Multiferroic materials providing both ferroelectric and ferromagnetic (or antiferromagnetic) properties are becoming increasingly important in view of their potential applications in sensor, actuation, and ferroelectric memory.^{[1– 4]} Meanwhile, they are promising materials in multiple-state memories, electric-field-controlled ferromagnetic resonance devices, and transducers with magnetically modulated piezoelectricity due to the magnetoelectric effect.^{[5– 7]} However, materials of this kind are very rare, as the two contrasting order parameters of ferroelectricity and magnetism are mutually exclusive.^{[2, 3]} Besides, most of them reveal multiferroic properties far below room temperature. So far, BiFeO₃ is the only one that exhibits multiferroism at room temperature with its Curie and Né el temperature of 1103  K and 643  K, respectively. However, there are high leakage current, inhomogeneous magnetic spin structure, and weak magnetoelectric coupling in BiFeO₃, ^{[8, 9]} which restrict its applications in devices. Hence, it is a great challenge to seek new multiferroic materials with strong magnetoelectric coupling at room temperature.

Furthermore, among multiferroics currently under investigation, rare-earth manganites have been extensively studied owing to their tendency toward a strong magnetoelectric coupling. Among them, R_EMnO₃-type oxides with R_E = Ho– Lu, ^{[10, 11]} belonging to hexagonal manganites, possess c-axis-oriented hexagonal ferroelectricity owing to the disappearance of a mirror image on the a– b plane with a tilting of the MnO₅ bipyramid units in a polar P6₃cm unit cell, including YMnO₃.^{[12, 13]} Note that YMnO₃ is considered to have another structure which is similar to R_EFeO₃, that is, nonferroelectric orthorhombic structure with the space group of Pbnm.^[14] Similarly, R_EFeO₃ may also exhibit hexagonal structure with dramatic multiferroic properties.^[15] The claim proved to be true. Hexagonal R_EFeO₃ can only be prepared by using methods other than conventional high-temperature solid state routes, for example solution-based precursor methods, ^[16] spray ICP, ^[17] or as an epitaxially-grown thin film.^{[18– 20]} Jeong et al.^{[18– 20]} first fabricated the hexagonal YbFeO₃, LuFeO₃, and YFeO₃ films epitaxially on Al₂O₃ (0001) substrate and testified the existence of ferroelectricity in these antiferromagnetic films. In YFeO₃ film, they attribute the electronic origin of this artificially imposed ferroelectricity to the asymmetric orbital overlapping along the c axis of P6₃cm over the T– M first Brillouin zone.^[20] Even so, the origin of multiferroism in YFeO₃ is controversial as it suffers from an insufficient understanding on leakage current, ferroelectric and magnetic mechanism.

In this study, we prepare the YFeO₃ film on Si(111) substrate by sol-gel spin-coating method, and investigate its crystal structure, surface morphology, leakage current characteristics, and ferroelectric properties. Meanwhile, its domain structures and switchable behavior are identified by piezoresponse force microscopy (PFM) also. The experimental results show that the hexagonal YFeO₃ film exhibits ferroelectricity at room temperature.

The YFeO₃ film was prepared by a conventional sol-gel spin-coating method. In the preparation experiment, n-type (111) orientation silicon wafers with a resistivity of 8– 10  Ω · cm are used as the starting substrates. Fe(NO₃)₃· 9H₂O and Y(NO₃)₃· 6H₂O were used as the raw materials and dissolved in ethylene glycol diethylether. The citric acid was added as the stabilizer to the solution subsequently, forming a solution mixture with 0.1  M Fe(NO₃)₃, 0.1  M Y(NO₃)₃, and 0.3  M citric acid. The solution was stirred for 3  h and then aged for 24  h at room temperature to yield a precursor solution. The precursor solution was dip-coated on Si substrate and then spun at 1500  rpm. The as-deposited film was dried in air at 453  K for 4  min. These steps were repeated five times. Finally, the film was pyrolyzed to evaporate the solvent and remove organic residuals at 723  K for 15  min and was sintered at 1223  K for 2  h in air. The film produced was approximately 160  nm thick, as measured by spectroscopic ellipsometry (SE, Spec EI-2000-VIS).

