With complex coupling of multiple degrees of freedom, transition metal oxides (TMOs) provide a promising platform to tune the magnetic property in heterostructures via the magnetic proximity effect. Recent realization of freestanding TMO thin films allows further extension of this technique to novel two-dimensional heterostructures by mechanically stacking with van der Waals materials. Here, we demonstrate the presence of significant magnetic exchange interactions in a heterostructure of 8 nm freestanding LaMnO3 and monolayer WS2. The high magnetization in freestanding LaMnO3 leads to valley degeneracy breaking in WS2, resulting in unbalanced valley polarization in the photoluminescence (PL). Further temperature-dependent PL measurements reveal the same transition behavior as the magnetization in the freestanding LaMnO3 film. Our results unlock new approaches for tuning the magnetism and the valley degree of freedom in ultrathin two-dimensional heterostructures.

The magnetic proximity effect is an efficient way to manipulate the spin degree of freedom in solids, which is not only a central issue in spintronics but also the foundation for many potential applications in devices with novel quantum functionality.1–5 For example, control of the interlayer exchange coupling and exchange bias has been widely used in magnetic storage technologies, such as giant magnetoresistance and magnetic tunnel junctions.6,7 In parallel, layered two-dimensional (2D) transition metal dichalcogenides (TMDs, MX2: M = Mo, W, X = S, Se, Te) are discovered to have a unique valley pseudospin degree of freedom, offering the potential to develop valleytronics.8–11 With 2D characteristics such as atomically thin film thickness and weakly coupled layers, the inherent properties of TMDs are highly sensitive to the substrate and therefore can be significantly tailored via the proximity effect. For example, tunable valley polarization and large valley splitting can be realized by the magnetic proximity effect in heterojunctions with other 2D materials.1–3 However, conventional magnetic materials with high magnetization such as stable transition metal oxides (TMOs) that can provide a strong magnetic exchange field have been rarely utilized in the heterostructure with monolayer TMDs because of the challenge in heterostructure fabrication.

Strongly correlated TMOs have been extensively studied because their inherent coupling of the lattice, charge, spin, and orbital degrees of freedom, which permit the tuning of disparate ground states through a variety of perturbations to induce colossal responses. Numerous novel physical phenomena have emerged in perovskite manganese oxides, such as colossal magnetoresistance,12 charge ordering,13 and metal–insulator transitions.14 However, until now, the magnetic proximity effects by constructing the ferromagnetic TMO/TMD heterostructure have been rarely reported, which are limited only in a couple of rare cases, including the strong many-body effect in the heterostructure composed of monolayer MoS2 and bulk YIG15 and the modulation of exciton states in the monolayer MoSe2/double-layered perovskite manganese oxide heterostructure.16 Recently, atomically flat freestanding perovskite films were developed on the sacrificial water-soluble Sr3Al2O6 layer,17 providing an efficient transfer way to fabricate 2D heterostructures with 2D van der Waals materials. Freestanding TMOs are flexible,18 elastic,19 and ultrathin20 and can be arbitrarily transferred while retaining the high-quality single crystal nature and the ferromagnetism of the bulk. Therefore, with freestanding TMO thin films, a significant magnetic proximity effect may be implanted in 2D TMD-based heterostructure to explore exotic functionalities.

Here, we study a TMD/TMO heterostructure by mechanically transferring monolayer WS2 onto a freestanding LaMnO3 thin film. The freestanding LaMnO3 thin film exhibits enhancements in the Curie temperature (Tc), coercive field, and remanent magnetization along the out-of-plane direction after being released from the substrate. Such enhancements favor the magnetic coupling to monolayer WS2 with perpendicular symmetry breaking. Consequently, in the WS2/LaMnO3 heterostructure, significant valley degeneracy breaking in monolayer WS2 was observed from the unbalanced valley polarization of the photoluminescence (PL) spectrum, evidencing the magnetic proximity effects at the interface. Furthermore, we found a transition behavior of valley polarizations via the temperature-dependent PL spectra, which is the same as the ferromagnetic transition in the freestanding LaMnO3 film. Our results provide an intriguing platform for investigating the magnetic proximity effects in ultrathin freestanding TMD/TMO heterostructures and extend the application of interfacial tuning techniques to 2D devices.

