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Multi-Fields Modulation of Physical Properties of Oxide Thin Films

Oxide thin films exhibit versatile physical properties such as magnetism, ferroelectricity, piezoelectricity, metal-insulator transition (MIT), multiferroicity, colossal magnetoresistivity, switchable resistivity, etc. More importantly, the exhibited multifunctionality could be tuned by various external fields, which has enabled demonstration of novel electronic devices. In this article, recent studies of the multi-fields modulation of physical properties in oxide thin films have been reviewed. Some of the key issues and prospects about this field are also addressed.

In many cases, combined external stimuli could enable fine tuning of physical properties in thin film oxides, which could be beneficial for practical applications. In the following paragraphs, we will discuss the response of thin film oxides upon the external stimulus such as electrical field, strain field, magnetic field and photo illumination. A special concern has been focused on multiferroics, perovskite manganites, and oxides with resistive switching (RS) properties.

Electric field effect
Electric field has been recognized as an efficient and scalable way for tuning the physical properties in various materials and structures. For example, in magnetic tunneling junctions, a spin polarized current could induce spin transfer torque effect, which has been intensively studied in MRAM devices [79][80][81][82] and microwave devices. [83] Upon the application of electric field, the polarization direction of ferroelectrics could be switched. This could result in either large electroresistance effect in ferroelectric 6 tunnel junctions [84,85] or switchable diode effect in ferroelectric diodes. [86] Noticeable strain effect associates with the electric-field-induced ferroelastic domain switching has also been intensively studied in magnetoelectric heterostructures. [87,88] In multiferroic materials where the electric polarization couples with the magnetization, applying an electric field modifies the electric polarization and tunes its magnetization through the magnetoelectrical (ME) coupling effect. [11,89,90] In RRAM devices, applying an electric field induces the RS effect by associating with several electric-field-induced effects: electric field induced ion transportation, [35] electric field induced phase transition, [91] and the Joule heating effect. [92]

Electric field control of magnetism
The interplay between electric polarization and magnetization has attracted wide interest for applications such as spintronics devices. [48,93] The presence of multiferroicity in some materials and structures has enabled several ways where the magnetic properties can be manipulated by applying an electric field: [94] (1) In single phase multiferroics where magnetic order and ferroelectricity couples, applying an electric field could alter the electric polarization along with the magnetization owing to the intrinsic ME coupling. [11] (2) Many multiferroics are antiferromagnets, by attaching it to a ferromagnetic layer, it is possible to construct an exchange bias system that could be controlled by applying electric field to the multiferroic layer, which is known as exchange-mediated ME coupling effect [12] . (3) In magnetoelectric composites composed of ferroelectric (FE)/ferromagnetic (FM) heterostructures, [95] applying an electric field in the FE layer would induce strain that are transmitted in to the FM layer and changes its magnetic properties through inverse magnetostrictive effect. (4) In FE/FM heterostructures, the electric field could also switch the polarization direction of the FE layer, which may tunes the carrier density and hence the magnetic properties of the FM layer, referring as charge-mediated ME coupling effect. [96] In single phase multiferroic materials, a tempting idea is to design multiferroics that combine both ferromagnetism and ferroelectricity, and the manipulation of its 7 magnetization by an electric field would be essential for the magnetoelectric control of spintronics devices. However, the exploring of both ferromagnetism and ferroelectricity in single phase multiferroic has been found unexpectedly difficult. [46] One promising single phase multiferroic that could be utilized is BiFeO 3 (BFO)a material that shows unambiguously ferroelectricity and anti-ferromagnetism above room temperature. [97] The principle that BFO can be utilized for an efficient magnetoelectric control of spintronics devices relies on the possibility of electric field controlled interfacial coupling that composed of BFO and a ferromagnetic layer. [12,98,99] Recently, Heron et al. demonstrated the deterministic magnetization reversal in BFO based heterostructures, [99] by utilizing the coupling between its weak magnetization arising from the Dzyaloshinskii-Moriya (DM) interaction and the ferroelectric polarization, [100,101] as well as the strong interfacial magnetic coupling. [102] Although previous theory [103] predicts that the direct reversal of ferroelectric polarization in BFO will leave the orientation of the canted magnetic moment unchanged, they found that ferroelectric-polarization reversal in a strained BFO film actually follows an indirect pathway, and proposed that the indirect switching path reverses the canted magnetic moment [ Fig. 2]. The ME coupling is determined by the DM interaction, where the weak ferromagnetism M c is determined by the DM vector which is given by D ~ d i × r i , where r i is a constant vector connects neighboring Fe 3+ ions and d i is the displacement of the intermediate oxygen atom from the mid-point of r i caused by the O 6 -rotation. Since the in-plane 71° and out-of-plane 109° switches are found to be capable of rotating D and the associated M c by 90° [ Fig. 2(b) and 2(c)], the reversal of D through sequential in-plane 71° and out-of-plane 109° switches would also be expected to reverse M c . Furthermore, they deposited a spin valve structure on top of the strained BFO, and realized the electric-field control of a spin-valve device at room temperature. After an in-plane 71° switch. E represents the applied electric field. (c) After an out-of-plane 109° switch. [99] The progresses of strain-mediated and charge-mediated ME effect will be further discussed in later sections.

