Synthesis of various magnetic nanostructures and the microwave characterizations 2

Chapter Characterization techniques Chapter Experimental techniques 2.1 Materials synthesis 2.1.1 Preparation of Fe/SiO2 core-shell particles by Stöber process Fe/SiO2 core-shell particles were prepared by a Stöber process.[1] Firstly, the raw commercial carbonyl iron powder was added into a mixture solution of anhydrous ethanol (160 ml) and water (40 ml). Then 3ml ammonia solution (NH3•H2O) and ml tetraethyl orthosilicate (TEOS) were added to the solution. After magnetic stirring about 12 hours, another ml TEOS and ml NH3•H2O were added with continuous magnetic stirring for another hours. Finally, the product was washed three times with anhydrous ethanol and dried at room temperature. 2.1.2 Preparation of Fe/Al flakes by ball milling and jet milling The starting materials are commercial iron powder and aluminum powder. Fe and Al powders were weighted and mixed in the nominal composition of Fe90Al10. Prior to morphology modification process, the solid solution of Fe(Al) was first obtained by a dry milling process by an 8000M-230 Spex mixer for 12 h. The steel balls with diameter of mm were used for grinding. The ball to powder weight ratio was 50:1. The collected powder was named as sample S0, it consisted of non-uniform Fe(Al) 26 Chapter Characterization techniques particles with irregular shapes. A second step was wet milling for the preparation of micron flaky particles. On the one hand, sample S0 was dissolved into anhydrous ethanol and further milled for 0.5 h and h, separately. The products were correspondingly named as Sample S1 and Sample S2. On the other hand, sample S0 was milled by the Micron-Master Jet Pulverizer (Model 02-612-WC). The jet milling period is determined by the gas pressure provided by the air compressor. The pressure Air compressor Jet miller Air drier Fig. 2.1 Photo image of the jet miller systems including air compressor, air drier as well as jet milling machine. is set at bars in our system. After jet milling, two kinds of products could be received due to their weight difference. The one is flaky Fe/Al powders in submicron scale (Sample S3), which are relatively light and accumulate on the top layer of the receiver. The other is spherical Fe/Al powders (Sample S4), which are relatively heavy and fall onto the bottom of the receiver after milling. Fig. 2.1 shows the photo image of the jet miller systems. Besides the jet milling machine, air compressor and 27 Chapter Characterization techniques air drier are indispensable accessories. 2.1.3 Synthesis of Fe3O4 nanoparticles by thermal decomposition method In the synthesis of Fe3O4 nanoparticles, iron(III) acetylacetonate [Fe(acac)3] and oleic acid were added to benzyl ether. The mixture was purged with nitrogen gas for 30 Table 2.1 The amounts of starting materials for synthesis of Fe3O4 nanoparticles with different sizes. Particles size (nm) 18 21 53 114 430 Fe(acac)3 (mmol) 10 11 12 14 16 24 Oleic acid (mmol) 40 40 40 40 40 40 40 Benzyl ether (mL) 50 50 50 50 50 50 50 Starting materials to remove air at room temperature, then slowly heated to 165 ℃ for 30 min, and further heated to 280 ℃ to reflux the mixture for 30 min. The mixture was cooled down to room temperature after reaction. Finally, Fe3O4 nanoparticles were separated by centrifugation in toluene solvent. For the separation of nanoparticles smaller than 20 nm, the mixture of hexane and ethanol with volume ratio of 1:2 was used as the solvent for centrifugation. The particle size was controlled by adjusting the amount of Fe(acac)3 precursor while keep the amounts of oleic acid and benzyl ether constant. Table 2.1 listed the amounts of starting materials for the synthesis of Fe3O4 nanoparticles with sizes ranging from 28 Chapter Characterization techniques nm to 430 nm. 2.1.4 Synthesis of Zn-ferrite nanoparticles by thermal decomposition method Table 2.2 The amounts of starting material for the synthesis of Zn-ferrite nanoparticles with various compositions and sizes. Sample No. Fe(acac)3 (mmol) Zn(acac)2 