Near-room-temperature exchange bias in surface-oxidized Fe₃GaTe₂ revealed by reflective magnetic circular dichroism

Abstract

Exchange bias, which typically requires a coupled ferromagnetic and antiferromagnetic interface, plays a key role in enhancing data storage technologies by reducing noise and improving signal readability. In this study, we investigate the exchange bias phenomenon in surface-oxidized van der Waals material Fe₃GaTe₂, which exhibits a blocking temperature of approximately 280 K — near room temperature. This behavior is particularly significant, as achieving room-temperature exchange bias has been challenging, potentially due to the lack of high-Néel-temperature van der Waals antiferromagnetic materials. Using ab initio calculations and magneto-optical methods, we propose a model to explain the origin of the exchange bias, thereby circumventing the need for complex fabrication typically required in transport measurements. Our findings suggest that surface oxidation plays a crucial role in realizing exchange bias behavior, which is strongly dependent on temperature, interfacial spin-pinning states, and the interplay between ferromagnetic and antiferromagnetic interactions. We also examine the stability and uniformity of this behavior in flakes of various thicknesses. These insights provide valuable understanding of near-room-temperature exchange bias in van der Waals materials, opening up possibilities for their integration into next-generation spintronic and data storage applications.

Graphical abstract

Keywords

exchange bias / surface-oxidized Fe ₃GaTe ₂ / reflective magnetic circular dichroism

1 Introduction

The pursuing of small-footprint and energy-efficient future spintronics has driven extensive research into room-temperature van der Waals ferromagnetic and antiferromagnetic materials, following the discovery of monolayer ferromagnetic CrI₃ and Cr₂Ge₂Te₆ flakes [1, 2]. These materials challenge the Mermin−Wagner’s theorem in two-dimensional limits. Recently, promising room-temperature ferromagnetic material candidates [3] such as Cr_1+xTe₂ and Fe₃GaTe₂, have been discovered, and spin switching devices [4-10] based on these materials have been demonstrated utilizing ferromagnetic layers with varying thicknesses and coercive fields. However, the exchange bias behavior, a key feature for stabilizing the storage layer in hard disk drives and enhancing the readability of stored data during read operations, has not been demonstrated at room temperature in van der Waals materials. Traditionally, thin-film magneto-devices employ an antiferromagnetic layer, such as cobalt oxides, to pin the spin direction of the storage layer, preventing it from flipping as the free layer switches at the coercive field (H_c). Although the antiferromagnetic/ferromagnetic interfaces in van der Waals materials [11-13] (e.g., Fe₃GeTe₂/CrCl₃, Fe₃GeTe₂/MnPS₃ and Fe₃GeTe₂/FePS₃) show effective coupling, the blocking temperatures are constrained by the low Néel temperature of the antiferromagnetic 2D materials, and the observed blocking temperature do not exceed 80 K. To date, no reports have confirmed near-room temperature exchange bias in van der Waals materials.

Given that iron-based van der Waals materials are air-sensitive [14, 15] and prone to oxidation, a previous study [16] reported that surface oxidized Fe₃GeTe₂ flakes exhibit a clear exchange bias behavior with a blocking temperature of 180 K, approximately 60 K lower than their Curie temperature. The bulk Curie temperature [17] of Fe₃GaTe₂ is reported to be between 350 and 380 K. A recent study [18] on proton-intercalated Fe₃GaTe₂ flakes observed the exchange bias in exfoliated Fe₃GaTe₂ below 80 K during Hall measurements. Due to the challenges of preparing air-sensitive sample for transport measurements, the oxidation induced exchange bias behavior in Fe₃GaTe₂ is still unclear. Compared to transport measurement, microscopic magneto-optical method has been proven to be a convenient probe for identifying out-of-plane ferromagnetism in van der Waals materials [1, 19, 20].

