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Polarization Switching and Photo-induced Dielectric, Ferroelectric Properties in YMnO3/La0.67Sr0.33MnO3 Heterostructure Yanan Zhao, Bingcheng Luo, Hui Xing, Chang-Le Chen, Jianyuan Wang, and Kexin Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15341 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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ACS Applied Materials & Interfaces

Polarization Switching and Photo-induced Dielectric, Ferroelectric Properties in YMnO3/La0.67Sr0.33MnO3 Heterostructure Yanan Zhaoa,b, Bingcheng Luoa, Hui Xinga, Changle Chena*, Jianyuan Wanga, Kexin Jina a

Shaanxi Key Laboratory of Condensed Matter Structures and Properties, Northwestern Polytechnical University,

Xi'an 710072, China b

College of Engineering management, Shaanxi Radio and Television University, Xi'an 710019, China

Abstract: YMnO3/La0.67Sr0.33MnO3 heterostructure was fabricated on SrTiO3 (110) substrate by pulse laser deposition technology. The photo-induced resistance is markedly decreased in the low temperature region (20K to 300K), especially arresting at the T’C of o-YMnO3 (~30K), the T’N of o-YMnO3 (~40K) and TN of h-YMnO3 (~80K). The variance tendency of dielectric constant anomaly near the T’N of o-YMnO3 progressively decreases as the frequency increases, while the anomaly near the TN of h-YMnO3 is not observed in the same frequency region. Additionally, the dielectric constant is suppressed under photo excitation, and the variation increases with the raise of temperature and frequency. A distinct photo-induced suppression in the ferroelectric hysteresis loops is observed, maybe the trapping of photo-induced electrons to incur the reorientation of domain or increase the leakage current density. The obvious polarization switching in phase and amplitude images are observed when poling at ±8 V DC bias by Piezoresponse Force Microscopy technique at room temperature. Keywords: ferroelectric,

YMnO3/La0.67Sr0.33MnO3, photo-induced resistance,

photo-induced

dielectric,

photo-induced

PFM

Introduction The interaction between electron and photon drives a multitude of technologically relevant material properties including ferroelectricity, thermoelectricity, and phase-change behavior.1 Particularly, YMnO3 (YMO) is the well-known multiferroic, which crystallizes in two structural phases. The hexagonal phase (h-YMO) belongs to the noncentrosymmetric (P63cm space group), with the ferroelectric Curie temperature TC~913K and the antiferromagnetic Neel temperature TN~80K.2–4

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Otherwise, the metastable orthorhombic phase (o-YMO) belongs to the distorted perovskite (the Pbnm space group), with the ferroelectric Curie temperature T’C~30K and the antiferromagnetic Neel temperature T’N~40K.5-7Moreover, YMO has similar bandgap magnitudes with the prototypical semiconductor GaAs, as a promising photo-ferroelectric oxide material.8-10 It would attract more attentions on the intriguing structural, optic-electric and magneto-electric properties of YMO. The research of YMO is mainly focused on the structure, magnetic, magneto-dielectric and ferroelectric properties.11-13 Craig J et al11found that YMO was an improper ferroelectric by studying of the structure phase transition of multiferroic YMO from first principles. Polycrystalline o-YMO displayed a remarkable dielectric constant anomaly below T’N, which was taken as a signature of magneto-electric coupling.12 Zhou et al13 have indicated that the c-oriented YMO film showed excellent ferroelectricity with remanent polarization (2Pr) of 0.52 µC/cm2 and Ec of 80 kV/cm at an applied electric field of 300 kV/cm, whereas the (110) oriented YMO film showed poor ferroelectricity. However, the resistance、dielectric and ferroelectricity response of YMO films under photo excitation have rarely been studied till to date, particularly the photo-induced effects in the coexisting of h-YMO and o-YMO phases in thin film. The research in resistance and ferroelectricity behavior of YMO thin films under photo excitation is one of the strategies to a new class of the photo-induced cooperative phenomenon. And the wide applications of noble photo-electric devices are such as photo-memory, ferroelectric random access memory, photo-switch and so on. 14-16 In this paper, the bilayers YMO/La0.67Sr0.33MnO3 (LSMO) was grown on SrTiO3 (STO) (110) substrate with pulse laser deposition technology (PLD). And on this basis, the photo-induced changes of the resistance, dielectric and ferroelectricity response of YMO/LSMO heterostructure were observed by the application of purple light of 405 nm at different temperatures and frequencies. The measurement of the domain structure and polarization switching of YMO thin films were tested by applying the positive and negative biases with Piezoresponse Force Microscopy (PFM) technique at room temperature (RT).


