Journal of Magnetism and Magnetic Materials 388 (2015) 22–27

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Influence of substrate in all-ferromagnetic superlattices

B.C. Behera a, P. Padhan a, W. Prellier b,n

a Department of Physics, Indian Institute of Technology Madras, Chennai 600036, Indiab Laboratoire CRISMAT, CNRS UMR 6508, ENSICAEN, 6 Bd du Marechal Juin, F-14050 Caen Cedex, France

a r t i c l e i n f o

Article history:Received 28 November 2014Received in revised form28 March 2015Accepted 5 April 2015Available online 8 April 2015

x.doi.org/10.1016/j.jmmm.2015.04.01853/& 2015 Elsevier B.V. All rights reserved.

esponding author. Fax: þ33 231951600.ail address: wilfrid.prellier@ensicaen.fr (W. Pr

a b s t r a c t

The Raman scattering and magnetization of the superlattices consisting of ultrathin layer of two metal-like ferromagnets La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) grown on (001)-oriented LaAlO3(LAO) andSrTiO3(STO) substrates, were studied. The Raman spectrum of LAO/[1-u.c. LSMO/2-u.c. SRO]x60 super-lattice shows modes which are shifted towards higher frequencies relative to that of LAO/[2-u.c. SRO/1-u.c. LSMO]x60 superlattice. However, the Raman spectra of these superlattices indicate the presence oforthorhombic structures of LSMO and SRO for both stacking orders. The STO/[1-u.c. LSMO/2-u.c. SRO]x60superlattices exhibit Curie temperature (TC) at �270 K and Neel temperature (TN) at �140 K. Surpris-ingly, TC of superlattice on LAO grown simultaneously with STO reduces to �209 K for in-plane magneticfield and to �99 K for out-plane field. But for the reverse stacking grown on LAO the TC increases to�121 and �218 K for in-plane and out-of-plane orientation of field, respectively. The superlatticesgrown on LAO do not show any signature of TN. Our result clearly indicates the influence of substrateinduced stress and stacking order on exchange coupling between the LSMO and SRO in the superlattices,providing a useful tool towards tailoring the magnetic properties of heterostructures.

& 2015 Elsevier B.V. All rights reserved.

The superlattice consisting of ferromagnetic constituentsLa0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) exhibits several func-tionalities like antiferromagnetic coupling [1–4], magnetocaloriceffect [5,6], magnetoresistance [7] etc.. Though there are severalexplanations for these physical properties, a detailed article re-ports that these physical properties are controlled by the structureof LSMO [3]. The LSMO can be stabilized in rhombohedral [8],orthorhombic [3,9] and mixture of both phases. The electronic andmagnetic properties observed in La1�xSrxMnO3 compounds arestrongly controlled by the Sr-substitution [10–14]. The chemicaldoping similar to hydrostatic pressure, induces the local latticedistortions in bulk manganite leading to the change in bondlengths and/or angles, and thus modified the crystal structures,electronic and magnetic properties. In case of thin films of LSMO,additional source of lattice distortion is the substrate-inducedstrain, which is also an effective tool to tailor the electron–latticecoupling and the magnetic properties. For example, Boschker et al.[9] have observed an orthorhombic crystal structure of LSMO filmsgrown on (LaAlO3)0.3-(Sr2AlTaO6)0.7 substrate. Yang et al. [15] haveobserved the decrease in the saturation magnetization, and mag-netoresistance with the increase in the tensile tetragonal distor-tions of the LSMO thin film. Similarly, Tsui et al. found a magneticanisotropy, and that Curie temperature of LSMO thin films is

ellier).

robust with lattice strain [16]. On the other hand, the magneticproperties of SRO and orthorhombic structure are also sensitive tothe substrate-induced strain [17]. Here, we have explored theepitaxial superlattices consisting of ultrathin layer of LSMO (1 unitcell (u.c.)) and SRO (2 u.c.) grown on (001)-oriented LaAlO3 (LAO)and SrTiO3 (STO) substrates using pulsed laser deposition techni-que. The crystal structure, electronic structure and magneticproperties of these superlattices with both stacking order arepresented in this articles.