Oleic acid Benzyl ether (mmol) Yielded composition ( from EDS) (mmol) (mL) ZF0 12 Fe3O4 28 20 ZF1 12 Zn0.189Fe2.811O4 28 20 ZF2 12 Zn0.290Fe2.710O4 28 20 ZF3 12 Zn0.387Fe2.613O4 28 20 ZF3_26.5 Zn0.38Fe2.62O4 28 20 ZF3_13.4 7.2 3.6 Zn0.38Fe2.62O4 28 20 ZF4 12 Zn0.468Fe2.532O4 28 20 ZF5 12 10 Zn0.522Fe2.478O4 28 20 ZF6 12 12 Zn0.524Fe2.476O4 28 20 ZF7 12 14 Zn0.527Fe2.472O4 28 20 Note: The particle sizes of samples ZF0 to ZF7 are all around 100nm, while the particle sizes of samples ZF3_26.5 and ZF3_13.4 are 26.5 nm and 13.4nm, respectively. In the synthesis of Zn-ferrite nanoparticles, both of iron(III) acetylacetonate [Fe(acac)3] and zinc acetylacetonate hydrate [Zn(acac)2·xH2O] were used as the precursors and dissolved into benzyl ether. Oleic acid was added as the surfactant. The mixture was heated to 120 ℃ and kept for 30 to eliminate the bonding 29 Chapter Characterization techniques water from precursors and finally heated to 280 ℃ at a rate of ℃/min to reflux the mixture for 30 min. The gray-black precipitates were collected by magnetic separation after the mixture was cooled down to ambient temperature naturally, and then washed by organic solvent (toluene or hexane) for several times. The molar ratio of Zn(acac)2·xH2O to Fe(acac)3 was adjusted to control the composition of as-synthesized Zn-ferrite, as listed in Table 2. Samples ZF0 to ZF7 were numbered according to the ratio of Zn/Fe precursors varying from to 1.25. The size of Zn-ferrite particles were controlled by scaling down/up the total amount of precursors. Take sample ZF3 for example. The particle size of sample ZF3 was about 104 nm. When the amounts of iron and zinc precursors were scaled down to mmol and mmol, respectively, the particle size of as-synthesized Zn-ferrite would reduce to 26.5 nm, thus this sample was named as ZF3_26.5. Similarly, Zn ferrite particles with size of 13.4 nm (sample ZF3_13.4) were obtained by further scaling down the iron and zinc precursors to 7.2 mmol and 3.6 mmol. 2.1.5 Synthesis of Fe3O4 nanoparticles via chemical reduction of α-Fe2O3 template 2.1.5.1 Synthesis of α-Fe2O3 nanoparticles with various shapes by hydrothermal route -Fe2O3 nanoparticles were synthesized by a hydrothermal treatment of FeCl36H2O (Sigma) in the presence of NH4H2PO4 (Sigma). For a typical synthesis of 110 nm 30 Chapter Characterization techniques -Fe2O3 nanorods, specific amount of FeCl3 and NH4H2PO4 aqueous solutions were mixed together. Then distilled water was added to the mixed solution to keep the final volume at 75 mL. After vigorous stirring for 30 minutes, the mixture was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 125 mL for hydrothermal treatment at 220 ℃ for 48 h. Then the autoclave was cooled down to room temperature naturally. The produced red precipitate was separated by centrifugation, then washed with absolute ethanol for times, finally dried in oven at 60 ℃. More than 100mg -Fe2O3 nanorods powder could be obtained after one single batch of experiment. The -Fe2O3 nanoparticles with different morphologies and sizes could be fabricated by adjusting the amounts of aqueous FeCl3 solution and aqueous NH4H2PO4 solution, as well as the volume of distilled water. In order to prepare -Fe2O3 nanoparticles with different structures, a serial of experiments were designed, as listed in table 2.3. 2.1.5.2 Synthesis of Fe3O4 nanoparticles via chemical reduction method using α-Fe2O3 nanoparticles as templates The -Fe2O3 templates (100 mg) were dispersed into 35mL of trioctylamine (TOA) solvent after ultrasonication for h. Then oleic acid (3.5 g) was added to the mixture and the final solution was transferred into a 100 mL three-neck flask with reflux condenser. The reaction was performed at 350 ℃ for h, then cooled down. The precipitates were collected and centrifuged with hexane (or toluene) for times and dried at room temperature. The whole process was under a flow of gas and with 31 Chapter Characterization techniques vigorous stirring. Pure Ar gas and a mixed gas involving 95%Ar with 5%H2 were employed for different reduction processes. The