LaMnO3 oxide thin film was grown on the sacrificial water-soluble layers of Sr3Al2O6, which was epitaxially grown on a SrTiO3 (001) substrate after etching using buffer HF acid and annealed in an oxygen atmosphere by pulsed laser deposition (PLD). Reflection high-energy electron diffraction (RHEED) was used to monitor the film growth process, and RHEED intensity oscillations were persistent throughout the growth, indicating a layer-by-layer growth mode. The accurately controlled thickness of LaMnO3 and the Sr3Al2O6 layer is 20 u.c. (8 nm) and 6 u.c. (9.6 nm), respectively. The whole sample clinging to the inverted SiO2/Si substrate was immersed in de-ionized water at room temperature to dissolve the Sr3Al2O6 layer. Then the freestanding LaMnO3 film without strain from SrTiO3 was transferred to the SiO2/Si substrate. Finally, monolayer WS2 was transferred onto the top of the freestanding perovskite ultrathin film.

Figure 1(a) shows the fabrication process of the WS2/LaMnO3 heterostructure, and more details can be found in the supplementary material. In Fig. 1(b), the optical microscope image of a freestanding WS2/LaMnO3 heterostructure indicates a clean and uniform surface, which is essential for interface coupling. The x-ray diffraction (XRD) shown in Fig. 1(c) reveals the same high-crystallinity of freestanding LaMnO3 as the epitaxial film. The out-of-plane lattice constant of freestanding LaMnO3 (∼3.94 Å) is slightly larger than that of the epitaxial one (∼3.93 Å) owing to the release of the strain from the substrate. Furthermore, the x-ray reflection (XRR) shown in Fig. 1(d) illustrates the similar roughness of the surface (∼3.0 Å) for freestanding and epitaxial films through the fitting (fitting results are listed in Table S1 of the supplementary material).

FIG. 1.

Synthesis and characterization of the WS2/LaMnO3 heterostructure with freestanding membranes. (a) Schematic diagram of the fabricated process of the WS2–LaMnO3 heterostructure sample. (b) Optical microscope images of the WS2–LaMnO3 heterostructure on a SiO2/Si substrate. The thickness of the LaMnO3 film is 8 nm. (c) X-ray diffraction (XRD) and (d) x-ray reflectivity (XRR) profiles of the 8 nm epitaxial LaMnO3 directly grown on the SrTiO3 substrate and the freestanding LaMnO3 film grown on the SiO2/Si substrate. “♦,” “◊,” and “*” indicate the peaks of SrTiO3, the Si substrate, and LaMnO3 films, respectively. The circles and solid lines represent experimental data and fits, respectively.

FIG. 1.

Synthesis and characterization of the WS2/LaMnO3 heterostructure with freestanding membranes. (a) Schematic diagram of the fabricated process of the WS2–LaMnO3 heterostructure sample. (b) Optical microscope images of the WS2–LaMnO3 heterostructure on a SiO2/Si substrate. The thickness of the LaMnO3 film is 8 nm. (c) X-ray diffraction (XRD) and (d) x-ray reflectivity (XRR) profiles of the 8 nm epitaxial LaMnO3 directly grown on the SrTiO3 substrate and the freestanding LaMnO3 film grown on the SiO2/Si substrate. “♦,” “◊,” and “*” indicate the peaks of SrTiO3, the Si substrate, and LaMnO3 films, respectively. The circles and solid lines represent experimental data and fits, respectively.

Close modal

We next characterized the magnetic properties of freestanding and epitaxial oxide thin films. The magnetization as a function of temperature for freestanding LaMnO3 under a 1000 Oe magnetic field after zero-field cooling shows a trifle distinction compared with epitaxial LaMnO3 films. Figure 2(a) shows the onset temperature (or Tc) of the ferromagnetic ordering that occurs at about 141 K for the freestanding film and about 127 K for the epitaxial film. In a small magnetic field of 5 K, the magnetization of the freestanding LaMnO3 is stronger than that of the epitaxial sample, which is 0.735 µB/Mn of the former and 0.304 µB/Mn of the latter. Although the difference in Tc and magnetization between the freestanding and epitaxial film is small, it still demonstrates that the out-of-plane direction magnetization increases after releasing the LaMnO3 film from the substrate. Therefore, it provides strong support to conceive a thin film with the ability to magnetize other samples, such as TMDs, oxide thin films, and so on, when forming a heterostructure. In contrast, the MT curves of these freestanding and epitaxial films are almost consistent along the in-plane direction, as shown in Fig. S1(a) of the supplementary material, which illustrates that the main magnetic properties such as Tc and magnetization remain the same for samples on and off the substrate.

