- Email: [email protected]
Multiferroic YCrO3 thin films: Structural, ferroelectric and magnetic properties
Accepted Manuscript Title: Multiferroic YCrO3 thin films: structural, ferroelectric and magnetic properties Authors: J.J. Gervacio-Arciniega, E. Murillo-Bracamontes, O. Contreras, J.M. Siqueiros, O. Raymond, A. Dur´an, D. Bueno-Baques, D. Valdespino, E. Cruz-Valeriano, C.I. Enr´ıquez-Flores, M.P. Cruz PII: DOI: Reference:
S0169-4332(17)32611-9 http://dx.doi.org/10.1016/j.apsusc.2017.09.011 APSUSC 37090
To appear in:
APSUSC
Received date: Revised date: Accepted date:
14-3-2017 20-6-2017 3-9-2017
Please cite this article as: J.J.Gervacio-Arciniega, E.Murillo-Bracamontes, O.Contreras, J.M.Siqueiros, O.Raymond, A.Dur´an, D.Bueno-Baques, D.Valdespino, E.Cruz-Valeriano, C.I.Enr´ıquez-Flores, M.P.Cruz, Multiferroic YCrO3 thin films: structural, ferroelectric and magnetic properties, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.09.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Multiferroic YCrO3 thin films: structural, ferroelectric and magnetic properties J. J. Gervacio‐Arciniega1,2,a), E. Murillo‐Bracamontes1, O. Contreras1, J. M. Siqueiros1, O. Raymond1, A. Durán1, D. Bueno‐Baques5, D. Valdespino3, E. Cruz‐Valeriano4, C. I. Enríquez‐Flores4, M. P. Cruz1 1
Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Km. 107 Carretera Tijuana-Ensenada, AP 14, Ensenada, B.C., México, C.P. 22860.
2
CONACYT-BUAP, Postgrado en Física Aplicada, Facultad de Ciencias Físico-Matemáticas, Benemérita Universidad Autónoma de Puebla, Av. San Claudio y Av. 18 sur, Col. San Manuel Ciudad Universitaria, Puebla, Pue. 72570, México. 3
Posgrado en Nanociencias, Centro de Investigación Científica y de Educación Superior de Ensenada, Carretera Ensenada-Tijuana 3918, Zona Playitas, Baja California, 22860, México. 4
5
CINVESTAV Unidad Querétaro, Lib. Norponiente 2000, Real de Juriquilla, 76230 Querétaro, Qro., México.
Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna Hermosillo No. 140, Saltillo, Coah., C.P. 25250, México.
Graphical Abstract
Highlights (for review)
Highly oriented YCrO3/SrTiO3 films grown by magnetron sputtering are reported. A structural, ferroelectric and magnetic study of YCrO3/SrTiO3 films is shown. These results will encourage further studies on origin of ferroelectricity in YCrO3.
Highly oriented and locally epitaxial multiferroic YCrO3 (001) thin films, 20 nm thick, were deposited by r.f. magnetron sputtering on SrTiO3 (110) substrates at 890ºC. The structure was investigated by x‐ray diffraction and cross section high resolution transmition electron microscopy, a clear local matching between the YCrO3 film and the substrate was observed. Ferroelectricity was confirmed by means of
a)
Author to whom correspondence should be addressed. Electronic mail: [email protected]
1
switching areas with opposite polarization directions, first and second harmonic electromechanical signals, and local hysteresis ferroelectric curves obtained by piezoresponse force microscopy. Additionally, below the Néel temperature, a clear ferromagnetic hysteresis loop was observed. These results will encourage further studies on the mechanism that promotes the ferroelectric nature in YCrO3 compound. Keywords: YCrO3 Highly oriented Multiferroic
1. Introduction Coexistence of ferroelectric and magnetic properties in a single compound gives rise to a new class of materials, called multiferroics, with promising applications in future technology. Among the few existing single phase multiferroics, YCrO3 has been one of the least studied mainly because its ferroelectric properties had been overshadowed by high dielectric loss [1–10]. It is well known that ferromagnetism in this compound is caused by a canted antiferromagnetic array; TN~140K [1,2,11], this property is quite reproducible in ceramics [2] and YCrO3 films, even as thin as 20nm, deposited on Pt [12]. Ferroelectricity, on the other hand, is associated to the peak around 430 K in the permittivity versus temperature curve, corresponding to a paraelectric‐ferroelectric transition. According to Ramesha et al [3], the origin of this transition is the local non‐centrosymmetry caused by the Cr displacement along the z‐direction of the perovskite‐type unit cell. However, polarization as function of voltage curves in bulk ceramics were so rounded [1] that it was not possible to elucidate whether polarization was due to free charge or to ferroelectricity. Yet, in a previous work on polycrystalline YCrO3 films grown on Pt/TiO2/Si substrates, it was demonstrated that decreasing film thickness down to approximately 20nm, diminishes free charge accumulation because of the reduction of grain boundaries [8]. Therefore, it would be reasonable to infer that, with no grain boundaries, as in epitaxial films, better ferroelectric properties should be attained, as was indeed found for other multiferroics, [13, 14]. Related to this matter, Seo and coworkers prepared YCrO3 films on Rh crystalline substrates and, even though epitaxy was not reached, better film orientation
