Magnetic modulation in epitaxial EuTiO₃ thin film via oxygen vacancies

ABSTRACT

Oxygen vacancies in magnetic materials are pivotal in tailoring their magnetic properties, offering a versatile pathway to manipulate their performance. This study focuses on the impact of oxygen vacancies on the magnetic properties of EuTiO₃ (ETO) thin films, demonstrating how these vacancies can induce ferromagnetism, a property not typically observed in its stoichiometric form. By controlling the background oxygen pressure during fabrication, we obtained ETO thin films with varying concentrations of oxygen vacancies and investigated their magnetic behavior. The results reveal that the manipulation of oxygen vacancies significantly influences the magnetic properties of ETO thin films. Films grown under low oxygen pressure exhibit a peak in the Curie temperature (T_c) around 4.1 K, indicating a transition to a ferromagnetic state. In contrast, films grown under high oxygen pressure show a T_c peak at approximately 2.5 K, suggesting an antiferromagnetic state. The control over oxygen vacancies provides a profound impact on the magnetic landscape of ETO, making it a critical handle in the engineering of magnetic properties for various applications, including multiferroic devices.

Keywords:
Oxygen vacancies; Antiferromagnetic; Ferromagnetic; Oxide film

1. INTRODUCTION

Oxygen vacancies in magnetic materials are critical in modulating their magnetic properties, offering a means to engineer performance [¹,²,³,⁴]. These vacancies, a result of oxygen deficiency in the crystal lattice, significantly alter the electronic structure and magnetic interactions within the material. For instance, in transition metal oxides like TiO₂, the introduction of oxygen vacancies can induce ferromagnetism, a property not typically observed in their stoichiometric forms [⁵]. The introduction of oxygen vacancies can induce ferromagnetism in transition metal oxides, a property not typically observed in their stoichiometric forms. This phenomenon has been attributed to the creation of mid-gap states near the fermi level, which can lead to the formation of bound magnetic polarons. Additionally, oxygen vacancies can modulate the magnetic exchange interactions, switching the magnetic ground state from antiferromagnetic to ferromagnetic in certain perovskite oxides [⁶,⁷,⁸,⁹].

The control of oxygen vacancies demonstrates the profound impact of these defects on the magnetic landscape of materials, making them a critical handle in the engineering of magnetic properties for various applications [¹⁰,¹¹,¹²,¹³]. The ability to manipulate oxygen vacancies provides a means to fine-tune the magnetic behavior of materials, which is essential for the development of advanced technological applications such as spintronics and multiferroic devices [¹⁴,¹⁵,¹⁶].

The findings enhance our understanding of the relationship between oxygen vacancies and magnetic properties in thin films and pave the way for the development of novel multiferroic materials with tailored properties for advanced technological applications. By controlling the concentration and distribution of oxygen vacancies, it is possible to engineer magnetic materials with desired properties, offering a versatile platform for the advancement of magnetic technologies [⁹].

EuTiO₃ (ETO) is a perovskite-structured material known for its unique magnetic and electronic properties. It crystallizes in the cubic Pm-3m space group and is characterized by its quantum paraelectric behavior. This means that at low temperatures, ETO exhibits behaviors similar to those of a ferroelectric material, but it does not undergo a true ferroelectric phase transition due to quantum fluctuations.

The material is also known to exhibit a G-type antiferromagnetic order below a certain temperature (Néel temperature), where the magnetic moments of europium ions order in an antiparallel fashion. This antiferromagnetic behavior is closely linked to the lattice structure and can be manipulated by external stimuli such as pressure or chemical doping.

ETO is of particular interest due to its strong magnetoelectric coupling, which means that its magnetic properties can be influenced by electric fields and vice versa [¹⁷,¹⁸,¹⁹,²⁰,²¹,²²,²³,²⁴]. This characteristic makes it a promising candidate for applications in multiferroic devices, where both magnetic and electric orders coexist and can be controlled by each other.

Research on ETO often focuses on understanding and controlling its magnetic and electric properties through various means, such as epitaxial strain, isotropic pressure, chemical doping, and electric fields. These external parameters can induce changes in the magnetic ordering of ETO, shifting from antiferromagnetic to ferromagnetic phases [²³, ²⁵, ²⁶].

