Defect-induced magneto-optic properties of MgO nanoparticles realized as optical-fiber-based low-field magnetic sensor Ch. N. Rao, Umesh T. Nakate, R. J. Choudhary, and S. N. Kale Citation: Appl. Phys. Lett. 103, 151107 (2013); doi: 10.1063/1.4824772 View online: http://dx.doi.org/10.1063/1.4824772 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v103/i15 Published by the AIP Publishing LLC.
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APPLIED PHYSICS LETTERS 103, 151107 (2013)
Defect-induced magneto-optic properties of MgO nanoparticles realized as optical-fiber-based low-field magnetic sensor Ch. N. Rao,1 Umesh T. Nakate,1 R. J. Choudhary,2 and S. N. Kale1,a) 1
Department of Applied Physics, Defence Institute of Advanced Technology (DIAT), Girinagar, Pune 411025, India 2 UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore, India
(Received 12 July 2013; accepted 26 September 2013; published online 8 October 2013) The spintronic applications of defect-magnetism in oxides have been explored for a long time. However, limited success has been obtained. We report on FCC-structured, magnesium oxide nanoparticles (20 nm) deposited on the mirror-surface of single-mode-optical-fiber as an effective low-field magnetic sensor. These show magnetic behavior and good magneto-opticKerr-effect signal. Red-shift phenomenon has been found in the birefringence pattern, when a magnetic field is applied. The sensitivity of red-shift is 202.4 pm/mT. Such red-shift phenomenon is ascribed to the influences of defect-induced magnetism on the optical-wave C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4824772] propagation. V
The success of the spintronics technology is largely dependent on the realization of room temperature magnetism at low magnetic fields. With this focus in mind, researchers all round the globe are putting tremendous efforts and are using different materials, dopants, and varied synthesis strategies to explore their properties. Development of magnetic semiconductors, as one class of such materials, is underway. In this, efforts are made to use a host semiconducting matrix and dope it with magnetic impurity so as to impart magnetism, keeping the optical property of host, intact. This is indeed tricky and many intriguing issues creep in, such as dopant clustering, interstitial-doping, and so on. While these efforts are going on, a different and interesting school-of-thought has emerged, which is intrinsic-defect-magnetism. Vacancies in a lattice structure, typically in wide-band-gap semiconductors, have shown magnetism.1–4 In these works, the materials are synthesized in nanocrystalline size and are explored as magnetic materials as well. The reports do exist; few of which are indeed encouraging. These show magnetism, along with their optical and semiconducting properties intact. However, the results are not always repeatable. Due to this, such materials are far from implementation in a real technology. A brief literature survey is depicted below. Intensive research is done on wide band gap semiconducting oxides, especially doped ZnO. Last decade has also witnessed dopants such as Mn, Co, Fe, Cr, and Cu, wherein the dopant percentage was varied from 1% to 7%.5,6 However, the progress has been limited due to the large disparities in experimental results and interpretations.7 Controversies have also originated due to the fact that the observed ferromagnetism was usually so weak that it was hard to distinguish it from the extrinsic sources or artefacts.7–9 Room temperature magnetism has been reported in the un-doped materials such as for Hafnium oxide,TiO2, MgO, and ZnO (nanoparticles, powders, nanorods). This magnetism, as documented by few groups, was not coming from any partially filled d-orbitals but from a)
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moments induced in the p-orbitals of the oxygen band.10–12 Concerning the origin of ferromagnetism in Hafnium oxide, the role of oxygen vacancies in the form of defects has been discussed.13,14 On the other hand, few researchers believe that lattice distortion plays the role in exhibiting magnetism.15,16 Though it has been observed in many different types of materials, this finding has raised a fundamental question about the origin of the magnetism in non-transition metal systems. Hence, even though the issue is of high scientific interest, harnessing material properties has remained a challenge. Interestingly, it has been predicted that the oxygen terminated MgO (111) surface manifests spin polarized state.17 Moreover, nanocrystalline MgO powder has also displayed ferromagnetic (FM) state which is attributed to the Mg vacancy defect near the surface.12 This is the motivation of our work. Assuming that the materials in their nanoforms could show magnetism, if one is able to harness them by optical manipulation, it can yield extremely good results. Furthermore, with an optical probe, the system approaches towards a practical spintronic device. In this work, we report a defect-induced magnetic sensor realized using optical sensing mechanism. MgO nanoparticles were synthesized using microwaveassisted co-precipitation method. The nanoparticles were further deposited on the mirror surface of single-mode optical fiber, which is a part of Y-coupler. The coupler connects one end connected to the C-Band source and other end connected to the optical spectral analyser (OSA). A sensitive magnetic sensor is hence demonstrated, with the sensitivity of 202.4 pm/Oe. The results have been correlated to the defectmagnetism evolved in the MgO nanocrystallites, which modulate the light propagating (reflecting from this active sensing-coat) through the probe, upon the applied field. Nanocrystalline MgO was synthesized using microwave irradiation method. The precursors used for synthesis were magnesium nitrate (Mg(NO3)26H2O, 99.5% pure) and sodium hydroxide (NaOH, 99.8% pure). All chemicals were procured from Thomas Baker. 1M NaOH in de-ionised (DI) water was added drop by drop to magnesium nitrate solution (0.1M) with continuous magnetic stirring. White precipitate
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FIG. 1. The experimental setup of the sensor assembly.
