Structure and Magnetization of Strontium Hexaferrite (SrFe12O19) Films Prepared by Pulsed Laser Deposition

M-type strontium hexaferrite (SrM) thin films show excellent magnetic properties and uniaxial magnetic anisotropy. We systematically investigated the magnetism of SrM films prepared by pulsed-laser deposition on different substrates [Al2O3 ( 1 1 ¯ 02 ), SrTiO3 (100), ZnO (0001), and LiNbO3 (0001)] at vacuum (10−4 Pa) and a substrate temperature of 800°C. Prepared films were annealed in air at a temperature of 1,000°C for 2 hours. This investigation determined the effect of annealing and different substrates on the morphology, strain, and hysteresis loops of the films. The prepared films were characterized using x-ray diffractometry, Raman spectroscopy, scanning electron microscopy, and superconducting quantum interference device (SQUID) magnetometry. X-ray diffraction analyses confirmed c-oriented growth along the out-of-plane direction in most films. We found that annealing causes enhanced crystallization in films and a significant increase in coercivity. The highest coercivity of ∼11 KOe was measured for the film deposited on the Al2O3 ( 1 1 ¯ 02 ) substrate.


INTRODUCTION
M-type hexagonal ferrites are promising materials for various applications, including microwave devices, magnetic field sensors, and data storage (Díaz-Castañón et al., 2001;Chen and Harris, 2012). They have favorable electrical properties (Tang et al., 2016), high chemical stability, low production cost (Zi et al., 2008), and unique magnetic properties such as high magnetization, high values of coercivity, and strong uniaxial magnetic anisotropy (Harris, 2012;Zhang et al., 2014). In hexaferrite thin films, the easy axis of magnetization is usually along the c-axis, and thin films with c-axis orientation find utility in specialized applications (Zhang et al., 2019). Thin films that can be used for microwave filters (Sun et al., 2016), phase shifters (Wu, 2012), and delay lines must be fabricated with in-plane orientation (IPCA). However, thin films used for circulators and isolators must possess an out-of-plane c-axis (OCA) orientation (Özgür et al., 2009;Meng et al., 2012). Strontium M-type hexagonal ferrite (SrM) belongs to the magnetoplumbite phase of ferrites (Pullar, 2012;Jotania, 2014). Researchers have classified hexagonal ferrites according to the location of their constituent subunit blocks. SrM has a hexagonal structure with a space group of P63/mmc and consists of four blocks (RSR*S*), where S Fe 6 O 2+ 8 and R MFe 6 O 2− 11 (Kimura, 2012;Chen et al., 2016). The asterisk (*) indicates that the subunit is rotated 180°around the crystallographic c-axis. At absolute zero temperature, the total magnetization of the unit cell is related to the number of Fe 3+ ions.
Fe 3+ ions are divided equally between the two blocks. In the R block with a hexagonal structure, five ions are in an octahedral position (three spins up the magnetic moment and two spins down the magnetic moments), and one spin is up the magnetic moment on the bipyramidal site. The S block has a spinel structure with 4 of 6 Fe 3+ ions in the octahedral position. The octahedral cations have spin-up moments with the two ions in the tetrahedral sites having spin-down moments (Kaur et al., 2006;Chen et al., 2017). There are eight spin-up and four spin-down moments in each unit cell, with the magnetic moment of each Fe 3+ ion being 5 μ B at absolute zero. Therefore, the magnetism of each unit cell is expected to be 4*5µ B 20µ B (Figure 1) (Zi et al., 2008;Harris, 2012;Izadkhah et al., 2017).
This work systematically investigated the effects of annealing on the structural and magnetic properties of SrM thin films deposited by PLD on various substrates. We discuss our results in terms of the effect of different magnetic anisotropy mechanisms on the structural and magnetic properties of SrM thin films.