Multitarget pulsed laser deposition with a KrF excimer laser(λ¼248 nm) has been used for the synthesis of thin films andsuperlattice structures. The details of the deposition process aredescribed in our previous articles [1–3]. The deposition rate forSRO and LSMO are �0.73 Å/pulse. The superlattices comprising of60 time repetitions of [1-u.c. LSMO/2-u.c. SRO] or [2-u.c. SRO/1-u.c.LSMO] bilayers were grown simultaneously on the (001)-orientedLAO and STO substrates. The structural characterizations of thesuperlattices were performed by using x-ray diffraction. The Ra-man spectra were recorded on a Jobin-Yvon LabRAM HR800UVspectrometer instrument equipped with highly efficient thermo-electrically cooled charge coupled device (CCD) [3]. The magneticproperties measurements were carried out using a super-conducting quantum interface device based magnetometer(Quantum Design MPMS-5). The magnetic field cooled tempera-ture dependent magnetization (M) measurement is performed inthe presence of 0.1 T external magnetic field (H) along the in-planeand out-of-plane directions of (001) oriented LAO and STO

Fig. 2. The low angle θ�2θ x-ray diffraction pattern and fit of (a) (001)STO/[1-u.c.LSMO/2-u.c. SRO]x60, (b) (001)LAO/[1-u.c. LSMO/2-u.c. SRO]x60 and (c) (001)LAO/[2-u.c. SRO/1-u.c. LSMO]x60 superlattices.

B.C. Behera et al. / Journal of Magnetism and Magnetic Materials 388 (2015) 22–27 23

substrates.Since the pseudocubic lattice parameter of the LSMO (3.88 Å)

and SRO (3.93 Å) are larger than that of the lattice parameter ofLAO (3.79 Å) substrate, the LAO substrate induces in-plane com-pressive strain for both LSMO and SRO with 2.37% and 3.69% latticemismatch, respectively. However, the STO substrate (3.905 Å)provides the in-plane tensile and compressive strain for the epi-taxial growth of LSMO and SRO with lattice mismatch �0.64% andþ0.64%, respectively, because its lattice parameter lies in betweenthe lattice parameters of LSMO and SRO. The θ�2θ x-ray patterns(Fig. 1) exhibit only the single phase (001)-oriented superlatticewith satellite reflections which confirm the long range periodicityof the superlattice structure with good crystallinity. To quantifythe number of unit cells present in these superlattice structures,we have carried out the quantitative refinement of x-ray diffrac-tion profile using calibrated thicknesses in DIFFaX program [1,2].Qualitative agreement between the simulated profiles with themeasured peak positions and relative intensities (Fig. 1) suggestthat the actual stacking periodicities are close to the designedvalues. The observed Kiessig fringes around the principal max-imum of the STO/[1-u.c. LSMO/2-u.c. SRO]x60 superlattice is shownin Fig. 1(b). The Kiessig fringes are extremely sensitive on thequality of the interface. The absence of Kiessig fringes in the caseof superlattices grown LAO is attributed to the twin surface of thesubstrate. The average out-of-plane lattice parameter calculatedfrom the 0th order satellite peak position of the STO/[1-u.c. LSMO/2-u.c. SRO]x60 superlattices is around 3.93 Å which is larger thanthe bulk value of LSMO. This indicates the presence of in-planecompressive strain for LSMO. The change in out-of-plane latticeparameters is therefore 0.43% with respect to the average bulk

Fig. 1. The θ�2θ x-ray diffraction and simulated patterns of (a) and (b) (001)STO/[1-u.c.u.c. SRO/1-u.c. LSMO]x60 superlattices.

lattice parameter of the bilayer. However, the observed out-of-plane lattice parameter calculated from the 0th order satellite peakposition of LAO/[1-u.c. LSMO/2-u.c. SRO]x60 and LAO/[2-u.c. SRO/1-u.c. LSMO]x60 superlattices is 4.088 Å and 4.085 Å, respectively. So,the change in out-of-plane lattice parameter with respect to theaverage bulk lattice parameter of the bilayer is 4.47% and 4.39%for LAO/[1-u.c. LSMO/2-u.c. SRO]x60 and LAO/[2-u.c. SRO/1-u.c.

