Magnetic Field Controllable Photocurrent Properties in BiFe0.9Ni0.1O3/La0.7Sr0.3MnO3 Laminate Thin Film

This paper reports a multifunctional magnetic‐photoelectric laminate device based on the integration of spintronic material (La0.7Sr0.3MnO3) and multiferroic (Ni‐doped BiFeO3), in which the repeatable modulation effect on the photoelectric properties were achieved by applying external magnetic fields. More obviously, photocurrent density (J) of the laminate was largely enhanced, the change rate of J up to 287.6% is obtained. This sensing function effect should be attributed to the low‐field magnetoresistance effect in perovskite manganite and the scattering of spin photoelectron in multiferroic material. The laminate perfectly combines the functions of sensor and controller, which can not only reflect the intensity of environmental magnetic field, but also modulate the photoelectric conversion performance. This work provides an alternative and facile way to realize multi‐degree‐of‐freedom control for photoelectric conversion performances and lastly miniaturize multifunction device.


Introduction
[34][35] (Also refer to Table S1, Supporting Information.)[53] Considering that multiferroics presents fascinating magnetic, ferroelectric properties and their coupling, it is interesting and meaningful to explore the effects of magnetic field on the photoelectric properties in "ferromagnetic/multiferroic" ME composite, which has been seldom investigated by now.
In this study, it is reported that obvious boosted effect of magnetic field on the photocurrent in the laminated thin film composed of Ni-doped BiFeO 3 (BFNO) and La 0.7 Sr 0.3 MnO 3 (LSMO).[60][61] Thus, the manipulation of photoelectric properties can be realized through magnetic field inducing the change of the coupling between ferromagnetic and multiferroic layers in BFNO/LSMO laminated thin film prepared by pulsed laser deposition (PLD) method.

Results
2.1.Fabrication and Characterization of BiFe 0.9 Ni 0.1 O 3 / La 0.7 Sr 0.3 MnO 3 Laminate Figure 1a presents the XRD diffraction patterns of the as-prepared BFNO film and BFNO/LSMO laminate film deposited in FTO substrate by PLD method (see Section 5 for more details).By comparing with the standard PDF cards of BiFe 0.9 Ni 0.1 O 3 and La 0.7 Sr 0.3 MnO 3 samples, respectively, no other obvious diffraction peaks can be indexed in the measuring precision of the X-ray diffractometer, indicating that pure phase BFNO/LSMO laminate film were achieved prepared by PLD method.Surface SEM images of the BFNO layer and the LSMO layer are shown in Figure 1b,c, respectively.The BFNO layer is uniformly distributed crystalline, with a grain size of about 100-200 nm and the boundary of tetragonal rule.The LSMO layer grown on the BFNO layer also has obvious crystals, which are spherical particles with the grain size of about 10-20 nm and the microstructure is dense in each crystalline unit.

