Spin-Glass-Like Behavior and Topological Hall Effect in SrRuO3/SrIrO3 Superlattices for Oxide Spintronics Applications

Angular-dependent magnetoresistance modulated by interfacial magnetic state in Pt/LSMO heterostructures

The interfaces between heavy metals and antiferromagnetic materials have garnered significant attention due to their interesting physical properties. La0.35Sr0.65MnO3 (LSMO), as a typical manganite, exhibits an antiferromagnetic ground state that can be controlled through epitaxial strain and interfacial spin-orbit coupling. In this work, we reported the diverse magnetoresistance, influenced by the interfacial magnetic state, in Pt (3 nm)/LSMO (6-20 nm) heterostructures. The strong spin-orbit coupling of Pt and Dzyaloshinskii-Moriya interaction alter the spin structure and enhance the electron scattering at the Pt/LSMO interface, resulting in positive magnetoresistance. The interfacial angular-dependent magnetoresistance modulated by the interfacial magnetic states was also observed in the Pt/LSMO (20 nm) heterostructures. Our findings contribute to a broader understanding of interfacial properties between heavy metals and antiferromagnetic manganites.

High-Temperature Ferroic Glassy States in SrTiO_{3}-Based Thin Films

Disordered ferroics hold great promise for next-generation magnetoelectric devices because their lack of symmetry constraints implies negligible hysteresis with low energy costs. However, the transition temperature and the magnitude of polarization and magnetization are still too low to meet application requirements. Here, taking the prototype perovskite of SrTiO_{3} as an instance, we realize a coexisting spin and dipole reentrant glass states in SrTiO_{3} homoepitaxial films via manipulation of local symmetry. Room-temperature saturation magnetization and spontaneous polarization reach ∼ 10  emu/cm^{3} and ∼ 25  μC/cm^{2}, respectively, with high transition temperatures (101 K and 236 K for spin and dipole glass temperatures and 556 K and 1100 K for Curie temperatures, respectively). Our atomic-scale investigation points out an underlying mechanism, where the Ti/O-defective unit cells break the local translational and orbital symmetry to drive the formation of unusual slush states. This study advances our understanding of the nature of the intricate couplings of ferroic glasses. Our approach could be applied to numerous perovskite oxides for the simultaneous control of the local magnetic and polar orderings and for the exploration of the underlying physics.

Growth and characterization of the magnetic topological insulator candidate Mn2Sb2Te5

We report a new member of topological insulator (TI) family i.e. Mn2Sb2Te5, which belongs to MnSb2Te4family and is a sister compound of Mn2Bi2Te5. An antiferromagnetic layer of (MnTe)2has been inserted between quintuple layers of Sb2Te3. The crystal structure and chemical composition of as grown Mn2Sb2Te5crystal is experimentally visualized by single crystal x-ray diffractometer and field emission scanning electron microscopy. The valence states of individual constituents i.e., Mn, Sb and Te are ascertained through x-ray photo electron spectroscopy. Different vibrational modes of Mn2Sb2Te5are elucidated through Raman spectroscopy. Temperature-dependent resistivityρ(T) of Mn2Sb2Te5resulted in metallic behavior of the same with an up-turn at below around 20 K. Further, the magneto-transportρ(T) vsHof the same exhibited negative magneto-resistance (MR) at low temperatures below 20 K and small positive at higher temperatures. The low Temperature -ve MR starts decreasing at higher fields. The magnetic moment as a function of temperature at 100 Oe and 1 kOe showed anti-ferromagnetism (AFM) like down turn cusps at around 20 K and 10 K. The isothermal magnetization showed AFM like loops with some embedded ferromagnetic/paramagnetic (PM) domains at 5 K and purely PM like at 100 K. The studied Mn2Sb2Te5clearly exhibited the characteristics of a magnetic TI.

