Photo-induced anomalous Hall effect in ferromagnetic (Co)/transition metal dichalcogenide (WS 2 ) heterostructure

doi:10.21203/rs.3.rs-3216100/v1

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Photo-induced anomalous Hall effect in ferromagnetic (Co)/transition metal dichalcogenide (WS 2 ) heterostructure

https://doi.org/10.21203/rs.3.rs-3216100/v1

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The photo-induced Hall effect is shown in the Co/WS₂ heterostructure. This heterostructure consists of a ferromagnetic metal layer that forms a Schottky junction to a transition metal dichalcogenide semiconductor layer. In this effect, under the magnetic field and light irradiation, the excited charge carriers from the WS₂ layer diffuse into the Co layer and are driven by Lorentz forces, then creates a transverse voltage in the open circuit. This open circuit voltage is a function of light and magnetic field intensity. The photo-induced voltage is nonlinear and hysteretic with the applied magnetic field and is in good agreement with magneto-optical Kerr's hysteresis loops. The observed results can provide a new technique for optical and magnetic sensing applications. In addition, it proposed a new technique to reconstruct the in-plane magnetic hysteresis loop for metallic thin films.

Physical sciences/Materials science

Physical sciences/Optics and photonics

Photo-induced Hall effect

Transition metal dichalcogenide (TMDC)

Interfaces

Thin films

Photo-induced Hall effect can occur in a Schottky diode containing a metal/semiconductor heterostructures ¹. Charge carriers are produced when photons, with energy greater than the band gap of the semiconductor, are absorbed in the semiconductor. In the semiconductor, the photogenerated charge carriers are separated by the space charge region in the Schottky barrier and drift at the metal/semiconductor interface. If an in-plane magnetic field is applied perpendicular to the direction of voltage measurement, large Lorentz forces are generated which can cause the photogenerated charge carriers to diffuse into the metal and a transverse open circuit voltage is generated ¹. This open circuit voltage is proportional to the magnetic field and light intensity. In these types of devices, without creating a net current, the internal potential in the Schottky barrier decreases ¹. The photo-induced Hall effect has been reported in several metal/semiconductor heterostructures. For example, it has been reported in a conventional device, in which Si is used as a semiconductor layer and Pt is used as a metal layer ¹. They observed that the photo-induced voltage has a linear relationship with the light intensity and the magnetic field. In another research, GaAs was used for the semiconductor layer while Pt was used as the metal layer ². They suggested that this new effect can be used as a new technique for magnetic field sensing and optical detection. In research, the Si layer is used as a semiconductor layer and nickel is used for metal layer ³. They observed that the photo-induced voltage has a non-linear and hysteretic relationship with the magnetic field. This non-linear and anomalous contribution to the photo-induced voltage arises from the spin-dependent scattering in the ferromagnetic metal ³.

Transition metal dichalcogenide (TMDC) belongs to a large family of semiconductors with surprising properties such as strong spin-orbit interaction, direct band gap for monolayers, and remarkable optical properties, which provide a unique opportunity for optical and spintronic applications ^4,5. For example, TMDCs have been used for ultrasensitive photodetectors and phototransistors ^6–8. Therefore, TMDC materials are attractive materials, especially for optical and optoelectronic applications ^8–10. Also, the heterostructure consisting of TMDC and ferromagnetic (FM) thin film can provide attractive opportunities such as interface-based magnetic manipulation and electronic transport ^11–13. For example, it has been observed that placing a layer of TMDCs on a layer of permalloy (NiFe) increases the anisotropic magnetoresistance ¹². It has also been observed that the connection of a layer of permalloy with a layer of TMDCs exhibits a Schottky response ¹³. In another work, increased magnetoresistance at room temperature has been observed in the FM/TMDC heterostructure ¹¹.

Using a TMDC layer with unique features such as a direct band gap in the monolayer, as a semiconductor layer instead of conventional semiconductors such as Si may help to understand the physical mechanism and increase the sensitivity of the photo-induced Hall effect. Also, the use of an FM layer such as cobalt, instead of a paramagnetic metal layer such as Pt, can lead to interesting results such as photo-induced anomalous Hall effect and adjustment of magnetic properties by light. In this work, in an FM/TMDC heterostructure, WS₂ as a member of TMDCs, which is a semiconductor with attractive properties, is used as the semiconductor layer. Also, a cobalt (Co) layer was used for the ferromagnetic layer, whose properties are well known. The cobalt thin film forms the Schottky contact with the WS₂ layer. The photo-induced Hall voltage is observed in the Co/WS₂ heterostructure. A comparison of photo-induced anomalous Hall voltage curves with the curves measured by the magneto-optical Kerr effect (MOKE) shows that the two rings are in good agreement.

