Perovskite manganites are well known for their complex magnetic and electronic properties. Their heterostructures are even more perplexing, specifically when interfaced with magnetic or nonmagnetic insulators as they offer particular promise for applications such as spin filter tunnel junctions. This study explores the structural and magnetic properties of one such heterostructure, namely, La0.7Sr0.3MnO3/LaCoO3 (LSMO/LCO) bilayers as the thickness of the LCO layer is varied while keeping that of the LSMO layer fixed. Exceeding a threshold value while gradually increasing the LCO layer thickness, prominent side lobes are observed in the reciprocal space map signifying the formation of periodic structural striped domains in LCO. The magnetic properties of the bilayers, specifically the interfacial couplings, are markedly influenced by the variation of the LCO thickness with noteworthy consequences that include a tunable negative exchange bias.

Heterostructures comprising perovskite oxides have been a topic of increasing interest because of their diverse range of functionalities (e.g., metallicity, ferromagnetism, etc.), potential applications in spintronics,1 and exotic physics of the interfaces.2–5 The interfaces, besides regulating the interlayer coupling between the layers, also dictate the overall properties of the systems. For instance, the interfaces between two non-ferromagnetic layers such as LaCrO3/LaFeO3 and LaMnO3/LaNiO3 on (111)-oriented substrates exhibit ferromagnetism that can be attributed to the superexchange interactions between the concerned magnetic moments at the interfaces.6,7

Heterostructures with manganites as one of the components have been paid specific attention primarily because of the promising properties of manganites [e.g., La0.7Sr0.3MnO3 (LSMO), La0.7Ca0.3MnO3] that include a nearly half-metallic and ferromagnetic behavior with an extremely high degree of spin polarization.8 Heterostructures with these metallic manganites have been reported to feature exotic interfacial properties.9–12 Specifically, those comprising manganites and ruthenates have garnered ample interest due to properties such as tunable antiferromagnetic coupling and exchange bias at the interface.12–14 Magnetic tunnel junctions, consisting of two half-metallic ferromagnets La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 separated by an insulating thin layer of SrTiO3 (STO), have also been extensively studied revealing their potential applicability in preparing magnetoresistive devices.15 

Another compound that has largely contributed to the field of heterostructures is LaCoO3 (LCO) which is a diamagnetic insulator in the bulk at low temperatures. It exhibits a ferromagnetic order under epitaxial tensile-strain near 85 K.16–20 The heterostructures involving LCO are also appreciated for their exotic physical properties and viability for practical applications including designing of spin-filtering tunnel barriers (in the heterostructures of LCO and STO).21 Stabilization of higher spin ordering due to octahedral distortions and enhancement of magnetic order has been observed in LCO/STO multilayers.22,23 Lucas et al.24 have demonstrated magnetic tunnel junctions in manganites and LCO heterostructures. A recent work from Cabero et al.25 has demonstrated that dramatic magnetic anisotropy can be induced by simply changing the growth sequence of the LSMO and LCO, suggesting the great opportunity of utilizing the structural distortion to modify the magnetic ground states of hybridized oxide heterostructures.

The above discussion highlights the diversity of electronic phenomena explored so far at the interfaces of heterostructures containing either LSMO or LCO and motivates us to study the bilayer (BL) systems made of LSMO and LCO together. In this work, we explore the structural and magnetic properties of these bilayers coherently grown on (001)-oriented STO substrates by pulsed laser deposition (PLD). We observe the signatures of periodic structural striped domains in the bilayers with increasing the LCO thickness. Our study reveals a strong ferromagnetic coupling at the interfaces between LSMO and LCO for thinner LCO layers. When the LCO thickness is increased, the magnetic order at the interface gets suppressed as a result of weakening of this interfacial coupling aided by the appearance of striped domains in the LCO layer. We also demonstrate the influence of varying the LCO thickness on the negative exchange bias (NEB) observed at the interfaces.

