Ultrafast demagnetization in magnetic nanoparticles has attracted considerable attention because of its potential applications such as spintronics. For such applications, it is important to develop materials in which ultrafast magnetization can be controlled by lower-power laser pulses. We developed a new method for fabricating Ag–Co hybrid nanoparticles in a TiO2 film using pulsed laser deposition, and elucidated ultrafast magnetization dynamics of these films by employing the pump–probe time-resolved Faraday rotation measurement. The measurements showed that localized surface plasmon resonance of the nanoparticles resulted in an enhancement of the ultrafast demagnetization of the films.
© 2014 Optical Society of America
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  and has since been observed in nanoscale magnetic materials, such as thin-film structures [1–6] or nanoparticles [7–10]. In spintronics , such as data storage , 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 [13,14]. The higher the excitation pulse energy, the larger the ultrafast demagnetization [8,15]. 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 . 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 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 Fig. 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 [16–23]. 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 [24–29]; 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 TiO2 as the matrix material, because TiO2 is transparent in the wide wavelength range, Ti is immiscible with Ag and Co, and TiO2 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.
The epitaxial thin films of (001)-oriented anatase TiO2 containing Ag–Co hybrid nanoparticles were prepared on LaSrAlO4 (LSAO) (001) single-crystal substrates using PLD. For ablation, a Kr–F excimer laser (wavelength = 248 nm) was operated with the laser fluence of 2 J cm−2 per pulse and the repetition rate of 2 Hz.
The sintered pellets of pure TiO2 and the mixtures of TiO2, CoO, and Ag2O (molar Ti:Co:Ag of 95:5:x, where x = 0, 5, 10, and 20) were used as PLD targets for the anatase TiO2 seed layers and (Agx, Co):TiO2 top layers containing Co and Ag, respectively. First, an anatase TiO2 seed layer was deposited on the LSAO substrate at the substrate temperature (Ts) of 650 °C and oxygen pressure (PO2) of 5 × 10−3 Torr. Then, the (Agx, Co):TiO2 top layer (x = 0, 5, 10, and 20) was grown at Ts = 300 °C and PO2 = 1.0 × 10−6 Torr. The prepared films are hereafter referred to as (Agx, Co):TiO2/TiO2 (x = 0, 5, 10, and 20). Then, we measured the optical properties of the synthesized samples. In addition, a thicker film for x = 20 was prepared to investigate the growth mechanisms of the Ag–Co hybrid nanoparticles (optical properties of the thicker film was not investigated).
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 magneto-optical 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, which was used as the probe beam. The pump beam, which wavelength was 400 nm, were generated by frequency doubling of the fundamental wave using a β–BaB2O4 crystal having 0.5 mm thickness. The pump and probe beam diameters, which were determined by knife-edge measurements of the Gaussian beam, were 191 ± 3 μm and 139 ± 4 μm, respectively. In our experiment, an inhomogeneous heated area is probed because the pump and probe beam diameters are of the same order, and probed signals correspond to averaged results in the Gaussian beam profile. Taking the inhomogeneity into accout, the observed amplitude of the Faraday effect is reduced to approximately 70% of that at the center of the pumped area. An external magnetic field of 9 kOe was applied perpendicular to the film surfaces. All measurements were performed at room temperature.