Magnetic and Ferroelectric
Properties of La 0.7 Sr 0.3 MnO 3 /BiFe 0.9 Ni 0.1 O 3 Laminate Figure 2a,b show the room temperature hysteresis loops of the BFNO layer, the LSMO layer, and the BFNO/LSMO laminate, respectively.The lower right of the two figures are the partial enlargement of the hysteresis loops in the low magnetic field area.When the magnetic field is perpendicular to the surface of the thin film, the saturation magnetization of the BFNO layer and the LSMO layer are 9.5 and 98 emu cm −3 , respectively, and the magnetization of the LSMO layer is much higher than that of the BFNO layer continuously during the magnetization stage in the magnetic field range of 0-5000 Oe, suggesting that the magnetic properties of the BFNO/LSMO laminate mainly contribute from the LSMO layer.The saturated magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) of the BFNO/LSMO laminate are 137 emu cm −3 , 27 emu cm −3 , and 221 Oe, respectively.Moreover, the saturation magnetization field is >5000 Oe, which indicates that it is possible to tune the magnetization of the BFNO/LSMO laminate within the 5000 Oe range.As shown in Figure 2b, when the magnetic field direction is in the membrane plane, the saturation magnetization of the BFNO layer and the LSMO layer are 8.5 and 105 emu cm −3 , respectively.The saturated magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) of the BFNO/LSMO laminate are 116 emu cm −3 , 65 emu cm −3 , and 218 Oe, respectively.In addition, the magnetization of BFNO/LSMO laminates is close to saturation within the size of 1000 Oe magnetic field.This indicates that when a magnetic field parallel to the film surface is applied, the magnetic field within 1000 Oe can significantly tune the magnetization of BFNO/LSMO laminates.
It should be noted that when calculating the magnetization of BFNO/LSMO laminates, the thickness of BFNO/LSMO laminates used is the superposition of the BFNO layer and the LSMO layer.However, it can be seen from the two figures that the magnetization degree mainly depends on the LSMO layer.This means that if the LSMO layer grown on the BFNO layer is compared with the LSMO layer grown on the FTO layer, the magnetization of the former layer will be twice as high as that of the latter.
Figure 2c presents the ferroelectric properties of the laminate thin film.Here, LSMO layer acts as electrode when performing the measurements of electric hysteresis loops.And the ferroelectric behavior should be contributed to the BFNO ferroelectric layer.The results show the distinct hysteresis characteristics of BFNO layer, which is benefit for us to study the ferroelectric photovoltaic effect and its field-modulation effect in BFNO/LSMO layer.From the figure, remnant polarization (Pr) of the BFNO thin film up to 13 μC cm −2 was obtained after polarized by 200 kV cm −1 polarization electric field.The coercivity field (E c ) up to 100 kV cm −1 can guarantee the stability of polarizability of the thin film after polarization.Considering that the laminate sample presents both magnetic, electric, and optical properties, it is meaningful to investigate external field modulated effect on the properties, especially, magnetic field modulated effect on the photoelectric character, which has been seldom studied by now.Different magnetic fields were applied to the laminate when performing the measurement of photocurrent density (J) dependent of applied magnetic field (H) at 3 V bias voltage (J-H(t) curves) and photocurrent density (J) dependent of bias voltage (V) (J-V curves).The measured schematic illustration was presented in Figure 3a.In order to realize in-situ J-V measurements when applying magnetic field, the set of in-situ magnetic field device to coordinate the semiconductor characterization system (Keithley 4200-SCS) was designed that equipped with the solar simulator (XES-40S1), where the size of magnetic field can be turned by changing the distance between NbFeB permanent magnet and the sample, as shown in Figure 3b.

Modulation of the
J-t curve corresponding to magnetic field changing between −1300 and 1300 Oe is shown in Figure 3c.It can be observed that the photocurrent changes regularly with the magnetic field being uniformly tuned between −1300 and 1300 Oe.Obviously, the magnetic field has a significant modulating effect on the photocurrent, which is probably related to the regulation of the magnetic field on the photogenerated carrier.Figure 4 presents the J-V curves with different magnetic field changing processes (The repeated measurement results with magnetic field changing from 0 to −1300 Oe were presented in Figure S2, Supporting Information).It can be observed that the dark current of the laminate is very small even under larger bias voltage (see from Figure 4a).However, obvious photocurrent can be obtained when applying sun light to the laminate.The value of J up to 40 μA cm −2 can be obtained when bias voltage is 3 V.Besides, the laminate presents asymmetric J-V character when applying positive and negative voltages to the thin film, respectively.This should be attributed to the different electrodes in both sides of the BFNO/LSMO laminate producing different Schottky barriers.
More importantly, from Figure 4, it is obvious that magnetic field makes large influence on photocurrent of the laminate, which is the key experimental result in this study.With magnetic field increasing, photocurrent changing with voltage becomes more obviously.When the bias voltage is −3 V, photocurrent density J changes from −40 μA cm −2 with H = 0 to −160 μA cm −2 with H = 1.3 kOe.That is, the laminate presents important magnetic-photocurrent (MPC) effect.In order to characterize this MPC effect in detail, the change rate of photocurrent density with and without a magnetic field under a certain bias voltage (MPC) V is defined as: where J(H) and J(0) presents photocurrent density with and without magnetic field, respectively.Based on Equation ( 1), the dependence of MPC on magnetic field (MPC-H curve) under bias voltage of AE3 V were plotted, as presented in Figure 5.It is observed from Figure 5a, with magnetic field increasing from 0 to 1.3 kOe (state 1 and 2 in Figure 5a), (MPC) 3 V gradually increases from 0% to 283.2%, and then decreases to 0 when magnetic field is back to 0. When inverted magnetic field is applied (state 3 and 4 in Figure 5a), (MPC) 3 V increases again until it reaches maximum value of 266.9% when magnetic field increase up to −1.3 kOe.Then, (MPC) 3 V decreases to near 0 with magnetic is back to 0. The rule of (MPC) −3 V changing with magnetic field is similar to that of (MPC) 3 V , as can be seen from Figure 5b.The results indicate that external magnetic fields with both positive and negative directions make much improvement to the photocurrent of the laminate.The largest MPC value up to 287.6% was obtained when applying −1.3 kOe magnetic field under bias voltage of −3 V. Besides, it can be observed from Figures 4 and 5 that, when magnetic field turn back to 0, photocurrent almost returns to its primary value, that is, this MPC effect is reversible, which is benefit to its potential application in photoelectric field.