Manipulating topological Hall-like signatures by interface engineering in epitaxial ruthenate/manganite heterostructures

Topologically protected non-trivial spin textures (e.g. skyrmions) give rise to a novel phenomenon called the topological Hall effect (THE) and have promising implications in future energy-efficient nanoelectronic and spintronic devices. Here, we have studied the Hall effect in SrRuO3/La0.42Ca0.58MnO3 (SRO/LCMO) bilayers. Our investigation suggests that pure SRO has hard and soft magnetic characteristics but the anomalous Hall effect (AHE) in SRO is governed by the high coercivity phase. We have shown that the proximity effect of a soft magnetic LCMO on SRO plays a critical role in interfacial magnetic coupling and transport properties in SRO. Upon reducing the SRO thickness in the bilayer, the proximity effect becomes the dominant feature, enhancing the magnitude and temperature range of THE-like signatures. The THE-like features in bilayers can be explained by a diffusive Berry phase transition model in the presence of an emergent magnetic state due to interface coupling. This work provides an alternative understanding of THE-like signatures and their manipulation in SRO-based heterostructures, bilayers and superlattices.

Interfacial charge transfer induced antiferromagnetic metals and magnetic phase transition in (CrO2)m/(TaO2)nsuperlattices

As a class of remarkable spintronic materials, intrinsic antiferromagnetic (AFM) metals are rare. The exploration and investigation of AFM metals are still in its infancy. Based on first-principles calculations, the interface-induced magnetic phenomena in the (CrO₂)_m/(TaO₂)_nsuperlattices are investigated, and a new series of AFM metals is predicted. Under different ratios ofm:nwith varying valence states of Cr, the (CrO₂)_m/(TaO₂)_nsuperlattices exhibit three different phases, including the AFM metal, the AFM semiconductor, and the ferromagnetic (FM) metal. In the AFM semiconducting phases, theintra-CrO₂-monolayer magnetic exchange interaction is systematically discussed, corresponding tom = 1 orm = 2. Both the localization of the Cr 3 dorbitals and the crystal-field splitting are crucial for magnetic ordering in super-exchange interactions. Based on the analyses of the AFM semiconducting phases withm = 1 andm = 2, the mechanisms of AFM metallic phases with radios ofm:n<1/2and1/2<m:n<1/1are discussed in detail. Additionally, the AFM metallic superlattices can be tuned into a FM metallic phase by applying strain in thec-direction, such as a compression of 7% in the (CrO₂)₁/(TaO₂)₃superlattice, and a tensile strain of 7% in the (CrO₂)₂/(TaO₂)₃superlattice. The phase diagram of the (CrO₂)_m/(TaO₂)_nsuperlattices is obtained as a function of the layer thickness. This work provides new insights about realizing and manipulating AFM metals in artificial superlattices or heterostructures in experiments.

Dual Effect of Superhalogen Ionic Liquids Ensures Efficient Carrier Transport for Highly Efficient and Stable Perovskite Solar Cells

Defect accumulation and nonradiative recombination at the interface of the electron-transport layer (ETL) and the photosensitive layer are inevitable obstacles to efficient and stable perovskite solar cells (PSCs). Herein, we reported a dual-effect interface modification strategy that employs potassium tetrafluoroborate (KBF₄) molecules for the simultaneous passivation of the SnO₂/perovskite interface and perovskite grain boundaries. The introduced highly electronegative BF₄^- enriched at the SnO₂ surface and the chemical bond interaction between them can effectively reduce the hydroxyl (-OH) group defects on the surface of SnO₂, improve electron mobility, and reduce nonradiative recombination. Meanwhile, partial K⁺ diffuses into the grain boundaries, causing the halogen ions to be uniformly distributed in the perovskite film and resulting in better crystallinity. Therefore, the performance of the experimental device was improved from 20.34 to 22.90% compared with the reference device, with a high electrical performance (J_SC = 25.1 mA cm^-2, V_OC = 1.137 V). In particular, the unencapsulated target PSCs retained 85% of their original PCE after aging for 1000 h under ambient conditions (70 ± 10% RH) in the dark.

