Interfacial ferromagnetism and exchange bias in CaRuO3/CaMnO3 superlattices

Unraveling the Correlation between the Interface Structures and Tunable Magnetic Properties of La1-xSrxCoO3-δ/La1-xSrxMnO3-δ Bilayers Using Deep Learning Models

Perovskite oxides are gaining significant attention for use in next-generation magnetic and ferroelectric devices due to their exceptional charge transport properties and the opportunity to tune the charge, spin, lattice, and orbital degrees of freedom. Interfaces between perovskite oxides, exemplified by La1-xSrxCoO3-δ/La1-xSrxMnO3-δ (LSCO/LSMO) bilayers, exhibit unconventional magnetic exchange switching behavior, offering a pathway for innovative designs in perovskite oxide-based devices. However, the precise atomic-level stoichiometric compositions and chemophysical properties of these interfaces remain elusive, hindering the establishment of surrogate design principles. We leverage first-principles simulations, evolutionary algorithms, and neural network searches with on-the-fly uncertainty quantification to design deep learning model ensembles to investigate over 50,000 LSCO/LSMO bilayer structures as a function of oxygen deficiency (δ) and strontium concentration (x). Structural analysis of the low-energy interface structures reveals that preferential segregation of oxygen vacancies toward the interfacial La0.7Sr0.3CoO3-δ layers causes distortion of the CoOx polyhedra and the emergence of magnetically active Co2+ ions. At the same time, an increase in the Sr concentration and a decrease in oxygen vacancies in the La0.7Sr0.3MnO3-δ layers tend to retain MnO6 octahedra and promote the formation of Mn4+ ions. Electronic structure analysis reveals that the nonuniform distributions of Sr ions and oxygen vacancies on both sides of the interface can alter the local magnetization at the interface, showing a transition from ferromagnetic (FM) to local antiferromagnetic (AFM) or ferrimagnetic regions. Therefore, the exotic properties of La1-xSrxCoO3-δ/La1-xSrxMnO3-δ are strongly coupled to the presence of hard/soft magnetic layers, as well as the FM to AFM transition at the interface, and can be tuned by changing the Sr concentration and oxygen partial pressure during growth. These insights provide valuable guidance for the precise design of perovskite oxide multilayers, enabling tailoring of their functional properties to meet specific requirements for various device applications.

Charge-Transfer-Induced Interfacial Ferromagnetism in Ferromagnet-Free Oxide Heterostructures

Due to the strong interlayer coupling between multiple degrees of freedom, oxide heterostructures have demonstrated exotic properties that are not shown by their bulk counterparts. One of the most interesting properties is ferromagnetism at the interface formed between "nonferromagnetic" compounds. Here we report on the interfacial ferromagnetic phase induced in the superlattices consisting of the two paramagnetic oxides CaRuO3 (CRO) and LaNiO3 (LNO). By varying the sublayer thickness in the superlattice period, we demonstrate that the ferromagnetic order has been established in both CaRuO3 and LaNiO3 sublayers, exhibiting an identical Curie temperature of ∼75 K. The X-ray absorption spectra suggest a strong charge transfer from Ru to Ni at the interface, triggering superexchange interactions between Ru/Ni ions and giving rise to the emergent ferromagnetic phase. Moreover, the X-ray linear dichroism spectra reveal the preferential occupancy of the d3z2-r2 orbital for the Ru ions and the dx2-y2 orbital for the Ni ions in the heterostructure. This leads to different magnetic anisotropy of the superlattices when they are dominated by CRO or LNO sublayers. This work clearly demonstrates a charge-transfer-induced interfacial ferromagnetic phase in the whole ferromagnet-free oxide heterostructures, offering a feasible way to tailor oxide materials for desired functionalities.

