Thesis (Ph. D.)--Lehigh University, 1997. / Includes vita. Includes bibliographical references.

Thin films and nanowires of Co-Pt have been prepared by means of electrodeposition. Composition, structure, microstructure and magnetic properties have been intensively studied using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometry and correlated to the deposition parameters such as electrolyte composition, deposition current and/or potential. Co rich Co-Pt films have been deposited at various current densities. A nearly constant composition of Co70Pt30 was achieved for current densities between 18 and 32 mA/cm². Detailed texture measurements confirmed an increasing fraction of the hexagonal phase with its c-axis aligned perpendicular to the film plane with increasing current density. Accordingly, magnetic properties are strongly affected by the magnetocrystalline anisotropy of the hexagonal phase that competes with the shape anisotropy of the thin film geometry. Co-Pt nanowires have been prepared within alumina templates at different deposition potentials between -0.6 and -0.9VSCE changing the composition from nearly pure Pt to Co. The composition Co80Pt20 was observed at a deposition potential of -0.7VSCE. Co-Pt nanowires are nanocrystalline in the as-deposited state. Magnetic measurements reveal changing fcc and hcp phase fractions within the wires as the effective anisotropy significantly differs from the expected shape anisotropy for nanowires with high aspect ratio. This change in effective anisotropy is attributed to the preferential alignment of the c-axis of hcp Co-Pt phase perpendicular to the nanowires axis. A promising alternative with much smaller feature sizes is the diblock copolymer template. Electrodeposition of Co and Co-Pt into these templates has been carried out. Inhomogeneities in the template thickness as well as a certain substrate roughness have been identified to be the reasons for inhomogeneous template filling. Thus magnetic properties are dominated by large deposits found on top of the template. Additionally, rolled-up tubes of several nm thick Au/Co/Au films have been characterized magnetically. Temperature dependent measurements show an exchange bias behaviour that is explained in terms of induced stresses during cooling. Changes of magnetic properties in the investigated samples are finally discussed in terms of competing effects of different magnetic anisotropies in various geometries. / Co-Pt Dünnschichten und Nanodrähte wurden mittels elektrochemischer Abscheidung hergestellt. Zusammensetzung, Struktur, Mikrostruktur und magnetische Eigenschaften wurden intensiv mit Röntgenbeugung, Rasterelektronenmikroskopie und Magnetometrie untersucht und mit den Depositionsparametern wie Elektrolytzusammensetzung, Abscheidestrom und/oder-potential korreliert. Co reiche Co-Pt-Filme wurden mit verschiedenen Stromdichten hergestellt. Eine nahezu konstante Zusammensetzung im Bereich Co70Pt30 wurde für Stromdichten zwischen 18 und 32 mA/cm² erreicht. Detaillierte Texturmessungen bestätigen einen zunehmenden Anteil an hexagonaler Phase mit senkrecht zur Filmebene ausgerichteter c-Achse mit zunehmender Stromdichte. Dementsprechend werden die magnetischen Eigenschaften stark von der magnetokristallinen Anisotropie der hexagonalen Phase beeinflusst, die mit der Formanisotropie der Dünnschicht-Geometrie konkurriert. Co-Pt-Nanodrähte wurden in nanoporöse Aluminiumoxidmembranen bei verschiedenen Potentialen zwischen -0,6 und -0.9 VSCE abgeschieden, wobei sich die Zusammensetzung von nahezu reinem Pt zu Co verändert. Die Zusammensetzung Co80Pt20 wurde bei einem Abscheidepotential von -0.7 VSCE erhalten. Die so hergestellten Co-Pt Nanodrähte sind nanokistallin. Magnetische Messungen weisen jedoch auf veränderte Phasenanteile der fcc und hcp Phase innerhalb der Drähte hin, da die effektive Anisotropie erheblich von der für Nanodrähte mit hohem Aspektverhältnis erwarteten Formanisotropie abweicht. Diese Änderung der effektiven Anisotropie ist auf die bevorzugte Ausrichtung der hexagonalen c-Achse des Co-Pt senkrecht zur Drahtachse zurückzuführen. Vielversprechende Template mit deutlich kleineren Dimensionen sind Diblockcopolymertemplate. Es wurden Versuche zur Abscheidung von Co und Co-Pt in diese Template durchgeführt. Als Gründe für die inhomogene Templatfüllung wurden Inhomogenitäten in der Schichtdicke sowie eine gewisse Rauhigkeit der Substrate identifiziert. Aufgrund der ungleichmäßigen Fülleg werden die magnetischen Eigenschaften durch große, halbkugelförmige Abscheidunge auf der Oberfläche des Templates bestimmt. Darüber hinaus wurden aus wenige nm dicken Au/Co/Au Filmen hergestellte Mikroröhren magnetisch charakterisiert. Temperaturabhängige Messungen zeigen ein Exchange Bias Verhalten, das durch beim Abkühlen induzierte Spannungen erklärt wird. Unterschiede im magnetischen Verhalten der untersuchten Proben werden abschließend im Hinblick auf die verschiedenen konkurrierenden magnetischen Anisotropien in verschiedenen Geometrien diskutiert.