In this study, we investigate the near-room-temperature exchange bias behavior of surface-oxidized Fe₃GaTe₂ flakes using the reflective magnetic circular dichroism (RMCD) method. The non-contact and damage-free nature of this optical probe facilitates easy sample preparation, allowing for better control of surface oxidation and characterization of thin layers. Through analysis of the magneto-optical responses, we have identified the blocking temperature, stability, and uniformity of these surface-oxidized van der Waals materials. We also find that the interfacial spin states play a key role in the exchange bias behavior. These findings provide critical insights for designing future 2D spintronic devices.

2 Methods

In this experiment, our Fe₃GaTe₂ crystals were grown using the self-flux method described in the literature [21]. Flakes for RMCD measurements were exfoliated on 2 nm/3 nm Ti/Au films on SiO₂/Si substrates inside an argon-filled glovebox [22]. Optical images of the samples were also taken in the glovebox. The flake thicknesses are characterized using atomic force microscopy and the results can be found in Supplementary Fig. S1. And sample preparation details can be found in Supplementary Note 1.

RMCD measurement reads the difference of absorption between the left circular polarized light and the right circular polarized light. The signal relates to the magnetic moments of the material indirectly via magneto-optical effect. Similar to previous studies [1, 19], in our measurements a linearly polarized laser (633 nm or 532 nm) was modulated between left and right circular polarization by a photoelastic modulator (PEM) at 50 kHz and a chopper at a frequency of 789 Hz. The modulated light was focused onto the sample surface at normal incidence. The beam spot size is around 1 μm in diameter. The reflected light was separated from the incident part by a beam splitter and detected by a photodetector. And the RMCD signal was determined by the ratio of photodetector response between 50 kHz and 789 Hz which reduces the instability of laser power fluctuation. The A.C. responses of the photodetector were measured with two lock-in amplifiers.

3 Results and discussion

Layered iron gallium telluride is proposed as a three-dimensional Ising ferromagnetic material that deviates from the itinerant Stoner mechanism and emphasizes a local-moment magnetism [23, 24]. The long-range interactions and the perpendicular magnetic anisotropy (PMA) [17] guarantee ferromagnetism at the 2D limit. As shown in Fig.1(a), the pristine Fe₃GaTe₂ has a hexagonal lattice structure, with the Fe₃Ga heterometallic layer consisting of two types of iron atoms sandwiched between tellurium layers. Because flakes will be exposed to air in one minute during the sample loading process for the RMCD measurement, a 5 nm Fe₃GaTe₂ flake is covered by a hexagonal boron nitride (h-BN) flake to prevent full oxidation of the thin layer as shown in Fig.1(b). The surface oxidation of a thick flake is also observed in the high-angle annular dark field image as shown in Fig.1(c). The gallium layer was deposited on top of the flake in the focused gallium ion beam-cutting process for the transmission electron microscope characterization. The cross-sectional image with the energy-dispersive X-ray measurements shows that the surface oxidation layer is thick possibly due to long term exposure in air or interface damage during the gallium deposition. Natural oxidation occurs in air and the reflection contrast dramatically changes as shown in Supplementary Fig. S2. However, Raman characterizations of a 10 nm thick flake before and after 3 hours of air exposure do not show any additional peaks (Supplementary Fig. S2), indicating that the oxidization process is limited and does not destruct the original crystal lattice of the Fe₃GaTe₂.

Four RMCD hysteresis loops, as shown in Fig.1(d), were measured at 40 K and 200 K with a polarization field set to 2 T and −2 T, respectively. The coercive field H_c is defined as (H_c+−H_c−)/2 and the exchanged bias H_ex as (H_c++H_c−)/2. At 40 K, the H_c and H_ex are 0.57 T (0.568 T) and 0.149 T (0.154 T) after positive(negative) polarization field settings, respectively. At 200 K, both H_c and H_ex reduce to 0.119 T (0.118 T) and 0.05 T (0.051 T) for positive and negative polarization settings, respectively. Since H_ex varies with the different thicknesses, we plotted the H_ex of flakes with thicknesses ranging from 5 nm to 224 nm at 40 K in Fig.1(e) for comparison. And it gets smaller as the thickness increases, similar to the exchange bias behavior reported in Fe₃GeTe₂ flakes [16]. These RMCD hysteresis loops at 40 K can be found in Supplementary Fig. S3.

