1 Introduction

Compared with conventional semiconductor devices, spintronic devices using both charge and spin properties of electrons have the advantages of high speed and large storage. Therefore, magnetic semiconductor materials with both magnetic and semiconductor transport properties have a wide prospect in application in the field of spintronics [1,2,3], which is also a crucial problem to be solved in the novel generation of spintronic devices. Hematite has a distribution of Fe3+ with a modulation length of 2 layers and ilmenite has a distribution of Fe2+ with a modulation length of 4 layers. This leads to complex magnetic interactions in the solid solutions; therefore, solid solutions of hematite and ilmenite (Hemo–Ilmenite, HI) are useful for investigating mixed-spin oxide systems [4,5,6,7].

The solid solution (1 − x)Fe2O3 − xFeTiO3 (0 < x < 1) is widely studied as a natural magnetic semiconductor material in the preparation of many oxides on an α-Al2O3(001) substrate by changing the substrate temperature (TS) [8,9,10,11]. Magnetization jumps and the exchange bias effect are simultaneously observed in the oxide (α-Fe2O3)0.1–(FeTiO3)0.9 of bulk materials at 2.0 K [12, 13]. Ilmenite (FeTiO3) and hematite (Fe2O3) are antiferromagnetic insulators. They have a hexagonal structure, in which oxygen ions are arranged in hexagonal dense packing lattice and 2/3 of the octahedral gap is occupied by cations. Fe2+ and Ti4+ layers are interlaced in FeTiO3, and the space group is \(R\overline{3 }\). However, Fe3+ replaces Fe2+ and Ti4+ layers in Fe2O3, and the space groups become \(R\overline{3 }c\). HI (1 − x)Fe2O3 − xFeTiO3(0 < x < 1) has ordered \(R\overline{3 }\) and disordered \(R\overline{3 }c\) construction [14]. Many people believe that the magnetism of HI is closely related to the ordered structure [15,16,17,18]. Notably, only films with ordered structure can acquire strong ferromagnetism. The magnetic moment of the film prepared by Dou et al. is close to that of the bulk sample, but it was not proved whether the film has an ordered structure [19]. The ordered HI film with x = 0.7 prepared by Hojo et al. has the same weak magnetism as the disordered structure [20]. The ordered structure of the film is usually determined by the appearance of the diffraction peaks (003) and (009) in an X-ray diffraction (XRD) chart. The complex magnetic interactions in the bulk sample of HI are mainly determined by the ordered structure, but the factors affecting magnetism are different in thin films.

Through PLD and by adjusting TS, we prepared epitaxial Fe1.1Ti0.9O3−δ(HI-9) thin films and demonstrated the effects of TS on the structure, surface morphology, optical properties, magnetic properties, and electrical properties of the thin films.

2 Experimental Methods

For PLD, thin epitaxial Fe1.1Ti0.9O3−δ films were prepared in a substrate. PLD target is made of 90% FeTiO3 powder and 10% Fe2O3 powder, which was fully mixed and ground, and then it was sintered using conventional solid reaction method. KrF pulse laser deposition was used in the experiment. The wavelength of the laser was 248 nm, the working frequency was 2 Hz, and the laser energy was 3 J/cm2. The intrinsic vacuum pressure of the PLD chamber was 5.0 × 10 − 5 Pa. The deposition atmosphere is Ar and the pressure was 1.0 × 10 − 5 Pa. The TS was 600–900 °C, and the temperature interval was 100 °C. The distance between the target and the substrate was 3.5 cm, and the thickness of the film was 60 nm. After deposition, the film was annealed for 0.5 h and cooled to room temperature naturally with constant pressure.

The phase structure of the films was analyzed via XRD with Compex Pro205. The surface morphology of the films was observed via NanoScope IV atomic force microscopy (AFM). The optical properties of the films were measured using a UV-3600 ultraviolet and visible (UV/VIS) spectrophotometer produced by Shimadzu, Japan. The magnetic and electrical properties of the films were measured using a physical property measurement system (PPMS) made by Quantum Design company.

