Open Access

Oana-Tereza Ciubotariu

• The present thesis focuses on the characterization of yttrium and thulium iron garnet thin films grown by pulsed laser deposition. The impact of various growth parameters (substrate type, film thickness, post-deposition annealing of amorphous or crystalline structures) on the morphologic, structural and magnetic properties is investigated. In the first part, the optimization steps carried out for the film growth are described. Chemical analysis reveals a strong dependence of the rare-earth/Fe ratio on the background oxygen pressure. Stoichiometric films are grown at low oxygen pressure, and increasing the chamber pressure during deposition results in rare-earth rich films. On the example of Tm3Fe5O12 films prepared on Gd3Sc2Ga3O12 substrates, it is shown that smooth epitaxial films can be achieved at moderate substrate temperatures (< 700 °C) and deposition rates below 0.04 nm/s. The cooling rate after deposition at elevated temperatures is observed to mainly influence the coerciveThe present thesis focuses on the characterization of yttrium and thulium iron garnet thin films grown by pulsed laser deposition. The impact of various growth parameters (substrate type, film thickness, post-deposition annealing of amorphous or crystalline structures) on the morphologic, structural and magnetic properties is investigated. In the first part, the optimization steps carried out for the film growth are described. Chemical analysis reveals a strong dependence of the rare-earth/Fe ratio on the background oxygen pressure. Stoichiometric films are grown at low oxygen pressure, and increasing the chamber pressure during deposition results in rare-earth rich films. On the example of Tm3Fe5O12 films prepared on Gd3Sc2Ga3O12 substrates, it is shown that smooth epitaxial films can be achieved at moderate substrate temperatures (< 700 °C) and deposition rates below 0.04 nm/s. The cooling rate after deposition at elevated temperatures is observed to mainly influence the coercive field. The following part focuses on the properties of Y3Fe5O12 (short YIG) thin films. Films grown on lattice-matching Gd3Ga5O12 substrates have an in-plane magnetic easy axis due to the dominant magnetic shape anisotropy. Substrate features such as terraces or sinusoidal structures are visible on the film surface for thicknesses up to 100 nm. Heat treatments on room temperaturedeposited YIG films carried out in ambient atmosphere were observed to lead to epitaxial films. Magnetic characterization indicates the film crystallization after annealing at 600 °C for 30 minutes. Additionally, films annealed at temperatures in the 600 °C ‒ 725 °C range possess similar properties as films grown at elevated temperatures: the garnet crystal structure, smooth surfaces characterized by roughness values of 0.1 nm, and coercive fields below 1 Oe. In the third part, Tm3Fe5O12 (TmIG) films are strained-grown on two substrate types: substituted-Gd3Ga5O12 (sGGG) and Gd3Sc2Ga3O12 (GSGG). The effect of film thickness on the structural and magnetic properties is investigated. Structural characterization reveals that the thin films are grown under out-of-plane compressive strain and increasing the film thickness results in the relaxation of the unit cell. The induced-magnetoelastic anisotropy is dominant below a critical film thickness and the films have an out-of-plane magnetic easy axis. The loss in strain with increasing film thickness results in the rotation of the easy axis of magnetization towards the film plane. The bulk TmIG ‒ substrate lattice mismatch influences the relaxation thickness: TmIG(70 nm)/sGGG shows perpendicular magnetic anisotropy, while an in-plane magnetic easy axis is reported for TmIG(37 nm)/GSGG. Additionally, the Gilbert damping parameter of TmIG(20 ‒ 300 nm)/sGGG is found to be around 0.02. Furthermore, the effect of post-deposition annealing of strained TmIG films is investigated. A heat treatment in ambient atmosphere at 600 °C for 30 minutes has a minor effect on the morphological, structural and magnetic properties of a 14 nm thick TmIG film. The crystallization of amorphous TmIG films via heat treatment carried out in ambient atmosphere is initiated around 670 °C. However, 15 ‒ 20 nm thick films show cracks and isotropic magnetic properties after annealing at this temperature. The topic of the last part is the growth of YIG films on strained TmIG layers. For this, 5 ‒ 20 nm thick YIG films were deposited at room temperature on 15 nm thick TmIG films, followed by a heat treatment at 600 °C for 30 minutes. Structural characterization carried out before and after the heat treatment confirms the crystallization of the YIG film at 600 °C. After annealing, no thulium is observed in the chemical analysis of the film surface indicating no or very weak intermixing between the YIG and TmIG films. Furthermore, magnetic characterization reveals that 5 ‒ 20 nm YIG films grown on strained TmIG films possess an out-of-plane easy axis of magnetization.…