gas flow rate was controlled by a gas Table 2.3 Synthesis conditions for -Fe2O3 nanoparticles with various shapes and sizes FeCl36H2O (×10-3M) NH4H2PO4 (×10-3M) Total volume (mL) Outer diameter (nm) Length 154-ring 0.72 40 154 ±15 117 ±12 117-ball 20 1.44 40 117 ±12 ------ 74-ring 10 0.36 40 74 ±9 64 ±9 70-tube 20 0.72 40 70 ±10 363 ±79 120-rod 26.7 0.48 30 120 ±14 366±55 98-rod 20 0.36 40 98 ±8 244 ±25 61-rod 13.3 0.24 60 61 ±5 136 ±21 55-rod 10.7 0.192 75 55 ±4 113 ±11 Samples (nm) Note: The size of -Fe2O3 particles is labeled according to the average value of outer diameter. mass flow meter with a maximum rate of 200 sccm (i.e. mL/min). After the chemical reduction process, the morphology of the -Fe2O3 templates could be well preserved. Therefore, the size and shape of as-obtained Fe3O4 nanoparticles were totally determined by that of the -Fe2O3 templates. The above synthesis processes were all employed by this work. The as-synthesized products were further characterized through different measurements. The following 32 Chapter Characterization techniques part would introduce the characterization methods and the sample preparation procedures for various characterizations. 2.2 Materials characterizations 2.2.1 Structural and microstructural analysis 2.2.1.1 X-ray Diffraction (XRD) The phase structure of crystals could be examined by means of X-ray diffraction. The instrument for X-ray diffraction involves an X-ray source and an X-ray detector. The incident X-ray beams are emitted from the source and impinges onto the solid samples. For a crystalline solid consisting of a regular array of atoms, the X-rays will be scattered from lattice planes separated by the interplanar distance d and the signal received by the detector. As shown in Fig. 2.2, when two X-ray beams are scattered, they remain in phase since the path length of each ray is equal to an integer n (n = 1, 2, … ) multiple of the wavelength. The path difference between the two beams is given by 2d sinθ. When they interference constructively, Bragg’s condition should be fulfilled as[2] nλ = 2d sinθ (n = 1, 2, … ) (2.1) where n is an integer corresponding to the order of the diffraction plane, λ is the wavelength of the incident X-ray beams, d is the distance of two adjacent diffraction planes and θ is the incident angle (or scattering angle) of X-ray beams relative to the 33 Chapter Characterization techniques diffraction planes (the black dash lines in Fig. 2.2). A diffraction pattern is obtained by recording the intensity of scattered X-ray beams as a function of scattering angle. The peaks in the X-ray diffraction patterns are corresponding to a set of the crystalline orientations (d values) of a given material, which is used to characterize the phase structure with referring to the existing diffraction files (PDF) compiled by the Joint Committee of Powder Diffraction Standards (JCPDS). Fig. 2.2 Schematic diagram of X-ray diffraction by a crystal. In this work, phase examination was performed using a Bruker D8 Advance X-ray diffractometer (Cu𝐾𝛼 radiation, λ~1.5406 Å). The θ to θ scan was adopted with the 2θ angel ranging from 10°to 90°in the step of 0.02°. Besides the phase confirmation, the crystalline size of the polycrystalline powder materials can be estimated using the Scherrer equation:[3] 𝐷= 𝜅𝜆 𝛽𝑐𝑜𝑠𝜃 (2.2) where 𝜅 is shape factor (for spherical shape particles, 𝜅 = 0.9 ), 𝜆 is X-ray wavelength, 𝛽 is line broadening at half the maximum intensity (FWHM) in radians 34 Chapter Characterization techniques and 𝜃 is scattering angle. D is mean diameter of crystallines, which may smaller or equal to the grain size. To be noted, the Scherrer equation is limited to nano-scale particles with grain size less than 100 nm.  Sample preparation for XRD measurement For Fe/Al alloys, the powders are dispersed in ethanol and ultrasonicated for half an hour. Then the solvent is taken out with pipette and dropped on glass substrates. After the ethanol volatilizes away, the samples are ready for measurement. The same procedure is applied to the sample preparation of Fe3O4 and Zn-ferrite nanoparticles for XRD measurement except that the solvent of toluene is used instead of ethanol. 