Spin Valve Systems for Angle Sensor Applications vom Fachbereich Material- und Geowissenschaften der Technischen Universität Darmstadt genehmigte Dissertation zur Erlangung des akademischen Grades eines Doktors der Ingenieurswissenschaften (Dr.-Ing.) eingereicht von M. Sc. Andrew Johnson aus Cincinnati, Ohio USA Referent: Prof. Dr.-Ing. Horst Hahn Korreferent: Prof. Dr.-Ing. Hartmut Fuess Tag der Einreichung: 25. Juli 2003 Tag der mündlichen Prüfung: 20. Januar 2004 Darmstadt 2004 D17 I Table of Contents 1 Introduction 1 2 Theory 5 2.1 Electrical Resistance and Magnetoresistance ......................................................5 2.2 Anisotropic Magneto Resistance (AMR).............................................................5 2.3 Giant Magneto Resistance (GMR).......................................................................7 2.4 Spin-Dependent Scattering ................................................................................10 2.4.1 Co/Cu Bandgap Structure........................................................................10 2.4.2 Mott Two-Current Model ........................................................................12 2.5 Spin Valve System.............................................................................................13 2.5.1 Magnetoresistance Characteristics of a Spin Valve.................................14 2.5.2 Uniaxial Anisotropy in the Free Layer and Pinned Layer.......................16 2.5.3 Interlayer Coupling in a Spin Valve System ...........................................18 2.5.4 Unidirectional Anisotropy: Exchange Bias .............................................19 2.5.5 Spin Valve Systems: Standard Materials and Microstructure. ................21 2.6 Exchange Bias Models.......................................................................................23 2.6.1 Ideal Interface Model...............................................................................24 2.6.2 Partial Domain Wall Model.....................................................................25 2.6.3 Random-Field Model...............................................................................26 2.6.4 Domain State Model................................................................................27 2.7 Synthetic Anti-Ferromagnet (SAF)....................................................................28 2.8 GMR 360° Angle Sensor ...................................................................................31 2.8.1 Design of GMR 360° Angle Sensor ........................................................31 2.8.2 Previous Research on GMR Angle Sensors ............................................33 2.8.3 Advantages of a GMR 360° Angle Sensor..............................................34 3 Experimental Methods 35 3.1 Sputter Deposition..............................................................................................35 3.1.1 The Sputtering Process ............................................................................35 3.1.2 Magnetron Sputtering ..............................................................................36 3.1.3 Unaxis Cyberite Sputtering System.........................................................37 3.1.4 Spin Valve Deposition Conditions ..........................................................39 3.2 Excimer-Laser....................................................................................................40 3.2.1 Theory and Basic Design of a Laser........................................................40 3.2.2 Excimer Lasers ........................................................................................41 4 Characterization Methods 45 4.1 Magnetoresistance measurements......................................................................45 4.1.1 Four-Point Probe......................................................................................45 II 4.1.2 Measurement Setup .................................................................................46 4.1.3 Analysis of the MR Rotation Curve ........................................................47 4.2 Structural Characterization ................................................................................48 4.2.1 X-Ray Diffraction....................................................................................48 4.2.2 X-Ray Reflectometry...............................................................................50 4.2.3 Auger Electron Spectroscopy ..................................................................51 4.3 Magnetic Characterization .................................................................................52 4.3.1 Alternating Gradient Magnetometer........................................................52 4.3.2 Magnetic Optical Kerr Effect ..................................................................53 5 Stoner-Wohlfarth Model: Spin Valve Systems 55 5.1 Applied Field Influence on Ferromagnetic Thin Film.......................................55 5.2 Model Description: Simple Spin Valve .............................................................55 5.3 Model for Spin Valve with SAF ........................................................................57 6 Results and Discussion 59 6.1 Cosine Dependence: Deviation Factors .............................................................61 6.1.1 AMR Effect .............................................................................................61 6.1.2 Interlayer Coupling..................................................................................65 6.1.3 Rotation of the Pinned Layer Magnetization..........................................70 6.1.4 Overview of the Cosine Deviation Factors..............................................73 6.1.5 Simulation of MR Rotation Curves.........................................................74 6.1.6 Cosine Deviation Factors: Implications for a 360° Angle Sensor...........80 6.1.7 Summary: Cosine Deviation Factors.......................................................82 6.2 Selection of Spin Valve System.........................................................................83 6.2.1 NiO Spin Valve System...........................................................................83 6.2.2 FeMn Spin Valve System .......................................................................86 6.2.3 IrMn Spin Valve System .........................................................................89 6.2.4 PtMn Spin Valve System.........................................................................91 6.2.5 PtMn Spin Valve System with SAF ........................................................94 6.2.6 Comparison Between the Different Spin Valve Systems......................101 6.2.7 Summary: Selection of Spin Valve System...........................................103 6.3 Multiple Deposition: Lift-off Method..............................................................104 6.3.1 Description of the Method.....................................................................104 6.3.2 Test of the Lift-off Method....................................................................104 6.3.3 Summary: Lift-off Method ...................................................................105 6.4 Ion Irradiation Method.....................................................................................106 6.4.1 Ion Irradiation of FeMn and IrMn Spin Valves.....................................106 6.4.2 Ion Irradiation of a Patterned Spin Valve Sample.................................108 6.4.3 Summary: Ion Irradiation Method........................................................110 6.5 Laser-writing Method ......................................................................................111 6.5.1 Laser Writing of FeMn and IrMn Simple Spin Valves .........................111 6.5.2 Reorientation Point: Gradual Change in Bias Direction .......................117 6.5.3 Complete Loss of GMR Effect and Exchange Bias .............................119 6.5.4 Source of the GMR Effect Reduction....................................................123 6.5.5 Domain State Model: Stability of the Exchange Bias Effect ................129 6.5.6 Laser Writing Experiments: PtMn Spin Valve with SAF .....................131 6.5.7 Antiferromagnetic Interlayer Coupling and the Reorientation Process.135 6.5.8 Induced Uniaxial Anisotropy of PtMn and the Reorientation Process..137 6.5.9 Induced Uniaxial Anisotropy: FeMn and IrMn Antiferromagnets........139 6.5.10Summary: Laser-Writing Method.........................................................141 7 Demonstrator of a 360° GMR Angle Sensor 143 III 7.1 Design and Fabrication ....................................................................................144 7.2 Demonstrator in Operation...............................................................................145 8 Summary of Conclusions 147 8.1 Summary ..........................................................................................................147 8.2 Future Work .....................................................................................................149 9 Zusammenfassung und Ausblick 151 9.1 Zusammenfassung............................................................................................151 9.2 Zukünftige Fragestellungen und Ausblick.......................................................153 10 Appendices 155 11 Citations 159 1 1Introduction Research on the Giant Magneto-Resistance (GMR) effect, large resistance changes due to an applied magnetic field, began