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Electrical-Transport and Magnetodielectric Properties in YMnO3/ La0.67Sr0.33MnO3 Heterostructure Yanan Zhao,†,‡ Yansheng Rao,† Bingcheng Luo,† Changle Chen,*,† Hui Xing,† Liwei Niu,† Jianyuan Wang,† and Kexin Jin† †

Shaanxi Key Laboratory of Condensed Matter Structures and Properties, Northwestern Polytechnical University, Xi’an 710072, China ‡ College of Engineering Management, Shaanxi Radio and Television University, Xi’an 710019, China S Supporting Information *

ABSTRACT: In this work, the heterostructure YMnO3/ La0.67Sr0.33MnO3 was deposited on SrTiO3(110) substrate by pulsed laser deposition. X-ray diffraction measurements indicated that YMnO3 film consists of coexisting hexagonal phase (h-YMnO3) and orthorhombic phase (o-YMnO3). In the low temperature range from 20 to 200 K, the transport mechanism is dominated by the Schottky emission model. In the high temperature range above 240 K, a space-chargelimited-current behavior is observed under high electric fields. Remarkable magnetoresistivity effects were detected for H = 0.8 T, with MR = −16% near the Néel temperature of oYMnO3, T′N, and MR = −54% near the Néel temperature of hYMnO3, TN. At f = 10 kHz, there exist two anomalies of the dielectric constant without an applied magnetic field at about 40 and 80 K, which correspond to the TN′ value of o-YMnO3 and the TN value of h-YMnO3, respectively, demonstrating the coupling between the antiferromagnetic and ferroelectric properties at low temperature. Additionally, the distinct magnetodielectric response of the film originates from the orthorhombic phase rather than the hexagonal phase.

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been taken as a signature of magnetoelectric coupling.10 Neutron diffraction measurements by Munoz et al. showed that the o-YMO perovskite presents a sinusoidal magnetic structure below TN′ ≈ 40 K.12 However, investigations of the electrical-transport and magnetodielectric properties in the coexisting h-YMO and o-YMO phases in YMO thin films are still insufficient. It is of general interest to analyze their relationship in the magnetoresistance (MR) and magnetodielectric (MD) properties of YMO thin films, especially the phase transition in the low-temperature region. Not only because of its fundamental interest but also it offer useful guidelines in multiphase films that may be well have potential applicability in magnetoelectric devices or optoelectronic devices. Presented herein is an investigation of the leakage, MR, and MD properties of YMO thin films consisting of coexisting hYMO and o-YMO phases obtained by the pulsed laser deposition (PLD) technique. It is expected to offer useful guidelines for further understanding the electrical-transport and magnetodielectric properties of YMO thin films.

INTRODUCTION Multiferroic materials, combining magnetic and ferroelectric orders in both single- and multiphase forms, have been extensively studied because of their potential applications in lower-power consumer and denser devices.1,2 In particular, YMnO3 (YMO) has attracted the most attention, with great interest regarding its intriguing structural and magnetoelectric properties. YMO crystallize in two structural phases: the hexagonal phase (h-YMO) and the metastable orthorhombic phase (o-YMO). h-YMO is the well-known multiferroic, which belongs to the noncentrosymmetric P63cm space group and has a ferroelectric Curie temperature of TC ≈ 913 K and an antiferromagnetic Néel temperature of TN ≈ 80 K.3−5 Otherwise, the distorted perovskite o-YMO (belonging to the Pbnm space group) also allows for the existence of ferroelectricity, with a ferroelectric Curie temperature of TC ≈ 30 K and an antiferromagnetic Néel temperature of T′N ≈ 40 K.6−8 Many previous reports have focused on the structural, magnetic, magnetodielectric, and nonlinear optical properties of single-phase YMO.9−12 Fiebig et al.9 visually observed the coupling of magnetic and ferroelectric domains in YMO by using the optical second harmonic generation technique. Polycrystalline o-YMO has been reported to display a remarkable dielectric constant anomaly below T′N that has © XXXX American Chemical Society

