Motivated by the recently observed topological Hall effect in ultra-thin films of SrRuO$_3$ (SRO) grown on SrTiO$_3$ (STO) [001] substrate, we investigate the magnetic ground state and anomalous Hall response of the SRO ultra-thin films by virtue of spin density functional theory (DFT). Our findings reveal that in the monolayer limit of an SRO film, a large energy splitting of Ru-$t_{2g}$ states stabilizes an anti-ferromagnetic (AFM) insulating magnetic ground state. For the AFM ground state, our Berry curvature calculations predict a large anomalous Hall response upon doping. From the systematic symmetry analysis, we uncover that the large anomalous Hall effect arises due to a combination of broken time-reversal and crystal symmetries caused by the arrangement of non-magnetic atoms (Sr and O) in the SRO monolayer. We identify the emergent Hall effect as a clear manifestation of the so-called crystal Hall effect in terminology of Šmejkal et al. arXiv:1901.00445 (2019), and demonstrate that it persists at finite frequencies which is the manifestation of the crystal magneto-optical effect. Moreover, we find a colossal dependence of the AHE on the degree of crystal symmetry breaking also in ferromagnetic SRO films, which all together points to an alternative explanation of the emergence of the topological Hall effect observed in this type of systems.
The proposal that very small Néel skyrmions can form in SrRuO$_3$/SrIrO$_3$ epitaxial bilayers and that the electric field-effect can be used to manipulate these skyrmions in gated devices strongly stimulated the recent research of SrRuO$_3$ heterostructures. A strong interfacial Dzyaloshinskii-Moriya interaction, combined with the breaking of inversion symmetry, was considered as the driving force for the formation of skyrmions in SrRuO$_3$/SrIrO$_3$ bilayers. Here, we investigated nominally symmetric heterostructures in which an ultrathin ferromagnetic SrRuO$_3$ layer is sandwiched between l…
Berry curvature plays a crucial role in exotic electronic states of quantum materials, such as intrinsic anomalous Hall effect. As Berry curvature is highly sensitive to subtle changes of electronic band structures, it can be finely tuned via external stimulus. Here, we demonstrate in SrRuO3 thin films that both the magnitude and sign of anomalous Hall resistivity can be effectively controlled with epitaxial strain. Our first-principles calculations reveal that epitaxial strain induces an additional crystal field splitting and changes the order of Ru d orbital energies, which alters the Berry …
Recent observation of quantum transport phenomena of Weyl fermions has brought much attention to 4d ferromagnetic perovskite SrRuO3 as a magnetic Weyl semimetal. Besides, the hump-like Hall effect anomaly, which might have a topological origin, has also been reported for this material. Here, we show that the emergence of such phenomena is governed by the degree of scattering determined by the defect density (Ru-deficiency- and/or interface-driven-defect scattering) and measurement temperature (phonon scattering), where the former is controlled by varying the growth conditions of the SrRuO3 fil…
The Hall effect in SrRuO$_3$ thin-films near the thickness limit for ferromagnetism shows an extra peak in addition to the ordinary and anomalous Hall effects. This extra peak has been attributed to a topological Hall effect due to two-dimensional skyrmions in the film around the coercive field; however, the sign of the anomalous Hall effect in SrRuO$_3$ can change as a function of saturation magnetization. Here we report Hall peaks in SrRuO$_3$ in which volumetric magnetometry measurements and magnetic force microscopy indicate that the peaks result from the superposition of two anomalous Hal…
Topological spin textures as an emerging class of topological matter offer a medium for information storage and processing. The recently discovered topological Hall effect (THE) is considered as a fingerprint for electrically probing non-trivial spin-textures. But the origin of THE in oxides has remained elusive. Here we report an observation of the THE in ultrathin (unit cells. u.c.) 4d ferromagnetic SrRuO3 films grown on SrTiO3(001) substrates, which can be attributed to the chiral ordering of spin structure (i.e., skyrmion-like) in the single SrRuO3 layer without contacting 5d oxide SrIrO3 …
Motivated by the recently observed unconventional Hall effect in ultra-thin films of ferromagnetic SrRuO$_3$ (SRO) we investigate the effect of strain-induced oxygen octahedral distortion in the electronic structure and anomalous Hall response of the SRO ultra-thin films by virtue of density functional theory calculations. Our findings reveal that the ferromagnetic SRO films grown on SrTiO$_3$ (in-plane strain of $-$0.47$\%$) have an orthorhombic (both tilting and rotation) distorted structure and with an increasing amount of substrate-induced compressive strain the octahedral tilting angle is…
