PLD Growth of Ferrimagnetic Tm3Fe5O12 Thin Film with Perpendicular Magnetic Anisotropy on GGG

Abstract

Thulium Iron Garnet (TIG), as an emerging hotspot in rare-earth iron garnet systems, possesses a large magnetostriction constant (λ₁₁₁) and a low damping coefficient. Therefore, it is possible to induce perpendicular magnetic anisotropy (PMA) through stress, which makes it more desirable for interfacial magnetic proximity or spin–orbit torque effects than Yttrium Iron Garnet (YIG). For achieving a high-quality TIG thin film and regulating its properties accordingly, understanding the effect of growth parameters on the film properties is essential. Using the Pulsed Laser Deposition (PLD) technique, we prepared TIG film on a Gadolinium Gallium Garnet (GGG) substrate. The correlations of its structural properties to the growth conditions are systematically studied, including the oxygen pressure and laser energy. With the annealing, a ferrimagnetic TIG thin film with PMA is successfully obtained. Our work provides a platform for achieving high-quality TIG thin films by experimentally regulating the growth factors.

1. Introduction

In recent years, with the deepening of spintronics research, the properties of ferrimagnetic insulators, such as long spin diffusion lengths, low damping, and the absence of heat generation, have garnered significant attention, demonstrating their potential for applications in fast and energy-efficient memory and logic devices [1,2,3,4,5]. Yttrium Iron Garnet (Y₃Fe₅O₁₂, YIG), as a typical representative, has been extensively studied in magneto-optics and spintronics due to its very weak spin wave damping (Gilbert damping parameter α: 3.0 × 10⁻⁵) [6] and low microwave loss characteristics [7,8,9,10,11]. With the emergence of various novel effects, such as spin–orbit torque [12] and the Rashba–Edelstein effect [13], the demand for materials with perpendicular magnetic anisotropy (PMA) has significantly increased. However, YIG is limited by its large shape anisotropy [13] and small magnetostriction constant (λ₁₁₁ = −2.4 × 10⁻⁶ in YIG [14,15]), which result in the magnetoelastic anisotropy induced by the lattice mismatching between YIG and the substrate being insufficient to overcome its shape anisotropy, often requiring additional doping to achieve PMA [16], which not only increases the difficulty of preparation but also risks introducing disorder and defects into the films.

Attention has thus turned to TIG [17,18], which belongs to the same rare-earth iron garnet as YIG. TIG also exhibits low Gilbert damping, with α ≈ 0.01 [19,20,21]. Compared to YIG, it has a larger magnetostriction constant (λ₁₁₁ = −5.2 × 10⁻⁶ in TIG [14,20]), making it easier to overcome shape anisotropy and achieve PMA [22,23]. The PMA fields of TIG film on Gadolinium Gallium Garnet (Gd₃Ga₅O₁₂, GGG) substrate can be modulated from 1429 Oe to 2439 Oe by varying the interfacial strain, which can be achieved by adjusting the film compositions [19]. Additionally, PMA with a coercive field of up to 77 Oe can obtained by precisely controlling the annealing temperature, thickness, and substrate surface roughness of the TIG films [24]. Additionally, the TIG film has a longitudinal spin Seebeck effect coefficient as high as 0.38 μV/K and a spin hall magnetoresistance of 0.0072% [25]. Furthermore, due to the magnetic proximity effect, TIG also provides a platform for studying novel topological magnetoelectric effects [26] and the complex interaction between topological spin textures [2], such as in the magnetism/Topological Insulator (TI) heterostructure TIG/(Bi_0.20Sb_0.80)₂Te₃, where a strong PMA was also observed in TI surface states, which provides a potential material platform for achieving a quantum anomalous Hall effect above room temperature [26]. To further explore novel topological magnetoelectric effects with TIG, obtaining high-quality TIG films is essential.

Pulsed laser deposition (PLD) is a typical film growth technique, especially for manufacturing high-quality complex oxides [27], commonly used for preparing films of the rare-earth iron garnet. In this work, we used GGG(111) as a substrate to prepare TIG films via PLD and systematically studied the effects of oxygen pressure and laser energy on the growth process of TIG films. By introducing a post-annealing process, the quality of TIG films was improved. By exploring and summarizing the corresponding growth laws, reliable experimental evidence was provided to further reduce the preparation difficulty of rare-earth iron garnet films.