To determine the blocking temperature of the exchange bias behavior, we measured hysteresis loops of Fe₃GaTe₂ flakes across a temperature range from 1.6 K to 300 K, as shown in Fig.2. While previous studies [17] have shown that the Curie temperature T_C decreases with decreasing thickness, for flakes thicker than 10 nm, T_C remains above 340 K. In our study, the 5 nm Fe₃GaTe₂ flake, protected by a h-BN layer, exhibited a Curie temperature above 300 K, while the 1 nm flake has been reported [24] with a much-reduced T_C of around 200 K, likely due to oxidation. Fig.2(a) shows hysteresis loops at different temperatures after positive (or negative) field polarization settings. The observed coercive field decreases with increasing temperature. The 82 nm sample near room temperature also shows labyrinthine domain-like RMCD behavior, similar to previous reports in Fe₃GeTe₂ and MnSb₂Te₄ flakes [15, 25]. Labyrinthine domains in Fe₃GaTe₂ flakes [21] have also been confirmed using a Lorentz transmission electron microscope and a magnetic force microscope at room temperature.

To analyze the exchange bias, we set the polarization setting field to be 2 T (or −2 T) as the temperature changes and the field scan loop is within ±1 T as shown in Fig.2(a), much smaller than the polarization field at each temperature. Fig.2(b) shows the coercive field and exchange bias as a function of temperature for the 82 nm sample, which was stored in the glove box for four months after the initial RMCD measurement. The sign of exchange bias is determined by the polarization of the setting field. With a fixed polarization field, H_ex decreases gradually with increasing temperature, likely due to enhanced thermal fluctuation. However, if the scan field increases to ±4 T at 1.6 K as shown in Fig.2(c), the exchange bias vanishes, indicating a loss of the pinning effect.

To explore the underlying mechanism of this usual exchange bias behavior, we investigated whether the oxidized Fe₃GaTe₂ structure exhibits antiferromagnetic characteristics. First, we exfoliated a 10 nm thick flake and exposed it to air at room temperature for 12 hours, followed by heating at 350 K for 2 days. The optical image and the RMCD results are shown in Supplementary Fig. S4. The RMCD results show a significant reduction in magnetization after the first exposure, which disappears upon complete oxidation. Since magnetic moments of iron atoms cannot be lost during the oxidation, this reduction is likely related to the formation of an antiferromagnetic phase. Moreover, as shown in the HAADF imaging in Fig.1(c), the oxidized layer forms a clear interface with the pristine Fe₃GaTe₂ layer, and the spatial distribution of Fe, Ga, and Te elements remains relatively uniform throughout the oxide layer. These observations suggest that the oxidation in air does not significantly compromise the integrity of the underlying Fe₃GaTe₂ lattice.

To further understand the nature of the oxidized layer, we constructed a model by incorporating oxygen atoms into the van der Waals layers of Fe₃GaTe₂. We subjected this O-Fe₃GaTe₂ model to unconstrained relaxation within the Vienna ab initio simulation package (VASP), the resulting structure is depicted in Fig.1(a). Before the oxidation, the interlayer Fe1 ions are ferromagnetically coupled. But when all Fe1 layers in the nearby layers are oxidized, the interlayer coupling switches to antiferromagnetic as depicted in Fig. S5. The calculated interlayer exchange coupling energy varies in partially oxidized Fe₃GaTe₂ are ferromagnetic as summarized in Supplementary Material Table 1 and calculation details can be found in Supplementary Note 2. In our experiment, the exchange bias can be tuned with the polarization fields far below the Curie temperature as shown in Fig.1(d) and S3. But the calculation of the interlayer exchange coupling as discussed in Supplementary Note 2 shows that a flip of the antiferromagnetic states in O-Fe₃GaTe₂ requires a field larger than 400 T, which is much larger than the 2 T polarization setting field. Therefore, the polarization field-tunable exchange bias behavior observed in Fig.1(d) and S3 cannot be simply explained by traditional antiferromagnetic/ferromagnetic coupling.

