Magnetic properties and high frequency characteristics of FeCoN thin films Tae-Jong Hwang, Joonsik Lee, Ki Hyeon Kim, and Dong Ho Kim Citation: AIP Advances 6, 055914 (2016); doi: 10.1063/1.4943358 View online: http://dx.doi.org/10.1063/1.4943358 View Table of Contents: http://aip.scitation.org/toc/adv/6/5 Published by the American Institute of Physics AIP ADVANCES 6, 055914 (2016) Magnetic properties and high frequency characteristics of FeCoN thin films Tae-Jong Hwang, Joonsik Lee, Ki Hyeon Kim, and Dong Ho Kima Department of Physics, Yeungnam University, Gyeongsan, 38541, Korea (Presented 15 January 2016; received November 2015; accepted 17 December 2015; published online March 2016) (Fe65Co35)N soft magnetic thin films were prepared by reactive RF magnetron sputtering with the sputtering power of 100 W on thermally oxidized Si substrate in various nitrogen partial pressures (PN2) A strong uniaxial in-plane magnetic anisotropy with the easy-axis coercive field as low as 1∼2 Oe was observed in films grown at PN2 in the range from 3.3% to 5.5% The saturation magnetizations for those films were about 20 KG Outside this range, almost isotropic magnetization curves were observed Vector network analyzer and grounded coplanar waveguide were used to measure the ferromagnetic resonance (FMR) signals up to 25 GHz The FMR signals were detected only in anisotropic films and their FMR frequencies were well fit to the Kittel formula The obtained g-values and damping parameters at magnetic fields >20 kOe for films grown at PN2 of 3.3%, 4.8% and 5.5% were 1.96, 1.86, 1.92 and 0.0055, 0.0047, 0.0046, respectively This low damping factor qualifies FeCoN thin films for high-frequency applications C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4943358] I INTRODUCTION FeCoN thin films are a promising material for high frequency applications because of high saturation magnetization and low coercivity since nitrogen in the films plays an important role in the microstructure of FeCoN.1–9 Magnetic characteristics of saturation magnetizations 4πMs above 20 kG, coercivities of only a few Oe, and high uniaxial in-plane anisotropy have been typically obtained when the nitrogen content is optimized Varying the gas flow rate of fixed argon/nitrogen gas mixture during the sputtering resulted in changes of static as well as dynamic properties of FeCoN films such as saturation magnetization, coercivity, damping parameter and ferromagnetic resonance (FMR) frequency.4 High frequency behavior and damping in such FeCoN thin films have been analyzed by using small signal solution of the Landau-Lifshitz-Gilbert (LLG) equation.1,10 FeCoN film with a selected seed layer showed soft magnetic properties because the seed layers released the stress of FeCoN from the substrate For example, FeCoN film prepared with Permalloy seed layer showed a FMR frequency over GHz at zero-bias field, which is very promising for applications in write heads and integrated inductors.1 Magnetic ripples in the film by inhomogeneity of the substrates in strains can lead to change of magnetic properties FeCoN thin films sputtered on the flexible Kapton substrate showed ultrawide frequency linewidth, which was explained by the dispersed magnetic ripples due to the flexible substrates.3 Control of the sputtering gas pressure can adjust the stress of film which has been shown to be effective in tuning the damping parameters.9 In this work, we show that magnetic properties and high frequency characteristics of FeCoN thin films are sensitive to the nitrogen partial pressure percentage (PN2) of sputtering gas a Author to whom correspondence should be addressed Electronic mail: dhkim@ynu.ac.kr 2158-3226/2016/6(5)/055914/5 6, 055914-1 © Author(s) 2016 055914-2 Hwang et al AIP Advances 6, 055914 (2016) II EXPERIMENT The FeCoN films were prepared by RF magnetron sputtering in Ar+N2 gas mixture with a base pressure lower than 3×10−7 Torr The target composition was Fe65Co35 The nitrogen content was varied by changing PN2 while the total pressure of Ar+N2 gas mixture was kept constant at mTorr during sputtering FeCoN films were deposited with six different PN2 (0%, 3.3%, 4.8%, 5.5%, 6.7%, and 7.8%) on thermally oxidized Si substrate without any heating The substrate holder was cooled by the same coolant that passes through the sputtering guns An external