Emergent Ferromagnetism in CaRuO3/CaMnO3 (111)-Oriented Superlattices

The boundary between CaRuO3 and CaMnO3 is an ideal test bed for emergent magnetic ground states stabilized through interfacial electron interactions. In this system, nominally antiferromagnetic and paramagnetic materials combine to yield interfacial ferromagnetism in CaMnO3 due to electron leakage across the interface. In this work, we show that the crystal symmetry at the surface is a critical factor determining the nature of the interfacial interactions. Specifically, by growing CaRuO3/CaMnO3 heterostructures along the (111) instead of the (001) crystallographic axis, we achieve a 3-fold enhancement of the magnetization and involve the CaRuO3 layers in the ferromagnetism, which now spans both constituent materials. The stabilization of a net magnetic moment in CaRuO3 through strain effects has been long-sought but never consistently achieved, and our observations demonstrate the importance of interface engineering in the development of new functional heterostructures.

Interface-related phenomena in epitaxial complex oxide ferroics across different thin film platforms: opportunities and challenges

Interfaces in complex oxides give rise to fascinating new physical phenomena arising from the interconnected spin, lattice, charge and orbital degrees of freedom. Most commonly, interfaces are engineered in epitaxial superlattice films. Of growing interest also are epitaxial vertically aligned nanocomposite films where interfaces form by self-assembly. These two thin film forms offer different capabilities for materials tuning and have been explored largely separately from one another. Ferroics (ferroelectric, ferromagnetic, multiferroic) are among the most fascinating phenomena to be manipulated using interface effects. Hence, in this review we compare and contrast the ferroic properties that arise in these two different film forms, highlighting exemplary materials combinations which demonstrate novel, enhanced and/or emergent ferroic functionalities. We discuss the origins of the observed functionalities and propose where knowledge can be translated from one materials form to another, to potentially produce new functionalities. Finally, for the two different film forms we present a perspective on underexplored/emerging research directions.

Atomic scale interfacial magnetism and origin of metal-insulator transition in (LaNiO[Formula: see text])[Formula: see text]/(CaMnO[Formula: see text])[Formula: see text] superlattices: a first principles study

Interfacial magnetism and metal-insulator transition at LaNiO[Formula: see text]-based oxide interfaces have triggered intense research efforts, because of the possible implications in future heterostructure device design and engineering. Experimental observation lack in some points a support from an atomistic view. In an effort to fill such gap, we hereby investigate the structural, electronic, and magnetic properties of (LaNiO[Formula: see text])[Formula: see text]/(CaMnO[Formula: see text])[Formula: see text] superlattices with varying LaNiO[Formula: see text] thickness (n) using density functional theory including a Hubbard-type effective on-site Coulomb term. We successfully capture and explain the metal-insulator transition and interfacial magnetic properties, such as magnetic alignments and induced Ni magnetic moments which were recently observed experimentally in nickelate-based heterostructures. In the superlattices modeled in our study, an insulating state is found for n=1 and a metallic character for n=2, 4, with major contribution from Ni and Mn 3d states. The insulating character originates from the disorder effect induced by sudden environment change for the octahedra at the interface, and associated to localized electronic states; on the other hand, for larger n, less localized interfacial states and increased polarity of the LaNiO[Formula: see text] layers contribute to metallicity. We discuss how the interplay between double and super-exchange interaction via complex structural and charge redistributions results in interfacial magnetism. While (LaNiO[Formula: see text])[Formula: see text]/(CaMnO[Formula: see text])[Formula: see text] superlattices are chosen as prototype and for their experimental feasibility, our approach is generally applicable to understand the intricate roles of interfacial states and exchange mechanism between magnetic ions towards the overall response of a magnetic interface or superlattice.

