie winkelabhängiger magnetooptischer Kerr Effekt Magnetometrie untersucht. Zur weiteren Vertiefung des Verständnisses noch kleinerer Strukturgrößen erfolgten mikromagnetische Simulationen, bei denen die magnetischen Wechselwirkungen lateral (benachbarte 3D Elemente) als auch vertikal (Wechselwirkungen ferromagnetischer Schichten innerhalb eines 3D Elementes) im Interesse standen, sowie deren Auswirkungen auf das Ummagnetisierungsverhalten des gesamten Feldes. Der Fokus des zweiten Teils liegt auf der Untersuchung des Riesenmagnetwiderstandseffektes in Systemen mit senkrechter Sensitivität. Dafür sind ferromagnetische Schichten mit senkrechter magnetischer Anisotropie nötig, wobei hier die chemisch geordnete L10-Phase der FePt Legierung und [Co/Pt] sowie [Co/Pd] Multilagen Anwendung fanden. Für eine chemische Ordnung der FePt Legierung sind hohe Temperaturen während der Schichtabscheidung notwendig, welche eine hinreichende Austauschentkopplung beider ferromagnetischer Schichten meist nicht gewährleisten. Grund dafür sind einsetzende Diffusionsprozesse als auch Legierungsbildungen mit dem Zwischenschichtmaterial. In der vorliegenden Arbeit konnte der GMR Effekt daher ausschließlich mit einer Ru Zwischenschicht in FePt basierten Trilagensystemen nachgewiesen und charakterisiert werden. Enorme Verbesserungen der magnetoresistiven Eigenschaften werden im Anschluss für [Co/Pt] und vor allem [Co/Pd] Multilagen vorgestellt. Diese Schichtsysteme mit senkrechter magnetischer Anisotropie können bei Raumtemperatur präpariert werden und stellen daher keine weiteren Anforderungen an das Zwischenschichtmaterial sowie die verwendeten Substrate. Hier wurden neben Systemen mit ausschließlich senkrechter magnetischer Anisotropie auch Systeme mit gekreuzten magnetischen Anisotropien intensiv untersucht, da diese durch einen linearen und weitgehend hysteresefreien R(H) Verlauf imHinblick auf Sensoranwendungen enorme Vorteile bieten. Letztendlich wurde die Korrosionsbeständigkeit in Abhängigkeit des Deckschichtmaterials als auch die mechanische Belastbarkeit von auf flexiblen Substraten abgeschiedenen GMR-Schichtstapeln untersucht. Zusätzlich wird in Kapitel 2.5.2 eine experimentelle Studie zum Surfactant-gesteuerten Wachstum der FePt Legierung mittels Molekularstrahlepitaxie vorgestellt. Als Surfactant dient Sb, wodurch die Kristallinität bei geringer Depositionstemperatur deutlich verbessert werden konnte. Die Oberflächensegregation von Sb wurde mittels Auger Elektronenspektroskopie und Rutherford Rückstreuspektrometrie verifiziert und die Charakterisierung magnetischer Eigenschaften belegt einen Anstieg der magnetischen Anisotropieenergie im Vergleich zu Referenzproben ohne Sb.