Manganese-based Heusler compounds display intriguing fundamental physical properties, determined by the delicate balance of magnetic interactions that give rise to real and reciprocal-space topology, sparking the interest in their potential application in the spin-based technology of the future. In this thesis, a thorough study of thin films of two Mn-based Heusler compounds, the hexagonal MnPtGa and inverse tetragonal Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) system, was performed. The observation of Néel-type skyrmions in single-crystalline MnPtGa motivated our interest in the growth and characterization of thin films of this compound. The films were deposited by magnetron sputtering on (0001)-Al2O3 single crystalline substrates, achieving the epitaxial growth of the Ni2In-type hexagonal crystal structure (P6_3/mmc space group, no. 194). Two thermally-induced magnetic transitions were identified in MnPtGa thin films: below the ordering temperature (T_C=273 K) the system becomes ferromagnetic, followed by a spin-reorientation transition at T_sr=160 K, adopting a spin-canted magnetic structure. Resorting to single-crystal neutron diffraction (SCND), we were able to resolve the magnetic ground state of our MnPtGa thin films. The Mn magnetic moments were found to tilt 20 degrees away from the c-axis, forming a commensurate magnetic structure with a ferromagnetic component along the crystallographic c-axis and a staggered antiferromagnetic one in the basal plane. This further demonstrated the applicability of a bulk technique, such as SCND, to the study of magnetic structures in thin films. Additionally, the perpendicular magnetic anisotropy (PMA) in the system was determined by magnetometry technique. Electrical magnetotransport measurements were performed in a thickness series of MnPtGa thin films. A non-monotonous anomalous Hall conductivity (AHC) was observed, whose intrinsic Berry-curvature origin was elucidated by means of first-principle calculations. We further observed by magnetic force microscopy technique the nucleation of irregular magnetic bubbles under the application of a magnetic field. We tentatively link their appearance to the onset of an additional electron scattering mechanism contributing to the transverse resistivity. In the second part of this thesis, the inverse tetragonal Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) system was investigated. The films were grown on MgO(100) single crystalline substrates, promoting the epitaxial growth of the tetragonal structure (I-4m2 space group, no. 119). We primarily focused on the impact of the systematic substitution of iridium on the structural, magnetic and electrical (magneto)transport properties of the system. A compression of the basal lattice parameters and elongation of the c-axis, accompanied by larger crystallographic disorder, was observed as the Ir content (x) increased, altering the Mn-Mn exchange interactions and therefore the magnetic properties of the compound. Mn2RhSn have two thermally-induced magnetic transitions: first, to a collinear ferrimagnetic state below the Curie temperature (T_C=280 K), followed by a spin-reorientation transition at T_sr=80 K to a noncollinear state, determined by two inequivalent Mn sublattices. A reduction of both T_C and T_sr was observed, as well as a tendency towards a hard-axis ferromagnet and therefore larger PMA as the Ir content of the films was increased. Additionally, a reduction of the saturation magnetization suggest a change of the magnitude of the spin canting upon Ir-substitution. The electrical magnetotransport properties of the Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) thin films were acquired and analyzed in a wide temperature and magnetic field range. A strongly temperature and composition dependent non-monotonous AHC was found, suggesting two regimes in the electronic transport: (i) a nearly x-independent regime dominated by intrinsic Berry-curvature and (ii) a strongly x-dependent regime suggesting a more relevant role from extrinsic