arXiv subjects

Jiahao Han

Publications and source records attributed to Jiahao Han.

12 recordsLinked to original sources

arXiv2026

Pronounced in-plane anomalous Hall effect with vanishing out-of-plane response in Cr1.2Te2

We report an unconventional anomalous Hall regime in the van der Waals ferromagnet Cr1.2Te2, in which the anomalous Hall effect (AHE) is present for in-plane magnetization but absent for out-of-plane magnetization. In this purely in-plane regime, the anomalous Hall signal exhibits a threefold angular dependence during both in-plane and out-of-plane rotations of the magnetization, which cannot be accounted for by the conventional dipolar contribution but instead requires an octupolar contribution. Although the octupolar term qualitatively captures the observed behavior, the experimentally extracted octupole differs quantitatively from first-principles calculations based solely on the intrinsic Berry-curvature mechanism, indicating an essential role for extrinsic scattering processes.

cond-mat.mtrl-sci↗

arXiv2026

Close correlation between giant magnetostriction and the microstructure in Fe-Ga melt-spun ribbons

Magnetoelastic anisotropy and <100> texture are crucial for promoting magnetostriction in Galfenol (Fe-Ga). Given that single-crystal Fe-Ga remains technically demanding and low magnetostriction of polycrystal, we explore melt-spun as an alternative, where rare-earth (RE) doping and cooling rates optimizing enable controllable <100> textured with required magnetoelastic anisotropy. Fe81Ga19 binary ribbons and RE-doped ribbons (0.2 at.% Pr, 1 at.% Pr and 1 at.% Ce) were fabricated at various cooling rates. Microstructural analyses reveal that RE elements preferentially dissolve into the matrix, while second-phase formation is suppressed at higher cooling rates. RE substitution increases the magnetostriction by enhancing magnetocrystalline distortion energy, while cooling rates act as an effective tuning knob to maximize the <100> texture. Notably, the decreased melting temperature associated with 1 at.% RE doping shifts the optimum texture to lower cooling rate compared with the binary alloy and 0.2 at.% RE doping sample. The magnetostriction as high as 688 ppm is achieved for the 1 at.% Ce doped ribbon fabricated at the speed of 1000 rotation per minute. These results demonstrate that RE-doped melt-spun ribbons are promising candidates for giant magnetostriction and establish a practical processing-texture-property guideline for designing highly magnetostrictive alloys.

cond-mat.mtrl-sci↗

arXiv2026

Electric field switching of altermagnetic spin-splitting in multiferroic skyrmions

Magnetic skyrmions are localized magnetic structures that retain their shape and stability over time, thanks to their topological nature. Recent theoretical and experimental progress has laid the groundwork for understanding magnetic skyrmions characterized by negligible net magnetization and ultrafast dynamics. Notably, skyrmions emerging in materials with altermagnetism, a novel magnetic phase featuring lifted Kramers degeneracy-have remained unreported until now. In this study, we demonstrate that BiFeO3, a multiferroic renowned for its strong coupling between ferroelectricity and magnetism, can transit from a spin cycloid to a Neel-type skyrmion under antidamping spin-orbit torque at room temperature. Strikingly, the altermagnetic spin splitting within BiFeO3 skyrmion can be reversed through the application of an electric field, revealed via the Circular photogalvanic effect. This quasiparticle, which possesses a neutral topological charge, holds substantial promise for diverse applications-most notably, enabling the development of unconventional computing systems with low power consumption and magnetoelectric controllability.

cond-mat.mtrl-sci↗

arXiv2025

Experimental Evidence of N\'eel-order-driven Magneto-optical Kerr Effect in an Altermagnetic Insulator

The magneto-optical Kerr effect (MOKE) is investigated in hematite, a collinear antiferromagnetic insulator, across a broad wavelength spectrum. By combining the optical measurements with magnetometry results, we unambiguously demonstrate that the N\'eel-order contribution dominates the MOKE signal, while contributions from net magnetization and external magnetic fields are negligible. This conclusion is quantitatively supported by first-principles calculations, and qualitatively by a symmetry analysis that the N\'eel contribution appears at the first order in spin-orbit coupling while the magnetization contribution starts only at the third order. This study clarifies the altermagnetic origin of the pronounced MOKE in hematite, underscoring the potential of altermagnets as a promising new class of magneto-optical materials.

