Microstruture high frequency property of fe co based nano grain thin films

MICROSTRUCTURE AND HIGH FREQUENCY PROPERTY OF Fe- & Co-BASED NANO-GRAIN THIN FILMS LIU YAN (B.Sc., JiLin University, China) A THESIS SUBMITTED FOR THE DEGREE OF DOCTORATE OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2007 ACKNOWLEDGEMENTS Having accomplished this thesis has brought to me a great sense of achievement and a fruitful gain of experiences. These would not have been possible without the guidance and encouragement of my supervisor Prof. Ong Chong Kim, to whom I wish to express my deepest appreciation. Prof. Ong is truly concerned for the well-being of his students and works tirelessly to make their research possible. I am very grateful to Dr. Chen Linfeng, Dr. Rao Xuesong, Dr. Tan Chin Yaw, and Dr. Liu Zhongwu, for their constructive opinions, invaluable discussions and great help in this research. I would also like to thank my friends Mr. Liu Huajun, Dr. Goh Wei Chuan, and Mr. Wang Peng, for the many helpful advices and discussions. I would like to thank Dr. Liu Binghai for his help in the transmission electron microscopy, to Mr. Yang Tao for his help in the magnetron sputtering, to Dr. Yan Lei for his helpful advice in the pulsed laser deposition, to Dr. Li Zheng Wen, Dr. Wu Yuping, and Mr. Lin Guoqing for their help in the ferrite study, and to Dr. Wang Shijie for his help in XPS measurement. Special thanks go to Mr. Tan Choon Wah and his team of staff at the machine workshop, Department of Physics, who had helped me to fabricate the fixtures required in my work. I would also like to thank my fellow colleagues at Centre of Superconducting and Magnetic Materials (CSMM), who have made my time there so enjoyable. These people are: Miss Mi Yanyu, Miss Lim Siew Leng, Miss Zhou Linlin, Miss Song Qing, Mr. Cheng Weining, Mr. Ning Min, Mr. Chen Xin, and Mr. Zhang Gufei. Lastly, I would like to thank my parents for their unfailing love and support, and most of all, my husband, for his love, patience, and support. This thesis is dedicated to them. ii This research was supported by Defence Science and Technology Agency (DSTA-NUS-DIRP/2004/02). iii TABLE OF CONTENTS ACKNOWLEDGEMENTS ii TABLE OF CONTENTS iv SUMMARY vii LIST OF TABLES ix LIST OF FIGURES x CHAPTER 1: INTRODUCTION 1.1 1.2 1.3 Magnetic thin films 1.1.1 Continuous magnetic thin films 1.1.2 Granular magnetic thin films 1.1.3 Patterned films 1.1.4 Thin films plated on hollow ceramic microspheres 1.1.5 Multilayer magnetic films Broad complex permeability characterizations of magnetic thin films 1.2.1 Pick-up coil method 1.2.2 Transmission-line perturbation method Objectives 13 References 15 CHAPTER 2: FABRICATION AND CHARACTERIZATION OF MAGNETIC THIN FILMS 2.1 11 26 rf magnetron sputtering 26 2.1.1 Glow discharge 26 2.1.2 rf sputtering 28 2.1.3 Magnetron sputtering 29 2.1.4 Reactive sputtering 30 2.2 Microstructure characterization by TEM 30 2.3 Magnetic property characterization 32 2.3.1 32 M-H loop tracer iv 2.3.2 2.4 Vibrating-Sample Magnetometer Electric property characterization by four-point resistance measurement 34 2.4.1 Four-point measurement 34 2.4.2 van der Pauw method for resistivity calculation 35 References 37 CHAPTER 3: HIGH FREQUENCY PERMEABILITY AND CHARACTERIZATION OF MAGNETIC THIN FILMS 3.1 38 Dynamic mechanisms of permeability 39 3.1.1 Magnetization precession without damping 39 3.1.1.1 Natural resonance and external field effect 40 3.1.1.2 Demagnetization field effect on resonance 41 3.1.2 3.1.3 3.2 34 Magnetization precession with damping 42 3.1.2.1 Landau-Lifchitz-Gilbert (LLG) equation 43 3.1.2.2 Bloch-Blombergen equations 47 Domain wall displacement 48 Permeability measurement using transmission-line perturbation 50 3.2.1 Derivation of permeability using shorted microstrip transmission-line perturbation 54 3.2.2 Improvement of measurement by a saturation magnetization method 59 3.2.3 Fabrication of permeability measurement fixture 61 3.2.4 Analysis by HFSS simulation 63 3.2.5 Characterization procedures of magnetic thin films 65 References CHAPTER 4: MICROSTRUCTURE, PROPERTIES, AND HIGH FREQUENCY PERMEABILITY OF FeCoN NANO-GRAIN THIN FILMS 71 74 4.1 Experiment 74 4.2 Effects of N on magnetic properties 76 4.3 Microstructure 79 4.4 Electrical resistivity 