FERROMAGNETIC COBALT-IRON THIN FILMS FOR LIGHT INDUCED MAGNONICS APPLICATIONS

Advances in electronics are hindered by energy dissipation from Joule losses associated with charge transport. In contrast, the processing of information based on spin waves propagation (magnons) in magnetic materials is free from such losses. For magnonic devices, materials with ultralow magnetic damping are required to ensure long propagation lengths of magnons. Ferrimagnetic Y₃Fe₅O₁₂ garnets (YIG) exhibit the lowest magnetic damping of all known materials. However, the lowest damping constant YIG materials require epitaxial growth on single crystal substrates of Gd₃Ga₅O₁₂ at elevated temperatures (900℃), hindering their CMOS integration in electronic devices. In the search for alternative material systems, polycrystalline ferromagnetic Co₂₅Fe₇₅ alloy films and ferrimagnetic spinel ferrites, such as MgAl_0.5Fe_1.5O₄ (MAFO), have emerged as potential candidates. The magnetic damping in these materials is comparable, although it is at least one order of magnitude higher than YIG’s. However, Co₂₅Fe₇₅ alloy thin film growth is CMOS compatible, and its magnon diffusion length is 20x longer than in MAFO. In addition, MAFO requires epitaxial growth on lattice-matched MgAl₂O₄ substrates. The work in this dissertation focuses on growth and characterization of Co_1-xFe_x ferromagnetic binary alloys and is considered as a material of choice for practical magnonic applications.

In this project, a narrow composition range of Co_1-xFe_x alloy thin films around 25 % Co have been fabricated and characterized to reveal unique trends in the Gilbert damping as the Co composition of the alloy is changed. Microstructural analysis of Co₃₆Fe₆₄ using STEM, revealed that the Cu interdiffusion takes place from the Cu buffer layer into the magnetic layer. This interdiffusion was found to be up to 7x higher at grain boundaries than in the bulk of the grains of the polycrystalline material. The presence of Cu negatively influences magnetic damping, as Cu modifies the magnetic exchange interactions of spins in the thin film, in particular at grain boundaries which is important for efficient magnon propagation. It is noted that Co₂₅Fe₇₅ epitaxially grown on single crystal MgO yielded magnetic damping parameters of 7.1 × 10^-4 [1]. This improvement is ascribed to the stronger ionic bonding in MgO vs metallic bonding in Cu, thereby preventing interdiffusion of either Mg or Oxygen into the CoFe thin film.

In addition, in this work, COMSOL optical modeling was conducted to investigate the generation of opto-magnetic fields driven by fs laser pulses in 30 nm diameter magneto-plasmonic resonators consisting of Au as the plasmonic material and Co₂₅Fe₇₅ as the ferromagnet. These nanopillars are proposed as magnon injection sources into CoFe waveguide thin films. An enhancement of the electric field at the waveguide interface generates ultrafast optomagnetic fields. Additionally, a prototype design of a magneto-plasmonic platform is proposed that provides dynamic magnon amplification and propagation re-configurability employing arrays of magneto plasmonic resonators independently addressed by fs laser pulses.

[1] Lee, A.J.; Brangham, J.T.; Cheng, Y.; White, S.P.; Ruane, W.T.; Esser, B.D.; McComb, D.W.; Hammel, P.C.; Yang, F. Metallic Ferromagnetic Films with Magnetic Damping under 1.4 × 10−3. Nat. Commun. 2017, 8, 234. https://doi.org/10.1038/s41467-017-00332-x.