Marjana Ležaić

Forschungszentrum Jülich

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Stefan Blügel

Forschungszentrum Jülich

K. Z. Rushchanskii

Forschungszentrum Jülich

Phivos Mavropoulos

National and Kapodistrian University of Athens

Gustav Bihlmayer

Forschungszentrum Jülich

Biplab Sanyal

Uppsala University

Ivetta Slipukhina

Forschungszentrum Jülich

Martin Schlipf

VASP Software (Austria)

S. Kamba

Czech Academy of Sciences, Institute of Physics

EŞ

E. Şaşıoğlu

Martin Luther University Halle-Wittenberg

Yuriy Mokrousov

Forschungszentrum Jülich

Cooperative Institutions

Forschungszentrum Jülich

Centre National de la Recherche Scientifique

Jülich Aachen Research Alliance

RWTH Aachen University

Czech Academy of Sciences, Institute of Physics

The University of Tokyo

SPring-8

Chinese Academy of Sciences

Tohoku University

Universitat Autònoma de Barcelona

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DFT description of orthorombic perovskite iridates: is there a gap?

Marjana Ležaić

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Tailoring the anomalous Hall effect of SrRuO3 thin films by strain: A first principles study

Journal of Applied Physics (2021)

Kartik Samanta Marjana Ležaić Stefan Blügel Yuriy Mokrousov

Motivated by the recently observed unconventional Hall effect in ultrathin films of ferromagnetic SrRuO3 (SRO), we investigate the effect of strain-induced oxygen octahedral distortion in the electronic structure and anomalous Hall response of the SRO ultrathin films by virtue of density functional theory calculations. Our findings reveal that the ferromagnetic SRO films grown on SrTiO3 (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 found to be suppressed gradually, with SRO films grown on NdGaO3 (in-plane strain of −1.7%) stabilized in the tetragonal distorted structure (with zero tilting). Our Berry curvature calculations predict a positive value of the anomalous Hall conductivity of +76 S/cm at −1.7% strain, whereas it is found to be negative (−156 S/cm) at −0.47% strain. We attribute the found behavior of the anomalous Hall effect to the nodal point dynamics in the electronic structure arising in response to tailoring the oxygen octahedral distortion driven by the substrate induced strain. We also calculate strain-mediated anomalous Hall conductivity as a function of reduced magnetization obtained by scaling down the magnitude of the exchange field inside Ru atoms finding good qualitative agreement with experimental observations, which indicates a strong impact of longitudinal thermal fluctuations of Ru spin moments on the anomalous Hall effect in this system.

Tetragonal crystal system

Orthorhombic crystal system

10.1063/5.0043742

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Crystal Hall and crystal magneto-optical effect in thin films of SrRuO3

Journal of Applied Physics (2020)

Kartik Samanta Marjana Ležaić M. Merte Frank Freimuth Stefan Blügel

Motivated by the recently observed topological Hall effect in ultra-thin films of SrRuO3 (SRO) grown on the SrTiO3 [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-t2g 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., Crystal Hall effect in collinear antiferromagnets (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 anomalous Hall effect 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.

Crystal (programming language)

10.1063/5.0005017

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Doped HfO_x nanoclusters: polar and resistive switching in the smallest functional units

physica status solidi (a) (2023)

K. Z. Rushchanskii Marjana Ležaić Stefan Blügel

In the study presented in this article, the impact of proton doping on the structural and electronic properties of hafnium oxide nanoclusters is investigated, with a focus on their potential for use in resistive and polar switching devices. In the results, it is shown that the incorporation of protons can stabilize the cage‐like crystalline structures of clusters, leading to reversible changes in electronic properties by varying oxygen stoichiometry. However, the full coverage of hafnia atoms by hydrogen removes in‐gap states, highlighting the importance of controlled moisture content in redox‐based memristive devices and neuromorphic units. In addition, in this study, the polar properties of these clusters are explored, illustrating possible polar switching in metastable pure , low‐barrier antiferroelectric‐like switching in carbon‐stabilized , and low‐barrier polar switching in . In these findings, the potential of clusters is revealed as active components for next‐generation high‐capacity nonvolatile electronic memory and beyond von Neumann computing in sub‐nanometer scale.

