Magnetic properties of Fe intercalation FexTaSe2

Intercalation of transition metal dichalcogenides with magnetic elements has been the subject of increasing research interest, aiming to explore novel magnetic materials with anisotropy and spin-orbit coupling. In this paper, two magnetic samples with varying Fe content have been prepared using different growth conditions via the chemical vapor transport method. A comprehensive investigation of the magnetic properties of the materials has been conducted using the Physical Property Measurement System (PPMS, EvercoolⅡ-9T, Quantum Design). The results reveal distinct features in the studied materials. Fe_0.12TaSe₂ exhibits significant ferromagnetism with a Curie transition temperature of 50 K. However, its in-plane magnetism is weak and no significant hysteresis loop is observed below the Curie temperature. On the other hand, Fe_0.25TaSe₂ exhibits antiferromagnetism without any hysteresis loop and has a Néel temperature up to 130 K. This finding is quite different from the intercalated iron in Fe_xTaS₂, where only an antiferromagnetic state occurs with x larger than 0.4. Our study thus provides updated insights into the magnetic properties of this new system and serves as a reference for future investigations of TaSe2 compounds with varying iron content.

1 Introduction

Transition metal dichalcogenides (TMDCs) are compounds of the type TX₂ (T = transition metal, X = S, Se, Te) that exhibit a variety of intriguing characteristics, such as topological semimetals [1], large magnetoresistance [2], superconductivity [3], and charge density wave (CDW) [4], etc. The layered structure of TMDCs consists of an X-T-X sandwich in each layer. Sandwiches come in two varieties: octahedral and triangular. The van der Waals force is used to link the layers. TMDCs often contain multiple polytypes [5], such as 1T, 2Ha, 2Hb, and 3R, due to different interlayer types and stacking arrangements. 2H-TaSe₂ is an intriguing TMD with a combination of CDW and superconductivity. It consists of selenium and transition metal-selenium sandwiches with a triatomic structure, and its unit cell can be considered as two layers of Ta-Se with 60-degree rotation, with neighboring layers having van der Waals forces [6]. 2H-TaSe₂ undergoes a second-order transition from the normal phase to the disproportional ordered phase at $T_{ICDW}=122K$, followed by a first-order locking transition to a 3 × 3 commensurate phase at $T_{CCDW}=90K$ [7]. The CDW properties of TaSe₂ and superconducting regulation have been extensively studied [8], such as strong enhancement of superconductivity at high pressures within the CDW states of 2H-TaSe₂ [9], CDW tuning in monolayer 1H-TaSe₂ by biaxial strain and charge doping [10], Interplay of CDWs, disorder, and superconductivity in 2H-TaSe₂ elucidated by NMR [11]. To date, relatively few studies have been reported on the preparation of magnetic materials from TaSe₂ by intercalation of ferromagnetic elements. However, the compound TaS₂, which is composed of elements of the same chalcogen group, has been studied extensively. Due to the coexistence of CDW, superconductivity, and 1T form of the glassy magnetic state, TaS₂ has been the subject of much research [12]. The intercalation of Fe in 2H-TaS₂ (Fe_xTaS₂) results in a transition of the magnetic properties of the system from spin glass to ferromagnetism at $x=0.2$, and to antiferromagnetism at $x=0.4$ [13–16]. Hardy et al. [13] discovered a significant increase in magnetoresistance (approximately 60%) in the Fe_0.28TaS₂ single crystal, as well as a slight change ($∆x=0.03$) in the ordered superstructure of Fe_0.25TaS₂. This large magnetoresistance change is nearly two orders of magnitude larger than the previously reported data on Fe_0.25TaS₂. The researchers attributed this phenomenon to spin-disorder scattering under strong spin-orbit coupling. More recently, Chen et al. reported an even larger reluctance of 140% in Fe_0.29TaS₂, which is more than twice the MR Value found by Hardy et al. Extensive reports on TaS₂ suggest that exploring the properties of TaSe₂ homologous substitution could also yield interesting results.