5 nm LSMO, 20 nm LCO single layer, and [5 nm-LSMO/t nm-LCO] bilayers (BLs) (t = 2, 4, 7, 10, 12, 15, 18) [see Fig. 1(a) for a schematic of the bilayers] are grown on the (001)-oriented STO substrates by means of PLD from stoichiometric ceramic targets of LSMO and LCO. During the deposition, the substrate temperature is kept at 700°C and the oxygen pressure is fixed at 0.2 mbar for LSMO and 0.4 mbar for LCO. The laser energy is fixed to 1.5 J/cm2. After deposition, the samples are annealed in oxygen at 600 mbar for 30 min.

FIG. 1.

(a) A schematic of the LSMO/LCO BL films. (b) Surface topography image of 5 nm-LSMO/10 nm-LCO BLs on the STO (001) substrate, indicates the presence of the smooth step edge. (c) High-resolution θ2θ scan around (002) reflection of 5 nm-LSMO/t nm-LCO BLs grown on the STO (001) substrate for t=2,4,7,10,12,15, and 18. The arrows indicate the (002) peak position of LCO and LSMO layers.

FIG. 1.

(a) A schematic of the LSMO/LCO BL films. (b) Surface topography image of 5 nm-LSMO/10 nm-LCO BLs on the STO (001) substrate, indicates the presence of the smooth step edge. (c) High-resolution θ2θ scan around (002) reflection of 5 nm-LSMO/t nm-LCO BLs grown on the STO (001) substrate for t=2,4,7,10,12,15, and 18. The arrows indicate the (002) peak position of LCO and LSMO layers.

Close modal

The surface topography of the BLs is verified by an atomic force microscope (AFM, Asylum Research). Structural characterization of the films has been performed at room temperature by using a Philips X'pert MRD diffractometer with Cu-Kα radiation (λ=1.54Å). Magnetic measurements are carried out by a Quantum Design SQUID (superconducting quantum interference device) magnetometer. The magnetizations are normalized by using the volume of the sample and an average unit cell size of (3.905Å×3.905Å× average out-of-plane lattice parameter of the film) for BLs grown on STO and expressed in Bohr magneton per unit cell (μB/μc).

A schematic diagram of the LSMO/LCO BLs is shown in Fig. 1(a) with the bottom and the top layer being LSMO and LCO, respectively. The topography of the 5 nm-LSMO/10 nm-LCO BL is shown in Fig. 1(b). The AFM shows low roughness with terrace-structured surface morphology and steps of unit cell height (4Å) evidencing a layer-by-layer growth of the film. Observation of Kiessig fringes in the X-ray reflectivity data (not shown) of the BLs indicates the smooth and well-defined interfaces between the LSMO and LCO layers (interface roughness is 5±2Å, which is roughly a unit cell thick). The θ2θ X-ray diffraction spectra for the BLs are shown in Fig. 1(c). The prominent (001) and (002) peaks between 20° and 55° clearly indicate the absence of any other phase in the BLs. The arrows indicate the (002) peak position for the LCO and LSMO layers.