Received: June 13, 2016 Revised: September 18, 2016

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DOI: 10.1021/acs.jpcc.6b05969 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

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EXPERIMENTAL PROCEDURES For this study, YMO and La0.67Sr0.33MnO3 (LSMO) targets were prepared by the sol−gel method and the solid-state reaction technique, respectively. YMO is on the edge of stability between orthorhombic and hexagonal phases, with the hexagonal phase being stable under normal synthesis conditions whereas the metastable orthorhombic phase can be prepared at high pressure13−15 or with epitaxial growth on different substrates16−18 and soft chemistry procedures.19,20 It has been reported that the phase stability of YMO thin films depends on the choice of suitable substrate and oxygen pressure.16 Hence, the bilayer YMO/LSMO thin film was successively deposited on SrTiO3(110) [STO(110)] substrate by the PLD method with a laser wavelength of 248 nm and a repetition rate of 1 Hz. An LSMO film as the bottom electrode was deposited first on the STO(110) substrate at 1073 K under an oxygen pressure of 8.0 Pa. Subsequently, the YMO film was deposited at 973 K under an oxygen pressure of 2.0 Pa and then annealed in situ at 973 K for about 30 min. The phase structures were characterized by X-ray diffraction (Rigaku D/max-2400 using Cu Kα radiation with a wavelength of 0.15432 nm). For electric measurements, Pt was sputtered on the surface as a top electrode by using a circular shadow mask at room temperature. The leakage current versus applied electric field (J−E) characteristics of the YMO/LSMO heterostructure were measured using Keithley electrometers (6487), and dielectric measurements were performed with an Agilent 4980E LCR meter. To study the MR and MD effects, a magnetic field was provided by an ordinary electromagnet applied parallel to the film; the maximum magnetic field intensity was 1 T. Temperatures ranging from 20 to 300 K were provided by a low-temperature system.

setting (a = 5.26 Å, b = 5.85 Å, and c = 7.36 Å in bulk YMO) and the P63cm setting (a = 6.14 Å and c = 11.4 Å in bulk YMO) from the JCPDS data card were used. According to a comparison with the XRD diffraction patterns of o-YMO and h-YMO, in the 2θ range from 32.5° to 34°, the peaks correspond to the (021) and (112) planes of o-YMO and the (112) plane of h-YMO, through a peak resolution and fitting technique; in the 2θ range from 67° to 69°, the peak corresponds to the (108) plane of h-YMO, as shown in the two insets. This indicates that the YMO thin film deposited on STO(110) is polycrystalline and consists of simultaneously coexisting h-YMO and o-YMO phases. However, for deposition on STO(100) and STO(111) substrates under the same conditions, the YMO thin films were found to be in the single o-YMO phase. Perhaps in the competing phase selection of the YMO film on STO(100), STO(110), and STO(111) substrates (Figures S1−S3), epitaxial stress is not the dominant factor,21 and other aspects, such as oxygen pressure or electronic interactions, could determine the YMO phase. One of the most important characteristics of multiferroic films is the leakage current behavior, because it provides information on the charge-transport mechanism, which directly affects the polarization properties of the material. Figure 2a shows the leakage current versus applied electric field (J−E) characteristics of the YMO/LSMO capacitor measured with the current perpendicular to the plane geometry of the sample at different temperatures from 20 to 300 K.The J−E curves are largely symmetric, which indicates the fine contact conductivity of LSMO as the bottom electrode. The leakage current was found to increase with increasing temperature, suggesting a thermally assisted conduction process. The temperature dependence of the leakage current level above 240 K changed significantly, indicating that different leakage mechanisms could exist in different temperature ranges. A large number of possible leakage-current-limiting mechanisms have been discussed in the literature.22−24 By plotting the data in various manners as a function of voltage, we found that two types of leakage mechanisms are responsible for the electrical transport in the YMO/LSMO heterostructure in different temperature ranges and electric fields. The first mechanism is interface-limited Schottky emission. The current density across a Schottky barrier is given by22