2.2.1.2 Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectrometer (EDS) SEM is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The electrons interact with the electrons within the material, producing various signals that can be detected and that contain information about the sample’s surface topography and composition. The types of signal produced include secondary electrons (SE), back-scattered electrons (BSE), primary electrons and characteristic X-rays. The morphology and composition of the samples were detected by a Field-emission scanning electron microscopy (FESEM; Zeiss Supra 40) equipped with an 35 Chapter Characterization techniques Energy-Dispersive X-ray Spectrometer (EDS). The Inlens detector was used for taking images at magnifications of 250x to 1,000,000x. The work distance was tuned between to mm. The working voltage could be adjusted. The commonly used voltage was kV. The EDS should be operated in the SE mode. And the work distance was kept at mm. The working voltage could be adjusted to fulfill a deadtime of 20% ~ 30%, which was good for signal collection. EDS spectra in the kinetic energy range of 0.1 keV to 20 keV were acquired for measured samples.  Sample preparation for SEM and EDS measurement Similar with the sample preparation for XRD measurement, Fe/Al alloys are dispersed in ethanol, while Fe3O4 and Zn-ferrite are dispersed in toluene. Dilute solutions are required for SEM measurement. After ultrasonication for a while, the solvent is taken out with pipette and dropped on Si wafer substrate. After the liquid (ethanol or toluene) volatilize away, the samples are ready for measurement. 2.2.1.3 Transmission Electron Microscopy (TEM) TEM is a sophisticated microscopy technique which can be used to obtain detailed microstructural information by high-resolution and high-magnification images or crystal structure information by diffraction patterns. Fig. 2.3 demonstrates the two operation modes of TEM, i.e. diffraction mode and imaging mode, depending on the used aperture.[4] When the electrons emitted are accelerated by the high voltage source, they can pass through the specimen. The electrons will be diffracted or 36 Chapter Characterization techniques scattered by the atoms within the specimen. A SAED aperture (or an objective aperture) is used in the diffraction mode (or imaging mode) to differentiate those electrons, determining which electrons configurate the image. In the diffraction mode, the image is configurated by the electrons which can fulfill the Bragg’s condition in Fig. 2.3 Schematic illustration of different working modes of transmission electron microscopy: (a) diffraction mode and (b) imaging mode. Eq. (2.1). The resultant image is either polycrystalline rings or single crystalline dots, as shown in Fig. 2.4. The lattice d spacing of the specimen could be determined from the diffraction patterns[5] d= 𝐿𝜆𝑒⁄ 𝑟 (2.3) 37 Chapter Characterization techniques where 𝐿 is camera length, 𝜆𝑒 is electron wavelength, 𝑟 is radius of the diffraction rings (for polycrystalline) or distance between diffraction spots (for single crystalline). In this work, a transmission electron microscopy (TEM, JEOL-2010 at 200 kV) was employed to detect the morphology, size and crystal structure of the nanoparticles. Both high resolution images and selected area electron diffraction (SAED) patterns were acquired. Fig. 2.4 Typical diffraction patterns for (a) single crystalline structure and (b) polycrystalline structure.  Sample preparation for TEM measurement Fe3O4/Zn-ferrite nanoparticles should be well dispersed in toluene prior to the sample preparation. Very dilute solution was required to get good TEM images. On drop of the diluted solution was taken out by pipette and dropped onto the carbon-coated copper grid for observation. 