2. Materials and Methods

2.1. Crystal Growth

In this study, the substrate used to prepare TIG films was commercial (111)-oriented GGG (Hefei Kejing Materials Technology Co., Ltd., Heifei, China) and the target material used was commercially purchased TIG target (Hefei Kejing Materials Technology Co., Ltd., Heifei, China). Due to the constraints of the PLD system, the GGG substrates were processed into 4 mm × 4 mm × 0.5 mm dimensions. The experimental protocol commenced with in situ heating of GGG substrates within the vacuum chamber (base pressure ~10⁻⁹ Torr) under continuous oxygen flow, with constant oxygen pressure being maintained throughout the heating–cooling cycling, film growth, and post-annealing processes. The temperature was elevated to the deposition temperature of 850 °C at a controlled ramping rate of 20 °C/min. Upon reaching the target temperature, a KrF excimer laser (λ = 248 nm, COMPex 205 F, Coherent Corp., Saxonburg, PA, USA) was activated for 5400 ablation pulses at a repetition rate of 3 Hz.

Following deposition, an in situ annealing procedure was immediately implemented, involving temperature elevation to 950 °C at 20 °C/min under sustained oxygen pressure. Post-annealing cooling to room temperature was conducted at 10 °C/min. For non-annealed specimens, the system was cooled directly from the deposition temperature to ambient conditions at the same 10 °C/min cooling rate after growth completion.

2.2. Characterization

The films were characterized using a high-resolution X-ray diffractometer (XRD, Rigaku Corporation, Tokyo, Japan) with a Cu Kα1 wavelength (λ = 1.54 Å) for structural analysis. Surface morphology was characterized by atomic force microscopy (AFM) using a MultiMode 8 instrument, Bruker Corporation, Billerica, MA, USA, operated in tapping mode.

The magnetic properties were characterized by a magneto-optical Kerr effect (MOKE) using a NanoMOKE3 (Durham Magneto Optics Ltd., Cambridge, UK), superconducting quantum interference device (SQUID) by a Magnetic Property Measurement System (MPMS) manufactured by Quantum Design, San Diego, CA, USA. The chemical stoichiometry of the films was determined by energy dispersive spectroscopy (EDS) using an EDS (X-Max^N 20) spectrometer, Oxford Instruments PLC, Oxford, UK. All characterizations were conducted at room temperature.

3. Results and Discussion

Previous research has shown that the provision of sufficient oxygen is essential in the preparation of oxide films using PLD; by adjusting the oxygen pressure, the shape of the plume can be altered, which in turn affects the properties, kinetic energy, and distribution of the material reaching the substrate [27]. Therefore, we first systematically investigated the effect of oxygen pressure during the growth process on the properties of TIG films. In this work, we sequentially studied the TIG film growth, starting with a lower oxygen pressure of 0.05 Torr and gradually increasing it to 0.20 Torr, while keeping other conditions constant (laser energy of 350 mJ, 5400 laser shots, and growth temperature of 850 °C). The corresponding XRD results are shown in Figure 1, and the surface morphologies are illustrated in Figure S1, indicating a roughness (Rq) in the range of 0.145–0.190 nm. The narrow sharp peak located at 51.01° demonstrates the diffraction signal from the GGG(444) substrate. We marked the film diffraction peak position using brown arrows, which gradually shifted to the right with increasing oxygen pressure, from 49.88° (0.05 Torr) to 50.34° (0.07 Torr) and 50.71° (0.10 Torr). When the oxygen pressure is 0.15 Torr, the diffraction peak of the film is masked by the strong substrate diffraction peak signal. A typical diffraction signal (located at the position of 51.3°) attributed to TIG(444) is observed when oxygen pressure increases to 0.20 Torr, as shown in Figure 1e. At lower oxygen pressures, the diffraction peak of the film appears to the left of the GGG(444) substrate peak, which is usually caused by a secondary phase generated by the element ratio away from the standard value. It has also been reported in PLD grown in other material systems [21], and the corresponding EDS results in our experiment also support this. As shown in Figure 1f, the Tm/Fe ratio of the sample at 0.05 Torr oxygen pressure is 0.709, which is far from the ideal Tm/Fe ratio (3:5 = 0.6). With the increase in oxygen pressure, the Tm/Fe ratio of the sample gradually decreased from the iron-deficient state to the iron-rich state, and the Tm/Fe ratio of the sample decreased to 0.551 at 0.20 Torr. The variation in Tm/Fe ratio and XRD peaks with oxygen pressure proved that, during the epitaxy of TIG samples by PLD, the Tm/Fe ratio can be significantly controlled by changing the oxygen pressure.