In order to know the in-plane lattice match between the layers and the substrates, a high-resolution X-ray reciprocal space mapping (RSM) around (103) reflection of the 5/4,5/10,5/15 and 5/18 LSMO/LCO BLs is performed and shown in Fig. 2(a) for the 5/15 BL. All the BLs are coherently strained on the STO substrate i.e., the in-plane lattice parameters of LSMO and LCO are the same with that of the STO substrate. The LSMO and LCO peaks are conspicuous. The out-of-plane lattice parameters (c) are calculated from the RSM. We find that the out-of-plane lattice parameter of LCO (cLCO), when measured as a function of the LCO layer thickness tLCO, shows a decreasing trend as evident from Fig. 2(b). The reduction in the c lattice parameter of the LCO layer with increasing LCO layer thickness is probably due to the interfacial octahedral rotation coupling.26–28 Since LCO has larger rhombohedral distortions than the underlying LSMO layer and STO substrates, because of the interfacial coupling, the thin LCO layer would appear to be pseudo-tetragonal-like and therefore, the Co-O-Co bond angle would be closer to 180° and the out-of-plane c lattice parameter will increase. Similar behavior has been observed in LSMO/STO27 and BFO/LSMO/STO heterostructures.26 Information about the in-plane strain state of LSMO and LCO in the BLs can be extracted by measuring the pseudocubic bulk lattice constant which is 3.87Å for LSMO29 and 3.81Å for LCO.16 The measurements reveal that LSMO and LCO, both are under tensile strain (+0.9% for LSMO and +1.9% for LCO) in the BLs. Side lobe diffraction peaks (satellites) are also observed in the RSM along the in-plane direction [marked by red arrows in Fig. 2(a)] which provide a clear indication of the periodic structural striped domain type modulations in the BLs along the in-plane direction with respect to the central peak. The lateral periodicity is 55 nm as calculated from the reciprocal lattice spacing. It should be noted that a similar kind of in-plane modulation has earlier been observed in ferroic systems.30–33 The in-plane arrangement of the satellite pattern can form ring like structures or align in the Qx and Qy direction or in particular, any azimuth.34 Here, we observe that the satellites are aligned along the in-plane direction.

FIG. 2.

(a) RSM around the (103) reflection of the 5 nm-LSMO/15 nm-LCO BL has satellites along the in-plane direction (red arrows) corresponding to the ordering of structural striped domains with periodicity 55 nm. (b) The out-of-plane lattice parameter cLCO of LCO is plotted as a function of its thickness tLCO.

FIG. 2.

(a) RSM around the (103) reflection of the 5 nm-LSMO/15 nm-LCO BL has satellites along the in-plane direction (red arrows) corresponding to the ordering of structural striped domains with periodicity 55 nm. (b) The out-of-plane lattice parameter cLCO of LCO is plotted as a function of its thickness tLCO.

Close modal

Among different reasons behind the origin of the in-plane satellites included are dislocations, grain boundaries, periodic structural discontinuities, or periodic domains.35 As LCO and LSMO, both are coherently strained with the STO substrates, dislocations are unlikely to form. The satellite peaks observed around the (103) and (013) diffraction peaks [(013) not shown] suggest the existence of periodic structural striped domains instead of random grain boundaries or defects. Besides, their appearance for only when tLCO is above 10 nm indicates the ferroelastic domain formation beyond a critical thickness of the LCO film. Such phenomena can be ascribed to the competition between the elastic energy (which grows with the thickness because the LCO layer is under tensile strain on the STO substrate), the magnetostatic energy from the built-in magnetic field, and the crystallographic anisotropy energy which together engage to create the domains beyond a certain value of tLCO.

In Fig. 3(a), we demonstrate the temperature dependence of the in-plane [μ0H//100] magnetization of the BLs at 0.1 T after field cooling (FC) the samples at 0.1 T and contrasted with that of the LCO single layers. The single layers of LCO films exhibit a ferromagnetic order in agreement with the previous reports.16 All the BLs of LSMO/LCO, on the other hand, show two distinct transitions. The transition temperatures are estimated from the derivative of the magnetization with respect to T. Upon approaching from high temperatures, a paramagnetic to ferromagnetic transition first takes place at around 320 K [TC(LSMO)]. Upon further cooling [below 85 K i.e., around TC(LCO)], when the LCO layers tend to order ferromagnetically, the net moment of the BLs starts increasing. These features evince an interfacial ferromagnetic (IFM) coupling between the adjacent LSMO and LCO layers. The IFM coupling temperature [TIFM or TC(LCO)] as well as the Curie temperature of LSMO TC(LSMO) gets strongly reduced with increasing tLCO [Fig. 3(b)]. An estimated change in TC is around 15 K for the former and 20 K for the latter with tLCO being varied from 2 nm to 18 nm in the BLs. The tendency is suggestive of the fact that the IFM coupling mediated by the Mn-O-Co bonds (and the interfacial magnetic order) enhances the magnetic order of the LSMO layer upon reducing the LCO thickness. The interfacial couplings are stronger in thinner LCO layers as also apparent from the magnetic order of the 5 nm LSMO single layer (TC240 K, not shown).