Electric field induced ion migration
RRAM has been widely investigated as next-generation nonvolatile memory devices due to its simple structure, high density, low power consumption and compatibility with complementary metal oxide semiconductor technology. [61,[104][105][106] Up to now, a large variety of materials have been investigated for RS including solid electrolytes, [107] perovskite oxides, [32,[108][109][110] binary oxides, [33,35,37,111] organic materials, [112][113][114] amorphous silicon, [115] and amorphous carbon. [116] Although the underlying mechanism is still controversial due to the diversity of RS behaviors, conductive filament (CF) model has gained much success and has been accepted extensively. [34] RRAM devices show a simple metal-insulator-metal sandwiched structure. For filament model based RRAMs, a sufficiently large electrical field between the two 9 metallic electrodes could induce a soft dielectric breakdown in the insulator, and set the device to a conductive state (also refer as an ON-state). In this process, due to the electric induced ion migration, a large amount of defects are formed inside the film and form CFs that connect the two terminal electrodes. The ON-state could also be electrically reset, leading to the OFF-state that corresponds to disruption of the CFs.
In the CF model, the formation and rupture of local conducting paths connecting the two-terminal electrodes dominates the RS process. [36,38,117,118] Liu et al. have performed real-time characterizations of the CF formation and dissolution processes of an oxide-electrolyte-based RRAM device, [36] which is based on a vertical Ag(Cu)/ZrO 2 /Pt structure with both small lateral sizes and ZrO 2 thickness to facilitate direct TEM studies. During the SET process, a dark nano-bridge region appears, signifying the formation of CF [ Fig. 3(a)]. Successive RESET process has been performed, and a first RESET voltage sweep is found to decrease the electrical current passing through the device drastically [ Fig. 3  of the TEM specimen conducted at the initial-, ON-and OFF-states. [36] It has been found that the nanoscale conduction channel not only dominates the RS process but also shows some other effects, such as quantum size effects and exchange bias effect. For example, quantized conductance has been realized in zinc-oxide based sandwich devices. [35] As can be seen from Forming and Set processes. [35] It has also been revealed that by controlling the migration of ions through electrical fields, the magnetism of some magnetic dielectrics can be modulated. For example, Chen et al. have successfully modulated the magnetization and coercive field of Co-doped ZnO thin films. [123] Similar behavior has been reported in other materials such as Mn:ZnO [124] and Fe 3 O 4 . [125] In CoFe 2 O 4 (CFO) thin films, nanoscale magnetization reversal via electrical ion-manipulation at room temperature has been reported. [126] As is shown in Fig. 5 [126]

Electric field induced MIT
In thin film oxides, besides ionic transportation process, the RS phenomenon is associated with other mechanisms such as self-Joule heating effect [92] and electric field induced electronic phase transition. [91] The electric field induced electronic phase 13 transition has been predicted theoretically in charge-orbital ordered manganites, where applying an electric field may suppress the charge ordering and drive the system from the antiferromagnetic charge-ordered state to the ferromagnetic metallic state, resulting in the well-known MIT. [127] The electric field induced magnetism in manganites shows a new way of realizing the electrical control over magnetism. [128,129] Another very interesting and widely studied correlated material with electric field induced MIT is vanadium dioxide (VO 2 ). Bulk VO 2 exhibits MIT at temperature ~340 K. [130] The electric field was found to be able to drive MIT in VO 2 , accompanying with a structural change [131] with large isothermal entropy change. [132] Various electronic devices have been proposed based on the electric field induced MIT in VO 2 . [133,134] However, the underlying mechanisms have not been fully clarified. [135][136][137][138]