LSMO/2-u.c. SRO]x60, (c) (001)LAO/[1-u.c. LSMO/2-u.c. SRO]x60 and (d) (001)LAO/[2-

B.C. Behera et al. / Journal of Magnetism and Magnetic Materials 388 (2015) 22–2724

LSMO]x60 superlattices, respectively. This difference in latticeparameters indicate that the substrate-induced stress experiencedby the LSMO–SRO superlattices on LAO is 10 times larger than thatof the superlattice on STO.

Fig. 2 shows the low angle θ�2θ x-ray scans of three super-lattices grown on STO and LAO substrates. All superlattices showrelatively broad first order Bragg's peaks. The broadness of the firstorder peak is due to the ultrathin bilayer thickness combinationsand the merging of Kiessig fringes. However, similar to the wideangle x-ray pattern, only STO/[1-u.c. LSMO/2-u.c. SRO]x60 super-lattice (see Fig. 2a) shows kiessig fringes in the range of 1°–3°. Toobtain layer thickness and interface roughness of these super-lattices we fitted the data using GenX code [18] based on Parrattformalism [19]. The fitting of the superlattice on STO indicate ahigh quality stacking with limited layer roughness of 0.6 Å andbilayer thickness of 10.6 Å. On the other hand a relatively highervalue of roughness 1.01 Å is found from the fit of superlatticesgrown on LAO though the bilayer thickness combinations arefound to be 12.11 Å and 10.8 Å for LAO/[1-u.c. LSMO/ 2-u.c. SRO]x60and LAO/[2-u.c. SRO/1-u.c. LSMO]x60, respectively. The bilayerthicknesses obtained from the fitting are consistent with the va-lues calculated from the satellite peak positions shown in Fig. 1.The relatively lower intensity of the Bragg's peak compared to thatof the fitted one of the superlattice on LAO is attributed to the twinsurface of the LAO substrate.

Fig. 3 shows the typical room temperature Raman spectra ofthe substrate, thin films of LSMO, and SRO and the superlatticewith both stacking order. These spectra show a common Ramanpeak at 487 cm�1 corresponding to the LAO substrate. In addition,the intensity of Raman mode at 487 cm�1 is found to be lower forthe thin film of LSMO and SRO and lowest for the superlatticescompared to that of the bare LAO substrate. This indicates that theRaman scattering from the LAO substrate becomes weaker as thefilm thickness increases. It is shown in [10] that La Sr MnOx x1 3− cancrystalize in orthorhombic D h2

16 and rhombohedral D d36 structures

depending on the doping concentration. For LSMO with rhombo-hedral structure, the factor group analysis shows that five modes(A1gþ 4Eg) are Raman active out of total 30 vibrational modes [8].The Raman spectrum of LSMO thin film shows (i) a shoulder to theRaman mode of LAO at around 202 cm�1, (ii) distinct Ramanmodes at around 274, 349, and 434 cm�1 and (iii) a broad bandaround 662 cm�1. By comparing with single crystal LSMO having a

Fig. 3. Raman spectra of (a) (001) LAO, (b) (001)LAO/(78-u.c.) LSMO, (c) (001)LAO/(100-u.c.) SRO, (d) (001)LAO/[1-u.c. LSMO/2-u.c SRO]x60 and (e) (001)LAO/[2-u.c.SRO/1-u.c. LSMO]x60.