2.2.1.4 X-ray Photoelectron Spectroscopy (XPS) XPS is a quantitative spectroscopic technique which is used to measure the elemental 38 Chapter Characterization techniques composition and chemical state of elements within a material. XPS spectra are obtained by irradiating a solid material with monoenergetic soft X-ray beams while simultaneously measuring the kinetic energy and number of electrons which escape from the top ~ 10 nm of the material. The resultant kinetic energy (KE) of the electrons can be measured as[6] KE = hν − BE − 𝜑𝑠 (2.4) where hν is the energy of the photon, BE is the binding energy of the atomic orbital from which the electron escapes and 𝜑𝑠 is spectrometer work function. Each element has a characteristic spectrum. In this work, the chemical states of iron were investigated by X-ray photoelectron spectroscopy (XPS, ESCA LAB 220i-XL spectrometer). The energy source was a non-monochromatic magnesium X-rays corresponding to a photon energy of 1253.6 eV. The total energy resolution was 900 meV. The XPS spectra were fitted by XPS peak4.1 software.  Sample preparation for XPS measurement All the measured samples are powders. Put a little bit of powder onto the carbon conductive tape on the sample holder. For each sample, the covered area of the conductive tape by the powder is about 3mm × 3mm. 39 Chapter 2.2.2 Characterization techniques Magnetic properties characterizations 2.2.2.1 Vibrating Sample Magnetometer (VSM) VSM measures the magnetic properties of samples using an induction technique. Fig. 2.5 shows the schematic illustration of a VSM system. During the measurement, the sample is placed on a sample holder in between the two electromagnets and oscillated at a constant frequency ω in a vertical direction driven by a resonator. The oscillation of the magnetic sample under a magnetic field leads to the change in magnetic flux, which induces an alternating voltage in the pick-up coils according to Faraday’s law of induction.[7] The effective voltage could be given by U = kωM (2.4) where U is the induced alternating voltage, which is detected and amplified using a lock-in amplifier, and then transferred to the magnetic moment of sample by linking to proper electric circuit. M is the magnetic moment of the sample and k is a coefficient determined by the calibration of a nickel standard sample. The magnetic hysteresis loop (M-H loop) is the common form of the recorded data. From a M-H loop, we can read the saturation magnetization Ms , the remanence Mr , the coercivity Hc and so on. For the materials under this study, Ms is a most concerned parameter. Vibrating sample magnetometer (VSM; Lakeshore, Model 7404) was used in this work to collect the room temperature M-H loops of as-prepared 40 Chapter Characterization techniques Fig. 2.5 Schematic illustration of a VSM system. magnetic particles. Before the measurement, a moment gain calibration was performed to make sure the accuracy of the obtained Ms value. A nickel ball with a moment value of 6.92 emu/g under the magnetic field of kOe was used as the standard sample. In the measurement, the full M-H loops were recorded by slowing sweeping the applied magnetic field from a maximum positive field (20 kOe) to a maximum negative field (-20 kOe) and reversing to the maximum positive field (20 kOe).  Sample preparation for VSM measurement Magnetic powders (~ 10 mg) were required to be wrapped in the aluminium foil before the measurement. Write down The weight of the powders was written down for further calculation of the magnetic moment per unit mass. 2.2.2.2 Mössbauer spectroscopy Mössbauer spectroscopy is a spectroscopic technique based on the Mössbauer effect. 41 Chapter Characterization techniques (a) (b) (c) Fig. 2.6 The effects of (a) the isomer shift; (b) the quadrupole splitting and (c) the magnetic splitting on the nuclear energy levels of 57Fe. The Mössbauer absorptions and the resulting spectra are also shown. 𝛅 represents isomer shift and 𝚫 represents the quadrupole splitting. This effect consists of the recoil-free, resonant absorption and emission of gamma rays in solid. It can probe tiny changes in the energy levels of an atomic nucleus in response to its environment. Typically, three types of nuclear interaction may be observed, i.e. isomershift, quadrupole interactions and magnetic splitting.