Discussion
As is known to all, photoconductive σ can be expressed as following, where n and μ e + h is carrier concentration and carrier mobility of the certain material, respectively.The expression indicates that both carrier concentration n and carrier mobility μ e + h can affect the photoconductive of the sample.Therefore, if n or μ e + h can be turned by magnetic field, MPC effect of the sample should be realized.Under light condition, the photon whose energy (E = hÁv) is larger than bandgap of the sample can produce non-equilibrium carriers (photo-producingcarriers). Obviously, if the bandgap of the sample can be changed under magnetic field, light injection of the sample will be affected, and therefore the concentration of the non-equilibrium carrier changes.Therefore, it is necessary to measure that the transmittance spectrums for the BFNO film under different external magnetic fields to investigate the effect of magnetic field on the bandgap of the film.The flow chart of the magnetic field tuning spectrum is shown in Figure 6a.The magnetic field is introduced into the UV-Vis Near IR measurements by using the Gauss meter to pre-calibrate the magnetic field intensity at magnet positions of different heights.In the transmission spectrum test, different magnetic fields were applied to the sample by varying the height of the magnet by changing the distance between the magnet and the sample.It can be considered that the magnetic field at the site of the sample is even since the thin film sample is small.Figure 6b,c presents the transmittance spectrums of the laminate under different magnetic fields.Obviously, in the measured optical wave regions, the transmittance of the laminated film increases with H decreasing from 200 to −175 Oe as shown in Figure 6b.This result indicates that magnetic field indeed affects the optical adsorption properties of the laminate.Moreover, when magnetic field changes back to 200 Oe, the transmittance almost resume to the initial state, indicating the change of transmittance modulated by magnetic field is reversible as shown in Figure 6c.
According to the experimental results presented in Figure 6, the relation between (αhv) 2 and hv can be calculated by Tauc formula.Figure 7 presents the (αhv) 2 -hv curves evolved from Figure 6.The bandgap of the laminate film can be deduced by linearly fitting the linear region of the curve in Figure 7.The deduced result was presented in Figure 8.It is can be seen that when the applied magnetic field H decreases from 200 to −175 Oe, the bandgap of the laminate film decreases from 2.286 to 2.205 eV, then increases back to near 2.286 eV with H returning to 200 Oe.This result indicates the reversible modulated effect of magnetic field on the bandgap of the laminate film.The chance of bandgap may cause the chance of eigen absorption limit λ 0 of the sample, which can be calculated by, where E g is bandgap of the sample.According to Equation ( 3), λ 0 chances between 544 and 562 nm due to the bandgap variation, which just provides an adjustable spectrum zone of 18 nm in green light region, and should not make much influence on the photon-produced carrier concentration n.Moreover, in the experimental results about the effect of magnetic field on bandgap, positive and negative direction of magnetic field presents opposite tuning effect on the bandgap, indicating that photon-generatedcarrier concentration n increases or decreases when the direction of magnetic field reverses.This result does not match the experimental results about the change rule between photocurrent density J and magnetic field H, where  J increases with H increasing, regardless of the direction of H, as had been presented in Figure 5. Therefore, the mechanism of the magnetic field modulation effect on photocurrent should not attribute from the bandgap change under magnetic field.
According to Equation (2), carrier mobility μ e + h can also affect the photoconductive σ of the material.Also, low resistivity is benefit to carrier migrating in the sample.Thus, if the resistivity of the laminate film can be modulated by magnetic field, carrier mobility μ e + h will be turned by magnetic field.It had been reported that perovskite LSMO thin film presented interesting low-field magnetoresistance (LFMR) effect in room temperature, [62][63][64] that is, the resistivity of LSMO thin film decreases when applying a magnetic field on it.Here, MR measurements were performed on the LSMO film as well, the results were presented in Figure 9.It is obviously that in low magnetic regions (from −200 to 200 Oe), the resistivity of the LSMO film decreases quickly with magnetic field increasing, presenting obvious LFMR effect.MR values up to −13%, −2%, and −2.5% were obtained under temperature of 77, 300, and 358 K, respectively.This experimental result indicates that external magnetic field can modulate the resistivity of the sample and therefore turn the carrier mobility μ e + h of the laminate.Consequently, external magnetic field modulating photocurrent effect may be realized by using MR effect of perovskite LSMO thin film in BFNO/ LSMO laminate.The quantitative relationship between magnetoresistance and magnetophotocurrent effect needs to be further studied in the subsequent research work.
For further investigating the mechanism of magnetic field modulated effect on photocurrent, considering that both LSMO and BFNO layers are spin-polarization, from spin photoelectron point of view, we analyze photoelectric processes of the laminate film under light condition and external magnetic field.As shown in Figure 10, energy band splitting occurs under the action of electron spin exchange in the LSMO layer. [59,65]The energy of the density of states of the spin-up (σ = ↑) band is generally lower than that of the spin-down (σ = ↓) band, so that the spin-up electrons become the majority of spin-electrons, and their number is higher than the minority of spin-electrons with spin-down state, which leads to the spontaneous magnetization of electrons.In LSMO layer, the spin-up and spin-down bands are completely separated, and the spin polarization reaches 100%.
As we known, BFNO can present spontaneous magnetization, [45,66,67] that is, the number of electrons with spin-up state may be larger than that of electrons with spindown state.Under light condition, the spinup electrons in the valence band (E V ) were excited and transfer to the conduction band (E C ) to form majority spin-photoelectrons (e maj ) in BFNO, therefore, the spin-down electrons act as minority spinphotoelectrons (e min ).The interaction between internal depolarization field (in BFNO) and interface electric field (in the interface of BFNO/ LSMO) makes the spin-photoelectrons transfer toward other adjacent ions, and then forming photogenerated current.In general, the conduction process in magnetic materials can be separated to two channels in which the direction of electron spin is up and down, respectively.And the total resistivity ρ T can be expressed as, [68] where ρ ↑ and ρ ↓ presents the resistivity corresponding to the contribution of electrons with spin-up state and spin-down state, respectively.As presented in Figure 10a, due to the weak spin polarization in BFNO, spin-related scatter is small, and the contribution of the two channels to the total resistivity is similar according to Equation (4).When applying an external magnetic field, as shown in Figure 10b, the degree of photoelectron spin polarization in BFNO layer is strengthened, consequently spinrelated scattering is enhanced, which reduces the resistivity of either channel (spin-up or spin-down), and thus reduces the total resistance ρ T according to Equation (4).Meanwhile, external magnetic field makes the directions of photoelectron spin polarization in BFNO layer and spintronic in LSMO layer reach unanimity, which reduces the interface scattering between BFNO layer and LSMO layer, therefore significantly promote photocurrent density.