rhombohedral structure, we assign the modes at 202 and434 cm�1 to the A1g and Eg symmetry [8]. These Raman modes arealso consistent with the computed zone center phonon fre-quencies for LSMO [20]. However, the presence of remaining Ra-man modes at around 274, 349 and 662 cm�1 do not belong to thereported rhombohedral structure suggesting that the thin film hasother crystal structures. Recently, it was reported that the or-thorhombic LSMO thin film can be grown under compressivestrain on (LaAlO3)0.3-(Sr2AlTaO6)0.7 substrate [9]. In addition,Kreisel et al. have observed the coexistence of rhombohedral andorthorhombic phases in the LSMO layer of LSMO/STO multilayers[21]. In orthorhombic structure, the MnO6 octahedra have differ-ent Mn–O bond lengths and are rotated around the cubic [010] and[101] axes. It is normally accepted that a cooperative static Jahn–Teller effect at the Mn3þ site is responsible for this distortedperovskite structure of manganites [22]. Notice that the fre-quencies of 274, 349 and 662 cm�1 modes are near the intense290, 326 and 643 cm�1 modes of the orthorhombic phase ofLaMnO3 [8,23]. Therefore, we attribute these peaks to scatteringinduced by orthorhombically distorted regions in the thin films. Byconsidering orthorhombic structure of LSMO, the Raman modes at273, 345 and 668 cm�1 can be assigned to the Ag, B3g, and B2gsymmetry, respectively [23]. Thus, the LSMO thin film shows themixture of orthorhombic and rhombohedral structure. Ramanspectrum of SRO thin film shows Raman lines at around 207, 238,284, 355, 374, 399, 586, and 729 cm�1, which clearly resembleswith the SRO orthorhombic structure [24,25]. Such Raman linesare further assigned to definite atomic vibrations based on theirsymmetry compared to the phonon frequencies predicted by lat-tice dynamics calculations [25]. Thus, the peaks at 207, 238, 284,374, and 586 cm�1 are all of Ag symmetry, while the peaks ataround 355, 399 and 729 cm�1 belong to B2g symmetry. The mostintense SRO phonon mode at 374 cm�1 assigned to Ag symmetryusually arise from the vibrations of apical oxygen atoms in SROthat modulate the Ru–O–Ru bond angle [25], whereas the Raman-active mode at higher frequency region 586 cm�1 represents thein-phase stretching vibration of Ru–O bond [25].

The LAO/[1-u.c. LSMO/2-u.c. SRO]x60 superlattice exhibits theRaman peaks at 206, 232, 298, 392 and 610 cm�1 (Fig. 3d). Thecomparison of Raman modes of this superlattice with the Ramanmodes of LSMO (Fig.3b) and SRO (Fig.3c) reveals (a) the Ramanpeaks at 206, 232, 392 and 610 cm�1 of the superlattice are closeto the Raman peaks of SRO thin films (Fig. 3c), indicating that theSRO layer of superlattice resembles orthorhombic structure,(b) the Raman peaks at 298 and 610 cm�1 of the superlattice areclose to the Raman peaks of LSMO thin films of orthorhombicstructure, and finally (c) the absence of most intense and dom-inating Raman peak at around 434 cm�1 (the characteristic spec-tral signature of rhombohedral structure of LSMO [8]) in the su-perlattice, rule out the possibility of the rhombohedral structure ofthe LSMO layer so that LSMO layer in the superlattice exhibits onlyorthorhombic structure. The LAO/[2-u.c. SRO/1-u.c. LSMO]x60 su-perlattice exhibits the Raman peaks at 210, 293, 386, and606 cm�1, (Fig.3e). Both LSMO and SRO layers of the LAO/[2-u.c.SRO/1-u.c. LSMO]x60 superlattice show the orthorhombic structuresimilar to that of the LAO/[1-u.c. LSMO/ 2-u.c. SRO]x60 superlattice.The intense peak in these superlattices at 232 or 210 cm�1 can beassigned to the Ag mode involving mainly Sr motion.[25] Similarly,the next intense peak at 392 or 386 cm�1 can be assigned to Ag orB2g mode. These modes correspond to the vibration of apicaloxygen which modulate Ru–O–Ru bond angle. While the next in-tense broad peak at 610 or 606 cm�1 can be assigned to the B2g inwhich, O2� of the plane vibrate in the Mn–O directions [23].However, we cannot neglect the contribution from the Ag modewhich represents the in-phase stretching vibration of Ru–O bond [25]. In general, the compressive strain leads to Raman shift

Fig. 4. Temperature dependent 0.1 T field cooled in-plane magnetization of (a) (001)LAO/(100-u.c.) SRO, (b) (001)LAO/(78-u.c.) LSMO, (c) (001)LAO/[1-u.c. LSMO/2-u.cSRO]x60 and (d) (001)LAO/[2-u.c. SRO/1-u.c. LSMO]x60. Temperature dependent 0.1 T field cooled out-of-plane magnetization of (e) (001)LAO/[1-u.c. LSMO/2-u.c SRO]x60 and(f) (001)LAO/[2-u.c. SRO/1-u.c. LSMO]x60.