The topological Hall effect (THE) has been discovered in ultrathin SrRuO3 (SRO) films, where the interface between the SRO layer and another oxide layer breaks the inversion symmetry resulting in the appearance of THE. Thus, THE only occurs in ultra-thin SRO films of several unit cells. In addition to employing a heterostructure, the inversion symmetry can be broken intrinsically in bulk SRO by introducing defects. In this study THE is observed in 60 nm thick SRO films, in which defects and lattice distortions are introduced by helium ion irradiation. The irradiated SRO films exhibit a pronoun…
Ionic substitution forms an essential pathway to manipulate the carrier density and crystalline symmetry of materials via ion-lattice-electron 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 carrier density and crystalline symmetry through the ionic liquid gating induced protonation. The insertion of protons electron-dopes SrRuO3, leading to an exotic ferromagnetic to paramagnetic phase transition along with the increase of proton concentr…
A controversy arose over the interpretation of the recently observed hump features in Hall resistivity $ρ_{xy}$ from ultra-thin SrRuO$_3$ (SRO) film; it was initially interpreted to be due to topological Hall effect but was later proposed to be from existence of regions with different anomalous Hall effect (AHE). In order to settle down the issue, we performed Hall effect as well as magneto-optic Kerr-effect measurements on 4 unit cell SRO films grown on SrTiO$_3$ (001) substrates. Clear hump features are observed in the measured $ρ_{xy}$, whereas neither hump feature nor double hysteresis loo…
In quantum matters hosting electron-electron correlation and spin-orbit coupling, spatial inhomogeneities, arising from competing ground states, can be essential for determining and understanding topological properties. A prominent example is Hall anomalies observed in SrRuO3 films, which were interpreted in terms of either magnetic skyrmion-induced topological Hall effect (THE) or inhomogeneous anomalous Hall effect (AHE). To clarify this ambiguity, we systematically investigated the AHE of SrRuO3 ultrathin films with controllable inhomogeneities in film thickness (tSRO). By harnessing the st…
In this work, the BiFeO3 (BFO)/SrRuO3 (SRO) heterostructure was fabricated and the anomalous Hall effect (AHE) was investigated the in BFO/SRO. It is found the nonmonotonic anomalous Hall resistivity behavior in BFO/SRO is originated from the inhomogeneous SRO layer instead of the topological Hall effect. It is surprised that the AHE in BFO/SRO structure can be manipulated by ferroelectric polarization of BFO. Moreover, an inhomogeneous phenomenological model has been applied on those structure. Furthermore, the modification of band structure in SRO under ferroelectric polarization was discuss…
Electron transport coupled with magnetism has attracted attention over the years as exemplified in anomalous Hall effect due to a Berry phase in momentum space. Another type of unconventional Hall effect -- topological Hall effect, originating from the real-space Berry phase, has recently become of great importance in the context of magnetic skyrmions. We have observed topological Hall effect in bilayers consisting of ferromagnetic SrRuO$_3$ and paramagnetic SrIrO$_3$ over a wide region of both temperature and magnetic field. The topological term rapidly decreases with the thickness of SrRuO$_…
Perovskite SrRuO$_3$ is a prototypical itinerant ferromagnet which allows interface engineering of its electronic and magnetic properties. We report synthesis and investigation of atomically flat artificial multilayers of SrRuO$_3$ with the spin-orbit semimetal SrIrO$_3$ in combination with band-structure calculations with a Hubbard $U$ term and topological analysis. They reveal an electronic reconstruction and emergence of flat Ru-4d$_{xz}$ bands near the interface, ferromagnetic interlayer coupling and negative Berry-curvature contribution to the anomalous Hall effect. We analyze the Hall ef…
Ferromagnetism and exotic topological structures in SrRuO$_3$ (SRO) induce sign-changing anomalous Hall effect (AHE). Recently, hump structures have been reported in the Hall resistivity of SRO thin films, especially in the ultra-thin regime. We investigate the AHE and hump structure in the Hall resistivity of SRO ultra-thin films with an SrTiO$_3$ (STO) capping layer and ionic liquid gating. STO capping results in sign changes in the AHE and modulation of the hump structure. In particular, the hump structure in the Hall resistivity is strongly modulated and even vanishes in STO-capped 4 unit …
Topological transport phenomena in magnetic materials are a major topic of current condensed matter research. One of the most widely studied phenomena is the ``topological Hall effect'' (THE), which is generated via spin-orbit interactions between conduction electrons and topological spin textures such as skyrmions. We report a comprehensive set of Hall effect and magnetization measurements on epitaxial films of the prototypical ferromagnetic metal SrRuO$_3$ the magnetic and transport properties of which were systematically modulated by varying the concentration of Ru vacancies. We observe Hal…
