Physical Review Materials

Oxide molecular beam epitaxy is a powerful synthesis technique capable of creating complex layered structures with elements in high oxidation states. The authros start with the Sr${}_{n+1}$Cr${}_n$O${}_{3n+1}$ Ruddlesden-Popper series. This system contains the magnetic Cr${}^{4+}$ cation which gives rise to electronic correlations that vary as a function of structural dimensionality: Sr${}_2$CrO${}_4$ and Sr${}_3$Cr${}_2$O${}_7$ possess enhanced spin and orbital ordering temperatures compared to the SrCrO${}_3$ end member. In this work, they synthesize films for $n=1$ to $n=5$, uncovering a metal to insulator transition. They seek the physical origins of the concomitant spin and orbital orderings – both experimentally with x-ray absorption spectroscopy measurements, and theoretically with density functional theory calculations. Their results unveil the basis of these exotic ground states, including additional structural distortions that play a key role in the system and enable the metal-insulator transitions.

Materials with strongly interacting electrons can exhibit remarkable behaviors, such as stripe ordering, where charge and spin form periodic patterns. These states often precede unconventional superconductivity, as in the bilayer nickelate La${}_3$Ni${}_2$O${}_7$ under pressure. Using correlated density-functional methods, the authors interrogate possible magnetic ground states of La${}_3$Ni${}_2$O${}_7$. The stripe-ordered ground state reproduces several experimental features, including insulating behavior at ambient pressure and strong antiferromagnetic coupling within the Ni-$d_{z^2}$ orbitals. Moreover, they find that the Ni-$d_{x^2-y^2}$ orbitals play a dominant role at low-energy. Overall, their findings provide fresh insights into the stripe ordering and its potential link to superconductivity in this fascinating material.

Perovskite cobaltite films and interfaces are of interest for a wide variety of reasons and applications, including magnetism. One fascinating aspect of such materials is their tendency to form ordered arrangements of oxygen vacancies. Here, the authors demonstrate that the ordering of oxygen vacancies in prototypical La${}_{1-x}$Sr${}_x$CoO${}_{3-\delta }$ can be precisely controlled through both heteroepitaxial strain and growth orientation. This in turn controls the oxygen vacancy depth profile, with profound consequences for electronic and magnetic behavior. In particular, dead layer effects can be effectively mitigated via this approach, enabling bulk-like metallic and ferromagnetic properties at few-unit-cell thicknesses.

Since the discovery of high-$T_c$ superconductivity in cuprates, transition-metal oxides have been a central subject of condensed matter physics to explore novel phenomena arising from strong electron-electron interactions. Recently, the combination of strong correlation and relativistic spin-orbit coupling effects has been recognized as a key ingredient to induce potential topological phenomena. This article reviews recent progress in correlated topological phenomena in transition-metal oxide thin films and heterostructures, such as magnetic skyrmions, topological Hall effects, and Dirac fermions. Future perspectives are also discussed including Weyl fermions, altermagnetism, and topological superconductivity.

The interface between γ-Al${}_2$O${}_3$ and SrTiO${}_3$ has for a decade held the record for highest mobility in SrTiO${}_3$-based interfaces. This has previously been attributed to a band-inversion between the d${}_{xy}$ and d${}_{xz/yz}$-bands. Such a band-inversion can be expected to reveal other effects, including a different superconducting phase diagram as a function of carrier densities. Elucidated by transport measurements, coexistence of high electron mobility, high carrier density, superconductivity and unpaired spins is presented in this article. The possibility of engineering interplay between these effect makes the γ-Al${}_2$O${}_3$/SrTiO${}_3$ a promising candidate for quantum and spintronics technologies.

Topochemistry enables material design by modifying the anion sublattices. In oxide heterostructures, these modifications can be layer-selective, resulting in different types of interface reconstructions with various electronic and magnetic properties. In the topochemical reduction of infinite-layer nickelate films, heteroepitaxy with the substrate or a capping layer plays an important role in stabilizing the superconducting phase. In this study, the authors investigate artificial superlattices with repeating interfaces between nickelate layers and layers of materials typically used as substrates or capping layers, using soft x-ray spectroscopy in combination with ab initio theory. They observe modulations in the nickel valence state and oxygen coordination disorder that correlate with electrical transport measurements.

$5d$ transition-metal oxides (TMOs) provide a promising platform for efficient spin-orbit torque (SOT) generation via the spin Hall effect. In this study, the authors fabricated binary IrO${}_2$ thin films with three distinct crystalline forms: epitaxial, polycrystalline, and amorphous states. Harmonic Hall measurements reveal that the SOT efficiency increases with decreasing crystallinity, from epitaxial to amorphous, along with an increase in electrical resistivity. Despite these variations, the spin Hall conductivity remains nearly constant, indicating the intrinsic spin Hall mechanism. These findings underscore the crucial role of crystallinity in SOT generation and open up possibilities for spintronic devices based on $5d$ TMOs.

Spin-polarized two-dimensional electron gases (2DEGs) are of particular interest for applications in functional oxide electronics. The 2DEG generated by redox reactions on the strontium titanate (SrTiO${}_3$ (STO)) side of a europium monoxide (EuO)/SrTiO${}_3$~(001) interface is a promising candidate for significant spin polarization. The authors have investigated this 2DEG using magnetic circular dichroism in the angular distribution of photo-emitted electrons and density functional theory calculations with a Hubbard $U$ term. They successfully demonstrated that the EuO/STO~(001) interface is spin-polarized due to its proximity to the strong Heisenberg ferromagnet EuO. Furthermore, they have shown that the spin polarization of the 2DEG depends on the thickness of the EuO thickness when approaching the 2D limit.