2
allowed the observation of an improved, less lossy, ferroelectric hysteresis loop [9]. Still, the obtainment of YCrO3 epitaxial films, aim of this work, is necessary to further enhance the ferroelectric properties that allow performing basic studies on ferroelectric domain structure, polarization switching and ferroelectric‐ magnetic coupling mechanisms. 2. Experimental details YCrO3 films were deposited on SrTiO3(110) substrates, by r. f. magnetron sputtering as described in [8] using a deposition temperature of 890ºC. This temperature was chosen based in a previous work on films deposited on Pt substrates at room temperature, where a heat treatment at ~900ºC was needed to achieved a pure, well define YCrO3 crystalline phase [8]. Film thickness and film‐substrate interface were characterized by cross‐sectional transmission electron microscopy (TEM) in a JEOL JEM‐2010 instrument, also employed for the acquisition of electron diffraction patterns. X‐ray diffraction (XRD) data were collected in a Panalitycal X’pert Pro MRD diffractometer with monochromatic Cu‐Kα1 radiation, the pole figures were measured for a ψ‐polar angle range between 30° and 60°, in steps of 0.7°, and Φ‐azimuthal angle scanning from 0 to 360°, in steps of 1°, and exposition times of 3 s for YCO and 1 s for STO. The magnetization data as function of the magnetic field were taken with a Quantum Design 6000 physical properties measurement system (PPMS) with a vibrating sample magnetometer (VSM), at 40Hz with the applied magnetic field oriented along the film plane. An XE‐70 Scanning Probe Microscope (SPM) from Park Systems was employed for the analysis of ferroelectric properties by piezoresponse force microscopy (PFM) in the contact resonance mode with an external SR865 Stanford Research Systems Inc. lock‐in amplifier. 3. Results and discussion As was expected, based in a previous publication on films grown on Pt substrates [8], a deposition temperature of 890 ºC resulted in a single phase YCrO3 (YCO), as is shown in the x‐ray diffraction pattern in Fig. 1(a), however, in this occasion, a perovskite monocrystalline (110)‐oriented SrTiO3 (STO(110)) substrate was used. The YCO film pattern, indexed according to the ICDD 340565 file, shows a highly (001)‐oriented film with an orthorhombic structure Pnma. Besides, in the uniform film thickness of around 20 nm (see Fig.
3
1(b)), an atomically sharp film‐substrate interface was observed, as depicted in the cross‐sectional high resolution transmission electron microscopy (HRTEM) image in Figure 1(c). The parameters for the YCO film lattice, calculated from the electron diffraction pattern shown in Fig. 1(d), were: aYCO=5.523 Å, bYCO=7.534 Å, and cYCO=5.242 Å, similar to those reported in the literature for bulk ceramic [15]. A simulation of the YCO structure is illustrated in Fig. 2(a). STO has a cubic structure with a lattice parameter of aSTO = 3.904 Å, thus the STO(110) substrate surface has a rectangular lattice of 3.904 Å in width and, along the [1 ‐1 0]STO direction, 5.521Å in length, as is indicated in Fig. 2(b). Therefore, the obtained aYCO = 5.523 Å value matches very well with the lattice parameter of the STO(110) surface along the [1 ‐1 0]STO, while bYCO = 7.534 Å is just a little bit shorter than 2aSTO by 0.274 Å. Consequently a YCO(001) films will grow incommensurately on STO(110) substrates, as is schematized in Figure 2(b), and the film would experience a tensile stress in the [010]YCO direction. Figures 2(c) and 2(d) are top and side views along [001]YCO and [010]YCO directions that illustrate the atomic distribution overlapping of the unit cells of the film and substrate. Due to the fact that the film is actually non‐epitaxial but highly oriented, as is appreciated in the HRTEM image on Fig. 1c, and confirmed by the pole figures shown of Fig. 3, the imperfections created at the substrate‐film interface would help to release the mentioned stress. To determine the epitaxial degree of the film growth, several pole figures for different crystallographic planes were measured. As representative cases, Figs. 3(a) and 3(b) show the high resolution (202) and (200)‐ pole figures of the YCO thin film and STO substrate measured at 2YCO = 47.8° and 2STO = 46.48°, respectively, calculated from the lattice parameters discussed above. As expected for the normal (001)YCO and (110)STO crystallographic orientations, only two peaks are observed in the pole figures, associated to the twofold symmetry of the YCO orthorhombic and STO cubic structures in such orientations; besides, the spatial positions of the {202} peaks for the YCO are in excellent agreement with the lattice parameters determined by electron diffraction. Moreover, the Φ‐scan obtained at ψYCO = 43.3° and ψSTO = 44° are illustrated also in Fig. 3(a) and 3(b) where the {202} peaks for the YCO films are very sharp as a strong confirmation of a very small in‐plane misorientation, in good correspondence with the local epitaxial growth at the YCO/STO interface observed in the HRTEM image in Fig. 1(c).