[8] The characteristic spectra are shown in Fig. 2.6. The simplest case produces a velocity shift of the peak in the transmission spectrum (Fig. 2.6a), called an isomer shift. The notations 1/2 and 3/2 refer to the nuclear spin angular momentum quantum number I. 42 Chapter Characterization techniques When the quadrupole moment at the nucleus interacts with the electric field gradient at the nucleus, it causes the 57 Fe Mössbauer spectrum to show a doublet (Fig. 2.6b). The phenomenon is called quadrupole splitting. If there is a magnetic field present at the nucleus, then hyperfine or Zeeman splitting takes place in the nuclear energy levels, producing a sextet in the Mössbauer spectrum, as shown in Fig. 2.6c. In this work, the inner magnetic structure of Fe3O4 and Zn-ferrite nanoparticles was demonstrated by zero-field Mössbauer spectra, which were recorded at room temperature, using a conventional constant acceleration spectrometer with a γ-ray source of 57Co/Pd embedded in palladium matrix. A spectrum of bcc-Fe was used for the calibration. Spectra of as-synthesized zinc ferrite nanocrystals were fitted based on Lorentzian site analysis by RECOIL software. 2.2.2.3 Vector network analyzer (VNA) A vector network analyzer is an instrument that measures the network parameters of electrical networks. In this work, it was employed to test the electromagnetic parameters of magnetic particles. The measurement requires that the samples are molded into a special shape. Firstly magnetic particles were mixed with paraffin wax (or epoxy resin) at a certain volume concentration. The mixture was then pressed into toroids with an outer diameter of 6.9 mm and an inner diameter of mm by suing a stainless steel mould under a pressure of MPa. The complex permeability (μr = μ′ + jμ′′) and permittivity (εr = ε′ + jε′′ ) of the composites were measured 43 Chapter Characterization techniques over the frequency range of 0.1-18 GHz using a vector network analyzer (Agilent PNA 8363B). The reflection loss (RL) was calculated from the measured μr and εr at given frequency and absorber thickness t according to the following equations:[9] RL = 20log|(Zin − Z0 )/(Zin + Z0 )| (2.5) Zin = Z0 √μr ⁄εr tanh{j(2πƒt⁄c)√μr εr } (2.6) where Zin is the input impedance at absorber surface, Z0 is the impedance of air, ƒ is the frequency of coming microwave, and c is the velocity of light. 2.3 References [1] W. Stöber, A. Fink, E. Bohn, J. Colloid. Interface Sci., 26, 62-69 (1968). [2] B. D. Cullity, “Elements of X-ray Diffraction”, Addison-Wesley, New York, 3rd Edn., p348 (1978). [3] H. P. Klug, L. E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, John wiley & Son Inc., New York, 2nd Edn., p687 (1974). [4] D. Tripathy, Half Metallic Fe3O4: An Experimental Study on Impurity Doping and the Giant Magnetoresistance, PhD Thesis, Nation University of Singapore, 2007. [5] S. Z. Wang, H. W. Xin, Radiat. Phys. Chem., 56, 567-572 (1999). [6] F. John, F. W. Sticle, E. P. Sobol, D. Kenneth, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, New York, p421 (1989). [7] F. T. Ulaby, Fundamentals of Applied Electromagnetics, New Jersy: Pentice Hall, p232 (2001). [8] P. E. Dominic, J. B. Frank, Mössbauer spectroscopy, Cambridge; New York: Cambridge University Press, p11, 1986. [9] Y. P. Duan, Y. Yang, H. Ma, S. H. Liu, X. D. Cui, H. F. Chen, J. Phys. D: Appl. Phys., 41, 125403 (2008). 44 [...]... 900 meV The XPS spectra were fitted by XPS peak4.1 software  Sample preparation for XPS measurement All the measured samples are powders Put a little bit of powder onto the carbon conductive tape on the sample holder For each sample, the covered area of the conductive tape by the powder is about 3mm × 3mm 39 Chapter 2 2 .2. 2 Characterization techniques Magnetic properties characterizations 2. 2 .2. 1 Vibrating... magnetic moment per unit mass 2. 2 .2. 2 Mö ssbauer spectroscopy Mö ssbauer spectroscopy is a spectroscopic technique based on the Mö ssbauer effect 41 Chapter 2 Characterization techniques (a) (b) (c) Fig 2. 