For the preparation of the Co/WS₂ heterostructure, Si/SiO₂ was used as the substrate. The substrates were ultrasonically cleaned with ethanol and acetone. First, the WS₂ layers were deposited on the Si/SiO₂ substrate by chemical vapor deposition (CVD) technique. Tungsten three oxide (WO₃) and sulfur (S) powders were used as raw materials and potassium chloride (KCl) as a catalyst. First, the furnace is kept at a temperature of 250°C for 10 minutes, then it is kept at a temperature of 820°C for 30 minutes. Also, Ar/H₂ gas mixture is injected during the reaction. After preparing the WS₂ layer, a cobalt layer with a thickness of 10 nm was deposited on WS₂ by the sputtering technique. The film thickness and deposition rate were monitored with a quartz crystal microbalance during the deposition. To identify the formed compounds in the prepared sample, Raman spectroscopy was performed using a Teksan Raman microscope spectrometer (Takram P50C0R10). To measure the electrical and Hall effect properties, two contacts were connected to the middle points of two opposite edges of the metal film, using silver paste. The measurements were done by the two-probe method using a Keithley source meter and an oscilloscope. An electromagnet was used to apply a magnetic field. A laser with a wavelength of 660 nm was used to illuminate the sample unless otherwise stated. All measurements were done at room temperature.

The optical microscopy image of the WS₂ film is shown in Fig. 1a. As can be seen the formed WS₂ grains have a triangular structure. We can see that the orientation of the triangles is disordered. Raman spectra for the WS₂ single layer and Co/WS₂ bilayer are shown in Fig. 1b. The spectrums show the Raman peaks corresponding to the in-plane (\({\text{E}}_{2\text{g}}^{1}\)) and out-of-plane (\({\text{A}}_{1\text{g}}\)) characteristic vibrational modes of WS₂ ¹⁴. In the reported works, the position and intensity ratio of two peaks \({\text{E}}_{2\text{g}}^{1}\) and \({\text{A}}_{1\text{g}}\) are used to identify the number of layers. By comparing with the reported works, we guess that the number of WS₂ layers is 6–8 layers ¹⁴. The Raman spectrum of the Co/WS₂ sample shows a wide peak at about 690 cm^− 1, which can be related to Co-oxide ¹⁵. Also, when a Co layer is deposited on top of WS₂, i.e. Co/WS₂ sample, the intensity of Raman peaks seems to be suppressed compared to the WS₂ sample. The sharp peak at 520 cm^− 1 corresponds to the Si substrate ¹⁶.

The schematic of the sample and the measurement configuration is demonstrated in Fig. 2a. The open circuit voltage is measured from the contacts that are connected to the middle points of the two opposite edges of the Co film. The light is uniformly irradiated along the z-direction, perpendicular to the sample plane. The magnetic field is applied in the plane of the sample in different directions to the open circuit voltage measurement. As shown in Fig. 2a, θ is the angle between the applied magnetic field and the direction of open circuit voltage measurement. Figure 2b shows the time-dependent open circuit voltage when exposing the sample to light with a power of 3 mW cm^− 2 and a constant magnetic field (µ₀H = 0.336 T) at different θ (0°, 90° and 270°) angles. As can be seen, when θ = 0°, the open circuit voltage is zero. For θ = 90°, the open circuit voltage is positive and for θ = 270°, it is negative, while their magnitudes are equal. So, the measured open circuit voltage for θ = 270° and 90° shows that the sign of the voltage changes with the sign of the applied magnetic field. This change of sign is due to the change of Lorentz force ¹. This behavior is a clear indication that the photo-induced Hall effect is the driving force behind the induction. One of the advantages of this device is that it needs two contacts, while conventional Hall sensors need four contacts, two contacts are used to measure current, and two contacts are used for voltage.