4
The ferroelectric behavior of the YCrO3 sample was verified by resonance piezoresponse force microscopy analysis of the out‐of‐plane component of the polarization. Polarization reversal was attained by applying a dc voltage of +10V in a 1.5x1.5μm2 area of the sample, and then ‐10V in a 0.5x0.5 μm2 zone in the center. An rms roughness of 0.52nm was calculated from Fig. 4(a). Polarization switching is inferred from the observed change in contrast, from dark to bright, of the phase‐PFM image in Fig. 4(c). On the other hand, the PFM hysteresis loops dependency on the Vac observed in Fig. 4(e), shows the expected behavior for ferroelectric samples [16], a coercive voltage of 4.5V was calculated from this loops. In order to discard any artifact in the contrast of the PFM images due to irreversible electrochemical effects, absence of topography damage was verified. For non‐ferroelectric effects, such as surface charges or electrostriction, the relative amplitude of the first and second harmonics of the piezoresponse were analyzed following the method proposed by Q. N. Chen et al [16] see Fig. 4(d). The electrostatic contribution to the hysteresis curves was also reduced was also reduced by using the “off‐dc mode” of the PFM hysteresis technique. Ferromagnetism was confirmed in the film by measurements of magnetization as a function of the applied magnetic field, at a temperature of 5 K. The hysteretic behavior, attributed to a canted antiferromagnetic ordering [1, 8] is clearly observed in the graph in figure 4(f). 4. Conclusions Locally epitaxial (001)‐oriented orthorhombic YCrO3 films, grown on SrTiO3(110) substrates, were obtained by r.f. magnetron sputtering. The films are multiferroic as polarization reversal, therefore ferroelectricity, was demonstrated through domain switching using PFM, and a clear ferromagnetic behavior was clearly displayed at 5 K. A coercive voltage of 4.5 V for a 20 nm thick film was calculated from a local phase‐PFM hysteresis curve. These results will be taken as a starting point for further studies about the relation of the film‐substrate strain and orientation and the ferroelectric domain structure, the magnetic properties and their coupling. Acknowledgments
5
Thanks are due to F. Ruiz, M. Adelaido Landaverde, Gilberto F. Hurtado López, J. López Mendoza, E. Aparicio, and P. Casillas for their technical assistance. This work has been supported in part by PAPIIT‐UNAM Proj. IN109016, IN107708, IN103016, IN104414, IN110315 and IN110315 and IN105317; and CONACyT proj. 174391 and 166286. References [1]
C.R. Serrao, A.K. Kundu, S.B. Krupanidhi, U. V. Waghmare, C.N.R. Rao, Biferroic YCrO3, Phys. Rev.
B ‐ Condens. Matter Mater. Phys. 72 (2005) 2–5. doi:10.1103/PhysRevB.72.220101. [2]
V. Bedekar, R. Shukla, A.K. Tyagi, Nanocrystalline YCrO3 with onion‐like structure and unusual
magnetic behaviour, Nanotechnology. 18 (2007) 155706. doi:10.1088/0957‐4484/18/15/155706. [3]
K. Ramesha, a Llobet, T. Proffen, C.R. Serrao, C.N.R. Rao, Observation of local non‐
centrosymmetry in weakly biferroic YCrO 3, J. Phys. Condens. Matter. 19 (2007) 102202. doi:10.1088/0953‐ 8984/19/10/102202. [4]