Quantum magnetic phenomena in engineered heterointerface of low-dimensional van der Waals and non-van der Waals materials

Investigating magnetic phenomena at the microscopic level has emerged as an indispensable research domain in the field of low-dimensional magnetic materials. Understanding quantum phenomena that mediate the magnetic interactions in dimensionally confined materials is crucial from the perspective of designing cheaper, compact, and energy-efficient next-generation spintronic devices. The infrequent occurrence of intrinsic long-range magnetic order in dimensionally confined materials hinders the advancement of this domain. Hence, introducing and controlling the ferromagnetic character in two-dimensional materials is important for further prospective studies. The interface in a heterostructure significantly contributes to modulating its collective magnetic properties. Quantum phenomena occurring at the interface of engineered heterostructures can enhance or suppress magnetization of the system and introduce magnetic character to a native non-magnetic system. Considering most 2D magnetic materials are used as stacks with other materials in nanoscale devices, the methods to control the magnetism in a heterostructure and understanding the corresponding mechanism are crucial for promising spintronic and other functional applications. This review highlights the effect of electric polarization of the adjacent layer, changed structural configuration at the vicinity of the interface, natural strain induced by lattice mismatch, and exchange interaction in the interfacial region in modulating the magnetism of heterostructures of van der Waals and non-van der Waals materials. Further, prospects of interface-engineered magnetism in spin-dependent device applications are also discussed.

Room-Temperature Ferromagnetism at an Oxide-Nitride Interface

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 (Cr_{2}O_{3})-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 Cr^{3+} 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.

Field-Free Improvement of Oxygen Evolution Reaction in Magnetic Two-Dimensional Heterostructures

Ferromagnetic (FM) electrocatalysts have been demonstrated to reduce the kinetic barrier of oxygen evolution reaction (OER) by spin-dependent kinetics and thus enhance the efficiency fundamentally. Accordingly, FM two-dimensional (2D) materials with unique physicochemical properties are expected to be promising oxygen-evolution catalysts; however, related research is yet to be reported due to their air-instabilities and low Curie temperatures (T_C). Here, based on the synthesis of 2D air-stable FM Cr₂Te₃ nanosheets with a low T_C around 200 K, room-temperature ferromagnetism is achieved in Cr₂Te₃ by proximity to an antiferromagnetic (AFM) CrOOH, demonstrating the accomplishment of long-ranged FM ordering in Cr₂Te₃ because the magnetic proximity effect stems from paramagnetic (PM)/AFM heterostructure. Therefore, the OER performance can be permanently promoted (without applied magnetic field due to nonvolatile nature of spin) after magnetization. This work demonstrates that a representative PM/AFM 2D heterostructure, Cr₂Te₃/CrOOH, is expected to be a high-efficient magnetic heterostructure catalysts for oxygen-evolution.

Impact of growth kinetics on the interface morphology and magnetization in La1/3Sr2/3FeO3/La2/3Sr1/3MnO3 heterostructures

The ability to create atomically perfect, epitaxial heterostructures of correlated complex perovskite oxides using state-of-art thin film deposition techniques has generated new physical phenomena at engineered interfaces. Here we report on the impact of growth kinetics on the magnetic structure and exchange coupling at the interface in heterostructures combining layers of antiferromagnetic La_1/3Sr_2/3FeO₃ (LSFO) and ferromagnetic La_2/3Sr_1/3MnO₃ (LSMO) on (0 0 1)-oriented SrTiO₃ (STO) substrates. Two growth orders are investigated, (a) LSMO/LSFO/STO(0 0 1) and (b) LSFO/LSMO/STO(0 0 1), where the LSFO layer is grown by molecular beam epitaxy and the LSMO layer by high oxygen pressure sputtering. The interface has been investigated using electron microscopy and polarized neutron reflectometry. Interdiffusion over seven monolayers is observed in LSMO/LSFO (a) with an almost 50% reduction in magnetization at the interface and showing no exchange coupling. However, the exchange bias effect ([Formula: see text] mT at 10 K) could be realized when the interface is atomically sharp, as in LSFO/LSMO (b). Our study therefore reveals that, even for well ordered and lattice-matched structures, the kinetics involved in the growth processes drastically influences the interface quality with a strong correlation to the magnetic properties.