Topological Hall effect in SrRuO3thin films and heterostructures

Transition metal oxides hold a wide spectrum of fascinating properties endowed by the strong electron correlations. In 4dand 5doxides, exotic phases can be realized with the involvement of strong spin-orbit coupling (SOC), such as unconventional magnetism and topological superconductivity. Recently, topological Hall effects (THEs) and magnetic skyrmions have been uncovered in SrRuO₃thin films and heterostructures, where the presence of SOC and inversion symmetry breaking at the interface are believed to play a key role. Realization of magnetic skyrmions in oxides not only offers a platform to study topological physics with correlated electrons, but also opens up new possibilities for magnetic oxides using in the low-power spintronic devices. In this review, we discuss recent observations of THE and skyrmions in the SRO film interfaced with various materials, with a focus on the electric tuning of THE. We conclude with a discussion on the directions of future research in this field.

Coupling Charge and Topological Reconstructions at Polar Oxide Interfaces

In oxide heterostructures, different materials are integrated into a single artificial crystal, resulting in a breaking of inversion symmetry across the heterointerfaces. A notable example is the interface between polar and nonpolar materials, where valence discontinuities lead to otherwise inaccessible charge and spin states. This approach paved the way for the discovery of numerous unconventional properties absent in the bulk constituents. However, control of the geometric structure of the electronic wave functions in correlated oxides remains an open challenge. Here, we create heterostructures consisting of ultrathin SrRuO_{3}, an itinerant ferromagnet hosting momentum-space sources of Berry curvature, and LaAlO_{3}, a polar wide-band-gap insulator. Transmission electron microscopy reveals an atomically sharp LaO/RuO_{2}/SrO interface configuration, leading to excess charge being pinned near the LaAlO_{3}/SrRuO_{3} interface. We demonstrate through magneto-optical characterization, theoretical calculations and transport measurements that the real-space charge reconstruction drives a reorganization of the topological charges in the band structure, thereby modifying the momentum-space Berry curvature in SrRuO_{3}. Our results illustrate how the topological and magnetic features of oxides can be manipulated by engineering charge discontinuities at oxide interfaces.

Surface-Step-Induced Magnetic Anisotropy in Epitaxial LSMO Deposited on Engineered STO Surfaces

Changes in stoichiometry, temperature, strain and other parameters dramatically alter properties of LSMO perovskite. Thus, the sensitivity of LSMO may enable control of the magnetic properties of the film. This work demonstrates the capabilities of interface engineering to achieve the desired effects. Three methods of preparing STO substrates were conducted, i.e., using acid, buffer solution, and deionized water. The occurrence of terraces and their morphology depend on the preparation treatment. Terraces propagate on deposited layers and influence LSMO properties. The measurements show that anisotropy depends on the roughness of the substrate, the method of preparing the substrate, and oxygen treatment. The collected results suggest that the dipolar mechanism may be the source of LSMO anisotropy.

Emergent Spin Glass Behavior Created by Self-Assembled Antiferromagnetic NiO Columns in Ferrimagnetic NiFe2O4

Spin glass (SG) is a magnetic state with spin structure incommensurate with lattice and charge. Fundamental understanding of its behavior has a profound impact on many technological problems. Here, we present a novel case of interface-induced spin glass behavior via self-assembly of single-crystalline NiO microcolumns in a single-crystalline NiFe₂O₄ matrix. Scanning transmission electron microscopy indicates that the hexagonal-shaped NiO columns are along their [211] direction and oriented along the [111] direction of the NiFe₂O₄ matrix. Magnetic force microscopy reveals magnetic anisotropy between NiO columns (antiferromagnetic transition temperature T_N ∼ 523 K) and NiFe₂O₄ matrix (ferrimagnetic transition temperature T_FI ∼ 860 K). This leads to spin disorder/frustration at atomically sharp NiFe₂O₄/NiO interfaces responsible for spin glass behavior below T_SG ∼ 28 K. Our results demonstrate that self-assembly of magnetically distinct microstructures into another crystalline and magnetically ordered matrix is an effective way to create novel spin states at interfaces.

