Appl. Phys. A (2015) 119:609–614 DOI 10.1007/s00339-015-8999-1
Effect of surface inhomogeneities on crystalline structure and magnetic properties of epitaxial Pr0.7Sr0.3MnO3/ La0.5Ca0.5MnO3 film on (001) SrTiO3 H. O. Wang • K. P. Su • Y. Q. Cao • L. W. Li • D. X. Huo • W. S. Tan • F. Xu Q. J. Jia • J. Gao
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Received: 5 December 2014 / Accepted: 13 January 2015 / Published online: 23 January 2015 Ó Springer-Verlag Berlin Heidelberg 2015
Abstract Epitaxil Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3 (PSMO/ LCMO) bilayer structure was grown on (001)-oriented SrTiO3 (STO) substrate by pulsed laser deposition. Highresolution synchrotron X-ray diffraction showed high quality of epitaxial layer. However, besides diffraction peaks from PSMO layer, LCMO layer and STO substrate, we observed an additional shoulder peak, which might stem from the inhomogeneities of composition in the film. Grazing incidence X-ray reflectivity (GIXRR) was employed to investigate the surface and interfacial crystalline structure of PSMO/LCMO. The fitting results of GIXRR indicated negligible interdiffusion and roughness at interface. Further atomic force microscopy measurement showed the presence of non-stoichiometric large particulates at surface, imparting an overall inhomogeneous composition to the film. This
H. O. Wang (&) K. P. Su Y. Q. Cao L. W. Li D. X. Huo Institute of Materials Physics, Hangzhou Dianzi University, Hangzhou 310018, China e-mail:
[email protected] H. O. Wang W. S. Tan (&) Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, China e-mail:
[email protected] F. Xu School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Q. J. Jia Institute of High Energy Physics, The Chinese Academy of Sciences, Beijing 100039, China J. Gao Department of Physics, The University of Hong Kong, Hong Kong, China
implied that the variation of crystalline structure of PSMO/ LCMO film occurred due to inhomogeneous composition. Moreover, studies on magnetic properties showed that surface inhomogeneities and ferromagnetic clusters at the PSMO/LCMO interface probably influenced the ferromagnetism of the bilayer film together, tuning exchange bias in the film.
1 Introduction The rare earth-doped perovskite manganites [1] have recently attracted considerable attention owing to their room temperature ferromagnetic (FM) properties and colossal magnetoresistance (CMR) effect. The display of such interesting magnetic properties makes these materials an attractive choice for magnetic field sensing and magnetic storage applications. The type of magnetic coupling systems is extremely rich, including magnetic metal/nonmagnetic metal, magnetic metal/nonmagnetic insulator and ferromagnet/antiferromagnet [2]. Special attentions have been paid to interfaces because it correlates with electrical and magnetic properties, particularly exchange bias (EB) which usually occurs in ferromagnetic/antiferromagnetic (FM/AFM) system. It should be pointed out that surface and interface play an important role in FM/AFM magnetic coupling effect, which affects significantly the physical properties of manganite multilayer [3, 4]. For example, phase separation can be controlled by the presence of magnetic nanodots at the surface. The exchange field at the interface is tunable with nanodot density and makes it possible to overcome strain effects in frustrated systems to greatly increase the metal–insulator transition temperature and magnetoresistance (MR) [5]. The physical properties of manganite multilayer such as Curie temperature,
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Metal–insulators translation temperature and MR can also be tuned by changing type and thickness of the constituent element layer [6, 7]. These results illustrate the essential role of interface in magnetic properties of manganite multilayer materials. Half-doped manganite La0.5Ca0.5MnO3 (LCMO) [8] exhibits phase separated state and a highly insulating CO state, which can transform to a conducting FM phase by applying external variables such as magnetic fields, introducing substrate-induced strain and photon exposure [9, 10]. Meanwhile, the interfaces are more complex especially when two competing ground states, the FM metallic state and the CO/OO insulating state are present [11]. The circumstance at interfaces between two manganites can allow some kind of new collective state to emerge such as phase separation, typically involving FM metallic and AFM charge-ordered (CO)/orbital-ordered (OO) insulating clusters. Thus, a CO LCMO thin film under a magnetic field along with FM thin film to form FM/CO system is ideally suitable for investigations on a variety of interesting physical phenomena. The performance of the advanced thin film device often depends on the microstructure, such as the thickness of component, mass density, interface and surface roughness and growth orientation. Characterization of the microstructure of film can help us optimize the growth conditions of thin films and provide us the mechanism foundation to understand the physical properties of film. In this work, we have performed high-resolution synchrotron X-ray diffraction (HRXRD) and grazing incidence X-ray reflectivity (GIXRR) measurements on epitaxial PSMO/LCMO bilayer (half-doped LCMO and PSMO is served as AFM layer and FM layer, respectively) on single crystalline SrTiO3 (001) substrate to investigate its crystalline structure and the in-plane growth orientation and then studied the relationship between the variation of structure and interlayer magnetic coupling.