81 4.5 High frequency permeability 82 4.6 Magnetization ripples 84 4.7 Summary on FeCoN films 87 v References CHAPTER 5: MICROSTRUCTURE, PROPERTIES, AND HIGH FREQUENCY PERMEABILITY OF FeCoSiN FILMS WITH FeCo NANO-GRAINS EMBEDDED IN Si-RICH MATRIX 89 91 5.1 Experiment 91 5.2 Microstructure 93 5.3 Magnetic properties and electrical resistivity 95 5.4 High frequency permeability 98 5.5 Effect of Si on the films 99 5.6 Effect of N on the films 104 5.7 Summary on FeCoSiN films 107 References 108 CHAPTER 6: MICROSTRUCTURE, PROPERTIES, AND HIGH FREQUENCY PERMEABILITY OF CoAlO NANO-GRAIN THIN FILMS 111 6.1 Experiment 111 6.2 Magnetic properties 112 6.3 Microstructure 112 6.4 Electrical resistivity 117 6.5 High frequency permeability 118 6.6 Summary on CoAlO films 120 References 121 CHAPTER 7: CONCLUSION 122 List of publications by author 128 vi SUMMARY A method based on shorted transmission-line perturbation model to measure permeability spectra of ferromagnetic thin films at high frequency was developed in this study. A prototype measurement fixture has been fabricated and the measurement method was improved by using a saturation magnetization method. With this improvement, the effect of the electrical properties of films on the measurement was removed and the measurement accuracy was enhanced. The Landau-Lifchitz-Gilbert equation was used to study the permeability spectrum. The parameters that affect the permeability spectrum were analyzed and the theoretical permeability model was applied to examine the reliability and accuracy of the measurement results. This thesis also presents a study on the microstructure, high frequency permeability as well as electrical resistivity and magnetic properties of FeCoN, FeCoSiN, and CoAlO thin films. FeCoN soft magnetic thin films with various nitrogen contents were fabricated by varying the gas flow rate (GFR) of the argon and nitrogen gas mixture during sputtering. The FeCoN thin films were found to have high value of permeability and the ability to change the ferromagnetic resonant (FMR) frequency over a wide range by adjusting the GFR during deposition, and can be an important candidate for microwave applications. Hoffmann’s ripple theory was used to explain the permeability spectra for the films with strong magnetization dispersion. It was found that the ripple effect was responsible for the large damping in the films and those films with larger coercivities have stronger magnetization dispersion. To increase the resistivity of the films, a granular structure was obtained in the sputter-deposited FeCoSiN film. The resistivity of the FeCoSiN film can be significantly enhanced to 1100 μΩ·cm, which is much higher compared to that of other FeCo-based films. Another important finding is that FeCoSiN granular films vii consist of ordered arrays of FeCo nano-grains embedded in a Si-rich matrix with certain Si content. Furthermore, the addition of Si in the FeCoSiN film accounted for the change of the crystal structure of FeCo grains from bcc to fcc structure, which has the lowest anisotropy energy in the films. This study of FeCoSiN films established a way not only to fabricate high-resistivity FeCo-based films with tunable magnetic properties and high frequency permeability by controlling the Si concentration but also to synthesize ordered ultra fine FeCo nano-grains with potential for applications in nano self-assembly system. In the last part of this thesis, granular film CoAlO was fabricated by rf magnetron sputtering. It was found that Co grains of fcc and hcp structures coexist in the films with relatively low (Al,O) content, whereas fcc Co grains are preferentially formed in the (Al,O)-rich films. Due to the microstructure change with increasing (Al,O) content in the films, coercivity and magnetization decreased but the anisotropy field increased. The microstructure change in the CoAlO films was the main reason for the abrupt jump of the FMR frequency from 2.1 to 2.8 GHz. Although CoAlO thin films not have high resistivity as expected in the soft magnetic range, this kind of film consisting of Co nano-grains surrounded by amorphous Al-O phase has potential for application in which such a jump on the resonant frequency is needed. viii LIST OF TABLES Table 5.1 6.1 6.2 7.1 Caption Magnetic properties and electrical resistivity of some FeCo-based films. Page 95 The exchange length and domain wall width of Co for different structures. 