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EXPERIMENTAL PROCEDURES

YMO and LSMO targets were prepared by sol-gel method and solid state reaction technique, respectively. The bilayer heterostructure YMO/LSMO was successively deposited on STO (110) substrate by the PLD method，the detailed deposition process has been described in our previous report.17 For electric measurements, Pt was sputtered on surface as top electrodes by using a circular shadow mask at room temperature. The resistance and dielectric property of YMO thin films are measured using 6487 Keithley electrometers and Agilent 4980E LCR meter, respectively. The ferroelectric hysteresis loops (P–E) were tested with ferroelectric test systems (Precision LC, Radiant Technologies Inc, USA). The temperature from 20K to 300K was provided by the temperature system. For studying the effects of optical radiation on YMO thin films, a semiconductor laser with wavelength of 405 nm and power density of 76 mW/cm2 served as a light source. The polarization switching, ferroelectric domain configuration and local piezoelectric response of YMO thin films were investigated by Atomic Force Microscope (AFM, MFP-3D-SA, Asylum Research) equipped with dual AC resonance tracking Piezoresponse Force Microscopy (DART-PFM). The vertical PFM measurements were taken in atmosphere condition at RT using a Gold-coated tip (AC240TM Electrical Cantilevers from Olympus with cantilever length l=240 µm, resonant frequency f~75 kHz, spring constant k~3 N/m). RESULTS and DISCUSSION Figure 1 shows the temperature dependence of resistances for YMO/LSMO heterostructure with and without photo excitation. Inset of Figure 1 shows the schematic diagram of the experimental setup, that the sample is illuminated with the purple light. The XRD pattern17 of heterostructure YMO/LSMO is shown on Figure S1 in supplementary data. It indicates that the YMO thin films deposited on STO (110) is polycrystalline in coexistence of the h-YMO and o-YMO phases simultaneously. Seen from the resistance curve without photo excitation, the phase transformations of YMO are clearly observed as the temperature changes, that the T’C of o-YMO(~30K), the T’N of o-YMO(~40K) and TN of h-YMO(~80K), as shown in Figure 1. Under the



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photo excitation, it is observed that the resistance decreases apparently, but still has the corresponding kinks, at the T’C of o-YMO (~30K), the T’N of o-YMO(~40K) and TN of h-YMO(~80K), respectively. The variation tendency indicates that YMO thin film has a dominant photosensitivity, in the form of a reduced resistance upon the purple light. The photo excitation may induce polarons break up to generate photo charge carriers to form jump conductance, hence the photo-induced resistance of YMO/LSMO heterostructure decreases in macro-scale.18 In addition, the phase transformations of YMO thin films move to higher temperature under photo excitation. Note that, it is a totally opposite of the phenomena that photo induced resistance of LSMO thin films, as shown in Figure S2. Figure 2 shows the dielectric constant εr and dielectric loss tanδ of YMO/LSMO heterostructure as the function of the temperature (20K to 300K) at f=100 kHz, 300 kHz, 500 kHz and 800 kHz with and without photo excitation, respectively. The εr-T curves as a whole decrease distinctly as the increasing temperature, all that clearly show the anomalies of dielectric constant near the T’N of o-YMO (~40K) at f=100 kHz,300 kHz,500 kHz and 800 kHz. The result is consistent with previous reports, which suggests that the anomaly is indicative of coupling between the ferroelectric and antiferromagnetic orders near Neel temperature of YMO thin films