The structure was analyzed by x-ray diffraction (XRD, D/Max2550VB+/PC, Rigaku) with a thin-film attachment and a CuK_α x-ray source was used. The incidence angle was fixed at 1.5° during scanning to increase the diffracted x-ray intensity of the film. The surface morphology and local piezoelectric response were investigated by atomic force microscope (AFM, MFP-3D-SA, Asylum Research) equipped with dual AC resonance tracking piezoresponse force microscopy (DART-PFM).^[21] Measurements were taken using a Pt-coated tip (AC240TM electri-levers from Olympus with cantilever length l of 240  μ m, resonant frequency of 70  kHz, spring constant k of 2  N· m^{− 1}). Vertical PFM measurements were performed at 800  kHz. The tip approaches the surface vertically until the deflection set point is achieved. The deflection set point used was 0.8  V. The leakage current and ferroelectric hysteresis-loop measurements were tested with ferroelectric test systems (Precision LC, Radiant Technologies Inc, USA) at room temperature. Here, Ag was pasted on the surface as top electrodes.

Figure  1 is the thin-film x-ray diffraction (XRD) pattern of the film deposited on Si(111) substrate. The main characteristic peaks corresponding to (100), (101), (102), and (106) planes of the hexagonal perovskite YFeO₃ film were observed at about 28.8° , 29.6° , 32.2° , and 55.4° , respectively, which exhibits the polycrystalline nature of the YFeO₃ film. The peaks deviate slightly from the standard pattern due to the lattice mismatch between YFeO₃ film and Si substrate. Besides, some Fe₂O₃ impurity was observed (marked with *), which does not show ferroelectric property.

Fig.  1.

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	Fig.  1. XRD pattern of the YFeO3 film on Si(111) substrates.

Typical current density (J) versus applied voltage (V) curve was measured from the Ag/YFeO₃/Si capacitor to study the leakage current behavior. Figure  2(b) shows the schematic of the test structure. Figure  2(a) shows that the J– V curve is substantially symmetric at low electric field and it becomes slightly different at high electric field. The maximal leakage current density under 5  V is about 5.4× 10^{− 6}  A/cm². It is speculated that the leakage current of pure YFeO₃ will be lower. As is well known, the impurity in thin film can influence its electrical properties notably. It has been reported that in BiFeO₃, ^[22] the second phase Fe₂O₃ originated from the non-stoichiometry is responsible for the high leakage of BiFeO₃, as it has lower resistivity and can constitute percolating conduction paths and other defects, e.g., at the interfaces between BiFeO₃ and Fe₂O₃.

Fig.  2.

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	Fig.  2. (a) J– V curve of the Ag/YFeO3/Si capacity, (b) schematic representation of capacity structure, (c) logarithm of the current density versus the logarithm of the voltage at positive bias, (d) logarithm of the current versus the logarithm of the voltage at negative bias.

The J– V characteristics of the ferroelectric film are to regard contributions from conductance itself and interfacial conductance between film and electrode. Five conduction mechanisms in ferroelectric oxide thin films were commonly considered, namely, thermal electric conduction, Schottky emission, space-charge-limited conduction, Poole– Frenkel emission, and Fowler– Nordheim tunneling.^{[21, 23]} Figures  2(c) and 2(d) show the dependence of log(J) on log(V) at positive and negative bias, respectively. As can be seen, the plots are linear at lower voltage. The thick solid lines are the linear fit and the slopes are both close to 1, indicating the normal Ohmic contact behavior. That is to say, the leakage current is dominated by thermal electric conduction itself. While at higher voltage above 4  V, log(J) versus log(V) plots have the slopes of 1.36 and 1.26, which agree well with space-charge-limited current (SCLC) theory. Thus, the mechanism of the leakage current of the YFeO₃/Si interface obeys an Ohmic behavior and then changes to an SCLC behavior by increasing the bias. This mechanism is similar to that of YMnO₃/n-Si interface reported by Parashar et al.^[23] Note that, the abrupt increase of the slope (> 2.64) occurs when the negative bias further increases (see inset in Fig.  2(d)) which could be explained by the FN tunneling model, indicating that the capacitor could be easy to breakdown after the SCLC process.

The hysteresis loops of the YFeO₃ film were measured at room temperature, subsequently. As presented in Figs.  3(a) and 3(b), the YFeO₃ film reveals ferroelectric characteristics. It can be seen that the polarization is weak with small remnant polarization and coercive field, which is similar to that of the hexagonal YFeO₃ epitaxial film by PLD method.^[19] Figure  3(a) shows the hysteresis curves at a frequency of 2  kHz under various applied electric fields. With the increase of electric fields, the polarizations increase accordingly. Under an applied electric field of 560  kV/cm, the remnant polarization (P_r) is about 0.07  μ C/cm² and the coercive field (E_c) is 100  kV/cm. The loops at different frequency were plotted in Fig.  3(b). It can be seen that polarization of the film is slightly different and the P_r value under low frequency is larger than that under high frequency. In general, polarization charges of ferroelectric contribute not only from intrinsic polarization, space charges which come from the substrate– film or film– electrode interfaces play a significant role as well. The space charge effect became indistinct with the increase of frequency, which induces the decrease of P_r under high frequency. Note that we could not obtain the saturated polarization (P_s) as the hysteresis loop became rounded under higher electric field (not shown here). A major problem encountered is that its ferroelectric property is hampered by its leakage behavior, related to impurity Fe₂O₃, boundaries, etc.

