Magneto-thermopower and Magnetoresistance of Co-Ni Alloy and Co-Ni/Cu Multilayered Nanowires. Dissertation zur Erlangung des Doktorgrades Department Physik Universität Hamburg vorgelegt von Tim Böhnert geb. in Hamburg Hamburg 2014 1 Gutachterin/Gutachter der Dissertation: Prof. Dr. Kornelius Nielsch Prof. Dr. Andy Thomas Gutachterin/Gutachter der Disputation: Prof. Dr. Kornelius Nielsch Prof. Dr. Hans Peter Oepen Datum der Disputation: 28.5.2014 Vorsitzende/Vorsitzender des Prüfungsausschusses: Prof. Dr. Daniela Pfannkuche Vorsitzende/Vorsitzender des Promotionsausschusses: Prof. Dr. Daniela Pfannkuche Dekan der MIN-Fakultät: Prof. Dr. Heinrich Graener 2 Table of Contents 3 Abstract The relationship of the magneto-thermopower and the anisotropic magnetoresistance/giant magnetoresistance (AMR/GMR) is investigated on individual Co-Ni alloy and Co-Ni/Cu multi- layered nanowires. A simple model is developed to distinguish the absolute thermopower con- tributions without relying on literature values. A versatile measurement setup is developed for the thermoelectric characterization of electro- chemically deposited nanowires. The measured thermopowers and electrical resistivities match reasonably well to those reported in the literature for bulk Co-Ni alloys and GMR thin films. The Co-Ni alloy composition is varied and AMR values as high as -6 % are measured at room tem- perature (RT). The multilayered nanowires with varying thickness of the Cu layers show typical current-perpendicular-to-plane GMR effects of up to -15 % at RT. A linear dependence between thermopower and electrical conductivity—with the magnetic field as an implicit variable—is found over a wide temperature range (50 K to 325 K). This observation is in agreement with the Mott formula under the assumption of a magnetic field independent thermopower offset, which is related to the absolute Seebeck coefficient of the contact materials. Utilizing this rela- tion, the absolute thermopower and the magneto-thermopower of the nanowires are deter- mined and equal absolute values of magnetoresistance and magneto-thermopower follow. This simple model is tested with different contact materials and compared to the absolute thermopower reported in the literature. Accordingly, the magnetic field independent energy derivative of the resistivity from the Mott formula is calculated. By changing the composition of the Co-Ni alloy, the thermoelectric power factor is increased by a factor of two as compared to the Ni nanowire. This can be further enhanced by 24 % in per- pendicular magnetic fields. The multilayered nanowires show smaller power factors, but are still competitive with high performance thermoelectric nanowires, which might pave the way for energy harvesting applications in the future. 3 4 Table of Contents Inhaltsangabe Die Beziehung zwischen Magneto-Seebeck Effekt und Anisotropen- bzw. Riesenmagnetowiderstand (AMR bzw. GMR) wird an einzelnen Co-Ni legierten und Co-Ni/Cu Multischicht Nanodrähte untersucht. Ein einfaches Modell wurde entwickelt, um die absoluten Thermospannungsbeiträge ohne Verwendung von Literaturwerten zu unterscheiden. Ein vielseitiger Messaufbau für die thermoelektrische Charakterisierung von elektrochemisch abgeschiedenen Nanodrähten wurde entwickelt. Die gemessenen Seebeck-Koeffizienten und elektrischen Widerstände passen gut zu den Literaturwerten für Bulk Co-Ni-Legierungen und GMR dünnen Filmen. Die Co-Ni Zusammensetzung wurde variiert und AMR Werte bis zu -6 % bei Raumtemperaturen (RT) gemessen. Die mehrschichtigen Nanodrähte mit unterschiedlicher Cu Schichtdicke zeigen typische GMR Effekte von bis zu -15 % bei RT mit dem Stromfluss senk- recht zur Schichtebene. Eine lineare Abhängigkeit zwischen Seebeck-Koeffizient und spezifi- scher Leitfähigkeit mit dem Magnetfeld als implizite Variable wurde über einen weiten Tempe- raturbereich (50 K bis 325 K) gefunden. Diese Beobachtung steht in Übereinstimmung mit der Mott Formel unter der Annahme eines vom Magnetfeld unabhängigen Thermospannungs- Offsets, der mit den absoluten Seebeck-Koeffizienten der Kontaktmaterialien verknüpft ist. Mit Hilfe dieser Beziehung können die absoluten Seebeck-Koeffizienten und der Magneto-Seebeck Effekt der Nanodrähte bestimmt werden und es folgen gleich große Beträge von Magneto- Seebeck Effekt und Magnetowiderstand. Dieses einfache Modell wird an unterschiedlichen Kontaktmaterialien getestet und mit absoluten Seebeck-Koeffizienten aus der Literatur vergli- chen. Die Magnetfeld unabhängige Ableitung des spezifischen Widerstands nach der Energie wird dementsprechend aus der Mott Formel berechnet. Durch Verändern der Co-Ni Zusammensetzung verdoppelt sich der thermoelektrische Power- faktor verglichen mit dem Wert des Ni Nanodrahtes. Eine Erhöhung um weitere 24 % ist in senkrechten Magnetfeldern möglich. Obwohl die multischichtigen Nanodrähte kleinere Power- faktoren zeigen, sind diese dennoch mit Nanodrähten aus thermoelektrischen Hochleistungs- materialien Vergleichbar, diese Erkenntnis könnte zukünftig zur Anwendung in der Energiege- winnung führen. 4 Table of Contents Table of Contents .................................................................................................................. 