FIG. 2.

Magnetic properties of freestanding and epitaxial oxide thin films. (a) Magnetization as a function of temperature measured after zero-field cooling along the out-of-plane direction, while warming of 1000 Oe of the freestanding LaMnO3 film released on silicon (red line) and epitaxially grown LaMnO3/SrTiO3(001) (blue line). The LaMnO3 film thickness is 8 nm. (b) The hysteresis loops measured at T = 5 K along the out-of-plane direction of the freestanding LaMnO3 film released on silicon (red line) and epitaxially grown LaMnO3/SrTiO3(001) (blue line).

FIG. 2.

Magnetic properties of freestanding and epitaxial oxide thin films. (a) Magnetization as a function of temperature measured after zero-field cooling along the out-of-plane direction, while warming of 1000 Oe of the freestanding LaMnO3 film released on silicon (red line) and epitaxially grown LaMnO3/SrTiO3(001) (blue line). The LaMnO3 film thickness is 8 nm. (b) The hysteresis loops measured at T = 5 K along the out-of-plane direction of the freestanding LaMnO3 film released on silicon (red line) and epitaxially grown LaMnO3/SrTiO3(001) (blue line).

Close modal

We also notice that there is a decrease in the saturation magnetization for the freestanding film compared with the epitaxial film. As shown in Fig. 2(b) of the magnetic field-dependent magnetization (MH) curve at 5 K, the saturation magnetization of the freestanding film reaches about 2.75 μB/Mn, while for the epitaxial film, the saturation magnetization is about 3.36 μB/Mn. Although the freestanding film presents no obvious superiority in saturation magnetization, both the coercivity and remanence are found to be stronger than those of the epitaxial film. The coercive field enhances to 695 Oe and the remnant magnetization increases to 0.446 µB/Mn along the out-of-plane direction in the freestanding film. The reason for the increased magnetic properties may be that the water dissolution process leads to the reduction in manganese valence on the top and bottom surfaces in the freestanding sample, which tends to rotate the magnetic axis easily to the out-of-plane direction.21 For the hysteresis loops along the in-plane direction, no obvious change is observed for the freestanding film since ∼720 Oe coercivity and ∼0.4 μB/Mn remanent magnetization are similar to those of the LaMnO3 film before released from the substrate, as shown in Fig. S1(b) of the supplementary material.

To prove that the magnetic proximity effect exists in the freestanding WS2/LaMnO3 heterostructure, we studied the valley-dependent light emissions from monolayer WS2 at cryogenic temperature. In the experiments, a 532 nm continuous wave laser is used as an excitation light source, which is above the bandgap of monolayer WS2.22 We first identified the excitonic optical transitions via Gaussian fitting to the unpolarized PL spectrum. As shown in Fig. S2 of the supplementary material, a dominant trion state (charged exciton) at 2.035 eV and a neutral A exciton at 2.111 eV were observed at 7 K, and a similar PL spectrum was observed at room temperature (297 K).23 In order to check whether the optical property or electronic structure is modulated by the coupling of freestanding LaMnO3 from charge transfer or interfacial doping, temperature-dependent PL was measured under the same condition from 7 to 297 K, as shown in Fig. S3 of the supplementary material. From the results, we found no distinctive peak position change or the appearance of additional new PL emission near the Tc of the LaMnO3 film. Only the trion peak is dominant within the studied temperature range, which is consistent with previous reports for n-type monolayer WS2.24,25 Therefore, it indicates that the valley-selective optical pumping and detection in TMDs are still valid and can be extended to the freestanding WS2/LaMnO3 heterostructure.