Fig.  3.

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	Fig.  3. (a) P– E ferroelectric hysteresis loops of the YFeO3 film under various applied electric fields, (b) P– E ferroelectric hysteresis loops of the YFeO3 film under different frequencies.

To further elucidate the ferroelectric properties of the YFeO₃ film, the topography and corresponding vertical piezoresponse images were captured by the standard PFM setup, as shown in Figs.  4(a)– 4(c). It can be seen from the topographic image that the film presents granular microstructure with good connectivity at the surface. YFeO₃ grains are irregular spherical in shape with some deformation and the average grain size is less than 100  nm. The corresponding root-mean-square roughness (R_RMS) measured at the surface evaluating surface roughness is about 15.8  nm. Such grains are characteristic of three-dimensional island growth (Volmer– Weber mode).

Fig.  4.

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	Fig.  4. PFM images of the ferroelectric domain structure of the YFeO3 thin film deposited on Si(111) substrate over the scanning size of 2  μ m× 2  μ m. (a) Topography image, (b) out-of-plane PFM amplitude image, (c) out-of-plane PFM phase image, (d) local amplitude loop and phase loop.

The amplitude image (Fig.  4(b)) provides the magnitude of the piezoelectric coefficient along the normal direction while the phase image (Fig.  4(c)) provides a great deal of information about the direction of polarization in the domains.^[24] The PFM phase image reveals a clear piezoelectric contrast associated with the direction of the polarization, with bright (yellow) and dark (violet) contrasts. Bright contrast indicates that the polarization of the domain is upward and perpendicular to the surface, and dark contrast corresponds to downward polarization. Obviously, the upward and downward domains are almost 180° apart. Besides, gray (orange) contrast occurs also, indicating the weak vertical piezoresponse signal. The gray constrast can be attributed to the following two aspects. First, it is a polycrystalline film and its grains show various orientations, so there may be domains with the polarization vector deviating from the direction normal to the film plane. According to Zeng, ^[25] different grain orientations of the film can result in the gray domain contrast. Second, the non-ferroelectric structure should be taken into account, based on the XRD data that reveal that the film contains the second phase of Fe₂O₃. Comparing PFM images simultaneously acquired with the topographic image, we find that it is not the one-to-one correlation between topographic and piezoresponse signals. A bright or dark contrast can belong to two or more grains, which means that grain boundary may not act as barriers for the domain-well motion.

Meanwhile, we measure the local amplitude and phase response loops under DC bias with a triangular waveform at a point in the desired domain regions, as shown in Fig.  4(d). A well-defined hysteresis phase loop as well as a butterfly-shape amplitude loop was observed, indicating the ferroelectricity in the YFeO₃ film further. The phase changes almost 180° (from 60° to 240° ), which implies a nearly complete polarization switching behavior.

Note that the asymmetric loops were observed. The amplitude loop as well as the phase loop is simultaneously shifted toward the positive voltage. Similar results have been reported elsewhere.^[26] The shift S value is 0.17  V, where S is defined as , with the right and left hand coercive voltage, respectively. This slight shift is caused partly by the asymmetry of the tip/film/bottom electrode. Secondly, the shift is considered as the evidence of the internal field by trapped space charges.^{[27, 28]} The asymmetric behavior suggests that the phase reversal phenomenon in the vertical phase images results not from the buckling effect of shear mode, but from the relaxation effect of ferroelectric domains.^[29]

In summary, YFeO₃ film about 160  nm with hexagonal structure was prepared on Si(111) substrate by sol– gel synthesis method. The leakage current density of the YFeO₃ film is 5.4× 10^{− 6}  A/cm² at a drive voltage of 5  V and shows good characteristics for practical applications. Three different regions, i.e., Ohmic contact, SCLC, and FN tunneling were observed in J– V characteristics. The P– E hysteresis loops at room temperature demonstrated ferroelectricity of the YFeO₃ film. Owing to the high leakage induced by the impurity of Fe₂O₃, the saturated polarization was not observed. Nanoscale domain structure and its reversal behavior of the film were also investigated by the PFM techniques, which further evidenced the polarization behavior of the YFeO₃ film. The slight shift of the amplitude loop and phase loop to positive voltage is caused by the asymmetry of the electrode and the interfacial-induced charges. The present work strongly promotes the need for better understanding of the multiferroism in the hexagonal ferrite film.