Probing topological quantum states at the atomic limit is a frontier in physics, yet it is often hindered by the “dead layer” effect in ultrathin films. The authors overcome this challenge by designing SrRuO${}_3$/SrIrO${}_3$ heterostructures, demonstrating that interfacial effects can stabilize metallicity and ferromagnetism in SrRuO${}_3$ down to a single unit cell. Angle-resolved photoemission spectroscopy provides direct evidence of its robust metallic band structure. This work not only revives exotic states in ultrathin SrRuO${}_3$ but also showcases a powerful pathway to tune the Berry curvature and anomalous Hall effect via interface engineering, opening new avenues for designing topological quantum devices in the two-dimensional limit.

Spin Hall magnetoresistance (SMR) describes how the resistivity of a heavy metal is modulated by the proximity of a magnetic material. In this work, the authors demonstrate SMR at the interface between platinum and an insulating ferroelectric altermagnetic candidate, Ba${}_2$CoGe${}_2$O${}_7$ – an emerging class of collinear compensated magnets with unique combined spin and crystal symmetries. They show that this heterostructure exhibits a relatively large SMR signal regardless of the unoptimized interface between bulk BCGO and Pt. Furthermore, the SMR is anisotropic with the crystal orientation of the current. Possible contributions, including the role of altermagnetism or ferroelectricity, are discussed. The results show a possible route towards tunable and electrically controllable SMR responses in spintronic devices.

Altermagnetism, a collinear magnetic phase distinct from ferro- and antiferromagnetism, has drawn attention with RuO₂ as a leading candidate. While theory predicted sizable Ru local moments driving antiferromagnetic order, a growing pile of experiments suggests that the bulk is essentially nonmagnetic, consistent with delocalized 4$d$ electrons and metallic screening. In this study, the authors reveal that the RuO₂(110) surface breaks this trend: symmetry breaking at the surface induces electronic redistribution and spontaneous magnetization. This surface magnetism produces spin-polarized states, distinctive scanning probe signatures, and potential spin-dependent transport, underscoring how subtle surface effects can radically alter the magnetic character of a nominally nonmagnetic material.

Transverse resistance (Hall effect) offers a sensitive probe into the properties of two-dimensional superconductors. In this work, the authors systematically investigate the transverse resistance in superconducting LaAlO${}_3$/KTaO${}_3$(111) interfaces and observe two distinct anomalies: a sharp transverse resistance peak during zero-field cooling, and a strongly even-symmetric signal under magnetic field sweeps–markedly departing from the conventional odd-symmetric Hall behavior. These features are explained by microscopically uneven vortex distributions, reflecting intrinsic inhomogeneity and unique vortex dynamics at the oxide interface.

The authors report a giant topological Hall effect at room temperature in permalloy/La${}_{0.65}$Sr${}_{0.35}$MnO${}_3$ (Py/LSMO) heterostructures, with resistivity reaching ~2.8 μΩ·cm, far exceeding that of single-layer Py. The effect arises from the interplay of LSMO ferromagnetism and Rashba spin–orbit coupling at the broken-symmetry interface. Magnetic imaging reveals the presence of skyrmion-like spin textures, which are further tunable with a ferroelectric spacer, as corroborated by theoretical modeling. These findings establish Py/LSMO heterostructures as a promising platform to manipulate interfacial spin textures and advance next-generation spintronic technologies.

The authors reveal a magnetic proximity effect in La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ (LSMO)/SrIrO${}_3$ (SIO) heterostructures arising from an interfacial reconstruction. When LSMO caps SIO, LSMO retains interfacial ferromagnetism, polarizing the iridate and producing an emergent, intrinsic AHE. Reversing the stack (SIO/LSMO) suppresses interfacial LSMO magnetism; the proximity effect and AHE vanish. These results identify interface-engineered chemistry and bonding as the lever controlling proximity magnetism and topological AHE at a 3d/5d oxide interface, highlighting a tunable interplay of topology and correlations with prospects for topological spintronics and spin-orbitronic devices.

This study demonstrates precise control over oxygen vacancy channel (OVC) orientations in brownmillerite La${}_{0.67}$Ca${}_{0.33}$MnO${}_{2.5}$ films through strain engineering achieved by tuning the crystallographic orientation of NdGaO${}_3$ substrates. Horizontal OVCs yield atomically sharp interfaces, whereas vertical ones develop interfacial layers to maintain oxygen connectivity. The films undergo reversible topotactic transitions between brownmillerite and perovskite phases, accompanied by a transformation between half-metallic ferromagnetism and insulating antiferromagnetism. The study elucidates how strain states, Mn valence, and Mn–O hybridization govern interfacial and electronic structures, providing design strategies for oxide electronics with tunable functionalities.

Superconductor/ferromagnet interfaces are fertile ground for emergent phenomena that result from competing interactions. All-oxide heterostructures based in high-$T_c$ superconducting cuprates and half-metallic manganites are especially appealing in this context, and a promising platform for developing superconducting spintronic devices. Based on those heterostructures, the authors realize vertical micro-junctions that show a unusual spin-valve effect, whose polarity can be reversed by temperature. This can be understood considering the interplay between superconducting proximity, spin-dependent scattering, and magnetic exchange effects.

It is shown that unexpectedly complex magnetic structures arise in epitaxial manganite films when their substrates undergo thermally driven structural transitions and electrically driven ferroelectric domain switching. This complexity may arise due to long-range strain fields between ferroelectric domains, and the resulting images of vector in-plane magnetization are beautiful. The image shown here is reminiscent of botanical structures. The magnetization in the green ‘leaves’ lies perpendicular to the magnetization in the brown twigs on which they ‘grow’. The ‘twigs’ are straight, and decorate ferroelectric domains in the substrate.