Effect of annealing temperature on crystallographic texture, magnetic and microwave properties of barium ferrite thin films

Due to it has large saturation magnetization (Ms), high magnetocrystalline anisotropy field and high ferromagnetic resonance (FMR) frequency, barium ferrite (BaM) has attracted more and more attentions in the fields of magnetic recording media, permanent magnets and microwave devices. Here, BaM thin films were deposited on Pt/TiO2/SiO2/Si substrates, and the effect of annealing temperature on the microstructure, magnetic and microwave properties of barium ferrite thin films was investigated in detail. It is found that when the BaM thin film was annealed at 1035 °C, it has good properties. The XRD data provide clear evidence that the BaM thin films have high c-axis orientation, and the Lotgering factor is as high as 0.96. The AFM morphology show that BaM grains are out of the film plane, and they are hexagonal. The magnetic hysteresis curves indicated that both saturated magnetization (4πMs), remanence ratio and coercive (Hc) for out of plane increase with increasing Ta first, then decreased, and get the maximum value at 1035 °C. The ferromagnetic resonance (FMR) measurement show that the FMR linewidth is 143 Oe@50 GHz, it means that this this sample has low microwave loss in millimeter wave loss, and the FMR absorption can be tuned by applied magnetic field. These results make sure that this BaM thin film is possible use in millimeter wave devices such as filers, circulators and isolators.


Introduction
Barium hexaferrite (BaFe 12 O 19 , BaM) has been widely applied in the fields of magnetic recording media, permanent magnets and microwave devices due to it has high magnetocrystalline anisotropy field (H A ), large saturation magnetization and moderate microwave loss [1][2][3].BaM has magnetoplumbite structure with space group of P63/mmc (No. 194) [4], and its magnetic easy axis is parallel to crystallographic c axis [5].The crystal structure of BaM can be represented as a stacking sequence of basic blocks RSR * S * , the asterisk indicates that R (or S) block is turned 180 °C around the easy axis.There are 64 ions including Ba 2+ , Fe 3+ and O 2− in the unit cell of BaM.The Fe 3+ ions are distributed on five different crystallographic sites, namely, 12k, 2a, 2b, 4f1 and 4f2 [6].It has been proved that the Fe 3+ ions have an important role on microwave and magnetic properties.For example, the H A can be tuned by ions substitution of Fe 3+ , it is increased when Fe 3+ is doped with Al 3+ [7], however, it is decreased when substituted with Ga 3+ [8], meanwhile, the ferromagnetic resonance frequency can be also adjusted in widen frequency range [9].This is attributed to the change of Fe 3+ -O-Fe 3+ super-exchange interactions.This convenience in designing microwave and magnetic properties makes BaM a candidate for microwave and millimeter wave devices (such as circulator, isolator) in next generation microwave integrated circuit.In microwave devices application, high crystallographic orientation is required to offer biasing magnetic field, and low microwave loss is also needed.
Typically, single crystal or highly oriented polycrystalline bulky BaM is used to offer biasing magnetic field for millimeter wave devices, which lead to a larger size and weight of BaM-based microwave device [10,11].Hence, it is not compatibility with the next generation MMIC technology which need miniaturized and self-biasing devices.To meet this requirement, it is thus necessary to grow highly oriented BaM thin film with selfbiasing and low microwave loss.There are in-plane and out-of-plane crystallographic orientation in BaM thin films, in order to meet the requirement of circulator and isolator, out-of-plane BaM are needed.Many works have been carried out to deposited BaM thin films, it was found that the properties of BaM are strongly depending on various experimental factors such as deposited processes, annealing atmosphere.For example, PLD is easy to deposit highly oriented BaM thin film [12], but the Mr/Ms of BaM is low, and the laser fluence play an important role on the stoichiometry, crystallographic texture, and magnetic properties of BaM [13].Magnetron sputtering are also a popular way to highly oriented BaM thin film [14], however, the quality of BaM is depend on the target, type of substrate, sputtering gas, sputtering power, substrate temperatures.Both PLD and magnetron sputtering need vacuum environment and expensive equipment.In addition, they are timeconsuming and energy-intensive.Sol-gel process is another way to grow thin film [15], it is high-efficiency and repeatability, this feature makes it feasible application industrial-level use.In this work, Sol-gel method was applied to grow highly oriented BaM thin film Pt/TiO 2 /SiO 2 /Si wafer, and the effect of annealing temperature (Ta) on the magnetic and microwave properties of BaM was investigated in detail.

Materials and methods
The starting materials used for depositing BaM thin films were Fe(NO 3 ) 3 , Ba(NO 3 ) 2 and ethylene diamine tetraacetic acid (EDTA).All of these starting materials are purchased from Macklin.The starting materials were mixed in DI water, and keep stirring for 6 h to get homogeneous solution, NH 3 .H 2 O and HCl were used to tailor pH value, while polyvinylpyrrolidone was used to adjust the viscosity.The solution was spin-coated on Pt/TiO 2 /SiO 2 /Si wafer, the sample was immediately dried on hot plates after spinning.These processes were repeated couple times to increase the thickness of BaM thin films.After that, all samples were annealed in N 2 at 950 °C firstly, the heating rate is 20 °C s −1 , and then annealed in N 2 for different temperature (950 °C, 1000 °C, 1035 °C and 1050 °C), and flow ratio of gas is 80 ml min −1 .
The x-ray diffractometer (XRD) is used to determine the composition, structure and crystallographic orientation.The topography images were characterized by atomic force microscope (AFM).Superconducting quantum interference device (SQUID) measurement was carried out to measure the magnetic properties with the applied magnetic field normal to film plane, and the applied magnetic field is varied from −50 kOe to +50 kOe.The microwave properties were measure by ferromagnetic resonance (FMR) system using short waveguide with the frequency changed from 30 GHz to 60 GHz, and the applied magnetic fields is perpendicular to plane of film.