Electrical modulation of the carrier density
The electronic structure of some functional oxides (such as manganites) depend critically on the doping level, hence enables electrostatic field tuning of their physical properties. [139,140] The charge-mediated tuning of functionality (such as electric transportation and magnetization) in oxides has been studied both theoretically [141][142][143][144] and experimentally [96,145,146] in a number of heterostructures.
In perovskite manganites, the electrostatic field could modify the charge carrier concentration without affecting the lattice distortion, which is useful to study of the influence of band filling on the electronic properties. The only challenge comes from the high carrier concentration in FM manganites (with carrier density as high as 10 21 carriers per cm 3 ) that result in an electronic screening length on the order of only 1 nm (the Thomas-Fermi screening limit), which has limited the tunability of electrostatic field imposed by utilizing conventional dielectrics like SiO 2 , SrTiO 3 (STO), and ferroelectrics such as lead zirconate titanate (PZT). [140,147] In recent years, the electric double-layer transistor (EDLT) has been used to achieve high electric fields, enabling charge accumulation in the channel surface at densities as high as 10 15 per cm 2 . [148] This is almost two orders of magnitude larger than that in conventional field effect 14 transitions, thus providing a more accessible way to the electrostatic modulation of electronic phases. The effective tunability by using EDLT has been demonstrated in manganite transistors such as (La,Ca)MnO 3 (LCMO) transistors, [149] (Pr,Sr)MnO 3 (PSMO) thin films, [150] and (Pr,Ca,Sr)MnO 3 (PCSMO) thin films. [151] For example, in PCSMO thin films, it is found that, in consistency with phase diagram, the electrostatic modulation of the carrier concentration could effectively modify the phase transition temperature. The electrostatic modulation could also co-play with external magnetic field, resulting in the fine tuning of electronic phase and phase transitions. [150,151] A MIT can be induced by a gating field in VO 2 , and the electrostatic effect caused by the gating field was found by Nakano et al. to have overcome the Thomas-Fermi screening limit. [152] They suggested that the Thomas-Fermi screening limit does not apply to materials such as VO 2 , which is characterized by inherent collective   [153] The mechanism associates with the electric field induced oxygen migration has also been suggested by Cui et al., [154,155]   compressive strain. [155]

Strain effect
Applying strain on thin film oxides has been proved an efficient way for tuning their physical properties. Exerting strain on thin films could induce distortions of the film unit cell, which changes the elastic energy and modifies their electro-magnetic properties. [156] In some cases, applying strain could alter the ground state of thin film oxides and novel phases are disclosed. There are three ways to apply strain in thin films: mechanical strain, [157] epitaxial strain, [158] and strain that due to the converse piezoelectric effect. [159] In recent years, inspired by the idea of wearable electronic devices, there has been increasing interest in the study of flexible electronic devices, [160][161][162][163][164] and the strain effect in these devices has been intensively investigated.

Strain tuning of the electronic phases
In some transitional metal oxides, the electronic phases are very sensitive to external strain, [165,166] resulting in highly sensitive electrical and magnetic response to external strain field. For example, the strain effect on the electrical properties of manganites thin films has been studied by using the converse piezoelectric effect, the resistance of manganite thin films is found to change dramatically under small strain, resulting in very large strain gauge factor. [159,[167][168][169] The electrical and magnetic response to external strain is essentially mediated by the piezostrain coupled orbital reoccupation [170] and electronic phase separation. [169] Zheng et al. investigated the influences of piezostrain on the electronic transport and magnetic properties of charge-ordered La 0.5 Ca 0.5 MnO 3 thin films. [169] An unprecedented large strain-induced resistance change is found to be magnetic-field tunable [ Fig. 8 Fig. 8(a) and Fig. 8(b)]. Besides the large strain induced resistivity change, applying anisotropic strain field is found to induce highly anisotropic resistivity in perovskite manganites. [171,172]  and (f) XRD θ-2θ scans for the ferroelectric substrate and LCMO film when the ferroelectric substrate was in the // r P and r P  states, respectively. [169] The substrate strain could sometimes induce new phases with dramatic electronic transportation properties. For example, it has been predicted theoretically that in 18 half-doped manganite, the ferromagnetic charge ordered insulating state, previously inaccessible to experiments, becomes stable under tensile strain. [173] Recent report on LSMO thin films deposited on DyScO 3 (DSO) substrate has evidenced a new phase at low temperature with dramatic anisotropic transportation properties [ Fig. 9(c), 9(d) and 9(e)]. [158] The new phase shows preferred occupancy of the Mn 3d x 2 -y 2 orbital at low temperature, which is consistent with the theoretically predicted A-type AFM phase [ Fig. 9(a)]. [165] However, abnormal in-plane transport anisotropy is observed in this low-temperature phase [ Fig. 9