4,19

or the

interfacial coupling with ferromagnetic order in LSMO thin films. However, the variation tendency of anomalies near the T’N of o-YMO progressively decreases with the increasing frequency, while the anomaly near the TN of h-YMO is not observed in the same frequency region. Besides, we have observed the anomaly near the TN of h-YMO at f=10 kHz in our previous paper.17 It may be caused that the ferroelectric Curie temperature (~913K) of h-YMO is well above room temperature, yet at the Neel temperature (~80K) the electric polarization is rigid.4 Therefore, the coupling between the ferroelectric and antiferromagnetic orders near TN of h-YMO is too much small to be apparent enough in the εr-T curves at lager frequency region. Figure 3 shows the dielectric constant εr and dielectric loss tanδ of YMO/LSMO heterostructure as the function of the frequency (100 kHz to 1 MHz) at T=40K,80K,110K,180K and 240K with and without photo excitation, respectively.


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The εr-f curves as a whole decrease distinctly as the increasing temperature, except for the abnormal εr-f curve at T=40K. Note that, the variation tendency of εr-f curves at different temperatures, especially at T=40K, that is consistent with the anomaly of εr-T curves near the T’N of o-YMO. Additionally, the peaks of εr-f curves shift to lower frequency parameters with the increasing temperature. For better observed the photo-dielectric effect of YMO/LSMO heterostructure, the variation of dielectric constant under photo excitation as the function of the temperature and frequency are plotted on Figure 4, respectively. The ∆εr is defined as (εr(laser)-εr(none))/εr(none), where εr(laser) and εr(none), are the dielectric constant with and without photo excitation. It evidently indicates that εr is suppressed under photo excitation, and the ∆εr increases with the raise of temperature and frequency. Additionally, the ∆εr-T curves also show dominant anomalies near the T’N of o-YMO (~40K) at f=100 kHz, 500 kHz and 800 kHz. Comparing the variation of amplitude in Figures 2, 3, and 4, the dielectric constant exhibits a faintish photo-induced effect. Considering these results, it concludes that there are obvious temperature and frequency dependency rather than photo-induced dependency in the dielectric constant and dielectric loss of YMO/LSMO heterostructure. As shown in Figure 5(a), it indicates the initial hysteresis loops of YMO thin films at different temperatures (T=40K, 90K, 160K, 220K and 300K) in fixed electric field of 40 kV/cm, at f=1 kHz. A gradually increase in coercive field (Ec) and remnant polarization (Pr) is observed with the increasing temperature. The Pr is 0.46 µC/cm2 and Ec is 30 kV/cm at T=300K, respectively. The hysteresis loops of YMO/LSMO heterostructure with photo excitation in the same testing parameters are shown in Figure 5(b). The sample could not reach fully saturations and exhibits poor loop preferably in higher temperature, may be due to the leakage current of the semiconducting nature in YMO and the resistivity of structure.20-22 Therefore, to eliminate the non-hysteretic polarization, the intrinsic Pr of YMO thin films was measured by the positive-up negative-down (PUND) method simultaneously, 23, 24 as shown in Figure S3. The 2Pr is 0.71 µC/cm2 in the applied electric field of 40 kV/cm at T=300K, is smaller than the Pr (Figure 5(a)) measured by a Sawyer-Tower circuit



