NANO LETTERS
Enhanced Magnetooptical Response in Dumbbell-like Ag−CoFe2O4 Nanoparticle Pairs
2005 Vol. 5, No. 9 1689-1692
Yanqiu Li,*,† Qiang Zhang,† Arto V. Nurmikko,† and Shouheng Sun‡ Departments of Physics and Chemistry, Brown UniVersity, ProVidence, Rhode Island 02912 Received May 2, 2005; Revised Manuscript Received July 14, 2005
ABSTRACT We study magnetooptical Faraday rotation in a colloidal solution of “dumbbell-like”, physically conjoined nanoparticle pairs composed of a ferromagnetic (CoFe2O4) and a noble metal (Ag). We show that outside the CoFe2O4 interband dominated spectral regime, the coupled dumbbells show significantly enhanced magnetooptic response relative to single CoFe2O4 nanoparticles, derived from plasmonic contribution by the Ag nanoparticles.
Enhancement of magnetooptical response has been reported in composite thin-film multilayer metallic structures that combine a ferromagnetic film with a free-electron-like metal that augments the optical response of the composite system. For example, Katayama et al. have shown that the Kerr rotation of a thin iron film on top of bulk copper is enhanced near the plasmon energy of copper.1 A similar phenomenon was observed by W. Reim et al. in a GdTbFeCo/Ag bilayer thin film.2 We were motivated by the question of magnetooptical response of composite nanoparticle materials, which can today be synthesized through organic chemistry as a “bottom-up” method. Here we report on studies of enhancement of magnetooptic behavior via spectral dependence of Faraday rotation in conjoined pairs of colloidal nanoparticles, composed of “dumbbell-like” pairs of Ag-CoFe2O4, henceforth labeled as “dimers”. The Ag-CoFe2O4 dimers were prepared as follows: CoFe2O4 nanoparticles made from the reaction between cobalt acetylacetonate and iron acetylacetonate3 were further treated with 1-hexadecanethiol in phenyl ether at reflux and were separated as in the synthesis of CoFe2O4. The treated CoFe2O4 nanoparticles were mixed with silver nitrate and tetrahydronaphthalene (tetralin) and heated at 100 °C for 1 h. The product was separated by adding ethanol and centrifugation and was dispersed in hexane. (The detailed synthesis and characterization of Ag-iron oxide dumbbelllike nanoparticles will be published separately.) For the magnetooptical study, we chose the dimers containing an Ag particle component approximately 6 nm in diameter and * Corresponding author. E-mail:
[email protected]. † Department of Physics. ‡ Department of Chemistry. 10.1021/nl050814j CCC: $30.25 Published on Web 07/28/2005
© 2005 American Chemical Society
Figure 1. Absorbance spectra of dimers and monomers in hexane. The volume concentration of CoFe2O4 in both solutions is ∼100 nM. The inset shows an electron microscope image of two dimers.
the soft ferromagnetic CoFe2O4 particle component approximately 14 nm in diameter with a size distribution less than 10%. (The ferromagnetic particle size was chosen to be sufficiently large to avoid complications that occur for smaller particles upon approach of the superparamagnetic regime). We also used the 14-nm CoFe2O4 nanoparticles, henceforth labeled as “monomers” as a baseline and for comparison. Figure 1 shows the absorbance spectra of hexane dispersed Ag-CoFe2O4 dimers and CoFe2O4 monomers at modest levels of nanoparticle concentration (∼100 nM) to avoid aggregation effects. The inset shows a transmission electron microscope image of two such pairs on a solid substrate.
Figure 2. Magnetooptical Faraday rotation of dimers and monomers in hexane at laser wavelengths of (a) 385 nm, (b) 421 nm, (c) 455 nm, (d) 532 nm, (e) 633 nm, and (f) 850 nm, respectively, (∼100 nM volume concentration; optical path length is 2 mm).
