Some photoinduced effects in the charge-ordering phase with a bicritical behavior competing with ferromagnetic phase have been observed. As shown in Fig. 1, the photoinduced persistent insulator–metal transition in Pr_0.55(Ca_0.75Sr_0.25)_0.45MnO₃ film has been revealed by Takubo et al.^[44] The transition is caused by the destruction of charge ordering in the immediate neighborhood of the excited sites by photoexcitation. Furthermore, as the reverse phenomenon above, the first persistent photoinduced metal-to-insulator transition has been found as shown in Fig. 2.^[45] They considered that the photoexcitation can destabilize the local lattice distortion by creating momentarily a uniform charge distribution and consequently cause the transition.

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. Temperature dependence of the resistance. Solid and dotted lines denote the cooling and warming processes, respectively. Insets show the resistance drop at individual pulses for various time scales.[44]

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (color online) (a) Temperature dependence of the resistance. Solid and dotted lines denote the cooling and warming processes, respectively. (b) Phase diagram of Pr0.55(Ca0.75Sr0.25)0.45MnO3 film. (c)–(e) Voltage change across the reference resistance caused by the laser illumination.[45]

Moreover, the photoinduced relaxation of resistance can be measured by the experimental setup shown in Fig. 3, which is composed of the triggering timing system and the resistance recording system.^[46–48] As shown in Fig. 4, the resistance nonlinearly decreases with the irradiation time when the laser is on and then the resistance restores the original value after irradiation. The experimental data are fitted using the formula 1 where R(t) is the resistance at the irradiation time t and τ is the time constant. The obtained time constant is the characterization of the photoinduced relaxation process, indicating the speed of the relaxation process, which is the process of delocalization.

In addition, the persistent and transient photoinduced effects have been observed in manganites, which originate from different mechanisms, such as the oxygen deficiency in most metallic states and the melting of charge-ordering (CO) state.^[49–51] The persistent photoinduced ratio is defined as PPC = (R₀ – R_b) ⨯ 100 / R₀, where R₀ and R_b are the initial resistance without the light illumination and the balanced resistance after the light illumination, respectively. Figure 5 shows the PPC effect for the phase-separated Sm_0.5Sr_0.5Mn_1−yCr_yO₃ (0 ≤ y ≤ 0.2) films under the light illumination as a function of the doping contents and temperatures.^[52] We observe an evolution of persistent and transient photoinduced effects induced by impurity doping effects and temperatures, which is closely related with the number of ferromagnetic clusters and paves a way for practical applications in photoelectric devices of all-oxides.

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. A schematic view of the photoinduced relaxation character experimental facility. PD is a photodiode.[46]

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. Photoinduced relaxation characters of the La0.5Ca0.5MnO3 films at different temperatures: (a) 80 K, (b) 140 K, (c) 180 K, (d) 230 K.[46]

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. (color online) PPC effect for the films under the light illumination as a function of the doping contents and temperatures.[52]

Fig. 6.

	Figure Option ViewDownloadNew Window
	Fig. 6. (color online) Photoinduced insulator to metal phase transition in strain-engineered thin film.[60]

