Self Assembly of and Plasmon Enhanced Ultrafast Magnetization in Ag Co Hybrid Nanoparticles

Ultrafast demagnetization in magnetic nanoparticles using pulsed laser has attracted considerable attention because of its potential applications in spintronics, such as data storage. In such applications, it is necessary to control magnetization using low energy laser pulses, however, this poses the problem of increasing the amount of energy from the excitation laser pulses to the spin subsystem. We take advantage of the phenomenon known as localized surface plasmon resonance (LSPR) to enhance the energy transfer from laser pulses to the spin subsystem. To induce LSPR, hybrid nanoparticles consisting of noble metal nanoparticles with LSPR absorption and magnetic metal nanoparticles are prepared using a novel method. Specifically, AgCo hybrid nanoparticles are prepared by a self assembly method using pulsed laser deposition. We performed measurements of the static Faraday and time-resolved Faraday effects using a pump probe technique on the AgCo hybrid nanoparticles with various AgCo ratios. The data suggest that the LSPR absorption and demagnetization amplitude increase with the increasing AgCo ratio. The results indicate that the amount of energy transferred from the laser pulses to the spin system of magnetic nanoparticles can increase via LSPR absorption.


Introduction
Ultrafast demagnetization, which is the reduction of magnetization within a few picoseconds after femtosecond laser excitation, has attracted considerable attention as a technique for the ultrafast manipulation of magnetization. Laser-induced ultrafast demagnetization was first observed in Ni thin films [1] and has since been observed in nanoscale magnetic materials, such as thin-film structures [1][2][3] or nanoparticles. [4,5,6,7] In spintronics [8] , such as data storage [9] , it is necessary to control the ultrafast magnetization of magnetic nanoparticles using low-energy laser pulses; however, this poses the problem of increasing the amount of energy transferred from the excitation laser pulses to the spin subsystem.
To overcome this problem, we took advantage of a phenomenon known as localized surface plasmon resonance (LSPR). LSPR is the collective oscillation of free electrons in metal nanoparticles, which is induced when the free electrons are coupled with electromagnetic waves of light at resonant frequency. When the frequency of incident light approaches that of LSPR, the amplitude of the electromagnetic waves near the nanoparticles drastically increases. Consequently, the cross-section of linear and nonlinear interactions, such as absorption, fluorescence, the Raman effect, and second-and third-harmonic generation, drastically increase. [10,11] The higher the excitation pulse energy, the larger the ultrafast demagnetization. [5,12] Thus, LSPR is expected to enhance the ultrafast demagnetization, because it increases the amount of energy from the excitation laser pulses to the spin subsystem. However, this has not been experimentally demonstrated to date.
Superparamagnetic nanoparticles are the most suitable magnetic materials for observing ultrafast demagnetization, because they have the lowest magnetic anisotropy. [4] However, unlike noble metal nanoparticles, magnetic metal nanoparticles do not exhibit strong LSPR.
Based on this fact, we prepared and used hybrid nanostructures consisting of noble metal and ferromagnetic metal nanoparticles. In such hybrid nanoparticles, the electric field strongly 3 increases near the noble metal nanoparticles via LSPR. Thus, the enhanced field is expected to couple with the nearby ferromagnetic metal nanoparticles, as shown in Figure 1.
Both superparamagnetism and LSPR require particles as small as several nanometers. In addition, the hybrid nanoparticles are preferably oriented because LSPR is sensitive to the shape and orientation of the noble metal nanoparticles. To date, most hybrid nanoparticles have been prepared by liquid-route techniques, such as decomposition of metallic salts. [13][14][15][16][17][18][19][20] The nanoparticles obtained by such methods are not oriented; moreover, they are randomly dispersed in the liquid. To obtain oriented fine structures, lithographic techniques have been used; [21][22][23][24][25][26] however, there is room for improvement. For example, photolithograpy has high throughput, but it requires a complicated multistep process. Electron-beam lithography produces fine structures of several nanometers, but it is time-consuming. Thus, a simple and expedient method for producing small and oriented hybrid nanoparticles is required. In this study, we developed and used a novel self-assembly method using pulsed laser deposition (PLD) to fabricate Ag-Co hybrid nanoparticles dispersed and aligned in a thin film. We chose TiO 2 as the matrix material, because TiO 2 is transparent in the wide wavelength range, Ti is immiscible with Ag and Co, and TiO 2 does not oxidize Ag or Co because Ti has smaller Gibbs free energy for oxidation. The obtained Co nanoparticles were small enough to show superparamagnetism and were adjoined to Ag nanoparticles. The hybrid nanoparticles were oriented in the matrix. To date, ultrafast demagnetization measurements have never been performed on such hybrid nanoparticles.
To examine the demagnetization enhancement via LSPR, we measured the static Faraday effect and time-resolved Faraday effect using a pump-probe technique. LSPR absorption and demagnetization amplitude were observed to increase with increasing Ag-Co ratio. The observations suggest that the demagnetization amplitude can be increased by increasing the LSPR absorption.