Nanoclusters

Metastability

Non-Volatile Memory

10.1002/pssa.202300404

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Engineering quantum anomalous Hall phases with orbital and spin degrees of freedom

Physical Review B (2013)

Hongbin Zhang Frank Freimuth Gustav Bihlmayer Marjana Ležaić Stefan Blügel

Combining tight-binding models and first principles calculations, we investigate the quantum anomalous Hall (QAH) effect induced by intrinsic spin-orbit coupling (SOC) in buckled honeycomb lattice with sp orbitals in an external exchange field. Detailed analysis reveals that nontrivial topological properties can arise utilizing not only spin but also orbital degrees of freedom in the strong SOC limit, when the bands acquire non-zero Chern numbers upon undergoing the so-called orbital purification. As a prototype of a buckled honeycomb lattice with strong SOC we choose the Bi(111) bilayer, analyzing its topological properties in detail. In particular, we show the emergence of several QAH phases upon spin exchange of the Chern numbers as a function of SOC strength and magnitude of the exchange field. Interestingly, we observe that in one of such phases, namely, in the quantum spin Chern insulator phase, the quantized charge and spin Hall conductivities co-exist. We consider the possibility of tuning the SOC strength in Bi bilayer via alloying with isoelectronic Sb, and speculate that exotic properties could be expected in such an alloyed system owing to the competition of the topological properties of its constituents. Finally, we demonstrate that 3d dopants can be used to induce a sizeable exchange field in Bi(111) bilayer, resulting in non-trivial Chern insulator properties.

Lattice (music)

10.1103/physrevb.87.205132

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Exchange interactions and local-moment fluctuation corrections in ferromagnets at finite temperatures based on noncollinear density-functional calculations

Physical Review B (2013)

Marjana Ležaić Phivos Mavropoulos Gustav Bihlmayer Stefan Blügel

We explore the derivation of interatomic exchange interactions in ferromagnets within density-functional theory (DFT) and the mapping of DFT results onto a spin Hamiltonian. We delve into the problem of systems comprising atoms with strong spontaneous moments together with atoms with weak induced moments. All moments are considered as degrees of freedom, with the strong moments thermally fluctuating only in angle and the weak moments thermally fluctuating in angle and magnitude. We argue that a quadratic dependence of the energy on the weak local moments magnitude, which is a good approximation in many cases, allows for an elimination of the weak-moment degrees of freedom from the thermodynamic expressions in favor of a renormalization of the Heisenberg interactions among the strong moments. We show that the renormalization is valid at all temperatures accounting for the thermal fluctuations and resulting in temperature-independent renormalized interactions. These are shown to be the ones directly derived from total-energy DFT calculations by constraining the strong-moment directions, as is done e.g. in spin-spiral methods. We furthermore prove that within this framework the thermodynamics of the weak-moment subsystem, and in particular all correlation functions, can be derived as polynomials of the correlation functions of the strong-moment subsystem with coefficients that depend on the spin susceptibility and that can be calculated within DFT. These conclusions are rigorous under certain physical assumptions on the measure in the magnetic phase space. We implement the scheme in the full-potential linearized augmented plane wave method using the concept of spin-spiral states, considering applicable symmetry relations and the use of the magnetic force theorem. Our analytical results are corroborated by numerical calculations employing DFT and a Monte Carlo method.

Hamiltonian (control theory)

Exchange interaction

10.1103/physrevb.88.134403

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Citations (49)

Probing Quasiparticle States Bound by Disparate Periodic Potentials

Physical Review Letters (2006)

Paolo Moras L. Ferrari Carlo Spezzani Sandra Gardonio Marjana Ležaić

Thin films of Ag(111) with two-dimensional crystallinity of large lateral coherence grow on Ge(111), free of in-plane registry with the underlying substrate. Ag s-p electrons forming two-dimensional quantum well states scatter coherently at the buried interface potential, resulting in an unexpected set of new quasiparticle states, as observed by angle-resolved photoemission. These new features originate from interactions among Ag quantum well bands, gaining a momentum equivalent to a reciprocal vector of the substrate lattice.