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RESULTS AND DISCUSSION Figure 1 shows the XRD pattern of the heterostructure YMO/ LSMO deposited on an STO(110) substrate. It can be

1/2 ⎤ ⎡ 3 Φ 1 ⎛ qU ⎞ ⎥ ⎢ ⎜ ⎟ − J = AT exp − ⎢ kBT kBT ⎝ 4πε0εd ⎠ ⎥⎦ ⎣ 2

(1)

where A is the Richardson constant, Φ is the height of the Schottky barrier, kB is the Boltzmann constant, ε is the optical dielectric constant of the film, and d is the sample thickness. The second mechanism is space-charge-limited conduction (SCLC). The current density for SCLC is23 Figure 1. XRD pattern of the YMO/LSMO heterostructure on STO(110) substrate. Insets: Enlargements of parts of the XRD pattern, corresponding to the 2θ ranges from 32° to 34° and from 67° to 69°.

JSCLC =

9με0εU 2 8d3

(2)

where μ is the carrier mobility and ε0 is the static dielectric constant of the film. According to eq 1, Schottky emission can be investigated by plotting the leakage data as ln J−E1/2 in the temperature range from 20 to 200 K, as shown in Figure 2b. It is necessary to extract the optical dielectric constant from the slopes of these plots. Although experimental data for the optical dielectric constant of YMO are not available, Huang et al.5 reported the value of the static dielectric constant (εr ≈ 15 from

observed that the peaks correspond to the STO (110) and (220) planes and the LSMO (110) and (220) planes, indicating the growth orientation of the LSMO thin film along the substrate orientation. Within the experimental resolution of analysis of the phase structures of the YMO film, the Pbnm B

DOI: 10.1021/acs.jpcc.6b05969 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. J−E curves of the YMO/LSMO heterostructure at different temperatures. (a) Leakage data. (b) ln J−E1/2 plots in the temperature range from 20 to 200 K. (c) ln J−ln E plots in the temperature range above 240 K. Inset: R−T curve of the LSMO thin film.

Figure 3. (a) Temperature dependence of the resistance at H = 0 T (black curve), 0.4 T (red curve), and 0.8 T (blue curve). Inset: Enlargement of part of the R−T curves. (b) Temperature dependence of MR at H = 0.4 T (red curve) and 0.8 T (blue curve).

150 to 230 K); hence, our fitted values of ε (∼7.5) are reasonable. Such values are indicative of the leakage mechanism being dominated by the Schottky model in the temperature range from 20 to 200 K, which was also found for the YMO/ GaN interface by Wu et al.24 Furthermore, considering the rapid increase of the leakage current of YMO in the temperature range above 240 K, the leakage current exhibits ohmic behavior at a low electric field, but the SCLC mechanism was found to be dominant at high electric field, as shown in Figure 2c. When the electric field was greater than 85 kV/cm at 240 K, greater than 90 kV/cm at 260 K, and greater than 95 kV/cm at 280 K, the slopes of the fitting curves were about 2,

in agreement with SCLC theory. Thus, the leakage mechanism of the heterostructure YMO/LSMO in the temperature range from 240 to 300 K exhibits ohmic behavior and then changes to SCLC behavior with increasing electric field. Figure 3a shows the resistance as a function of temperature from 20 to 300 K under magnetic fields of H = 0, 0.4, and 0.8 T, where the bias voltage was applied from −1 to 1 V. Through a comparison with the resistance measured without an external magnetic field, it was found that the resistances were apparently decreased at H = 0.4 and 0.8 T, especially near the TN′ value of o-YMO (∼40 K) and TN value of h-YMO (∼80 K). At temperatures above 150 K, all of the curves decreased C