During the growth of rare iron garnet film including TIG, oxygen pressure is critical for its cation stoichiometry, since oxygen pressure significantly regulates the shape and kinetic energy of the plume, ensuring that oxides can be transferred stoichiometrically, avoiding the formation of a secondary phase [27,28,29,30].

In addition to the oxygen pressure, we found that the laser energy during the growth process also has a significant effect on the composition of the film. In the process of preparing epitaxial thin films by PLD, films have the same composition as the target when the focused laser energy density is chosen properly [27]. In our work, we also investigated the influence of laser energy on the TIG film’s growth. We vary the laser energy from 350 mJ to 550 mJ (oxygen pressure of 0.20 Torr). Figure S2 exhibits the surface morphologies of the samples grown at different laser energies (350 mJ, 425 mJ, 450 mJ, 480 mJ, 500 mJ, and 550 mJ), indicating a roughness (Rq) in the range of 0.161 nm–0.217 nm. The corresponding surface morphologies are illustrated in Figure S2, and their XRD measurement results are also summarized in Figure 2.

The samples grown at laser energies lower than 480 mJ (350 mJ–450 mJ) produce the diffraction peaks around the bulk of the TIG diffraction peak position (51.3°), which is at the right of the sharp GGG substrate peak. These diffraction peaks show slight shifts around the bulk TIG peak position. The EDS results of the samples at different laser energies are shown in Figure S3. Our EDS studies demonstrate that the Tm/Fe radios of these samples show different values: Tm/Fe = 0.551, 0.572, 0.590 at the laser energies 350 mJ, 425 mJ, 450 mJ, respectively. The slight change in the TIG(444) peak positions might be attributed to a small variation in rare-earth/Fe composition, consistent to previous reports [28]. While the laser energy is higher than 480 mJ, the diffraction peaks of the films shift to the left of the narrow substrate diffraction peak but are located at different positions (50.89° and 50.64°). Additionally, the Tm/Fe ratios are 0.629 and 0.647 for the samples grown at the laser energies of 500 mJ and 550 mJ, respectively. It indicates that higher laser energy increases the Tm/Fe composition ratio and induces the XRD peak leftward shifts accordingly. At the laser energy of 480 mJ, the diffraction curve shows a broad shoulder at the right of the GGG substrate peak, and our EDS measurement shows its Tm/Fe composition ratio with a value of 0.613. The results indicate that the XRD diffraction peaks strongly correlate to the stoichiometry. Increasing the laser energy induces a monotonic increase in the Tm/Fe ratio. The Tm/Fe ratio of the TIG film prepared at a laser energy of 450 mJ is closest to the ideal value 0.6.

In order to investigate the magnetic properties of the TIG films, we conducted out-of-plane magnetic characterizations using MOKE and SQUID. Figure 3a displays the MOKE results of the TIG film grown under the conditions of oxygen pressure at 0.20 Torr and laser energy at 450 mJ. The well-defined square loop in the vertical direction infers the PMA of the TIG film [31]. However, in our SQUID measurements, no obvious hysteresis loops were detected in either the in-plane or out-of-plane directions. The reason is probably due to the inhomogeneity of the as-prepared samples prior to annealing and the fundamental differences in magnetic detection methodologies between SQUID and MOKE techniques.

In order to further improve the quality of the TIG films and to achieve more uniform films, we post-annealed the TIG films at 950 °C. We selected the samples annealed for 10 min, 30 min, and 60 min for MOKE and SQUID characterizations, respectively. All the annealed samples exhibit remarkable well-defined rectangular M-H loops in out-of-plane MOKE measurements in Figure 3b–d. However, the SQUID measurements exhibit different results. For the films annealed with 0 min, 10 min and 30 min, no obvious hysteresis loop is observed. For the film annealed with 60 min, the plot presents clear square hysteresis loop in the out-of-plane SQUID measurements (Figure 4c), inferring the PMA of the TIG film. The thickness of the film is 15.5 nm (Figure S4). The saturation magnetization (M_s) of the TIG film is approximately 100 emu/cm³, which is close to the M_s of bulk TIG [14,32]. Additionally, the coercive force is approximately 10 Oe, in accordance with other reports [22,23,33].