3 Results and Analysis

3.1 Phase Analysis

Figure 1(a) shows the XRD patterns of the films prepared under different TS conditions. The diffraction peaks (006) and (0012) of the ordered \(R\overline{3 }\) and disordered \(R\overline{3 }\) structures can be observed in the figure, whereas diffraction peaks (003) and (009) of the ordered \(R\overline{3 }\) structure cannot be observed, which indicates that the films have a disordered structure. No other impure peaks were observed, which indicates that the films were well epitaxial. The intensity of diffraction peak (006) of the TS = 600 °C film is the weakest, and its crystallization is poor.

Fig. 1
figure 1

(a) XRD diagram of the HI-9 thin films prepared under different TS, boxes and black dots denotes the diffraction peak of Al2O3 and the film HI-9, respectively. (b) Magnified view of the diffraction peak of the film (006)

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Figure 1(b) shows that the diffraction peak of (006) gradually moves to a small angle with increasing TS, which indicates that the lattice constant, c, increases. However, the (006) diffraction peak of the TS = 800 °C films becomes abnormal, and c decreases as the diffraction peak shifts to a larger angle. This is because of the increase of TS, and thus the oxygen loss of the film increases, Fe3+ changes into Fe2+ (the radius of Fe2+ is larger than Fe3+), and the Coulomb gravitational interaction of Fe2+/3+  − O2− –Fe2+/3+ changes into Fe2+ –Fe2+ Coulomb repulsive interaction. The diffraction peak of the film prepared at TS = 800 °C changes abnormally, and it could be owing to the following reasons: owing to lattice mismatch (HI bulk a = 5.086 Å, Al2O3 a = 4.763 Å), there is in-plane compressive stress in the film, TS increases, lattice relaxation occurs, and c decreases. c decreases with the increase of crystallinity.

3.2 Analysis of the Surface Morphology of the Film

Figure 2 shows the AFM diagram of films prepared at different TS. The scanning range is 1 × 1 μm. It is observed that the size of the film particles is uniform, and the particles gradually become larger with increasing TS. The measured root mean square deviation (RMS) of the films prepared at TS = 600–900 °C were 0.253, 0.228, 0.285, and 0.289 nm. RMS first decreases and then increases, but the difference is not significant, indicating that the film surface is smooth.

Fig. 2
figure 2

Surface morphology of the HI-9 films prepared at different TS, (a) TS = 600 °C, (b) TS = 700 °C, (c) TS = 800 °C, and (d) TS = 900 °C

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3.3 Analysis of the Optical Properties

The transmittance of the film samples was measured using a UV/VIS spectrophotometer in the wavelength range of 220–2600 nm. The absorptivity of sample was obtained using the transmittance of the film sample. The absorption coefficient of the sample is α = 2.303 × A/t, where t is the thickness of the film, A is the absorptivity of the film. α and the phonon energy, hυ, of direct band gap semiconductor satisfy the relative formula \((\alpha hv{)}^{2}\approx (hv-{E}_{g})\) [21]. According to this formula, we can calculate the optical band gap (Eg) of the thin film sample, that is, we can make a \((\alpha hv{)}^{2}-hv\) graph according to the absorption coefficient of the sample, and then determine that Eg is the linear part of the extrapolated curve and the intersection point \((\alpha hv{)}^{2}=0\) of the coordinate axis hυ.

Figure 3 shows the (αhυ)2 variation with hυ of thin films prepared at different TS conditions. The results indicate that the films have a wide band gap. With the increase of TS, the Eg of the film is 3.78, 3.87, 3.95, and 3.90 eV. The Eg of the film is wider than that of the pure FeTiO3 film (3.55 eV) [21], which is consistent with the change of XRD. The films grown in Ar gas have more oxygen vacancies, and the Eg becomes larger, which is similar to literature reports [19]. The Eg of the films prepared at TS > 600 °C was almost similar, and it was not affected by lattice relaxation (stress change), which indicates that the content of oxygen vacancy in the films is almost similar. The thin film at TS = 600 °C has a relatively narrow Eg owing to low substrate temperature, much oxygen loss, and oxygen vacancy.