For the manganite systems with the typical metal–insulator transition, the photo-irradiation results in an increase of the resistivity of the film in the metallic state and a decrease in the insulating state.^[53–58] Generally, the light can excite the downspin e_g electrons, which will destroy the ferromagnetic coupling between the upward-spin e_g and t_2g electrons in Mn³⁺ ions. As a result, it weakens the double exchange effect and increases the resistivity. Namely, this is the so-called photoinduced demagnetization. Moreover, according to the double-exchange effect, the probability of the e_g electron transferring from Mn³⁺ to the neighboring Mn⁴⁺ is proportional to cos(θ/2) (θ being the angle between the neighboring spins). Supposing that the interaction of the photo-irradiation causes a change δ of angle θ, the relative change in probability is defined as follows: 2 One obtains the simplified result from Eq. (2) 3 where δ/2 and θ/2 vary from 0 to π/2. The relative change in Eq. (3) is negative, indicating that the double exchange effect becomes weaker. According to Eq. (3), if the change δ is a fixed constant, the larger value of θ, the larger relative change. Therefore, the relative change in the resistance becomes larger. In the experiments, the condition of each irradiation is identical, accordingly the change δ is a certain value. This model can be used to explain the relation between the relative change in the resistance and the temperature or the strain. Meanwhile, the photoinduced resistance can be erased by a magnetic field.^[59] Furthermore, the photoinduced insulator-to-metal phase transition has been observed in strain-engineered La_2/3Ca_1/3MnO₃ film, as shown in Fig. 6.^[60] As temperature decreases, photoexcitation yields a step-like conductivity progression until it reaches the hidden metallic state at 80 K. With increasing temperature (without the photoexcitation), the conductivity returns to the insulating phase at 130 K. Recently, some scientists also discovered many emergent physical phenomena and properties. For examples, Li et al.^[61] successfully fabricated a novel spin transistor with an optical gate by partially illuminating the Hall channel with the blue light of a light-emitting diode (LED). And the study found that this optical gating effect is significantly enhanced with the increase of the optical power and shows repeatable cycling characteristics. Their team also focused on the optical control of the magnetic properties of La_1/2Sr_1/2MnO_3−δ. They manipulated the anisotropic magnetoresistance under the illumination of a common red LED and the window of the anisotropic magnetoresistance curve under the scanning magnetic field is enlarged under red light, indicating an increase in saturation magnetization. The experimental results also showed that the optical control of magnetization is reversible and strongly depends on the light intensity and Mn⁴⁺ content of La_1/2Sr_1/2MnO_3−δ.^[62] Sun et al.^[63] found that in the bilayered perovskite manganite La_2−2xSr_1+2xMn₂O₇ (x = 0.5 and 0.6), laser irradiation can lead to persistent modifications of both resistance and microstructure in the phase-separated (PS) states. In situ laser irradiation can modify the PS nature and strengthen the charge-ordering state. Yada et al.^[64] performed double-pulse pump experiments in Pr_0.6Ca_0.4MnO₃. The results showed that compared with the single-pulse pump, the photoinduced melting of charge-ordering is enhanced by ∼ 60% by the double-pulse pump. This also indicated that the decrease in crystal anisotropy is a key factor in enhancing the efficiency of photoinduced CO melting. Elovaara et al.^[65] reported that the laser-irradiated Pr_1−xCa_xMnO₃ (x = 0.4) film achieved a nearly nine orders of magnitude magnetoresistive insulator-to-metal switching of the film in a significantly reduced magnetic field compared to darkness. And this combined effect of light and magnetic field is called magneto-optical resistance (MPR). Yue et al.^[66] studied a large photo-induced enhanced MR in La_2/3Ca_1/3MnO₃ films on n-Si substrates. The film was irradiated with a 808 nm CW laser in a magnetic field of 6.4 kOe, the MR ratio was sharply increased from 0.54% to 18%. Wang et al.^[67] reported an ultrafast optical pump-probe measurement with linearly and circularly polarized laser pulses in the manganite La_0.67Ca_0.33MnO₃ thin film and detected the basic process of light–matter interaction in the La_0.67Ca_0.33MnO₃ film induced by femtosecond laser excitation. Esposito et al.^[68] comprehensively analyzed the photoinduced transition of magnetoresistive manganites. They used femtosecond x-ray diffraction to study the structural response of charge and orbitally ordered Pr_1−xCa_xMnO₃ films across a phase transition induced by 800 nm laser pulses. These provide new ideas and directions for further study of manganites, especially about the interaction between light and matter.

