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Tailored Magnetic Properties of Exchange-Spring and Ultra Hard Nanomagnets

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Tailored Magnetic Properties of Exchange-Spring and Ultra Hard Nanomagnets Max-Planck-Institute für Intelligente Systeme Stuttgart Tailored Magnetic Properties of Exchange-Spring and Ultra Hard Nanomagnets Kwanghyo Son Dissertation An der Universität Stuttgart 2017 Tailored Magnetic Properties of Exchange-Spring and Ultra Hard Nanomagnets Von der Fakultät Mathematik und Physik der Universität Stuttgart zur Erlangung der Würde eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung Vorgelegt von Kwanghyo Son aus Seoul, SüdKorea Hauptberichter: Prof. Dr. Gisela Schütz Mitberichter: Prof. Dr. Sebastian Loth Tag der mündlichen Prüfung: 04. Oktober 2017 Max‐Planck‐Institut für Intelligente Systeme, Stuttgart 2017 II III Contents Contents ..................................................................................................................................... 1 Chapter 1 ................................................................................................................................... 1 General Introduction ......................................................................................................... 1 Structure of the thesis ....................................................................................................... 3 Chapter 2 ................................................................................................................................... 5 Basic of Magnetism .......................................................................................................... 5 2.1 The Origin of Magnetism ........................................................................................ 5 2.1.1 Magnetic moment ................................................................................................................................. 5 2.1.2 Magnetization and Field ........................................................................................................................ 6 2.2 The Classes of Magnetic Materials ......................................................................... 7 2.2.1 Ferromagnetism .................................................................................................................................... 7 2.2.2 Antiferromagnetism and Ferrimagnetism ............................................................................................. 9 2.2.3 Paramagnetism .................................................................................................................................... 10 2.2.4 Diamagnetism ..................................................................................................................................... 11 2.3 Basics of Ferromagnetic Hysteresis loop .............................................................. 12 2.3.1 The Stoner-Wohlfarth (S-W) model ................................................................................................... 14 2.3.2 Preisach model and First Order Reversal Curves ................................................................................ 15 2.4 Micromagnetism .................................................................................................... 17 2.4.1 Magnetic free energy .......................................................................................................................... 17 2.4.2 Domain and Domain Walls ................................................................................................................. 19 2.4.3 Single- and Multi-Domain particles .................................................................................................... 20 2.4.4 Coercivity and Microstructural parameters ......................................................................................... 22 2.5 X-ray and Magnetism ............................................................................................ 24 2.5.1 X-ray Absorption Spectroscopy (XAS) .............................................................................................. 24 2.5.2 X-ray Magnetic Circular Dichroism (XMCD) .................................................................................... 26 2.5.3 Sum rules ............................................................................................................................................ 28 IV Chapter 3 ................................................................................................................................. 33 Basic of Nanosized Exchange Spring Magnets .............................................................. 33 3.1 Nanomagnets ......................................................................................................... 33 3.2 Exchange-Spring magnet ...................................................................................... 36 3.2.1 Soft and Hard Ferromagnetic materials ............................................................................................... 37 3.2.2 Exchange-coupling effect ................................................................................................................... 39 3.2.2.1 Critical size for exchange-coupling effect ..................................................................................... 40 3.3 L10-FePt / Co composition ES magnet .................................................................. 41 3.3.1 Phase diagram and Crystal Structures of FePt and Cobalt .................................................................. 42 3.3.1.1 FePt ............................................................................................................................................... 42 3.3.1.2 Cobalt ............................................................................................................................................ 44 Chapter 4 ................................................................................................................................. 45 Experimental methods: Fabrication & Characterization................................................. 45 4.1 Sample preparation ................................................................................................ 45 4.1.1 Sputter deposition of thin films (Magnetron Sputtering) .................................................................... 45 4.1.2 Manufacture of regular nanopatterns .................................................................................................. 46 4.1.3 Structure and microstructure characterization method ........................................................................ 49 4.1.3.1 X-ray diffraction (XRD) ............................................................................................................... 49 4.1.3.2 Atomic / Magnetic force microscopy (AFM, MFM)..................................................................... 50 4.1.3.3 Scanning electron microscopy (SEM) ........................................................................................... 51 4.1.3.4 Wavelength-dispersive X-ray analysis (WDX_EPMA) ................................................................ 51 4.1.3.5 Transmission electron microscopy (TEM) .................................................................................... 51 4.2 Characterization of magnetic properties via SQUID............................................. 53 4.2.1 SQUID magnetometry ........................................................................................................................ 53 4.2.1.1 Correction of Diamagnetism ......................................................................................................... 54 4.2.2 Magnetic parameters ........................................................................................................................... 54 4.2.2.1 Saturation polarization JS .............................................................................................................. 54 4.2.2.2 Anisotropy constant K1 ................................................................................................................. 55 4.2.2.3 Exchange stiffness constant A ....................................................................................................... 56 4.2.2.4 Microstructural parameter, α, and Neff ........................................................................................... 57 4.2.3 First order reverse curves (FORC) measurement ................................................................................ 58 4.2.3.1 FORC density analysis .................................................................................................................. 58 4.3 Characterization of magnetic systems via XMCD ................................................ 60 4.3.1 Synchrotron Radiation - WERA beamline at ANKA .......................................................................... 60 4.3.1.1 Total Electron Yield (TEY) mode ................................................................................................. 63 4.3.1.2 Correction of Saturation effect _self-absorption ........................................................................... 64 V Chapter 5 ................................................................................................................................. 67 Results: Artificially structured exchange-spring nanomagnets ...................................... 67 5.1 L10-FePt / Co nanopatterned samples ................................................................... 68 5.1.1 Characterization of the thin film structure..........................................................................................
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