Magnetic anisotropy control in rare earth iron garnet thin films for spintronic devices and all-optical ultrafast manipulation of magnetization

Spin-based memory and logic devices might provide a promising route for fast, nonvolatile and power-efficient operation using magnetic insulators with minimal Joule heating and ultrafast spin dynamics, which can be controlled all optically. There are very few magnetic insulator iron garnets, whose magnetic properties cannot be tuned easily and limit spintronic device applications. Thus, new iron garnets with perpendicular magnetic anisotropy and low saturation fields are needed for ultra-low power spintronics. In this thesis, we present theory, modelling and experimental studies on controlling magnetic anisotropy in epitaxial iron garnet thin films and all-optical control of magnetization dynamics in quantum-confined metallic nanolayers. First, we developed an anisotropy model for describing the magnetic anisotropy characteristics of insulating rare earth iron garnet (REIG) films (Re3Fe5O12, Re; rare-earth ions Y, Tm, Dy, Ho, Er, Yb, Tb, Gd, Sm, Eu). We construct an effective anisotropy energy using shape, magnetocrystalline, and magnetoelastic terms for ten different REIG films grown epitaxially and lattice-matched on five commercially-available garnet substrates. We calculated their magnetic easy axes and predict that 20 different pairs out of 50 to possess out-of-plane magnetic easy axis (PMA). Only 7 of them were experimentally tested and confirmed. We predict that the magnetic saturation fields of PMA garnets could span two orders of magnitude (300 Oe to 12.6 T), significantly expanding the available PMA garnet class. To test our predictions, we grew 420 and 67 nm-thick Holmium iron garnet films on Gd3Ga5O12 and Tb3Ga5O12 substrates using pulsed laser deposition at 800°C and 650°C. X-ray diffraction and magnetic hysteresis loop measurements indicate phase purity and PMA, respectively, for 67 nm HoIG, confirming our prediction. Using a modified microscopic three temperature model, we investigate the effect of laser pulse parameters and magnetic elemental metal thin film properties on the femto- and picosecond magnetization dynamics. We model the coupled energy transfer between electrons, phonons and spin baths. A magnetization quenching (in sub-200 fs) and recovery (a few ps) was found for metals with high Curie temperature (Fe, Co, and Ni). In the quantum-confined thickness regime (t < 50 Å), spin-phonon scattering in magnetic metals are significantly reduced due to the reduced density of states. Thus, THz spin wave emission might become feasible with three orders of magnitude lower laser fluence compared with the previously reported experimental values. Our models and experiments could expand the available PMA iron garnets with minimal Joule heating for spintronics and help control ultrafast spin dynamics much more efficiently.