Detection of the Chiral Spin Structure in Ferromagnetic SrRuO3 Thin Film

SrRuO₃ (SRO) thin films and their heterostructure have attracted much attention because of the recently demonstrated fascinating properties, such as topological Hall effect and skyrmions. Critical to the understanding of those SRO properties is the study of the spin configuration. Here, we conduct resonant soft X-ray scattering (RSXS) at the oxygen K edge to investigate the spin configuration of a four-unit-cell SRO film that was grown epitaxially on a single-crystal SrTiO₃. The RSXS signal under a magnetic field (∼0.4 tesla) clearly shows a magnetic dichroism pattern around the specular reflection. Model calculations on the RSXS signal demonstrate that the magnetic dichroism pattern originates from a Néel-type chiral spin structure in this SRO thin film. We believe that the observed spin structure of the SRO system is a critical piece of information for understanding its intriguing magnetic and transport properties.

Field-induced slow magnetic relaxation in mixed valence di- and tri-nuclear CoII-CoIII complexes

Two novel mixed valence Co^II-Co^III complexes, namely [Co^IICo^III(L1)(ab)(mb)₂(H₂O)]·dmf (1) and [CoCo^II(L2)₄(H₂O)₄]·2H₂O (2) [H₂L1 = (E)-2-((1-hydroxybutan-2-ylimino)methyl)-6-methoxyphenol, ab = 2-amino-butan-1-ol anion, mb = p-methyl benzoate, H₂L2 = 3-((2-hydroxy-3-methoxy-benzylidene)-amino)-propionic acid, and dmf = N,N-dimethyl-formamide], were synthesized and characterized by single crystal X-ray diffraction and magnetic studies at low temperature. The structure determination reveals that both complexes belong to the monoclinic system with P21/c (1) and I2/a (2) space groups. Complex 1 is a dinuclear Co^IIICo^II compound with distorted octahedral cobalt centers showing different coordination environments. In 2, a bent trinuclear CoCo^II complex, the coordination environments around the two terminal Co^III sites are alike, whereas they are different in the central Co^II ion. Alternating current/direct current (ac/dc) magnetic studies revealed that both complexes show field-induced slow magnetic relaxation. The dc magnetic susceptibility and magnetization data were analyzed with the following Hamiltonianwhere D and E are the axial and rhombic zero-field splitting (zfs) parameters, respectively, and a good agreement between experimental and simulated results was found using the parameters g_⊥ = 2.585, g_∥ = 2.437, D = +98.1 cm^-1, E/D = 0.008 and F = 8.2× 10^-5 for 1 and g_⊥ = 2.580, g_∥ = 2.580, D = +55.4 cm^-1, and E/D = 0.000 for 2.

Electronic Inhomogeneity Influence on the Anomalous Hall Resistivity Loops of SrRuO3 Epitaxially Interfaced with 5d Perovskites

SrRuO₃, a 4d ferromagnet with multiple Weyl nodes at the Fermi level, offers a rich playground to design epitaxial heterostructures and superlattices with fascinating magnetic and magnetotransport properties. Interfacing ultrathin SrRuO₃ layers with large spin-orbit coupling 5d transition-metal oxides, such as SrIrO₃, results in pronounced peaklike anomalies in the magnetic field dependence of the Hall resistivity. Such anomalies have been attributed either to the formation of Néel-type skyrmions or to modifications of the Berry curvature of the topologically nontrivial conduction bands near the Fermi level of SrRuO₃. Here, epitaxial multilayers based on SrRuO₃ interfaced with 5d perovskite oxides, such as SrIrO₃ and SrHfO₃, were studied. This work focuses on the magnetotransport properties of the multilayers, aiming to unravel the role played by the interfaces with 5d perovskites in the peaklike anomalies of the Hall resistance loops of SrRuO₃ layers. Interfacing with large band gap insulating SrHfO₃ layers did not influence the anomalous Hall resistance loops, while interfacing with the nominally paramagnetic semimetal SrIrO₃ resulted in pronounced peaklike anomalies, which have been lately attributed to a topological Hall effect contribution as a result of skyrmions. This interpretation is, however, under strong debate and lately alternative causes, such as inhomogeneity of the thickness and the electronic properties of the SrRuO₃ layers, have been considered. Aligned with these latter proposals, our findings reveal the central role played in the anomalies of the Hall resistivity loops by electronic inhomogeneity of SrRuO₃ layers due to the interfacing with semimetallic 5d⁵ SrIrO₃.