Oxygen vacancies in ETO can indeed induce a transition from antiferromagnetism to ferromagnetism [²⁴]. This phenomenon is of significant interest due to the potential control over magnetic properties in this material, which is inherently antiferromagnetic at low temperatures. The presence of oxygen vacancies can significantly modify the magnetic ordering in ETO, leading to the emergence of ferromagnetic behavior.

In this study, by precisely controlling the background oxygen pressure during the fabrication of ETO thin films, samples with varying concentrations of oxygen vacancies were obtained. By comparing the magnetic properties of different high-quality oxide thin films, the role of oxygen vacancies in the regulation of the magnetic properties was further summarized. This research delved into how the manipulation of oxygen vacancies can significantly influence the magnetic behavior of these thin films, demonstrating the potential for defect engineering as a tool for tailoring the magnetic properties of complex oxides [²⁷,²⁸,²⁹].

2. MATERIALS AND METHODS

2.1. Target preparation

Mix powders of nanoscale titanium dioxide (TiO₂) with a purity of 99.9999% and europium oxide (Eu₂O₃) in a molar ratio of 2:1 to obtain a 5 g mixture. After ball-milling the mixture for 48 hours, a powder with a particle size of 10nm is achieved. This powder is then compacted into a fixed shape under a pressure of 80 MPa. The compacted samples are sintered in an oxygen atmosphere at a heating and cooling rate of 3°C per minute for 48 hours at 1000°C to obtain the pre-reactant Eu₂Ti₂O₇. Subsequently, Eu₂Ti₂O₇ is sealed in a tube furnace and subjected to a mixed reducing gas of 5% hydrogen and 95% argon, sintered at a heating and cooling rate of 3°C per minute for 24 hours at 1180°C to obtain the polycrystalline target, EuTiO₃(ETO). Figure 1 presents the XRD pattern of the ETO target, which reveals sharp ETO diffraction peaks. This indicates that through the aforementioned process, we have successfully obtained high-quality polycrystalline target. After analysis, the lattice constant of EuTiO₃ in the target material should be 3.905 Å.

Figure 1.
(a) XRD θ-2θ scans of the ETO target.

2.2. Sample preparation

The as-deposited ETO thin films were fabricated on SrTiO₃ (STO) substrates with (001) orientation via pulsed laser deposition (PLD). The ETO polycrystalline target is then placed into a pulsed laser deposition system. A 10 mm × 10 mm (100) oriented, one-side polished STO single crystal substrate is selected. After placing the substrate into the deposition chamber of the PLD, the chamber’s pressure is first evacuated to 10Pa using a mechanical pump, and then further reduced to different target pressure (1 × 10⁻⁴ Pa, 5 × 10⁻⁴ Pa, 1 × 10⁻³ Pa, 5 × 10⁻³ Pa) using a molecular pump. Concurrently, the substrates were heated to 750°C at a rate of 15°C per minute. The polycrystalline target of ceramic ETO was irradiated by a laser (Physik, 248 nm, 3 Hz, 2 J/cm²), operating at a frequency of 3Hz for a deposition time of 1 hour, resulting in the formation of 100nm oxide thin films.

2.3. Sample characterization

The crystal structure of the FETO film was ascertained using X-ray Diffraction (XRD, Bruker D8 Advance), employing θ−2θ, Φ, and ω scans. The in-plane lattice constants were determined through reciprocal space mapping (RSM, PANalytical Empyrean). X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher Scientific 230XI) was utilized to examine the valence states of the constituent elements. Magnetic characterization was performed using a Physical Properties Measurement System (PPMS-9, Quantum Design), with dc magnetic fields applied parallel to the film surface to derive the temperature-dependent magnetization (M-T) and direct current magnetic field-dependent magnetization (M-Hdc) curves at 2 K of the ETO film on the STO substrate.

3. RESULTS

The θ-2θ scan was employed to confirm the out-of-plane epitaxial relationship between various films and substrates, as shown in Figure 2a. In the ETO and FETO films, only the peaks of ETO 00l (l = 1, 2, 3) and STO 00n (n = 1, 2, 3) are observed, with no impurity phases detected, indicating single-phase oriented growth. Additionally, the peaks of the ETO films grown under oxygen pressures of 1 × 10⁻³Pa and 5 × 10⁻³Pa are overshadowed by those of the STO substrate, demonstrating that the films grown at these or higher oxygen pressures exhibit no tensile strain along the c-axis. As Table 1 shows, the out-of-plane lattice constants for the various films are 3.933 Å (1 × 10⁻⁴Pa), 3.925 Å (5 × 10⁻⁴Pa), 3.911 Å (1 × 10⁻³Pa), and 3.907 Å (5 × 10⁻³Pa). The rocking curve of the ETO (002) peak in Figure 2b displays a full-width at half-maximum (FWHM) of 0.047°, signifying the high crystallinity of the films.