In dieser Dissertation wird die gute Einstellbarkeit von synthetischen Antiferromagneten mit dem kollektiven Surface Spin-Flop-Verhalten kombiniert. Es wird der Einfluss der Gesamtschichtdicke untersucht, welche mit dem Abstand der magnetische Oberflächen korreliert. Zudem werden die Dicken der ferromagnetischen Untereinheiten an den Außenseiten verändert, womit die Beiträge der Oberflächen unterdrückt oder verstärkt werden können. Darauffolgend wird die Kontrolle der Oberflächenbeiträge angewendet, um Exchange Bias-Strukturen auf Basis synthetischer Antiferromagnete zu erzeugen. Da diese nicht aus Heterostrukturen intrinsischer Antiferromagnete und Ferromagnete bestehen, wird nicht nur eine gute Abstimmbarkeit erreicht, sondern auch die Materialwahl wird potentiell vereinfacht. Zudem kann der Exchange Bias in synthetischen Antiferromagneten vollständig bei Raumtemperatur beobachtet und gesteuert werden. Im Weiteren wird ein zuvor untersuchtes Konzept zur Stabilisierung der vom Surface Spin-Flop erzeugten vertikalen antiferromagnetischen Domänenwände erweitert. Es wird demonstriert, wie ein Paar koexistierender antiferromagnetischer Domänenwände in Abwesenheit äußerer Magnetfelder und bei tiefen Temperaturen stabil gehalten werden kann. Damit können in Erweiterung der ursprünglichen Konzeptes nun acht anstatt sechs remanenter Zustände durch geeignete Magnetfeldroutinen eingestellt werden.:1. Introduction 2. Theoretical background 2.1. Micromagnetic energy terms 2.1.1. Zeeman energy 2.1.2. Demagnetization energy 2.1.3. Anisotropy energy 2.1.4. Exchange energy 2.2. Magnetic multilayers 2.2.1. Magnetic anisotropy in magnetic multilayers 2.2.2. Synthetic antiferromagnets 2.3. Exchange Bias 2.4. The bulk and surface spin-flop 3. Methods 3.1. Sputter deposition 3.2. X-ray diffraction and reflectometry 3.3. Magnetometry 3.4. Magnetic force microscopy 3.5. Micromagnetic simulations 4. Results 4.1. From collective reversal to exchange bias 4.1.1. Total thickness dependency of the surface spin flop 4.1.2. Influence of the surface block thickness 4.1.3. Exchange bias in synthetic antiferromagnets 4.2. Tailoring the surface spin flop 4.2.1. Coexistence of two vertical domain walls 4.2.2. Alternative anisotropy profile 5. Conclusions and Outlook A. Supplemental material A.1. Supplemental material for Section 2.4 A.2. Supplemental material for Section 4.1.1 A.3. Supplemental material for Section 4.1.2 A.4. Supplemental material for Section 4.1.3 A.5. Supplemental material for Section 4.2.1 A.6. Supplemental material for Section 4.2.2 A.7. Supplemental material for the outlook in Chapter 5 A.7.1. Synthetic ferrimagnets ans ferromagnetic resonance A.7.2. Synthetic antiferromagnets based on Co/Ni A.7.3. Initial magneto-resistance measurements A.8. Micromagnetic simulations MuMax3 code B. Bibliography C. List of Samples D. Selbstständigkeitserklärung E. Danksagung F. Lebenslauf G. Publikationsliste / In this thesis, the high degree of tunability in the SAFs is combined with the collective surface spin-flop reversal. The influence of the total thickness and thus the distance of the magnetic surfaces is explored. Furthermore, the thickness of the ferromagnetic surface subunits is altered to selectively suppress or enhance the surface contribution. The control of the surface contribution is subsequently employed to create magnetic exchange bias structures based on the synthetic antiferromagnets. If compared to conventional exchange bias systems in heterostructures of intrinsic antiferromagnetic and ferromagnetic materials, an exchange bias with full room temperature operation, high tunability and a potential potential much more flexible choice of materials becomes available. Additionally, a previously established concept to stabilize the vertical antiferromagnetic domain walls that originate from the surface spin-flop at remanence is extended to a coexisting pair of antiferromagnetic domain walls. At low temperatures, the coexisting vertical antiferromagnetic domain walls can be stabilized at remanence, too. Furthermore, the total number of different remanent states, which are accessible through different field routines, can be increased from six in the original concept to eight in the more sophisticated concept presented here.:1. Introduction 2. Theoretical background 2.1. Micromagnetic energy terms 2.1.1. Zeeman energy 2.1.2. Demagnetization energy 2.1.3. Anisotropy energy 2.1.4. Exchange energy 2.2. Magnetic multilayers 2.2.1. Magnetic anisotropy