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a shows an X-ray diffraction (XRD) θ-2θ scan of a [BFO 5 /LSMO 20 ] 10 superlattice, where 5 and 20 represent the number of unit cells of BFO and LSMO layers and 10 is the number of bilayer repetitions. Narrow superlattice peaks, as evidenced by seven orders of Kiessig fringes, indicate epitaxial growth of a high quality superlattice. Well-defined interfaces and uniform chemical compositions within individual layers were also confirmed with X-ray reflectivity (XRR) (Figure S1, Supporting Information). Specifically, we obtained a root-meansquare (rms) roughness of 4.0 ± 0.2 Å for the BFO/LSMO interfaces averaged over the coherence of the X-ray beam projected on the sample's surface (≈tens of millimeters). Atomic force microscopy confirmed an atomically flat surface composed of step-and-terrace features with a rms roughness 4.2 ± 0.2 Å (inset of a) over a large area of 3 μm × 3 μm. The superlattice is coherently strained by the STO substrate. c,d shows representative high-angle annular dark-field (HAADF) images of a BFO/LSMO superlattice taken in scanning transmission electron microscopy (STEM) mode. These images show epitaxial growth with chemically sharp and coherent interfaces. [29,30] e shows spatially resolved electron energy-loss spectroscopy (EELS) elemental maps for the Bi-M 4,5 , Fe-L 2,3 , La-M 4,5 , Sr-L 2,3 , and Mn-L 2,3 edges, indicating the chemical uniformity within the individual layer. Chemical sharpness was ascertained using EELS profiling, as shown in
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![Figure 3. Magnetometry measurements on [BFOx/LSMO20]6 superlattices. a)...](profile/Er-Jia-Guo/publication/317385673/figure/fig3/AS:507331214364672@1497968595783/Magnetometry-measurements-on-BFOx-LSMO206-superlattices-a-Field-dependent-and-b_Q320.jpg)
In this work, we obtained a quantitative depth dependent magnetization profile across the planar interfaces in BiFeO3/La0.7Sr0.3MnO3 (BFO/LSMO) superlattices using polarized neutron reflectometry (PNR). We observed an enhanced magnetization of 1.83 ± 0.16 μB/Fe in BFO layers when they are interleaved between two manganite layers. The enhanced magne...
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In the bulk, LaCoO3 (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heter...
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... Therefore, with the increase of the exchange-coupling J int strength, the remanent and saturation magnetization increases, as shown in Fig. 6a. As a result, [62,63]. Furthermore, it's confirming the reported high magnetization value for BFO on LSMO compared to LSMO (or BFO) alone [64,65]. ...
... The plasma is sputtered onto the heated single crystal substrates, as shown in Fig. 2(a), [44] and the sample can be subsequently har-088106-2 vested in the form of thin films, [45][46][47][48][49][50][51] heterostructures, [52][53][54][55] and superlattices. [56][57][58][59] Meanwhile, in order to identify the insitu growth process of low-dimensional structures, a variety of monitoring techniques have been elaborately developed, such as the oblique incidence reflectance difference, [60] surface photo-absorption, [61] spectral ellipsometry, [62] p-polarized reflectance spectroscopy, [63] and reflection high-energy electron diffraction (RHEED). [64] Among these advanced techniques, the RHEED outcompetes for its high sensitivity to variations of the surface topography and in-situ monitoring of the inplane crystalline structure simultaneously. ...
Photons with variable energy, high coherency, and switchable polarization provide an ideal tool-kits for exploring the cutting-edge scientific questions in the condensed matter physics and materials sciences. Over decades, extensive researches in the sample fabrication and excitations have employed the photon as one of the important means to synthesize and explore the low-dimensional quantum materials. In this review, we firstly summarize the recent progresses of the state-of-the-art thin-film deposition methods using excimer pulsed laser, by which syntactic oxides with atomic-unit-cell-thick layers and extremely high crystalline quality can be programmatically fabricated. We demonstrate the artificially engineered oxide quantum heterostructures exhibit the unexpected physical properties which are absence in their parent forms. Secondly, we highlight the recent work on probing the symmetry breaking at the surface/interface and weak couplings among nanoscale ferroelectric domains using optical second harmonic generation. We clarify the current challenges in the in-situ characterizations under the external fields and large-scale imaging using optical second harmonic generation. The improvements in the sample quality and the non-contact detection technique further promote the understanding of the mechanism of the novel properties emerged at the interface and inspire the potential applications, such as the ferroelectric resistive memory and ultrahigh energy storage capacitors.
