Effect of Film Thickness on Microstructural and Magnetic Properties of Lithium Ferrite Films Prepared on SrTiO₃ (001) Substrates

1. Introduction

Lithium ferrite (LiFe₅O₈) has drawn widespread attention of research because of its remarkable physical properties, such as high saturation magnetization, high Curie temperature, large electric resistivity, low loss at high frequencies, and good chemical and thermal stability [1,2], which make it have potential application in components of microwave device and spintronics [3,4]. LiFe₅O₈ has the inverse spinel structure, where the tetrahedral sites are occupied by Fe³⁺, and the octahedral sites are shared by Li⁺ and the rest Fe³⁺ in a ratio of 1:3 (denoted as Fe[Li_0.5Fe_1.5]O₄). The antiparallel aligned magnetic spin between the Fe³⁺ distributing at tetrahedral sites and octahedral sites leads to a high magnetic moment of 2.5 µB per formula unit [1,5]. Compared to the bulk material, spinel thin films exhibit microstructural variations such as the presence of planar defects, which can alter the electrical and magnetic structures of the films [6,7]. Thus, research efforts concentrating on the growth, structure, property, and applications of the spinel thin films have proliferated over the last decades [8,9,10].

Generally, during the film deposition process, many degrees of freedom can be used to modify the structural and physical properties of the film [11,12,13]. Among them, changing the film thickness is a common method to manipulate the strain state of the film [14,15]. Particularly, tuning strain states not only cause the formation of oriented domains [16,17], but also lead to the different density of antiphase boundaries in spinel films [18,19], which influence the magnetic properties of the films consequently [20]. Moreover, enhanced magnetic moments are present in ultrathin films (e.g., NiFe₂O₄ and CoFe₂O₄) prepared on spinel-type MgAl₂O₄ substrates [21,22,23]. In contrast, there are limited investigations on the microstructural characteristic and magnetic behavior of LiFe₅O₈ films with different thickness prepared on perovskite-type substrates that are widely used as substrates for growing functional films in device application.

In the present work, the microstructural and magnetic properties of LiFe₅O₈ films with two different thicknesses prepared on SrTiO₃ substrates have been investigated by aberration-corrected (scanning) transmission electron microscopy ((S)TEM) and superconducting quantum interference device (SQUID). The twin boundaries (TBs), orientation domains, and interface dislocation in the films have been determined by high-angle annular dark-field (HAADF) imaging. The magnetic properties of the films have been characterized by magnetization measurement in a SQUID magnetometer, and the effect of film thickness and structure defects on the magnetic properties of the LiFe₅O₈ films has been discussed. This investigation provides a way of changing the thickness to manipulate the microstructure and the magnetic properties of LiFe₅O₈ films, making them adapt to diverse technological applications, e.g., electrode for rechargeable lithium-ion batteries, various components in microwave devices, and magnetic insulators for spin filtering in spintronics [24,25,26].

2. Materials and Methods

LiFe₅O₈ ceramic target was prepared by a standard solid-state reaction method with the initial reactants Fe₂O₃ and LiCO₃ (ratio 5:2). The LiFe₅O₈ films with different thicknesses were fabricated on single-crystalline SrTiO₃ (001) substrates by a high-pressure sputtering system at the substrate temperature of 800 ℃. The working pressure was 0.5 mbar with the mixed ambient of Ar and O₂ at the ratio of 1:1.

(S)TEM specimens were prepared by focused ion beam (FIB) lift-out technique using an FEI Helios600i FIB/SEM system. FIB lamellae were cut along the <110> orientations of the SrTiO₃ substrate. TEM and HAADF-STEM experiments were performed on a JEOL-ARM200F with a probe aberration corrector, operated at 200 kV. In STEM mode, a probe size of 0.1 nm at semi-convergence angle of 22 mrad was used for HAADF-STEM imaging. The HAADF detectors covered angular ranges of 90-176 mrad. The magnetic hysteresis (M-H) loops were measured by a SQUID (Quantum Design) with magnetic field applied along STO [100] and [001] directions, respectively. The quartz paddle and brass half-tube were used as sample holders for M-H loops along in-plane (STO [100] direction) and out-of-plane (STO [001] direction), respectively.