Unusual exchange bias in Sr2FeIrO6/La0.67Sr0.33MnO3 multilayer

Here, we study interface induced magnetic properties in a 3d-5d based multilayer made of La_0.67Sr_0.33MnO₃ and double perovskite Sr₂FeIrO₆, respectively. Bulk La_0.67Sr_0.33MnO₃ is metallic and shows ferromagnetic (FM) ordering above room temperature. In contrast, bulk Sr₂FeIrO₆, is an antiferromagnet (AFM) with a Néel temperature around 45 K ([Formula: see text]) and exhibits an insulating behavior. Two set of multilayers have been grown on SrTiO₃ (1 0 0) crystal with varying thickness of FM layer. A multilayer with equal thickness of La_0.67Sr_0.33MnO₃ and Sr₂FeIrO₆ (∼10 nm) shows exchange bias (EB) effect both in conventionally field cooled (FC) as well as in zero field cooled (ZFC) magnetic hysteresis measurements which is rather unusual. The ZFC EB effect is weakened both with increasing maximum field during initial magnetization process at low temperature and with increasing temperature. Interestingly, a multilayer with reduced thickness of La_0.67Sr_0.33MnO₃ (∼5 nm) does not exhibit ZFC EB phenomenon, however, the FC EB effect is strengthened showing much higher value. We believe that an AFM type exchange coupling at the interface and its evolution during initial application of magnetic field causes this unusual EB in present multilayers.

Interfacial Engineering of Ferromagnetism in Epitaxial Manganite/Ruthenate Superlattices via Interlayer Chemical Doping

Interfacial charge transfer and structural proximity effects are the two essential routes to trigger and tune numerous functionalities of perovskite oxide heterostructures. However, the cooperation and competition of these two interfacial effects in one epitaxial system have not been fully understood. Herein, we fabricate a series of La_0.67Ca_0.33MnO₃/CaRuO₃ superlattices and introduce various chemical doping in the nonmagnetic CaRuO₃ interlayers. We found that Ti, Sr, and La doping in the CaRuO₃ layer can effectively tune the interfacial charge transfer and octahedral rotation, thus modulating the ferromagnetism of the superlattices. Specifically, the B-site Ti doping depletes the Ru 4d band and suppresses the interfacial charge transfer, leading to a decay of ferromagnetic Curie temperature ( T_C). In contrast, the A-site Sr doping maintains a sizable charge transfer and meanwhile suppresses the octahedral rotation, which facilitates ferromagnetism and significantly enhances the T_C up to 291 K. The La doping turns out to localize the itinerant electrons in the CaRuO₃ layer, which suppresses both the interfacial charge transfer and ferromagnetism. The observed intriguing interfacial engineering of magnetism would pave a new way to understand the collective effects of interfacial charge transfer and structural proximity on the physical properties of oxide heterostructures.

Ultrathin Scale Tailoring of Anisotropic Magnetic Coupling and Anomalous Magnetoresistance in SrRuO3-PrMnO3 Superlattices

A strong perpendicular magnetocrystalline anisotropy (PMA) in antiferromagnetically coupled SrRuO₃(17 uc (unit cell))/PrMnO₃( n uc) superlattices effectively reconstructs the interfacial spin ordering. The occurrence of significant anisotropic interfacial antiferromagnetic coupling between the Ru and Mn ions is systematically tuned by varying the PrMnO₃ layer thickness in ultrathin scale from 3 to 12 uc, which is associated with a rise in PMA energy from 0.28 × 10⁶ to 1.60 × 10⁶ erg/cm³. The analysis using the Stoner-Wohlfarth model and density functional theory confirm that the exchange anisotropy is the major contribution to the PMA. The superlattices with PrMnO₃ layer thickness ≥7 uc exhibit the tunneling-like transport of Ru 4d electrons, which is rather expected in the stronger antiferromagnetically coupled superlattices with thinner PrMnO₃ layer. Tunneling-like transport at thicker spacer layer in the SrRuO₃-PrMnO₃ superlattice system is an unique feature of two ferromagnet-based superlattices. Our investigations show that the technologically important interfacial magnetic coupling, PMA, and tunneling magnetoresistance could be achieved in a periodically stacked bilayer and can be precisely manipulated by the size effect in ultrathin scale.

Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces

Polar discontinuities occurring at interfaces between two materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates, or charge transfer. In the latter case, one may expect a local electronic doping of one material: one example is the two-dimensional electron liquid (2DEL) appearing in SrTiO3 once covered by a polar LaAlO3 layer. Here, it is shown that tuning the formal polarization of a (La,Al)1-x (Sr,Ti) x O3 (LASTO:x) overlayer modifies the quantum confinement of the 2DEL in SrTiO3 and its electronic band structure. The analysis of the behavior in magnetic field of superconducting field-effect devices reveals, in agreement with ab initio calculations and self-consistent Poisson-Schrödinger modeling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on the interface total charge densities. These results strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO3/SrTiO3 interface and demonstrate an effective tool for tailoring the electronic structure at oxide interfaces.

Orientation Control of Interfacial Magnetism at La0.67Sr0.33MnO3/SrTiO3 Interfaces

Understanding the magnetism at the interface between a ferromagnet and an insulator is essential because the commonly posited magnetic "dead" layer close to an interface can be problematic in magnetic tunnel junctions. Previously, degradation of the magnetic interface was attributed to charge discontinuity across the interface. Here, the interfacial magnetism was investigated using three identically prepared La_0.67Sr_0.33MnO₃ (LSMO) thin films grown on different oriented SrTiO₃ (STO) substrates by polarized neutron reflectometry. In all cases the magnetization at the LSMO/STO interface is larger than the film bulk. We show that the interfacial magnetization is largest across the LSMO/STO interfaces with (001) and (111) orientations, which have the largest net charge discontinuities across the interfaces. In contrast, the magnetization of LSMO/STO across the (110) interface, the orientation with no net charge discontinuity, is the smallest of the three orientations. We show that a magnetically degraded interface is not intrinsic to LSMO/STO heterostructures. The approach to use different crystallographic orientations provides a means to investigate the influence of charge discontinuity on the interfacial magnetization.

Interface-Induced Phenomena in Magnetism

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

Interfacial Symmetry Control of Emergent Ferromagnetism at the Nanoscale

The emergence of complex new ground states at interfaces has been identified as one of the most promising routes to highly tunable nanoscale materials. Despite recent progress, isolating and controlling the underlying mechanisms behind these emergent properties remains among the most challenging materials physics problems to date. In particular, generating ferromagnetism localized at the interface of two nonferromagnetic materials is of fundamental and technological interest. Moreover, the ability to turn the ferromagnetism on and off would shed light on the origin of such emergent phenomena and is promising for spintronic applications. We demonstrate that ferromagnetism confined within one unit cell at the interface of CaRuO3 and CaMnO3 can be switched on and off by changing the symmetry of the oxygen octahedra connectivity at the boundary. Interfaces that are symmetry-matched across the boundary exhibit interfacial CaMnO3 ferromagnetism while the ferromagnetism at symmetry-mismatched interfaces is suppressed. We attribute the suppression of ferromagnetic order to a reduction in charge transfer at symmetry-mismatched interfaces, where frustrated bonding weakens the orbital overlap. Thus, interfacial symmetry is a new route to control emergent ferromagnetism in materials such as CaMnO3 that exhibit antiferromagnetism in bulk form.

Magnetic Interactions at the Nanoscale in Trilayer Titanates

We report on the phase diagram of competing magnetic interactions at the nanoscale in engineered ultrathin trilayer heterostructures of LaTiO_{3}/SrTiO_{3}/YTiO_{3}, in which the interfacial inversion symmetry is explicitly broken. Combined atomic layer resolved scanning transmission electron microscopy with electron energy loss spectroscopy and electrical transport have confirmed the formation of a spatially separated two-dimensional electron liquid and high density two-dimensional localized magnetic moments at the LaTiO_{3}/SrTiO_{3} and SrTiO_{3}/YTiO_{3} interfaces, respectively. Resonant soft x-ray linear dichroism spectroscopy has demonstrated the presence of orbital polarization of the conductive LaTiO_{3}/SrTiO_{3} and localized SrTiO_{3}/YTiO_{3} electrons. Our results provide a route with prospects for exploring new magnetic interfaces, designing a tunable two-dimensional d-electron Kondo lattice, and potential spin Hall applications.