... However, the discovery oxide-nitride interfaces have never been discussed previously in the literature. The challenge of fabricating stoichiometric nitride thin films lies in the precision of controlling stoichiometry and crystallinity in this class of emergent heterojunctions, as is also the case with perovskite oxide interfaces and epitaxial metal-oxide interfaces [21,22]. We have recently developed a novel method for depositing stoichiometric nitride films using plasmaassisted pulsed laser deposition [23,24], paving the way for the fabrication of artificially structured SLs composed of transition metal nitrides and oxides. ...
Heterointerfaces have led to the discovery of novel electronic and magnetic states because of their strongly entangled electronic degrees of freedom. Single-phase chromium compounds always exhibit antiferromagnetism following the prediction of the Goodenough-Kanamori rules. So far, exchange coupling between chromium ions via heteroanions has not been explored and the associated quantum states are unknown. Here, we report the successful epitaxial synthesis and characterization of chromium oxide (Cr2O3)-chromium nitride (CrN) superlattices. Room-temperature ferromagnetic spin ordering is achieved at the interfaces between these two antiferromagnets, and the magnitude of the effect decays with increasing layer thickness. First-principles calculations indicate that robust ferromagnetic spin interaction between Cr3+ ions via anion-hybridization across the interface yields the lowest total energy. This work opens the door to fundamental understanding of the unexpected and exceptional properties of oxide-nitride interfaces and provides access to hidden phases at low-dimensional quantum heterostructures.
... The challenge of fabricating stoichiometric nitride thin films lies in the precision of controlling stoichiometry and crystallinity in this class of emergent heterojunctions, as is also the case with perovskite oxide interfaces and epitaxial metal/oxide interfaces 21,22 . We have recently developed a novel method for depositing stoichiometric nitride films using plasma-assisted pulsed laser deposition 23,24 , paving the way for the fabrication of artificially structured SLs comprised of transition metal nitrides and oxides. ...
Heterointerfaces have led to the discovery of novel electronic and magnetic states because of their strongly entangled electronic degrees of freedom. Single-phase chromium compounds always exhibit antiferromagnetism following the prediction of Goodenough-Kanamori rules. So far, exchange coupling between chromium ions via hetero-anions has not been explored and the associated quantum states is unknown. Here we report the successful epitaxial synthesis and characterizations of chromium oxide (Cr2O3)-chromium nitride (CrN) superlattices. Room-temperature ferromagnetic spin ordering is achieved at the interfaces between these two antiferromagnets, and the magnitude of the effect decays with increasing layer thickness. First-principles calculations indicate that robust ferromagnetic spin interaction between Cr3+ ions via anion-hybridizations across the interface yields the lowest total energy. This work opens the door to fundamental understanding of the unexpected and exceptional properties of oxide-nitride interfaces and provides access to hidden phases at low-dimensional quantum heterostructures.
... Most publications concerning ME coupling in BFO-based heterostructures explore exchange-bias effects using either transition metal or metal oxide ferromagnets layered on BFO thin films [115,122]. Heterostructures of BFO and LSMO are a popu-lar model system of a multiferroic coupled to an metal oxide ferromagnet [122][123][124][125]. The interface magnetism and exchange-bias coupling in this system are being studied extensively [124]. ...
... The interface magnetism and exchange-bias coupling in this system are being studied extensively [124]. Guo et al. used PNR to locally resolve the magnetization in BFO-LSMO superlattices and found a magnetization of 1.86 µ B /Fe atom [125]. They "exclude charge transfer, intermixing, epitaxial strain, and octahedral rotations/tilts as dominating mechanisms for the large net magnetization" and instead suggest strong orbital reconstruction between Fe and Mn across the interfaces as the origin. ...
