Christian Binek

Axion electrodynamics, originally proposed by Frank Wilczek in the context of high energy physics, can manifest in condensed matter systems as an isotropic linear magnetoelectric (ME) response. Yet, a continuous and purely monopolar ME response has until now remained experimentally inaccessible. We realize such a desired response by converting the topologically trivial antiferromagnet chromia into an isotropic ME medium. This conversion is demonstrated through low-frequency AC ME susceptometry and theoretically supported by Monte Carlo simulations. Field-cooling a chromia single crystal into an antiferromagnetic single-domain state, followed by powdering and solidification while keeping the grains in the antiferromagnetically ordered state, suppresses ME quadrupolar contributions and yields an isotropic ME tensor α_ij=θδ_ij. The parameter θ is temperature dependent and crosses zero at 168 K, consistent with the axion contribution in crystalline chromia and our theoretical findings. Above the Néel temperature, the pure axion response becomes irretrievable, revealing an intrinsic asymmetry between ME annealing before and after powdering. By realizing and controlling the monopolar ME state, we establish a condensed-matter platform for axion electrodynamics and open pathways toward ME Hall, magnetophotovoltaic, and monopolar-based devices.

This perspective article explores the potential of two-dimensional electron and hole gases at oxide/oxide interfaces as promising conduction channels in energy-efficient spintronic devices for memory and logic applications. Magneto-electric and ferro-electric device architectures are considered, highlighting their advantages in integrating non-volatility with ultra-fast switching capabilities. We emphasize the significance of spin–orbit coupling in generating robust spin currents and examine how the Rashba and inverse Edelstein effects can be harnessed in device operations. Specific heterostructures, including HfO₂/SrTiO₃, Cr₂O₃/TiO_2-x, and ZrO_2-x-based systems, are reviewed in terms of their material properties, interface stability, and suitability for practical implementation.

We provide evidence of incipient edge-state formation in graphene-on-chromia heterostructures, resulting from the presence of substrate-induced spin-orbit coupling (SOC). Low-temperature spin-Hall measurements of the graphene exhibit large fluctuations of the nonlocal resistance (R_nl) as the Fermi level is swept through its bands. These features persist to much higher temperatures (>60 K) than is typical for mesoscopic effects in normal metals and semiconductors, suggesting that they are associated with a very different mechanism to normal charge-based transport. The fluctuations of the nonlocal resistance are moreover invariant to the application of large magnetic fields (as high as 7 T), a characteristic that speaks to a significant edge-like component to transport. To provide insight into these results, we have formulated a (Landauer-Büttiker) model of quantum transport in graphene in the presence of strong SOC. Our calculations capture the key features of experiments, connecting the large quantum fluctuations in R_nl to the impact of the substrate-induced SOC. More specifically, the spin-orbit interaction is found to imbue the spin diffusion with an edge-like character, displacing injected spins toward the edges of the graphene channel while retaining a coupling to disorder within its interior. It is this interplay of edge-like conduction, with more conventional transport within the interior of the two-dimensional channel, which gives rise to the observed fluctuations in R_nl as the Fermi level is varied.

A bulk gadolinium (Gd) single crystal exhibits virtually zero remnant magnetization, a common trait among soft uniaxial ferromagnets. This characteristic is reflected in our magnetometry data showing virtually hysteresis free isothermal magnetization loops with large saturation magnetization. The absence of hysteresis allows to model the measured easy axis magnetization as a function of temperature and applied magnetic field, rather than a relation, which permits the application of Maxwell relations from equilibrium thermodynamics. Demagnetization effects broaden the isothermal first-order transition from negative to positive magnetization. By analyzing magnetization data within the coexistence regime, we deduce the isothermal entropy change and the field-induced heat capacity change. Comparing the numerically inferred heat capacity with relaxation calorimetric data confirms the applicability of the Maxwell relation. Analysis of the entropy in the mixed phase region suggests the presence of hitherto unresolved nanoscale magnetic structures in the demagnetized state of Gd. To support this prediction, Monte Carlo simulations of a 3D Ising model with dipolar interactions are performed. Analyzing the cluster size statistics and magnetization from the model provides strong qualitative support of our analytic approach.

R. Ramesh(*rr73@rice.edu) R. Ramesh(*rr73@rice.edu)

Chromia (Cr ₂ O ₃ ) is a magnetoelectric oxide that permits voltage‐control of the antiferromagnetic (AFM) order, but it suffers technological constraints due to its low Néel Temperature ( T _N ≈307 K) and the need of a symmetry‐breaking applied magnetic field to achieve reversal of the Néel vector. Recently, boron (B) doping of Cr ₂ O ₃ films led to an increase T _N >400 K and allowed the realization of voltage magnetic‐field free controlled Néel vector rotation. Here, the impact of B doping is directly imaged on the formation of AFM domains in Cr ₂ O ₃ thin films and elucidates the mechanism of voltage‐controlled manipulation of the spin structure using nitrogen‐vacancy (NV) scanning probe magnetometry. A stark reduction and thickness dependence of domain size in B‐doped Cr ₂ O ₃ (B:Cr ₂ O ₃ ) films is found, explained by the increased germ density, likely associated with the B doping. By reconstructing the surface magnetization from the NV stray‐field maps, a qualitative distinction between the undoped and B‐doped Cr ₂ O ₃ films is found, manifested by the histogram distribution of the AFM ordering, that is, 180 ° domains for pure films, and 90 ° domains for B:Cr ₂ O ₃ films. Additionally, NV imaging of voltage‐controlled B‐doped Cr ₂ O ₃ devices corroborates the 90 ° rotation of the AFM domains observed in magnetotransport measurement.

Chromia (Cr2O3) is a magnetoelectric oxide which permits voltage-control of the antiferromagnetic (AFM) order, but it suffers technological constraints due to its low Neel Temperature (TN ~307 K) and the need of a symmetry breaking applied magnetic field to achieve reversal of the Neel vector. Recently, boron (B) doping of Cr2O3 films led to an increase TN > 400 K and allowed the realization of voltage magnetic-field free controlled Néel vector rotation. Here, we directly image the impact of B doping on the formation of AFM domains in Cr2O3 thin films and elucidate the mechanism of voltage-controlled manipulation of the spin structure using nitrogen vacancy (NV) scanning probe magnetometry. We find a stark reduction and thickness dependence of domain size in B-doped Cr2O3 (B:Cr2O3) films, explained by the increased germ density, likely associated with the B doping. By reconstructing the surface magnetization from the NV stray-field maps, we find a qualitative distinction between the undoped and B-doped Cr2O3 films, manifested by the histogram distribution of the AFM ordering, i.e., 180 degree domains for pure films, and 90 degree domains for B:Cr2O3 films. Additionally, NV imaging of voltage-controlled B-doped Cr2O3 devices corroborate the 90 degeree rotation of the AFM domains observed in magnetotransport measurement.