ZnO is an important wide band gap semiconductor with a bandwidth of 3.37 eV. In general, it favors the n-type conduction and possesses a large exciton binding energy. The divalent alkaline-doped manganites exhibit p-type conduction. Thus, the heterostructures combining manganites with n-type ZnO will create novel physical properties. Until now, there are many studies on manganites/ZnO heterostructures, involving electrical characteristics, positive magnetoresistance, and charge-transport mechanism, etc.^[73–80] La_0.7Sr_0.3MnO₃ (LSMO) and ZnO layers on LaAlO₃ (100) substrates have been fabricated by using a pulsed laser deposition method.^[81,82] The schematic illustration and the current–voltage characteristics of the heterostructure at different temperatures are shown in Fig. 7. The positive bias is defined by the current flowing from LSMO to ZnO. The good rectifying behavior of the p–n diodes is observed, which is similar to that of conventional p–n junctions. The diffusion potential V_D is changed from 0.4 V at 277 K to 0.6 V at 77 K and the slope of the I–V curves becomes steeper in the forward mode as the temperature increases. It is known that the carriers can be driven over the energy barrier at the ZnO/LSMO interface by smaller applied voltage as the temperature is increased. Therefore, V_D decreases and the leakage current increases with increasing temperature; consequently, the slope coefficient becomes larger. Furthermore, the photovoltaic voltages with a 248 nm laser irradiation at the energy density of 0.3 mJ/mm² as a function of time at 138 K, 238 K, and 358 K are shown in Fig. 8. It is noted that the rise time is about 10 ns. The peak photovoltaic voltages and the full width at half maxima (FWHM) of the heterostructure irradiated by the laser as a function of temperature are shown in the inset. The peak photovoltaic voltages and the FWHM nonlinearly decrease with increasing temperature. It is known that only the photogenerated carriers, located at or reaching the depletion region by diffusion, directly contribute to the photovoltage. As the temperature increases, more carriers pass through the barrier and thereby the accumulated electrons and holes decrease, which results in the decrease in the photovoltages.

Fig. 7.

	Figure Option ViewDownloadNew Window
	Fig. 7. The I–V characteristics of the heterostructure at different temperatures.[81]

Fig. 8.

	Figure Option ViewDownloadNew Window
	Fig. 8. (color online) Photovoltaic voltage as a function of time at 138 K, 238 K, and 358 K.[82]

As an important semiconductor, Si is an indispensable element of modern information industry. Considering the practical applications in devices, it is of special significance to study the integration of manganites with Si. So far, a large amount of phenomena based on manganites/Si heterostructures have been observed, such as photovoltaic effect,^[83–87] rectification characteristics,^[88] and magnetoresistance.^[89–91] For example, Chen et al.^[92] prepared a La_0.88Te_0.12MnO₃/Si heterostructure using a pulsed laser deposition method. The heterostructure has a photovoltaic effect and good rectification. Xing et al.^[93] investigated the side-illumination-induced enhancement of photovoltaic response in La_0.9Sr_0.1MnO₃/Si heterostructures. The photovoltaic responsivity can reach 6.87 mV·mW⁻¹ under the side illumination, which is much larger than 0.17 mV·mW⁻¹ under the normal illumination.

We have deposited the La_0.7Sr_0.3MnO₃ (LSMO) film on a single-crystal Si (100) substrate.^[94] The temperature dependence of the resistivity is shown in the inset of Fig. 9, in which the LSMO film exhibits the typical metal–insulator transition at T = 143 K. It is interesting to note that a local minimum of photovoltage also seems to occur at T = 143 K, which is consistent with the transition temperature. This indicates that the phase transition of the LSMO film affects the photovoltaic effect. The photovoltages are usually related to the accumulation of the photogenerated carriers and the width of the depletion layer. When the temperature is increased, the accumulation of the photogenerated carriers is reduced because of the stronger thermal fluctuation, resulting in a decrease of the photovoltages. At T = 143 K, the LSMO film undergoes a change in the band structure accompanying a metal–insulator transition, i.e., the up-spin and down-spin subbands split. The spin-polarized carriers tend to occupy the lower subbands, which may cause the thinner thickness of the depletion layer at the heterostructure. Thus, this will result in an appearance of the local minimum of the photovoltages at T = 143 K.

Fig. 9.