10.1103/physrevlett.97.206802

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Spin and orbital magnetism by light in rutile altermagnets

npj Spintronics (2024)

T. Adamantopoulos M. Merte Frank Freimuth Dongwook Go Lishu Zhang

Abstract While the understanding of altermagnetism is still at a very early stage, it is expected to play a role in various fields of condensed matter research, for example spintronics, caloritronics and superconductivity. In the field of optical magnetism, it is still unclear to which extent altermagnets as a class can exhibit a distinct behavior. Here we choose RuO 2 , a prototype metallic altermagnet with a giant spin splitting, and CoF 2 , an experimentally known insulating altermagnet, to study the light-induced magnetism in rutile altermagnets from first-principles. We demonstrate that in the non-relativisic limit the allowed sublattice-resolved orbital response exhibits symmetries, imposed by altermagnetism, which lead to a drastic canting of light-induced moments. On the other hand, we find that inclusion of spin-orbit interaction enhances the overall effect drastically, introduces a significant anisotropy with respect to the light polarization and strongly suppresses the canting of induced moments. Remarkably, we observe that the moments induced by linearly-polarized laser pulses in light altermagnets can even exceed in magnitude those predicted for heavy ferromagnets exposed to circularly polarized light. By resorting to microscopic tools we interpret our results in terms of the altermagnetic spin splittings and of their reciprocal space distribution. Based on our findings, we speculate that optical excitations may provide a unique tool to switch and probe the magnetic state of rutile altermagnets.

Magnetism

Rutile

10.1038/s44306-024-00053-0

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EuO Ready for Spintronics---Controlling Epitaxial EuO/Si Spin Interfaces with Passivation Techniques

arXiv (Cornell University) (2015)

Christian Caspers A. Gloskovskii Mihaela Gorgoi Claire Besson M. Luysberg

Epitaxial monolayers of magnetic Europium Oxide (EuO) directly on silicon wafers are perfect candidates for adding spin filter tunneling to silicon technology. However, the inherent chemical reactivity of EuO and Si prevents a straightforward synthesis by reactive molecular beam evaporation (Oxide MBE) without significant contamination of the interface region by Eu silicide and Si oxide formation. We present a solution to this long-standing problem by applying two complementary passivation techniques for the reactive Si (001) surface: \emph{in situ} hydrogen passivation of Si (001) and an oxygen-protective Eu monolayer, without using additional buffer oxides. The interface-engineered EuO/Si spin contacts are chemically analyzed by hard x-ray photoemission spectroscopy, proving that both Eu silicides and Si oxides can be suppressed to the sub-monolayer regime at the same time. A clear EuO \emph{fcc} crystal structure of the 2 nm-EuO/Si (001) spin contacts is evidenced by lateral and cross-sectional electron diffraction techniques. These complementary interface passivations permit highly chemically clean, epitaxial and ferromagnetic EuO/Si (001) spin contacts, rendering possible a one-process \emph{in situ} integration of spin-functional magnetic oxides seamless on silicon wafers.

Passivation

Europium

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Charge Transfer: Identifying Ionic and Electronic Charge Transfer at Oxide Heterointerfaces (Adv. Mater. 4/2021)

Advanced Materials (2021)

Marc‐André Rose Břetislav Šmíd Mykhailo Vorokhta Ivetta Slipukhina Michael Andrä

In article number 2004132, Marc-André Rose, David N. Mueller, Felix Gunkel, and co-workers identify how ion and electron transfer naturally balance at the LaAlO3/SrTiO3 oxide heterointerface, affecting the band alignment and magnetic signature of the interface. They utilize near-ambient-pressure X-ray photoelectron spectroscopy, showing that Sr ions are more mobile at the interface than in the bulk, and implicating a high importance of ionic charge transfer in oxide heterostructures.

10.1002/adma.202170022

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