Figure 2.
a) XRD θ-2θ scans of the ETO on a STO (001). (b) Omega scan of FETO (100) peak. (c) Phi scans of STO (013) and FETO (013) peaks. (d) RSM scan around the STO (310) peak.

Table 1
Samples deposited under different oxygen pressures.

TEMPERATURE	LASER ENERGY	PRESSURE	LATTICE CONSTABNT
750°C	1.5 J/cm2	1 × 10−4 Pa	3.933 Å
750°C	1.5 J/cm2	5 × 10−4 Pa	3.925 Å
750°C	1.5 J/cm2	1 × 10−3 Pa	3.911 Å
750°C	1.5 J/cm2	5 × 10−3 Pa	3.907 Å

Figure 2c presents the phi scan of the ETO film and the STO substrate. Both the (012) planes of the ETO film and STO substrate exhibit four-fold symmetry, appearing at every 90° interval, further suggesting that the ETO film is epitaxially grown on the LAO substrate. It can be deduced that the ETO film maintains a cubic-cubic growth mode on the substrate, with epitaxial relationships of ETO ₍₀₀₁₎//STO ₍₀₀₁₎ and FETO _[100]//LAO _[100]. Figure 2d displays the reciprocal space mapping (RSM) pattern of the ETO film grown at an oxygen pressure of 1 × 10⁻⁴ Pa around the (310) peak on the STO substrate. Based on this map, the in-plane lattice constants are calculated to be a = b = 3.905 Å, and the out-of-plane lattice constant c = 3.933 Å, which is consistent with the θ-2θ scan results. The in-plane lattice constant remains unchanged due to the constraint from the substrate.

This detailed analysis not only confirms the epitaxial growth of the ETO films on the STO substrate but also highlights the impact of oxygen pressure during deposition on the crystallographic properties of the films. The precise control over oxygen pressure allows for the fine-tuning of the films’ lattice constants, which is crucial for optimizing their structural and functional characteristics. The high crystallinity, as evidenced by the narrow FWHM of the rocking curve, is a key factor in achieving films with desired magnetic and electronic properties, making them suitable for advanced applications in spintronics and other related fields.

Figure 3 presents the XPS spectra of samples grown under different oxygen pressures. As shown in Figure 3a, the XPS spectra of the Eu 4d orbital indicate that as the oxygen pressure decreases, the overall energy levels of Eu increase, the proportion of non-magnetic Eu³⁺ particles gradually decreases, as Table 2 shows. leading to enhanced magnetism, which is consistent with the magnetic measurement results discussed below. Figure 3b displays the Ti 2p orbital spectra, revealing that the Ti 2p orbitals of different samples are identical, suggesting that the introduction of oxygen vacancies only affects the super-exchange interaction between Eu-O-Eu, while the Ti-O vacancies remain constant, indicating that the introduction of oxygen vacancies influences the ferromagnetic (FM) interaction between Eu-Eu, making it stronger than the antiferromagnetic (AFM) super-exchange interaction of Eu-Ti-Eu, thereby rendering the film ferromagnetic.

Figure 3.
X-ray photoemission spectrum of (a) Eu 4d, (b) Ti 2p.

Table 2
The ratio of Eu²⁺to Eu³⁺.

PRESSURE	RATIO
1 × 10−4 Pa	3.6:2.0
5 × 10−4 Pa	3.2:2.5
1 × 10−3 Pa	3.4:2.8
5 × 10−3 Pa	3.1:2.6

Expanding on this, the XPS analysis provides a detailed insight into the electronic structure changes of the samples as a function of oxygen pressure during growth. The decrease in Eu³⁺ ions and the corresponding increase in Eu²⁺ ions with reduced oxygen pressure suggest a shift in the magnetic properties towards ferromagnetism. This is attributed to the fact that Eu²⁺ ions have a single unpaired electron in their 4f orbital, which contributes to the net magnetic moment, unlike Eu³⁺ ions which have a half-filled 4f shell, making them non-magnetic. The consistency in the Ti 2p spectra across samples implies that the titanium sublattice remains unaffected by the changes in oxygen pressure, reinforcing the notion that the magnetic transition is primarily driven by alterations in the europium oxidation state and the associated modification of the Eu-O-Eu super-exchange pathways. The reduction in oxygen vacancies with decreasing oxygen pressure further supports the idea that these vacancies play a pivotal role in mediating the magnetic interactions within the film.