114 Ms, Hk / Heff, and α used in the calculation for complex permeability spectra. 119 The grain size, resistivity ρ, saturation magnetization Ms, the minimum coercivity Hc in each series of films, the anisotropy field Hk, the resonant frequency fr, and the permeability at low frequency μ’in of the investigated films. 123 ix LIST OF FIGURES Figure Caption Page 2.1 A schematic diagram of dc glow discharge. 28 2.2 A schematic diagram of planar-magnetron. B and E represent magnetic field and electric field, respectively. 29 2.3 A schematic diagram of an M-H loop tracer. 33 2.4 A circuit diagram illustrating the four-point measurement setup. 35 2.5 A schematic diagram of a rectangular van der Pauw configuration. 36 3.1 Resonant frequency as a function of the applied field H both parallel and perpendicular to the preferred axis in a material having uniaxial anisotropy. 41 Schematic diagram of a magnetic thin film in the microwave magnetic field. 43 Complex permeability spectra calculated with α=0.01, 0.03, and 0.05, Ms=1.5 T, and Hk=5 mT. 45 Complex permeability spectra calculated with Ms=1.0 T, 1.5 T, and 2.0 T, α=0.01, and Hk=5 mT. 45 Complex permeability spectra calculated with Hk=2 mT, mT, and mT, α=0.01, and Ms=1.5 T. 46 Model of domain-wall displacement under the influence of a longitudinal ac magnetic drive field. 48 Microstrip circuits for characterization of magnetic thin films. (a) Transmission approach and (b) reflection approach. 52 Electric field (solid line) and magnetic field (dashed line) distribution at the cross section of the region with the magnetic thin film sample. 53 Cross section of the microstrip circuit (a) loaded with substrate without thin film and (b) loaded with substrate coated with thin film. 58 (a) Photograph and (b) schematic diagram of shorted microstrip line fixture. 61 Magnetic field distributions of the fixture at GHz. (a) Empty fixture. (b) Fixture loaded with a dielectric substrate without magnetic film. (c) Fixture loaded with a dielectric substrate coated with magnetic thin film with zero conductivity. (d) Fixture loaded with a dielectric substrate coated with magnetic thin film with conductivity σ=1000 S/m. 63 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 x From Figure 6.1 and Figure 6.2, those CoAlO films with larger grain size have lower Hc and higher Hk. According to Herzer,3 if the grain size D exceeds the domain wall width, the magnetization process is determined by domain wall pinning at the grain boundaries, and the Hc decreases according to 1/D law. On the other hand, for small grain size, Hc increases following a D6 power law. The domain wall width is given by δB=π Lex=π (A/K1)1/2, where Lex is the ferromagnetic exchange length, A B denotes the exchange stiffness and is 3×10-11 J/m3 for Co,4 and K1 is the magnetocrystalline anisotropy constant. The Lex and δB of Co for different structures B are calculated and listed in Table 6.1.5 In the CoAlO films, although the grain size of the Co grains is smaller than the domain wall width, the Hc is obviously not following D6 power law but is closer to the 1/D law. This is because in the CoAlO films, the Co grains are surrounded by amorphous Al-O phase, and hence the extension of the grain boundary results in the increase of the effective grain size. In addition, the separation of the Co grains leads to a weaker magnetic coupling, which also attributes to the decrease of the Hc. The effective anisotropy is calculated by taking the average over several grains within the volume V= Lex3 and is reduced in magnitude by averaging out the magneto-crystalline anisotropy. With the increase of the grain size, the number of the grains within V decreases and thus the effective anisotropy increases as well. Thus, the increased Hk as well as decreased Hc with the increase of the (Al,O) content in the CoAlO films are due to the increased grain size. Table 6.1 The exchange length and domain wall width of Co for different structures Co structure a Anisotropy constant (J/cm3)a Lex (nm) δB (nm) B hcp Ku1=4.4×105 Ku2=1.3×105 8.25 25.9 fcc K1=-5.7×104 K2=1.3×104 22.9 71.9 Reference 5. 