Fig. 5. Temperature dependent 0.1 T field cooled in-plane magnetization of (001)STO/[1-u.c. LSMO/2-u.c. SRO]x60 superlattice.

B.C. Behera et al. / Journal of Magnetism and Magnetic Materials 388 (2015) 22–27 25

towards higher wave number and reverse for the case of tensilestrain. So, the observed Raman shift of both the superlattices tohigher wave number compare to thin film of LSMO and SRO is dueto the large compressive strain as confirmed from the x-ray dif-fraction results. However, we were unable to distinguish the Ra-man modes of STO/(78-u.c.) LSMO, STO/(100-u.c.) SRO, STO/[1-u.c.LSMO/2-u.c. SRO]x60 from that of the bare STO.

Fig. 4 displays the 0.1 T field cooled (FC) temperature depen-dent magnetization (M(T)) of the thin films and superlattices afterdiamagnetic correction for the substrate. The in-plane M(T) curve(Fig.4a) shows the onset of ferromagnetism of the SRO thin film atthe Curie temperature (Tc)�160 K, which is same as Tc of bulk SRO.Similar features are also found in the in-plane M(T) curve of LSMOthin film (Fig. 4b). The onset of ferromagnetism of LSMO thin filmarises at Tc� 352 K which is close to the Tc of bulk LSMO. The in-plane M(T) of (001)LAO/[1-u.c. LSMO/2-u.c. SRO]X60 superlattice(Fig.4c) shows two ferromagnetic transitions. That means, as thetemperature decreases from 360 K the first onset of spontaneousmagnetization occurs at around 209 K above which the sample

becomes paramagnetic. This temperature (209 K) below which thesuperlattice exhibits ordered magnetic moment is the TC of thesuperlattice. On further cooling the superlattice a sharp rise ofmagnetization is observed below temperature �61 K where theSRO layer becomes ferromagnetic. The temperature below whichSRO becomes ferromagnetic is denoted as TC

⁎. However, the out-of-plane M(T) curve of this superlattice (Fig.4e) reveals only a singleferromagnetic transition around 99 K which is the characteristicfeature of the superlattice. Qualitatively, similar in-plane and out-of-plane M(T) curves are observed for (001)LAO/[2-u.c. SRO/1-u.c.LSMO]x60 superlattice (see Figs. 4d and 4f). Nevertheless, the TC

and TC⁎ of (001)LAO/[2-u.c. SRO/1-u.c. LSMO]x60 superlattice is

significantly larger than that of the (001)LAO/[1-u.c. LSMO/2-u.c.SRO]X60 superlattice. Interestingly, these superlattices do not showany decrease of in-plane magnetization below the TC of SRO [1–4].The possible sources that can influence TC and TC

⁎ of these super-lattices are substrate induced strain, oxygen stoichiometry, inter-facial disorder, size effect and crystal structure of constituents. Inorder to find further insight of these effects we have measured themagnetization of LSMO–SRO superlattice grown on STO simulta-neously with LAO. The in-plane FC-M(T) of the (001)STO/[1-u.c.LSMO/2-u.c. SRO]x60 superlattice (Fig. 5) shows TC at 270 K and agradual decrease in magnetization below 140 K, which we havemarked as TN (Neel Temperature). The drop in magnetization be-low 140 K is due to the interfacial magnetic and electronic dis-orders in the interfacial region [1,2]. The TN and TC of this super-lattice is relatively higher and lower than the value observed in[26] for (001)STO/[1-u.c. LSMO/1-u.c. SRO]x22, respectively. Inter-estingly, the comparison of the in-plane FC-M(T) curves of thesame superlattice grown on the (001)-oriented LAO than STOsubstrates reveal; (i) reduced TC, (ii) broader ferromagnetic toparamagnetic transition and (iii) disappearance of in-plane anti-ferromagnetic coupling in the case of LSMO–SRO superlatticegrown on LAO. Since these superlattices are grown simultaneously in two successive run with same growth parameter wecan neglect the influence of oxygen stoichiometry on TC and TC

⁎.

Fig. 6. Field dependent magnetization at 10 K with field oriented along (a) and (b) in-plane and (c) and (d) out-of-plane of (001)LAO/[1-u.c. LSMO/2-u.c. SRO]x60 and (001)LAO/[2-u.c. SRO/1-u.c. LSMO]x60 superlattices, respectively.