	Figure Option ViewDownloadNew Window
	Fig. 9. Temperature dependence of the maximum photovoltages. Inset shows the temperature dependence of resistivity of the LSMO film.[94]

In addition, we also investigated the photoinduced effect on carrier transport properties in the LSMO/Si heterostructures.^[95] Figure 10 shows the temperature dependence of the resistance and the relative change of the photoinduced resistance in the current-perpendicular-to-plane (CPP) geometry. The heterostructure without laser irradiation has a metal–insulator transition at T = 270 K. And the heterostructure in the CPP geometry exhibits a transient photoconductivity effect. The maximum relative change of photoinduced resistance in LSMO/Si heterostructures at a temperature of 270 K is approximately 6200%, and the relative change can reach 5674% at room temperature. The giant change in resistance is attributed to laser-created carriers in the heterostructure. As we know, the band gaps of LSMO and Si are 1.0–1.3 eV and 1.12 eV, respectively.^[96] The photon energy of the laser (about 2.34 eV) in the experiments is larger than their band gaps. Under laser irradiation, the created electrons and holes in LSMO and Si move to the Si and LSMO sides, respectively. Therefore a drastic reduction in resistance (by more than three orders of magnitude) occurs. The resistance of the heterostructure irradiated by laser as a function of the power level at 100 K and 240 K is shown in the inset, which clearly shows a nonlinear behavior. This relation is described by a simple power law R ∝ P⁻ⁿ, where R is the resistance and P is the power level of the laser. The solid lines are the fitting curves in the inset of Fig. 10, from which we obtain that the exponent n is about 0.13 at T = 100 K and 240 K. This indicates that the recombination rate is exponentially proportional to the concentration of laser created carriers.

Fig. 10.

	Figure Option ViewDownloadNew Window
	Fig. 10. (color online) Temperature dependence of resistance and PR of the heterostructure in CPP geometry. The resistance is given on a natural log scale. The resistance as a function of power level at 100 K and 240 K is shown in the inset and the solid lines are the fitted curves using a simple power law.[95]

As early as 2003, the multiferroic properties of single-phase BFO were observed.^[97] Then, researches on perovskite single-phase multiferroic materials attracted much attention again.^[98,99] In particular, the BFO has excellent photoelectric properties.^[100–104] Nevertheless, the magnetism of BFO is very weak and thus researchers want to enhance it through compositing with other materials. We have prepared BFO/La_1/3Sr_2/3MnO₃ heterostructures.^[105] The results show that BFO/La_1/3Sr_2/3MnO₃ heterostructures retain multiferroic properties and furthermore magnetic properties are enhanced because of the interfacial effect. In addition, the photovoltaic effect was also observed in the heterostructures.^[106,107] It is observed that the short-circuit current is inverted with the change of polarization of the BFO layer. Figure 11(a) shows the time dependence of the short-circuit photocurrent at an incident laser density of 91.4 mW/cm² and zero bias. We can see that the photocurrent increases suddenly to a transient maximum and then reaches a steady state. When the BFO is poled along the two opposite directions, the photocurrent and photovoltage are switched (see Fig. 11(b)), indicating that the ferroelectric polarization has a dominant role in the photovoltaic effect. On the other hand, asymmetric properties between the negatively poled film and the positively poled film are observed, which may be caused by the contribution from the electrode/film interface. Therefore, the photovoltaic effect in the heterostructure is due to the contribution from both the ferroelectric polarization and the electrode/film interface. Figure 12 shows the I–V curves taken in the dark, under 160 mW/cm² and 200 mW/cm² green-light illumination (λ =532 nm), suggesting that photoconductivities increase with enhancing light intensity. The resulting current of photoexcited carriers, driven by the intrinsic polarization, can be described by the following equation:^[108] 4 where σ_d and σ_ph represent the dark and light components of the conductivity, respectively. In principle, thermal variation induced by visible-light illumination can contribute to the photoconductivity, but the decrease of the resistance due to a light-induced temperature increasing causes a less negative photoconductivity increasing. These findings are consistent with the model present in Eq. (4) and serve as an evidence of the photoconductivity effect in BFO.

Fig. 11.