Strain control of ferroelectric domain structures
In ferroelectric materials, the formation of ferroelectric domain structures is a consequence of minimizing the elastic and electrostatic energy of a ferroelectric system. Hence applying strain can effectively tune the ferroelectric domain structures, which could be useful for further exploring the ME properties in multiferroic systems.
Both theoretical and experimental studies have been performed on controlling the ferroelectric domain structures of BFO thin films, and strain constraints have been found to be an effective way for controlling the domain structures of BFO thin films.
Zhang et al. [40]  Chu et al. [175] demonstrated the creation of ordered arrays of the prototypical domain structures predicted before. [176]  Piezoelectric materials with large piezoelectric coefficients (such as lead-based 20 perovskites PZT, PMN-PT, and PZN-PT) are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary, [177] and an electrically switching from one phase to the other. [178] By using a combination of epitaxial growth techniques in conjunction with theoretical approaches, Zeches et al.
has shown the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric BFO films. [41] The tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, making this new lead

Strain mediated electrical control over magnetism
A number of works have been devoted to the studying of composite multiferroic films, [96,179,180] most of which are composed of ferroelectrics and magnetic materials with layered heterostructures. In these composite multiferroic films, changes in magnetic anisotropy and coercivity can be realized by applying a strain, [181,182] which is caused by applying an electric field to the piezoelectric layer. The strain-mediated electric field control of magnetism has been observed in a number of epitaxial heterostructures, such as ferromagnetic LSMO films, [183] ferromagnetic CFO films [184] and metals grown on PMN-PT single crystal substrate, and ferromagnetic LSMO films [95] and metals [185] grown on BTO substrate. A giant electrical-field-induced magnetic anisotropy has been evidenced in these layered multiferroic heterostructures, and the strong ME coupling is used to control the orientation of magnetization and thus enable dynamically tuning of magnetoresistance in giant magnetoresistance (GMR) and anisotropic magnetoresistance (AMR) devices. [186,187] In most strain mediated electrical control over magnetism case, the M-E loop is butterfly-shaped, tracking the butterfly-shaped piezo-strain curve of the ferroelectric substrate, which clearly demonstrates the vital part of the elastic strains for the converse ME response in the heterostructure. For the application of information storage, tunable and nonvolatile converse ME effects are two important requirements, 22 hence loop-like behavior of the magnetic response to electric field is desirable.
Therefore great efforts have been devoted to the non-volatile control over magnetism. [88,[188][189][190][191][192]  heterostructures. [180,190] Yang et al. reported piezostrain-mediated nonvolatile rotations of magnetic easy axis (MEA) in the Co/PMN-PT multiferroic heterostructure at room temperature. [193] They found a reversible and non-volatile 90° rotation of magnetization [ Fig. 11A  used to return the magnetization to stage 1 from stage 4. [193] Another way of realizing non-volatile electric-field-controlled magnetization is by inducing two different remnant strain states in the single crystal piezoelectric substrate. [194] However, such a way is not practical for real applications, as the remnant strain states strongly depends on the history of applied electric fields.
In the (001)-oriented PMN-PT substrate, a bipolar loop-like non-volatile strain has been reported, and this is ascribed to the ferroelastic domain switching that constructed a third way for realizing the non-volatile strain state. [87,88] Although much remains to be done for a precise control of the ferroelastic domain switching process, the non-volatile strain associates with it has been proven promising for realizing the non-volatile control over magnetism. For example, S. Zhang and co-authors [195] reported a large and nonvolatile bipolar electric-field-controlled magnetization in  . [195] 25