The valley-dependent light emissions from the freestanding WS2/LaMnO3 heterostructure are shown in Fig. 3. We used left-circularly polarized (LCP) (σ+) or right-circularly polarized (RCP) light (σ) to excite and detect. To simplify, the intensity of four excitation/detection configurations is defined as follows: I++ = σ+/σ+ (σ+ excited and σ+ detected), I+− = σ+/σ+ (σ+ excited and σ detected), I−− = σ/σ (σ excited and σ detected), and I−+ = σ/σ+ (σ excited and σ+ detected). In addition, we defined the valley polarization as P+ = (I++I+−)/(I++ + I+−) and P= (I−−I−+)/(I−− + I−+). The unbalanced valley polarization due to the valley degeneracy breaking can therefore be represented as ∆P = P+P. As shown in Fig. 3(a), under the excitation at room temperature, almost the same P+ and P are observed, showing zero value for ∆P. However, at 7 K, a significant difference in the PL intensity for the four excitation/detection configurations was observed. As shown in Fig. 3(b), ∆P is estimated to be ∼13%, which is strong evidence for the existence of the interfacial magnetic proximity effect in the heterostructure.

FIG. 3.

PL spectrum of WS2 on the freestanding and epitaxial LaMnO3 film. (a) PL spectrum for I++ (red solid), I+− (red dashed), I−− (blue solid) and I−+ (blue dashed) circularly polarized PL from monolayer WS2 at room temperature on the freestanding LaMnO3 film. (b) Same as (a), but at low temperature. Inset of (b): Experimental geometry under luminescence, which is excited and collected separately for σ+ and σ polarization showing monolayer WS2 on the ferromagnetic insulator. (c) and (d) Schematic band structure of WS2 near the K and K′ points for T > Tc and T < Tc, respectively.

FIG. 3.

PL spectrum of WS2 on the freestanding and epitaxial LaMnO3 film. (a) PL spectrum for I++ (red solid), I+− (red dashed), I−− (blue solid) and I−+ (blue dashed) circularly polarized PL from monolayer WS2 at room temperature on the freestanding LaMnO3 film. (b) Same as (a), but at low temperature. Inset of (b): Experimental geometry under luminescence, which is excited and collected separately for σ+ and σ polarization showing monolayer WS2 on the ferromagnetic insulator. (c) and (d) Schematic band structure of WS2 near the K and K′ points for T > Tc and T < Tc, respectively.

Close modal

Intuitively, the nonzero ∆P can be understood as a result from the magnetic Zeeman-type tuning on the excitonic optical transitions in different valleys, as sketched in Figs. 3(c) and 3(d), with a schematic band structure of monolayer WS2 at the K and K′ points. Previous studies have shown that the K/K′ valley energy in WS2 can be shifted under an effective magnetic exchange field, resulting in spectrum-distinguishable light emissions with opposite circular polarizations.26 In our heterostructure, when the temperature is below Tc, the LaMnO3 serves as a ferromagnetic substrate and provides the magnetic exchange field, which introduces an excitonic valley Zeeman splitting energy ∆, as shown in Fig. 3(d). However, the remanence of freestanding LaMnO3 thin film under zero magnetic field is relatively small at only 0.446 µB/Mn, equaling an effective magnetic field at ∼0.08 T, which leads to very small ∆ in the K and K′ valleys. Therefore, significant valley Zeeman splitting cannot be observed in the PL spectrum of the heterostructure, as seen in Fig. 3(b), where the trion peaks are almost at the same position for the four excitation/detection configurations. Nonetheless, this splitting ∆ can still lead to significant distinction for the intervalley scattering of optically excited photocarriers in K and K′ valleys.

The valley polarization in TMDs under continuous-wave excitation can be described as P = P0/(1 + 2τ/τK),22 where P0 is the theoretical limit of PL polarization and τ and τK denote the exciton lifetime and valley lifetime, respectively. For monolayer WS2, τ is much smaller than τK. The valley polarization P is therefore mainly determined by τK. At the same temperature, τK depends on the intervalley scattering rate, denoted as τinter, via the involvement of phonons that can flip the valley pseudospin while fulfilling the conservation of momentum and energy. In our heterostructure, with the presence of nonzero ∆ at 7 K, the τinter from the K to K′ valley should be tuned distinctively to τ′inter (scattering rate from the K′ to K valley) due to the valley energy difference, resulting in different τK and τK’ for photocarriers in K/K′ valleys. Therefore, unbalanced P+ and P are observed in the PL measurements, as shown in Fig. 3(b). It should also be pointed out that our results are consistent with previous theoretical explanations and experimental observations on the valley degeneracy breaking under an applied external magnetic field.3,4,10,27–30