	Figure Option ViewDownloadNew Window
	Fig. 11. (color online) (a) The time dependence of the photo current under zero bias and an incident laser intensity of 91.4 mW/cm2, (b) J–V curves measured with laser incident.[106]

Fig. 12.

	Figure Option ViewDownloadNew Window
	Fig. 12. Dark, 160 mW/cm2, and 200 mW/cm2 green-light current density as a function of applied field for the heterostructure.[107]

Titanate-based oxide, a perovskite material with a high dielectric constant, has potential applications for dynamic random access memory (DRAM) devices, and its dielectric constant can also be adjusted by an external field. At the same time, manganites-based heterostructures have also attracted wide attention with the gradual deepening study of different functional devices with different structures.^[109–114] Recently, many researchers have conducted in-depth studies on the photoelectric effect of heterostructures, such as manganite/Nb:SrTiO₃ heterostructures.^[115–118] We prepared phase-separated manganite heterostructures (Pr_0.65(Ca_0.75Sr_0.25)_0.35MnO₃ (PCSMO)/Nb:SrTiO₃ by the pulsed laser deposition. The optoelectronic properties of the heterostructures under the application of a magnetic field were investigated. As shown in Fig. 13, it is found that the relative change in photovoltaic (PV) values at 140 K reaches 63% under 1 T magnetic field, indicating that the magnetic field is an important factor affecting the photovoltaic effect.^[119] At the same time, a monovalent Ag-doped manganite La_0.8Ag_0.2MnO₃ (LAMO)/Nb: SrTiO₃ heterostructure has also been prepared and exhibits good rectification characteristics.^[120] The photovoltage and light intensity satisfy the classical exponential relationship with the increase of the light intensity, as shown in Fig. 14. Apparently, the photovoltage grows with the increase in power, and tends to saturate (∼ 0.76 V) once the power reaches 0.3 W. When the light power increases, more electron–hole pairs are excited, which results in an increase in V_oc. However, the photovoltage is limited by the original barrier height, and the maximum V_oc is close to the diffusion voltage. In the power range of −0.3 W, the data analysis shows very poor agreement with the exponential relationship. However, the fitting in the range 0–0.1 W shows good agreement. This indicates that the idealized p–n diode can partly describe the photovoltaic effect of the LAMO/Nb:SrTiO₃ heterojunction. Furthermore, the rectifying characteristics and photovoltaic effect of TbMnO₃ (TMO)/Nb:SrTiO₃ heterostructures were studied systematically. Figure 15 shows the relationship between the photovoltage and time in the heterostructure irradiated by a 248 nm light at the temperature of 300 K. The maximum photovoltage is about 0.119 V at 0.71 mJ/mm². That work provides the possibility for TMO applications in ultraviolet (UV) light.^[121] In addition, Sheng et al. deposited 24 nm LSMO thin films on the (001) and (110) crystal planes of SrTiO₃-Nb. From Fig. 16, it can be seen that the correlation gap E_g plays a decisive role in the amplitude J_ph, its temperature and magnetic adjustability of heterostructures.^[122]

Fig. 13.

	Figure Option ViewDownloadNew Window
	Fig. 13. Temperature dependence of (a) photovoltage of heterostructure irradiated by a laser, (b) PV for PCSMO/Nb:SrTiO3 under different magnetic fields, respectively. Inset: the schematic diagram of heterostructure under magnetic field and light.[119]

Fig. 14.

	Figure Option ViewDownloadNew Window
	Fig. 14. (color online) Photovoltage versus power under the illumination of a 473 nm laser beam at 20 K. Inset is the structure of the film.[120]

Fig. 15.

	Figure Option ViewDownloadNew Window
	Fig. 15. (color online) Photovoltages of the heterostructure under the irradiation of a laser (248 nm) at T = 300 K as a function of time. The inset shows the peak photovoltages as a function of the power density.[121]

Fig. 16.

	Figure Option ViewDownloadNew Window
	Fig. 16. (color online) A schematic image of the relationship between JSC and correlated gap in correlated junctions. The insets show the schematic band diagrams for (left) LSMO (001) junctions, (middle) LSMO (110) junction under high magnetic field (6 T) or high temperature (HT), and (right) LSMO (110) junction without magnetic field (0 T) or low temperature (LT), respectively.[122]

Fig. 17.