S. Krishnan, N. Kalarikkal, Synthesis of YCrO3 nanoparticles through PAA assisted sol‐gel route, J.
Sol‐Gel Sci. Technol. 66 (2013) 6–14. doi:10.1007/s10971‐013‐2959‐z. [5]
J. Bahadur, D. Sen, S. Mazumder, V.K. Aswal, V. Bedekar, R. Shukla, a. K. Tyagi, Effects of sintering
on microstructure and dielectric response in YCrO3 nanoceramic, Pramana. 71 (2009) 959–963. doi:10.1007/s12043‐008‐0207‐9. [6]
J.H.K.I.M. Ã, H.S. Hin, S.K. Im, J.M. Oon, B.L. Ee, Formation of YCrO3 Thin Films using Radio‐
Frequency Magnetron Sputtering Method for a Wide Range Thermistor Application, Jpn, J. Appl. Phys. 42 (2003) 575–578. doi:10.1143/JJAP.42.575. [7]
Z.X. Cheng, X.L. Wang, S.X. Dou, H. Kimura, K. Ozawa, A novel multiferroic system: Rare earth
chromates, J. Appl. Phys. 107 (2010) 7–10. doi:10.1063/1.3360358. [8]
M.P. Cruz, D. Valdespino, J.J. Gervacio, M. Herrera, D. Bueno‐Baques, A. Durán, J. Muñoz, A.C.
García‐Castro, F.J. Espinoza‐Beltrán, M. Curiel, J.M. Siqueiros, Piezoelectric and ferroelectric response
6
enhancement in multiferroic YCrO3 films by reduction in thickness, Mater. Lett. 114 (2014) 148–151. doi:10.1016/j.matlet.2013.10.009. [9]
J.D. Seo, J.Y. Son, Room temperature ferroelectricity of YCrO3 thin films on Rh single crystals, J.
Cryst. Growth. 375 (2013) 53–56. doi:10.1016/j.jcrysgro.2013.04.016. [10] Y. Sharma, P. Misra, R.S. Katiyar, Unipolar resistive switching behavior of amorphous YCrO 3 films for nonvolatile memory applications, 084505 (2014) 1–6. doi:10.1063/1.4893661. [11] Y. Sharma, S. Sahoo, W. Perez, S. Mukherjee, Phonons and magnetic excitation correlations in weak ferromagnetic YCrO3, 183907 (2014). doi:10.1063/1.4875099. [12] C.M. Araujo, S. Nagar, M. Ramzan, R. Shukla, O.D. Jayakumar, A.K. Tyagi, Y. Liu, J. Chen, P. Glans, C. Chang, A. Blomqvist, E. Holmstro, L. Belova, J. Guo, R. Ahuja, K. V Rao, Disorder‐induced Room Temperature Ferromagnetism in Glassy Chromites, (2014) 1–6. doi:10.1038/srep04686. [13]
Y.H. Chu, L.W. Martin, Q. Zhan, P.L. Yang, M.P. Cruz, K. Lee, M. Barry, S.Y. Yang, R. Ramesh,
Epitaxial Multiferroic BiFeO3 Thin Films: Progress and Future Directions, Ferroelectrics. 354 (2007) 167– 177. doi:10.1080/00150190701454867. [14] H. Paik, H. Kim, J. Hong, Applied Surface Science Epitaxial growth and multiferroic properties of cation‐ engineered (Bi0.45La0.05Ba0.5)(Fe0.75Nb0.25)O3 thin film on Ir‐buffered (0 0 1) MgO substrate, Appl. Surf. Sci. 334 (2015) 52–57. doi:10.1016/j.apsusc.2014.08.012. [15]
Powder Diffraction File, Card No. 340365, Natl. Bur. Stand. (U. S.) Monogr. 23, 19, 83 (1982).
[16]
N. Balke, P. Maksymovych, S. Jesse, A. Herklotz, A. Tselev, C. Eom, I.I. Kravchenko, P. Yu, S. V
Kalinin, Differentiating Ferroelectric and Nonferroelectric Electromechanical Effects with Scanning Probe Microscopy, ACS Nano. 9 (2015) 6484–6492. doi:10.1021/acsnano.5b02227. [17] Q.N. Chen, Y. Ou, F. Ma, J. Li, Mechanisms of electromechanical coupling in strain based scanning probe microscopy, Appl. Phys. Lett. 104 (2014) 2–6. doi:10.1063/1.4884422.
7
Figure 1. (a) XRD pattern; cross section (b) TEM and (c) HRTEM images and (d) electron diffraction pattern for the YCrO3(001) film grown on a SrTiO3(110) substrate.
8
Figure 2. (a) YCO unit cell, (b) schematics of the YCO cells on the (110)STO plane. (c) top and (d) side views along the [001]YCO and [010]YCO directions, respectively, of the atomic overlapping of YCO and STO lattices.
Fig. 3. Pole figures and Φ‐scans at the maximum of the (a) {202} reflections and (022) pole, and (b) {200} reflections and (200) pole, for the YCO film and STO substrate, respectively.