Conclusion
In summary, the La 0.7 Sr 0.3 MnO 3 /BiFe 0.9 Ni 0.1 O 3 laminate thin film, which was deposited on FTO substrate by PLD, was used to construct a solar cell structure where BiFe 0.9 Ni 0.1 O 3 film was acted as light absorbance layer and La 0.7 Sr 0.3 MnO 3 film as electrode.The experimental results indicate that the solar cell presents good ferromagnetic, and ferroelectric properties.Especially, obvious photoelectric conversion character was detected.Under sun light condition, photocurrent density J up to 40 μA cm −2 was obtained when applying a 3 V bias voltage to the sample.More importantly, magnetic field modulating photocurrent (MPC) effect was firstly detected in this solar cell structure.This MPC effect presents following three features, 1) Photocurrent density increases with magnetic field increasing; 2) MPC effect has no concern with the direction of the magnetic field; and 3) This MPC effect is repeatable, i.e., photocurrent recovers to primary value when magnetic field decreases to 0. In this solar cell structure, MPC value up to 287.6% was obtained when magnetic field of −1.3 kOe was applied.The experimental results aiming to detect the mechanism of MPC effect indicates that, instead of magnetic-field turning bandgap of the BFNO thin film, both low-field magnetoresistance (LFMR) effect in LSMO electrode and the scattering of spin photoelectron in BFNO light absorption layer under external magnetic field, should be the mechanism of MPC effect.But the quantitative relation between LFMR effect and MPC effect should be further investigated later.The present results about reversible magnetic-field turning photocurrent effect in La 0.7 Sr 0.3 MnO 3 /BiFe 0.9 Ni 0.1 O 3 laminate thin film provides an optional road to manipulate photoelectric conversion properties of multiferroic thin films with solar-cell structure.