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focused on rotating target with energy density about 3 J/ cm2, and the base pressure in the growth chamber was pumped to about 2 9 10-5 mbar. The temperature of substrate was 720 °C, and a pure O2 of 5 9 10-1 mbar was maintained throughout the deposition. The repetition rate of deposition was 2 Hz. The nominal thickness of the PSMO layer and LCMO layer was 36 and 24 nm, respectively. The deposition rates of PSMO and LCMO were accurately calibrated so as to control the thickness. After deposition, the bilayer film was annealed in 1 atm pure oxygen for half an hour and cooled down slowly in the same oxygen pressure to avoid possible oxygen deficiency. Atomic force microscopy was applied to investigate the surface morphology. The crystal structures were characterized by using GIXRR and HRXRD. All X-ray diffraction experiments were performed at the X-ray diffuse scattering station on the 4W1C beam line in Beijing Synchrotron Radiation Facility (BSRF). The X-ray wavelength was 0.15405 nm, and the energy resolution was 4.4 9 10-4. The individual layer thickness, the average roughness at every interface and the average density of individual layer were obtained by the simulation of GIXRR curves by using Bede REFS MERCURY software. Magnetic measurements were performed by a commercial Physical Property Measurement System (PPMS) (Quantum Design, Inc) in magnetic fields up to 9 T with a temperature range of 1.9–400 K.
3 Results and discussion Figure 1 shows the HRXRD h–2h scan of the (002) Bragg’s diffraction peak for the PSMO/LCMO thin film on
2 Experimental details Conventional solid-state reaction was applied to synthesize the PSMO and LCMO targets using high-purity (C99.99 %) raw powders of Pr6O11, SrCO3, La2O3, CaCO3 and MnO2 [12]. Epitaxial PSMO/LCMO bilayer film with PSMO as top layer was deposited on (001)-oriented single crystal SrTiO3 (STO) substrate by pulsed laser deposition (PLD) of bulk PSMO and LCMO targets. Phase purity in target was confirmed by powder X-ray diffraction patterns (not shown here). According to results of Rietveld refinements, the structures of PSMO and LCMO were regarded as pseudocubic ones with lattice parameter of 0.3874 and 0.3822 nm, respectively. The beam of KrF laser (k = 248 nm) was
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Fig. 1 HRXRD symmetric h–2h scan centered around the STO (002) peak for PSMO (36 nm)/LCMO (24 nm) bilayer film on STO (001) substrate
Effect of surface inhomogeneities on crystalline structure and magnetic properties
STO (001) substrate. Only (002) diffraction peaks of PSMO layer and LCMO layer were observed along with this for the single crystal STO substrate, suggesting the epitaxial relationship of the film in relation to the substrate. The small lattice mismatches among PSMO (pseudo-cubic, aPSMO = 0.3874 nm) and LCMO (pseudo-cubic, aLCMO = 0.3822 nm) and STO (cubic, aSTO = 0.3905 nm) allowed for the epitaxial growth of PSMO/LCMO thin film on STO substrate. Interestingly, besides PSMO layer and LCMO layer diffraction peaks, there is an additional shoulder peak labeled with S1 in Fig. 1. The emergence of shoulder peak is usually association with several factors, such as defect, strain, dislocation, and diffusion. The origin of the shoulder peak need further to be investigated. Figure 2 shows the asymmetric 2h–x scan about PSMO (103) plane for PSMO/LCMO thin film on STO (001) substrate. However, no such shoulder peak can be seen in the asymmetric scan in Fig. 2, indicating that the appearance of peak S1 is not related with the diffusion at the PSMO/LCMO interface. The out-of-plane (a\) and in-plane (ak) lattice parameters for PSMO layer and LCMO layer were calculated from the symmetric h–2h scans about PSMO (002) plane and LCMO (002) plane (Fig. 1) and the asymmetric 2h–x scans about PSMO (103) plane and LCMO (103) plane (Fig. 2), respectively. Subsequently, the in-plane strain ek and out-of-plane strain e\ were calculated using the formula e = (a - a0)/a0, where a is ak or a\ and a0 is the bulk unstressed lattice parameter as measured from power XRD pattern. For PSMO layer, the ak (0.3893 nm) was slightly larger than a\ (0.3825 nm), indicating an in-plane stretching of the PSMO lattice. The corresponding strain values (e\ = -1.26 % and ek = 0.49 %) indicated that PSMO layer had slight tensile strain. Like PSMO, for LCMO layer, ak (0.3872 nm) was larger than a\