5 1 Introduction ................................................................................................................... 7 2 Theoretical Background .................................................................................................. 9 2.1 Magnetoresistance ........................................................................................................... 9 2.2 Thermopower ................................................................................................................. 16 2.3 Magnetism...................................................................................................................... 27 3 Nanowire Synthesis ...................................................................................................... 33 3.1 Anodization .................................................................................................................... 33 3.2 Hard Anodized Aluminum Oxide Membranes ............................................................... 35 3.3 Atomic Layer Deposition ................................................................................................ 36 3.4 Preparation Steps ........................................................................................................... 37 3.5 Electrodeposition ........................................................................................................... 38 3.6 Electrodeposition of Multilayers .................................................................................... 40 3.7 Release of the Nanowires .............................................................................................. 43 4 Measurement Platform ................................................................................................ 45 4.1 Microscopic Contact Design ........................................................................................... 45 4.2 Macroscopic Circuit ........................................................................................................ 48 4.3 Measurement Equipment .............................................................................................. 49 4.4 Measurement Routine ................................................................................................... 51 4.5 Applications of the Seebeck Setup ................................................................................. 53 4.6 Secondary Effects ........................................................................................................... 56 5 Thermoelectric Transport in Anisotropic Magnetoresistance Nanowires ....................... 61 5.1 Magnetoresistance ......................................................................................................... 62 5.2 Magneto-Thermopower ................................................................................................. 71 5.3 The Mott Formula–S vs. R-1 ............................................................................................ 75 5 6 Table of Contents 5.4 Permalloy Nanowires ..................................................................................................... 77 5.5 Conclusion AMR and MTEP of Co-Ni alloy Nanowires ................................................... 79 6 Thermoelectric Transport in Giant Magnetoresistance Nanowires ................................. 81 6.1 Magnetoresistance ......................................................................................................... 83 6.2 Magneto-Thermopower ................................................................................................. 88 6.3 The Mott Formula–S vs. R-1 ............................................................................................ 93 6.4 Conclusion Co-Ni/Cu Multilayered Nanowires ............................................................ 100 7 Conclusion ...................................................................................................................101 Appendix: Seebeck Measurement Software ........................................................................105 Appendix: TEM analysis ......................................................................................................106 Appendix: Hall and Nernst effect ........................................................................................109 Appendix: Overview of Measurement Results at RT ............................................................110 Appendix: Publication List ..................................................................................................111 8 Bibliography ................................................................................................................114 6 1 Introduction The discovery of the giant magnetoresistance (GMR) is not only a story of a magnificent break- through,1,2 but also of an application with a significant impact on several aspects of society. Storage devices, which were based previously on the anisotropic magnetoresistance (AMR) sen- sors, were improved in cost, size, and power efficiency. This development accelerated the trend of hard disk miniaturization beyond the initial GMR technology to a point that today