114 From Figure 6.2, it can be seen that as the amorphous Al-O content increases in the granular films, the structure of Co grains changes from a mixture of hcp and fcc structures to entirely fcc structure. In Figure 6.2 (b), (d), both fcc and hcp d-spacings were found, whereas in Figure 6.2 (f) only fcc d-spacings can be found. Although many values of the hcp Co and fcc Co d-spacings are too similar to be distinguished, e.g. d-spacing of hcp Co (002) is 2.03 Å, while fcc Co (111) is 2.04 Å, two d-spacings of hcp Co (101) 1.92 Å and (102) 1.48 Å not appear in fcc Co, and also the dspacing of fcc Co (200) 1.78 Å is not found in the hcp structure. In addition, Figure 6.2 (d) and (f) show a small amount of CoO content. The phase constitution was further investigated by HRTEM. Figure 6.3 shows HRTEM images of CoAlO films with various number of Al2O3 chips on the target. In Figure 6.3 the Co grains with lattice space of about ~2.0 Å due to fcc Co (111) or hcp Co (002) were observed, and the amorphous Al-O phase surrounding the Co grains with different width in (a) (b), and (c) was also observed. 115 Figure 6.3 HRTEM images of CoAlO films sputtered with (a) chip, (b) chips, and (c) 12 Al2O3 chips on the target. From above results it can be deduced that as amorphous Al-O phase increases, the formation of fcc Co grains are preferred. The easy axis for hcp Co is along [001] direction, whereas the easy axis for fcc Co is along [111] direction. The anisotropy energy for the [111] orientation is reported to be the lowest in the Co structure.1,6 This 116 fcc structure results in a lower Hc and a higher in-plane Hk compared to hcp structure. The lower Hc and higher in-plane Hk of the fcc structure are generally beneficial for microwave application. 6.4 Electrical resistivity Figure 6.4 Resistivity of CoAlO films with various numbers of Al2O3 chips on the target. The resistivity of CoAlO films with various numbers of Al2O3 chips on the target is from tens to hundred, as shown in Figure 6.4, which is in the range of low oxygen content when compared to reference 1. The low resistivity also indicates that the metallic conduction still exists in the films and this is further confirmed by the TEM images shown in Figure 6.2 in which the grains are interconnected with each other. In this experiment, 12 chips of Al2O3 almost covered the most sputtered ring of the inch Co target, so it is possible that further increase of Al2O3 chips will not be of much help for increasing resistivity as the increase of the oxygen gas low rate does1. 117 6.5 High frequency permeability Figure 6.5 Complex permeability spectra measured and calculated for CoAlO film sputtered with (a) 1, (b) 2, (c) 4, (d) 8, and (e) 12 Al2O3 chips on the target. Figure 6.5 shows the measured and calculated permeability spectra for CoAlO films with various numbers of Al2O3 chips on the target. For the calculation, the values of Ms were obtained from the M-H loops, the first three values of Hk obtained from the M-H loop, and the last two were adjusted to an appropriate Heff according to the 118 experimentally measured results of permeability for the magnetization ripples that could appear as discussed in Chapter 4. Figure 6.5 indicates that the calculations using the LLG equation are in good agreement with the experimental results. The Hk, α used in the calculation, and the permeability values from the experimental results are listed in Table 6.2. As discussed in Chapter 3, α affects fr slightly, the tradeoff of the decreased Ms and the increased Hk play a dominant role in the change of fr. Furthermore, the fr of the first three samples is about 2.1 GHz and jumps to 2.8 GHz for the last two samples. The dominant reason is the increase of the effective anisotropy field. This suggests that the magnetic anisotropy in the ferromagnetic CoAlO films, in addition to the induced uniaxial magnetic anisotropy by an applied field during sputtering, results from the anisotropic coupling between the magnetic granules. Furthermore, the fcc Co grains contributes more than the hcp Co grains to the anisotropy field. Table 6.2 Ms, Hk / Heff, and α used in the calculation for complex permeability spectra Sample Al2O3 on the target Hk / Heff (Oe) α µin′a µmax″b fr (GHz) (a) chip 46 0.03 324 582 2.17 (b) chips 