Piezoelectric and ferroelectric response enhancement in multiferroicYCrO3 films by reduction in thickness

M.P. Cruz a,n, D. Valdespino b, J.J. Gervacio a, M. Herrera a, D. Bueno-Baques c, A. Durán a,J. Muñoz d, A.C. García-Castro d, F.J. Espinoza-Beltrán d, M. Curiel a, J.M. Siqueiros a

a Centro de Nanociencias y Nanotecnología (CNyN), Universidad Nacional Autónoma de Mexico (UNAM), Km. 107 Carretera Tijuana-Ensenada,Ensenada, B.C., C.P. 22860, Mexicob Posgrado en Ciencia e Ingeniería de Materiales, UNAM, Mexicoc Centro de Investigación en Química Aplicada (CIQA), Blvd. Enrique Reyna Hermosillo No. 140, Saltillo, Coah., C.P. 25250, Mexicod Centro de Investigación y de Estudios Avanzados (CINVESTAV)-IPN, Libramiento Norponiente No. 2000, Real de Juriquilla, Qro., C.P. 76230, Mexico

a r t i c l e i n f o

Article history:Received 30 July 2013Accepted 1 October 2013Available online 9 October 2013

Keywords:FerroelectricsMagnetic materialsPiezoelectric materialsThin films

a b s t r a c t

A study of the piezoelectric response and polarization switching of YCrO3 films as function of thickness, ispresented. The films, 20 to 180 nm thick, were deposited on Pt/TiO2/Si substrates by r.f. sputtering atroom temperature, and then annealed at 900 1C for 1 h in air. Better grain coalescence and reduction ofcharge accumulation at grain boundaries was observed in thinner films, leading to the appearance andenhancement of the piezoresponse. A piezoelectric coefficient of d33�6.4 pm/V and a coercive voltage ofVc�12.5 V were obtained in the 20 nm film, which also showed a magnetic hysteresis loop. Therefore,while maintaining the ferromagnetic behavior, ferroelectricity, hence multiferroicity, is induced in YCrO3

when film thickness is reduced to 20 nm.& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Materials that combine ferroelectric and ferromagnetic proper-ties, called multiferroics, have attracted great attention in recentyears because of the possibility of novel applications. However,due to the fact that ferroelectricity and ferromagnetism aremutually excluding phenomena, many aspects about the obtain-ment as well as the explanation of the simultaneous presence ofboth properties, remain unsolved. As a consequence, even themost studied multiferroic material, BiFeO3 [1], is still subject ofnumerous studies. In contrast, other proposed systems, such asYCrO3, have been shallowly investigated [2–10]. Ceramics of thisorthochromite exhibit antiferromagnetism and a weak ferroelec-tricity below TN¼140 K and TFE¼473 K, respectively [2]. Theferromagnetic response is caused by a canted antiferromagneticordering [2]. Ferroelectricity, on the other hand, is a consequenceof the local non-centrosymmetry originated by the chromiumdisplacements along the z-direction [3]. However, high dielectriclosses have impeded the acquisition of clear experimental evi-dence of the ferroelectric behavior, particularly when the sample isin thin film form [2,4–5]. Nonetheless, properties in films arestrongly related to thickness. In BiFeO3 for example, ferroelectri-city is difficult to measure in ceramics because of the dielectric

loss. In contrast, a 100 nm thick film of BiFeO3 shows excellentferroelectric and piezoelectric properties brought about for themore ordered crystal growth, free from porous and grain bound-aries, and a strained structure caused by the film-substrate latticemismatch [1]. In YCrO3, however, films reported by other authorswere around 70 nm [4] and 300 nm in thickness [5], thick enoughto achieve relaxation and too thick to reduce the accumulationcharge. In other works film thickness was not even mentioned [2].

Consequently, as seen in the case of BiFeO3, it is reasonable topropose the growth of very thin films of YCrO3 as a means toimprove their piezoelectric and ferroelectric properties, which isthe aim of this work.

2. Experimental procedure

YCrO3 films 20 to 550 nm thick were grown by rf magnetronsputtering at room temperature, 13.0 mT of a mixture of argon(70%) plus oxygen (30%), 200 W of power supplied and a target–substrate distance of 6.3 cm. Pt(1500 Å)/TiO2(300 Å)/SiO2/Si(1 0 0)substrates from Radiant Tech. Inc. were used. The YCrO3 target wasprepared by the solid state reaction of Y2O3 and CrO3 powders(99.99% and 99.97% purity, respectively), from Alfa Aesar, sinteredat 1450 1C. After deposition, films were heat treated in air at 900 1Cfor 1 h. X-ray diffraction (XRD) patterns were collected in a PhillipsX'pert powder diffractometer with the Cu-Kα1 radiation. A JEOLJMS-5300 scanning electron microscope (SEM) was used for the

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Materials Letters

0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.matlet.2013.10.009

n Corresponding author. Tel.: þ52 6461744602 ext. 349.E-mail address: mcruz@cnyn.unam.mx (M.P. Cruz).