... This is in part due to the inaccessibility of detailed local states of strain, electronic configuration and magnetic order. In recent years, this gap is being slowly filled through the use of advanced sample characterization techniques such as atomically resolved HR-TEM [30,125,217], X-ray magnetic circular dichroism [217] and polarized neutron reflectometry (PNR) [30,125,[217][218][219], complemented by density functional theory calculations. In particular PNR has gained much interest in the research of BFO based heterostructures, as this measurement technique allows the formulation of depth-resolved magnetic profiles, even for small magnetic moments such as the small ferromagnetic moment induced Dzyaloshinski-Moriyainteraction in canted anti-ferromagnets such as BFO. ...
The presented thesis explores the magnetoelectric (ME) coupling in multiferroic thin film multilayers of BaTiO3 (BTO) and BiFeO3 (BFO). Multiferroics possess more than one ferroic order parameter, in this case ferroelectricity and anti-ferromagnetism. Cross-coupling between these otherwise separate order parameters promises great advantages in the fields of multistate memory, spintronics and even medical applications. The first major challenge in this field of study is the rarity of multiferroics. Second, most known multiferroics, both intrinsic and extrinsic in nature, possess very low ME coupling coefficients. In previous studies conducted by our group, BTO-BFO multilayers deposited by pulsed laser deposition (PLD) showed a ME coupling coefficient αME enhanced by one order of magnitude, when compared to single-layers of the intrinsic multiferroic BFO. However, the mechanism of ME coupling in such heterostructures is poorly understood until now. In this thesis, we used a selection of structural, chemical, electrical and magnetic measurements to maximize the αME-coefficient and shed light on the origin of this enhanced ME effect. The comparison of BTO-BFO multilayers over single-layers revealed not only enhanced ME-coupling, but also reduced mosaicity, roughness and leakage current density in multilayers. Following a parametric sample optimization, we achieved an atomically smooth interface roughness and vast improvements in the ferroelectric properties by introducing a shadow mask in the PLD process. We measured the highest αME-value so far of 480 Vcm-1Oe-1 for a multilayer with a double-layer thickness of only 4.6 nm, two orders of magnitude larger than the coefficient of 4 Vcm-1Oe-1 measured for BFO single-layers. The αME-coefficient in these multilayers stands in an inverse correlation with the double-layer thickness ddl. The influence of oxygen pressure during growth and BTO-BFO ratio on αME was shown to be neglible in comparison to that of ddl. From the characteristic dependencies of αME on magnetic bias field, temperature and ddl, we concluded the existence of an interface-driven coupling mechanism in BTO-BFO multilayers.
... From the PNR data, the magnetization exists exclusively in the SRO layer and the LNO layer does not have an in-plane-oriented magnetic moment. Previous work has shown that interfacial interaction between transition metal oxides can induce a complex magnetic structure in a nonmagnetic material [56][57][58][59]. A profound interfacial magnetism in the paramagnetic LNO layers was reported in LNO/LaMnO 3 superlattices [56]. ...
... However, to explore the exact symmetry of ultrathin SRO layers is extremely difficult. The 034033-5 symmetry of a SRO ultrathin single layer could be monoclinic [30,46] or orthorhombic [59]. Both RSM and STEM results suggest the capped SRO layers are most likely to be the tetragonal structure. ...
Materials with large perpendicular magnetic anisotropy (PMA) are candidates for spintronic devices, such as magnetic random-access memory, etc., due to their stable magnetic reference states. Because of shape anisotropy, the magnetic easy axis of oxide thin films favors in-plane orientation. In this Paper, we demonstrate a convenient means to control the magnetic anisotropy of SrRuO3 (SRO) ultrathin layers from in-plane to out-of-plane by capping with nonmagnetic materials. Tuning the anisotropy is achieved by imposition of symmetry mismatch at the interface-induced structural transition of SRO with suppressed octahedral tilt. These results suggest a potential direction for engineering magnetic oxide thin films with flexible tunable PMA using capping-layer-induced dissimilar symmetry.