50 0.04 336 401 2.11 (c) chips 60 0.04 242 343 2.11 (d) chips 90c 0.03 215 314 2.80 (e) 12 chips 100c 0.03 141 297 2.85 a µin′ represents µ′ at low frequency. µmax″ represents µ″ at resonant frequency. c Adjusted Heff b 119 6.6 Summary on CoAlO films This study shows that nano-granular CoAlO thin films consisting of Co nano- grains surrounded by amorphous Al-O phase can be fabricated by sputtering. It was found that the Co grains of fcc and hcp structures with a mean diameter of 5-30 nm coexist in the films with relatively low (Al,O) content, whereas fcc Co grains are preferentially formed in the (Al,O)-rich films. Due to the microstructure change with increasing (Al,O) content in the films, coercivity and magnetization decrease but the anisotropy field increases. The increase of the anisotropy field in the CoAlO films is the dominant reason in the abrupt increase of the ferromagnetic resonant frequency from 2.1 to 2.8 GHz. 120 References S. Ohnuma, H. Fujimori, S. Mitani, and T. Masumoto, "High-frequency magnetic properties in metal-nonmetal granular films", Journal of Applied Physics, 79, 5130 (1996). Y. Liu, L. F. Chen, C. Y. Tan, H. J. Liu, and C. K. Ong, "Broadband complex permeability characterization of magnetic thin films using shorted microstrip transmission-line perturbation", Review of Scientific Instruments, 76, 063911 (2005). G. Herzer, "Grain-Size Dependence of Coercivity and Permeability in Nanocrystalline Ferromagnets", IEEE Transactions on Magnetics, 26, 1397 (1990). N. Dao, M. J. Donahue, I. Dumitru, L. Spinu, S. L. Whittenburg, and J. C. Lodder, "Dynamic susceptibility of nanopillars", Nanotechnology, 15, S634-S638 (2004). D. Weller, G. R. Harp, R. F. C. Farrow, A. Cebollada, and J. Sticht, "Orientation Dependence of the Polar Kerr-Effect in Fcc and Hcp Co", Physical Review Letters, 72, 2097 (1994). A. Hosono and Y. Shimada, "Crystal-Structure and Magnetic Softness of Fe-Si Polycrystalline Films", Journal of Applied Physics, 67, 6981 (1990). 121 CHAPTER CONCLUSION In order to apply magnetic thin films at high frequency range, it is essential to measure the high frequency permeability and understand the electromagnetic properties that can control the high frequency permeability. In this thesis, the permeability measurement at high frequency for soft magnetic thin films was investigated; three types of magnetic thin films including FeCoN, FeCoSiN, and CoAlO were fabricated, and their microstructure, properties and high frequency permeability were studied. Thin films with high value of permeability are achieved with resonant frequency tunable from 0.45 to 2.99 GHz. A measurement fixture based on shorted transmission-line perturbation model has been designed, fabricated and improved upon by a saturation magnetization method. LLG equation was applied to examine the reliability and accuracy of the measurement results, and the theoretical calculation and measurement results agree with each other. The shorted transmission-line perturbation method with improved accuracy developed in this study provides a valuable means of characterizing magnetic thin films at high frequency. Three series of magnetic films were mainly studied in this work, FeCoN, FeCoSiN, and CoAlO. Under certain deposition conditions and some compositions, good soft magnetic properties with low coercivity, high saturation magnetization, high anisotropy field, and good μ-f response can be obtained, but high electrical resistivity was only achieved in FeCoSiN films. The addition of insulating element Si was the key reason for the high resistivity, but the spatial separation of the FeCo nano-grains by Si-rich matrix also plays a part. Furthermore, the FeCoSiN granular structure was 122 found to consist of ordered arrays of FeCo nano-grains embedded in Si-rich matrix and this establishs a way to synthesize ordered arrays of ultra fine nano-grains with potential for applications in nano self-assembly system. Phase transition of bcc to fcc structure of FeCo in the FeCoSiN films and a mixture of hcp and fcc to pure fcc structure in the CoAlO films were found. The reason for these two phase transitions is that in the film state, fcc structure which was found to have the lowest anisotropy energy was preferred, as the Si or Al-O nonmagnetic phase which