Riding atop the large intrinsic absorption coefficient by CoFe2O4 at short wavelengths, the absorption of AgCoFe2O4 pairs showed an additional surface plasmon resonance absorption peak at ∼421 nm due to the Ag nanoparticles (well known from earlier work on single Ag nanoparticles of comparable size). We assume that the prepared volume concentrations of CoFe2O4 monomers and Ag-CoFe2O4 dimers were indeed comparable, since the transmission of both solutions was comparably high in the long wavelength of our spectral window where the contributions of CoFe2O4 and Ag separately are expected to be small. We note that the very strong optical absorption at short wavelengths for both monomers and dimers is dominated by the interband transition of cobalt ions in the crystalline CoFe2O4 nanoparticles. For magnetooptical characterization, we performed experiments in standard Faraday rotation configuration. A combination of lasers was employed to provide six different wavelengths at 385, 421, 455, 532, 633, and 850 nm, respectively, to acquire data from the violet to the nearinfrared. The solutions were filled in cuvettes of 2 mm optical path length. Figure 2 shows a comparison of the wavelengthdependent Faraday rotation for the monomers and dimers, respectively. At short wavelengths, the magnitude of the rotation and the shape of the hysteresis loops were quite comparable for the two types of particles of common particle 1690
concentration, with coercivity below 50 Oe and with a saturation field of approximately 500 Oe. We interpret this similarity originating from the dominant effects by the CoFe2O4 interband transitions to the magnetooptical tensor (diagonal and off-diagonal terms) in the highly absorptive violet/blue wavelength regime. A dramatic contrast emerges between the magnetooptical response for the Ag-CoFe2O4 dimers and CoFe2O4 monomers at longer wavelengths, outside the CoFe2O4 interband transition-dominated regime. Although the overall magnitude of the Faraday rotation decreases away from the absorption edge of CoFe2O4, the rotation becomes significantly enhanced for the dimers relative to the monomers by nearly an order of magnitude near 633 nm. This approximate wavelength range also corresponds to a “crossover” regime, where the sign of the Faraday effect changes for the monomer nanoparticles, while the dimers remain unaffected in this regard. We interpret this strong contrast in spectral behavior as being due to the dielectric contribution of the Ag-nanoparticle component in the Ag-CoFe2O4 dimer. The crossover behavior for the CoFe2O4 monomer in particular, absence in the dimer case, occurs in the Ag nanoparticle plasmon tail where the dielectric contribution by Ag to the dimer appears to produce a significant additive contribution to the overall magnetooptical response of the composite nanoparticle Ag-CoFe2O4 particle pairs. Nano Lett., Vol. 5, No. 9, 2005
Classically, and in homogeneous bulk material, the magnetooptical Faraday rotation originates from the complex offdiagonal elements of σxy ) σ1xy + iσ2xy of the optical conductivity tensor, as well as the diagonal elements σxx ) σ1xx + iσ2xx ) -iω(1 - xx)/4π as follows θF ) -
2πl0 nσ1xy - kσ2xy c n2 + k2
(1)
1xx ) n2 - k2 and 2xx ) 2nk where θF is the Faraday rotation angle for material thickness l0, 1xx and 2xx are the real and imaginary parts of the diagonal elements of the dielectric tensor with n and k the index of refraction and extinction coefficients, respectively.4 This suggests there are two different ways to increase θF. One involves excitations of highly spin-polarized electronic states that increase the off-diagonal element, σxy. Alternatively, the material may be optimized by using the strong dependence of θF on optical constants n and k in the denominator of eq 1. We underscore that the latter “dielectric” enhancement effects need not be directly correlated in photon energy with the plasma edge of a metal1 but can impact the outcome at photon energies where the optical constants (n, k) of the transmission medium remain small (i.e., the plasmon tail). In the case of our composite nanoparticle Ag-CoFe2O4 pairs in a solution, their size is 1 order of magnitude smaller than the optical wavelength. Hence to lowest approximation, the dielectric response of the particle pairs can be treated as an effective optical conductivity tensor that sums contributions by both particles. That is, although the off-diagonal elements are dominated by the CoFe2O4 particles, the diagonal elements obtain a strong contribution to n and k from the Ag particles in the plasmon tail. We note that quantitatively modeling or estimating the modified conductivity tensor of our colloidal nanoparticle pairs dispersed in hexane is more broadly connected with the theoretical challenges of various approaches, such as the “homogenization theory”.5,6 For magnetooptical media, the material case typically involves a thin-film composite that is viewed as an “alloy” in the effective medium theory (EMT) with two isotropic components such as Co/Pt multilayers.7 To qualitatively gauge the optical response of our dumbbell nanoparticles in a lowest-order approximation, a simple weighted summation of the dielectric constants of each component, that is, Ag, CoFe2O4, and hexane, according to their volume fraction factors will give a reduced value of n and k of the composite material as n and k of Ag in the plasmon tail, which is smaller than that of CoFe2O4 and hexane. Furthermore, the optical response of Ag-CoFe2O4 dimers is more enhanced than CoFe2O4 monomers alone, which will give a much larger weighting factor for Ag when summing the dielectric constants of each component. Beyond that, a serious effort at modeling is an advanced challenge here, with the geometry defined by the dumbbell structure on the nanoscale as a three-component dielectric system (Ag, CoFe2O4, and hexane solvent). Because of the fact that the Nano Lett., Vol. 5, No. 9, 2005
Figure 3. Transient transmission change ∆T(t)/T in hexane solution of dimers and monomers. The concentration of CoFe2O4 in both solutions is about 500 nM. The optical path length is 100 µm. The incident pump fluence for dimers and monomers are 10 µJ/cm2 and 400 µJ/cm2, respectively.