To further confirm the magnetic proximity effect in the heterostructure, we carried out the temperature-dependent study. The freestanding LaMnO3 film is ferromagnetic and provides a magnetic exchange field that leads to the change in the degree of the valley polarization in monolayer WS2. Therefore, the temperature-dependent ∆P can be used as an index to reflect the ferromagnetic phase transition of the LaMnO3 film along the out-of-plane direction. Here, we performed the PL spectrum measurements from 7 to 297 K and summarized the temperature-dependent ∆P together with the M-T curve in Fig. 4. For directly confirming the magnetic influence of the freestanding LaMnO3 film on the valley pseudospin, we converted the magnetic moment into an equivalent magnetic field. The maximum equivalent magnetic field of the freestanding LaMnO3 film along the out-of-plane direction reached about 0.116 T. The transition behavior found in the ∆PT curve is similar to that in the MT curve of the freestanding LaMnO3 film. The small difference in the ∆P-T and M-T curves might be caused by sample differences or magnetic domains of the manganese oxide film.31,32 The transition behavior in the ∆PT curve further confirms the strong magnetic proximity effect in the as-fabricated freestanding WS2/LaMnO3 heterostructure and also indicates that the interfacial magnetic exchange field is proportional to the magnetization of the ferromagnetic substrate.33 

FIG. 4.

Temperature dependence of the degree of PL circular polarization from 7 to 300 K. The temperature dependence of ∆P of WS2 on the freestanding LaMnO3 film.

FIG. 4.

Temperature dependence of the degree of PL circular polarization from 7 to 300 K. The temperature dependence of ∆P of WS2 on the freestanding LaMnO3 film.

Close modal

In summary, we successfully fabricated the freestanding WS2/LaMnO3 heterostructure and studied its magnetic proximity effect by polarization-dependent PL spectroscopy. The ferromagnetic order in the freestanding LaMnO3 film lifts the valley degeneracy in WS2, leading to a population imbalance in the two valleys of WS2 when optically pumped. The 2D heterostructure formed by TMD and TMO proves to be a practical approach in which the magnetic exchange field can be introduced via the remanence of magnetic TMO without the need to apply a large external magnetic field, which favors the application of flexible spintronics. In addition, it opens up the possibility of exploring exotic physical phenomena by the magic twist between monolayer TMD and TMO materials.

See the supplementary material for the following details: sample fabrication and characterization, magnetic properties along the in-plane direction, Gaussian fitting of the excitonic optical transitions, temperature-related PL evolution, and XRR fitting results for freestanding and epitaxial LaMnO3 thin films.

We acknowledge the support from the National Key R&D Program of China (Grant No. 2022YFA1403000), the CAS Project for Young Scientists in Basic Research (Grant No. YSBR-049), the National Science Foundation of China (Grant Nos. 52072244 and 12104305), the Science and Technology Commission of Shanghai Municipality (Grant No. 21JC1405000), the Fundamental Research Funds for the Central Universities (Grant Nos. WK3510000013 and WK2030020032), the Anhui Initiative in Quantum Information Technologies (Grant No. AHY170000), the ShanghaiTech Startup Fund, and the Double First-Class Initiative Fund of ShanghaiTech University.

The authors have no conflicts to disclose.

Q.L., X.L., and J.F. contributed equally to this work.

Qinwen Lu: Data curation (lead); Formal analysis (lead); Investigation (equal); Methodology (equal); Writing – original draft (equal). Xunyong Lei: Data curation (equal); Formal analysis (equal); Writing – original draft (equal). Jun Fu: Data curation (equal); Methodology (equal). Qing Wang: Data curation (equal); Methodology (equal). Xiaoyu Mao: Data curation (equal); Methodology (equal). Long Cheng: Data curation (equal); Methodology (equal). Xiaofang Zhai: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Writing – original draft (equal); Writing – review & editing (equal). Hualing Zeng: Conceptualization (equal); Funding acquisition (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.

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Supplementary Material