could be considered as inclusion, appears and increases other types of energy. Table 7.1 The grain size, resistivity ρ, saturation magnetization Ms, the minimum coercivity Hc in each series of films, the anisotropy field Hk, the resonant frequency fr, and the permeability at low frequency μ’in of the investigated films. Magnetic thin films FeCo FeCoNa FeCoSiNb CoAlOc Grain size (nm) – e.g. 3-5 12 – 7.5 – 28.5 ρ (μΩ⋅cm) 72.7 167 – 211 152 – 1100 52.3 – 125.2 Ms (T) 2.38 1.83 – 1.6 1.2 – 0.5 1.48 – 1.02 10.9 / 6.55 3.02 / 3.54 5.84 / 2.36 22.0 / 6.5 Hk (Oe) 37.5 40 – 44 46 – 48 – 80 fr (GHz) 2.09 2.16 – 2.99 2.1 – 0.45 2.1 / 2.8 μ’in 680 580 – 245 400 336 –141 Hc (Oe)_minimum easy axis/hard axis a The grain size is obtained from the sample deposited at GFR=7 sccm; ρ, Ms, Hk, fr and μ’in are listed as N content increases in the films. b The grain size, ρ, Ms, Hk, and fr are listed as Si content increases in the films; μ’in is obtained from the sample deposited with chips of Si. c The grain size, ρ, Ms, Hk, and μ’in are listed as Al-O content increases in the films; fr jumps from 2.1 to 2.8 GHz as the Al-O content increases in the films The grain size, resistivity ρ, saturation magnetization Ms, the minimum coercivity Hc in each series of films, the anisotropy field Hk, the resonant frequency fr, 123 and the permeability at low frequency μ’in are summarized in Table 7.1. The magnetic properties including Ms, Hc, and Hk were found to be strongly related to the microstructure of the magnetic thin films, while the high frequency properties including fr and μ’in were related to the static magnetic properties. The influences of the various parameters on these properties are briefly described below. Influence of microstructure on Hc in the magnetic thin films: Low Hc can be obtained at a certain value of nitrogen concentration in FeCoN films, e.g. when GFR=3 to 10 sccm, Hc < Oe. Depending on the nitrogen concentrations, FeCoN films can form several iron nitrides with different structures and properties. Within the interstitial nitrogen solubility limit in the FeCo crystalline lattice, N acted as a grain refiner to decrease the growth of the FeCo grain size and may occupy interstitial sites in an ordered way in some FeCo phase, e.g. α”- FeCo phase appears. This is the reason for the lower Hc. In FeCoSiN films, Si was firstly found to preferentially combine with N rather than FeCo due to the higher binding enthalpy, and soft magnetic properties were not found in the films. With the increase of Si, the soft magnetic properties are improved as the formation of SiN amorphous phases defines the FeCo grain boundary and decreased the grain size. This leads to a lower Hc according to Herzer’s D6 power law in which Hc increases with the increase of the grain size D. However, further addition of Si in the films tends to lead to the formation of superparamagnetic films in which the exchange coupling is much weakened by the separation of magnetic nano-grains. 124 Al-O content in the CoAlO films does not seem to change Hc significantly in the investigated range of this study; however, superparamagnetic properties can be expected with higher content of Al-O. Influence of microstructure on Hk in the magnetic thin films: Hk does not change much in the FeCoN films, while significant change in FeCoSiN films and CoAlO films were found. Herzer’s random anisotropy model was used to explain the relationship between Hk and the grain size. The effective anisotropy is calculated by taking the average over several grains within the volume V= Lex3 and is reduced in magnitude by averaging out the magneto-crystalline anisotropy. With the increase/decrease of the grain size, the number of the grains within V decreases/increases and thus the effective anisotropy increases/decreases as well. Thus, the decrease of Hk in FeCoSiN films is due to decreased grain size and the increase of Hk in CoAlO films is due to increased grain size. Influence of microstructure on Ms in the magnetic thin films: FeCo film still has the highest Ms among the entire investigated films, up to 2.38 T. For the nitrogenated FeCoN films, the α”- FeCo phase which was reported to have high Ms of 2.4-2.9 T was found in the FeCoN film deposited at GFR=7 sccm, which leads to a maximum Ms 1.83 