9
Figure 4. (a) Topography, (b) amplitude‐PFM, (c) phase‐PFM, (d) first and second PFM harmonics, (e) hysteresis loops in the off dc mode at different Vac and (f) magnetization as a function of magnetic field, at 5K, of the YCrO3 film grown on a SrTiO3(110) substrate.
10
S0169-4332(17)32611-9 http://dx.doi.org/10.1016/j.apsusc.2017.09.011 APSUSC 37090
To appear in:
APSUSC
Received date: Revised date: Accepted date:
14-3-2017 20-6-2017 3-9-2017
Please cite this article as: J.J.Gervacio-Arciniega, E.Murillo-Bracamontes, O.Contreras, J.M.Siqueiros, O.Raymond, A.Dur´an, D.Bueno-Baques, D.Valdespino, E.Cruz-Valeriano, C.I.Enr´ıquez-Flores, M.P.Cruz, Multiferroic YCrO3 thin films: structural, ferroelectric and magnetic properties, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.09.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Multiferroic YCrO3 thin films: structural, ferroelectric and magnetic properties J. J. Gervacio‐Arciniega1,2,a), E. Murillo‐Bracamontes1, O. Contreras1, J. M. Siqueiros1, O. Raymond1, A. Durán1, D. Bueno‐Baques5, D. Valdespino3, E. Cruz‐Valeriano4, C. I. Enríquez‐Flores4, M. P. Cruz1 1
Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Km. 107 Carretera Tijuana-Ensenada, AP 14, Ensenada, B.C., México, C.P. 22860.
2
CONACYT-BUAP, Postgrado en Física Aplicada, Facultad de Ciencias Físico-Matemáticas, Benemérita Universidad Autónoma de Puebla, Av. San Claudio y Av. 18 sur, Col. San Manuel Ciudad Universitaria, Puebla, Pue. 72570, México. 3
Posgrado en Nanociencias, Centro de Investigación Científica y de Educación Superior de Ensenada, Carretera Ensenada-Tijuana 3918, Zona Playitas, Baja California, 22860, México. 4
5
CINVESTAV Unidad Querétaro, Lib. Norponiente 2000, Real de Juriquilla, 76230 Querétaro, Qro., México.
Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna Hermosillo No. 140, Saltillo, Coah., C.P. 25250, México.
Graphical Abstract
Highlights (for review)
Highly oriented YCrO3/SrTiO3 films grown by magnetron sputtering are reported. A structural, ferroelectric and magnetic study of YCrO3/SrTiO3 films is shown. These results will encourage further studies on origin of ferroelectricity in YCrO3.
Highly oriented and locally epitaxial multiferroic YCrO3 (001) thin films, 20 nm thick, were deposited by r.f. magnetron sputtering on SrTiO3 (110) substrates at 890ºC. The structure was investigated by x‐ray diffraction and cross section high resolution transmition electron microscopy, a clear local matching between the YCrO3 film and the substrate was observed. Ferroelectricity was confirmed by means of
a)
Author to whom correspondence should be addressed. Electronic mail: [email protected]
1
switching areas with opposite polarization directions, first and second harmonic electromechanical signals, and local hysteresis ferroelectric curves obtained by piezoresponse force microscopy. Additionally, below the Néel temperature, a clear ferromagnetic hysteresis loop was observed. These results will encourage further studies on the mechanism that promotes the ferroelectric nature in YCrO3 compound. Keywords: YCrO3 Highly oriented Multiferroic
1. Introduction Coexistence of ferroelectric and magnetic properties in a single compound gives rise to a new class of materials, called multiferroics, with promising applications in future technology. Among the few existing single phase multiferroics, YCrO3 has been one of the least studied mainly because its ferroelectric properties had been overshadowed by high dielectric loss [1–10]. It is well known that ferromagnetism in this compound is caused by a canted antiferromagnetic array; TN~140K [1,2,11], this property is quite reproducible in ceramics [2] and YCrO3 films, even as thin as 20nm, deposited on Pt [12]. Ferroelectricity, on the other hand, is associated to the peak around 430 K in the permittivity versus temperature curve, corresponding to a paraelectric‐ferroelectric transition. According to Ramesha et al [3], the origin of this transition is the local non‐centrosymmetry caused by the Cr displacement along the z‐direction of the perovskite‐type unit cell. However, polarization as function of voltage curves in bulk ceramics were so rounded [1] that it was not possible to elucidate whether polarization was due to free charge or to ferroelectricity. Yet, in a previous work on polycrystalline YCrO3 films grown on Pt/TiO2/Si substrates, it was demonstrated that decreasing film thickness down to approximately 20nm, diminishes free charge accumulation because of the reduction of grain boundaries [8]. Therefore, it would be reasonable to infer that, with no grain boundaries, as in epitaxial films, better ferroelectric properties should be attained, as was indeed found for other multiferroics, [13, 14]. Related to this matter, Seo and coworkers prepared YCrO3 films on Rh crystalline substrates and, even though epitaxy was not reached, better film orientation
2
