arXiv subjects

Angela Wittmann

Publications and source records attributed to Angela Wittmann.

8 recordsLinked to original sources

arXiv2026

Chirality-induced spin selectivity without intrinsic spin-orbit coupling: Role of current-induced molecular orbital moment

The microscopic origin of the chirality-induced spin selectivity (CISS) in helical molecules remains an open question. Recent experiments suggest that a significant contribution to CISS arises from the molecule itself, which is disregarded in existing interfacial or scattering based theories. Here we present an alternative theory of CISS to address this molecular contribution. The mechanism is based on the circulation of charge current in molecular loops that generates a molecular orbital moment (MOM). The direction of the MOM is governed by the gauge field arising from the structural distortion of the molecule and is associated with the handedness of the molecule. Such a MOM can produce finite CISS magnetoresistance and magnetochiral conductance asymmetries that are even in bias voltage, without violating the Onsager-Casimir reciprocity relations. Depending on the Fermi level and bias voltage, the MOM can be controlled externally, which can result in additional crossings of the enantiomer $I-V$ curves. Finally we explain the origin of the electrical magnetochiral anisotropy within the same framework, which establishes its generic applicability.

cond-mat.mes-hall↗

arXiv2026

Magnetic Microscopy of Skyrmions in Magnetic Thin Films with Chiral Overlayers

Topologically nontrivial magnetic textures such as skyrmions offer promising opportunities for spintronic applications. In recent years, it has been shown that the magnetic properties of layered materials can be affected by depositing chiral molecules on the surface, while the influence of chiral overlayers on skyrmion properties such as their stability and interactions remains largely unexplored. To address this challenge, we employ wide-field nitrogen-vacancy (NV) magnetometry to directly image skyrmions in chiral-molecule-functionalized magnetic thin films, enabling quantitative mapping of magnetic stray fields over extended areas under ambient conditions. Using pixel-resolved optically detected magnetic resonance (ODMR) combined with controlled magnetic fields, we reproducibly nucleate and probe skyrmion states in CoFeB ferromagnetic samples, enabling quantitative investigation of their properties. We find evidence for enantioselective and magnetic-field-polarity-dependent modifications of skyrmion diameter, spacing, and shape, pointing to a possibility of molecular control of topological spin textures via magneto-chiral coupling.

cond-mat.mtrl-sci↗

arXiv2026

Topological textures and emergent altermagnetic signatures in ultrathin BiFeO3

Magnetoelectric multiferroics, materials with intrinsically coupled electric polarization and magnetic order, promise ultralow-power switching, nonvolatile memory, and energy-efficient signal transduction. Yet practical deployment demands ultrathin films down to the atomic limit, where both orders typically degrade. Maintaining both order parameters at the thinnest scales in complex oxides remains a tremendous challenge, as uncompensated bound charge drives nanoscale depolarization in most ferroelectrics, while off-stoichiometry, reduced anisotropy, and charge transfer can produce magnetic dead layers in ultrathin oxides at substrate interfaces. Here, we realize a multiferroic phase of BiFeO3 that not only sustains both order parameters at room temperature with no dead layer but also exhibits signatures of emergent altermagnetism in the four-unit-cell, ultrathin limit. First-principles calculations, spin symmetry analysis, atomic-resolution imaging, and angle-resolved magnetic imaging reveal that short-circuit electrostatic boundary conditions, together with epitaxial strain, drive a continuous second-order, thickness-driven phase transition that enables the formation of multiferroic topological textures. Moreover, the imposed boundary conditions stabilize a d-wave altermagnetic time-reversal symmetry breaking, with corresponding signatures observed in magnetic circular dichroism. Collectively, these results establish a pathway to stabilize unconventional multiferroicity at device-relevant thicknesses, reframing scaling limits for oxide electronics.

cond-mat.mtrl-sci↗

arXiv2025

Quantum Imaging of Ferromagnetic van der Waals Magnetic Domain Structures at Ambient Conditions