6 The effects of (a) the isomer shift; (b) the quadrupole splitting and (c) the magnetic splitting on the nuclear energy levels of 57Fe The Mö ssbauer absorptions and the resulting spectra are also... temperature M-H loops of as-prepared 40 Chapter 2 Characterization techniques Fig 2. 5 Schematic illustration of a VSM system magnetic particles Before the measurement, a moment gain calibration was performed to make sure the accuracy of the obtained Ms value A nickel ball with a moment value of 6. 92 emu/g under the magnetic field of 5 kOe was used as the standard sample In the measurement, the full M-H loops... spectra are shown in Fig 2. 6 The simplest case produces a velocity shift of the peak in the transmission spectrum (Fig 2. 6a), called an isomer shift The notations 1 /2 and 3 /2 refer to the nuclear spin angular momentum quantum number I 42 Chapter 2 Characterization techniques When the quadrupole moment at the nucleus interacts with the electric field gradient at the nucleus, it causes the 57 Fe Mö ssbauer... sweeping the applied magnetic field from a maximum positive field (20 kOe) to a maximum negative field ( -20 kOe) and reversing to the maximum positive field (20 kOe)  Sample preparation for VSM measurement Magnetic powders (~ 10 mg) were required to be wrapped in the aluminium foil before the measurement Write down The weight of the powders was written down for further calculation of the magnetic. .. voltage in the pick-up coils according to Faraday’s law of induction.[7] The effective voltage could be given by U = kωM (2. 4) where U is the induced alternating voltage, which is detected and amplified using a lock-in amplifier, and then transferred to the magnetic moment of sample by linking to proper electric circuit M is the magnetic moment of the sample and k is a coefficient determined by the calibration... measures the magnetic properties of samples using an induction technique Fig 2. 5 shows the schematic illustration of a VSM system During the measurement, the sample is placed on a sample holder in between the two electromagnets and oscillated at a constant frequency ω in a vertical direction driven by a resonator The oscillation of the magnetic sample under a magnetic field leads to the change in magnetic. .. ⁄εr tanh{j (2 ƒt⁄c)√μr εr } (2. 6) where Zin is the input impedance at absorber surface, Z0 is the impedance of air, ƒ is the frequency of coming microwave, and c is the velocity of light 2. 3 References [1] W Stö A Fink, E Bohn, J Colloid Interface Sci., 26 , 62- 69 (1968) ber, [2] B D Cullity, “Elements of X-ray Diffraction”, Addison-Wesley, New York, 3rd Edn., p348 (1978) [3] H P Klug, L E Alexander, X-ray... state of elements within a material XPS spectra are obtained by irradiating a solid material with monoenergetic soft X-ray beams while simultaneously measuring the kinetic energy and number of electrons which escape from the top 1 ~ 10 nm of the material The resultant kinetic energy (KE) of the electrons can be measured as[6] KE = hν − BE − 𝜑 𝑠 (2. 4) where hν is the energy of the photon, BE is the binding... calibration of a nickel standard sample The magnetic hysteresis loop (M-H loop) is the common form of the recorded data From a M-H loop, we can read the saturation magnetization Ms , the remanence Mr , the coercivity Hc and so on For the materials under this study, Ms is a most concerned parameter Vibrating sample magnetometer (VSM; Lakeshore, Model 7404) was used in this work to collect the room temperature . 28 20 ZF3 12 6 Zn 0.387 Fe 2. 613 O 4 28 20 ZF3 _26 .5 8 4 Zn 0.38 Fe 2. 62 O 4 28 20 ZF3_13.4 7 .2 3.6 Zn 0.38 Fe 2. 62 O 4 28 20 ZF4 12 8 Zn 0.468 Fe 2. 5 32 O 4 28 20 . Zn 0.468 Fe 2. 5 32 O 4 28 20 ZF5 12 10 Zn 0. 522 Fe 2. 478 O 4 28 20 ZF6 12 12 Zn 0. 524 Fe 2. 476 O 4 28 20 ZF7 12 14 Zn 0. 527 Fe 2. 4 72 O 4 28 20 Note: The particle sizes of samples ZF0 to ZF7. further scaling down the iron and zinc precursors to 7 .2 mmol and 3.6 mmol. 2. 1.5 Synthesis of Fe 3 O 4 nanoparticles via chemical reduction of α-Fe 2 O 3 template 2. 1.5.1 Synthesis of