Flexible electronic devices
In recent years, flexible electronic devices, which offers advantages of being lightweight, foldable, and stretchable, has attracted special attention. Although a great deal of flexible electronic devices was made of organic materials, there has been an increasing trend in the exploration of flexible inorganic devices, such as thin-film transistor, [196][197][198][199] magnetoelectronics, [164,200,201] and memories. [202][203][204][205] In this part, we merely review the progress on flexible RRAM devices based on oxides.
In RRAM devices, the RS is dominated by the formation and rupture of the CF, as the CF is of small size and perpendicular to the film plane, hence the RS phenomenon can endure moderate strain along the film plane. There have been some reports of flexible memristive memories with a simple cross-point-type array based on various oxide such as GeO x /HfON, [206] aluminum oxide, [207] zinc oxide, [202,208] and TiO 2 , [209,210] Fig. 13(a)]. [212] The ultrathin WO 3   respectively. Inset is a digital photograph of a bent RRAM device. [212]

Magnetic field effect
The study of magnetic field on the physical properties of oxide thin films is an important issue in condensed matter physics, which also shows temping prospect of applications in spintronic devices and magnetic sensors. The discovery of colossal magnetoresistance (CMR) effect, as well as the investigation of full oxide magnetic tunnel junctions (MTJ) has greatly prompt the study of magnetic field on thin film oxides. The effect of magnetic field on the electrical properties of oxide thin films has been studied in order to combine charge-tuning devices with spintronics devices. 27

Magnetoresistance effect
In perovskite manganites, applying magnetic field usually facilitates double exchange interaction which favors metallic conductivity, inducing the well-known CMR effect. [213] The CMR effect has been widely studied in manganite, [26,28,29,[214][215][216] and it always shows a peak value near the phase boundary, which reflects the fact that the phase competition is susceptible to external magnetic field. [27] In recent years, it has been found that despite the near cubic structure of perovskite manganites, the electrical transportation behavior actually depends strongly on the magnetic field direction, resulting in very large AMR effect in manganite single crystals [217,218] and thin films. [219][220][221][222][223] The AMR effect shows different magnetic-field and temperature dependence from that in 3d ferromagnetic alloys, suggesting different mechanisms behind. [217,224,225] The AMR effect in manganite thin films can be tuned by epitaxial strain and has attracted lots of attentions. [221,[226][227][228][229]

Magnetic coupled RRAM devices
The magnetic field couples with RRAM devices in two different ways. One is by the effective tuning of the resistance switching process, and the other is by realizing more resistance states through magnetoresistance effect. Thakare et al. [230] reported a bipolar RRAM of CFO/LSMO which can be gated with high sensitivity by low magnetic fields. The switching shows a strong CFO thickness dependence only under applied magnetic field, suggesting an effective coupling with the interfacial magnetoresistive LSMO layer. Li et al. demonstrated three-state RS in films combining multiferroic CFO/PZT bilayer with semiconductor ZnO layer. [231] The three states are distinguished by various stimuli combination of electric voltage pulse and magnetic bias, and are attributed to changes in the charge carrier states modulated by magnetoelectric coupling between CFO and PZT layers, as well as interface polarization coupling between PZT and ZnO layers. In a Si-SiO 2 -MgO device, the nonvolatile RS effect can be controlled by applying a magnetic field. [232] The magnetic field will delay the transition from HRS to LRS, and could even suppress 28 this transition. Das et al. [233] observed sharp switching between low and high conducting states having a large ratio of currents (~10 7 ) in NiO film. Besides the switching effect, an extraordinary large magnetoconductance (~98% at 20 K) has been found in the device, signifying multifunctional applications.
The RS effect can be utilized as a simple and controllable way to produce nanoscale devices, such as an artificial MTJ. [234] The spin transportation in memristive devices has also been reported in a number of MTJs, [235][236][237][238] it is found that applying an electric field may induce stable, reversible two-resistance states with substantial magnetoresistance change. Li et al. reported the use of composite barrier layers of CoO-ZnO to fabricate the spin memristive Co/CoO-ZnO/Co MTJ. [239] They found a bipolar resistance switching ratio high up to 90 [ Fig. 14(a)], and the tunnel magnetoresistance (TMR) ratio of the high resistance state reaches 8% at room temperature [ Fig. 14(b)]. At low resistance state, the TMR ratio vanishes [inset of Fig.   14(b)]. The bipolar resistance switching is explained by the MIT of CoO 1-x layer due to the migration of oxygen ions between CoO 1-x and ZnO 1-x [ Fig. 14(c)]. These novel phenomena enable integrating different functionalities into a single device, which has potential applications including multi-state storage devices and logic circuit. [238]  Co/CoO-ZnO/Co junctions. [239]

