Enhancement of the magneto-optical activity via surface plasmon resonance on Au-Co nanocomposite thin films K. Yang 1 , C. Clavero 1 , J. R. Skuza 2,* and R. A. Lukaszew 1,2 1 Department of Applied Science, College of William and Mary, Williamsburg, VA 23185, USA 2 Department of Physics, College of William and Mary, Williamsburg, VA 23185, USA *Currently at: National Institute of Aerospace, Hampton, VA 23666, USA ABSTRACT Appropriately designed metallic nanoparticle arrays and thin films are suitable platforms for sensing applications based on their optical, magneto-optical (MO) and magneto- transport properties. For example most current biosensing schemes based on optical excitation of surface plasmon resonance (SPR) are "passive", i.e., they are based on changes in the optical properties at the sensor surface - typically gold- when a biological specimen to be detected is bound to it and SPR is excited. We have shown that it is possible to enhance the sensitivity of these thin films by combining Au with other materials that exhibit changes under suitable external modulation hence developing “active” plasmonic surfaces. In such scenario, optimal combination of materials can exhibit enhanced optical as well as electrical transport properties. Here, such physical properties, i.e. optical and electric transport, have been investigated in Au-Co nanocomposite thin films after modifying growth parameters such as growth temperature and relative Au:Co concentration. We found that suitable combinations of these two materials can exhibit enhanced MO effects and in some cases also substantial magneto- resistance, the latter of interest for sensing magnetic media. Thus, here we have applied theoretical models to simulate the optical properties in the nanocluster-matrix aggregation and compared with the experimental optical data. Correlation between the nanocomposite films’ microstructure, morphology, optical, MO enhancement and magneto-transport is discussed. Keywords: surface plasmon resonace, magneto-optical Kerr efffect, plasmonics, magneto-transport 1 INTRODUCTION Gold-cobalt is an interesting metallic binary system where no actual alloying occurs 1 and where the microstructure and composition can be tuned, particularly in thin film form, in order to exploit properties of greatest benefit for sensing applications. In this system one of the constituents, i.e. gold, can sustain sharp surface plasmon resonances (SPR) when optically excited while the other constituent while also susceptible of SPR excitation although considerably damped due to stonger optical absorption, also exhibits magnetic and MO response that can be modulated with modest external magnetic fields. Suitable compositions of these two materials can be tailored in thin film geometry to also favor antiferromagnetic coupling between Co regions such that giant magneto- resistance can also be observed. Our preliminary work with this sytem indicates that correlation between these optical and magneto-transport phenomena warrants further study. With respect to optical properties, surface plasmon polaritons (SPPs) are surface electromagnetic waves that propagate at the interface between a metal and a dielectric. These evanescent waves excited at the boundary of the metal and the external medium are very sensitive to any dielectric change at this interface. Thus, optical sensors, based on SPPs at planar gold surfaces, are fast becoming a preferred method in many bio-sensing applications. Nonetheless, in order to improve sensitivity it is highly desirable to develop “active” plasmonic systems such that some optical property can be modulated under application of an external field. Transition metals alone such as Fe, Ni and Co exhibit MO effects accessible at relatively low fields, but their absorption coefficients are higher than those of Au and therefore their SPPs are over-damped. Thus, composite noble-metal/ferromagnetic-metal sensing elements can overcome the high absorption problem while exhibiting high MO activity. In fact, we as well as other researchers have observed that carefully tailored multilayer films of Co and Au do exhibit enhanced MO activity related to SPPs excitation. 2 In order to explore less stringent materials designs, we have considered here the possibility of using nano-composite Au-Co films. Codeposition of these two metals can yield different nanocluster-matrix aggregates depending on their relative concentration and the growth temperature, and thus their properties, i.e magneto-plasmonic, magneto-transport, etc. can be optimized. Correlation between magneto-trasnport properties and optical properties can thus be investigated to establish possible composition and microstructure parameters that can enhance/optimize both, optical and magneto-transport properties in Au-Co nano-composite thin films. 2 EXPERIMENT SETUP The Au-Co nano-composite films were prepared using DC magnetron sputtering codeposition in an ultra-high vacuum chamber with base pressure in the low 10 -9 Torr NSTI-Nanotech 2011, www.nsti.org, ISBN 978-1-4398-7142-3 Vol. 1, 2011 460