In addition, the open circuit voltage measured in a constant magnetic field (µ₀H = +/- 0.336 T) at θ = 90°, under illumination with different light intensities with the wavelength of 660 nm is shown in Fig. 3a. As can be seen, the magnitude of the open circuit voltage increases with the enhance of the light intensity, which is due to the increase of photogenerated charge carriers. As the sign of the applied magnetic field changes, the sign of the open circuit voltage changes. Also, excellent repeatability is observed in photo-switch conditions. The magnitude of measured open circuit voltage as a function of the light intensity is presented in Fig. 3b. It shows a linear behavior in our measurement range. Therefore, in the Co/WS₂ device, the bias voltage is replaced by the light bias, and the light bias voltage is proportional to the intensity of the light irradiated on the sample. To study the effect of light wavelength on the observed photo-induced Hall effect, the open circuit voltage was measured at two different wavelengths, 532 and 660 nm, under the same light power of 3 mW cm^− 2. The results are shown in Fig. 3c. The open circuit voltage increases with decreasing wavelength because at higher photon energies, the probability of transition from the valence band to the conduction band in the WS₂ semiconductor increases.

It has been proved that the existence of a Schottky barrier is a necessary condition for the existence of the photo-induced Hall effect ¹. To check the Schottky barrier between WS₂ and Co layer, I-V curves were measured in a dark state and under light illumination with a wavelength of 660 nm. The measurement configuration is shown in the inset of Fig. 4. The recorded curves are demonstrated in Fig. 4. As can be seen, the metallic Co forms a Schottky contact with the semiconductor WS₂. It also shows the effect of light illumination on the Schottky behavior, increasing the conductivity. When the light reaches the interface, it is converted into electron-hole pairs in the WS₂ semiconductor. Photogenerated holes pass through the interface and combine with cobalt electrons. But the photogenerated electrons are trapped in the interface and create an optical potential that has the opposite sign of the internal potential of the Schottky barrier and decays with the inverse of the distance from the interface and can cause the edge of the Schottky barrier to be rounded ¹. Therefore, in this case, electrons can be injected into the cobalt. Without applying a magnetic field, the currents of holes and electrons are equal, so there is no net current. But when the magnetic field is applied perpendicular to the direction of voltage measurement, the charge transfer in the sample plane is not uniform and Hall voltage is generated. It should be noted that non-polarized light was used in this research, so other effects such as effects related to circularly polarized light and valley-dependent spin polarization are rejected ^17,18.

To study the effect of magnetic field intensity on the photo-induced voltage V_H, the V_H vs H curves under illumination with different light intensities were recorded (Fig. 5a). As can be seen, the photo-induced voltage is nonlinear and hysteretic concerning the applied magnetic field. While for paramagnetic metals such as Pt, linearity has been observed ¹. The nonlinearity of the photo-induced Hall voltage can be caused by the spin-dependent scattering in the ferromagnetic cobalt layer ³. Also, increasing the light power increases the photo-induced voltage, which is due to the increase of photogenerated charge carriers. For comparison, the Kerr magneto-optical effect (MOKE) of the Co/WS₂ sample was investigated, which can be seen in Fig. 5b. A direct comparison of the photo-induced voltage curve and the MOKE curve is shown in Fig. 5c. As can be seen, there is a good match between the curves. In other words, the photo-induced voltage curve mimics the magnetic hysteresis loop, so it can be used as a magnetometric technique.

In conclusion, we have shown the photo-induced anomalous Hall effect in Co/WS₂ Schottky heterojunction. It was observed that when the Co layer is magnetized along the x-axis, an open-circuit voltage is detected along the y-axis, while the light is incident on the sample along the z-axis. The sign of the measured open circuit voltage is changed by changing the sign of the magnetic field. The photo-induced voltage is increased linearly with the light intensity while it is nonlinear and hysteretic with the applied magnetic field intensity. This nonlinearity is due to the ferromagnetic property of the cobalt layer. These results proposed a new technique for optical detection and magnetic field sensing. In addition, the results show that the photo-induced voltage vs applied magnetic field intensity curves are consistent with the obtained magneto-optical Kerr's hysteresis loops, which presented a magneto-metric technique to reconstruct the magnetic hysteresis loop of metallic thin films.

Author contributions

S.J. conceptualization, visualization, methodology, investigation, data curation, writing–original draft, resources, software, writing–review and editing. A.A. conceptualization, methodology, writing–review and editing, formal analysis. A.R. investigation, resources. M.M.T. writing–review and editing, project administration, supervision.

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

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No competing interests reported.

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Photo-induced anomalous Hall effect in ferromagnetic (Co)/transition metal dichalcogenide (WS 2 ) heterostructure

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