Phase separation enhanced magneto-electric coupling in La0.7Ca0.3MnO3/BaTiO3 ultra-thin films

We study the origin of the magnetoelectric coupling in manganite films on ferroelectric substrates. We find large magnetoelectric coupling in La0.7Ca0.3MnO3/BaTiO3 ultra-thin films in experiments based on the converse magnetoelectric effect. The magnetization changes by around 30-40% upon applying electric fields on the order of 1 kV/cm to the BaTiO3 substrate, corresponding to magnetoelectric coupling constants on the order of α = (2-5) · 10(-7) s/m. Magnetic anisotropy is also affected by the electric field induced strain, resulting in a considerable reduction of coercive fields. We compare the magnetoelectric effect in pre-poled and unpoled BaTiO3 substrates. Polarized neutron reflectometry reveals a two-layer behavior with a depressed magnetic layer of around 30 Å at the interface. Magnetic force microscopy (MFM) shows a granular magnetic structure of the La0.7Ca0.3MnO3. The magnetic granularity of the La0.7Ca0.3MnO3 film and the robust magnetoelastic coupling at the La0.7Ca0.3MnO3/BaTiO3 interface are at the origin of the large magnetoelectric coupling, which is enhanced by phase separation in the manganite.

Interfacial magnetism in complex oxide heterostructures probed by neutrons and x-rays

Magnetic complex-oxide heterostructures are of keen interest because a wealth of phenomena at the interface of dissimilar materials can give rise to fundamentally new physics and potentially valuable functionalities. Altered magnetization, novel magnetic coupling and emergent interfacial magnetism at the epitaxial layered-oxide interfaces are under intensive investigation, which shapes our understanding on how to utilize those materials, particularly for spintronics. Neutron and x-ray based techniques have played a decisive role in characterizing interfacial magnetic structures and clarifying the underlying physics in this rapidly developing field. Here we review some recent experimental results, with an emphasis on those studied via polarized neutron reflectometery and polarized x-ray absorption spectroscopy. We conclude with some perspectives.

Electric Field Control of Interfacial Ferromagnetism in CaMnO_{3}/CaRuO_{3} Heterostructures

New mechanisms for achieving direct electric field control of ferromagnetism are highly desirable in the development of functional magnetic interfaces. To that end, we have probed the electric field dependence of the emergent ferromagnetic layer at CaRuO_{3}/CaMnO_{3} interfaces in bilayers fabricated on SrTiO_{3}. Using polarized neutron reflectometry, we are able to detect the ferromagnetic signal arising from a single atomic monolayer of CaMnO_{3}, manifested as a spin asymmetry in the reflectivity. We find that the application of an electric field of 600 kV/m across the bilayer induces a significant increase in this spin asymmetry. Modeling of the reflectivity suggests that this increase corresponds to a transition from canted antiferromagnetism to full ferromagnetic alignment of the Mn^{4+} ions at the interface. This increase from 1 μ_{B} to 2.5-3.0 μ_{B} per Mn is indicative of a strong magnetoelectric coupling effect, and such direct electric field control of the magnetization at an interface has significant potential for spintronic applications.

Electronic control of interface ferromagnetic order and exchange-bias in paramagnetic-antiferromagnetic epitaxial bilayers

The hetero-epitaxially engineered magnetic phases, formed due to entanglement of the spin, charge and lattice degrees of freedom, at the atomically sharp interfaces of complex oxide heterostructures are indispensable for devising multifunctional devices. In the quest for novel and superior spintronics functionalities, we have explored the interface magnetism in the epitaxial bilayer of atypical magnetic and electronic states, i.e., of paramagnetic metallic and antiferromagnetic (AFM) insulating phases. In this framework, we observe an unusually strong ferromagnetic order and large exchange-bias fields generated at the interface of the bilayers of metallic CaRuO3 and AFM insulating manganite. The magnetic moment of the interface ferromagnetic order increases linearly with increasing thickness (7-90 nm) of the metallic CaRuO3 layer. This linear scaling signifying an electronic (non-magnetic) control of the interface magnetism and a non-monotonic dependence of the exchange-bias on metallic layers evolve as novel spintronics attributes in atypical bilayers.

