Control of Magnetic Properties of Barium Ferrite Thin Films With Unusual Valence Fe

Thin films of BaFe1-x M x O3 (M = Hf, Zr, and Ce; 0.0 ≤ x ≤ 0.75) were fabricated using pulsed laser deposition and their magnetic properties were investigated. X-ray diffraction analysis indicated that oxygen-deficient BaFeO x (x < 3.0) with a monoclinic structure was formed when the deposition was conducted using a non-substituted target (x = 0.0). The as-grown BaFeO x films were converted into fully oxidized BaFeO3 with a perovskite structure by low-temperature oxidation in an ozone atmosphere. In contrast, the as-deposited films of Hf, Zr, and Ce-substituted films exhibited a perovskite structure, and their crystallinity did not change after low-temperature ozone annealing. The magnetic transition temperature T c of the BaFeO3 film was 115 K, whereas the substituted BaFeO3 films showed ferromagnetic behavior even at 300 K. These results can be attributed to the weakening of the antiferromagnetic super-exchange coupling among Fe ions owing to the lattice expansion in the substituted BaFeO3. In addition, the magnetization of the films was found to increase with the decreasing ionic ratio of Fe4+/Fe3+, suggesting that the inherent carrier-induced ferromagnetic interaction is dominant in the films.


INTRODUCTION
Oxide materials with unusual valence ions have attracted much attention for a long time because of their intriguing electrical and magnetic properties (Watanabe, 1957;Chen et al., 2012). In this study, we focused on perovskite-type BaFeO 3 with unusually high-valence Fe 4+ ions. Barium ferrite, BaFeO 3-δ , has various crystal structures depending on the values of the oxygen vacancy δ. For decades, a number of studies have been conducted on the crystal structures and magnetic properties of oxygen-defective BaFeO 3-δ (K. Mori et al., 2007, S. Mori 1966. However, owing to the large ionic radius of Ba, the synthesis of bulk BaFeO 3 (BFO) with a perovskite structure was not successful until recently. It has also recently been shown that fully oxidized BaFeO 3 can be obtained from BaFeO x (x < 3.0) using a low-temperature oxidation procedure (Hayashi et al., 2011). The authors reported that the obtained BaFeO 3 is an A-type spiral magnet with a Curie temperature (T c ) of 111 K. Likewise, Chakraverty et al. reported the successful growth of epitaxial BaFeO 3 films by pulsed laser deposition (PLD) followed by low-temperature oxidation treatment (Chakraverty et al., 2013). They reported that the M s and T c values for the films are 3.2 μ B /formula unit and 115 K, respectively. Thus, the T c value of the BaFeO 3 films and bulk BaFeO 3 is far below room temperature. Enhancing the T c of BaFeO 3 is crucial from an applied viewpoint. Recently, it has been reported that the T c of BaFeO 3 can be elevated by the substitution of Fe by other elements with larger ionic ratios, such as Zr and Sn (Matsui et al., 2005;Matsui et al., 2008;Kanatani et al., 2010;Shinoda et al., 2014). However, there are few systematic studies on substituted BaFeO 3 in the literature. In this study, we fabricated thin films of Hf-, Zr-, and Ce-substituted BaFeO 3 films and investigated their magnetic properties.

EXPERIMENTAL
Thin films of BaFe 1-x M x O 3 (BFMO) (M Hf, Zr, and Ce; 0.0 ≤ x ≤ 0.75) were fabricated by PLD followed by a low-temperature oxidation process. PLD targets were prepared via a standard solid-state reaction. A SrTiO 3 (STO) (001) single-crystal plate was used as the substrate. During the film deposition, the substrate temperature and oxygen pressure were maintained at 700°C and under 0.1 Pa, respectively. The as-grown films were annealed at 200°C in an O 3 atmosphere for 3 h. The crystalline structures of the films were analyzed by X-ray diffraction (XRD) using an X-ray diffractometer (Empyrean system, Malvern Panalytical) with a Cu Kα source (λ 0.154 nm) operated at 40 kV and 40 mA. The magnetic properties of the films were measured using a superconducting quantum interference device (MPMS, Quantum Design) over 10-300 K. To obtain detailed information about the oxidation states of the films, X-ray photoelectron spectroscopy (XPS) was conducted using an XPS system (JPS-9010 MC, JEOL) with a monochromatic Al Kα radiation source (1,486.7 eV). Optical absorption measurements were performed using a UV-Vis spectrometer (V-670, JASCO).