a third of the world’s population has access to personal computers and the internet. The GMR is known to be an early application in the promising field of nanotechnology and has provided the foun- dation of the research field of spin transport electronics—called spintronics.3 In addition to the charge of an electron, a second fundamental property—the spin—is utilized in spintronics for advanced magnetic memories and sensors.4-7 A second topic of current technological interest is the thermoelectricity, which describes the interaction of heat and charge transport. The major material property of thermoelectricity—the Seebeck coefficient S—describes the diffusion of charge carriers due to an applied temperature gradient and was found by Thomas Johann See- beck in 1821. The behavior of S in the free-electron model can be described by the Mott formu- la.8 Only the relative Seebeck coefficient is experimentally accessible. Therefore, S is ultimately calculated from observations of the Thomson heat.9,10 By combining spintronics and thermoe- lectricity the so called spin-caloritronics evolved, which investigates spin caloric effects of spin polarized currents in magnetic nanostructures. The influence on magnetoresistance, thermal transport, and magnetic states is of particular interest of this topic.11 Due to the observation of the novel spin-Seebeck effect by Uchida et al.12 in 2008 this research field has grown at high pace in spite of critical publications toward the initial finding.13 A recent systematic study pub- lished by Schmidt et al.14 indicates the existence of the spin-Seebeck effect at a much smaller magnitude. Nevertheless, research motivated the development of necessary measurement techniques to investigate conventional transport properties of nanostructures. Measurements of the thermopower (Seebeck coefficient) and magneto-thermopower (spin-dependent See- beck effect) on single nanowires15,16 as well as nanostructures like magnetic tunnel junc- tions17,18 or spin valves19,20 show the interest in the thermoelectric properties of nanostructures in particular. Recently, Heikkilä et al.21,22 introduced the concept of spin heat accumulation in perpendicular-to-plane transport in spin valve or multilayered structures, which describes the spin dependent effective electron temperature that might lead to a violation of the Wiedemann-Franz law. This effect increases in low dimensional structures. Therefore, multi- layered nanowires are one of the few systems that can be employed to experimentally verify this effect. 7 8 Introduction Motivated by these fruitful developments in the scientific community, the aim of this work is to contribute to the spin-caloritronics by investigating the thermopower of individual magnetic nanowires. The Co-Ni alloy and Co-Ni/Cu multilayered nanowires are electrochemically deposit- ed into nanoporous alumina templates. Electrical contacts are lithographically defined on top of a single nanowire on a glass substrate. In contrast to measurement approaches performed on platforms,23-26 in which the particular nanowire has to be assembled on top of a pre-defined structure. The alloy nanowires exhibit the AMR effect, while the magnetic behavior of the mul- tilayered nanowires is dominated by the GMR effect. The high aspect ratio of the nanowires results in a defined magnetization behavior of the alloy nanowires due to pronounced shape anisotropy. Therefore, the composition dependent AMR and magneto-thermopower can be studied under defined magnetization conditions. These effects show magnitudes up to 6.5 % in bulk literature27,28 as well as in the presented nanowire experiments. In multilayered systems GMR values of 80 % can be achieved by physical deposition,29,30 while electrochemically depos- ited nanowires show current-perpendicular-to-plane GMR values of up to 35 %. The magnetic field dependency of S in materials—showing AMR or GMR effects—can be explained by the Mott formula,8 which describes the diffusive part of the thermopower.31-39 A direct relation be- tween S and σ is predicted, while experimental results do not obey these clear predictions and a more complicated relationship is often presumed. The major experimental difficulty is that only relative Seebeck coefficients are accessible and to obtain the absolute sample value the contact material contributions have to be corrected. Since the thermopower is very sensitive to impuri- ties40 and shows size effects,41,42 deviations between literature values and experimental materi- als have to be considered. This work tries to determine absolute thermopowers utilizing a sim- ple model based on the Mott formula, without relying on literature values. The magnetoresistance, the thermopower, the magnetism, the nanowire synthesis, and the meas- urement setup are explained in the following. Subsequently, measurement results on AMR, GMR and magneto-thermopower of Co-Ni alloy nanowires and Co-Ni/Cu multilayered nan- owires are presented. Finally, the resistance and the thermopower are correlated through the Mott formula with the aim to distinguish the different thermopower contributions. 