in magnetic multilayers 2.2.2. Synthetic antiferromagnets 2.3. Exchange Bias 2.4. The bulk and surface spin-flop 3. Methods 3.1. Sputter deposition 3.2. X-ray diffraction and reflectometry 3.3. Magnetometry 3.4. Magnetic force microscopy 3.5. Micromagnetic simulations 4. Results 4.1. From collective reversal to exchange bias 4.1.1. Total thickness dependency of the surface spin flop 4.1.2. Influence of the surface block thickness 4.1.3. Exchange bias in synthetic antiferromagnets 4.2. Tailoring the surface spin flop 4.2.1. Coexistence of two vertical domain walls 4.2.2. Alternative anisotropy profile 5. Conclusions and Outlook A. Supplemental material A.1. Supplemental material for Section 2.4 A.2. Supplemental material for Section 4.1.1 A.3. Supplemental material for Section 4.1.2 A.4. Supplemental material for Section 4.1.3 A.5. Supplemental material for Section 4.2.1 A.6. Supplemental material for Section 4.2.2 A.7. Supplemental material for the outlook in Chapter 5 A.7.1. Synthetic ferrimagnets ans ferromagnetic resonance A.7.2. Synthetic antiferromagnets based on Co/Ni A.7.3. Initial magneto-resistance measurements A.8. Micromagnetic simulations MuMax3 code B. Bibliography C. List of Samples D. Selbstständigkeitserklärung E. Danksagung F. Lebenslauf G. Publikationsliste

The present thesis is about the investigation of ferromagnetic thin film systems with respect to exchange coupling, magnetization reversal behavior and effects appearing in magnetic heterostructures, namely the exchange bias and the giant magnetoresistance effect. For this purpose, DC magnetron sputtered thin films and multilayers with perpendicular magnetic anisotropy were prepared on single crystalline and rigid as well as flexible amorphous substrates. The first part concentrates on magnetic data storage applications based on the combination of the concept of bit patterned media and three dimensional magnetic memory, consisting of at least two exchange decoupled ferromagnetic storage layers. Here, [Co/Pt] multilayers, revealing different magnetic anisotropies, have been applied as storage layers and as spacer material Pt and Ru was employed. By the characterization of the magnetization reversal behavior the exchange coupling in dependence of the spacer layer thickness was studied. Furthermore, with regard to the concept of bit patterned media, the layers were also grown on self-assembled silica particles, leading to an exchange decoupled single-domain magnetic dot array, which was studied by magnetic force microscope imaging and angular dependent magneto-optic Kerr effect magnetometry to evaluate the reversal mechanism and its dependence on the array dimensions, mainly the diameter of the silica particles and layer thicknesses. To complete the study, micromagnetic simulations were performed to access smaller dimensions and to investigate the dependence of intralayer as well as interlayer coupling on the magnetization reversal of the dot array with multiple storage layers. The second part focuses on the investigation of the giant magnetoresistance effect in systems with perpendicular magnetic anisotropy, where L10 -chemically ordered FePt alloys and [Co/Pt] as well as [Co/Pd] multilayers were utilized. In case of FePt, where high temperatures during the deposition are necessary to induce the chemical ordering, diffusion and alloying of the spacer material often prevent a sufficient exchange decoupling of the ferromagnetic layers. However, with Ru as spacer material a giant magnetoresistance effect could be achieved. Large improvements of the magnetoresistive behavior of such trilayer structures are presented for [Co/Pt] and [Co/Pd] multilayers, which can be deposited at room temperature not limiting the choice of spacer as well as substrate material. Furthermore, in systems consisting of one ferromagnet with perpendicular magnetic anisotropy and one ferromagnet with in-plane magnetic easy axis, a linear and almost hysteresis-free field dependence of the electrical resistance was observed and the behavior for various thickness series has been intensively studied. Finally, the corrosion resistance in dependence of the capping layer material as well as the magnetoresistance of a strained flexible pseudo-spin-valve structure is presented. In addition, in chapter 2.5.2 an experimental study of an improved crystal growth of FePt at comparable low temperatures by molecular beam epitaxy and further promoted by a surfactant mediated growth using Sb is shown. Auger