RESULTS AND DISCUSSION
The out-of-plane XRD patterns of the films are shown in Figure 1A-E. As shown in the black data in Figure 1A, upon using a non-substituted barium ferrite target, the resulting asgrown film comprises (001)-oriented single-crystalline BaFeO x with a pseudocubic lattice parameter of 4.12 Å. After annealing in O 3 , the BaFeO x phase is converted to a (001)-oriented, singlecrystalline, and cubic perovskite-type BaFeO 3 phase with a lattice parameter of 3.99 Å, as shown in the red data in Figure 1A. Figure 1B shows the XRD patterns of the as-grown and annealed BaFe 0.5 Hf 0.5 O 3 films. Unlike in the case of pristine BaFeO 3 , the asgrown film shows a cubic phase, and no obvious change in the crystal structure is observed after annealing in O 3 , indicating that the cubic perovskite phase is stabilized by the substitution of Hf 4+ . This result is consistent with the fact that the Gibbs formation energy of BaHfO 3 is smaller than that of BaFeO 3 (https:// materialsproject.org/materials/mp-19035/, https:// materialsproject.org/materials/mp-998552/). As shown in Figure 1C, clear Laue fringes are observed even for the asgrown film, which suggests the high quality of the films. The lattice parameter of the as-grown and annealed films are 4.19 Å and 4.18 Å, respectively. The slightly smaller lattice parameter of the annealed film was due to oxidation. It should be noted that Ce-or Zr-substituted BaFeO 3 films with a cubic perovskite structure can also be fabricated by PLD without O 3 annealing as shown in Figure 1D. As Figure 1E shows, the (002) peak shifts to a lower angle with an increase in the substitution level x. This result indicates the lattice expansion of films by the substitution of M 4+ for Fe 4+ . Figure 1F shows the dependence of lattice parameters of as-grown BFMO films on the substitution level x. The lattice parameter obeys Vegard's law for M Hf, Zr, and Ce. In addition, as clearly shown in Figures 1D,F, the lattice parameter increases upon increasing the ionic radius of the substituted ions [Hf 4+ (0.78 Å), Zr 4+ (0.84 Å), and Ce 4+ (0. 97 Å)]. These results suggest that the M 4+ ions substitute the Fe 4+ ions at the B-site of the perovskite structure in the films. As shown in the reciprocal space map (RSM) of Figure 1G, all the films are relaxed because of the large lattice mismatch (5.2-8.6%) between the film and the STO substrate.
We found that the optical properties of the BFMO films were peculiar compared to the solid-solution system. Figure 2A shows the optical transmission spectra of the BaFe 1-x Hf x O 3 films. The increased absorption of BaFeO 3 in the visible region is attributed to O2p-Fe3d charge-transfer transitions, as suggested by firstprinciples calculations (Li et al., 2012;Mizumaki et al., 2015). Figure 2A shows the Tauc plots of the optical absorption of the BaFe 1-x Hf x O 3 films. The bandgap energy of the BFO films is estimated to be 1.8 eV, which is close to the previously reported value (Chakraverty et al., 2013). The tails observed in the lower energy region for x 0-0.5, in the Tauc plots can be attributed to defects in the films. As shown in Figure 2C, E g exhibits a bowing behavior. The relation between the bandgap and the substitution level x can be described by the following equation based on Vegard's law (Cardona 1963;Androulidaki et al., 2006): where b g is the so-called bowing parameter, and E g (BaFe 1- x Hf x O 3 ), E g (BaHfO 3 ), and E g (BaFeO 3 ) are the bandgaps of BaFe 1-x Hf x O 3 , BaHfO 3 Kim et al. (2017), and BaFeO 3 , respectively. The dependence of the optical bandgap on x is well fitted with b g 8.7 eV, as shown by the dotted curve in Figure 2C. XPS measurements were performed to validate the valence of the Fe ions in the films. The Fe-2p core level XPS spectra of the BaFeO x and BMFO films are shown in Figure 3A. The 2p 1/2 and 2p 3/2 peaks of the BFMO films are located at 727.0 and 709.9 eV, respectively. These peak positions are nearly the same for all BFMO films. However, the 2p 1/2 and 2p 3/2 peaks can be seen at 729.9 and 711.3 eV, respectively, both of which are slightly higher than those of the BFMO films, suggesting the different valence states of Fe between the BaFeO x and BFMO. As Figure 3B shows, when a Gaussian fitting method is used, the 2p 3/2 peak of the BaFeO x film is deconvoluted into two