Fig. 3
figure 3

(αhυ)2 varied diagram of the HI-9 thin films with the phonon energy hυ prepared at different TS

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3.4 Analysis of the Magnetic Properties

Figure 4 shows the varied curves of coercivity and saturation magnetization of the thin film sample with TS, which the magnetic field is parallel to the film surface. The figure shows that the coercivity of the film at TS = 600 °C is 15 KOe, and it decreases rapidly with the increase in TS. Notably, anisotropy, oxygen vacancy, and stress affect the coercivity of thin films mainly. According to the previous analysis of the Eg, oxygen vacancy content in the films prepared at TS > 600 °C is the same; therefore, the effect of oxygen vacancy on the coercivity of the films is similar. However, the film prepared at TS = 600 °C has less oxygen loss and oxygen vacancy, and the in-plane compressive stress of the film is the largest. Therefore, the stress in the HI-9 film sample has a significant influence on the coercivity of the film. Lattice relaxation occurs with the increase of TS, and then the stress decreases gradually, and the coercivity decreases rapidly. Similar to the previous analysis in XRD, the film prepared at TS = 600 °C has poor crystallization; therefore, its magnetism is the weakest. With the increase in TS, the crystallization of the film gradually increases. The crystallization of the film prepared at TS = 800 °C is the best, and the saturation magnetization (Ms) reaches 172.5 emu/cc.

Fig. 4
figure 4

Curve of coercivity and saturation magnetization at 5 K

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3.5 Measurement of the Electrical Properties of the HI-9

As shown in Fig. 5, the film at TS = 600 °C is not conductive, which is because of the poor crystallization of the film. In contrary, the film at TS = 700, 800, or 900 °C can conduct electricity. The resistivity of the film increases with the decrease in temperature, indicating that the film has semiconductor conductivity. In addition, the resistivity of the film at TS = 800 °C is the lowest, and the resistivity of the film at TS = 700 °C is slightly lower than that of the film at TS = 900 °C film, which is consistent with the rule of Ms changing with TS. This indicates that the film at TS = 800 °C has a high degree of order, and the resistivity of the film is smaller than that of the HI-9 bulk sample, which is owing to the oxygen vacancy in the hypoxia film, and it changes Fe3+ into Fe2+ . We established that the conductivity of the film satisfies the Arrhenius conduction mechanism, \(\sigma ={\sigma }_{0}\mathrm{exp}(-E/2kT)\), at high temperature, where E is the activation energy, k is the Boltzmann constant, and \({\sigma }_{0}\) is the prefactor [22]. When the temperature is low, the film satisfies both the Mott program transition and the Arrhenius conduction mechanism,\(\sigma ={\sigma }_{1}\mathrm{exp}[-({\mathrm{T}}_{1}/T{)}^{0.5}]+{\sigma }_{2}(E/2kT)\), where T1 is the characteristic temperature [17]. Figure 5 inset shows the conductivity curve of the film at TS = 800 °C as a function of temperature, which is consistent with both the Mott program transition and the Arrhenius conduction mechanism. That is, at the critical temperature, the conduction mechanism of the film changes from a thermally activated transition to a programmatic transition, which indicates that the carriers of the film are localized at low temperature. The critical temperatures of the films at TS = 700 and 800 °C are 208 and 195 K, respectively. A minimum activation energy of 102.15 meV is required for the thermal activation transition of the film at TS = 800 °C, and the minimum activation energies of the films at TS = 700 and 900 °C are 114.95 and 114.30 meV.