allowed the observation of an improved, less lossy, ferroelectric hysteresis loop [9]. Still, the obtainment of YCrO3 epitaxial films, aim of this work, is necessary to further enhance the ferroelectric properties that allow performing basic studies on ferroelectric domain structure, polarization switching and ferroelectric‐ magnetic coupling mechanisms. 2. Experimental details YCrO3 films were deposited on SrTiO3(110) substrates, by r. f. magnetron sputtering as described in [8] using a deposition temperature of 890ºC. This temperature was chosen based in a previous work on films deposited on Pt substrates at room temperature, where a heat treatment at ~900ºC was needed to achieved a pure, well define YCrO3 crystalline phase [8]. Film thickness and film‐substrate interface were characterized by cross‐sectional transmission electron microscopy (TEM) in a JEOL JEM‐2010 instrument, also employed for the acquisition of electron diffraction patterns. X‐ray diffraction (XRD) data were collected in a Panalitycal X’pert Pro MRD diffractometer with monochromatic Cu‐Kα1 radiation, the pole figures were measured for a ψ‐polar angle range between 30° and 60°, in steps of 0.7°, and Φ‐azimuthal angle scanning from 0 to 360°, in steps of 1°, and exposition times of 3 s for YCO and 1 s for STO. The magnetization data as function of the magnetic field were taken with a Quantum Design 6000 physical properties measurement system (PPMS) with a vibrating sample magnetometer (VSM), at 40Hz with the applied magnetic field oriented along the film plane. An XE‐70 Scanning Probe Microscope (SPM) from Park Systems was employed for the analysis of ferroelectric properties by piezoresponse force microscopy (PFM) in the contact resonance mode with an external SR865 Stanford Research Systems Inc. lock‐in amplifier. 3. Results and discussion As was expected, based in a previous publication on films grown on Pt substrates [8], a deposition temperature of 890 ºC resulted in a single phase YCrO3 (YCO), as is shown in the x‐ray diffraction pattern in Fig. 1(a), however, in this occasion, a perovskite monocrystalline (110)‐oriented SrTiO3 (STO(110)) substrate was used. The YCO film pattern, indexed according to the ICDD 340565 file, shows a highly (001)‐oriented film with an orthorhombic structure Pnma. Besides, in the uniform film thickness of around 20 nm (see Fig.
3
1(b)), an atomically sharp film‐substrate interface was observed, as depicted in the cross‐sectional high resolution transmission electron microscopy (HRTEM) image in Figure 1(c). The parameters for the YCO film lattice, calculated from the electron diffraction pattern shown in Fig. 1(d), were: aYCO=5.523 Å, bYCO=7.534 Å, and cYCO=5.242 Å, similar to those reported in the literature for bulk ceramic [15]. A simulation of the YCO structure is illustrated in Fig. 2(a). STO has a cubic structure with a lattice parameter of aSTO = 3.904 Å, thus the STO(110) substrate surface has a rectangular lattice of 3.904 Å in width and, along the [1 ‐1 0]STO direction, 5.521Å in length, as is indicated in Fig. 2(b). Therefore, the obtained aYCO = 5.523 Å value matches very well with the lattice parameter of the STO(110) surface along the [1 ‐1 0]STO, while bYCO = 7.534 Å is just a little bit shorter than 2aSTO by 0.274 Å. Consequently a YCO(001) films will grow incommensurately on STO(110) substrates, as is schematized in Figure 2(b), and the film would experience a tensile stress in the [010]YCO direction. Figures 2(c) and 2(d) are top and side views along [001]YCO and [010]YCO directions that illustrate the atomic distribution overlapping of the unit cells of the film and substrate. Due to the fact that the film is actually non‐epitaxial but highly oriented, as is appreciated in the HRTEM image on Fig. 1c, and confirmed by the pole figures shown of Fig. 3, the imperfections created at the substrate‐film interface would help to release the mentioned stress. To determine the epitaxial degree of the film growth, several pole figures for different crystallographic planes were measured. As representative cases, Figs. 3(a) and 3(b) show the high resolution (202) and (200)‐ pole figures of the YCO thin film and STO substrate measured at 2YCO = 47.8° and 2STO = 46.48°, respectively, calculated from the lattice parameters discussed above. As expected for the normal (001)YCO and (110)STO crystallographic orientations, only two peaks are observed in the pole figures, associated to the twofold symmetry of the YCO orthorhombic and STO cubic structures in such orientations; besides, the spatial positions of the {202} peaks for the YCO are in excellent agreement with the lattice parameters determined by electron diffraction. Moreover, the Φ‐scan obtained at ψYCO = 43.3° and ψSTO = 44° are illustrated also in Fig. 3(a) and 3(b) where the {202} peaks for the YCO films are very sharp as a strong confirmation of a very small in‐plane misorientation, in good correspondence with the local epitaxial growth at the YCO/STO interface observed in the HRTEM image in Fig. 1(c).