... 233 BFO coupling to manganites such as La 0.7 Sr 0.3 MnO 3 233,234 has received considerable attention because of their good structural compatibility and potential for a well-matched, high-quality interface. Guo et al. 235 have examined LSMO / BFO superlattices using a combination of analytical STEM and polarized neutron reflectometry (PNR). As shown in Figure 16.A, they examined the structure and chemistry of the film layers, revealing minimal intermixing less than 1-2 unit cells. ...
Thin film oxides are a source of endless fascination for the materials scientist. These materials are highly flexible, can be integrated into almost limitless combinations, and exhibit many useful functionalities for device applications. While precision synthesis techniques, such as molecular beam epitaxy (MBE) and pulsed laser deposition (PLD), provide a high degree of control over these systems, there remains a disconnect between ideal and realized materials. Because thin films adopt structures and chemistries distinct from their bulk counterparts, it is often difficult to predict what properties will emerge. The complex energy landscape of the synthesis process is also strongly influenced by non-equilibrium growth conditions imposed by the substrate, as well as the kinetics of thin film crystallization and fluctuations in process variables, all of which can lead to significant deviations from targeted outcomes. High-resolution structural and chemical characterization techniques of the kind described in this volume, are needed to verify growth models, bound theoretical calculations, and guide materials design. While many characterization options exist, most are spatially-averaged or indirect, providing only partial insight into the complex behavior of these systems. Over the past several decades, scanning transmission electron microscopy (STEM) has become a cornerstone of oxide heterostructure characterization owing to its ability to simultaneously resolve structure, chemistry, and defects at the highest spatial resolution. STEM methods are an essential complement to averaged scattering techniques, offering a direct picture of resulting materials that can inform and refine the growth process to achieve targeted properties. There is arguably no other technique that can provide such a broad array of information at the atomic-scale, all within a single experimental session.
... BFO/La 1−x Sr x MnO 3 has recently been proposed as a candidate to engineer three-dimensional topological insulators [21]. Induced interface ferromagnetism as well as exchange bias have been reported in La 0.7 Sr 0.3 MnO 3 /BFO [8,[22][23][24] and in La 0.5 Ca 0.5 MnO 3 /BFO [11]; however, the charge-transfer process was only reported by Liang et al. from the Fe 2p core level x-ray photoemission spectra (XPS) at the manganite/BFO interface [25]. ...
... The orbital and spin moments of the interfacial BFO are M L = 0.23 µ B and M S = 0.88 µ B , respectively, with the total magnetic moment M = 1.11 µ B /Fe atom (Supplemental Material [37]). This interfacial magnetism of Co/BFO is commensurate with that observed at the interfaces of Co 0.4 Fe 0.4 B 0.2 /BFO (1 ± 0.5 µ B /Fe atom) [12] and of La 0.7 Sr 0.3 MnO 3 /BFO superlattices (0.3 μ B /Fe atom [23] and 1.83 ± 0.16 µ B /Fe atom [24]). Note that the theoretical value for the total magnetic moment in bulk iron is 1.2 µ B /Fe atom and the interfacial Fe 2+ shows an approximately 40-fold enhancement compared to that in bulk BFO (0.03 µ B /Fe atom). ...
The interplay between ferroelectricity and magnetism in multiferroic materials is of great scientific and technological interest, allowing magnetic control of ferroelectric properties and electric control of magnetic properties through the magnetoelectric coupling from interfacial strain, exchange bias, or charge-transfer process. We report the charge transfer at the Co/BiFeO3 interface from which the interfacial exchange coupling is achieved with the formation of Fe2+. The x-ray linear dichroism and x-ray magnetic circular dichroism results reveal that the strain as well as the exchange coupling Jex in BiFeO3 (BFO) with periodic 109° domains are stronger than that in BFO with 71° domains, resulting in the higher coercive field of Co/BFO with 109° domain samples. The possible electric field control of the charge-transfer process at the Co/BFO interface enables an alternative class of electrically controllable magnetization, exchange bias, and giant magnetoresistive response of spintronic devices.