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measurement, confirming the part played by the leakage current. It is important to note that, whatever using the PUND measurement or a Sawyer-Tower circuit measurement, the variation tendencies of photo-ferroelectric effect of YMO thin films are the same. For better analysis of photo-ferroelectric effect of YMO/LSMO heterostructure, we show the ∆Pr-T curve at different temperatures in inset of Figure 5(b). Here, ∆Pr is defined as (Pr(laser)-Pr(none))/Pr(none), where Pr(laser) and Pr(none), are the remnant polarization with and without photo excitation. A giant photo-induced ferroelectric effect is found that the P-E loops are suppressed distinctly under photo excitation in fixed electric field. The ∆Pr is found to be decreased when the temperature increases. Hence, the change of ferroelectric polarization of YMO/LSMO heterostructure is unlikely to originate from the temperature increased by photo-induced. The possible reason of photo-induced changes in ferroelectric materials, such as crystal BaTiO3, polycrystalline PZT and BiFeO3 thin films, which are demonstrated based on the trapping of the photo generated charge carriers at domain/grain boundaries and the mobility of ferroelectric domains under photo excitation.

25-28

Therefore, the

photo-induced hysteresis suppression of YMO thin films could be accounted for the result of the trapping photo-generated charges create a space-charge field, which possibly minimizes the remnant polarization. Figure 6(a), (b), (c) show the surface topography, PFM amplitude and phase imagines of YMO thin films over a scanning area of 2×2 µm2, respectively. The PFM imagines are detected under the Dual AC model with drive AC voltage (1.5 V) and drive frequency (296 kHz) by applied between the tip and the bottom electrode. Figure 6(a) shows atomically flat surface with the corresponding root mean square (RMS) roughness of 360 pm. Note that, domains with up (P+) and down (P−) polarizations give rise to bright (yellow) and dark (violet) contrasts.29, 30 The OP phase images in different colors associated with different local polarization orientations, it obviously reveals the intrinsic ferroelectricity of YMO thin films, as shown in Figure 6(c). Comparing the topographic and phase images simultaneously, the bright or dark


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domain belongs to two or more YMO grains. It means that grain boundaries may not act as barriers for the domain wall motion. Presentable butterfly-shape amplitude loop and hysteresis phase loop at RT are observed, as shown in Figure 6(d), indicating a switchable spontaneous polarization in YMO thin films. The phase change is about 180°, which implies a complete polarization switching behavior. Moreover, the amplitude loop as well as the phase loop is simultaneously shifted toward the negative voltage. The voltage shift of the hysteresis loops in PFM measurement to a certain polarization state can be explained by the formation of space charges at the film/electrode interface. 31, 32 Therefore, the loops of YMO thin films would be shifted toward negative voltages in a positively polarized capacitor. Note that, the defects of YMO thin films, such as the oxygen vacancy or space charge would affect the PFM domain images characterization.33 To more deeply understand the evolution of the ferroelectric domain polarization switching in our sample, we applied a series of DC biases (±2 V, ±4 V, ±6 V, and ±8 V) on the tip to switch the domain orientation. The sequential polarization switching tests clearly show that the higher voltages (±6 V and ±8 V) could cause distinct PFM phase changes, while the lower voltages (±2 V and ±4 V) were too small to reverse the domain(Figure S4~S7). Figure 7 shows the OP PFM images of YMO thin films obtained after 3×3 µm2 was poled with ±8 V DC bias by following the pattern of the capital initial letter of Northwestern Polytechnical University (NPU). The pattern of NPU is distinct in phase image, which is the rectangular boundary between the violet and yellow tone. The phase in the unpoled region is roughly monodomain of downward on account of the smaller grain size, 34, 35 while the region poled under ±8 V is successfully switched upward. Moreover, the uncomplete domain switching may be ascribed to local stress across the grain boundary, domain wall densities and domain states (monodomain or polydomain) in different grains and impurities, etc.36 The amplitude phenomenon of ±8 V polarized also shows obvious differences, which proves the polarization switching and piezoelectric response of YMO thin films. CONCLUSIONS