4
The ferroelectric behavior of the YCrO3 sample was verified by resonance piezoresponse force microscopy analysis of the out‐of‐plane component of the polarization. Polarization reversal was attained by applying a dc voltage of +10V in a 1.5x1.5μm2 area of the sample, and then ‐10V in a 0.5x0.5 μm2 zone in the center. An rms roughness of 0.52nm was calculated from Fig. 4(a). Polarization switching is inferred from the observed change in contrast, from dark to bright, of the phase‐PFM image in Fig. 4(c). On the other hand, the PFM hysteresis loops dependency on the Vac observed in Fig. 4(e), shows the expected behavior for ferroelectric samples [16], a coercive voltage of 4.5V was calculated from this loops. In order to discard any artifact in the contrast of the PFM images due to irreversible electrochemical effects, absence of topography damage was verified. For non‐ferroelectric effects, such as surface charges or electrostriction, the relative amplitude of the first and second harmonics of the piezoresponse were analyzed following the method proposed by Q. N. Chen et al [16] see Fig. 4(d). The electrostatic contribution to the hysteresis curves was also reduced was also reduced by using the “off‐dc mode” of the PFM hysteresis technique. Ferromagnetism was confirmed in the film by measurements of magnetization as a function of the applied magnetic field, at a temperature of 5 K. The hysteretic behavior, attributed to a canted antiferromagnetic ordering [1, 8] is clearly observed in the graph in figure 4(f). 4. Conclusions Locally epitaxial (001)‐oriented orthorhombic YCrO3 films, grown on SrTiO3(110) substrates, were obtained by r.f. magnetron sputtering. The films are multiferroic as polarization reversal, therefore ferroelectricity, was demonstrated through domain switching using PFM, and a clear ferromagnetic behavior was clearly displayed at 5 K. A coercive voltage of 4.5 V for a 20 nm thick film was calculated from a local phase‐PFM hysteresis curve. These results will be taken as a starting point for further studies about the relation of the film‐substrate strain and orientation and the ferroelectric domain structure, the magnetic properties and their coupling. Acknowledgments
5
Thanks are due to F. Ruiz, M. Adelaido Landaverde, Gilberto F. Hurtado López, J. López Mendoza, E. Aparicio, and P. Casillas for their technical assistance. This work has been supported in part by PAPIIT‐UNAM Proj. IN109016, IN107708, IN103016, IN104414, IN110315 and IN110315 and IN105317; and CONACyT proj. 174391 and 166286. References [1]
C.R. Serrao, A.K. Kundu, S.B. Krupanidhi, U. V. Waghmare, C.N.R. Rao, Biferroic YCrO3, Phys. Rev.
B ‐ Condens. Matter Mater. Phys. 72 (2005) 2–5. doi:10.1103/PhysRevB.72.220101. [2]
V. Bedekar, R. Shukla, A.K. Tyagi, Nanocrystalline YCrO3 with onion‐like structure and unusual
magnetic behaviour, Nanotechnology. 18 (2007) 155706. doi:10.1088/0957‐4484/18/15/155706. [3]
K. Ramesha, a Llobet, T. Proffen, C.R. Serrao, C.N.R. Rao, Observation of local non‐
centrosymmetry in weakly biferroic YCrO 3, J. Phys. Condens. Matter. 19 (2007) 102202. doi:10.1088/0953‐ 8984/19/10/102202. [4]
S. Krishnan, N. Kalarikkal, Synthesis of YCrO3 nanoparticles through PAA assisted sol‐gel route, J.
Sol‐Gel Sci. Technol. 66 (2013) 6–14. doi:10.1007/s10971‐013‐2959‐z. [5]
J. Bahadur, D. Sen, S. Mazumder, V.K. Aswal, V. Bedekar, R. Shukla, a. K. Tyagi, Effects of sintering
on microstructure and dielectric response in YCrO3 nanoceramic, Pramana. 71 (2009) 959–963. doi:10.1007/s12043‐008‐0207‐9. [6]
J.H.K.I.M. Ã, H.S. Hin, S.K. Im, J.M. Oon, B.L. Ee, Formation of YCrO3 Thin Films using Radio‐
Frequency Magnetron Sputtering Method for a Wide Range Thermistor Application, Jpn, J. Appl. Phys. 42 (2003) 575–578. doi:10.1143/JJAP.42.575. [7]
Z.X. Cheng, X.L. Wang, S.X. Dou, H. Kimura, K. Ozawa, A novel multiferroic system: Rare earth
chromates, J. Appl. Phys. 107 (2010) 7–10. doi:10.1063/1.3360358. [8]
M.P. Cruz, D. Valdespino, J.J. Gervacio, M. Herrera, D. Bueno‐Baques, A. Durán, J. Muñoz, A.C.