3. Results and Discussions

Figure 1a and 1b are the low-magnification bright-field (BF) TEM images of LiFe₅O₈ thin film on SrTiO₃(001) substrates with a thickness of 7.5 nm and 30 nm, respectively. The film-substrate interfaces are marked by horizontal arrows. The contrast variation within the film can be discerned in both films. In Figure 1b, the oblique contrast lines shown by blue arrows are apparent. Figure 1c and 1d display the corresponding selected area electron diffraction (SAED) pattern of the heterostructure in Figure 1a and 1b, respectively, recorded along the [1$\overline{1}$0] zone axis of SrTiO₃. In Figure 1c, apart from the diffraction spots of the SrTiO₃ substrate, two sets of diffraction spots from the LiFe₅O₈ film can be distinguished, resulting in two film-substrate orientation relationships (ORs) as [1$\overline{1}$0](001)_film//[1$\overline{1}$0](001)_substrate (cube-on-cube) and [1$\overline{1}$0](111)_film//[1$\overline{1}$0](001)_substrate. Considering the four-fold symmetry of the SrTiO₃(001) substrate surface, there exists an equivalent OR having a 90° in-plane orientation relation to the latter OR. In Figure 1d, the LiFe₅O₈ film adopts the cube-on-cube OR with the substrate. Instead of forming crystalline orientation domains in the 7.5-nm-thick film, there present some {111} TBs in the 30-nm-thick film. Taking the lattice parameter of SrTiO₃ substrate (0.3905 nm) as the calibration standard [27], the in-plane and out-of-plane lattice parameter of the 7.5-nm-thick film is calculated to be 0.8319 nm and 0.8353 nm, respectively. Similarly, in-plane and out-of-plane lattice parameter of the 30-nm-thick film is calculated to be 0.8301 nm and 0.8359 nm, respectively. All the parameters are close to that of the bulk material, indicating that the considerable mismatch strain of both films is relaxed, leaving tender compressive strain in the film [10].

In order to further investigate the microstructure and strain relaxation behaviors, high-resolution HAADF-STEM experiments have been performed. Figure 2a–2c are the atomic-resolution HAADF-STEM images showing the interfaces of the 7.5-nm-thick film, viewed along [1$\overline{1}$0] zone axis of the SrTiO₃ substrate. Misfit dislocations form at the interface in both heterostructures. For the grain with the cube-on-cube OR (Figure 2a), the projected Burgers vector of misfit dislocations can be determined as (a_f/4)[110] (a_f is the lattice parameter of LiFe₅O₈). For the [1$\overline{1}$0](111)_film//[1$\overline{1}$0](001)_substrate OR, the misfit dislocations occur at the interfaces, as shown in Figure 2b and 2c. The projected Burgers vectors are determined to be (a_f/8)[11$\overline{2}$] and (a_f/4)[1$\overline{1}$0], respectively. In contrast, for the heterostructure of the 30-nm-thick film on the SrTiO₃(001) substrate, only a number of {111} TBs appear within the LiFe₅O₈ film as demonstrated in Figure 2d. It should be noted that abnormal contrast has not been observed in any HAADF images, indicating there is no chemical segregation in the film.

Additionally, for the heterostructure of the 7.5-nm-thick film prepared on SrTiO₃(001) substrate, the occurrence of two types of film-substrate ORs would form a columnar grain structure in the film. The coalescence of these grains inevitably leads to the formation of grain boundaries (GBs). Figure 3a–3c are the HAADF-STEM images containing such GBs. The boundaries appear curved through the film as traced by white dashed lines. It should be emphasized that there is no secondary phase or obvious element segregation at the boundaries.

In the LiFe₅O₈/SrTiO₃ heterostructure, the lattice mismatch is calculated to be about +6.2% for cube-on-cube epitaxy, using the formula [(a_f-2a_s)/2a_s]*100%. For the LiFe₅O₈(111)/SrTiO₃(001) epitaxy, the lattice mismatch along [110]_f direction is the same as that of the cube-on-cube epitaxy, whereas the film-substrate lattice mismatch along [11$\overline{2}$]_f direction is much large. Based on the TEM results, different strain relaxation behaviors occur in the LiFe₅O₈ films through different defect configurations [28,29]. The appearance of oriented grains and misfit dislocations releases the compressive strain in the 7.5-nm-thick film on SrTiO₃ substrate. In contrast, the formation of a high density of twins within the film mainly contributes to the strain relaxation in the 30-nm-thick film.