FIG. 3.

Temperature-dependent magnetization (M) along H//100 (in-plane) of the (a) LCO and LSMO/LCO BLs, recorded at 0.1 T during warming after field cooling the BLs. The interfacial ferromagnetic coupling (IFM) is visible in the bilayers below the TC=85 K. (b) The Curie temperature of LSMO [TC(LSMO)] and IFM temperature [TC(LCO)] are plotted as functions of the LCO layer thickness (tLCO) in the LSMO/LCO BLs, extracted from (a).

FIG. 3.

Temperature-dependent magnetization (M) along H//100 (in-plane) of the (a) LCO and LSMO/LCO BLs, recorded at 0.1 T during warming after field cooling the BLs. The interfacial ferromagnetic coupling (IFM) is visible in the bilayers below the TC=85 K. (b) The Curie temperature of LSMO [TC(LSMO)] and IFM temperature [TC(LCO)] are plotted as functions of the LCO layer thickness (tLCO) in the LSMO/LCO BLs, extracted from (a).

Close modal

The magnetic hysteresis curves M(H) have been recorded at 10 K in the film plane along a pseudocubic [100] direction. Figure 4(a) shows the typical ferromagnetic loop for the single layer of LCO and LSMO films. In BLs, as argued before, the magnetic order gets enhanced gradually with reducing the LCO thickness tLCO [Fig. 4(b)]. At a certain value of tLCO, the loop shows a two-step switching and completely changes to single ferromagnetic nature for the 5/18 BL. Also, the saturation magnetization (MS) and the coercive field (HC) increase with reducing tLCO [Figs. 4(c) and 4(d)]. The phenomena indicate that for ultrathin LCO layers, a strong IFM coupling is at play, however, that is weakened in thicker LCO layers. The changes in MS and HC are closely related to that of the LCO lattice constant cLCO. As evident in our BLs, the striped domains grow in density in proportion to the LCO layer thickness i.e., with increasing the monoclinic/rhombohedral distortion. The formation of domains also affects the magnetocrystalline anisotropy of the LSMO layer and, as a result, the overall magnetization of the bilayers reduces at higher values of tLCO. A negative exchange bias (NEB) is observed in the BLs [Fig. 4(d)] because of this IFM coupling which can be ascribed to the Mn-O-Co superexchange across the LSMO/LCO interface. The exchange bias field (Heb) is defined by the horizontal shift of the loops measured after field cooling the samples at 0.1 T. The reduction of Heb with increasing tLCO clearly demonstrates the suppression of the ferromagnetic order at the interface with increasing the LCO thickness in the BLs.

FIG. 4.

Isothermal magnetizations along H//100 (in-plane) of the (a) single layer of LSMO (5 nm-thick) and LCO (20 nm-thick), and (b) LSMO/LCO BLs (all grown on (001)-oriented STO substrates), measured at 10 K in the field-cooling mode under an applied field of 0.1 T. (c) The saturation magnetization (MS), coercive field (HC), and negative exchange bias (Heb) of the BLs decrease with the LCO thickness.

FIG. 4.

Isothermal magnetizations along H//100 (in-plane) of the (a) single layer of LSMO (5 nm-thick) and LCO (20 nm-thick), and (b) LSMO/LCO BLs (all grown on (001)-oriented STO substrates), measured at 10 K in the field-cooling mode under an applied field of 0.1 T. (c) The saturation magnetization (MS), coercive field (HC), and negative exchange bias (Heb) of the BLs decrease with the LCO thickness.

