| dc.description.abstract | Spin waves are the collective wave excitations in the magnetically ordered system, which have the frequencies typically ranged from GHz up to even THz. In recent years, the study of spin waves, which is referred to as “magnonics”, has been significantly advanced; and the low-damping coherent spin waves are considered as a suitable candidate for performing rapid data processing and wave computing. The scalability of such spin-wave based computing devices is rather promising due to the possibilities of exciting spin waves with wavelengths down to the nanometer range. Magnonic crystals are various forms of spatial modulation of magnetic properties that can be seen as magnetic metamaterials. The magnonic crystals, as a widely used approach to tailor the spin-wave band structure and an effective way to control the spin-wave propagation, have been studied extensively. In this dissertation, we explore the possibilities of utilizing various kinds of magnonic crystals for the controllable spin-wave dynamics in different magnetic systems.
We first develop a description of spin waves in a 3D quantum XY antiferromagnet (AFM) in terms of macroscopic variables, magnetization and Néel vector densities. We consider a layered AFM with spins located on the honeycomb lattice. We show that, in the discussed system, the spectrum of spin waves consists of four modes, all well captured by our macroscopic description. The gapless mode of the spin waves, i.e., magnons, is described by a system of equations, which has a structure general for the Goldstone mode in AFMs. We demonstrate that the parameters in the spin Hamiltonian can be evaluated by fitting the experimental data with the results obtained for the four modes using the macroscopic variable approach. The description of AFM in terms of macroscopic variables can be easily extended to the case when the lattice of the magnetic substance is deformed by an external strain or acoustic wave.
Next, we study the spin-wave dynamics in such a layered AFM in the presence of a periodic lattice deformation. We suggest to use spatially modulated strain (a type of magnonic crystals) for the control of a spin wave propagating inside a bulk AFM. The modulation with the wave vector q, by virtue of magnetoelasticity, mixes spin waves with wave vectors near q/2 and −q/2. This leads to lifting the degeneracy of the symmetric and antisymmetric eigenstate combinations of these waves. Therefore, a moving spin wave being subjected to the lattice modulation after some time alters its propagation direction to the opposite one, and so on. The resulting picture reminds one of a tunneling particle in a symmetric double-well potential. The effect can be utilized for the control of the spin-wave propagation that can be useful for magnonic applications. The control may include a delay line element, filtering, and waveguide of the spin waves in AFM.
For a ferromagnet (FM), we investigate its spin-wave dynamics under a switchable current-induced magnonic crystal. In this case, we consider a ferromagnetic (FM) sample with a metallic meander pattern (whose spatial modulation is described by a wave vector q) fabricated on its top surface. The magnonic crystal will be switched on and off by applying a current to the meander structure. For a conventional magnonic crystal with direct current (DC) supply, the spin waves around q/2 are resonantly coupled to the waves near −q/2, and similar to the periodically deformed AFM, a band gap is opened at k = ±q/2. We further demonstrate that if instead of the DC current the magnonic crystal is supplied with an alternating current (AC), then the band gap is shifted to k satisfying |ωs(k) −ωs(k − q)| = ωac; here ωs(k) is the dispersion of the spin wave, while ωac is the frequency of the AC modulation. The resulting gap in the case of the AC magnonic crystal is the half of the one caused by the DC with the same amplitude of modulation. The time evolution of the resonantly coupled spin waves controlled by properly suited AC pulses can be well interpreted as the motion on a Bloch sphere. The tunability of the AC magnonic crystal broadens the perspective of spin-wave computing. | |