mechanisms contributing to the AHC. On the other hand, the Mn2Rh(0.95)Ir(0.05)Sn bulk system is known to host magnetic skyrmion and antiskyrmion phases. We indirectly assessed the impact of the systematic Ir-substitution on the (anti)skyrmionic phases through the analysis of the sign of the topological Hall effect in our thin films. A tendency towards the suppression of the low-T skyrmion phase stabilized by magnetic dipole-dipole interaction, and subsistence of the high-T antiskyrmion phase in Mn2Rh(1-x)Ir(x)Sn thin films was found as x > 0.2, which can be interpreted as a change of magnitude of the anisotropic DMI in this tetragonal D_2d system upon Ir-substitution. We have thus demonstrated that the magnetic and topological properties of the Mn2Rh(1-x)Ir(x)Sn system can be tailored upon chemical substitution, showing a strongly intertwined relation between composition, crystal and electronic structure, with the emergence of exotic magnetic phases, ultimately reflected in their electrical transport signatures.:Abstract iii Abbreviations iv Symbols vi Preface xii 1 Fundamentals 1 1.1 Noncollinear magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Magnetic interactions in solids . . . . . . . . . . . . . . . . . . . 2 1.1.1.1 Exchange interaction . . . . . . . . . . . . . . . . . . . 2 1.1.1.2 Dzyaloshinsky-Moriya interaction . . . . . . . . . . . . 3 1.1.1.3 Magnetic anisotropy . . . . . . . . . . . . . . . . . . . 4 1.1.1.4 Magnetic dipolar interaction . . . . . . . . . . . . . . . 5 1.1.2 Spin-reorientation transition . . . . . . . . . . . . . . . . . . . . 5 1.1.3 Magnetic skyrmions and antiskyrmions . . . . . . . . . . . . . . 6 1.1.3.1 Antiskyrmions in Heusler compounds . . . . . . . . . . 8 1.2 Magnetic Heusler compounds . . . . . . . . . . . . . . . . . . . . . . . 9 1.2.1 Cubic crystal structure . . . . . . . . . . . . . . . . . . . . . . . 10 1.2.2 Distorted crystal structures . . . . . . . . . . . . . . . . . . . . 10 1.2.2.1 Tetragonal Heusler compounds . . . . . . . . . . . . . 11 1.2.2.2 Hexagonal Heusler compounds . . . . . . . . . . . . . 11 1.3 Charge and spin transport in ferromagnets . . . . . . . . . . . . . . . . 13 1.3.1 The two-current model . . . . . . . . . . . . . . . . . . . . . . . 13 1.3.2 The Hall effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.3.2.1 Anomalous Hall effect . . . . . . . . . . . . . . . . . . 15 1.3.2.2 Topological Hall effect . . . . . . . . . . . . . . . . . . 17 1.4 Neutron scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.4.1 Thermal Neutrons . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.4.1.1 Scattering cross sections . . . . . . . . . . . . . . . . . 19 1.4.1.2 The four-circle diffractometer . . . . . . . . . . . . . . 23 xv 1.4.2 Magnetic neutron scattering . . . . . . . . . . . . . . . . . . . . 24 2 Experimental Techniques 29 2.1 Magnetron sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.1.1 Thin films growth modes . . . . . . . . . . . . . . . . . . . . . . 32 2.1.2 Thin films microstructure . . . . . . . . . . . . . . . . . . . . . 33 2.2 X-ray characterization of thin films . . . . . . . . . . . . . . . . . . . . 34 2.2.1 Geometry of the X-ray diffractometer . . . . . . . . . . . . . . . 35 2.2.2 Radial θ-2θ scan . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.3 ϕ -scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.4 Rocking curves (ω-scans) . . . . . . . . . . . . . . . . . . . . . . 36 2.2.5 X-ray reflectivity (XRR) . . . . . . . . . . . . . . . . . . . . . . 37 2.3 Composition analysis: energy dispersive X-ray spectroscopy (EDS) . . . 38 2.4 Surface characterization: atomic and magnetic force microscopy . . . . 38 2.5 D10 thermal neutron diffractometer . . . . . . . . . . . . . . . . . . . . 39 2.6 SQUID-VSM magnetometry . . . . . . . . . . . . . . . . . . . . . . . . 40 2.7 Electrical (magneto-)transport measurements . . . . . . . . . . . . . . 41 3 Noncollinear magnetism in MnPtGa epitaxial thin films 43 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.2 MnPtGa thin films: growth and characterization . . . . . . . . . . . . . 