Results and discussion
The representative XRD patterns of BaM thin films annealed at different temperature were shown in figure 1.It can be seen that these thin films exhibit typical diffraction peaks of the BaM phase, and all diffraction peaks are in (00l) direction, which indicated that these samples are single phase and have high c-axis orientation.Moreover, the Lotgering factor (LF) was used to quantify degree of orientation for texture samples [16].The LF of BaM thin films increased from 92% to 96% with increasing the annealing temperature (Ta), and reached a maximum at 1035 °C, then LF decreased to 93% when Ta is further raised to 1050 °C.These results shown that the degree of orientation is sensitive to the Ta, and the orientation play an important role on magnetic and microwave properties.(Will be discussed later) In addition, there are not diffraction peaks shift in these samples, which indicated that no other ions were introduced into the crystal structure of BaM, and Pt is good buffer to block the ion diffusion between BaM and Si substrate.
Figure 2 are the AFM morphology of BaM thin films.It is observed that lots of striped grains in BaM thin film was annealed in 950 °C.However, more and more hexagonal grains generated when the Ta turned to 1035 °C, and the grains size is between 100 nm and 300 nm.It is also observed that the BaM grains are out of the film plane, which is agree with XRD results.The 3D AFM images of BaM thin film is shown in figure 3. It can be seen that the film is flat and smooth, and surface average roughness is 6.53 ± 1.2nm, and the quadratic square root is 7.52 ± 1.86nm, this data is obtained from AFM system.
The in plane (IP) and out of plane (OP) magnetic hysteresis curves of the BaM films are shown in figure 4. It is clear that the hysteresis loops for IP and OP are quite different, the IP hysteresis loop is hard to saturate, but later one is easy to get saturation.The summary of typical magnetic parameters is shown in table 1.Both saturated magnetization (4πMs) and OP coercive (Hc) increase with increasing Ta, and get the maximum value at 1035 °C.The relationship between out-of-plane remanence ratio SQ (SQ = Mr/Ms) and Ta is also revealed in table 1.The SQ increased from 0.737 to 0.919, it agrees well with the LF tendency.This is attributed to the higher temperature improves the texture orientation of the grains, and lead to an increase in remnant magnetism (Mr).The SQ is as high as 0.919, which show that this BaM film has nice self-biasing.This phenomenon means that once the BaM thin film gets saturated magnetized, 91.9% of Ms remaining even the external magnetic field is taken off.Hence, when it is used in microwave devices, it could control the microwave without the help of large external magnetic field.
In order to evaluate the practical feasibility of these BaM thin films, the ferromagnetic resonance (FMR) spectra of them are measured.The applied magnetic field (H ex ) was swept while the frequency is fixed during the measurement, the frequency scan of FMR was also carried out.The H ex is applied perpendicular to sample plane due to the easy axis is perpendicular to sample plane.The FMR relationship is given as [17]: Where f is resonance frequency, g is gyromagnetic ratio, H ex is applied magnetic field, H A is. anisotropic field, M S is saturation magnetization.same as those for single crystal BaM [19].The strongest FMR absorption peak can be observed at around 64 GHz when the frequency sweep was increased from 30 to 65 GHz, as shown in figure 5(c).According to equation (1), the FMR absorption peaks can be tuned by applied magnetic field, which is agree with figure 5(c).These data make this BaM thin film is possible use in adjustable millimeter wave devices.

Conclusion
In summary, this study demonstrated that highly c-axis oriented BaM film can be successfully grown on Pt/TiO 2 /SiO 2 /Si substrate by sol-gel way.The annealing temperature has a significant influence on the crystallographic texture, magnetic properties and microwave loss of BaM thin films.It is found that when the BaM thin film was annealed at 1035 °C, it has high c-axis orientation and low microwave loss.The Lotgering factor is 0.96, FMR linewidth is 143 Oe@50 GHz and anisotropy field is 16.2kOe.

Figure 1 .
Figure 1.The representative XRD patterns of BaM films annealed at different temperature.

Figure 3 .
Figure 3.The 3D AFM images of BaM thin film.

Figure 4 .
Figure 4.The in plane and out of plane hysteresis loops of BaM film annealed at (a) 1035 °C and (b) 1050 °C.
The typical FMR absorption spectra measured at 50 GHz for BaM thin films annealed at 1035 °C is indicated in figure 5(a), and the relation between annealed temperature and FMR linewidth is plotted in figure 5(b), and the frequency scan of FMR is shown in figure 5(c).It is found that the FMR [18]width decreased with increasing the Ta, it decreased from 163 Oe at 950 °C to 143 Oe at 1035 °C, then it increased to162 Oe at 1050 °C.It means that this this sample has low microwave loss, and lower than those for sputtered BaM thin film[18].It has been proved that the microwave loss is sensitive to the porosity and nonuniformity.Therefore, the smaller microwave loss proved the thin film is dense.The g was deduced to be 2.8 GHz/kOe from the linear relation between f versus H ex .Since 4ππMs was obtained from the hysteresis loop, the H A can be calculated from equation (1), as shown in table1.The H A for thin film annealed at 1035 °C is as