Angle-Dependent transmission for visible light of magnetic granular thin films - Giap Van Cuong

Journal of Science and Technology 54 (5A) (2016) 27-33 ANGLE-DEPENDENT TRANSMISSION FOR VISIBLE LIGHT OF MAGNETIC GRANULAR THIN FILMS Giap Van Cuong1, 2, Tran Trung2, Nguyen Anh Tuan1, * 1International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology, 1 Dai Co Viet Road, Hai Ba Trung District, Hanoi, Vietnam 2Hung Yen University of Technology and Education (UTEHY); Dan Tien, Khoai Chau, Hung Yen, Vietnam *Email: tuanna@itims.edu.vn Received: 15/7/2016; Accepted for publication: 12 December 2016 ABSTRACT The transmission for visible light (T), with wavelengths from 560 to 695 nm, was investigated at various light incident angles of 0 ÷ 45 degree for the surface normal of samples (φ) as a function of Co contents for magnetic granular films Cox-(Al2O3)1-x, where x = 0.10 ÷ 0.45. This investigation was carried out under an external magnetic field fixed at 4 kOe to ensure for all the Co-granule magnetization being kept in a fixed incident light propagating direction. Results showed a rather remarkable angular dependence of T(φ) on the Co content for different wavelengths. It was attributed the behavior of this to be dependent on the so-called magnon- photon interaction. Keywords: magnetic granular film, visible light, angle-dependent transmission, magnon-photon interaction. 1. INTRODUCTION Most of extraordinary behaviors from the optical electron manipulations have been recorded for noble metallic nanoparticles, such as Ag or Au, in which different plasmonic properties and modifiability or enhancement via nanoparticle resonance have been acquired [1–4]. Recently, it was predicted that one can perform an optical spin manipulation [5]. This prediction suggests a potential of considerable applications by acting optically on electronic spin. In recent years, magnon–photon coupling subject has gained interest [6] and has shown the potential to use new applications in quantum information [7, 8]. In this work, we report on some results investigating the angle-dependent transmittance of Co-Al2O3 magnetic granular films (MGFs) with different Co contents using visible light, which is for the purpose of finding about the optical magnetic manipulations through the spin-photon interaction. Giap Van Cuong, Tran Trung, Nguyen Anh Tuan 28 2. EXPERIMENTAL The Cox-(Al2O3)1-x MGFs were deposited on glass substrates by radiofrequency (rf) sputtering techniques (Alcatel SCM 400), with basic vacuum of p ~ 10-9 bar, in an argon (5N) environment with sputtering pressure of pAr ~ 5×10-6 bar. A 4N alumina (Al2O3) target disk of 7.5 cm in diameter with 4N Co pieces attached on was used. A 7.5 cm-target-substrate distance and a rf sputtering power of 300 W, corresponded to the rf power density of ~7 W/cm2 was established for the sputtering. Average deposition rate for Co-Al2O3 was determined to be about 0.3 Å/s. A thickness of ~90-100 nm, determined by an IQ Alpha-Step of the KLA Tencor, for all samples studied with different Co contents of x = 0.10, 0.15, 0.25, 0.35, 0.40 and 0.45, determined by X-ray energy dispersive spectroscopy (EDS), were selected. More detail and typical informations on the microstructure and magnetic properties of the Cox-(Al2O3)1-x thin films can be found out somewhere else [9]. Experimental schematic set up for the transmission measurements is presented in Figure 1. In this installation, a visible light beam generated from a source of 12V-20W (1) is projected to and focused by a lens (2) on a monochromator (3) to be select different wavelengths from λ = 575 nm to 695 nm, then is aimed to a chopper (4), and a polarizer (5) passing the sample (6) placed in the center of a electromagnet (7) setting at a fixed magnetic field intensity of 4 kOe. After interacting with the sample, a transmitted monochromatic light beam is analyzed (by (5’) analyzer) and focused (by (2’) lens) on a CdS photodetector (8). The speed of the chopper (4) can be controlled by Lock-in DSP 7225 (9) for production of light pulses. The MGFs are placed in the center of electromagnet so that the magnetic field direction is parallel to the sample surface normal. Meanwhile, the whole of the sample-electromagnet system can be turned around an axis so that the incident light beam composes an angle α versus the sample surface normal. The φ angle can change from 5o to 45o. The polarizer and the analyzer are included to remove the Kerr and Faraday phenomena by magnetic field, used to appear in the case of ferromagnetic materials. The transmittance is determined as the ratio of J/J0 ≡ T, where JT is light intensity transmitted on the photodetector and measured, and J0 is incident light intensity on the surface of the samples. However, in this study, the results of transmission intensity, presented in Figure 2, were used base on JT as a power converted to photodiode currents. Figure 1. Schema of experimental setup for visible light transmission measurements of the Co-Al2O3 films. Transmittance spectroscopy of polarized flght depend on direction of external magnetic field... 29 3. RESULTS AND DISCUSSION Figure 2 shows transmission T as a function of the φ angle varied in interval of 0 ÷ 45°. For the first, it can be seen that only a significant increase of T in the interval of φ = 0 ÷ 10° for the low-x samples (10 ÷ 25 Co at.%) at wavelength of 560 nm, after that there is a slow decrease again, as seen in Figures 2(a)-(c). For these samples, at long-wavelengths of 575 ÷ 695 nm the increase is fewer. Meanwhile for the high-x samples (35 ÷ 45 Co at.%), a significant increase of T in the interval of φ = 0 ÷ 10° is observed for all the wavelengths, as seen in Figures 2(d)-(f). It is also found that for the high-x cases, the angle-dependent transmission has a reducing trend at 0 10 20 30 40 50 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Tr an sm is si on Angle (Deg.) 695 nm 665 nm 650 nm 575 nm 560 nm 10 at.%Co B = 4 kOe (a) 0 10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 695 nm 665 nm 650 nm 575 nm 560 nm Tr an sm is si on Angle (Deg.) 15 at.%Co B = 4 kOe (b) 0 10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 695 nm 665 nm 650 nm 575 nm 560 nm Tr an sm is si on Angle (Deg.) 25 at. %Co B = 4 kOe (c) 0 10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 (d) 695 nm 665 nm 650 nm 575 nm 560 nm Tr an sm is si on Angle (Deg) 35 at.% Co B = 4 kOe 0 10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 695 nm 665 nm 650 nm 575 nm 560 nm Tr an sm is si on Angle (Deg.) 45 at.%Co B = 4 kOe (f) 0 10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 (e) 695 nm 665 nm 650 nm 575 nm 560 nm Tr an sm is si on Angle (Deg) 40 at.% Co B = 4 kOe Figure 2. Variation of transmittance of visible light at different wavelengths with angle between the direction of incident light beam and the surface normal φ for the granular thin films Cox-(Al2O3)1-x, where x is varied in arrange from 0.10 to 0.45, corresponding to 10 to 45 Co at.%. Giap Van Cuong, Tran Trung, Nguyen Anh Tuan 30 short-wavelengths, for example at 560 nm, and an enhancing trend at long-wavelengths, for example at 575 - 695 nm. An inversion trend versus the behaviors in the low-x cases even is shown for wavelengths of 560 nm and 575 nm, as seen in Figure 2(d). In addition, it can also be noted that the trend continues with an increase of the transmission in large angles of φ > 20° for the high-x samples at almost all visible wavelengths, as shown clearly in Figure 2(e)-(f). Finally, another behavior of the angular-dependence of the transmission was also observed in this study (if glancing through from Figure 2(a) to (f). That is the enhancing trend of the transmission level with increase in the Co content from 10 to 45 at.