To find new magnetic materials, transition metal diselenides are an ideal alternative because of their weaker van der Waals forces between layers and easy intercalation of transition metals [17–19]. We can try to obtain new magnetic materials by intercalating magnetic elements between the 2H-TaSe₂ layers. In this study, we use chemical vapor phase transport to prepare the samples. Using the chemical vapor transport method (CVT) [20], we prepared two Fe samples with different Fe contents and determined their crystal orientation and elemental composition through XRD and SEM energy spectroscopy analyses. In addition, the magnetic properties of Fe_xTaSe₂ were investigated using a PPMS setup, providing experimental measurements. We have found that the two samples with varying Fe content constitute distinct ferromagnetic phases. Specifically, one phase exhibits ferromagnetism, while the other phase exhibits antiferromagnetism. These results indicate that Fe_xTaSe₂ single crystals have a rich potential for applications in magnetism and deserve further theoretical and experimental studies.

2 Experimental section

The CVT method, which involves using a carrier gas to transport the sample in the vapor phase to a cooler location where it is deposited as a solid, was used to prepare the sample. A small amount of iodine was added to the starting materials in a sealed tube to act as a transport agent, and the tube was heated to a temperature high enough to evaporate both the sample and the iodine [21].

Sample 1: A mixture of pure Fe (51.6 mg, 99.9%), Ta (506.4mg, 99.9%), and Se (442.0mg, 99.5%) powders with a stoichiometric molar ratio of 1:3:6 was loaded into a quartz tube. Vacuum-sealed in quartz tubes with I₂ (93.9mg, 99.99%) as a transport carrier. The evacuated quartz tube’s source zone was heated to 900 °C, while the crystal growth zone was warmed to 700 °C. The whole system was kept at a set temperature for 5 days and then cooled to room temperature naturally [22].

Sample 2: Similarly, pure Fe (76.4 mg, 99.9%), Ta (493.7 mg, 99.9%), and Se (430.8 mg, 99.5%) powders were loaded into a quartz tube in a stoichiometric molar ratio of 1:3:6. The quartz tube was then vacuum-sealed, and I₂ (133.9 mg, 99.99%) was used as the transport carrier. The quartz tubes were evacuated and sealed before being placed in a tube furnace equipped with a two-temperature zone. The high-temperature end was set at 830°C while the low-temperature end was set at 730 C. The tubes were initially heated to the desired temperature and held at that temperature for 12 h. Subsequently, they were maintained at this temperature for 12 days before being allowed to cool down naturally to room temperature.

3 Results and discussion

Sample 1: Figure 1A shows the schematic principle of CVT, where the feedstock is located at the high-temperature end and the sample in the gas phase is transported by iodine vapor to the low-temperature end for solid-phase deposition. To further characterize the crystal quality, the structure of Fe_0.12TaSe₂ single crystal was characterized by X-ray diffractometry (XRD) [23]. Figure 1B illustrates the XRD diffraction data analysis of the single crystals. Strong and prominent diffraction peaks in each crystal direction are observed for single crystals with excellent crystalline quality. The inset shows the optical image of the Fe_0.12TaSe₂ single crystal, which exhibits a regular polygonal shape and metallic luster. The elemental composition of Fe_0.12TaSe₂ crystals was characterized using scanning electron microscopy (SEM) and energy spectrometry (EDS). The morphology and dimensions of the samples were examined by SEM. Figure 1C displays the EDS energy spectrum obtained by comparing the ratio of integrated intensities of each element’s peaks. The estimated composition of this data is Fe_0.12TaSe₂. The Fe_0.12TaSe₂ bulk was verified in polytype using Micro-Raman spectroscopy, and the test results are shown in Figure 1D. The experiments were performed in a backscattering configuration with an excitation laser wavelength of 532 nm. Figure 1D shows the information bands at 208 cm^-1 (E_2g) and 234 cm^-1 (A_1g), which are consistent with the previous reports on 2H-TaSe₂ [24].

FIGURE 1

FIGURE 1. (A) Schematic diagram of the principle of CVT. (B) XRD spectra of Fe_0.12TaSe₂. The inset is the optical image of the Fe_0.12TaSe₂ single crystal. (C) EDS spectrum of Fe_0.12TaSe₂, inset shows actual atomic ratio. (D) Raman spectra of Fe_0.12TaSe₂ single crystals.