45 3.2.1 Growth conditions . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.2.2 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.3 Magnetic properties of MnPtGa thin films . . . . . . . . . . . . . . . . 49 3.3.1 Thermal evolution of the magnetic structure . . . . . . . . . . . 49 3.3.2 Field dependent magnetization . . . . . . . . . . . . . . . . . . 50 3.3.3 Single-crystal neutron diffraction in MnPtGa thin films . . . . . 52 3.3.3.1 Ferromagnetic phase . . . . . . . . . . . . . . . . . . . 54 3.3.3.2 Noncollinear phase . . . . . . . . . . . . . . . . . . . . 55 3.4 Electronic band structure of h-MnPtGa . . . . . . . . . . . . . . . . . . 57 3.5 Electrical magnetotransport properties of MnPtGa thin films . . . . . . 59 3.5.1 Zero field longitudinal resistivity . . . . . . . . . . . . . . . . . . 60 3.5.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.5.3 Magnetic transitions under a magnetic field . . . . . . . . . . . 64 3.6 Intrinsic origin of the anomalous Hall effect . . . . . . . . . . . . . . . . 65 3.6.1 Scaling of the anomalous Hall conductivity vs. σxx . . . . . . . 68 3.7 Spin textures in MnPtGa thin films . . . . . . . . . . . . . . . . . . . . 73 3.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4 Tuning the magnetic and topological properties of Mn2Rh1−xIrxSn epitaxial thin films 83 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.2 Growth and characterization of Mn2Rh1−xIrxSn thin films . . . . . . . 86 4.2.1 Growth conditions and Ir substitution . . . . . . . . . . . . . . 86 4.2.2 Crystal structure of Mn2Rh1−xIrxSn . . . . . . . . . . . . . . . . 87 4.3 Tuning the magnetic properties of the Mn2Rh1−xIrxSn system . . . . . 91 xvi 4.3.1 Thermal magnetic transitions . . . . . . . . . . . . . . . . . . . 91 4.3.2 Increasing the magnetic anisotropy under Ir-substitution . . . . 92 4.4 Electrical (magneto-)transport properties of Mn2Rh1−xIrxSn thin films 94 4.4.1 Zero-field longitudinal resistivity and spin reorientation transition 94 4.4.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.4.3 Hall effects: from ordinary to anomalous & topological . . . . . 96 4.4.3.1 Ordinary Hall effect . . . . . . . . . . . . . . . . . . . 97 4.4.3.2 Anomalous Hall effect . . . . . . . . . . . . . . . . . . 98 4.4.3.3 Competing mechanisms in the AHC of the Mn2Rh1−xIrxSn system . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.4.3.4 Scaling of the AHC with the magnetization . . . . . . 101 4.4.3.5 Topological Hall effect . . . . . . . . . . . . . . . . . . 102 4.5 Tuning the (Anti-)Skyrmion phases . . . . . . . . . . . . . . . . . . . . 106 4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 5 Conclusions & Outlook 111 List of Figures 117 List of Tables 120 List of Publications 124 Aknowledgements 124 Bibliography 127 Eigenständigkeitserklärung 147

Durch die laterale Modifizierung der magnetischen Eigenschaften von austauschgekoppelten NiFe/IrMn-Schichten wurden weichmagnetische Schichten geschaffen, die sowohl neue statische als auch dynamische hybride Eigenschaften zeigen. Als laterale Strukturierungsmethoden wurden hierbei die lokale Oxidation sowie Ionenimplantation verwendet. Mit Hilfe dieser Verfahren ist es gelungen spezifische magnetische Domänenkonfigurationen mit Streifenstrukturen nominell antiparalleler Magnetisierungsausrichtung in die Schichten einzuprägen. In Abhängigkeit der Strukturorientierung sowie Streifenperiode konnte direkt das Ummagnetisierungsverhalten sowie die magnetische Resonanzfrequenz und Dämpfung der Schichten modifiziert werden. Die neuen dynamischen Eigenschaften wie z.B. eine hybride Resonanzfrequenz werden hierbei im Rahmen der Kopplung über dynamische Ladungen und die direkte Beeinflussung des effektiven Feldes des künstlich eingebrachten Domänenzustandes diskutiert. Die vorgestellten Ergebnisse belegen somit das große Potential der lateralen Magneto-Strukturierung zur Einstellung spezifischer statischer wie auch dynamischer Eigenschaften magnetisch dünner Schichten.