DOI: 10.1021/acs.jpcc.6b05969 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 4. (a) Temperature dependence of the dielectric constant of YMO film at f = 10 kHz at different magnetic fields: H = 0 T (black curve), 0.4 T (red curve), and 0.8 T (blue curve). (b) Temperature dependence of the dielectric loss of YMO film at f = 10 kHz at different magnetic fields: H = 0 T (black curve), 0.4 T (red curve), and 0.8 T (blue curve). Insets: Enlargements of parts of the (a) εr−T and (b) tan δ−T curves.

laser deposition technique. The leakage mechanism agrees with the Schottky model from 20 to 200 K, but it exhibits a transformation from ohmic to SCLC behavior with increasing electric fields in the temperature range above 240 K. An apparent magnetoresistance effect is observed, with MR = −16% at 40 K and −54% at 80 K for H = 0.8 T. The dielectric constant curves without a magnetic field show two kink-like anomalies near the TN′ value of o-YMO (∼40 K) and the TN value of h-YMO (∼80 K), indicating coupling between the antiferromagnetic and ferroelectric orderings at low temperature. Additionally, the results indicate a stronger magnetodielectric response in the orthorhombic phase than in the hexagonal phase in a magnetic field.

monotonically with increasing temperature. It should be noted that there is a phase transition of the YMO/LSMO heterostructure at about 150 K, which might affect the resistance variation. The magnetoresistance (MR) is defined as [(RH − R0)/R0] × 100%, where RH is the resistance of the YMO film under an applied magnetic field and R0 is the resistance without a magnetic field, as shown in Figure 3b. The MR of the YMO film was found to be negative and to increase markedly near the T′N value of o-YMnO3 and the TN value of hYMnO3, with values of −17% and −54%, respectively, at H = 0.8 T. Figure 4 shows the temperature dependence of the dielectric constant εr and the dielectric loss tan δ of YMO film under different magnetic fields of H = 0, 0.4, and 0.8 T at f = 10 kHz. As shown in Figure 4, the εr−T and tan δ−T curves without an external magnetic field both clearly show two anomalies near the T′N value of o-YMO (∼40 K) and the TN value of h-YMO (∼80 K). The result is consistent with previous reports, which suggested that the anomalies are indicative of coupling between the ferroelectric and antiferromagnetic orders near the Néel temperature in the YMO film.5,25 Below the TN value of hYMO, εr increases slowly and passes through a maximum at about 75 K. However, below the TN′ value of o-YMO, εr increases rapidly and does not pass through a maximum because of the limiting condition of the low-temperature system. From the increasing tendency, we conclude that the increase in εr below the antiferromagnetic phase of o-YMO is much larger than that of h-YMO. This indicates that the effects are small in the hexagonal phase but huge in the orthorhombic phase, as reported in previous studies.5,26,27 We also observed that the anomaly of o-YMO was still evident under nonzero magnetic fields, whereas the anomaly of h-YMO disappeared, as shown in Figure 4. This can be explained by the fact that the ferroelectric Curie temperature (∼913 K) of h-YMO is well above room temperature. Yet, at the Néel temperature (∼80 K), the electric polarization is rigid,5 and the magnetodielectric response is therefore much too small to be apparent in the curves under any effects of magnetic order or additional magnetic fields.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b05969. XRD patterns of YMO thin film on STO(100), STO(110), and STO(111) substrates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+86 13193308139. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 61471301, 51172183, 51402240, 51471134, and 51572222), National Natural Science Foundation of Shaanxi Province in China (No. 2015JQ5125), and Fundamental Research Funds for the Central Universities (No. 3102015ZY078).

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CONCLUSIONS In summary, a polycrystalline YMnO3 film consisting of coexisting h-YMO and o-YMO phases was successfully deposited on La0.67Sr0.33MnO3/SrTiO3(110) by the pulsed D

DOI: 10.1021/acs.jpcc.6b05969 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b05969 J. Phys. Chem. C XXXX, XXX, XXX−XXX