Fig. 2 HRXRD asymmetric 2h-x scan about the (103) plane of PSMO (36 nm)/LCMO (24 nm) thin film on STO (001) substrate
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(0.3726 nm). The corresponding strain values (e\ = -2.51 % and ek = 1.29 %) suggested that LCMO layer had tensile strain in order to match the slightly larger lattice parameter of STO (a = 0.3905 nm). The calculated strain values indicated an out-of-plane compressive strain but an in-plane tensile strain in PSMO/LCMO film. In order to study the in-plane epitaxy of the PSMO/LCMO on STO substrate, we performed the u scans of the sample around asymmetric (103) planes (the scans of (103) PSMO, (103) LCMO and (103) STO reflections starting with the same u azimuth). The reflection intensity from the asymmetric (103) planes of PSMO and LCMO and STO was shown in Fig. 3. The presence of four symmetric peaks with 90° interval confirms the fourfold symmetry of pseudo-cubic perovskites. The reflection peaks of (103) PSMO, (103) LCMO and (103) STO appeared at the same u angle, representing an epitaxial relationship of STO [100]//PSMO [100]//LCMO [100]. As shown in Figs. 1, 2 and 3, we studied the structure of PSMO/LCMO bilayer grown on (001) STO substrate with nondestructive HRXRD techniques and found the following epitaxial orientation relationship with respect to substrate surface: STO (001) [100]//PSMO (001) [100]//LCMO (001) [100]. The epitaxial growth of the bilayer film tends to be perfect, which probably correlated with low lattice mismatch rate between film and substrate. To investigate the microstructure at the surface and interface of film, grazing incident X-ray reflectivity (GIXRR) and atomic force microscopy were also applied. The roughness of surface and interface, the X-ray absorption and diffusion at PSMO/LCMO interface will affect GIXRR of the bilayer film. Figure 4 shows grazing incidence specular reflectivity of PSMO/LCMO bilayer film. The
Fig. 3 The u scan patterns of (103) PSMO and (103) LCMO and (103) STO substrate reflections of PSMO (36 nm)/LCMO (24 nm)/ STO starting from the same azimuth
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fitting result of GIXRR suggests smooth surface [surface root mean square (rms) roughness of about *2.68 nm], negligible interlayer diffusion and roughness at the interface. Atomic force microscopy was also employed to analyze the film’s surface morphology and calculate its surface rms roughness. Figure 5 shows the atomic force microscopy topographic image of PSMO/LCMO bilayer film grown on STO substrate. The scan was shown using the area of 4 9 4 lm2. In Fig. 5, we could see a smooth surface with rms roughness about *2.62 nm. This result was well consistent with above GIXRR fitting results within experimental error. In addition, we could see the presence of large particulates (near top right-hand corner), indicating an atomic collapse model [13]. Such surface deformities (large particulates) are typically associated with non-stoichiometric deposition of island growth film [14, 15]. The shoulder peak S1 in the HRXRD pattern in Fig. 1 is usually associated with some possibilities such as defect, strain and dislocation, whereas no such shoulder peak was seen in the asymmetric scan in Fig. 2. This proved that the emergency of shoulder peak S1 was not related with interlayer diffusion and interfacial structure. Further, the atomic force microscopy image shows the presence of large particulates. Such large particulates are typically observed in PLD-deposited films and are nonstoichiometric in nature. The presence of such non-stoichiometric particulates can impart an overall inhomogeneous composition to the films. Shoulder peak as seen in Fig. 1 could be associated with inhomogeneities of composition in the film which seems to be the case here. Therefore, the variation of crystalline structure of PSMO/ LCMO film occurred due to inhomogeneities of composition induced by surface deformities.