Figure 2 exhibits the magnetic measurements associated with Fe_0.12TaSe₂. Figures 2A, B depict the magnetization intensity versus temperature measured under zero-field cooling (ZFC) and field cooling (FC) with an applied magnetic field of 0.1T parallel to the c-axis (M_c) and along the ab-plane (M_ab) direction, respectively [25]. Due to the strong magnetic crystal anisotropy of Fe_0.12TaSe₂ (with the c-axis serving as the easy magnetization axis), the measured M-T is different between the two directions [26]. The bifurcation of the two curves in the low-temperature part originates from the characteristics of the spin glass [27]. As the temperature increases, the Fe_0.12TaSe₂ single crystal becomes paramagnetic, and the zero-field cooling curve coincides with the field cooling curve. The inset of Figure 2A shows the dM/dT curve of ZFC, where the minimum point of the derivative is the Curie temperature. By deriving the data, we can deduce that the Curie temperature of Fe_0.12TaSe₂ is 50 K. Figure 2C shows the magnetization curves at different temperatures when the magnetic field is perpendicular to the ab-plane, the maximum value of the scanning magnetic field is 7 T, and obvious rectangular hysteresis loops can be observed below the Curie transition temperature. The rectangular hysteresis loop is most obvious when the temperature is $T=10K$, the coercivity is $H_c=1.15T$, and the saturation field is $H_s=2.5T$ [28]. As the temperature increases, the single crystal transforms into a paramagnetic material with no rectangular switch [29, 30]. Figure 2D shows the magnetization curves at different temperatures when the magnetic field is parallel to the ab-plane, and it can be seen that the in-plane magnetism is weak and not easily saturated. By comparing the in-plane and out-of-plane magnetization curves, we can see that Fe_0.12TaSe₂ is a ferromagnetic material with an easy magnetization axis parallel to the c-axis and a weak in-plane magnetism.

FIGURE 2

FIGURE 2. (A) Temperature-dependent magnetization measured of zero-field cooling (red) and field-cooling (black) with $H\parallel c$. The inset shows the dM/dT curve for zero field cooling with $H\parallel c$. (B) Temperature-dependent magnetization measured of zero-field cooling (red) and field-cooling (black) with $H\parallel ab$. (C) Magnetization curves at different temperatures out of the plane. (D) Magnetization curves at different temperatures in the plane.

Sample 2: Figure 3A illustrates the X-ray diffraction (XRD) pattern of sample 2. The crystalline orientation is prominent and robust, suggesting that the single crystal exhibits high crystalline quality. The inset is an optical image of a single crystal of Fe_0.25TaSe₂, showing a regular polygonal shape and metallic luster. The elemental composition of the samples was determined using scanning electron microscopy (SEM), as depicted in Figure 3B. The inset shows the atomic proportions of the elements. The scanning electron microscopy (SEM) results reveal that the Fe content in this single crystal is 0.25, indicating its chemical formula as Fe_0.25TaSe₂. Similarly, the polytype of the Fe_0.25TaSe₂ block was confirmed using micro-Raman spectroscopy. The test results are presented in Figure 3C. The experimental pattern using the 532 nm wavelength laser is consistent with that of Sample 1. Figure 3C displays the characteristic bands observed at 212 cm^-1 (E_2g) and 234.27 cm^-1 (A_1g), which confirm that Sample 2 corresponds to the 2H-TaSe₂ polytype [24]. We obtained two samples with varying Fe content, allowing us to proceed with individual measurements of the magnetic properties of each sample for a comprehensive investigation of their respective characteristics.

FIGURE 3

FIGURE 3. (A) XRD spectra of Fe_0.25TaSe₂. The inset is the optical image of the Fe_0.25TaSe₂ single crystal. (B) EDS spectrum of Fe_0.25TaSe₂, inset shows actual atomic ratio. (C) Raman spectra of Fe_0.25TaSe₂ single crystals.

Figures 4A–D shows the magnetic measurements associated with Fe_0.25TaSe₂. Figures 4A, B show the zero-field cooling (ZFC) and field cooling (FC) curves at 0.1 T magnetic field parallel to the c-axis (M_c) and along the ab-plane (M_ab) direction, respectively. Antiferromagnetic behavior is observed in Figures 4A, B, suggesting that Fe_0.25TaSe₂ exhibits the characteristics of an antiferromagnetic material [31, 32]. The inset in Figure 4A displays the derived curve of the zero-field cooling (ZFC) magnetization versus temperature, with the lowest point serving as the critical temperature for the antiferromagnetic to paramagnetic transition, known as the Néel temperature (T_N) [33, 34]. Based on the inset, the Néel temperature (T_N) of Fe_0.25TaSe₂ is determined to be 130 K. Figures 4C, D present the magnetization curves for in-plane and out-of-plane orientations at various temperatures, exhibiting a linear trend that is indicative of antiferromagnetism. This behavior is characterized by the absence of saturation, making it challenging to reach a maximum magnetization value.