Materials Letters 114 (2014) 148–151

morphology. In the same microscope, electron beam inducedcurrent (EBIC) images were taken with a Keithley-428 amplifierwhile the beam was kept at 15 keV and 1 nA. A Veeco Dimension3100 Nanoscope IV atomic force microscope (AFM), with Cr/Ptcantilevers (Multi75E-G from BudgetSensors), was employed inthe piezoresponse force microscopy (PFM) and switching-PFM(S-PFM) analysis. Magnetic measurements were carried out in aQuantum Design 6000 physical properties measurement system(PPMS) with a vibrating (40 Hz) sample magnetometer (VSM).

3. Results and discussion

X-ray diffraction patterns of 550 nm thick films grown at roomtemperature, post-annealed in air at 800 and 900 1C for 1 h, arepresented in Fig. 1. The as-deposited amorphous films acquire thepolycrystalline YCrO3 phase after the heat treatment at 800 1C, aresult in agreement with that obtained by Durán et al. [6] inpowders synthesized by the combustion method. Crystallinity isimproved further by increasing the annealing temperature to 900 1C,as evidenced by the sharpness of the (2 0 0) and (0 0 2) reflections.

Hence 900 1C was the annealing temperature used for the subse-quent series of studied samples.

A granular morphology is observed in the 180 nm and thickerfilms. Island-like grains are formed in the 90 and 45 nm samplesand, as a result of the higher influence of the substrate surfaceenergy, graininess gives way to a smoother surface in the 20 nmfilm (see Fig. 2).

The electron beam induced current (EBIC) images (not shownhere) coincide exactly with those in Fig. 2 but grain boundariesappear as lighter zones which, according to Maestre et al. [11], areassociated to charge accumulation. This result is in agreement withthe literature where it has been reported that grain boundaries inceramics are regions of enhanced charge carrier recombination due tothe presence of electric charge [7]. Movement of the charge carriesgives rise to dielectric losses at low frequencies [7]. In particularin YCrO3 ceramics, dielectric loss is so high [8–10] that roundedhysteresis loops are obtained [2]. However, Bahadur et al. [7] foundthat in YCrO3 nanoceramics, dielectric loss at low frequenciesdecreases with total porosity. In the same manner, the improvementof homogeneity by thickness reduction in YCrO3 films, shown inFig. 2, leads to the reduction of the charge carrier accumulation.Moreover, such carrier accumulation at the grain boundaries of the180 nm thick film could be creating depolarizing electric fields thatreduce to the point of disappearing, the net polarization in grains [7].This would be the reason for the homogeneous “neutral” contrast ofthe corresponding phase-PFM image on Fig. 3, associated to theabsence of a piezoresponse [12]. When thickness is reduced, apiezoresponse comes into sight getting stronger, as made evidentby thewell defined contrast for the 45 and 20 nm samples (see Fig. 3).

Polarization reversal was achieved in the 20 nm sample byapplying þ12 V in a 2�2 μm2 area, and then �12 V in a1�1 μm2 square inside, as can be determined by the change incontrast of the phase-PFM image on Fig. 4a. Such ferroelectricbehavior was further confirmed by the phase and amplitudePFM-signalsvs. voltage curves in Fig. 4b, from which a coercive voltage,Vc�12.5 V, similar to that used to switch the polarization onFig. 4a, and a piezoelectric coefficient d33�6.4 pm/V, werecalculated using the method described by Hong et al. [13].

Fig. 1. XRD of 550 nm films as deposited and after annealing in air at 800 and900 1C.

Fig. 2. SEM micrographs of YCrO3 films of different thickness (same scale in all images).

M.P. Cruz et al. / Materials Letters 114 (2014) 148–151 149

Fig. 3. Phase-PFM images of YCrO3 films of different thickness (same scale in all images).

Fig. 4. (a) Phase-PFM image after applying712 V, (b) phase and amplitude PFM-signals vs. voltage and (c) magnetization vs. magnetic field curves of a 20 nm thickYCrO3 film.

M.P. Cruz et al. / Materials Letters 114 (2014) 148–151150

In addition, and despite the low thickness of the sample, ahysteresis loop is appreciable in the magnetization vs. magneticfield measurements, after the substrate signal subtraction, per-formed at a temperature of 50 K (see Fig. 4c).

In conclusion, a clear piezoelectric response and polariza-tion reversal was achieved in polycrystalline YCrO3 films whenfilm thickness is reduced to 20 nm. The improved performanceis associated to the reduction of charge accumulation at grainboundaries, a result of the more homogenous growth. Besidesbeing piezoelectric (d33�6.4 pm/V) and ferroelectric (Vc�12.5 V),the film is ferromagnetic, therefore multiferroicity is attained inYCrO3 when film thickness reaches 20 nm. These are promisingresults that encourage further investigation of YCO3 films as amagnetoelectric multiferroic option.

Acknowledgments

The work was supported by grants from Universidad NacionalAutónoma de México (UNAM), Projs. PAPIIT 107312 and 103213;and Consejo Nacional de Ciencia y Tecnología (CONaCyT), Projs.174391 and 166286. Thanks are due to E. Aparicio, P. Casillas and I.Gradilla for their technical help.

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