3. Results and discussion
The cross-sectional and planar STEM–EDX and STEM–HADDF images of (Agx, Co):TiO2/TiO2 (x = 0 , 5, 10, and 20) films, shown in Fig. 2(a)–2(j), confirmed that the films consisted of Ag–Co hybrid nanoparticles embedded in the anatase TiO2 matrix. The thicknesses of the seed layer and top layer, determined by these images, were 5 nm and 28 nm for x = 0, respectively, while the thicknesses of the seed layers and top layers were 4 nm and 24 nm for x = 5, 10, and 20 (thin), 6 nm and 36 nm for x = 20 (thick). As seen in the cross-sectional images (Fig. 2(b), 2(d), 2(f), 2(h), and 2(j)), the Co nanoparticles formed at the interface between the seed and top layers, whereas the Ag nanoparticles formed on top of the Co nanoparticles, as seen in Figs. 2(c)–2(j). The comparison between the thinner and thicker (Ag20, Co):TiO2/TiO2 films in Figs. 2(g)–2(j) disclosed that even though both films had Ag–Co hybrid nanoparticles consisting of Co nanospheres and Ag nanorods with almost the same number density (0.0057 nm−2 for the thicker and 0.0084 nm−2 for the thinner), the Co nanospheres were larger and the Ag nanorods were longer in the thicker film. This suggests that the Co atoms have stronger chemical affinity for the TiO2 seed layer than the Ag atoms, the nuclei of the Ag–Co hybrid nanoparticles form at the beginning of the film growth, and the Co and Ag atoms are always phase-separated and never mix. The images in Figs. 2(a)–2(j) show that the Co nanoparticles are cylindrical for films with x = 0 and 5, and spherical forfilms with x = 10 and 20. Moreover, the size of the Co nanoparticles decreased with the increasing Ag content (x).
In the films, the average volume of the Co nanoparticles, obtained from Figs. 2(a)–2(h), was 1900 nm3 (10.4 ± 0.4 nm in diameter and 22.0 ± 2.9 nm in height) for x = 0, 450 nm3 (6.7 ± 0.3 nm in diameter and 12.8 ± 0.8 nm in height) for x = 5, 91 nm3 (5.6 ± 0.1 nm in diameter) for x = 10, and 45 nm3 (4.4 ± 0.1 nm in diameter) for thin x = 20 film. The results strongly suggest that the content of Ag atoms affects the diffusion and aggregation rate of Co atoms, resulting in changes in the nucleation density of the Co nanoparticles. The amount of Ag adjacent to Co nanoparticles increased with x (Figs. 2(a)–2(j)).
Figure 3(a) shows a high-resolution TEM image of the thicker (Ag20, Co):TiO2/TiO2 film. The TiO2 matrix and Ag nanoparticles appear as bright and dark regions, respectively. Lattice fringes of 0.19 nm spacing corresponded to d200 of anatase TiO2, those of 0.20 nm to d200 of fcc-Ag, and those of 0.24 nm to d111 of fcc-Ag. No TEM lattice images of Ag nanoparticles or Co nanoparticles for the (Agx, Co):TiO2/TiO2 (x = 5, 10, and 20) films were observed, because the nanoparticles were very small and at the boundary of the anatase TiO2 seed layer and (Agx, Co):TiO2 top layer. In the (Ag0, Co):TiO2/TiO2 film , 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 (Agx, Co):TiO2/TiO2 films (x = 5, 10, and 20) possess the fcc structure. The XRD patterns of (Agx, Co):TiO2/TiO2 (x = 0, 5, 10, and 20) in Fig. 3(b) indicate that the anatase TiO2 films grew epitaxially on the LSAO substrate with the (001) plane of anatase parallel to the (001) plane of LSAO. No diffraction peaks from fcc-Ag or fcc-Co were detected in the XRD patterns. This is probably because Ag and Co were randomly oriented and their concentration was too small.
Figures 4(a)–4(c) depict the Faraday ellipticity spectra, the Faraday ellipticity vs magnetic field curves measured at 800 nm, and the absorption spectra of the films, respectively. The Faraday ellipticity spectra of (Agx, Co):TiO2/TiO2 (x = 0, 5, 10, and 20) films were similar in shape and amplitude to one another, indicating that the amount of Co incorporated in each film was nearly identical. The (Ag0, Co):TiO2/TiO2 film showed a hysteresis loop with coercivity value of ~1 kOe. In contrast, the (Agx, Co):TiO2/TiO2 (x = 5, 10, and 20) films 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 around 450 nm were observed for the (Agx, Co):TiO2/TiO2 (x = 10 and 20) films, which 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 . The peak intensity increased with increasing amount of Ag. Furthermore, in our samples, Ag nanoparticles are formed by phase-separation between Ag atoms and TiO2 matrix, therefore a certain amount of Ag forms a solid solution with TiO2 matrix. This may also make intensity of LSPR absorption not proportional to x. The strong absorption in the shorter wavelength region (<350 nm) originated from the TiO2 matrix. To excite LSPR, the pump pulse wavelength was set to 400 nm in the time-resolved measurements.