Experimental Section
Photocurrent Properties of BiFe 0.9 Ni 0.1 O 3 /La 0.7 Sr 0.3 MnO 3 Laminate by Magnetic Field

Figure 2 .
Figure 2. Room temperature magnetic hysteresis loops of the LSMO layer, the BFNO layer, and the BFNO/LSMO laminate thin film.The directions of the magnetic field were a) perpendicular and b) parallel to the surface of the thin film, respectively.c) Room temperature electric hysteresis loops of the BFNO/LSMO laminate thin film.

Figure 3 .
Figure 3. a) Schematic diagram of BFNO/LSMO laminate and its photoelectric measurement structure.b) Structure chart of in-situ magneto-photoelectric effect testing platform.c) J-t curve corresponding to magnetic field changing between −1300 and 1300 Oe under light condition and a bias voltage of 3 V.

Figure 4 .
Figure 4. J-V curves of BFNO/LSMO laminate with light condition measured under different magnetic field processes a) H changes from 0 to 1.3 kOe.b) H changes from 1.3 kOe to 0. c) H changes from 0 to −1.3 kOe.d) H changes from −1.3 kOe to 0.

Figure 5 .
Figure 5.The MPC dependence on magnetic field under bias voltage of a) 3 V and b) −3 V with different magnetic field changing processes.The dash line is a guide to the eyes.

Figure 6 .
Figure 6.The transmittance spectrums of the BFNO/LSMO laminate with different magnetic fields.a) Structural diagram of in-situ UV-Vis Near IR measurements with different magnetic fields.b) H changes from 200 to −175 Oe. c) H changes from −175 to 200 Oe. ρ Devices fabrication: One hundred nanometer thick BiFe 0.9 Ni 0.1 O 3 thin film was first grown on a FTO wafer by PLD method.The following technical condition and processes are performed to preparing high quality BFNO/LSMO laminate.

Figure 8 .
Figure 8.The dependent of bandgap on the magnetic field for the BFNO/ LSMO laminate.

Figure 9 .
Figure 9. Low-field magnetoresistance effect of LSMO thin film at different temperatures.