Interfacial Control of Ferromagnetism in Ultrathin SrRuO3 Films Sandwiched between Ferroelectric BaTiO3 Layers

Interfaces between materials provide an intellectually rich arena for fundamental scientific discovery and device design. However, the frustration of magnetization and conductivity of perovskite oxide films under reduced dimensionality is detrimental to their device performance, preventing their active low-dimensional application. Herein, by inserting the ultrathin 4d ferromagnetic SrRuO₃ layer between ferroelectric BaTiO₃ layers to form a sandwich heterostructure, we observe enhanced physical properties in ultrathin SrRuO₃ films, including longitudinal conductivity, Curie temperature, and saturated magnetic moment. Especially, the saturated magnetization can be enhanced to ∼3.12 μ_B/Ru in ultrathin BaTiO₃/SrRuO₃/BaTiO₃ trilayers, which is beyond the theoretical limit of bulk value (2 μ_B/Ru). This observation is attributed to the synergistic ferroelectric proximity effect (SFPE) at upper and lower BaTiO₃/SrRuO₃ heterointerfaces, as revealed by the high-resolution lattice structure analysis. This SFPE in dual-ferroelectric interface cooperatively induces ferroelectric-like lattice distortions in RuO₆ oxygen octahedra and subsequent spin-state crossover in SrRuO₃, which in turn accounts for the observed enhanced magnetization. Besides the fundamental significance of interface-induced spin-lattice coupling, our findings also provide a viable route to the electrical control of magnetic ordering, taking a step toward low-power applications in all-oxide spintronics.

Reversible manipulation of the magnetic state in SrRuO3 through electric-field controlled proton evolution

Ionic substitution forms an essential pathway to manipulate the structural phase, carrier density and crystalline symmetry of materials via ion-electron-lattice coupling, leading to a rich spectrum of electronic states in strongly correlated systems. Using the ferromagnetic metal SrRuO3 as a model system, we demonstrate an efficient and reversible control of both structural and electronic phase transformations through the electric-field controlled proton evolution with ionic liquid gating. The insertion of protons results in a large structural expansion and increased carrier density, leading to an exotic ferromagnetic to paramagnetic phase transition. Importantly, we reveal a novel protonated compound of HSrRuO3 with paramagnetic metallic as ground state. We observe a topological Hall effect at the boundary of the phase transition due to the proton concentration gradient across the film-depth. We envision that electric-field controlled protonation opens up a pathway to explore novel electronic states and material functionalities in protonated material systems.

Magnetic and electrical properties of LuFe2O4 epitaxial thin films with a self-assembled interface structure

Thin film of LuFe₂O₄, one of multiferroics, deposited on an yttria-stabilized zirconia substrate shows a unique interface structure, leading to an exchange bias effect. The thin film itself undergoes spin glass or cluster glass transition.

Polarity and Spin-Orbit Coupling Induced Strong Interfacial Exchange Coupling: An Asymmetric Charge Transfer in Iridate-Manganite Heterostructure

Charge transfer is of particular importance in manipulating the interface physics in transition-metal oxide heterostructures. In this work, we have fabricated epitaxial bilayers composed of polar 3d LaMnO₃ and nonpolar 5d SrIrO₃. Systematic magnetic measurements reveal an unexpectedly large exchange bias effect in the bilayer, together with a dramatic enhancement of the coercivity of LaMnO₃. Based on first-principle calculations and X-ray absorption spectroscopy measurements, such a strong interfacial magnetic coupling is found closely associated with the polar nature of LaMnO₃ and the strong spin-orbit interaction in SrIrO₃, which collectively drive an asymmetric interfacial charge transfer and lead to the emergence of an interfacial reentrant spin/superspin glass state. Our study provides a new insight into the charge transfer in transition-metal oxide heterostructures and offers a novel means to tune the interfacial exchange coupling for a variety of device applications.