Figure 5(a) shows the raw time-resolved differential Faraday ellipticity data for the (Ag20, Co):TiO2/TiO2 film under polarities opposite to those of the external magnetic field (Δη ( + 9 kOe): blue line andΔη (−9 kOe): orange line) and without the external magnetic field (Δη (0 kOe): green line). The polarity of the Faraday ellipticity depends on the direction of the external magnetic field. To find the magnetic and nonmagnetic components, the formulae Δη = (Δη ( + 9 kOe) − Δη (−9 kOe))/2 and Δηnonmag = (Δη ( + 9 kOe) + Δη (−9 kOe))/2 were used, respectively. As shown in Fig. 5(a), the nonmagnetic component (Δηnonmag: black line) was exactly the same as Δη (0 kOe). Figure 5(b) shows the time-resolved differential Faraday ellipticity normalized by the saturation value of the static Faraday ellipticity Δη/η and Time-resolved differential transmittance normalized by the transmittance without the pump pulse ΔT/T of the (Ag20, Co):TiO2/TiO2 film. Δη/η exhibited ultrafast decreases within subpicoseconds and fast recovery over timescales in several picoseconds. As shown in , Faraday ellipticity η is described by η = fη Μ, where M is the magnetization and fη is a coefficient which depends on the optical response of the material to the probe beam, and can be presented in terms of the complex refractive index ñ. Correspondingly, Δη is described as Δη ≈fη ΔΜ (t) + Δfη (t) Μ , which means that Δη depends on both ΔΜ (t) and Δfη (t); theformer is the photoinduced magnetization change, and the latter is a coefficient that depends on the photoinduced change of the complex refractive index Δñ. Note that the behavior of ΔΜ (t) does not depend on the probe pulse wavelength . Amplitude of Δ ñ depends on the probe pulse wavelength; thus the contribution of Δfη (t) tends to be large at some resonances, such as LSRP. In contrast, the first term fη ΔΜ (t) do not depend on Δñ. Therefore, in order to extract the contribution of ΔΜ (t), we set the probe pulse wavelength at 800 nm to be off-resonant to the LSRP of the samples. In fact, as shown in Fig. 5(b), ΔT/T, which directly reflects the behavior of Δñ, is much smaller than Δη/η and does not exhibit fast recovery over timescales less than several picoseconds. Therefore, we can consider that the contribution of the second term, Δfη (t) Μ, to Δη is negligible and Δη indicates the behavior of magnetization change ΔΜ (t).
Figure 6(a) shows Δη/η of the (Agx, Co):TiO2/TiO2 films measured under a pump laser fluence of 0.06 mJ cm−2. The observed demagnetization times were similar to those reported for Fe3O4  and Co nanoparticles , whereas the observed magnetization recovery times were much shorter than those reported for Fe3O4, CoxFe3−xO4, and Co [7–10] (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 Fe3O4, CoxFe3−xO4, and Co (>1 mJ cm−2) [7–10]. 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 Fig. 6(b), even though the saturation value of the static Faraday ellipticity was independent of x, as shown in Fig. 4(a). Two factors are presumably responsible for the enhanced demagnetization: the decrease in the size of the Co nanoparticles with increasing x and the increase in the LSPR absorption intensity of the Ag nanoparticles with increasing x. However, the contribution of the former is probably small. In Fig. 6(b), the demagnetization amplitude, the normalized volume of each Co nanoparticle, and the LSPR absorption intensity at 400 nm are plotted against x. The demagnetization amplitude varied widely for x = 5 to 20, whereas it hardly varied for x = 0 to 5. This behavior is similar to that of the LSPR absorption intensity but differs from that of the volume of each Co nanoparticle.