field of about 400 Oe was applied to the in-plane direction of the film to induce uniaxial in-plane anisotropy The sputtering power and time was 100 W and min, respectively, which yielded films of thicknesses between 70 and 78 nm Crystal structure was analyzed by X-ray diffraction (XRD) and magnetic properties including the magnetization were characterized by vibrating sample magnetometer (VSM) High frequency characteristics, i.e FMR frequency and frequency linewidth, were obtained by using vector network analyzer (HP8510C) and grounded coplanar waveguide in the frequency range from 100 MHz to 26.5 GHz An external magnetic field up to kOe has been applied parallel to the magnetization easy axis and the RF field along the hard axis III RESULTS AND DISCUSSION The role of nitrogen on the crystal structure of FeCoN thin film was investigated by XRD study For pure FeCo thin film, a broad α-Fe(Co,N) (110) peak was observed Adding optimal amount of nitrogen, the main (110) peak was replaced by a broad bump for three samples with PN2 = 3.3%, 4.8%, and 5.5% The broadening of the (110) peak, that corresponds to a decrease of average grain size, with increasing PN2 has been commonly observed6–8 and attributed to formation of an amorphous phase in the film caused by interstitial nitrogen atoms For higher nitrogen content, different crystalline phases other than α-Fe(Co,N) (110) were observed: For PN2 = 6.7%, a ε-Fe(Co)3N (101) peak, and for PN2 = 7.8%, a paramagnetic ζ-Fe(Co)2N (021, 211) peak were detected The role of nitrogen on the magnetic properties of FeCoN thin film can be clearly seen in the hysteresis loops of FeCoN thin films measured by VSM as shown in Fig The solid lines are easy-axis loops and the dotted lines are hard-axis loops Here hard-axis loop means the hysteresis curve measured when the external in-plane field is applied to the 90◦ from the easy axis With proper addition of nitrogen to FeCo, soft nature of magnetic properties as well as in-plane uniaxial anisotropy was greatly enhanced for the three samples with PN2 = 3.3%, 4.8%, and 5.5%, in which amorphous phase was formed The coercivities along the easy and hard directions, Hce and Hch, reached minimum values of 1.1 Oe and 2.1 Oe, respectively, for PN2 = 5.5%, an order of magnitude smaller than those in the pure FeCo and higher PN2 samples Even with formation of amorphous phase, the saturation magnetization 4πMs showed no degradation but even a slight increase for the two out of three samples compared to that of pure FeCo thin films, reaching the highest 4πMs of slightly higher than 20 kG at PN2 of 4.8% All those FeCoN thin films with very soft magnetic properties in the same time showed clear uniaxial in-plane anisotropy Films prepared with the other PN2 had no in-plane anisotropy The softness of magnetic properties of three FeCoN thin films prepared with PN2 = 3.3%, 4.8%, and 5.5% may occur due to very small grains Earlier work on soft nanocrystalline ferromagnets revealed that coercivities had very strong grain-size dependence if the grain size is smaller than 50 nm.11 Above this limit, the corecivities decrease with grain size Applying the Scherrer equation to the (110) peaks of XRD data of the pure FeCo thin film, the average grain sizes were estimated as 13 nm For films with isotropic loops prepared at higher PN2 = 6.7% and 7.8%, the grain sizes were found to be around 15 nm Since (110) peaks of the three anisotropic FeCoN thin films with PN2 = 3.3%, 4.8%, and 5.5% were much broader, these films must have much smaller grains with sizes less than 10 nm and, in this size range, strong dependence of the coercivities on the grain size is expected Thus, very low coercivities of around Oe in these films are supposedly due to the formation of very small nano-sized grains Nano-sized grains with grain size of about 10 nm or Hwang et al 055914-3 AIP Advances 6, 055914 (2016) FIG Magnetization hysteresis loops of FeCoN thin films prepared with partial nitrogen pressure (a) 0%, (b) 3.3%, (c) 4.8%, (d) 5.5%, (e) 6.7%, and (f) 7.8% The solid lines are easy-axis loops and the dotted lines are hard-axis loops Three films of (b), (c) and (d) showed soft magnetic properties as well as uniaxial in-plane anisotropy 3-5 nm grain size embedded in an amorphous matrix have been identified in FeCoN thin films by using high-resolution electron