Emergent Topological Hall Effect in La0.7Sr0.3MnO3/SrIrO3 Heterostructures

Recently, perovskite oxide heterostructures have drawn great attention because multiple and complex coupling at the heterointerface may produce novel magnetic and electric phenomena that are not expected in homogeneous materials either in the bulk or in films. In this work, we report for the first time that an emergent giant topological Hall effect (THE), associated with a noncoplanar (NC) spin texture, can be induced in ferromagnetic (FM) La_0.7Sr_0.3MnO₃ thin films in a wide temperature range of up to 200 K by constructing La_0.7Sr_0.3MnO₃/SrIrO₃ epitaxial heterostructures on (001) SrTiO₃ substrates. This THE is not observed in La_0.7Sr_0.3MnO₃ single-layer films or La_0.7Sr_0.3MnO₃/SrTiO₃/SrIrO₃ trilayer heterostructures, indicating the relevance of the La_0.7Sr_0.3MnO₃/SrIrO₃ interface, where the Dzyaloshinskii-Moriya interaction due to strong spin-orbital coupling in SrIrO₃ may play a crucial role. The fictitious field associated with THE is independent of temperature in La_0.7Sr_0.3MnO₃/SrIrO₃ heterostructures, suggesting that the NC spin texture may be magnetic skyrmions. This work demonstrates the feasibility of using SrIrO₃ to generate novel magnetic and transport characteristics by interfacing with other correlated oxides, which might be useful to novel spintronic applications.

Metal-Insulator Transition of LaNiO3 Films in LaNiO3/SrIrO3 Heterostructures

LaNiO₃/SrIrO₃ (LNO/SIO) heterostructures were deposited epitaxially on (001) SrTiO₃ substrates. Transport characteristics of these LNO/SIO heterostructures were investigated as functions of LNO and SIO thickness. It has been observed that interfacing with SIO induces a metal-insulator transition at about 20 K in a 10 unit cell thick LNO film, which is otherwise metallic down to 2 K. In addition, this metal-insulator transition is irrelevant to the thickness of SIO, indicative of an interfacial effect. X-ray absorption measurements reveal an electron transfer from LNO to SIO across the interface. Meanwhile, the observation of a spin-glass-like state manifests the importance of spin-dependent scattering. The metal-insulator transition is discussed in terms of Kondo effect by random scattering from impurity spins associated with the interfacial electron transfer and the Dzyaloshinskii-Moriya interaction due to strong spin-orbit coupling inherent in 5d perovskite SIO.

Tuning the Dzyaloshinskii-Moriya interaction in Pt/Co/MgO heterostructures through the MgO thickness

The interfacial Dzyaloshinskii-Moriya interaction (DMI) in ferromagnetic/heavy metal ultra-thin film structures has attracted a lot of attention thanks to its capability to stabilize Néel-type domain walls (DWs) and magnetic skyrmions for the realization of non-volatile memory and logic devices. In this study, we demonstrate that magnetic properties in perpendicularly magnetized Ta/Pt/Co/MgO/Pt heterostructures, such as magnetization and DMI, can be significantly influenced by the MgO thickness. To avoid the excessive oxidation of Co, an ultrathin Mg layer is inserted to improve the quality of the Co-MgO interface. By using field-driven domain wall expansion in the creep regime, we find that non-monotonic tendencies of the DMI field appear when changing the thickness of MgO. With the insertion of a monatomic Mg layer, the strength of the DMI could reach a high level and saturate. We conjecture that the efficient control of the DMI is a result of subtle changes of both Pt/Co and Co/MgO interfaces, which provides a method to optimize the design of ultra-thin structures achieving skyrmion electronics.