FIGURE 4

FIGURE 4. (A) Temperature-dependent magnetization measured of zero-field cooling (red) and field-cooling (black) with $H\parallel c$. The inset shows the dM/dT curve for zero field cooling with $H\parallel c$. (B) Temperature-dependent magnetization measured of zero-field cooling (red) and field-cooling (black) with $H\parallel ab$. (C) Magnetization curves at different temperatures out of the plane. (D) Magnetization curves at different temperatures in the plane.

The inset of Supplementary Figure S1A illustrates the schematic diagram of the device configuration after the mechanical stripping of the sample into a thin film. The four-terminal method was used to pass a constant current under zero-field conditions by integrating the setups of Keithley 2400 and Keithley 2182. The variation curve of longitudinal resistance from 10K to 300K is obtained by this method. Supplementary Figure S1B shows the temperature dependence of the resistance of the Fe_0.12TaSe₂ sample in the presence of zero field. The resistance decreases with decreasing temperature and exhibits metallic behavior. In the vicinity of the ferromagnetic and paramagnetic transition temperatures, the resistance varies significantly with the temperature slope. At higher temperatures, the linearity of R_xx (T) weakens, suggesting a decline in the metal’s properties. Conversely, below 50 K, it maintains consistency with the loss of spin-disorder scattering in the FM state. This corresponds to the Curie temperature of the magnetic measurement data, indicating that Fe_0.12TaSe₂ has a Curie temperature of 50 K [35]. Supplementary Figure S1C shows the temperature dependence of the resistance of the Fe_0.25TaSe₂ sample in the presence of zero field. The longitudinal resistance R_XX also exhibits metallic properties. The resistance exhibits a reduction as the temperature decreases, with a notable alteration in the slope of resistance observed near $T=130K$ [36]. This observation suggests that the antiferromagnetic sample with a Fe content of 0.25 has a Néel temperature of 130 K, in excellent agreement with prior magnetic properties measurements.

4 Conclusion

We prepared two samples of Fe_xTaSe₂ with different Fe contents using the CVT technique. By measuring their magnetic properties, we determined that the two samples exhibit ferromagnetism and antiferromagnetism, respectively. Fe_0.12TaSe₂ is ferromagnetic with a Curie temperature of 50 K. At low temperatures, there is a well-defined rectangular saturation magnetization curve outside the surface. As the temperature increases, the opening of the magnetization curve becomes smaller, and it gradually transforms into a linear magnetization curve at the Curie temperature. The magnetic properties within the ab-plane are weak and there is no obvious rectangular magnetization curve, which is linear. Significantly, Fe_xTaSe₂ exhibits an intriguing antiferromagnetic behavior when x is equal to 0.25, which is in contrast to the phase diagram of Fe_xTaS₂. Remarkably, the Néel temperature associated with the antiferromagnetic phase of Fe_0.25TaSe₂ can reach an impressive value of 130 K. The magnetization curves of Fe_0.25TaSe₂ are all linear curves and difficult to saturate. Finally, the temperature dependence of the sample resistance was investigated and the results verified that the magnetic transition temperature corresponds to the magnetic measurements for both samples. This finding holds significant value for future investigations focused on elucidating the phase diagram of Fe_xTaSe₂ and determining the critical ferromagnetic-antiferromagnetic threshold of this material.

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.

Author contributions

Q-QF: Investigation, Methodology, Validation, Conceptualization, Data curation, Visualization, Writing–original draft. J-JG: Investigation, Data curation, Writing–original draft. M-ZZ: Conceptualization, Funding acquisition, Project administration, Writing–review and editing. Z-YL: Conceptualization, Funding acquisition, Project administration, Writing–review and editing, Formal Analysis. BL: Project administration, Writing–review and editing, Supervision. X-GW: Project administration, Supervision, Writing–review and editing, Conceptualization, Funding acquisition. Y-ZN: Conceptualization, Project administration, Writing–review and editing, Methodology. Q-LX: Methodology, Project administration, Writing–review and editing, Funding acquisition, Investigation, Validation. G-HG: Conceptualization, Data curation, Funding acquisition, Project administration, Writing–review and editing.

Funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work was supported by the National Natural Science Foundation of China (Nos. 12274467, 12274469, and 12204549), and the Fundamental Research Funds for the Central Universities of Central South University (No. 2023ZZTS0700). The project was also supported by the State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fphy.2024.1371171/full#supplementary-material

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