Is it possible that the polarization field-tunable exchange bias behavior is related to a formation of the soft magnetic/hard magnetic interface? If so, the double step shape full field loop should be observed as shown in Figs. S6(a) and (d). But as the scanning magnetic field is symmetrically increased to ±4 T (or ±6 T), the exchange bias disappears without additional spin reversal steps, as shown in Fig.2(c) and S6(f). These results rule out the explanation of soft magnetic/hard magnetic interactions. A miss of the double steps in the full field loop-scans is also consistent with the conclusion that if the exchange bias simply originates from the interface between fully oxidized antiferromagnetic Fe₃GaTe₂ and ferromagnetic Fe₃GaTe₂, a 4 T (6 T) magnetic field would still be insufficient to overcome the antiferromagnetic coupling in the system.

However, our RMCD measurements of Fe₃GaTe₂ with varying degrees of oxidation are shown in in Fig. S4. As the oxidation increases, the overall RMCD signal amplitude decreases, and the RMCD loops do not ramp linearly with the applied magnetic field. This suggests the presence of strong antiferromagnetic coupling in oxidized Fe₃GaTe₂ and the spin configuration in the antiferromagnetic layer is not being directly controlled by the 6 T external field, which is also supported by the first-principles calculations.

Given the nonuniform nature of the oxidation process in FGT, we propose that partially oxidized Fe₃GaTe₂ plays a critical role in the exchange bias mechanism. Based on experimental results and first-principles calculations, we developed a model to explain this behavior as illustrated in the insets of Fig.2(c) and (d). This model comprises a fully oxidized layer, a partially oxidized pinning layer, and a pristine ferromagnetic layer. According to first-principles calculations (Supplementary Table S1 and Fig. S5) indicate a gradual transition from FM to AFM interlayer coupling with increasing oxygen coverage, highlighting those intermediate oxidized states still exhibit FM interactions. This aligns well with our experimental observations, where partial oxidation forms a pinning layer that retains FM coupling without fully saturating into an AFM state. This means that ferromagnetic coupling and antiferromagnetic coupling coexist between the fully oxidized layer and the partially oxidized layer as shown in Fig.1(a). For the partially oxidized layer, the intralayer ferromagnetic coupling and interlayer ferromagnetic coupling significantly weakens the effect of its antiferromagnetic coupling with the fully oxidized layer acting as the pinning layer, thereby allowing the unpaired spin layer to be influenced by the external magnetic field. For example, under a 2 T polarization setting field, most spins in the pinning layer align with the external field. During small field scans (±1 T), exchange bias arises due to the interaction between the pinning layer and the ferromagnetic interface, as shown in Fig.2(d). When the field cycling range increases to ±4 T, the pinning layer is symmetrically and fully reversed, leading to the disappearance of exchange bias and the absence of antiferromagnetic step features, as shown in Fig.2(c). In sum, the polarization field is strong enough to influence the direction and size of complex magnetic domains at the interface, thereby determining the direction of the exchange bias [26].

This model also explains the polarization history-dependent exchange bias at fixed temperatures. Fig.3(a) shows hysteresis loops measured at 130 K and 150 K after five successive field settings. Fig.3(b) illustrates the spin states in each layer after these settings. The bottom layer represents the ferromagnetic interface, the middle layer is the pinning layer, and the top layer is the antiferromagnetic layer. The long vertical arrows indicate the polarization field direction, with values labeled nearby. In the first loop at 130 K, after applying a 1.5 T polarization field, the pinning layer aligns upwards, resulting in a H_ex of −0.1195 T. Although the subsequent −1.5 T field attempts to flip the pinning layer down, the antiferromagnetic layer impedes this, leaving most of the pinning layer aligned upwards and producing a smaller negative exchange bias (H_ex = −0.0173 T) rather than a positive one. A third, larger field of −4 T fully flips the pinning layer, yielding a H_ex of 0.1143 T. The fourth loop, after applying a minor −1.5 T field, does not change the H_ex (0.1027 T), despite a slight temperature increase to 150 K. Finally, the fifth loop, after applying a +1.5 T field, cannot flip the majority of the pinning layer, leaving a positive H_ex of 0.0268 T. These results highlight the sensitivity of exchange bias to the local state of the pinning layer, with the competition between the external magnetic field and the antiferromagnetic layer contributing to fluctuations in exchange bias after polarization switching.