Experimental Section
The epitaxial thin films of (001)-oriented anatase TiO 2 containing Ag-Co hybrid nanoparticles were prepared on LaSrAlO 4 (LSAO) (001) single-crystal substrates using PLD. The crystallinity and crystallographic orientation of the prepared films were evaluated by X-ray diffraction (XRD). The size and distribution of the Ag-Co hybrid nanoparticles were examined by transmission electron microscopy (TEM) and scanning TEM (STEM) equipped with an energy-dispersive X-ray detector (STEM-EDX) or with high-angle annular dark-field imaging (STEM-HAADF). The magneto-optical properties were measured using a magnetooptical spectrometer (BH-M800UV-KC-KF; Neoark Corp., Tokyo, Japan). Ultrafast demagnetization dynamics were established by measuring the time-resolved Faraday effect using a pump-probe technique. A regenerative amplified Ti:sapphire laser system (RegA9000, Coherent Inc.) operating at 120 kHz was used. The fundamental wavelength was 800 nm and the pulse duration was 220 fs. The wavelength of the probe pulses was set to 800 nm and that of the pump pulses, which was generated by frequency doubling, was 400 nm. The pump and probe beam diameters were approximately 0.2 mm. An external magnetic field of 9 kOe was applied perpendicular to the film surfaces. All measurements were performed at room temperature.

Results and Discussion
The cross-sectional and planar STEM-EDX and STEM-HADDF images of (Ag x , Co):TiO 2 /TiO 2 (x = 0 [27] [27] , the Co nanoparticles have the fcc structure, which is the most stable structure for small Co nanoparticles. Thus, we speculate that the Co nanoparticles in the (Ag x , Co):TiO 2 /TiO 2 films (x = 5, 10, and 20) possess the fcc structure. The XRD patterns of (Ag x , Co):TiO 2 /TiO 2 (x = 0, 5, 10, and 20) in  were superparamagnetic with small coercivity (~0.05 kOe or less). This is attributable to the reduction in the size of the Co nanoparticles with increasing Ag. In the absorption spectra, peaks at ~450 nm were observed for the (Ag x , Co):TiO 2 /TiO 2 (x = 10 and 20) films, which 7 indicate LSPR in the Ag nanoparticles. The red-shifting of the peak wavelength with increasing x was also observed, suggesting that the Ag nanoparticles increased in size with x, as is typically seen with metal nanoparticles. [28] The peak intensity increased with increasing amount of Ag. The strong absorption in the shorter wavelength region (<350 nm) originated from the TiO 2 matrix. To excite LSPR, the pump pulse wavelength was set to 400 nm in the time-resolved measurements.  to those reported for Fe 3 O 4 [5] and Co nanoparticles [4] , whereas the observed magnetization recovery times were much shorter than those reported for Fe 3 O 4 , Co x Fe 3−x O 4 , and Co [4,5,6,7] (several hundreds of picoseconds). The reason for the shorter recovery times in this study is probably that the pump laser energy was lower (0.06 mJ cm −2 ) than that used for Fe 3 O 4 , Co x Fe 3−x O 4 , and Co (>1 mJ cm −2 ). [4,5,6,7] Discussing the ultrafast magnetization mechanism is beyond the scope of this study. Thus, we focus on how the LSPR affects the ultrafast magnetization.
The peak demagnetization amplitude strongly depended on the Ag-Co ratio for x = 5 to 20, whereas it did not depend for x = 0 to 5, as shown in Figure 6(b), even though the saturation value of the static Faraday ellipticity was independent of x, as shown in Figure 4 The results confirm that LSPR enhances the demagnetization amplitude of the nanoparticles.
Ultrafast demagnetization is considered the result of the thermalization of photoexcited hot electrons. [4] In general, the photoexcitation of the electronic subsystem of sparsely distributed ferromagnetic metal nanoparticles is inefficient. However, the absorption cross-section of the as-synthesized nanostructures, consisting of noble metal and ferromagnetic nanoparticles, increased via LSPR, leading to the efficient coupling of light with the electron subsystem of the Co nanoparticles. [29] Figure 6(c) shows the demagnetization amplitude of (Ag x , In all films, the magnetization recovery process was effectively expressed by combining fast ( ≈ 2 ps) and slow ( ≈ 10 ps) exponential decay components. Figure 6(a) shows the experimental data (colored lines) and fitting curves (black lines).

Conclusion
In this study, we found that ultrafast demagnetization could be enhanced by taking advantage