Recently discovered 2D van der Waals magnetic materials, and specifically Iron-Germanium-Telluride ($\rm Fe_{5}GeTe_{2}$), have attracted significant attention both from a fundamental perspective and for potential applications. Key open questions concern their domain structure and magnetic phase transition temperature as a function of sample thickness and external field, as well as implications for integration into devices such as magnetic memories and logic. Here we address key questions using a nitrogen-vacancy center based quantum magnetic microscope, enabling direct imaging of the magnetization of $\rm Fe_{5}GeTe_{2}$ at sub-micron spatial resolution as a function of temperature, magnetic field, and thickness. We employ spatially resolved measures, including magnetization variance and cross-correlation, and find a significant spread in transition temperature yet with no clear dependence on thickness down to 15 nm. We also identify previously unknown stripe features in the optical as well as magnetic images, which we attribute to modulations of the constituting elements during crystal synthesis and subsequent oxidation. Our results suggest that the magnetic anisotropy in this material does not play a crucial role in their magnetic properties, leading to a magnetic phase transition of $\rm Fe_{5}GeTe_{2}$ which is largely thickness-independent down to 15 nm. Our findings could be significant in designing future spintronic devices, magnetic memories and logic with 2D van der Waals magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

Chiral-induced unidirectional spin-to-charge conversion

The observation of spin-dependent transmission of electrons through chiral molecules has led to the discovery of chiral-induced spin selectivity (CISS). The remarkably high efficiency of the spin polarizing effect has recently gained significant interest due to the high potential for novel sustainable hybrid chiral molecule magnetic applications. However, the fundamental mechanisms underlying the chiral-induced phenomena remain to be understood fully. In this work, we explore the impact of chirality on spin angular momentum in hybrid metal/ chiral molecule thin film heterostructures. For this, we inject a pure spin current via spin pumping and investigate the spin-to-charge conversion at the hybrid chiral interface. Notably, we observe a chiral-induced unidirectionality in the conversion. Furthermore, angle-dependent measurements reveal that the spin selectivity is maximum when the spin angular momentum is aligned with the molecular chiral axis. Our findings validate the central role of spin angular momentum for the CISS effect, paving the path toward three-dimensional functionalization of hybrid molecule-metal devices via chirality.

cond-mat.mes-hall↗

arXiv2022

Role of substrate clamping on anisotropy and domain structure in the canted antiferromagnet $\alpha$-Fe$_2$O$_3$

Antiferromagnets have recently been propelled to the forefront of spintronics by their high potential for revolutionizing memory technologies. For this, understanding the formation and driving mechanisms of the domain structure is paramount. In this work, we investigate the domain structure in a thin-film canted antiferromagnet $\alpha$-Fe$_2$O$_3$. We find that the internal destressing fields driving the formation of domains do not follow the crystal symmetry of $\alpha$-Fe$_2$O$_3$, but fluctuate due to substrate clamping. This leads to an overall isotropic distribution of the N\'eel order with locally varying effective anisotropy in antiferromagnetic thin films. Furthermore, we show that the weak ferromagnetic nature of $\alpha$-Fe$_2$O$_3$ leads to a qualitatively different dependence on magnetic field compared to collinear antiferromagnets such as NiO. The insights gained from our work serve as a foundation for further studies of electrical and optical manipulation of the domain structure of antiferromagnetic thin films.

cond-mat.mes-hall↗

arXiv2020

Tuning Spin Current Injection at Ferromagnet/Non-Magnet Interfaces by Molecular Design

There is a growing interest in utilizing the distinctive material properties of organic semiconductors for spintronic applications. Here, we explore injection of pure spin current from Permalloy into a small molecule system based on dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene (DNTT) at ferromagnetic resonance. The unique tunability of organic materials by molecular design allows us to study the impact of interfacial properties on the spin injection efficiency systematically. We show that both, spin injection efficiency at the interface as well as the spin diffusion length can be tuned sensitively by the interfacial molecular structure and side chain substitution of the molecule.

physics.app-ph↗

arXiv2014

Characterization of spin-orbit interactions of GaAs heavy holes using a quantum point contact

We present transport experiments performed in high quality quantum point contacts embedded in a GaAs two-dimensional hole gas. The strong spin-orbit interaction results in peculiar transport phenomena, including the previously observed anisotropic Zeeman splitting and level-dependent effective g-factors. Here we find additional effects, namely the crossing and the anti-crossing of spin-split levels depending on subband index and magnetic field direction. Our experimental observations are reconciled in an heavy hole effective spin-orbit Hamiltonian where cubic- and quadratic-in-momentum terms appear. The spin-orbit components, being of great importance for quantum computing applications, are characterized in terms of magnitude and spin structure. In the light of our results, we explain the level dependent effective g-factor in an in-plane field. Through a tilted magnetic field analysis, we show that the QPC out-of-plane g-factor saturates around the predicted 7.2 bulk value.

cond-mat.mes-hall↗