Doctor of Philosophy
Prof. Michael G. Cottam
Motivated by recent experimental developments, we present a theoretical study of some linear and nonlinear properties of spin waves in ferromagnetic nanostructures under conditions of microwave pumping. A microscopic (or Hamiltonian-based) approach is followed including terms for both the short-range exchange and the long-range dipole-dipole interactions, as well as the effects of an external magnetic ﬁeld, single-ion anisotropy, biquadratic exchange and the Ruderman-KittelKasuya-Yosida interactions, as appropriate. In ultrathin ﬁlms and nanowires with thickness or lateral dimensions less than about 100 nm, the discreteness of the quantized spin waves (or magnons) and their spatial distributions become modiﬁed, making it appropriate to employ a microscopic approach to the nonlinear dynamics based on a lattice model. First an application is made to the magnetization dynamics in trilayer nanowires, consisting of Permalloy layers separated by a nonmagnetic Ru spacer layer. The experimental results for different stripe widths are successfully analyzed using our microscopic theory. Next we investigate the nonlinear spin-wave properties of ultrathin ﬁlms and ferromagnetic nanowires with rectangular cross sections as well as nanowires and nanotubes with circular cross sections. Numerical results are deduced for the spin-wave frequencies as a function of wave vector, for either parallel or perpendicular external ﬁeld orientations. Also numerical applications are presented for the dependence of the threshold microwave ﬁeld amplitudes for instability on the static applied ﬁeld (the analog of the butterﬂy curves). Effects due to the discreteness of the spin-wave branches lead to structural features in the butterﬂy curves that are signiﬁcantly different, due to edge effects, in nanowires compared with those in the bulk and for ultrathin ﬁlms. Further, we examine the instability thresholds for nonlinear processes in ferromagnetic stripes and ﬁlms under perpendicular pumping with an microwave ﬁeld. Finally the quantum statistical properties are investigated for microwave-driven magnon system in the presence of four-magnon interactions. In particular, collapse-and-revival-phenomena for the time evolution of the average magnon number, squeezing, and super-Poissonian statistics of magnons are studied. Bulk systems are analyzed initially, and then the results are generalized to consider cross correlation and other behavior in two-mode magnon systems.
Haghshenasfard, Zahra, "Linear and Nonlinear Dynamics of Spin Waves in Ferromagnetic Nanowires" (2017). Electronic Thesis and Dissertation Repository. 4467.