cond-mat.other↗

arXiv2025

Observation of ferron transport in ferroelectrics

Ferroelectrics feature spontaneous electric dipolar order reconfigurable via electric fields. Recent theoretical studies of the collective excitations of this electric dipolar order give rise to the hope that "ferron" quasiparticles may complement the magnons of magnetic materials in information and heat management technologies. Yet direct experimental evidence of ferron transport remains elusive. Here we demonstrate efficient ferron injection and detection enabled by ferromagnetic metal contacts, achieving nonlocal signal transmission over micrometer distances in a prototypical ferroelectric PMN-PT. The transmission efficiency can be switched by external magnetic fields that couple to the contacts and gate electric fields that control the ferron excitations. Ferron-based devices open new power saving strategies that employ ferroelectric materials in a future sustainable information society.

physics.app-ph↗

arXiv2025

Navigating Chiral Spin Architectures in Non-Collinear Antiferromagnetic Thin Films

Noncollinear antiferromagnets offer much promise for antiferromagnetic spintronics and neuromorphic applications with a plethora of functional properties surpassing many competing magnetic systems. Films grown on mismatched substrates may relieve strain by the creation of slip-plane defects that strongly manipulate global physical properties important for application. This work demonstrates that a post growth annealing strategy results in near-defect-free, structurally robust films that reveal the underlying thermal evolution of the magnetic order symmetry. Beyond a critical film thickness, the spin structure transitions between two right-handed irreducible chiral representations via left-handed chiral ordering producing a striking change in the sign and angular dependence of the Anomalous Hall coefficient. The previously established mechanism of spin rotations in the (111) plane cannot fully explain the transition in applied magnetic field, and using a macrospin model this work finds that rotations along the chirality-inverting [1-10] direction are energetically preferable under certain conditions. These observations suggest that both left-handed and right-handed chiral order can be accessed in a single system, opening new routes to engineer devices by controlled switching of magnetic chirality allowing selection of associated functional properties as governed by symmetry.

cond-mat.mtrl-sci↗

arXiv2025

Unveiling the Landscape of LLM Deployment in the Wild: An Empirical Study

Large language models (LLMs) are increasingly deployed through open-source and commercial frameworks, enabling individuals and organizations to self-host advanced LLM capabilities. As LLM deployments become prevalent, particularly in industry, ensuring their secure and reliable operation has become a critical issue. However, insecure defaults and misconfigurations often expose LLM services to the public internet, posing serious security and system engineering risks. This study conducted a large-scale empirical investigation of public-facing LLM deployments, focusing on the prevalence of services, exposure characteristics, systemic vulnerabilities, and associated risks. Through internet-wide measurements, we identified 320,102 public-facing LLM services across 15 frameworks and extracted 158 unique API endpoints, categorized into 12 functional groups based on functionality and security risk. Our analysis found that over 40% of endpoints used plain HTTP, and over 210,000 endpoints lacked valid TLS metadata. API exposure was highly inconsistent: some frameworks, such as Ollama, responded to over 35% of unauthenticated API requests, with about 15% leaking model or system information, while other frameworks implemented stricter controls. We observed widespread use of insecure protocols, poor TLS configurations, and unauthenticated access to critical operations. These security risks, such as model leakage, system compromise, and unauthorized access, are pervasive and highlight the need for a secure-by-default framework and stronger deployment practices.

cs.CR↗

arXiv2023

Bright nonblinking photoluminescence with blinking lifetime from a nanocavity-coupled quantum dot

Colloidal semiconductor quantum dots (QDs) are excellent luminescent nanomaterials for a broad range of optoelectronic applications. Their photoluminescence blinking, however, hinders their practical use in many aspects. It has been shown that coupling QDs to plasmonic nanostructures may provide a viable way to suppress blinking. Nevertheless, the underlying mechanism of blinking suppression remains unclear and debated. Here, by deterministically coupling a single QD to a plasmonic nanocavity, we clarify the mechanism of blinking suppression, and demonstrate unprecedentedly bright emission from a single colloidal QD. In particular, we report for the first time that the coupled system exhibits nonblinking photoluminescence with blinking lifetime, which shows that the elimination of photoluminescence blinking originates from enhanced quantum yield of the charged states. We identify that the radiative decay rate is boosted from (48 ns)-1 to (0.7 ns)-1, which outcompetes Auger processes and enables similar quantum yields for charged and neutral excitons. Moreover, we demonstrate ultrabright photoluminescence of up to 17 million detected photons per second from a single QD. This work sheds new light on the goal of achieving ultrabright nonblinking QDs and may benefit a variety of QD-based applications.

