Confined magnon modes and anisotropic exchange interaction in ultrathin Co films : dissertation zur erlangung des doktorgrades der naturwissenschaften (Dr. rer. nat.) der Naturwissenschaftlichen Fakultät II Chemie, Physik und Mathematik der Martin-Luther

Long description: A fundamental need in wireless communication and modern computer processors is to fabricate faster, smaller, and lower power-consumption circuits. A promising approach is the utilization of spin waves, or rather, magnons in ferromagnetic films. Quantum confinement in ultra-thin films permits the coexistence of several exchange-dominated magnon modes with terahertz-range frequencies and sub-nanometer length scales. By means of spin-polarized electron energy loss spectroscopy (SPEELS), these exchange-dominated terahertz magnons are directly probed in ultra-thin cobalt films on Ir(001), Cu(001) and Pt(111) single crystal surfaces. The dispersion relation of the quantized magnon modes depends particularly on the interatomic exchange interaction in individual layers. By tuning the exchange interaction, modes with opposite group velocities, and thus an opposite propagation direction of the wave packets, can be generated. Ab initio theoretical calculations as well as an analytical Heisenberg model reveal a spatial localization of the modes at the surface, interior, and interface of the film that opens an experimental access to the layer-dependent magnetic properties. In itinerant ferromagnets like cobalt the magnetic properties depend sensitively on many-body correlation effects in the electronic structure. Here, it is shown for the first time that spin-dependent correlations lead to a pronounced renormalization of the energy of the highest magnon mode by up to 260 meV, explaining the significant overestimation of theoretically predicted magnon energies and interatomic exchange interaction found in literature 1 Introduction 7 2 Background 13 2.1 Magnons in localized moment systems 13 2.1.1 The quasi-classical counterpart of a magnon 13 2.1.2 A magnon in quantum mechanics 16 2.1.3 Magnons in thin ferromagnetic films 18 2.2 Stoner excitations in itinerant-electron ferromagnets 22 2.3 Electron scattering 24 2.3.1 Low-energy inelastic electron scattering 24 2.3.2 Spin-dependent effects in elastic electron scattering 25 3 Experimental techniques 29 3.1 Spin-polarized electron energy loss spectroscopy 29 3.1.1 Spin polarized inelastic electron scattering 29 3.1.2 Principles of SPEELS 31 3.2 The experimental setup 33 3.2.1 The ultra high vacuum system 33 3.2.2 SPEEL-spectrometer 34 3.2.3 The GaAs-photocathode 36 4 Results 39 4.1 The Co/Ir(001) system 39 4.1.1 Sample preparation and characterization 39 4.1.2 MOKE measurements 42 4.1.3 SPEELS measurements 46 4.2 The Co/Cu(001) system 55 4.2.1 SPEELS measurements 55 4.3 The Co/Pt(111) system 59 4.3.1 Sample preparation and characterization 59 4.3.2 MOKE measurements 60 4.3.3 SPEELS measurements 61 5 Discussion 69 5.1 Film structure and substrate dependence 69 5.1.1 "Acoustic" magnons in Co and Fe films 69 5.1.2 Confined magnon modes in the 3 ML Co films 73 5.2 Confined magnon modes in the Heisenberg model 77 5.3 Layer-dependent exchange parameters 82 5.4 Impact of the electronic structure on the magnons 91 5.5 The magnon lifetime 95 5.5.1 "Acoustic" magnons in Co and Fe films 95 5.5.2 Confined magnon modes 98 6 Conclusions and outlook 101 A Description of spin-polarized electrons 105 Magnetic Excitations,Magnons,Spin Waves,Exchange Interaction,Spin-Polarized Electron Energy Loss Spectroscopy A fundamental need in wireless communication and modern computer processors is to fabricate faster, smaller, and lower power-consumption circuits. A promising approach is the utilization of spin waves, or rather, magnons in ferromagnetic films. Quantum confinement in ultra-thin films permits the coexistence of several exchange-dominated magnon modes with terahertz-range frequencies and sub-nanometer length scales. By means of spin-polarized electron energy loss spectroscopy (SPEELS), these exchange-dominated terahertz magnons are directly probed in ultra-thin cobalt films on Ir(001), Cu(001) and Pt(111) single crystal surfaces.