Magneto-optical characterization of ultrathin ferrimagnetic insulator bilayers

Abstract

The demands for information storage and processing technologies continue to grow. We continue to press on for higher stability and density memory, and faster computation rates. We are even moving beyond classical computing models. Magnetic devices are prized for information storage owing to the stability of magnetization states in the absence of driving power and over many on-off cycles. Magnetic storage, however, has not been able to scale as quickly as other computing technologies. Devices such as hard disk drives tend to be one of the slowest parts of a computer because they require mechanical components and because they can only read/write from a single head. In the field of spintronics, researchers are exploring different alternative forms of storage technologies such as racetrack and skyrmion memory. These new memory devices allow for all-electrical reading and writing of bits in magnetic materials. These memory devices were first created using metallic ferromagnet films. The use of ferrimagnetic insulators as a platform for chiral domain wall motions and skyrmions are recent developments. Magnetic insulators have the potential to reduce the amount of charge current present in a working device and therefore reduce the amount of energy loss through mechanisms such as Joule heating. These materials also have several other useful characteristics such as smaller magnetizations, high tunability of anisotropy, and the potential for higher than GHz computing frequencies. However, studying ferrimagnetic thin films at the ultrathin limit necessary for room temperature skyrmion creation presents outstanding challenges in the characterization and optimization of these films. This thesis focuses on combining electrical transport and magneto-optical experimental methods in the study of thulium iron garnet (TmIG) ultrathin film systems. Detecting the presence of skyrmions in ultrathin magnetic insulating films is challenging. A standard electrical-read out method relies in the spin Hall-topological Hall effect. A heavy metal thin film, such as Pt, is deposited on the TmIG thin film and through the spin-Hall and inverse spin-Hall effects, we can detect changes to the transverse resistivity in this film generated by what is correlated to spin textures in TmIG. By using an ultra-high sensitivity Sagnac interferometer, we are able to measure the hysteresis curve for these ultrathin Pt/TmIG bilayers. The functional form of the hysteresis takes the same form as the spin-Hall anomalous Hall effect. By combining the optically determined spin Hall-anomalous Hall function with the Hall resistivity data, the anomalous contribution can be removed. For the first time, a fully quantified, room temperature spin Hall-topological Hall resistivity is presented for Pt/TmIG bilayers. A phenomenological expression for estimating skyrmion densities using the spin Hall-topological Hall resistivity is presented. The estimated skyrmion density suggests that the skyrmions in this system are on the order of 10s of nanometers in diameter. We also present work on the magnetic anisotropy of the ultrathin films as a function of magnetic layer thickness and show the large effect of the Pt layer induced interfacial anisotropy. This is necessary because one of the main parameters used to engineer a skyrmion phase is magnetic anisotropy. This work was performed by measuring three thickness of TmIG films with and without Pt, showing the anisotropy evolution. For the thicker films, in-plane hysteresis is measured using the polar MOKE configuration. The in-plane field sweeps indicate that the magnetization rotation is coherent and gives some information about in-plane saturation fields. These measurements show that without Pt, the TmIG films continue to exhibit perpendicular magnetic anisotropy to the thinnest film thickness. The interfacial anisotropy from Pt is shown to be comparable in magnitude to the volumetric anisotropy of the thickest film and produces an easy-plane anisotropy in the thinnest film. These results are discussed and estimates of the interfacial anisotropy magnitude are presented. Further avenues of study for the anisotropy are discussed with the goal of completely understanding the nature of the anisotropy for films exhibiting skyrmion phases at room temperature.Physic