T. On the other hand, Ms decreases greatly as the nonmagnetic phase appears in the FeCoSiN and CoAlO films, as shown in Table 7.1. Influence of magnetic properties on fr in the magnetic thin films: Kittel’s Equation, f r = γ 2π H ( H + 4π M s ) (see section 3.1.1.2) can be used to roughly estimate the relation of Hk and Ms to fr since it is obtained from the 125 magnetization precession with no damping. LLG equation is used to examine measurement results, with the effective field and damping factor appropriately chosen. In this study, both FeCoN and FeCoSiN were found to have a tunable resonant frequency from 2.16 to 2.99 GHz and from 2.1 to 0.45 GHz, respectively. For FeCoN thin film deposited at GFR of 10 sccm and above, it was found that the measured permeability results can not be well fitted with the LLG equation using Hk. The reason was that the anisotropy in these films was dispersed, and the deviation from homogeneous magnetization due to local anisotropies, so-called magnetization ripple, caused an intrinsic demagnetization field in the films. Hoffmann’s ripple theory was used to explain the permeability spectra for the films with strong magnetization dispersion. By fitting the measured results to the calculated transverse bias permeability, the ripple constant which describes the strength of the ripple effect is larger for FeCoN thin film with the GFR of sccm than that of film with GFR of 15 sccm. This explained the strong ripple effect occurring in the sample with GFR of 15 sccm. It was also found that the ripple effect is responsible for the large damping in the films and those films with larger coercivities have stronger magnetization dispersion. For CoAlO films, a jump in fr was found and it was believed to result from the change of microstructure. Influence of magnetic properties on μ’in in the magnetic thin films: Stoner and Wolfarth theory gives the value of the static permeability as μ ′ = M s H , and it can be used to roughly estimate μ’in from Hk and Ms. FeCo film k 126 still has the highest μ’in among the investigated films due to its highest Ms. With the addition of N and Si, μ’in decreases accordingly. 127 List of publications by author [1]. Liu Y., Tan C. Y., Liu Z. W., and Ong C. K., “FeCoSiN films with ordered FeCo nanoparticles embedded in a Si-rich matrix”, Applied Physics Letters, vol. 90, 112506, 2007. [2]. Liu Y., Tan C. Y., Liu Z. W., and Ong C. K., “Microstructure and high frequency properties of nano-granular CoAlO thin films”, Journal of Applied Physics, vol. 101 (2), 023912, 2007. [3]. Liu Y., Liu Z. W., Tan C. Y., and Ong C. K., “High frequency characteristics of FeCoN thin films fabricated by sputtering at various (Ar+N2) gas flow rates”, Journal of Applied Physics, vol. 100 (9), 093912, 2006. [4]. Liu Y., Chen L. F., Tan C. Y., Liu H. J., and Ong C. K., “Broadband complex permeability characterization of magnetic thin films using shorted microstrip transmission-line perturbation”, Review of Scientific Instruments, vol. 76 (6), 063911, 2005. [5]. Liu Z. W., Liu Y., Ma Y. G., Tan C. Y., Ong C. K., “Co-based nanogranular thin films on flexible substrate for gigahertz applications”, Journal of Magnetism and Magnetic Materials, vol. 313, 37, 2007. [6]. Liu Z. W., Phua L. X., Liu Y., and Ong C. K., “Microwave characteristics of low density hollow glass microspheres plated with Ni thin-film”, Journal of Applied Physics,, vol. 100 (9), 093902, 2006. 128 [7]. Ma Y. G., Liu Y., Tan C. Y., Liu Z. W., and Ong C. K., “Magnetic anisotropy and high frequency permeability of multilayered nanocomposite FeAlO thin films”, Journal of Applied Physics, vol. 100 (5), 054307, 2006. [8]. Liu Z. W., Liu Y., Yan L., Tan C. Y., and Ong C. K., “Thickness-dependent properties of FeTaN thin films deposited on flexible substrate”, Journal of Applied Physics, vol. 99 (4), 043903, 2006. 129 [...]... anisotropy The combination of these important properties of Fe- and Co- based magnetic thin films is most desirable at high frequency applications and thus the concept of FeCo -based films was proposed.19-24 FeCoN thin films with high saturation magnetization have been explored by several researchers Wang24 reported that the FeCoN thin films had Ms up to the range of 2.5-2.7 T with 5-25 at.