	Figure Option ViewDownloadNew Window
	Fig. 17. (color online) Dielectric constant (a) and its relative variation (b) as a function of temperature for the BST/La0.67Sr0.33MnO3 heterostructure exposed to light of different intensities with an interval of 5 mW/mm2. Inset: the sketches of the crystal lattice structure for the La0.67Sr0.33MnO3 and barium titanate layers at different temperature ranges.[123]

Besides, we systematically studied the microstructure, leakage current, and tunability of the dielectric constant of the barium titanate/manganite heterostructures induced by an optical field. As shown in Fig. 17, it is worth noted that the relative change in the dielectric constant has a positive to negative transition at 150 K when the heterostructure is irradiated by a light.^[123] In the experiment, the photon energy of about 2.34 eV (532 nm) is smaller than the band gap of Ba_0.6Sr_0.4TiO₃ (BST) (∼ 3.0 eV) and larger than that of La_0.67Sr_0.33MnO₃. Moreover, the transmission of the Pt film (electrode material) is about 36% at 532 nm. Accordingly, the light can generate the carriers in the La_0.67Sr_0.33MnO₃ layer by passing through the BST layer and Pt film when the heterostructure is irradiated. And the photoinduced carriers at the BST/La_0.67Sr_0.33MnO₃ interface change the interfacial polarization and thus affect the dielectric properties.^[124] These results may be useful in developing potential applications of heterojunctions for optoelectronic detections.^[125–131]

The combination of a manganite thin film and a ferroelectric PMN-PT single crystal is a promising platform. It is considered as a convenient method to modulate the physical properties of films.^[132–137] An electric field can be applied to the substrate and thus the lattice constant of PMN-PT can be continuously adjusted by utilizing the electrostriction effect. So, the physical properties of the film can be continuously controlled.^[138–142] If an ultra-thin film is grown, the interface polarization of PMN-PT will play a dominant role. It can also be considered as a ferroelectric field effect transistor, in which manganite and PMN-PT substrates serve as conductive path and insulated gate, respectively. The electric field-induced polarization of the ferroelectric layer can not only generate the lattice distortion, but also generate charge accumulation or depletion at the interface.

Here, a Pr_0.65(Ca_0.75Sr_0.25)_0.35MnO₃ (PCSMO) thin film was deposited on the piezoelectric PMN-PT substrate by pulsed laser deposition. Besides the observed persistent magnetoresistance effect,^[143] the PCSMO/PMN-PT heterostructure exhibits a reversible bistability in the photoinduced change in resistance at T < 80 K as the voltages are increased.^[144] We define the photoinduced change in the resistance (PR), PR = ( R_dark – R_light ) / R_dark × 100%. As shown in Fig. 18(a), the film exhibits the transient photoinduced effect (TPE) and the PR is reduced under the applied voltage, demonstrating that the applied voltage seems to impose an active suppression on the TPE. The TPE becomes more complex with the decrease of temperature. It should be noted that the values of normalized PR undergo a significant crossover from positive to negative when the applied voltages increase up to a critical value. It indicates that the electric field modulates the TPE and induces an appearance of the bisable states. Meanwhile, the transition voltages are increased with increasing temperature. In other words, the bistable state of PR requires the larger voltage at the higher temperatures. The electric field modulation of bistability in the PCSMO/PMN-PT heterostructure provides us with some deep insights into understanding the physical mechanisms of coupling effects of external fields in electric phase separation manganites.

Fig. 18.

	Figure Option ViewDownloadNew Window
	Fig. 18. (color online) (a) Resistance as a function of elapsed time upon the application of light irradiation and bias voltage 40 V at 150 K. (b) and (c) Variation of normalized PR under the different voltages at 65 K and 80 K, respectively. (d) Temperature dependence of PR for the PCSMO film by applying different bias voltages. The inset shows the temperature dependence of VT.[144]