8 2 Theoretical Background Currently, the interest in the magneto-thermopower (MTP) of ferromagnetic nanostructures is high, as measurements on single nanowires,15,16 tunnel junctions,17,18 and spin valves19,20 show. Especially, multilayered nanowires are the perfect model system for the experimental investiga- tion of spin dependent perpendicular-to-plane (CPP) transport. The CPP transport is of particu- lar interest in the concept of spin heat accumulation, which is proposed to cause a violation of the Wiedemann-Franz law—ratio of thermal conductivity and electrical conductivity.21 Crucial to understand the magnetotransport in the nanowire are the resistivity and the magnetoresistance (MR),43-45 which describes the change of the electrical resistance in external magnetic fields. The resistivity is related to the thermopower by the Boltzmann transport equa- tions or in first approximation by the Mott formula.8 The theoretical background on magnetoresistance, thermopower, and magnetism is provided in this chapter. 2.1 Magnetoresistance Magnetoresistance (MR) effects are a well-known research topic that has been intensively in- vestigated during the last few decades.43-45 The magnetoresistance describes the change of the electrical resistance in external magnetic fields and is usually given as the relative change: MR  1 (2.1-1) , H 0 with the zero magnetic field resistivity ρ and the resistivity in the magnetic field ρ . A slightly 0 H different definition is occasionally used, called inflated or “optimistic” MR due to possible val- ues above 100 %:* MR   1 (2.1-2) . inf 0 H The most common MR effect is the “ordinary” or the positive magnetoresistance, which shows an increase of the resistivity with the square of the applied magnetic field in metallic materials (MR(H)∼H2). This effect can be explained in the simple picture of circular motions of the con- duction electrons due to the Lorenz force in the applied magnetic field. Therefore, the mean free path between scattering events is effectively reduced and thus the resistivity increases.46 The positive MR is commonly dominated in ferromagnetic materials by negative * The negative MR value can be converted into a negative MR value via: MR=(1-MR )-1-1 and MR =1-(MR+1)-1. inf inf inf 9 10 Chapter 2 Theoretical Background magnetoresistance effects. Scattering of the conduction electrons due to spin-disorder, so- called “magnons”, causes a negative magnetoresistance. This magnon magnetoresistance (MMR) depends linearly on the applied magnetic field (MR(H)∼H).47 In transition metals like nickel, iron and cobalt an additional anisotropic magnetoresistance (AMR) effect appears.43-45 The effects is distinguished between the transversal and the longitudinal magnetoresistance depending on the alignment (perpendicular or parallel) between magnetic field and current direction. In multilayers of ferromagnetic and non-magnetic layers the so-called giant magnetoresistance (GMR)1,2 can be observed. The terms current-in-plane (CIP) and current- perpendicular-to-plane (CPP) are used to describe the alignment of the current with respect to the multilayers. The AMR and the GMR effects depend on the magnetization, the direction of the magnetic dipole moments, rather than on the applied magnetic field. Therefore, both are generally negative quadratic effects with the magnetic field (MR(H)∼-H2) and saturate at the characteristic saturation field, when all magnetic dipole moments are aligned with the magnetic field. In the following, the different effects of the measured samples are discussed. 2.1.1 Anisotropic Magnetoresistance (AMR) The anisotropic magnetoresistance (AMR) occurs in ferromagnetic materials like the 3d transi- tion metals nickel, cobalt and iron. The effect was found by Thomson (also known as Lord Kel- vin)48 in 1857 and describes the change of the resistivity dependent on the angle between elec- trical current and magnetization. The origin of this mechanism in ferromagnetic 3d metals is explained in detail in the textbook by O'Handley.49 To give a simple explanation it is important to understand the different scattering channels in the transition metals. The 4s and the 3d bands are contributing to the electrical conductivity in these metals. Due to much lower effec- tive mass, the 4s electrons carry most of the current.50 Due to exchange coupling, the 3d-band splits spin dependent and results in the electron distribution scetched in Figure 2-1. Mott’s two- current model describes each of the two spins as a separated conduction path with distinct re- sistivity.51-53 The scattering from s↑ in d↑ states is at first negligible since the d↑ band is com- pletely filled. It is reasonable that the resistivity of the s↑ electrons is small compared to s↓ electrons and the spin-up channel carries the majority of the current. Small changes in the scat- tering behavior of the majority channel will have a strong influence on the overall resistivity. Due to the spin-orbit coupling, spin flip scattering is possible and s↑ electrons can scatter in the d↓ band as well as d↑ electrons can scatter in s↓ states. These two mechanisms open the pos- sibility for s-d scattering of the majority channel, which increases the resistivity significantly. The s-d scattering probability depends on the angle α between the magnetic moments and the 10