was formed at pH 13. Microwave-assisted hydrolysis was performed with frequency of 2.45 GHz and 350 W. Precipitate solution was kept in the micro-oven at 80 C for 30 min under stirring. Whitish viscous gel was formed which was cooled to room temperature. Final precipitate was washed with DI water using centrifuge (3000 rpm) and then dried at 100 C for 20 min. This amorphous powder was annealed at 450 C for 2 h in muffle furnace. The well characterized MgO samples were then used for further studies. X-ray diffraction (XRD) technique, magneto-optic Kerr effect (MOKE) system, superconducting quantum interference device (SQUID-VSM, Quantum Design), and atomic force microscopy (AFM) have been used for analysis. For coating of MgO on the optical fiber tip, PVA (Poly Vinyl Alcohol) was used as binder. 100 mg of PVA was dissolved in 40 ml DI water at 100 C. The mixture was continuously stirred to yield a viscous gel. 20 mg of MgO powder was then added to 2 ml of PVA gel and grinded well to form uniform mixture of MgO and PVA. This gel was dip-coated (the thickness was 15 lm) to deposit on the mirrored-surface of the optical fiber coupler. The schematic of the sensor setup is shown in Fig. 1. The optical fiber used in this work was a single-mode fiber (SMF), which was in the form of a Y-coupler (SMF-28 (3 dB, 1 2) coupler), in which, from one side, laser source (C-band (ALS-10-B-FC)) was connected. The beam propagated towards the coupler’s island point. With such couplers, the power is directed to the mirrored surface and reflected power goes to the detector. MgO was deposited on this mirrored surface. The beam of light was expected to reflect from the MgO-modified-mirrored surface and interfere with the beam which was going towards the detector (an OSA). An interference pattern, so formed, was further modulated with the application of external magnetic field. The sensor head was then subjected to the applied magnetic field. Fig. 2 shows the XRD pattern of the MgO nanocrystallites. Face-centered-cubic structure was observed.18 Based on Scherrer’s formula, the particle size was determined as about 20 nm. The MgO gel showed similar XRD pattern (not shown). The inset of Fig. 2 shows the AFM image of the MgO nanoparticles. Clear dispersion of MgO particles of the size of 20 nm was seen in the image, which agreed well with the calculated XRD analysis. MOKE is a well suited tool to study thin film magnetism, specifically called as SMOKE (surface MOKE).19–21 In our experiments (results not shown), the applied magnetic field was in the range of 1500 to þ1500 Oe. A distinct Kerr
FIG. 2. The XRD pattern of MgO nanocrystallites. Inset: the AFM imaging of the MgO nanoparticles of the size of 20 nm are seen in the image.
signal, with hysteresis loop, was seen with the applied field which clearly indicated that the sample exhibit ferromagnetic properties. SQUID-VSM measurements further confirmed the magnetic nature of the sample. Fig. 3 shows the room temperature M-H curve of the MgO sample. A small hysteresis loop was observed at the low field range, as shown in the inset of Fig. 3. The diamagnetic response was also seen in Fig. 3 in the high field range. Such observation is documented by many research groups.22,23 The magnetism in the region of 1 kOe to þ1 kOe was 0.025 emu/g, which was appreciably large. Inset of Fig. 3 shows the region near origin, expanded. Small hysteresis loop with coercivity and remnence was seen. The magnetism was mainly ascribed to the defect-induced effects itself. Many references do point out to such observations. Andriotis et al.22 and Xing et al.23 have explored the magnetism in ZnO material. They have attributed these to the defect-induced magnetism. The samples having more defects (such as synthesized using sol-gel method) showed stronger room temperature magnetism, as compared to high-quality (MBE grown) films. They attributed their results to zinc vacancies and also compared their results to first-principles density functional theory calculations. Maoz et al.24 have
FIG. 3. The room temperature M-H curve of the sample. The complete scan shows magnetic moment in the lower fields, which decreases at higher fields.