% Co contents deposited... (c), (f) HRTEM image of the FeCoN thin films sputtered at GFR of 7 sccm and 20 sccm, respectively 79 4.5 Resistivities of FeCoN thin films deposited at various GFR 81 4.6 Measured and calculated permeability spectra of FeCoN thin films at GFR of (a) 7 sccm and (b) 15 sccm 82 Ferromagnetic resonant frequency and permeability at low frequency of the FeCoN thin films as a function of GFR 84 Measured and... CrystalStructure of High Moment Fetan Materials for Thin- Film Recording-Heads", Journal of Applied Physics, 73, 6573 (1993) 18 N Ishiwata, C Wakabayashi, and H Urai, "Soft Magnetism of High- NitrogenConcentration Fetan Films" , Journal of Applied Physics, 69, 5616 (1991) 19 V Bekker, K Seemann, and H Leiste, "Development and optimisation of thin soft ferromagnetic Fe- Co- Ta-N and Fe- Co- Al-N films with in-plane... that the growth of α″-(FeCo)16N2 phase in the films resulted in the high saturation magnetization Similar results were reported in Kuo’s20 paper in which FeCoN thin films with Ms of 2.39 T was fabricated and the high saturation magnetization achieved was attributed to the combination of high magnetic moment nitrides of α -Fe In addition, Sun23 reported FeCoN thin films possessing low Hc of 3 Oe and there... between coercivity and the grain size In these three studies, the FeCoN thin films possessed high saturation magnetization and low coercivity, and while these properties showed that FeCoN is an ideal material for high frequency application, until now no detailed high frequency characterization has been done 1.1.2 Granular magnetic thin films Granular films consist of magnetic metallic nano- grains embedded... target 101 Resistivity of FeCoSiN films with various numbers of Si chips on the target 101 Coercivity Hc of both easy axis and hard axis, and saturation magnetization Ms of the FeCoSiN films with various number of Si chips on the target 102 Anisotropy field Hk and resonant frequency fr of the FeCoSiN films with various number of Si chips on the target 103 The variation of coercivity of both easy axis and... (μ=μ′-jμ″) of magnetic thin films in the high frequency range; • To fabricate magnetic thin films based on Fe- and Co- based with good soft magnetic properties, such as high saturation magnetization, low coercivity, high anisotropy as well as high resistivity to reduce eddy current in the high frequency applications; • To tune the resonant frequency and permeability values by adjusting the composition... effect of the electrical properties of the films on the measurement, the use of this saturation magnetization method which reduces the measurement error on the measurement is an important contribution to the accuracy in characterization This study mainly focuses on the Fe- and Co- based magnetic thin films, typically FeCoN, FeCoSiN, CoAlO, due to their well-known soft magnetic properties The results of. .. results of the Fe- and Co- based magnetic films in this study, including results of soft magnetic properties and permeability measurement results using the method mentioned above, are likely to be a useful source of information for the application of the magnetic thin films at high frequency In the next chapter, fabrication details and the characterization methods of Feand Co- based magnetic thin films will... films and their potentials for application will be discussed 2 1.1 Magnetic thin films The current research in the magnetic thin films can be broadly classified into five types, continuous films, granular films, patterned films, thin films plated on hollow ceramic microspheres, and multilayer films This study mainly focuses on the continuous films and granular films 1.1.1 Continuous magnetic thin films . MICROSTRUCTURE AND HIGH FREQUENCY PROPERTY OF Fe- & Co -BASED NANO-GRAIN THIN FILMS LIU YAN (B.Sc., JiLin University, China) A THESIS SUBMITTED FOR THE DEGREE OF DOCTORATE OF PHILOSOPHY. coercivity, whereas Co -based films exhibit strong uniaxial anisotropy. The combination of these important properties of Fe- and Co -based magnetic thin films is most desirable at high frequency applications. Continuous magnetic thin films The first type of magnetic thin films to be presented here is the continuous films, which are mostly based on Fe and Co elements. This kind of films has high saturation