physics.optics↗

arXiv2022

Observation of the unidirectional magnetoresistance in antiferromagnetic insulator Fe2O3/Pt bilayers

Unidirectional magnetoresistance (UMR) has been observed in a variety of stacks with ferromagnetic/spin Hall material bilayer structures. In this work, we reported UMR in antiferromagnetic insulator Fe2O3/Pt structure. The UMR has a negative value, which is related to interfacial Rashba coupling and band splitting. Thickness-dependent measurement reveals a potential competition between UMR and the unidirectional spin Hall magnetoresistance (USMR). This work revealed the existence of UMR in antiferromagnetic insulators/heavy metal bilayers and broadens the way for the application of antiferromagnet-based spintronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Manipulation of coupling and magnon transport in magnetic metal-insulator hybrid structures

Ferromagnetic metals and insulators are widely used for generation, control and detection of magnon spin signals. Most magnonic structures are based primarily on either magnetic insulators or ferromagnetic metals, while heterostructures integrating both of them are less explored. Here, by introducing a Pt/yttrium iron garnet (YIG)/permalloy (Py) hybrid structure grown on Si substrate, we studied the magnetic coupling and magnon transmission across the interface of the two magnetic layers. We found that within this structure, Py and YIG exhibit an antiferromagnetic coupling field as strong as 150 mT, as evidenced by both the vibrating-sample magnetometry and polarized neutron reflectometry measurements. By controlling individual layer thicknesses and external fields, we realize parallel and antiparallel magnetization configurations, which are further utilized to control the magnon current transmission. We show that a magnon spin valve with an ON/OFF ratio of ~130% can be realized out of this multilayer structure at room temperature through both spin pumping and spin Seebeck effect experiments. Thanks to the efficient control of magnon current and the compatibility with Si technology, the Pt/YIG/Py hybrid structure could potentially find applications in magnon-based logic and memory devices.

cond-mat.mtrl-sci↗

arXiv2018

Current-induced domain wall motion in compensated ferrimagnet

Due to the difficulty in detecting and manipulating magnetic states of antiferromagnetic materials, studying their switching dynamics using electrical methods remains a challenging task. In this work, by employing heavy metal/rare earth-transition metal alloy bilayers, we experimentally studied current-induced domain wall dynamics in an antiferromagnetically coupled system. We show that the current-induced domain wall mobility reaches a maximum close to the angular momentum compensation. With experiment and modelling, we further reveal the internal structures of domain walls and the underlying mechanisms for their fast motion. We show that the chirality of the ferrimagnetic domain walls remains the same across the compensation points, suggesting that spin orientations of specific sublattices rather than net magnetization determine Dzyaloshinskii-Moriya interaction in heavy metal/ferrimagnet bilayers. The high current-induced domain wall mobility and the robust domain wall chirality in compensated ferrimagnetic material opens new opportunities for high-speed spintronic devices.

cond-mat.mtrl-sci↗

arXiv2017

Room temperature spin-orbit torque switching induced by a topological insulator

Recent studies on the magneto-transport properties of topological insulators (TI) have attracted great attention due to the rich spin-orbit physics and promising applications in spintronic devices. Particularly the strongly spin-moment coupled electronic states have been extensively pursued to realize efficient spin-orbit torque (SOT) switching. However, so far current-induced magnetic switching with TI has only been observed at cryogenic temperatures. It remains a controversial issue whether the topologically protected electronic states in TI could benefit spintronic applications at room temperature. In this work, we report full SOT switching in a TI/ferromagnet bilayer heterostructure with perpendicular magnetic anisotropy at room temperature. The low switching current density provides a definitive proof on the high SOT efficiency from TI. The effective spin Hall angle of TI is determined to be several times larger than commonly used heavy metals. Our results demonstrate the robustness of TI as an SOT switching material and provide a direct avenue towards applicable TI-based spintronic devices.

cond-mat.mtrl-sci↗