Fig. 5
figure 5

Resistivity curves of the films at TS = 700, 800, and 900 °C varied with temperature; fitting diagram of the conductivity of the film at TS = 800 °C

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The resistivity curve of the HI-9 bulk sample under different magnetic fields varies with temperature (Fig. 6). The figure shows that the resistivity of the HI-9 bulk sample increases gradually with the decrease in temperature, indicating significant semiconductor characteristics. As shown by the arrow in Fig. 6(a), the resistance shows a clear turning point at 60 K, which is consistent with the magnetic ordered transition temperature of Fe2+ . Below 60 K, the spin of Fe2+ is antiparallel. At the same temperature, the resistance decreases gradually with the increase in external magnetic field. We can use the Arrhenius conduction mechanism to fit the conductivity with temperature variation under zero magnetic field, the expression is \(\sigma ={\sigma }_{0}\mathrm{exp}(-E/2kT)\) [22], as same as the films with high temperature. As shown in Fig. 6(b), the conductivity of the HI-9 bulk follows the thermal activation conductive mechanism, and the activation energy is 117.55 meV.

Fig. 6
figure 6

lnρ-T plots of the HI-9 bulk sample measured at different magnetic fields; (a) a local enlargement of the lnρ-T plots; (b) a conductivity fitting plot

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The resistivity of the thin film samples increases with the decrease in temperature and it has significant semiconductor conductivity. We also prepared Hi-9 films in the oxygen environment. It was found that the films showed the characteristics of insulators when the oxygen pressure was high, and the films were conductive only when the oxygen pressure was low. The resistivity of the film samples increased with the increase of oxygen pressure. The conductivity of the films prepared at low oxygen pressure is better than that of the HI-9 bulk sample, and the phenomenon is similar for FeTiO3 ± δ [23] and Fe1.5Ti0.5O3 ± δ [24]. The conductivity of the sample can be enhanced when cations of the same element appear in multiple valence states [25]. As the oxygen pressure decreases, oxygen vacancies form in the films, and some Fe3+ become Fe2+. The change in oxygen vacancy and Fe valence state is the main reason for the change in electrical conductivity. The resistivity of the film we prepared is relatively high, as compared with the resistivity of the HI-9 film prepared by Zhou et al. (3.93 Ω cm) [26]. This phenomenon is also found in many other oxides [27]; therefore, oxygen vacancies are crucial for the HI solid solution systems.

Contrast the HI-9 bulk sample, we established that the conduction mechanism of the film changes from the thermal activation transition to the variable-range hopping with decreasing temperature, which indicates that the carriers of films are localized at low temperature. Oxygen vacancies cause the changes of electrical properties. Magnetic and electrical measurements show that the film at TS = 800 °C has the highest saturation magnetization and the best electrical conductivity, indicating that it has a relatively high degree of order, and its rule of the saturation magnetization, remanent magnetization, and coercivity changes consistently with those of the bulk material.

In HI-9 system, antiferromagnetic Fe3+ clusters are nested in antiferromagnetic Fe2+ lattices and form a triangular lattice. In this structure, 2/3 of the magnetic moments, all Fe3+ cations, order antiferromagnetically with each other. The remaining 1/3, all Fe2+ cations, are “confused.” This structure forms a partially disordered antiferromagnetic state. The partially disordered antiferromagnetic state transforms into a ferromagnetic state, and it will induce the antiferromagnetic spins of the Fe2+ ions to enter a ferromagnetic state, resulting in the optical and transport properties of Hi-9 [12].

4 Conclusion

In this study, Fe1.1Ti0.9O3−δ thin films were prepared on an α-Al2O3 (001) substrate and in Ar gas via PLD. The thin films are epitaxial growing with a symmetrical structure and a smooth surface. By measuring and analyzing the optical and magnetic properties of the thin films, we established that there were more oxygen vacancies in the thin films prepared in Ar atmosphere, and the Eg of the thin films widened significantly. TS has a significant influence on the crystallization and magnetism of thin films. The crystallization of the films prepared at TS = 800 °C is the best, and the saturation magnetization reaches 172.5 emu/cc. As the TS increases, the stress decreases and lattice relaxation occurs. The coercivity of the film decreases rapidly from 15 to 4.4 kOe. Oxygen vacancies and stress affected the optical, magnetic, and electrical properties of the films.