The magnetic properties of the LiFe₅O₈ films have been characterized by the magnetic hysteresis loops using the SQUID system. The effect of pure SrTiO₃ substrate has been carefully eliminated. Figure 4a and 4b present the M-H hysteresis loops measured along in-plane and out-of-plane directions of 7.5-nm- and 30-nm-thick film separately. The in-plane saturation magnetization (Ms) of the 7.5-nm-thick film is about 583 emu/cc and the out-of-plane Ms is 465 emu/cc (experimental error for the magnetization (± 1 emu/cc)). Both values are significantly higher than that of bulk LiFe₅O₈ (2.5 µB/formula unit ~ 320 emu/cc) [5]. The in-plane and the out-of-plane Ms of the 30-nm-thick film are about 204 emu/cc and 154 emu/cc, respectively. The 7.5-nm-thick film exhibits considerable high Ms compared with the values in literature (see Table I in Supplementary Material). The in-plane and out-of-plane coercive fields (Hc) of the 7.5-nm-thick film are about 50 Oe and 101 Oe, respectively, which are slightly smaller than that of the 30-nm-thick film (254 Oe and 140 Oe) (experimental error for experimental error for coercivity values (± 10 Oe)). Our measurement of the magnetic properties shows apparent thickness dependence of LiFe₅O₈ thin films prepared on SrTiO₃(001) substrates.

The enhancement of the magnetization and the decrease of the coercive field has been reported in thinner spinel films, e.g., NiFe₂O₄ and CoFe₂O₄ [21,22,23] and LiFe₅O₈ on MgAl₂O₄ substrates [10]. The anomalous cation distribution among the tetrahedral and octahedral sites of the spinel structure has been invoked to account for this phenomenon [14,21]. In our LiFe₅O₈ thin films, no chemical modulation or second phase has been observed during TEM investigations, ruling out the anomalous Fe³⁺ distribution as the origin of the enhanced Ms. Thus, the most likely factor responsible for the thickness-dependent magnetic properties is the strain state and the microstructure of the film. It is considered that the enhanced Ms in the 7.5-nm-thick film is due to the distinct column grain structure and the possible oxygen vacancies at the GBs. The presence of oxygen vacancies is associated with the reduced ions Fe²⁺, which can mitigate the antiparallel aligned spin of Fe³⁺ at tetrahedral sites and octahedral sites, resulting in enhancement of the net magnetic moment [1,30]. In contrast, the appearance of a high density of TBs with antiferromagnetic coupling [7] in the 30-nm-thick film will weaken the Ms of the film [31].

LiFe₅O₈ is a negative magnetostrictive material with saturation magnetostriction ~27.8 ppm [10]. The compressive strain favors the in-plane orientation of the magnetization [22,32]. Although strain relaxations occur in our LiFe₅O₈ films, the tetragonal lattice distortions appear in both films under compressive strain, resulting in anisotropic magnetization in the both films. The coercive fields (Hc) of 30 nm film are slightly higher than that of 7.5nm film. The occurrence of antiferromagnetic defects and effect of magnetic domain wall pinning induced by those defects are likely to make it difficult to turn over the magnetic domain during magnetization process, which leads to the larger coercive field in 30 nm film. Overall, varying thicknesses of the LiFe₅O₈ films on SrTiO₃(001) substrate can effectively modify the microstructural and magnetic properties of the film.

4. Conclusion

The epitaxial LiFe₅O₈ thin films with the thickness of 7.5 nm and 30 nm have been grown on SrTiO₃ (001) substrate. Microstructural investigations show that the (111)_film//(001)_substrate and (001)_film//(001)_substrate ORs appear in the 7.5-nm-thick film, and TBs occur in the 30-nm-thick film, respectively, which contributes to the lattice misfit strain. Importantly, the 7.5-nm-thick film displays a larger saturation magnetization and a relatively lower coercive field in comparison with the 30-nm-thick film. Our results demonstrate that changing the film thickness could effectively tune the microstructure and magnetic properties in epitaxial LiFe₅O₈ thin film.