We investigate structural and transport properties of highly Ru-deficient SrRu0.7O3 thin films prepared by molecular beam epitaxy on (001) SrTiO3 substrates. To distinguish the influence of the two types of disorders in the films, Ru vacancies within lattices and disorders near the interface, SrRu0.7O3 thin films with various thicknesses (t = 1-60 nm) were prepared. It was found that the influence of the former dominates the electrical and magnetic properties when t > 5-10 nm, while that of the latter does when t < 5-10 nm. Structural characterizations revealed that the crystallinity, in terms…
We investigated the structural and magnetic properties of bare SrRuO$_3$ (SRO) ultra-thin films and SrRuO$_3$/SrIrO$_3$/SrZrO$_3$ (SRO/SIO/SZO: RIZ) trilayer heterostructures between 10 K and 80 K, by comparing macroscopic data using magneto-optical Kerr effect (MOKE) and magneto-transport (anomalous Hall effect: AHE), with nanoscale fingerprints when applying non-contact scanning force microscopy (nc-SFM) and magnetic force microscopy (MFM). SRO and RIZ ultra-thin films were epitaxially grown at 650C onto vicinal SrTiO$_3$ (100) single-crystalline substrates to a nominal thickness of 4 and 4/…
Manipulation of octahedral distortion at atomic length scale is an effective means to tune the physical ground states of functional oxides. Previous work demonstrates that epitaxial strain and film thickness are variable parameters to modify the octahedral rotation and tilt. However, selective control of bonding geometry by structural propagation from adjacent layers is rarely studied. Here we propose a new route to tune the ferromagnetic response in SrRuO3 (SRO) ultrathin layers by oxygen coordination of adjacent SrCuO2 (SCO) layers. The infinite-layered CuO2 in SCO exhibits a structural tran…
The magnetization of single crystal SrRuO3 is studied as a function of temperature along different crystallographic directions. The magnetocrystalline anisotropy and behavior near the critical transition temperature are analyzed in detail. The magnetization vs temperature is found to vary more like $T^2$ rather than $T^{3/2}$ expected for spin waves.
Influence of substrate miscut on magnetization dynamics in SrRuO$_3$ (SRO) thin films was studied. Two films were grown on SrTiO$_3$ substrates with high ($\sim1^{\circ}$) and low ($\sim0.1^{\circ}$) miscut angles, respectively. As expected, high miscut angle leads to suppression of multi-variant growth. By means of SQUID magnetometry, comparable relaxation effects were observed in both the multi-variant and the nearly single-variant sample. Differences in the magnetization reversal process were revealed by magnetic force microscopy. It showed that the multi-variant growth leads to higher dens…
A SrRuO$_3$ (SRO) thin film and its heterostructure have brought 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 oxygen K-edge to investigate the spin configuration of a 4 unit-cell SRO film that was grown epitaxially on a single crystal SrTiO$_3$. The RSXS signal under a magnetic field (~0.4 Tesla) clearly shows a magnetic dichroism pattern around the specular reflectio…
Due to the strong lattice-property relationships which exist in complex oxide epitaxial layers, their electronic and magnetic properties can be modulated by structural distortions induced at the atomic scale. The modification and control can be affected at coherent heterointerfaces by epitaxial strain imposed by the substrate or by structural modifications to accommodate the film-substrate symmetry mismatch. Often these act in conjunction with a strong dependence on the layer thickness, especially for ultrathin layers. Moreover, as a result of these effects, the temperature dependence of the s…
The effect of Cr substitution in a SrRuO3 epitaxial thin film on SrTiO3 substrate was investigated by measuring the magnetic and transport properties and the electronic states. The ferromagnetic transition temperature of the SrRu0.9Cr0.1O3 film (166 K) was higher than that of the SrRuO3 film (147 K). Resonant photoemission spectroscopy experimentally revealed that the Cr 3dt2g orbital is hybridized with the Ru 4dt2g orbital in the SrRu0.9Cr0.1O3 film, supporting the assumption that the enhancement of the ferromagnetic transition temperature through Cr substitution stems from the widening of en…
High-mobility two-dimensional carriers originating from surface Fermi arcs in magnetic Weyl semimetals are highly desired for accessing exotic quantum transport phenomena and for topological electronics applications. Here, we demonstrate high-mobility two-dimensional carriers that show quantum oscillations in magnetic Weyl semimetal SrRuO3 epitaxial films by systematic angle-dependent, high-magnetic field magnetotransport experiments. The exceptionally high-quality SrRuO3 films were grown by state-of-the-art oxide thin film growth technologies driven by machine learning algorithm. The quantum …
One of the key technologies in spintronics is to tame spin-orbit coupling (SOC) that links spin and motion of electrons, giving rise to intriguing magneto-transport properties in itinerant magnets. Prominent examples of such SOC-based phenomena are anomalous and topological Hall effects. However, controlling them by electric field has remained unachieved since electric field tends to be screened in itinerant magnets. Here we demonstrate that both anomalous and topological Hall effects can be modulated by electric field in oxide heterostructures consisting of ferromagnetic SrRuO$_{3}$ and nonma…