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A markedly photo-induced resistance response of YMO/LSMO heterostructure was observed in the temperature range from 20K to 300K, particularly near the phase transformations of o-YMO and h-YMO. The anomaly of dielectric constant near the T’N of o-YMO was observed at f=100 kHz~1 MHz, while the anomaly near the TN of h-YMO was not observed. The variation tendency of anomaly of dielectric constant progressively decreased as the frequency increased. Note that, the dielectric constant of heterostructure YMO/LSMO was less sensitive with photo-induced than with frequency and temperature ． In addition ， a distinct photo-induced ferroelectric polarization

suppression of YMO thin films was found in fixed electric field at

different temperatures. A clear patterns of NPU in phase and amplitude images were obtained, when the sample poling at ±8 V DC bias by PFM technique at room temperature. ASSOCIATED CONTENT Support information The XRD pattern of YMO/LSMO heterostructure on STO (110); the photo-induced resistance of LSMO thin films; PUND measurements of YMO/LSMO heterostructure; the Out-of-plane PFM images of YMO thin films after poling at ±2 V、±4 V、± 6V and ±8 V DC bias AUTHOR INFORMATION Corresponding Author *Changle Chen. Tel:+86 13193308139. Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (No. 61471301, 61078057, 51172183, 51402240, 51572222 and 51471134), and Fundamental Research Funds for the Central Universities (No. 3102015ZY078). REFERENCES (1) Jiang,M.P.; Trigo,M.; Fahy,S.; Murray,É.D.; Savić.I. Photoinduced Suppression of the Ferroelectric Instability in PbTe. Physics.2015.


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Figure and figure captions Figure 1.Temperature dependence of the resistance of YMO/LSMO heterostructure with and without the photo excitation. The Black line indicates the resistance measured without photo excitation, the red line indicates the resistance measured with photo excitation. Inset: the schematic diagram of experimental measurements. Figure 2. (a)The dielectric constant εr of YMO/LSMO heterostructure as function of the temperature (20K to 300K) at f=100 kHz, 100 kHz, 300 kHz, 500 kHz and 800 kHz. (b) The dielectric loss tanδ of YMO/LSMO heterostructure as function of the temperature (20K to 300K) at f=100 kHz, 100 kHz, 300 kHz, 500 kHz and 800 kHz. The circular curves indicate εr and tanδ without photo excitation, while the triangular curves indicate εr and tanδ with photo excitation. Figure 3. (a)The dielectric constant εr of YMO/LSMO heterostructure as function of the frequency (100 kHz to 1 MHz) at T=40K, 80K, 110K, 180K and 240K. (b) The dielectric loss tanδ of YMO/LSMO heterostructure as function of the frequency (100 kHz to 1 MHz) at T=40K, 80K, 110K, 180K and 240K. The circular curves indicate εr and tanδ without photo excitation, while the triangular curves indicate εr and tanδ with photo excitation. Figure 4. (a) Variation of dielectric constant of YMO/LSMO heterostructure as function of the temperature at f=100 kHz, 500 kHz and 800 kHz with photo excitation. (b) Variation of dielectric constant of YMO/LSMO heterostructure as function of the frequency at T=40K, 80K and 180K with photo excitation. Figure 5. (a) The initial hysteresis loops of YMO thin films at different temperatures. (b)The hysteresis loops of YMO thin films with photo excitation at different temperatures. Inset: The variation of Pr of YMO thin films under photo excitation at different temperatures. Figure 6. PFM images of YMO thin films over the scanning size of 2×2 µm2 at RT. (a) Topography image. (b) Out-of-plane PFM amplitude image. (c) Out-of-plane PFM phase image. (d)Local amplitude and phase loops. Figure 7. PFM images of YMO thin films over 5×5 µm2 after poling at ±8 V at RT. (a) Out-of-plane PFM amplitude image. (b) Out-of-plane PFM phase image. Note that


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the poling area of 3×3 µm2 is smaller than the scanning area.

Figure 1.

(b)

(a)

Figure 2.

(a)

(b)

Figure 3.



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(b)

(a)

Figure 4.

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(b)

Figure 5.

(a)

(b)

(c)

(d)

Figure 6.


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(a)

(b)

Figure 7.



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Polarization Switching and Photoinduced ... - ACS Publications

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