Magnetoelectric and magnetocapacitance effects
As has been mentioned above, the interplay between electric polarization and magnetization in multiferroic materials is very important for both fundamental physics and spintronic devices. Manipulation of magnetism by electric polarization has been manifested both in single-phase multiferroics and composites. Conversely, the possibility of electric polarization reversal induced by magnetic fields has been reported in multiferroics. Keeney et al. demonstrated switching of ferroelectric polarization by magnetic fields at room temperature in multiferroic Aurivillius phase thin films. [240] For practical applications, the weak ME coupling in single-phase multiferroics is always a big challenge. [241] Lu et al. reported a remarkable magnetically induced electric polarization in DyMnO 3 thin films, and they found an enhancement of electric polarization up to 800% upon 2 Tesla at 2 K. [42] An efficient coupling between magnetism and dielectric properties results in phenomenon such as magnetocapacitance effect. This phenomenon has been studied both in single phase materials [47,[242][243][244][245][246] and composite systems. [247][248][249][250] For example, the magnetocapacitance effect has been found in perovskite heterostructures such as LCMO/BTO [247] and LCMO/Ba 1-x Sr x TiO 3 [251] superlattices, manifesting an effective ME coupling in these structures. Colossal magnetocapacitance has been reported in electronic phase separated manganite thin films, and the competition of microscopic ferromagnetic metallic and charge ordered insulating clusters is believed to play a major role in the exhibited colossal magnetocapacitance effect. [245]

Photoinduced effect
Photo-coupled phenomena have attracted wide interest due to the special advantages of light, such as high speed, low power consumption and green energy, etc.
In thin film oxides, photoinduced effect such as photostrictive effect, [77] photoinduced electronic phase transition, [252] photovoltaic effect, [253] and photoinduced spin dynamics, [254] indicates a diversity of degrees of freedoms that couples effectively with light.

Photo-coupled RRAM devices
Light-controlled RS memory has photon-charge bifunction. The resistance state can be modified not only by voltage pulses, but also by means of light. M. Ungureanu et al. [255] reported a new type of RS memory device with a simple metal/Al 2 O 3 /SiO 2 /Si structure. In order to verify the performance of the devices when used as core of a binary memory cell, they observe that data can be accurately written, has also been found in ZnO NRs/Nb:STO Single Crystal Diodes [256] and ITO/ZnO NR/Au junction [257] .  [255] Recently, a multifunctional optoelectronic RS memory has been demonstrated in a simple ITO/CeO 2-x /AlO y /Al structure [ Fig. 16(a)]. [258] As a result of the photo-induced detrapping, electrode-injection and retrapping of electrons in the pW/μm 2 and 60 pW/μm 2 , respectively, renders promising multilevel storage capabilities. [258]

Photoinduced phase transitions
Photoinduced phase transitions have attracted special interest because they can 33 change the properties of complex materials on the ultrafast timescale. [259][260][261][262] In perovskite manganites, photoinduced insulator-to-metal transition has been frequently reported, [263][264][265][266][267][268] and the photo illumination has been found to coupled coherently in these materials, resulting in modifications of a variety of the physical properties due to the strong coupling between spin, charge, orbital and lattice. In PCSMO thin films, both photoinduced metal-to-insulator transition and photoinduced insulator-to-metal transition, have been confirmed. [269] This is a demonstration of photons as an effective way in overcoming the large potential barrier due to the long-range elastic energy between the insulator and metallic phase. Possibly due to the high energies associated with photons, the photoinduced state could differ greatly from states produced with stimuli close to equilibrium. For example, in a charge-orbital ordered Nd 0.5 Sr 0.5 MnO 3 thin film, the photoinduced state was found to be structurally ordered, homogeneous, metastable insulating state with crystallographic parameters different from any thermodynamically accessible state. [270] In vanadium dioxide (VO 2 ), a first-order transition between a high-temperature metallic phase and a low-temperature semiconducting phase has been evidenced. [130] A nonthermal driven ultrafast phase transition can be achieved under photo illumination. [271] The transition takes place near room temperature, and has been heavily studied for both fundamental physics and possible applications such as switching devices. [58,272] There has been hot debate regarding the driving force of this insulator-metal transition, and contributions of both electron-lattice interactions and electron-electron interactions have been proposed. [273,274] Advanced time-resolved methods have been developed to capture the phase transition processes, and much has been gained about the details during the transition processes. [274,275] For instance, Morrison et al. utilized ultrafast electron diffraction to monitor the crystal structure of VO 2 and simultaneously measured its optical properties to monitor the electronic state, and they have found an interesting metastable state that preserves the initial semiconducting crystal structure while shows metal-like properties. [275] 34