microscope2,4,6 or by applying the Scherrer equation to XRD data.5 Beside soft magnetic nature, FeCoN thin films usually have relatively high resistivity, which is advantage to use them for RF shield Relatively high resistivity of ∼160 µΩ·cm was reported in single-layer FeCoN films made with 10% nitrogen flow rate in the total sputtering gas.6 When FeCoN films are grown with Permalloy nanolayer seeds, a resistivity of 50 µΩ·cm was reported.1 We observed relatively high resistivity of 170 µΩ·cm to 210 µΩ·cm for PN2 from 3.3% to 5.5% Detailed properties of the three FeCoN films are listed in Table I The high-frequency dynamics of magnetization can be well described by the phenomenological LLG equation with a characteristic precession and the Gilbert damping associated with the precession When the frequency of RF field is equal to the precession frequency f res, resonance absorption so called the ferromagnetic resonance takes place In this work, the FMR parameters were obtained from standard microwave S-parameter measurements by using vector network analyzer (VNA) and grounded coplanar waveguide (CPW) as a function of frequency at static magnetic fields up to kOe TABLE I Magnetic and high frequency properties of FeCoN thin films prepared with nitrogen partial pressure of 3.3%, 4.8%, and 5.5% N2 Coercivity– Resonance Frequency Damping partial Saturation easy and frequency at Landé linewidth at parameter at pressure Thickness Resistivity magnetization hard axis Oe and g- Anisotropy Oe and Oe and (%) (nm) (µΩ·cm) (kG) (Oe) kOe (GHz) factor field (Oe) kOe (GHz) kOe 3.3 4.8 5.5 77 74 76 170 180 210 20.1 20.4 18.4 3.9/4.4 1.4/2.4 1.1/2.0 2.64/23.0 2.19/22.0 2.04/21.7 1.96 1.86 1.92 45.0 35.0 37.0 0.519/0.239 0.016/0.0055 0.349/0.196 0.011/0.0047 0.375/0.182 0.013/0.0046 055914-4 Hwang et al AIP Advances 6, 055914 (2016) FIG (a) Resonance frequencies as a function of external magnetic field of three FeCoN thin films prepared with nitrogen partial pressure of 3.3%, 4.8%, and 5.5% The lines are fit to the Kittel formula of equation (1) (b) Damping parameters as a function of external magnetic field for the same films FMR signals were detected only in three FeCoN thin films that exhibit uniaxial in-plane anisotropy The data were analyzed assuming that the dominant CPW signals were the quasi-transverse electromagnetic mode FMR frequency f res and the frequency linewidth ∆ f at the half maximum of the resonance peak were determined from fit of the absolute transmission parameter S21 obtained by VNA to the Lorentzian curve for spectra measured at high magnetic fields However, low-field resonance peak showed deviations from the Lorentzian shape In this case, the phase information of S21 which corresponds to the real part of susceptibility was used to determine f res and ∆ f ; the zero-crossing frequency was assigned to f res and peak-to-peak frequency span to ∆ f The external field dependence of FMR frequency for a magnetic thin film with uniaxial in-plane anisotropy at saturation along the easy axis is given by the Kittel formula written as12 γ (Hext + Hk ) (Hext + Hk + 4πMs ) (1) f res = 2π where Hk is the anisotropy field and γ = gµ B is the gyromagnetic ratio with the Landé g-factor and the Bohr magneton µ B Fig 2(a) shows the f res as a function of external magnetic field The solid lines are fit of the data to equation (1) We fixed 4πMs as 20.1 kOe, 20.4 kOe, and 18.4 kOe for PN2 = 3.3%, 4.8%, and 5.5%, respectively, which were obtained from the VSM measurements, and used g and Hk as fitting parameters The results were g = 1.96, 1.86, and 1.92, and Hk = 45.0 Oe, 35.0 Oe, and 37.0 Oe for PN2 = 3.3%, 4.8%, and 5.5%, respectively The experimental value of the Gilbert damping parameter α of the LLG model can be obtained from ∆ f by applying the following relation α= 2π∆ f γ [2 (Hext + Hk ) + 4πMs ] (2) Fig 2(b) shows the experimentally determined α from ∆ f through equation (2) as a function of external magnetic field The values of α for all the three thin films showed similar field dependence: They sharply decreased with Hext from the range of 0.011-0.016 at a zero field to 0.0046-0.0055 at kOe A same kind of dependence on Hext of the damping parameter has been observed in FeCoB thin films.13 The intrinsic α is assumed to be independent of Hext, thus, large α, equivalently broad ∆ f , for Hext