Effect of Co component in CoAg granular thin films on visible-light reflection applied by magnetic field

The granular ferromagnetic thin films (example: Co-Ag; Co-Cu; Fe-Cu, etc.) has been studied through the phenomenon of giant magneto resistance (GMR). Metal nanoparticles (Au) in the glass have also been known to stimulate surface plasmon (Maier and Atwater, 2005). Ferromagnetic nanoparticles may have magnetic plasmonic phenomenon (spinplasmonic) (Garcia, 2011). Plasmonic materials are more widely used (Maier and Atwater, 2005; Garcia, 2011; Maier, 2007; Wang et al., 2004), as photonic components for optical information technology. On the surface nano-heterojunction, the surface plasmon excitation depends on the spin state density (Chau et al., 2007; Sapienza and Zerulla, 2009). This effect has opened a new way for the development and application of various fields such as electronics, plasmonics, spintronics and photonics (Chau et al., 2007; Elezzabi, 2007). In this paper, the dependence of the reflection of visible light (wavelength from 350 to 800 nm) on magnetic granular films with an external static magnetic field was study. The structure of similar type thin films was observed, as a few nanometers (Cullity, 1972). Simultaneously, the dependence of reflection (R) on the percentage of Co in the thin films was observed.


INTRODUCTION
The granular ferromagnetic thin films (example: Co-Ag; Co-Cu; Fe-Cu, etc.) has been studied through the phenomenon of giant magneto resistance (GMR).Metal nanoparticles (Au) in the glass have also been known to stimulate surface plasmon (Maier and Atwater, 2005).Ferromagnetic nanoparticles may have magnetic plasmonic phenomenon (spinplasmonic) (Garcia, 2011).Plasmonic materials are more widely used (Maier and Atwater, 2005;Garcia, 2011;Maier, 2007;Wang et al., 2004), as photonic components for optical information technology.On the surface nano-heterojunction, the surface plasmon excitation depends on the spin state density (Chau et al., 2007;Sapienza and Zerulla, 2009).This effect has opened a new way for the development and application of various fields such as electronics, plasmonics, spintronics and photonics (Chau et al., 2007;Elezzabi, 2007).
In this paper, the dependence of the reflection of visible light (wavelength from 350 to 800 nm) on magnetic granular films with an external static magnetic field was study.The structure of similar type thin films was observed, as a few nanometers (Cullity, 1972).Simultaneously, the dependence of reflection (R) on the percentage of Co in the thin films was observed.

EXPERIMENTAL
The granular thin films Cox-Ag1-x, with thickness of 150 nm have Co components x from 0 to 35 at % atomic, and fabricated by flash evaporation technique in vacuum 10 -9 bar.Experimental schematic is presented in Figure 1.White light source (1) 12V-21W, white light is projected to dispersive system (2), monochromatic light emerged from the dispersive system can be adjusted by adjusting the rotation angle of the lens system.Rotational speed of chopper (3) can be controlled by Lock-in DSP 7225 (8), the chopper can produce light pulses.Detector ( 7) is a photo diode CdS.The magnetic field is generated by an electromagnet with magnetic induction B from 0 to 500 Gs.Granular thin films is placed so that the magnetic field is parallel to the surface of thin films.To remove the phenomenon, Kerr and Faraday appear in the case of ferromagnetic, samples are placed in magnetic field and polarizer (4) is included.
The magnetic induction B and current intensity I have a linear relation as a function: B(Gs) ~2000×I(A).The light intensity of CdS obtained is displayed on the screen of Lock-in DSP 7225.In this study, the reflection on the surface is defined as the ratio J/J0, with J as light intensity on detector (7), J0 as light intensity direct.

RESULTS AND DISCUSSION
To interpret these results, magnetic properties of granular thin films Co x -Ag 1-x with x from 0% to 35 at % were considered.Figure 2 show the full hysteresis loops of thin films with x = 7 and 26%.As shown in Figure 2, the thin films with x = 7% characterized behavior of superparamagnetic material and the coercivity (H C ) reached 10 Oe.With x = 26%, thin films characterized behavior of ferromagnetic properties (H C = 60 Oe).According to previous reasearch (Wang et al., 2004), the Co particles displayed superparamagnetic properties when the particle size was in the range of 50-60 nm (Chau et al., 2007;Sapienza and Zerulla, 2009).
Figures 3 are the reflection of visible light (wavelength from 380 to 760 nm) without and with the applied magnetic field.Black line on the graph is the reflection (R) without magnetic field, the red line is the reflection (R B ) with external magnetic field B = 500 Gs.The reflection (R) reach a maximum value corresponding to a wavelength of 660 nm without magnetic field applied to the thin films.When magnetic field applied to the thin films with visible light has a smaller wavelength (under 570 nm), reflection (R) is almost unchanged.However, with wavelength longer than 570 nm, the increase of the reflection (R) can be seen clearly, especially at peak wavelength of 660 nm corresponding to the increase which is most pronounced.Figure 4 shows more clearly Int.J. Phys.Sci. the influence of the magnetic field of thin films Co x -Ag 1-x with x = 7%.Figure 4a shows the reflection depends strongly on the magnetic field strength.However in Figure 4b, this effect is not monotonous: the B increased from 0 to 300 Gs, R decreases, but when B increases further to 400 and 500 Gs, R increased.Results for all other thin films were similar.Figure 5 shows the reflection depends on the percentage of Co, while B = 0 and B = 500 G. R depends strongly on percentage of Co.
Moreover, R dependence on the percentage of Co is not monotonous (Figure 5c and d).
In the Co-Ag thin films, RKKY interaction between particles Co appears (Bruno and Chappert, 1993).This interaction has changed the characteristics and relative distribution of the thin films even in the absence of an external magnetic field effects.In addition, the spin accumulation at the surface Co/Ag induced plasmon oscillations depends on spin (spin-plasmonic).

Conclusion
Dependence of reflection on the applied external magnetic and percentage of Co field of the granular thin films Co-Ag have been studied.Initial results showed that, an optical environment composed of ferromagnetic metal nanoparticles with the size and density appropriate, can be controlled by an external magnetic field to change the reflection of surface of thin films.The above results may be a sign that spinplasmonics phenomenon has been studied in model systems as Co-Au nanoparticles with band THz.

Figure 4 .Figure 5 .
Figure 4. (a) Reflection of visible light depend on the magnetic field of granular thin film Co x -Ag 1-x with x = 7% and B = 0 ÷ 500 Gs.(b).Reflection of visible light depend on the magnetic field of granular thin film Co x -Ag 1-x with x = 7% and B = 0 ÷ 500 Gs (>575 nm).350 400 450 500 550 600 650 700 750 800 0.0 0.1 The thickness of thin films are determined by technical Alpha-Step, rate component of Co are determined by technical EDS.