Close modal

The different behavior of the BLs can be explained in terms of their different magnetic responses. A number of parameters are responsible for the enhanced magnetism in the BLs with reducing the LCO thickness, namely (i) the biaxial strain of the layers, (ii) interfacial structure of the adjacent LSMO and LCO layers, and (iii) structural striped domains in LCO; their contributions are analyzed systematically. Regarding point (i): the LSMO and LCO layer are fully strained for all the BLs with the in-plane strain value of LSMO and LCO being the same for all of them (tensile strain: +0.9% for LSMO, +1.9% for LCO). This indicates that the strain is unlikely to be the reason for the different magnetic order in the BLs. About point (ii): lowering the thickness of LCO facilitates a superexchange interaction between the Co and Mn atoms giving rise to a strong overall ferromagnetic order in the BLs. As the LCO layer thickness is increased towards its bulk limit, the antiferromagnetism preponderates in the bulk of LCO, but the interface still fosters a ferromagnetic order. Due to this intriguing competition between the ferromagnetic and the antiferromagnetic order, the overall magnetization of the BLs decreases with the LCO thickness.

Besides the biaxial strain and the interfacial effects, structural striped domains in the LCO film are also responsible to suppress the magnetic order of the BLs. The striped domains start to appear with increasing the LCO thickness beyond a certain value (10 nm). Knížek et al.36 and Ravindran et al.37 have earlier proved by density-functional calculations that such domains emerge in the LCO film with increasing rhombohedral distortions i.e., when the Co-O-Co bond angle differs from 180° giving rise to a tilt of the CoO6 octahedra. Such rhombohedral distortions can strongly affect the ferromagnetic state of the LCO film.31 In our bilayers, the striped domains grow in density in proportion to the LCO layer thickness i.e., with increasing rhombohedral distortions. The domains also affect the magnetocrystalline anisotropy of the LSMO layer and, as a result, the overall magnetization of the bilayers is reduced at the higher values of tLCO.

In summary, we present a detailed study of the structural and magnetic properties of ultrathin LSMO/LCO bilayers. We further investigate the effects of varying the LCO thickness ranging from 2 nm to 18 nm while keeping the thickness of the LSMO layer fixed on the STO substrates. According to our observation, the structural and magnetic properties at the interfaces are highly influenced by such a variation. The BLs, which are coherently strained on the STO substrates, feature an interfacial ferromagnetic coupling and exhibit signatures of periodic structural striped domains when the LCO thickness is gradually increased. The saturation magnetization, TC, of LSMO and the interfacial coupling (TC of LCO) are greatly enhanced for lower thicknesses of LCO. We also notice a negative exchange bias as a result of this interfacial ferromagnetic coupling. The observed reduction of the exchange bias field with the LCO thickness is noteworthy from the tunability perspectives. The results together encourage us to conclude that the bilayer heterostructures consisting of LSMO and LCO can be put to efficient use in a variety of device fabrication.

S.T. acknowledges the facilities provided by the Science and Engineering Research Board, Department of Science and Technology (SERB/DST), under the Young Scientists Scheme (YSS; Ref. No. YSS/2014/000340). S.T. also acknowledges the support from the Department of Atomic Energy, Board of Research in Nuclear Sciences (DAEBRNS), under the Young Scientist Research Award (YSRA; 34/20/02/2015/BRNS) scheme. S.T. further acknowledges the Fund for Improvement of S&T Infrastructure in Universities and Higher Educational Institutions (FIST) programme of the DST, India, for partial support of this work (Ref. No. SR/FST/PSII-020/2009). K.R. was supported in part by the National Science Foundation under Grant No. NSF PHY17-48958. Z.H.C. acknowledges the startup grant from the Harbin Institute of Technology, Shenzhen, China, under Project No. DD45001017.

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