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studied highly defective nanosheets of MgO. These sheets have exhibited ferromagnetism. Using electron paramagnetic resonance experiments, they have confirmed that the sample comprised strongly interacting spin clusters, concentrated along extended defects, possibly as unpaired electrons trapped at oxygen vacancies. Kumar et al.25 have done work on MgO nanoparticles and have observed ferromagnetism in their samples, which they attributed to Mg vacancies. On similar lines, many other researchers reported that the bulk MgO was nonmagnetic and MgO nanocrystals showed room temperature ferromagnetism, which probably originates due to the loss (Mg vacancies) of donor charge of O atoms, which will form the 2p holes at the surface of the nanograins. Based on the first-principles calculations, Gao et al.26 investigated defect induced magnetism in MgO and suggested that the induced magnetic moment was due to the spin- polarization of 2p electrons of O atoms near Mg vacancies. These calculations showed that the pure MgO bulk is nonmagnetic. Oxygen vacancies do not induce ferromagnetism. The ferromagnetism in MgO nanomaterial is not only due to the vacancies of Mg but also the distribution/positions of the vacancies. The magnetic moment of MgO increases with the increasing percentage of Mg vacancies. For the sensing studies, the OSA was continuously monitored as the sensing material was being deposited. The fringence pattern was observed only after 5 times of dipcoating. The laser beam propagated through the fiber and reflected back from the mirror surface. Since it was a SMF, the fringence pattern emerged only when the laser beam propagated through multiple reflections from the silica and coated-silica layers of the coated material. The velocity of laser beam, being different in silica and in MgO coat (refractive index of silica is 1.46 and MgO is 1.72), and the reflection modes from these two types of surfaces was different. This resulted in birefringence pattern, which is shown in Fig. 4(a) (the curve for 0 mT). Fig. 4(a) shows the response (reflected power, in dB) of the sensor assembly to the applied magnetic field. A clear shift in the birefringence pattern was observed as a function of applied field. The elaborated graph is depicted in Fig. 4(b). The sensitivity was found to be 202.4 pm/mT, which was better than the best documented value of 131 pm/mT.27 It was however observed that beyond 1000 Oe, the response saturated. The results could be understood from the SQUID-VSM data, which also shows ferromagnetic behavior until around 1000 Oe, and later shows its diamagnetic response. It was exactly in this region that the sensor also showed the birefringence shift. As the sample moved in its diamgnetic regime, the response also saturated. The results agreed well with the MOKE data. Considering references quoted above,22–26 it is proposed here that MgO nanocrystallites exist in the form of defects. These defects are either due to Mg or O vacancies at the grain boundaries. The refractive index of MgO is 1.72 and that of silica fiber is 1.46. As the beam propagates towards the mirrored surface, it will traverse from the rarer medium to denser medium and will reflect back from various depths of the MgO coat. Several additional modes would emerge depending upon the difference in refractive index, and hence the velocity of the beam which is reflected back in the fiber.
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FIG. 4. (a) The response of the sensor assembly to the applied magnetic field. The elaborated graph is depicted in (b).
These will form a fringence pattern. On application of magnetic field, the MgO nanocrystallites respond to modify the reflected beam. This will lead to the variation in mode coupling and hence the fringence shift. Defect magnetism, as is realized here, can be actually useful in a magnetic field sensing. The coupling of photonics physics to conventional sensing techniques gives us a handle to tap the shifts as low as pm and nm (per Oe). In conclusion, defect magnetism has been used to fabricate a low-field magnetic sensor with the sensitivity of 202.4 pm/mT. It demonstrates promising applications in the area of magnetoelectronics and spintronics. The authors from DIAT thank the funding from ER-IPER, DRDO to “DIAT-DRDO Programme on Nanomaterials.” The authors also thank Dr. V Raghevendra Reddy, UGC-DAECSR, Indore, for his help with MOKE instrumentation. Thanks also to Mr. P. Bankar and Dr. S. S. Datar for their help in AFM imaging.
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