components at 711.7 and 709.9 eV, which can be assigned to Fe 3+ and Fe 2+ , respectively (Temesghen and Sherwood, 2002;Li et al., 2021). The ionic ratio of Fe 3+ /Fe 2+ is calculated to be 13.3, indicating that most of Fe ions in the BaFeO x films are in a 3 + state. However, as shown in Figure 3C, the Fe 2p3/2 peak of the BFMO films consists of two major components at 711.7 and 709.6 eV, which can be assigned to Fe 3+ and Fe 4+ , respectively (Bocquet et al., 1992;Lombardi et al., 2019). Thus, Fe ions in the BFMO films are in the mixed valence state of Fe 4+ /Fe 3+ , which strongly affects the magnetic properties of the films, as discussed later. The ionic ratio of Fe 4+ /Fe 3+ in the annealed film (Fe 4+ /Fe 3+ ∼ 2.0-2.5) was larger than that of the as-grown films (Fe 4+ /Fe 3+ ∼ 1.5-1.7), suggesting the oxidation of the film after annealing in O 3 .
The results of the magnetic measurements are shown in Figure 4. As shown in Figure 4A, the T c of the BFO film is estimated to be 115 K, which agrees well with the reported value for the bulk BFO (111 K) (Hayashi et al., 2011). As shown in the inset of Figure 4A, the saturation magnetization of the BFO film is ×1.0 10 -3 emu/cm 3 at 10 K. The magnetic properties were found to change drastically upon substitution. Figure 4B shows the temperature dependence of the magnetization of the BFMO films measured under a persistent magnetic field of 1 kOe. There was no significant downward trend in the magnetization of any of the films. This result provides evidence that the Curie temperature T C of the films is well above 300 K. High temperature measurements are scheduled using the MPMS oven option to determine the value of T C for BFMO films. As shown in Figure 4C, we confirmed that all the BFMO films displays ferromagnetic behaviors with coercive field of ∼20-30 Oe at 300 K. Figure 4D shows the magnetic field Hdependence of the magnetization M of the BaFe 1-x Ce x O 3 films at room temperature (300 K). It can be clearly seen that all these compositions exhibited room-temperature ferromagnetic behavior. This enhanced magnetization could be attributed to the lowered antiferromagnetic (AFM) coupling among Fe ions in the substituted films, as explained later. The magnetic properties  Figure 4E shows the dependence of the saturation magnetization on the ionic ratio Fe 4+ /Fe 3+ , which was calculated from the results of Gaussian fittings in the XPS analysis. As shown in Figure 4E, the saturation magnetization tends to decrease with an increase in the Fe 4+ /Fe 3+ ratio. This result indicates that the Fe 4+ /Fe 3+ mixed valence state enhances the ferromagnetism in the films owing to the double exchange mechanism. However, super-exchange coupling among Fe ions should also be considered. According to Harrison's relation, the super-exchange interaction J is expressed as J ∝ 1/r 14 , where r is the distance between Fe ions, while the double exchange interaction depends significantly less on r (Harrison 1989).
Therefore, AFM coupling is drastically reduced and FM interaction becomes more prominent by substituting Fe 4+ with M 4+ with larger ionic radii in the films, resulting in the observed ferromagnetism at room temperature. In summary, thin films of Hf, Zr, and Ce-substituted BaFeO 3 epitaxial thin films were fabricated using PLD. The highly oriented epitaxial thin films of BFMO films with a cubic perovskite structure were found to grow without annealing treatment in contrast to the pure BaFeO 3 film. The bandgap of the films can be varied over a wide range owing to the large bandgap bowing effect. XPS measurements revealed that the Fe ions in the BFMO films were in the mixed valence state of Fe 4+ / Fe 3+ . The BaFeO 3 film was a ferromagnet with a T c of 115 K. On the other hand, ferromagnetic behaviors were observed at room temperature for the BFMO films. The saturation magnetization of the BFMO films increased with an increase in the lattice parameters and ionic ratio of Fe 4+ /Fe 3+ in the films. The ferromagnetic behaviors of the BFMO films could be explained by the double exchange interaction and lowered AFM coupling among Fe ions caused by the substitution of Fe 3+ by M 4+ with a larger ionic ratio. Our findings indicate the potential of BFMO films as room-temperature ferromagnetic semiconductors.

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.