To conduct a thorough comparison of magnetic properties across different films, a systematic investigation was undertaken. As shown in Figure 4a, the temperature-dependent magnetization (M-T) curves of the ETO film, measured under a 1000 Oe direct current (dc) magnetic field applied in the in-plane direction, indicated no significant difference between the zero-field cooling (ZFC) and field cooling (FC) processes. With the temperature T decreasing, the magnetization M increased in a monotonic fashion. Notably, films grown under low oxygen pressure displayed a peak in the Curie temperature (T_c) around 4.1 K, suggesting a transition to a ferromagnetic state at this temperature. In contrast, films grown under high oxygen pressure exhibited a T_c peak at approximately 2.5 K. This difference is believed to stem from the inadequate introduction of oxygen vacancies in the high oxygen pressure samples, which results in magnetic properties that more closely resemble those of the bulk material, specifically its Néel temperature of 2.5 K, indicating a transition to an antiferromagnetic state.

Figure 4.
(a) M–T at 1000 Oe curves for the FETO thin films. (b) M–Hdc at 2 K and (c) shows a ferromagnetic loop under the low magnetic field..

Figures 4b and 4c present the dc magnetic field-dependent magnetization (M-Hdc) curves of the ETO film on the STO substrate at 2 K, measured under a direct current (dc) magnetic field applied in the in-plane direction, which reveal a sharp increase at 200 Oe and saturation at 1.8 T. The coercive field and saturation magnetization are 26 Oe and 3.12 μ_b/Eu, respectively. These curves provide clear evidence of the film’s ferromagnetic behavior, as the magnetization increases rapidly with the application of a magnetic field and reaches a plateau at higher fields, indicating that the magnetic moments are aligned with the field. A distinct hysteresis loop, observable under low magnetic fields (< 200 Oe), further confirms the ferromagnetic behavior within the film. This hysteresis loop indicates that the film retains its magnetic state even after the removal of the external magnetic field, a characteristic of ferromagnetic materials. Additionally, due to the lack of oxygen vacancies, a tendency towards antiferromagnetic (AFM) behavior is observed in films grown under high oxygen pressure. Consistent with Figure 3a.

It becomes evident that when the concentration of oxygen vacancies reaches a critical threshold, These vacancies induce local distortions in the lattice, enhancing the super-exchange interaction between Eu and O, which in turn modifies the electronic structure and affects magnetic interactions. they facilitate ferromagnetic coupling between Eu ions. Consequently, the competition between ferromagnetic and antiferromagnetic interactions is transferred, enabling the initial antiferromagnetic material to exhibit ferromagnetism.

4. CONCLUSIONS

This study underscores the pivotal role of oxygen vacancies in modulating the magnetic properties of ETO thin films. Through a systematic exploration of how oxygen vacancies affect the electronic structure and magnetic interactions within these films, we have demonstrated a clear correlation between vacancy concentration and the magnetic behavior of ETO.

Our findings indicate that the introduction and control of oxygen vacancies can effectively induce a transition from antiferromagnetic to ferromagnetic states in ETO thin films. This transition is marked by a shift in the Curie temperature (T_c), with films grown under low oxygen pressure exhibiting a higher T_c, indicative of ferromagnetic behavior, compared to those grown under high oxygen pressure, which show a Néel temperature of 2.5 K, aligning with the antiferromagnetic properties of bulk ETO. The introduction of oxygen vacancies leads to an enhancement of the Eu-O-Eu super-exchange interaction, weakening the antiferromagnetic super-exchange interaction of Ti-O-Ti, thereby resulting in ferromagnetic films. Furthermore, the introduction of additional oxygen vacancies has been shown to enhance the magnetic properties of the ferromagnetic films.

5. ACKNOWLEDGMENTS

The authors thank the National Natural Science Foundation of China (NSFC) for their financial support. The authors also acknowledge the Suzhou University of Science and Technology for the measurements of Magnetic properties.

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Publication Dates

• Publication in this collection
  24 Feb 2025
• Date of issue
  2025

History

• Received
  09 Nov 2024
• Accepted
  20 Jan 2025