EXPERIMENTAL
The films were deposited by PLD onto various single-crystal substrates from a sintered SrFe 12 O 19 target prepared by the solidstate method. A KrF excimer laser producing monochromatic light at a wavelength of 248 nm at 25 ns pulses was used to produce a laser fluence of about 1.5 J/cm 2 at the surface of the ceramic target. A pulse repetition rate of 10 Hz was employed. The base pressure in the PLD chamber was 2 × 10 −4 Pa, and the substrate temperature was 800°C. After deposition, the films were annealed at 1,000°C in air for 2 hours to further complete the film's crystallinity.
It is expected that the crystal structure and orientation of the substrate play an important role in determining the texture and properties of the films. Therefore, here we deposited SrM thin films on different substrates: Al 2 O 3 (1102), STO (100), LiNbO 3 (0001), and ZnO (0001), which are assigned the following abbreviations in this article: SAlO, SSTO, SLNO, and SZnO, and the films annealed at 1,000°C are designated as AAlO, ASTO, ALNO, and AZnO. The thickness of all films was ∼50 nm. The objective of the study was to determine optimum process parameters that would yield films of the highest crystal quality, magnetization, and magnetic anisotropy. θ−2θ X-ray diffraction (XRD) was carried out to evaluate thin film crystallinity, orientation, and strain. Moreover, Raman spectroscopy was also employed to investigate the strain in the thin films. The surface morphology of the films was examined using scanning electron microscopy (SEM). Most magnetic measurements were made using a 5 T SQUID magnetometer (MPMS 5 XL, Quantum Design) on films mounted in clear plastic straws with the magnetic field applied parallel or perpendicular to the film plane. Figure 2 shows the XRD patterns of films deposited on different substrates before and after heat treatment. This allows for the evaluation of the impact of the annealing treatment on the structural properties of SrM thin films, which indicates annealing can be used to improve crystallinity that may result in improved superior magnetic properties (Zheng et al., 2016). Also, the effect of different substrates is observed. The X-ray patterns and the known JCPDS card (01-080-1197) were compared, and the presence and identification of all diffraction peaks confirmed the hexagonal structure . The X-ray diffraction pattern of the sample SAlO showed two different peaks being indexed to the (h0hl) and (000l) planes of SrM, but after annealing AAlO film 000l peaks remained , and this film exhibited good crystallinity and good out-of-plane orientation of the c-axis. The film deposited on STO (100) indicated both in-plane (hh2h0) and out-of-plane (000l) orientations. The structure of the substrate plays a vital role in the formation of the film and its properties. STO substrate has a cubic structure, so the difference between the film's structure and the substrate and the mismatch of their lattice parameter increase the strain in the thin film and cause a scattered orientation. Figure 2B shows the XRD patterns of ASTO. As can be observed, the intensity of (000l) peaks increased, and a new peak, 00010, appeared, indicating improvement of out-of-plane orientation after annealing. The XRD pattern of AZnO represents a highly oriented (000l) direction due to the same hexagonal crystal structure of SrM and ZnO.