Fig. 4 Experimental (open circle) and simulated (solid line) X-ray specular reflectivities of PSMO (36 nm)/LCMO (24 nm) bilayer film
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The magnetic measurements for the thin films were taken in the in-plane configuration with the magnetic field applied parallel to the film plane. We measured the M–H loops at T = 5 K to investigate the interlayer coupling. Specially, a clear shift of the center of magnetic loop toward the negative field direction at 5 K is observed after field cooling from room temperature under the magnetic field of 500 Oe, as shown in Fig. 6. While for the ZFC process, the hysteresis is normal. The behavior put toward the EB effect in PSMO/LCMO thin film at low temperature. In contrast, the EB disappears at higher temperature such as at 100 K. Our previous researches [12] confirmed that charge-ordering (CO) phase separation surely appeared in the half-doped LCMO and magnetic properties were influenced by metallic FM clusters at interface in inhomogeneous manganite films. The absence of bias effect at 100 K supports the importance of FM cluster at the PSMO/ LCMO interface in decreasing interfacial exchange coupling. When the temperature decreases, the coupling between FM and AFM phases is energetically modulated. At 5 K, the EB field of 46 Oe and coercive field of 234 Oe were obtained in sample under 500 Oe cooling field, as shown in Fig. 6. According to the random-field model of exchange biasing proposed by Malozemoff [16], EB field fJ (HEB) could be expressed as HEB / 2Mf dj FM aL, where Mf and dFM are the magnetization and thickness of the ferromagnet, respectively, fi is a parameter related to randomness of spin orientations with order unity, J is the atomic interfacial exchange coupling, a is the atomic spacing, and L is the domain size. Enhanced ferromagnetism (Mf) due to both phase separation and FM clusters percolation at PSMO/LCMO interface maybe takes responsibility for the disappearance of EB at higher temperature. In addition, surface roughness [17, 18] may decrease interfacial exchange coupling (J) between the ferromagnet and the antiferromagnet and therefore lead to the disappearance of EB. Interestingly, EB was observed at lower temperature (*5 K). We conclude that non-stoichiometric particulates at the surface of PSMO/LCMO film can impart inhomogeneous chemical composition in the film. The inhomogeneous composition probably induces inhomogeneities of magnetic phase at lower temperature, in turn, lead to the decrease in Mf and then enhance the EB effect. EB was not observed in PSMO/LCMO/PSMO previously [12]. Unlike PSMO/LCMO/PSMO, the inhomogeneous composition is more significant in our PSMO/LCMO bilayer film (see Figs. 1, 5), which supports the emergency of EB effect. The results give further demonstrate that EB effect in the bilayer film at lower temperature is closely related to the inhomogeneity in the film. The magnetic properties are also influenced by other factors, such as strain and disorder in the film.
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Fig. 5 Atomic force microscopy contact mode image plotted for an area of 4 9 4 lm2 of PSMO (36 nm)/LCMO (24 nm) bilayer film on STO substrate
Acknowledgments This work was supported by Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ14E010005) and the National Natural Science Foundation of China (Grant Nos. 11079022, 51271093, 10904071, U1332106) and the Scientific Research Foundation of Hangzhou Dianzi University (Grant No KYS205614013). We would like to thank colleagues from Shanghai Synchrotron Radiation Facility (SSRF) and Beijing Synchrotron Radiation Facility (BSRF) for their help in XRD experiments.
References
Fig. 6 Magnetic hysteresis loops of PSMO (36 nm)/LCMO (24 nm) thin film after zero-field-cooled (ZFC) and field-cooled (FC) in 500 Oe. Temperature was fixed at temperature of 5 K during loop measurements
4 Conclusions In summary, high-quality PSMO/LCMO bilayer thin film was grown on STO (100) substrate using PLD. A detailed analysis of the crystalline structure using XRD asymmetric/symmetric scans revealed that the variation of crystalline structure of PSMO/LCMO film occurred probably due to inhomogeneous composition in film. But this did not destroy the epitaxial growth of PSMO/LCMO film, there exists epitaxial orientation relationship: STO (001) [100]// PSMO (001) [100]//LCMO (001) [100]. The emergency of EB (about *46 Oe) at 5 K suggests that the inhomogeneities of PSMO/LCMO film can reduce ferromagnetism and then enhance EB. In contrast, interfacial coupling between FM PSMO layer and CO LCMO layer can induce FM cluster percolation, lead to the enhancement of ferromagnetism and the disappearance of EB. This finding is of practical importance in developing thin film materials with tuned magnetic properties, particularly EB for use in spintronics and sensor devices.
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