To further assess the stability of the hysteresis loop, we also repeated the loop measurement ten times following a single polarization field setting, as illustrated in Fig.4. The magnetic field scan rate was fixed at 0.004 T/s and each loop measurement took approximately 10 minutes, with continuous measurements. The time between two loops is less than 1 minute. Since the laser spot size is approximately 1 μm, fixed at the center of the sample and the laser power at the sample chamber entrance is 10 μW, with a fluctuation of ±0.2 μW. Once the measurement started, the temperature fluctuation is within ±0.001 K throughout the whole process. At 130 K, the absolute values of both the coercive field and the exchange bias decrease after the first scan and stabilize after the third scan as shown in Fig.4(a). However, at 280 K, instability is evident, with both the coercive field and the exchange bias drifting after the third scan as shown in Fig.4(b) and (c). The deviation of exchange bias could come from the pinned interfacial spins [26] or a formation of domain boundary [27, 28] during the hysteresis loop scans. And the thermal fluctuations increase the possibility of the spin polarization change in the pinning layer, altering the exchange bias behavior at higher temperatures. Based on temperature dependent exchange bias behavior and the stability analysis, we conclude that the blocking temperature of the exchange bias phenomenon is close to 280 K, significantly higher than previously reported values at the heterostructure interface of van der Waals materials [11-13, 16, 20, 29-33].

To confirm that the spin flip occurs within one single flake under the same external field, we conducted an RMCD mapping experiment, adjusting the magnetic field strength near the transition point, as shown in Fig.5. The flake used in the mapping has multiple steps, with thickness varied from 13.5 nm to 39 nm. Initially, the magnetic field was set to zero. Then at 0.64 T, an RMCD mapping was performed by scanning the flake position, as depicted in Fig.5(a). When the field was increased to 0.65 T, the RMCD mapping showed a sudden sign change in the middle of the scanning, showing a different contrast before and after. A subsequent mapping at 0.65 T confirmed that the spin-flip across the entire flake was complete. Similarly, when the field was increased from 0.55 T to 0.56 T, the spin-flip occurred at the same external field. Additional hysteresis scan loops of this flake can be found in Supplementary Fig. S7. Fig.5(b) illustrates the six spin states as the field scans relating to the RMCD measurements in Fig.5(a).

Although long term oxidation increases the thickness of buried interface, as long as the interface between pristine Fe₃GaTe₂ and the oxidized layer is robust and the interface position is within the penetration depth of the laser beam, the exchange bias is always observable. To estimate the penetration depth, we have calculated the dielectric constant as a function of photon energy in both Fe₃GaTe₂ and O-Fe₃GaTe₂ using VASP under the independent particle approximation. For the 532 nm laser, the estimated δ_p of the fully oxidized Fe₃GaTe₂ is about 75 nm and the estimated δ_p of the Fe₃GaTe₂ is about 31 nm. For the 633 nm laser, the estimated δ_p of the fully oxidized Fe₃GaTe₂ is about 23 nm and the estimated δ_p of the Fe₃GaTe₂ is about 93 nm. Detailed analysis can be found in Supplementary Fig. S8 and Supplementary Note 3.

4 Conclusion

In conclusion, we have investigated the exchange bias phenomenon in surface-oxidized van der Waals material Fe₃GaTe₂, which exhibits a blocking temperature of approximately 280 K, close to room temperature. Through ab initio calculations and magneto-optical methods, we uncover the origin of the exchange bias, thereby eliminating the need for complex fabrication typically required in transport measurements. This highlights its potential as a practical alternative to conventional magnetic thin films. These findings underscore the significance of exchange bias in improving data storage technologies by reducing noise and enhancing signal readability. Our work not only advances the understanding of van der Waals ferromagnetic materials but also paves the way for innovative applications in spintronic devices, potentially revolutionizing future data storage solutions.

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