Magnetization Dynamics in Quantum Material/Ferromagnet Heterostructures

Mukhopadhyay, Suchetana (2025) Magnetization Dynamics in Quantum Material/Ferromagnet Heterostructures. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Suchetana Mukhopadhyay (20RS143)) 20RS143.pdf - Submitted Version Restricted to Repository staff only Download (10MB)

Abstract

In this thesis, the femtosecond laser-induced ultrafast spin dynamics and nanosecond magnetization precessional dynamics have been studied in various quantum material/ferromagnet (FM) heterostructures, with a view to utilizing their unique material properties to achieve controllable manipulation of the dynamical magnetic phenomena at both timescales. Chapter 1 provides an introduction to the fields of spin-orbitronics and ultrafast magnetism and touches on the niche offered by quantum materials in spin-orbitronic applications. Chapter 2 provides a short background on femtosecond laser-induced magnetization dynamics followed by a brief introduction to TIs and graphene, the material systems that we consider in the later chapters. In Chapter 3 we describe the methodologies we have used in this thesis. In Chapter 4, before delving into the magnetization dynamics of FM-based spin-orbitronic heterostructures, we investigate the intercorrelation between ultrafast demagnetization and damped magnetization precession as a function of laser excitation fluence in FM thin films of cobalt (Co), nickel (Ni), and permalloy (Py, Ni₈₀Fe₂₀) deposited under identical conditions using all-optical time-resolved magneto-optical Kerr effect (TRMOKE) measurements. Numerical modeling of demagnetization dynamics suggests that fluence-dependent electron-lattice and electron-spin coupling parameters reflect the influence of nonthermal electrons on magnetization dynamics at low laser fluences. We confirm that the Curie temperature to magnetic moment ratio acts as a figure of merit for the demagnetization time, show that the demagnetization times and damping factors are directly correlated as a function of laser fluence, and that they present an apparent sensitivity to the density of states at the Fermi level for a given system. In Chapter 5, using all-optical TRMOKE magnetometry, we demonstrate strong spin pumping in BiSbTe₁.₅Se₁.₅(BSTS)/Co₂₀Fe₆₀B₂₀(CoFeB) heterostructures at room temperature. By studying the BSTS- and CoFeB-thickness-dependence of Gilbert damping, the spin diffusion length in BSTS and the interfacial spin mixing conductances are determined. We show that for BSTS thicknesses much greater than the spin diffusion length, in the so-called "perfect spin sink" regime, ultrahigh interfacial spin transparencies close to 0.9 can be obtained, promoting the system as an exceptionally attractive candidate for topological spin-orbitronic applications. In Chapter 6 we describe a follow-up work, in which we demonstrate laser excitation fluence-dependent tunability of ultrafast demagnetization and spin pumping in BSTS/CoFeB thin films. An inverse correlation of the demagnetization time and Gilbert damping is found in BSTS/CoFeB due to interfacial spin accumulation-driven pure spin current transport. The fluence-dependent enhancement of damping modulation at the BSTS/CoFeB interface surpasses that arising from interfacial two-magnon scattering, consistent with enhanced spin pumping at higher pump fluences and a factor-of-two enhancement of spin mixing conductance. In Chapter 7, we study the temperature dependence of spin pumping in a BSTS/CoFeB bilayer using ferromagnetic resonance (FMR) spectroscopy to explore the role of the topological surface states (TSS) in spin detection. A pronounced temperature-dependent change in both the Gilbert damping and the resonance condition is observed, closely corresponding to the emergence of the TSS-dominated transport regime. In this low-temperature window, both damping and resonance condition show substantial changes indicating enhanced efficiency of the dynamical exchange-driven spin pumping, manifesting in a twofold increase of the spin mixing conductance from its room temperature value. In Chapter 8, we investigate the spin pumping phenomenon in heterostructures of indium (In)-doped (Bi₀.₃Sb₀.₇)₂]Te₃ (IBST) TI with amorphous ferromagnetic CoFeB in a spin battery structure. Upon doping the ternary TI (Bi₀.₃Sb₀.₇)₂]Te₃ with indium, weakened bulk spin-orbit coupling and finite-size effects modify low-temperature transport to generate a trivial insulating phase at the highest doping level. Enhanced spin pumping and ultrafast demagnetization rate are observed in the presence of TSS, confirming their role as an additional spin relaxation channel. These findings underscore nonmagnetic doping-induced structural phase transition as a viable measure for disentangling surface and bulk spin transport channels and further enable tunability of ultrafast spin- and magnetization dynamics for topological spin-orbitronic applications. In Chapter 9, we use TiOx barrier layers (BLs) to structurally modify the interfacial spin conductance and further isolate the spin transport and magnetic proximity effects in a single layer graphene/FM system where no functional relationship of the defect density to the BL thickness is obtained. TRMOKE measurements are used to demonstrate significant tunability of ultrafast demagnetization time and Gilbert damping, by varying the BL thickness. We show that dielectric BLs of suitable thickness, can eliminate interfacial modification of graphene by FM metals while allowing efficient pure spin current transport and enable substantial control over spin angular momentum dissipation in graphene/FM thin-film heterostructures. Finally, in Chapter 10, we summarize how we have addressed the specific research problems included in this thesis, before concluding with some future perspectives.

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