Spin dynamics in magnetic thin films and eletromagnetic properties of metamaterials

In this work, I have investigated the high frequency magnetic properties of a variety of novel materials by using the microwave techniques. The work consists of two parts: (1) spin dynamics study in magnetic multilayer thin films, (2) fabrication and characterization of novel magnetic materials.;In the first part, we have observed nonlinear behaviors of the normal Gilbert damping G0 and the effective spin-mixing conductance g↑↓ in Pt/NiFe/Pt thin films when the incident microwave power is above a critical ac field hrf of 1.6 Oe. Both G0 and g↑↓ are affected by the coupling between spin coherent precession and spin wave modes. Our work is the first experimental demonstration of nonlinear behavior of the effective spin-mixing conductance g↑↓ . It suggests the nonlinear spin wave modes excited at high incident microwave power is detrimental to the spin pumping effect and should be avoided in future spin battery design. We have also studied the magnetization dynamic in IrMn/FeCo/Cu/NiFe/Cu spin valve through the Gilbert damping. Our results show that the Gilbert damping constant of NiFe is enhanced in antiparallel configuration when the magnetizations of both FM layers are precessing. This enhancement is induced by the dynamic exchange between the magnetizations of NiFe and FeCo layers.;We have observed the dc voltage generation across the tunneling barrier while the spin precession is excited in the ferromagnetic free layer by the microwave field. The magnitude of the dc voltage peak is around few muVs with AlOx tunneling barrier. Our results directly indicate that spin current can be pumped through the tunneling barrier thus generates the dc voltage across the barrier. The results raise an important question about the role of the F/I barrier interface in spin pumping mechanism. More detailed experiment and theory studies are certainly needed, especially in MgO barrier based MTJ that could become a good candidate for realizing spin battery device.;In the second part, we have experimentally observed, in addition to conventional PBG from Bragger scattering, two groups of magnetically tunable PBGs in 2D magnetic photonic crystals due to magnetic surface plasmon and spin-wave resonance bands, respectively. The former is particularly interesting because of its analogy to surface plasmon in metals. Simulations on transmission coefficients are in a good agreement with experimental results. Our results thus provide a first demonstation of the concept of the magnetic SP bands and illustrate the fundamental mechanism for EM propogation at subwavelength confinement via surface plasmons. We have also experimentally studied the high frequency properties of the novel metamaterials, including NiFex/Polymer 100-x nanocomposites and magnetic left-handed materials (m-LHMs). In the nanocomposites study, we found the microwave transmission in NiFe x/Polymer100-x nanocomposites can be improved in the presence of an external magnetic field. It provides us a potential method to make a controllable microwave absorber for wave transmission at microwave frequency, which has various applications in microwave devices and communication. In m-LHM study, we have theoretically investigated the possibility of realizing LHM in metallic magnetic system. We have found that these thin magnetic films consist of metallic entities that may exhibit left-handed behaviors near the vicinity of the ferromagnetic resonant frequency o0. Experimentally, we observed the thickness dependent peak shifts in both multilayer [NiFe(10nm)/SiO2(2nm)]N thin films and granular Fe 30(SiO2)70 thin films. This suggests a negative refraction index.