Electric-field (E-field) control of magnetism enabled by multiferroics has the potential to revolutionize the landscape of present memory devices plagued with high energy dissipation. To date, this E-field controlled multiferroic scheme at room temperature has only been demonstrated using BiFeO3 (BFO) films grown on DyScO3 (refs 1 and 2), a unique and expensive substrate, which gives rise to a particular ferroelectric domain pattern in BFO. Here, we demonstrate reversible E-field-induced switching of the magnetic state of the Co layer in Co/BFO (001) thin film heterostructures fabricated on Sr…
Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy magnetization axis depending on the epitaxial strain and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism (XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to non-magnetic around 3monolayer thickness, consistent with previous magnetization and magneto-optical Kerr eff…
The spin Hall magnetoresistance (SMR) and anomalous Hall effect (AHE) are observed in a Cr2O3/Ta structure. The structural and surface morphology of Cr2O3/Ta bilayers have been investigated. Temperature dependence of longitudinal and transverse resistances measurements confirm the relationship between SMR and AHE signals in Cr2O3/Ta structure. By means of temperature dependent magnetoresistance measurements, the physical origin of SMR in the Cr2O3/Ta structure is revealed, and the contribution to the SMR from the spin current generated by AHE has been proved. The so-called boundary magnetizati…
The anomalous Hall effect, observed in conducting ferromagnets with broken time-reversal symmetry, offers the possibility to couple spin and orbital degrees of freedom of electrons in ferromagnets. In addition to charge, the anomalous Hall effect also leads to spin accumulation at the surfaces perpendicular to both the current and magnetization direction. Here we experimentally demonstrate that the spin accumulation, subsequent spin backflow, and spin-charge conversion can give rise to a different type of spin current related magnetoresistance, dubbed here as the anomalous Hall magnetoresistan…
To date, magnetic proximity effect (MPE) has only been conclusively observed in ferromagnet (FM) based systems. We report the observation of anomalous Hall effect and anisotropic magnetoresistance in angular dependent magnetoresistance (ADMR) measurements in Pt on antiferromagnetic (AF) $α$-Fe$_2$O$_3$(0001) epitaxial films at 10 K, which provide evidence for the MPE. The Néel order of $α$-Fe$_2$O$_3$ and the induced magnetization in Pt show a unique ADMR compared with all other FM and AF systems. A macrospin response model is established and can explain the AF spin configuration and all main …
Magnetic lanthanide half-Heuslers ($R$PtBi; $R$ being the lanthanide) represent an attractive subgroup of the Heusler family and have been identified as ideal candidates for time reversal symmetry breaking topological Weyl semimetals. In this paper, we present the detailed analysis of the magnetotransport properties of frustrated antiferromagnet TbPtBi. This material shows large, non-saturating magnetoresistance (MR) with unusual magnetic field dependence. The MR of TbPtBi is significantly anisotropic with respect to the magnetic field, applied along different crystallographic directions and i…
Spintronics-based nonvolatile components in neuromorphic circuits offer the possibility of realizing novel functionalities at low power. Current-controlled electrical switching of magnetization is actively researched in this context. Complex oxide heterostructures with perpendicular magnetic anisotropy (PMA), consisting of SrRuO$_3$ (SRO) grown on SrTiO$_3$ (STO) are strong material contenders. Utilizing the crystal orientation, magnetic anisotropy in such simple heterostructures can be tuned to either exhibit a perfect or slightly tilted PMA. Here, we investigate current-induced magnetization…
Complex oxide heterostructures are hot candidates for post CMOS multi-functional devices. Especially in spintronics applications ferromagnetic oxides may play a key role because they can exhibit extraordinary high spin polarization. Indeed, there are already plenty of examples in spintronics, notably in the area of spin pumping and inverse spin Hall effect (ISHE) \cite{Azevedo2011, Czeschka2011, Hahn2013, Obstbaum2014}. Although complex oxides have been used in these experiments as a source of spin currents, they have never been demonstrated to act as a spin sink that exhibits ISHE. Here we sh…
Giant Domain-Wall Hall Magnetoresistance in Magnetic Topological Semimetal
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University（华南师范大学）
We investigate the spin Hall magnetoresistance in thin film bilayer heterostructures of the heavy metal Pt and the antiferromagnetic insulator NiO. While rotating an external magnetic field in the easy plane of NiO, we record the longitudinal and the transverse resistivity of the Pt layer and observe an amplitude modulation consistent with the spin Hall magnetoresistance. In comparison to Pt on collinear ferrimagnets, the modulation is phase shifted by 90° and its amplitude strongly increases with the magnitude of the magnetic field. We explain the observed magnetic field-dependence of the spi…