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 . 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 . Figure 6(c) shows the demagnetization amplitude of (Agx, Co):TiO2/TiO2 (x = 0 and 20) films as a function of the pump laser fluence. In the applied range of pump laser fluence, we did not observe saturation of the demagnetization signal, whereas we clearly observed enhancement in the demagnetization signal resulting from the existence of Ag nanoparticles.
In all films, the magnetization recovery process was fitted by combining fast (τ: 2 ps) and slow (τ: 10 ps) exponential decay components: a – b∙exp(-t/2) - c∙exp(-t/10) (a, b, c: fitting parameters, t: delay [ps]). Figure 6(a) shows the experimental data (colored lines) and fitting curves (black lines). Figure 7(a) shows the values of the fitting parameters for the (Agx, Co):TiO2/TiO2 films as a function of x. As shown in the figure, the amplitude of the slower recovering component, c, was almost the same in all films. However, the amplitude of the faster recovering component, b, increased for x = 5 to 20. The behavior of the amplitude of the faster recovering component is similar to that observed for the demagnetization amplitude. This confirms that the acceleration of the relaxation process was caused by the Ag nanoparticles near the Co nanoparticles. Such fast recovery has never been reported for single magnetic nanoparticles. The demagnetization of (Agx, Co):TiO2/TiO2 (x = 0 and 20) films was measured at various pump laser fluences. The results are shown in Fig. 8(a) and (b), and the fitted values of the fast- and slow-recovering components are summarized in Fig. 7(b). As the pump laser fluence increased, only the amplitude of the slower recovering component increased, as in Fe3O4 nanoparticles . The amplitude of the faster recovering component was almost constant.
Based on the above results, we propose the following demagnetization mechanism for the Ag–Co hybrid nanoparticles. The pump laser energy is absorbed mainly by the Ag–Co hybrid nanoparticles. When the Ag–Co hybrid nanoparticles are irradiated with the pump laser pulse,strong LSPR is induced at the interface between the Co and Ag nanoparticles as they are intensively heated, as shown in Fig. 1. That leads to an increase of the spin temperature of the Co nanoparticles, resulting in ultrafast demagnetization. Large amounts of energy are transferred from the pump laser pulse to the Ag–Co hybrid nanoparticles with increasing x, because the LSPR absorption intensity increases. In addition, the appearance of the fast recovery component due to the proximity of the Ag nanoparticles implies a new, fast cooling process of spin temperature in Co nanoparticles. Recently, hot-electron generation and injection at the interface between metal nanoparticle and metal-oxide is attracting the attentions ; it would have an important role to consider the cooling process of this system. The detailed mechanism for the efficient cooling process for the Ag–Co hybrid nanoparticles is unclear, but a possible scenario is like this: electron temperatures in the Ag and Co nanoparticles are increased rapidly by the photoexcitation, then electron temperature in the Ag nanoparticle is decreased very fast (in less than 50 fs) through hot-electron injection process at the interface between Ag nanoparticle and TiO2 . Electron and spin system in Co nanoparticles are strongly coupled. Thus, spin temperature of Co nanopartilce can be rapidly decreased because of the contact with the cooled Ag nanoparticle through the electron system in Co nanopartilce. The slower recovery component probably results from the cooling of the heated TiO2 matrix.
In this study, we found that ultrafast demagnetization could be enhanced by taking advantage of the LSPR phenomenon even for low laser pulse energies. Suitably prepared hybrid nanoparticles significantly contributed to such behavior. The as-synthesized Ag–Co hybrid nanoparticles exhibited superparamagnetic properties and produced LSPR, which enabled us to examine how LSPR affected the ultrafast demagnetization. The results of this study can help to better understand and design new strategies for controlling ultrafast magnetization in nanostructures.
We would like to thank N. Kanda for performing the ultrafast magnetization measurements, and T. Higuchi for a fruitful discussion. This research is supported by the Photon Frontier Network Program, KAKENHI (20104002), Project for Developing Innovation Systems of the Ministry of Education, Culture, Sports, Science and Technology, Japan and by JSPS through its FIRST Program.
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