García‐Castro, F.J. Espinoza‐Beltrán, M. Curiel, J.M. Siqueiros, Piezoelectric and ferroelectric response
6
enhancement in multiferroic YCrO3 films by reduction in thickness, Mater. Lett. 114 (2014) 148–151. doi:10.1016/j.matlet.2013.10.009. [9]
J.D. Seo, J.Y. Son, Room temperature ferroelectricity of YCrO3 thin films on Rh single crystals, J.
Cryst. Growth. 375 (2013) 53–56. doi:10.1016/j.jcrysgro.2013.04.016. [10] Y. Sharma, P. Misra, R.S. Katiyar, Unipolar resistive switching behavior of amorphous YCrO 3 films for nonvolatile memory applications, 084505 (2014) 1–6. doi:10.1063/1.4893661. [11] Y. Sharma, S. Sahoo, W. Perez, S. Mukherjee, Phonons and magnetic excitation correlations in weak ferromagnetic YCrO3, 183907 (2014). doi:10.1063/1.4875099. [12] C.M. Araujo, S. Nagar, M. Ramzan, R. Shukla, O.D. Jayakumar, A.K. Tyagi, Y. Liu, J. Chen, P. Glans, C. Chang, A. Blomqvist, E. Holmstro, L. Belova, J. Guo, R. Ahuja, K. V Rao, Disorder‐induced Room Temperature Ferromagnetism in Glassy Chromites, (2014) 1–6. doi:10.1038/srep04686. [13]
Y.H. Chu, L.W. Martin, Q. Zhan, P.L. Yang, M.P. Cruz, K. Lee, M. Barry, S.Y. Yang, R. Ramesh,
Epitaxial Multiferroic BiFeO3 Thin Films: Progress and Future Directions, Ferroelectrics. 354 (2007) 167– 177. doi:10.1080/00150190701454867. [14] H. Paik, H. Kim, J. Hong, Applied Surface Science Epitaxial growth and multiferroic properties of cation‐ engineered (Bi0.45La0.05Ba0.5)(Fe0.75Nb0.25)O3 thin film on Ir‐buffered (0 0 1) MgO substrate, Appl. Surf. Sci. 334 (2015) 52–57. doi:10.1016/j.apsusc.2014.08.012. [15]
Powder Diffraction File, Card No. 340365, Natl. Bur. Stand. (U. S.) Monogr. 23, 19, 83 (1982).
[16]
N. Balke, P. Maksymovych, S. Jesse, A. Herklotz, A. Tselev, C. Eom, I.I. Kravchenko, P. Yu, S. V
Kalinin, Differentiating Ferroelectric and Nonferroelectric Electromechanical Effects with Scanning Probe Microscopy, ACS Nano. 9 (2015) 6484–6492. doi:10.1021/acsnano.5b02227. [17] Q.N. Chen, Y. Ou, F. Ma, J. Li, Mechanisms of electromechanical coupling in strain based scanning probe microscopy, Appl. Phys. Lett. 104 (2014) 2–6. doi:10.1063/1.4884422.
7
Figure 1. (a) XRD pattern; cross section (b) TEM and (c) HRTEM images and (d) electron diffraction pattern for the YCrO3(001) film grown on a SrTiO3(110) substrate.
8
Figure 2. (a) YCO unit cell, (b) schematics of the YCO cells on the (110)STO plane. (c) top and (d) side views along the [001]YCO and [010]YCO directions, respectively, of the atomic overlapping of YCO and STO lattices.
Fig. 3. Pole figures and Φ‐scans at the maximum of the (a) {202} reflections and (022) pole, and (b) {200} reflections and (200) pole, for the YCO film and STO substrate, respectively.
9
Figure 4. (a) Topography, (b) amplitude‐PFM, (c) phase‐PFM, (d) first and second PFM harmonics, (e) hysteresis loops in the off dc mode at different Vac and (f) magnetization as a function of magnetic field, at 5K, of the YCrO3 film grown on a SrTiO3(110) substrate.
10