Dynamics Of Magnetic Materials In Microwave And Tearhertz Frequency Regime

The dynamics of magnetic materials in microwave and terahertz range have beenstudied and utilized for dacades. Most of attentions were focused on the insulatingmagnetic materials. Recently, due to the development of magnetic photonic crystals,negative refractive index materials and ultrafast laser pulse techniques, there have beenincreasing interests in the microwave guiding system such as magnetic surface plasmawaveguide and split ring resonator. On the other hand, the demands for theever-increasing speed of data storage in magnetic media have triggered intense searchesfor innovative methods to control magnetization, ultrafast magnetization switching bypulsed laser excitation can be viewed as one of the prospective technique.We experimentally studied magnetically controllable photonic band gaps (PBGs)in two-dimensional magnetic photonic crystals (MPCs) consisting of ferrite rods.Besides the conventional PBG that relates to Bragg scattering, another two types ofPBGs, resulting from magnetic surface plasmon resonance and spin-wave resonance,respectively, are observed. The PBG due to magnetic surface plasmon resonance isparticularly interesting because of its analogy to surface plasmon in metal, furthermore,it is shown to be completely tunable by an external static magnetic field from bothexperimental and theoretical point of view.We also have demonstrated experimentally a one-way magnetic surface plasmonelectromagnetic waveguide in the microwave range based on the MPCs The waveguideexhibits asymmetric transmission of electromagnetic waves in the frequency range nearthe magnetic surface plasmon resonance for an MPC, such that a significant one-waypropagation can be observed in the channel between the two MPC slabs, each in anexternal static magnetic field (ESMF) of opposite directions. The one-way waveguide isnot only immune to interstitial metal defects,but also robust against the disorder of rodposition. Furthermore, its working frequency can be flexibly tuned by an ESMF, whichmakes it more favorable for the design of electromagnetic devices. The physics isrelated to the broken time reversal symmetry of the magnetic surface plasmon bandstates and the excitation of a giant circulation of the energy flow, similar to the case in the quantized Hall Effect.Based on the understanding of the magnetic resonance properties of split ringresonator no longer suffers from half-wavelength requirement for resonance, wedemonstrated that for coplanar split ring resonator, the wavelength of the resonancemode is about10times larger than geometrical size of the ring; for non-coplanar splitring resonator, the resonance frequency depends linearly on the ring-ring separation. Werealized the non-coplanar split ring system using sputtering and etching techniques, it'sgeormetrical size is only1/120of the wavelength at the lowest resonance mode. Thisdemonstration opens a door to miniaturization of resonator.It is believed that ultrafast demagnetization process in ferromagnetic film canproduce terahertz (THz) emission. We present an experimental demonstration that,following ultrafast optical excitation, the magnitude of terahertz electromagnetic pulsesemitted from a ferromagnetic film is proportional to the Gilbert damping constant,which is conventionally used to describe the damping of magnetization precession.Different damping factors are obtained by varying the normal metal film adjacent to themagnetic film via spin pumping. It is shown that the larger the Gilbert damping, thelarger emitted THz signal and therefore, the faster demagnetizing process. On the otherhand, the nonlocally adjusted damping constant via spin pumping in this experimentalso indicates that the spin pumping theory may still be valid in THz regime. We expectthis study would lead to new perspectives of the ultrafast manipulation of magneticorder.