In order to further explore the effect of annealing on the properties of TIG films, we also compared the XRD results of TIG with different annealing times, arranged upward with the increasing annealing time presented in Figure 4a. It can be seen that, as the annealing time gradually increases, the intensity of the characteristic diffraction peak of the TIG film gradually increases. The Laue oscillations near the characteristic diffraction peak becomes more obvious, indicating that annealing improves the quality of the crystal [34]. As the annealing time increases from 10 min to 30 min to 60 min, the peak position has a slight rightward shift, from 51.45° (annealing for 10 min) to 51.47° (annealing for 30 min) to 51.55° (annealing for 60 min), indicating that, in the growth direction, the lattice of TIG film is further distorted and the lattice constant is further reduced. According to the Bragg equation, the lattice constant of TIG film in the growth direction after annealing for 60 min is 1.227 nm (the original TIG lattice constant is 1.232 nm). This is due to the recrystallization of TIG film during post-annealing, and the TIG film is prone to following the lattice of the substrate with the post-annealing [24,35,36]. The in-plane lattice expansion leads to compressive strain in the growth direction. In order to rule out that the shift in the XRD diffraction peak was caused by the change in the Tm/Fe ratio, EDS tests are carried out on films with different annealing times, and the results are shown in Figure 4. It was found that the Tm/Fe ratio of the films did not change significantly even after annealing at 950 °C for 60 min. It is proven that the annealing operation had a significant effect on the crystal quality of TIG film and did not change the Tm/Fe ratio in the sample.

4. Conclusions

This study employs the PLD technique to fabricate TIG films on GGG substrates. By meticulously adjusting the growth parameters and incorporating a post-deposition annealing process, we have successfully obtained high-quality TIG films with PMA. Our research establishes a correlation between growth parameters, diffraction peak positions, and Tm/Fe ratios, thereby providing a robust experimental framework for the convenient synthesis of rare-earth iron garnet materials, including TIG, using PLD technology. Innovatively, we introduced annealing into the PLD growth process of TIG films and discovered its critical role in achieving high-quality TIG films with PMA. A period of annealing operation (950 °C for 60 min) significantly enhances the crystalline quality of the TIG films. Concurrently, this annealing process increases internal strain, leading to a further reduction in the lattice constant of the TIG films. This reduction amplifies the contribution of magnetoelastic anisotropy, ultimately resulting in the attainment of PMA. Our work guides the way for obtaining high-quality TIG films by adjusting the film growth parameters and promotes its potential applications in spintronics.

Author Contributions

Conceptualization, Z.L. and Y.L. (Ying Liu); methodology, Z.L.; software, Z.L. and X.W.; validation, Z.L., Y.X. and Y.Z.; formal analysis, Z.L.; investigation, Z.L. and D.W.; resources, Z.L., H.Y. and H.Z.; data curation, Z.L.; writing—original draft preparation, Z.L.; writing—review and editing, Z.L. and X.W.; visualization, Z.L.; supervision, Z.L.; project administration, Z.L.; funding acquisition, Y.L. (Yunliang Li) and Y.L. (Ying Liu). All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures

Figure 1. XRD results of the films at different oxygen pressures. (a) 0.05 Torr, (b) 0.07 Torr, (c) 0.10 Torr, (d) 0.15 Torr, (e) 0.20 Torr. And (f) Tm/Fe ratios of the films at different oxygen pressures.

Figure 2. XRD results of the films at different laser energies.

Figure 3. MOKE results for samples annealed at different times; (a) 0 min, (b) 10 min, (c) 30 min, (d) 60 min.

Figure 4. (a) XRD results of TIG films with different annealing times. The dashed line denotes the peak position of the bulk TIG. (b) Tm/Fe ratios of the films with different annealing times. (c) M-H curves of TIG film.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/cryst15030234/s1: Figure S1: AFM results of the samples at different oxygen pressures. Figure S2: AFM results of the samples at different laser energies. Figure S3: EDS results of the samples at different laser energies. Figure S4: The XRR curve and corresponding fitting profile of the TIG film annealed for 60 min are presented.

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