The work presented in this thesis encompasses the design, development, realization and testing of novel magnetoelectric thin film elements that do not rely on ferromagnets, but are based entirely on magnetoelectric antiferromagnets such as Cr2O3. Thin film spintronic elements, and in particular magnetoelectric transducers, are crucial building blocks of high efficiency data processing schemes that could complement conventional electronic data processing in the future. Recent developments in magnetoelectrics have revealed, that exchange biased systems are ill-suited to electric field induced switching of magnetization due to the strong coupling of their ferromagnetic layer to magnetic fields. Therefore, ferromagnet-free magnetoelectric elements are proposed here in an effort to mitigate the practical problems associated with existing exchange biased magnetoelectric elements. This goal is achieved by establishing an all-electric read-out method for the antiferromagnetic order parameter of thin films, which allows to omit the ferromagnet from conventional exchange biased magnetoelectric elements. The resulting ferromagnet-free magnetoelectric elements show greatly reduced writing thresholds, enabled operation at room temperature and do not require a pulsed magnetic field, all of which is in contrast to state-of-the-art exchange biased magnetoelectric systems. The novel all-electric read-out method of the magnetic field-invariant magnetization of antiferromagnets, so-called spinning-current anomalous Hall magnetometry, can be widely employed in other areas of thin film magnetism. Its high precision and its sensitivity to previously invisible phenomena make it a promising tool for various aspects of thin solid films. Based on this technique, a deep understanding could be generated as to what physical mechanisms drive the antiferromagnetic ordering in thin films of magnetoelectric antiferromagnets. As spinning-current anomalous Hall magnetometry is an integral probe of the magnetic properties in thin films, it offers no intrinsic scale sensitivity. In order to harness its great precision for scale related information, a statistical framework was developed, which links macroscopic measurements with microscopic properties such as the antiferromagnetic domain size.:TABLE OF CONTENTS Abbreviations 9 1 Introduction 11 1.1 Motivation 11 1.2 Objectives 12 1.3 Organization of the thesis 13 2 Background 15 2.1 History of magnetoelectric coupling 15 2.2 Long range magnetic ordering 16 2.2.1 Magnetic order parameter and field susceptibility 17 2.2.2 Magnetic proximity effect 19 2.2.3 Exchange bias 20 2.3 Phenomenology of magnetoelectric coupling 21 2.3.1 The linear magnetoelectric effect 21 2.3.2 Magnetoelectric pressure on the antiferromagnetic order parameter 22 2.3.3 Switching the antiferromagnetic order parameter 23 2.4 Realized magnetoelectric thin film elements 24 2.4.1 BiFeO3/CoFe system 24 2.4.2 Cr2O3/Co/Pt system 25 3 Experimental methods 27 3.1 Development of ferromagnet free magnetoelectric elements 28 3.1.1 The substrate 29 3.1.2 The Cr2O3 bulk and top surface 31 3.1.3 The V2O3 or Pt bottom electrodes 33 3.1.4 Epitaxial relationships 34 3.1.5 The Cr2O3 bottom interface 39 3.1.6 Twinning of Cr2O3 39 3.1.7 Hall crosses and patterning processes 43 3.2 Magnetotransport measurements 44 3.2.1 Hall effects 45 3.2.2 Anomalous Hall effect 46 3.2.3 Magnetoelectric writing 47 3.2.4 All electric read out 49 3.3 The experimental setup 50 3.3.1 Temperature control 50 3.3.2 Magnetic field control 51 4 Spinning-current anomalous Hall magnetometry 53 4.1 Characteristics of the technique 53 4.1.1 Operational principle 53 4.1.2 Advantages 55 4.1.3 Magnetic hysteresis loops and field-invariant magnetization 55 4.1.4 Measurement of field-invariant magnetization 56 4.1.5 Limitations 58 4.2 Application of SCAHM to Cr2O3(0001) thin films 59 4.2.1 Criticality and distribution of the antiferromagnetic phase transition 61 4.2.2 Evaluation of the magnetic proximity effect 64 4.3 SCAHM with thin metallic antiferromagnetic IrMn films 65 4.3.1 [Pt/Co]4/IrMn exchange bias system 65 4.3.2 Isolated antiferromagnetic IrMn thin films 67 5 Magnetoelectric performance 69 5.1 Magnetoelectric field cooling 69 5.2 The gate bias voltage 71 5.3 Isothermal binary magnetoelectric writing in Cr2O3 72 6 Order parameter selection in magnetoelectric antiferromagnets 77 6.1 Uncompensated magnetic moment 77 6.2 Extrinsic causes for broken sublattice equivalence 81 6.3 The V2O3 gate electrode 83 7 Measurement of microscopic properties with an integral probe 87 7.1 Interentity magnetic exchange coupling 87 7.2 Ensemble formalism for the entity size determination 90 7.3 Estimation of the entity sizes 94 7.4 Microscopic confirmation of the ensemble model 97 8 Summary and Outlook 101 8.1 Goal-related achievements 101 8.1.1 All-electric read-out of the AF order parameter 101 8.1.2 Electric field induced writing of the AF order parameter 102 8.2 Further achievements 103 8.2.1 Foreseen impact of SCAHM on thin film magnetism 103 8.2.2 Practical optimization routes of magnetoelectric Cr2O3 systems 104 8.2.3 Theoretical work 105 8.3 Future directions 105 8.3.1 Development of Cr2O3-based magnetoelectric systems 105 8.3.2 Applications of SCAHM 106 References 107 Erklärung 113 Acknowledgements 115 Curriculum Vitae 117 Scientific publications, contributions, patents 119