%. As seen from these figures, an significant enhancement level of the transmission at all wavelengths of 560-695 nm for the high-x cases is obviously higher than the one for the low-x cases presented in Figures 2(a)-(c). On the origin of the above phenomena for the angular-dependence of the transmission by visible light in MGFs, which are typically by Co-Al2O3 thin films, we suggest a preliminary model based on the mechanism of magnon-photon interaction. This mechanism is also a background for the so-called spin-plasmonic phenomenon [10, 11]. As known that a metallic nanoparticle (supposed a sphere) are excited by photons with energy corresponding to wavelengths that are bigger than its particle size R, the plasmonic resonance phenomenon can occur in the entire volume of the particle by the electrical field components E0 of electromagnetic radiation (EMR), as illustrated in Fig. 3(a). Plasmonic resonance is quite the electrostatic nature with dielectric constant of the metallic nanoparticles ε < 0 because of pinning and oscillation phenomenon for electrons trapped/confined in the metallic nanoparticles by EM radiation. Interaction between EMR field and the confined electrons will lead to the emergence of a located-E' electric field oriented in the same direction of E0 [12], as denoted in Fig. 3(b), and causes intensive transmission of EMR field. Consequence of this interaction for the Co- Al2O3 MGFs is to form a dipole moment p = αE’, where α is polarizability of the Co particle, dependent on Co particle-size R, the complex dielectric function of Co, εCo = RelεCo + iImεCo, and the dielectric constant of Al2O3, εAl2O3 by express as follows [13]: (*) 2 4 32 323 OAlCo OAlCoR εε εε πα + − = . Condition of the plasmonic resonance is just a minimum for the denominator of Eq.(*), determinated that ε’Co = -2εAl2O3. Meanwhile, the B0 component of the EM wave also effects on the spins of the polarized electrons, so that makes a precession for magnetization M of Co particles (Fig. 3(c)), which will be mentioned later on. On the other hand, in this case an interaction process between photons with the free conduction electrons appeared on the surface of metallic nanoparticles will take place. The electrons receive energy from photons to move to a state of excitation. However, the state is excited only in a very short period, and then return back to the ground state. Then, secondary photons with energies smaller than the ones of the incident photons, corresponding to longer wavelengths, are emitted (Fig. 3(d)) [11], and add to the beam of transmitted photons, to enhance the background intensity for trans-mission. When x increases, which means the amount and particle size of Co particles increases, the number of electrons excited at the surface of the Co particles increases also, causing increase of the secondary photons due to the photon-electron interaction. Transmittance spectroscopy of polarized flght depend on direction of external magnetic field... 31 When Co content overcomes the threshold, the particles tend to bind together to larger rafts [14], and then developed gradually until forming a continuous metallic film as x  1 (100 % Co). Thus the plasmon resonance phenomenon occurred primarily when x 0.5 the plasmon phenomenon almost loses. This can be recognized if the high-x case is analysed more thoroughly. Note that the high-x thin films can be considered as continued Co films included Figure 3. (a) EM wave with the electric E0 and magnetic B0 components of wavelength λ larger than particle size R. (b) Plasmonic mechanism and induction of the E’-field by the E0 for a normal metallic particle with ε < 0. (c) Spin-plasmonic mechanism for a ferromagnetic particle, where the B0 component polarizes plasmonic spins. (d) Mechanism for generation of second photon due to photon-electron interaction (After [13]). Al2O3 inclusions with nano-sizes. These inclusions are similar to dielectric nanovoids dispersed into the Co matrix. In other words, a shell-type model (for example, see Refs. [15], [16]) can be used to simulate approximately for the high-x Cox-Al2O3 thin films, where, considering entirely, the Al2O3 nanovoids are reproduced as though a nanosphere in role of a “core” of radius a, and the Co matrix in role of a “shell” of thickness b so that a << b, even b can be compared with λ. Then, in this case, surface plasmons arised that strongly localized to the Co void-edge surface propagate parallely to the surface but decay exponentially into large Co metallic bulk and away from the interface Co/Al2O3 [13]. 