B.C. Behera et al. / Journal of Magnetism and Magnetic Materials 388 (2015) 22–2726

Also we can neglect the influence of size effect on the TC and TC⁎ of

the superlattices on LAO as (i) the superlattices are simultaneouslygrown with that of the STO and (ii) the bilayer thickness obtainedfrom the fit of low angle x-ray are very close to each other. So, thereduced TC and TC

⁎ of the superlattices grown on LAO could be dueto the substrate induced stress and the absence of anti-ferromagnetic interfacial coupling could be due to the twin surfaceof LAO.

Fig. 6 shows zero-field-cooled (ZFC) field dependent magneti-zation M(H) curves measured at 10 K of LAO/[1-u.c. LSMO/2-u.c.SRO]x60 and [2-u.c. SRO/1-u.c. LSMO]x60 superlattices. The mag-netization is hysteretic for both directions of the field, and doesnot saturate even at 4.5 T field. The magnetization at 10 K under4.5 T field of LAO/[1-u.c. LSMO/2-u.c. SRO]x60 superlattice is 1.48and 1.51 μB/u.c. while that of [2-u.c. SRO/1-u.c. LSMO]x60 super-lattice is 1.73 and 1.27 μB/u.c. for in-plane and out-of-plane or-ientations, respectively. Though the magnetization at 4.5 T field ofboth superlattices is close to that of SRO (1.6 μB/Ru [27]), it isnoticeably lower than that of LSMO (3.34 μB/Mn [28]). Note thatthe STO/[20-u.c. LSMO/n-u.c. SRO]x15 superlattices with n¼2–12exhibit enhanced saturation magnetization [1]. The observedmonotonic increase of high field magnetization and quenchedmoment can be explained from the x-ray (Figs. 1 and 2) and Ra-man spectra (Fig. 3). The presence of in-plane compressive stressdrives the vibrations of apical oxygen atoms in SRO and LSMOorthorhombic structures to higher frequency which modulates thebond angle and bond length, and may lead to spin pinning/biasingor spin canting. The coercive filed (HC) at 10 K of LAO/[1-u.c. LSMO/2-u.c SRO]x60 superlattice is 0.063 and 0.26 T while that of [2-u.c.SRO/1-u.c. LSMO]x60 superlattice is 0.07 and 0.46 T for in-planeand out-of-plane orientations, respectively. Interestingly, the HC

values of both superlattices are larger than that of the LSMO. ButHC of LAO/[1-u.c. LSMO/2-u.c SRO]x60 superlattice is relativelylower. However, the out-of-plane HC of [2-u.c. SRO/1-u.c. LSMO]x60superlattice is larger than that of the SRO [3].

In conclusion, superlattices consisting of two metal-like ferro-magnets La0.7Sr0.3MnO3 and SrRuO3 with both stacking ordergrown on (001)-oriented LaAlO3 and SrTiO3 were investigated. Theexistence of relatively higher substrate induced stress on the su-perlattices grown on LAO compared to that of the STO is confirmedby the x-ray diffraction spectra and the shift of Raman modestowards higher frequency. In addition, the Raman spectra of thesesuperlattices grown on LAO strongly indicate the stabilization oforthorhombic structures of LSMO and SRO irrespective of thestacking order. The superlattices grown on LAO show reduced TCand absence of antiferromagnetic coupling compared to that of thesimultaneously grown superlattice on STO. The reduced TC and TC

⁎

of the superlattices grown on LAO could be due to the substrateinduced stress and the absence of antiferromagnetic interfacialcoupling could be due to the twin surface of LAO. However, the TCof the LAO/[1u.c. LSMO/2u.c SRO] superlattice is smaller than theTC of the superlattice with other stacking order. Our results in-dicate strong interfacial coupling between layers in such super-lattices providing a useful tool towards tailoring the magneticproperties of heterostructures.

We greatly acknowledge Indo-French collaboration through thefinancial support of both the Laboratoire Franco-Indien pour leCooperation Scientifique (LAFICS) and the IFPCAR/CEFIPRA (Projectno. 3908-1). Partial support from PIA2-FCN2 Medilight17 is alsoacknowledged.

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