RESULTS AND DISCUSSIONS
The XRD patterns of SLNO and ALNO show diffraction peaks that support the existence of out-of-plane crystal texture. Still, in the pattern of the annealed film, the intensity of SrM diffraction features diminish, indicating that most SrM has evaporated during annealing and only a small volume of the ferrite remains on the substrate. We discuss this point in the following sections.
In-plane lattice parameter a and out-of-plane lattice parameter c of the SrM film were calculated using the formula: where d is the interplanar distance and h, k, and l are Miller indices. The bulk (target) lattice parameters are a 5.914 and c 23.283. We calculated the stain ratio for each film. The results are shown in Table 1. The in-plane and out-of-plane lattice parameters of the films on ALO, STO, and LNO substrates are less than the bulk value, indicating compressive strain. On the other hand, the films on the ZNO substrate are under tensile strain (Malek et al., 2015). As can be seen from Table 1, the strain increased after annealing. In our work, we used Raman spectra to investigate the effect of strain on thin films after annealing (Wang et al., 2004) (Figure 3). In Raman spectroscopy, the incident phonons either gain quanta or lose quanta by interacting with the vibrational modes of the material. If it gains energy, it gets blue-shifted, and if it loses, it is red-shifted. The amount of the shift determines the energy of the phonon in the material. Raman spectroscopy is a powerful tool for ascertaining lattice strain (Lisfi and Williams, 2003). If the material lattice experiences compressive strain, the Raman shift increases; if it is under tensile strain, the Raman shift decreases. These shifts are called blue shift and red shift, respectively. Since the lattice constants of SrFe 12 O 19 are larger than the AlO substrate, the strain created in the film should be compressive. The Raman spectra of the thin films (shown in Figure 3A) deposited on Al 2 O 3 show a substantial peak at 672 cm −1   (SAlO) related to the A 1g mode of SrM cm −1 SAlOwhich shifts to 715 cm −1 after annealing (AAlO), confirming the existence of a pronounced compressive strain (Kreisel et al., 1998;Zhang et al., 2017). Also, a blue shift is observed for films on STO substrates that increases with the annealing of the sample, but the shift is less than that for the film on the Al 2 O 3 substrate. It can be observed in  Figure 3B that the peak (695 cm −1 ) associated with the trigonal site of the SrM is relatively sharp, which shows that strontium ferrite films have an improved crystalline structure after annealing. In contrast, the Raman spectra shown in Figure 3C illustrate a red shift because the lattice parameter of ZnO is larger than SrM, and therefore, a tensile strain exists in these films, which increases with annealing. Figure 3D shows that for the films on LNO, the Raman peaks do not change markedly after annealing. Figure 4 shows SEM micrographs of the surface morphology of as-deposited thin films and the films annealed at 1,000°C. It is known that ferrite microstructure depends on various parameters such as annealing temperature and time, substrate type, and deposition temperature. The SEM images illustrate an increase in the average grain sizes upon annealing. It has been shown by others that strontium ferrite grains often appear acicular-like or platelet-like (Xu et al., 2013b). According to the literature, we can determine the orientation of the c-axis from the shape and alignment of grains in SEM images. Platelet-like grains tend to have an out-of-plane orientation of the c-axis, while acicular-like grains have either an in-plane or a random orientation of the c-axis (Meng et al., 2014b). Therefore, nearly all the samples studied here have an out-of-plane orientation except for two samples SSTO and ASTO, whose grains are distributed randomly without a pronounced crystalline texture. As previously mentioned, among the films deposited on LNO, including those that were annealed, the SrM mostly evaporates, with few large grains visible on the substrate surface.
The magnetic properties of the strontium ferrite thin films were measured by a superconducting quantum interference device (SQUID) magnetometer. Magnetic hysteresis loops were measured with magnetic fields applied along the perpendicular and the in-plane directions to the films at room temperature ( Figure 5H). The normalized hysteresis curves are shown in Figure 5. The magnetic parameters such as saturation magnetization (M s ), remanent magnetization (M r ), and coercivity (H c ) were determined from the M-H loops and are tabulated in Table 2. It was found that AAlO exhibits the maximum H c (∼11 KOe) compared to other film samples. Figure 5 shows that annealing generally leads to increased H c . We attribute this H c enhancement to an increase in strain and crystallization in SrM thin films as confirmed by Raman spectroscopy (Figure 3) and SEM micrographs (see Figure 4). The SAlO film exhibits an inplane easy axis ( Figure 5A), but the c-axis of this sample is along the out-of-plane direction. The anisotropy in films was determined by competing anisotropy mechanisms, for instance, magnetocrystalline anisotropy, shape anisotropy, and magnetostriction (i.e., magnetic response to strain). In AlO, the in-plane anisotropy induced by the sample shape (i.e., surface dipoles) and strain, and the out-of-plane anisotropy induced by magnetocrystalline anisotropy are comparable. It seems the shape anisotropy energy begins to dominate, and this sample shows an in-plane easy direction. The shape anisotropy changes the magnetization direction to where the magnetostatic energy is minimal; thus, thin films have an inplane shape anisotropy. In this sample, there is a lattice mismatch between the SrFe 12 O 19 film and the substrate. The film lattice parameter is larger than the substrate. As a result, compressive strain appears in this sample results. It seems AAlO is more isotropic than other films, and the hysteresis loop of this film displays a characteristic reduced amplitude at low field. The out-of-plane hysteresis loop of AAlO presents an apparent twostep behavior, with the magnetization decreasing at a small return field. This anomaly in the magnetic hysteresis loop can be described by the participation of a surface anisotropy, different from the bulk because of the broken symmetry at the film surface (Lavorato and Winkler, 2016). This also can indicate a 2phase material that could have resulted from the diffusion of Fe into the AlO and the formation of interface soft spinel . On the other hand, this change in slope signals the switching of a soft magnetic phase that is decoupled from the harder SrM phase.
The SSTO film indicates an in-plane anisotropy ( Figure 5C), but the anisotropy changes after annealing and ASTO shows an out-of-plane anisotropy ( Figure 5D). Alternatively, SZnO and AZnO illustrate very little coercivity (H c ). After annealing, the magnetic hysteresis loop of the sample AZnO shows little coercivity, and the remanence is nearly zero. The hysteretic magnetization is followed by the opening of the loop at high fields possibly due to the influence of uniaxial anisotropy (Gao et al., 2009) (Figure 5E).
The SLNO film does not show magnetic anisotropy. Since the LNO substrate is paramagnetic and a small amount of strontium ferrite material remains on the substrate surface after annealing, no magnetic hysteresis loops were observed for ALNO.

CONCLUSION
M-type strontium hexaferrite (SrM) thin films prepared by pulsed laser deposition on Al 2 O 3 (1102), SrTiO 3 (100), ZnO 2 | Magnetic characteristics of the SrM films grown on different substrates in an applied magnetic field, parallel to (in-plane) or perpendicular to (out-of-plane) the c-axis of SrM.

Thin films
In-plane (0001), and LiNbO 3 (0001)) substrates were annealed in air at a temperature of 1,000°C and characterized by Raman spectroscopy, X-ray diffractometry, scanning electron microscopy, and SQUID magnetometry. These investigations indicated that annealing and different substrates have a critical effect on the morphology, strain, and hysteresis loops of the films. X-ray diffraction analyses confirmed the c-axis-oriented growth along the out-of-plane direction. We found that annealing causes enhanced crystallization of films and a significant increase in coercivity. The highest coercivity of ∼11 KOe was measured for the film on Al 2 O 3 (1102) substrate.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.