electron spectroscopy as well as Rutherford backscattering spectrometry were carried out to confirm the surface segregation of Sb and magnetic characterization revealed an increase of magnetic anisotropy in comparison to reference layers without Sb. / Die vorliegende Dissertation beschäftigt sich mit der Untersuchung ferromagnetischer Dünnschichtsysteme im Hinblick auf die Austauchkopplung, das Ummagnetisierungsverhalten und Effekte wie z.B. den Exchange Bias Effekt oder den Riesenmagnetwiderstandseffekt (GMR), welche in derartigen Heterostrukturen auftreten können. Die Probenpräparation erfolgte mittels DC Magnetronsputtern, wobei auf einkristallinen aber auch flexiblen sowie starren amorphen Substraten abgeschieden wurde. Im ersten Teil der Arbeit werden Untersuchungen mit dem Hintergrund einer Anwendung als magnetischer Datenträger vorgestellt. Konkret werden hier die Konzepte Bit Patterned Media (BPM) und 3D Speicher miteinander kombiniert. Letzteres Konzept basiert auf der Verwendung wenigstens zweier austauschentkoppelter ferromagnetischer Schichten, für welche [Co/Pt] Multilagen mit unterschiedlicher magnetischer Anisotropie verwendet wurden. Als Zwischenschichtmaterial diente Pt und Ru. Durch die Charakterisierung des Ummagnetisierungsverhaltens wurde die Austauschkopplung in Abhängigkeit der Zwischenschichtdicke untersucht. Darüber hinaus wurden jene Schichtstapel zur Realisierung des BPM-Konzeptes auf selbstangeordnete SiO2 Partikel mit unterschiedlichen Durchmessern aufgebracht, durch welche sich lateral austauschentkoppelte, eindomänige magnetische Nanostrukturen erzeugen lassen. Zur Untersuchung des Ummagnetisierungsverhaltens und der jeweiligen Größenabhängigkeiten (maßgeblich Durchmesser und Schichtdicke) wurden diese mittels Magnetkraftmikroskopie sowie winkelabhängiger magnetooptischer Kerr Effekt Magnetometrie untersucht. Zur weiteren Vertiefung des Verständnisses noch kleinerer Strukturgrößen erfolgten mikromagnetische Simulationen, bei denen die magnetischen Wechselwirkungen lateral (benachbarte 3D Elemente) als auch vertikal (Wechselwirkungen ferromagnetischer Schichten innerhalb eines 3D Elementes) im Interesse standen, sowie deren Auswirkungen auf das Ummagnetisierungsverhalten des gesamten Feldes. Der Fokus des zweiten Teils liegt auf der Untersuchung des Riesenmagnetwiderstandseffektes in Systemen mit senkrechter Sensitivität. Dafür sind ferromagnetische Schichten mit senkrechter magnetischer Anisotropie nötig, wobei hier die chemisch geordnete L10-Phase der FePt Legierung und [Co/Pt] sowie [Co/Pd] Multilagen Anwendung fanden. Für eine chemische Ordnung der FePt Legierung sind hohe Temperaturen während der Schichtabscheidung notwendig, welche eine hinreichende Austauschentkopplung beider ferromagnetischer Schichten meist nicht gewährleisten. Grund dafür sind einsetzende Diffusionsprozesse als auch Legierungsbildungen mit dem Zwischenschichtmaterial. In der vorliegenden Arbeit konnte der GMR Effekt daher ausschließlich mit einer Ru Zwischenschicht in FePt basierten Trilagensystemen nachgewiesen und charakterisiert werden. Enorme Verbesserungen der magnetoresistiven Eigenschaften werden im Anschluss für [Co/Pt] und vor allem [Co/Pd] Multilagen vorgestellt. Diese Schichtsysteme mit senkrechter magnetischer Anisotropie können bei Raumtemperatur präpariert werden und stellen daher keine weiteren Anforderungen an das Zwischenschichtmaterial sowie die verwendeten Substrate. Hier wurden neben Systemen mit ausschließlich senkrechter magnetischer Anisotropie auch Systeme mit gekreuzten magnetischen Anisotropien intensiv untersucht, da diese durch einen linearen und weitgehend hysteresefreien R(H) Verlauf imHinblick auf Sensoranwendungen enorme Vorteile bieten. Letztendlich wurde die Korrosionsbeständigkeit in Abhängigkeit des Deckschichtmaterials als auch die mechanische Belastbarkeit von auf flexiblen Substraten abgeschiedenen GMR-Schichtstapeln untersucht. Zusätzlich wird in Kapitel 2.5.2 eine experimentelle Studie zum Surfactant-gesteuerten Wachstum der FePt Legierung mittels Molekularstrahlepitaxie vorgestellt. Als Surfactant dient Sb, wodurch die Kristallinität bei geringer Depositionstemperatur deutlich verbessert werden konnte. Die Oberflächensegregation von Sb wurde mittels Auger Elektronenspektroskopie und Rutherford Rückstreuspektrometrie verifiziert und die Charakterisierung magnetischer Eigenschaften belegt einen Anstieg der magnetischen Anisotropieenergie im Vergleich zu Referenzproben ohne Sb.