two nanoparticles are in close proximity (i.e., in optical near field), it is probably necessary to go beyond EMT, to include the nearest-neighbor interaction terms and so forth. One approach might be through methods developed via paircluster theory.8 We have further investigated the contrast in optical properties between the dumbbell nanoparticle Ag-CoFe2O4 pairs and the CoFe2O4 monomers by performing an ultrashort laser pulse pump-probe experiment (in zero magnetic field). Such a nonlinear method can provide another window and spectroscopically sensitive access to details of a material’s dielectric response and was employed here as a complement to our magnetooptical measurements in characterizing the optical properties of the colloidal dimers. In these types of experiments, each subpicosecond pulse creates energetic “hot” electrons inside the individual nanoparticles, which subsequently relax to equilibrium though electron-phonon interaction, followed by thermal transport across the particlesolvent interface. The occupancy within the available density of electronic states is tracked, the results depending on the probe laser wavelength. We employed 100-fsec laser pulses for excitation at 400 nm and probing at 532 nm, respectively, where the time dependent changes in the transmission of the probe (∆T) were measured as a function of the pump-probe delay. The volume concentration was about 500 nM, and the pumpprobe optical overlap defined a path length of 100 µm. Although the absorption of the pump light is comparable for both the dimers and monomers (from Figure 1), Figure 3 shows that for a comparable nonlinear signal (peak value of ∆T) an approximately 40 times larger excitation level was required for the CoFe2O4 monomers in solutions of approximately the same molar concentration. This observation supports the argument that the dielectric contribution by the Ag-nanoparticles in the dimer enhances the coupling of the probe to the media (and provides an overall enhancement in the nonlinear optical response). Note that the transient decay 1691
times of the nonlinear transmission signals are comparable for dimers and monomers and do not contain physical information of direct relevance to this paper (the temporal oscillations in the monomer are related to high-frequency acoustic “breathing mode” oscillations in the CoFe2O4 nanoparticles, triggered by the picosecond impulsively induced thermal expansion9 and are dampened for the AgCoFe2O4 dimer; a point of comparison beyond the scope of subject matter in this paper). In conclusion, we have investigated a novel magnetic and metal nanoparticle pair system, the Ag-CoFe2O4 dimer, by attempting to understand the magnetooptical response through spectroscopic examination. It appears that with a noble metal nanoparticle Ag in physical contact with a ferromagnetic nanoparticle CoFe2O4, the effective optical constants of the composite are distinct from the magnetooptical response of the individual ferromagnetic nanoparticle CoFe2O4 only. Although not investigated in this letter, the sizes of both Ag and CoFe2O4 are both readily controllable by chemical synthesis, enabling in principle the optimization and tailoring of the magnetooptical response of the dimer system to match
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specific optical sources. From an applied point of view, this offers advantages in the development of magnetic nanoparticles as candidates for next generation high-density storage media. Acknowledgment. This research was supported by the National Science Foundation, DOE, and DARPA. References (1) Katayama, T.; Suzuki, Y.; Awano, H.; Nishihara, Y.; Koshizuka, N. Phys. ReV. Lett. 1988, 60, 1426. (2) Weller, D.; Reim, W. Appl. Phys. Lett. 1989, 49, 599. (3) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2004, 126, 273. (4) Reim, W.; Schoenes, J. In Ferromagnetic Materials: A Handbook on the Properties of Magnetically Ordered Substances; Wohlfarth, E. P., Ed.; 1980; Vol. 5, p 145. (5) Bergman, D. J. Phys. Rep. 1978, 43, 377. (6) Choy, T. C. EffectiVe Medium Theory, Principles and Applications; Oxford University Press: Oxford, U.K., 1999. (7) You, C. Y.; Shin, S. C.; Kim, S. Y. Phys. ReV. B 1997, 55, 5953. (8) Sheng, P. Phys. ReV. B 1980, 22, 6364. (9) Nelet, A.; Crut, A.; Arbouet, A.; Del Fatti, N.; Valle´e, F.; Portale`s, H.; Saviot, L.; Duval, E. Appl. Surf. Sci. 2004, 226, 209.
NL050814J
Nano Lett., Vol. 5, No. 9, 2005