Photovoltaic effect in ferroelectric thin films
Another important consequence of photo illumination is the photovoltaic effect, which has been frequently studied in ferroelectric thin films. The ferroelectric-photovoltaic device, in which a homogeneous ferroelectric material is used as a light absorbing layer, has been investigated with numerous ferroelectric oxides. [276][277][278] The ferroelectric-photovoltaic effect is distinctly different from the typical photovoltaic effect in semiconductor p-n junctions in that the polarization electric field is the driving force for the photocurrent in ferroelectric-photovoltaic devices. [279,280] In addition, the anomalous photovoltaic effect, in which the voltage output along the polarization direction can be significantly larger than the bandgap of the ferroelectric materials, has been frequently observed in ferroelectric-photovoltaic devices. [253,281] For most ferroelectric materials, the relatively wide band gap limited the photovoltaic efficiency, and utilization of narrow-band-gap ferroelectrics is therefore a promising route towards their application in both the novel optoelectronic and the solar energy devices. [282,283] Multiferroic material BFO, which possesses a narrow band gap (~2.2 eV) offers an exciting opportunity for such applications. A significant photovoltaic effect has already been observed in single crystalline BFO in both bulk [279,284,285] and thin film forms, [286][287][288][289][290] but still needs improving. [253,[291][292][293] Yang et al. [253] studied the photovoltaic effect on the BFO film with ordered domain strips and lateral device configuration [ Fig. 17 the net polarization direction for the entire device structure. [253] In the ferroelectric based photovoltaic devices, it is suggested that the depolarization field may be the dominating driving force for the separation of photo-generated charge carrier-pairs. This depolarization field should be closely related to the degree of screening of the spontaneous polarization, which depends on both the properties of ferroelectric materials and electrode materials. Chen et al. [292] 36 studied the anomalous photovoltaic effect in the device with a structure of Au/BFO/Au and found that the photocurrent output was increased 25 times when the Au electrode was replaced by ITO. The great enhancement of photocurrent was attributed to the increased depolarizing field.
Furthermore, the photovoltaic effect in ferroelectric films has also provided a new method to sense the polarization direction non-destructively in ferroelectric memory.
For example, Guo et al. reported a novel approach to create a non-volatile memory based on the ferroelectric polarization-dependent photovoltaic effect. [294] Recently, Wang et al. presented a new non-destructive readout by using photo-recovered surface potential contrast. [295] By introducing laser illumination, they found that the surface potential contrast between the adjacent oppositely polarized domains can be ‗‗on'' and ‗‗off'' represent laser on and laser off, respectively. [295]

Conclusions and outlooks
Oxide thin films exhibit interesting and sometimes unexpected property changes upon external stimulus. These novel phenomena provide the prototype for the next generation of devices. Although many progresses have been made in the systems, there are still many challenges and opportunities. As an example, the emergent phenomenon at oxide interface is an interesting yet not well explored subject. [62,66,73] In multiferroic based devices, a central idea is the electric control of magnetism that would enable scalable and green energy spintronic devices [296] . Although the electric induced energy-efficient control of a spin-valve device at room temperature has been demonstrated in BFO based heterostructures, [99] there is challenge in improving 38 reliability of the devices, possibly due to oxidation of the ferromagnetic metal at the interface under the large electric field. In this sense, study of full oxide spintronics would be of importance to make the device robust. [98,297] In RRAM based devices, the study of ion transportation have brought about new direction toward physics in nanoscale, and it could be useful in designing novel electronic devices with nanoscale sizes. Difficulties lie in effective characterizations of the ionic channels, and an effective way for improving the stability and uniformity of the nanoscale devices.
Given the strong composition-structure-property relationship in many complex oxides, thin film oxides show sizable and sometimes unexpected response to external stimuli, making them a wonderful playground for studying fundamental physics and exploring novel devices. In this case, in-situ structural and chemical characterizations during film growth are desirable.