4. CONCLUSION Angle-dependent transmission for visible light of the magnetic granular thin films Cox- (Al2O3)1-x with different Co contents was investigated as a function of the variant wavelengths under applying by a fixed dc external magnetic field. For this dependence, there are three dominant features observed. (i) A significant increase of T in the interval of φ = 0 ÷ 10° for the low-x samples (10 ÷ 25 Co at.%) at wavelength of 560 nm, after that is a slow decrease again. However, for the high-x samples (35 ÷ 45 Co at.%), a significant increase of T in the interval of φ = 0 ÷ 10° is observed for all wavelengths. In the high-x cases, the angle-dependent transmission has a reducing trend at short-wavelengths, and an enhancing trend at long-wavelengths. (ii) E’ 2R E0 B0 λ/2 (a) (d) (b) (c) Giap Van Cuong, Tran Trung, Nguyen Anh Tuan 32 Increasing trend of the transmission in large angles of φ > 20° for the high-x samples at almost all visible wavelengths is received. (iii) Enhancing trend of the transmission level with increase in the Co content from 10 to 45 at.% is observed in this study. Initial mechanism of features is attributed to intensification in magnon-photon interaction indicated by spin-magnon polarization in Co nanoparticles. These results may be a sign of a spin-plasmonic phenomenon. However, it must be continued with more careful and detail studies, as well as it need to find out a suitable model for explaining thoroughly in next time. Acknowledgment: This research was funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02.2015.04. REFERENCES 1. Bacherlier G. and Mlayah A. - Surface plasmon mediated Raman scattering in metal nanoparticles, Phys. Rev. B 69 (2004) 205408. 2. Maier S. A., Atwater H. A. - Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures, J. Appl. Phys. 98 (2005) 011101. 3. Toudert J., Simonot L., Camelio S., Babonneau D. - Orthogonal metals: The simplest non- Fermi liquids, Phys. Rev. B 86 (2012) 045415. 4. Singh V., Aghamkar P. - A self-healing dielectric elastomer actuator, Appl. Phys. Lett. 104 (2014) 111112. 5. 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TÓM TẮT SỰ TRUYỀN QUA PHỤ THUỘC GÓC ĐỐI VỚI ÁNH SÁNG CỦA CÁC MÀNG MỎNG TỪ DẠNG HẠT Giáp Văn Cường1, 2, Trần Trung2, Nguyễn Anh Tuấn1, * 1Viện Đào tạo Quốc tế về Khoa học Vật liệu (ITIMS), Trườn Đại học Bách khoa Hà Nội, Số 1 Đại Cồ Việt, Quận Hai Bà Trưng, Hà nội, Việt Nam 2Trường Đại học Sư phạm Kĩ thuật Hưng Yên (UTEHY); Dân Tiến, Khoái Châu, Hưng Yên, Việt Nam *Email: tuanna@itims.edu.vn Sự truyền qua của ánh sáng (T), với bước sóng từ 560 đến 695 nm, đã được khảo sát ở các góc tới tạo với pháp tuyến bề mặt mẫu biến thiên trong khoảng φ = 0 ÷ 450 đối với các màng mỏng dạng hạt Cox-(Al2O3)1-x có tỉ lệ Co, x, thay đổi từ 10 % đến 45 % nguyên tử. Nghiên cứu này được tiến hành trong điều kiện các mẫu được đặt trong một từ trường tĩnh không đổi ở 4 kOe nhằm đảm bảo cho tất cả các hạt Co đều có hướng từ độ được duy trì ở một góc xác định so với phương lan truyền của ánh sáng tới. Các kết quả cho thấy một sự phụ thuộc góc đáng chú ý của quan hệ T(φ) vào tỉ lệ hạt Co đối với các bước song khác nhau trong dải nhìn thấy. Hành vi của sự phụ thuộc này đã được sơ bộ cho rằng đến từ tương tác magnon-photon. Từ khóa: màng mỏng từ dạng hạt, ánh sáng trong dải nhìn thấy, sự truyền qua phụ thuộc góc, tương tác magnon-photon.