... This kind of approach is often utilized for interfaces in thin-film heterostructures of metals and alloys [1,2], while it is yet little exploited for complex oxides. Recent examples are the selection of electronic spin polarization in tunnel junctions with a LaFeO 3 barrier [3,4] and the impact of interfacial magnetism on the exchange bias exerted by magnetoelectric BiFeO 3 [5][6][7]. For both interface types, an insulating antiferromagnet (LaFeO 3 , BiFeO 3 ) shows induced in-plane magnetization at the interface to a ferromagnet (La 0.7 Sr 0.3 MnO 3 ). ...
Interfaces of ferroic oxides can show complex magnetic textures which have strong impact on spintronics devices. This has been demonstrated recently for interfaces with insulating antiferromagnets such as BiFeO3. Here, noncollinear spin textures which can be switched in very low magnetic field are reported for conducting ferromagnetic bilayers of La0.7Sr0.3MnO3−SrRuO3 (LSMO-SRO). The magnetic order and switching are fundamentally different for bilayers coherently grown in reversed stacking sequence. The SRO top layer forms a persistent exchange spring which is antiferromagnetically coupled to LSMO and drives switching in low fields of a few milliteslas. Density functional theory reveals the crucial impact of the interface termination on the strength of Mn-Ru exchange coupling across the interface. The observation of an exchange spring agrees with ultrastrong coupling for the MnO2/SrO termination. Our results demonstrate low-field switching of noncollinear spin textures at an interface between conducting oxides, opening a pathway for manipulating and utilizing electron transport phenomena in controlled spin textures at oxide interfaces.
... This implies, after subtraction of the MSLD, that a reduction of the NSLD smaller than the value of the STO (3.525 × 10 −6 Å −2 ) is found in the oxygen-deficient layer. Compared with the reported larger NSLD value for a stoichiometric La 0.7 Sr 0.3 MnO 3 sample, [27][28][29] such a distinct difference in NSLD makes it a prospect to resolve oxygen composition variations inside the film using neutron scattering. The stoichiometry of other elements was checked by Rutherford backscattering spectroscopy to make sure the difference is only due to oxygen ( Figure S4, Supporting Information). ...
The vacancy distribution of oxygen and its dynamics directly affect the functional response of complex oxides and their potential applications. Dynamic control of the oxygen composition may provide the possibility to deterministically tune the physical properties and establish a comprehensive understanding of the structure–property relationship in such systems. Here, an oxygen‐vacancy‐induced topotactic transition from perovskite to brownmillerite and vice versa in epitaxial La0.7Sr0.3MnO3−δ thin films is identified by real‐time X‐ray diffraction. A novel intermediate phase with a noncentered crystal structure is observed for the first time during the topotactic phase conversion which indicates a distinctive transition route. Polarized neutron reflectometry confirms an oxygen‐deficient interfacial layer with drastically reduced nuclear scattering length density, further enabling a quantitative determination of the oxygen stoichiometry (La0.7Sr0.3MnO2.65) for the intermediate state. Associated physical properties of distinct topotactic phases (i.e., ferromagnetic metal and antiferromagnetic insulator) can be reversibly switched by an oxygen desorption/absorption cycling process. Importantly, a significant lowering of necessary conditions (temperatures below 100 °C and conversion time less than 30 min) for the oxygen reloading process is found. These results demonstrate the potential applications of defect engineering in the design of perovskite‐based functional materials. Oxygen vacancy dynamics and ordering in La0.7Sr0.3MnO3−δ epitaxial films are identified by real‐time X‐ray diffraction. A novel intermediate phase with a noncentered crystal structure is observed. Huge changes of the physical properties (i.e., ferromagnetic metal and antiferromagnetic insulator) are found just by altering the oxygen content. Reverse switching among distinct topotactic phases is achieved at remarkably reduced temperatures.
