This thesis discusses spin-transfer torques in MgO-based magnetic tunnel junctions. The voltage-field switching phase diagrams have been experimentally determined for in-plane CoFeB/MgO/CoFeB magnetic tunnel junctions. In order to limit the effect of thermal activation, experiments have been carried out using nanosecond voltage pulses, as well as at low-temperature (4.2 K).
The bias-dependence of the two spin-torque terms (Slonczewski-like and field-like) has been determined from thermally-excited ferromagnetic resonance measurements, yielding values which are in good agreement with previous reports. Additionally, material parameters such as the effective magnetisation and the damping factor have also been extracted.
Using these values as input, the switching voltages as function of the applied magnetic field have been calculated numerically and analytically by solving the modified Landau-Lifshitz-Gilbert equation. Unlike previous studies, the field-like spin-torque has also been included. Moreover, different configurations have been considered for the magnetic anisotropy directions of the reference and free layer, respectively.:1 Introduction
2 Fundamentals
2.1 Magnetoresistance
2.1.1 Giant magnetoresistance
2.1.2 Tunnel magnetoresistance
2.2 Spin-transfer torque effect
2.2.1 Physical picture of the STT
2.2.2 In-plane and perpendicular STT
2.3 Equation of motion for the magnetisation
2.3.1 The Landau-Lifshitz-Gilbert equation
2.3.2 Extension including spin-transfer-torque (LLGS)
2.4 Applications of MR and spin-transfer torque
2.4.1 Read heads in hard disk drives
2.4.2 Spin-transfer torque magnetic random access memory
2.5 STT effects in magnetic tunnel junctions
2.5.1 Current-induced switching
2.5.2 Magnetisation precession
2.5.3 Bias-dependence of STT
2.5.4 Back-hopping
3 Experimental
3.1 Samples
3.1.1 Stack composition
3.1.2 Properties of samples used in this work
3.2 Experimental setup
3.2.1 Overview of equipment for the different measurement techniques
3.2.2 Electromagnet and Kepco power supply
3.2.3 Contacting of the sample
3.2.4 Principle specifications of equipment
3.3 Experimental techniques
3.3.1 Measurement of DC R-H and R-I loops
3.3.2 Measurement of phase diagrams: off and on-pulse
3.3.3 Thermally-excited ferromagnetic resonance
4 Results and discussion
4.1 Switching phase diagrams of MTJs
4.1.1 Theory: Calculating the phase diagram
4.1.2 Experimental phase diagrams
4.2 Thermally excited ferromagnetic resonance
4.2.1 Smoothing and fitting of raw data
4.2.2 Determination of Ms
4.2.3 Signal evolution with bias voltage
4.2.4 Analysis of peak position: perpendicular STT
4.2.5 Analysis of peak linewidth
5 Summary and outlook
A Appendix
List of figures
List of tables
Bibliography / Diese Arbeit befasst sich mit Spin-Transfer-Torque-Effekten in MgO-basierten magnetischen Tunnelstrukturen. Die Phasendiagramme als Funktion von Spannung und Magnetfeld von CoFeB/MgO/CoFeB-Tunnelstrukturen mit Magnetisierung in der Ebene wurden experimentell bestimmt. Um thermische Anregungseffekte zu limitieren, wurden die Experimente einerseits mit nanosekundenlangen Spannungspulsen und andererseits bei niedrigen Temperaturen (4.2 K) durchgeführt.
Die Spannungsabhängigkeit der beiden Spin-Torque-Parameter (in-plane und senkrechter Spin-Transfer-Torque) wurde aus Messungen der thermisch angeregten ferromagnetischen Resonanz bestimmt, wobei sich Werte ergaben, die gut mit vorangegangenen Untersuchungen übereinstimmen. Zusätzlich wurden Werte für Materialparameter wie die effektive Magnetisierung und den Dämpfungsparameter gewonnen.
Unter Verwendung der erhaltenen Werte wurden die Schaltspannungen als Funktion des angelegten Magnetfeldes analytisch und numerisch berechnet, indem die erweiterte Landau-Lifshitz-Gilbert-Gleichung gelöst wurde. Im Gegensatz zu vorangegangenen Untersuchungen wurde der senkrechte Spin-Transfer-Torque dabei mit einbezogen. Darüber hinaus wurden verschiedene Konfigurationen für die Richtung der magnetischen Anisotropie der freien und fixierten Schicht berücksichtigt.:1 Introduction
2 Fundamentals
2.1 Magnetoresistance
2.1.1 Giant magnetoresistance
2.1.2 Tunnel magnetoresistance
2.2 Spin-transfer torque effect
2.2.1 Physical picture of the STT
2.2.2 In-plane and perpendicular STT
2.3 Equation of motion for the magnetisation
2.3.1 The Landau-Lifshitz-Gilbert equation
2.3.2 Extension including spin-transfer-torque (LLGS)
2.4 Applications of MR and spin-transfer torque
2.4.1 Read heads in hard disk drives
2.4.2 Spin-transfer torque magnetic random access memory
2.5 STT effects in magnetic tunnel junctions
2.5.1 Current-induced switching
2.5.2 Magnetisation precession
2.5.3 Bias-dependence of STT
2.5.4 Back-hopping
3 Experimental
3.1 Samples
3.1.1 Stack composition
3.1.2 Properties of samples used in this work
3.2 Experimental setup
3.2.1 Overview of equipment for the different measurement techniques
3.2.2 Electromagnet and Kepco power supply
3.2.3 Contacting of the sample
3.2.4 Principle specifications of equipment
3.3 Experimental techniques
3.3.1 Measurement of DC R-H and R-I loops
3.3.2 Measurement of phase diagrams: off and on-pulse
3.3.3 Thermally-excited ferromagnetic resonance
4 Results and discussion
4.1 Switching phase diagrams of MTJs
4.1.1 Theory: Calculating the phase diagram
4.1.2 Experimental phase diagrams
4.2 Thermally excited ferromagnetic resonance
4.2.1 Smoothing and fitting of raw data
4.2.2 Determination of Ms
4.2.3 Signal evolution with bias voltage
4.2.4 Analysis of peak position: perpendicular STT
4.2.5 Analysis of peak linewidth
5 Summary and outlook
A Appendix
List of figures
List of tables
Bibliography
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:27640 |
| Date | 03 February 2014 |
| Creators | Bernert, Kerstin |
| Contributors | Cuniberti, Gianaurelio, Faßbender, Jürgen, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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