Positive exchange-bias and giant vertical hysteretic shift in La0.3Sr0.7FeO3/SrRuO3 bilayers

The exchange-bias effects in the mosaic epitaxial bilayers of the itinerant ferromagnet (FM) SrRuO₃ and the antiferromagnetic (AFM) charge-ordered La_0.3Sr_0.7FeO₃ were investigated. An uncharacteristic low-field positive exchange bias, a cooling-field driven reversal of positive to negative exchange-bias and a layer thickness optimised unusual vertical magnetization shift were all novel facets of exchange bias realized for the first time in magnetic oxides. The successive magnetic training induces a transition from positive to negative exchange bias regime with changes in domain configurations. These observations are well corroborated by the hysteretic loop asymmetries which display the modifications in the AFM spin correlations. These exotic features emphasize the key role of i) mosaic disorder induced subtle interplay of competing AFM-superexchange and FM double exchange at the exchange biased interface and, ii) training induced irrecoverable alterations in the AFM spin structure.

Interfacial ferromagnetism in LaNiO3/CaMnO3 superlattices

We observe interfacial ferromagnetism in superlattices of the paramagnetic metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO3. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of CaMnO3 at the interface. Together these results suggest that ferromagnetism can be attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.

An oil droplet template method for the synthesis of hierarchical structured Co3O4/C anodes for Li-ion batteries

Superposed cobalt(II)-cobalt(III) layered double hydroxide (Co(II)-Co(III)-LDH) nanoplates were synthesized by an oil droplet template method, in which the main steps are as follows: LDH nanosheets were first assembled on an oil droplet template to form a multishell sphere, and then the oil droplet was easily removed under centrifugal force due to its very different density from that of the assembled LDH shell. This resulted in the multishell spheres being split open to create superposed LDH nanoplates. The resulting material has a three-stage architecture, namely, the primary building blocks of nanosheets, the secondary architecture of shells derived from the nanosheets, and the long-range architecture of superposed nanoplates assembled from the vertically stacked shells. Most importantly, the as-fabricated LDH-based hierarchical structure can be readily converted to a Co3O4/C composite via calcination, without obvious structural alteration, where the residual surfactant is the source of the carbon. When used as an anode material for Li-ion batteries, the Co3O4/C electrode exhibits an excellent electrochemical performance, which is attributed to the unique hierarchically porous structure.

Corrosion/fragmentation of layered composite cathode and related capacity/voltage fading during cycling process

The Li-rich, Mn-rich (LMR) layered structure materials exhibit very high discharge capacities exceeding 250 mAh g(-1) and are very promising cathodes to be used in lithium ion batteries. However, significant barriers, such as voltage fade and low rate capability, still need to be overcome before the practical applications of these materials. A detailed study of the voltage/capacity fading mechanism will be beneficial for further tailoring the electrode structure and thus improving the electrochemical performances of these layered cathodes. Here, we report detailed studies of structural changes of LMR layered cathode Li[Li0.2Ni0.2Mn0.6]O2 after long-term cycling by aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The fundamental findings provide new insights into capacity/voltage fading mechanism of Li[Li0.2Ni0.2Mn0.6]O2. Sponge-like structure and fragmented pieces were found on the surface of cathode after extended cycling. Formation of Mn(2+) species and reduced Li content in the fragments leads to the significant capacity loss during cycling. These results also imply the functional mechanism of surface coatings, for example, AlF3, which can protect the electrode from etching by acidic species in the electrolyte, suppress cathode corrosion/fragmentation, and thus improve long-term cycling stability.