arXiv subjects

Delin Zhang

Publications and source records attributed to Delin Zhang.

At least 19 recordsLinked to original sources

arXiv2025

Modeling Finite Deformations in Alloying Electrodes -- A Closer Look at Cracks and Pores During Phase Transformation

Nanostructured electrodes with voids or interconnected pores accommodate large volume changes, shorten ion diffusion pathways, and enhance the structural reversibility of alloying electrodes. While these nanoporous features improve the performance of architected electrodes over bulk electrodes, they also act as geometric irregularities that localize and concentrate internal stresses. In this work, we investigate the hierarchical interplay between phase boundaries and nanoporous features at the microstructural scale and their collective role in mitigating chemo-mechanical failure at the engineering scale. Using Sb$\to$Li$_2$Sb$\to$Li$_3$Sb as a model system, we develop a continuum framework coupling lithium diffusion and reaction kinetics with the finite deformation of alloying electrodes. We analytically show that large volume changes in the Sb$\rightarrow$Li$_2$Sb transformation induce fracture, which nanoporous geometries can mitigate. Building on this, we develop a micromechanical model using a hyper-elastic neo-Hookean material law to predict the deformations accompanying the Li$_2$Sb$\to$Li$_3$Sb transformation. Our results reveal how diffusion and reaction kinetics shape phase boundary morphology, identify crack geometries likely to propagate, and show how carefully architected electrodes relieve stresses. These findings highlight critical design principles to optimize electrode lifespan and demonstrate a potential application of our continuum model as an electrode design tool.

cond-mat.mtrl-sci↗

arXiv2025

Lattice Compatibility and Energy Barriers in Intercalation Compounds

We present a continuum model for symmetry-breaking phase transformations in intercalation compounds, based on Ericksen's multi-well energy formulation. The model predicts the nucleation and growth of crystallographic microstructures in Li$_{2}$Mn$_{2}$O$_{4}$ -- a representative intercalation compound -- with twin boundary orientations and volume fractions that closely match experimental observations. Our chemo-mechanically coupled model not only generates geometrically accurate microstructures through energy minimization, but also reveals a subtle interplay between twinned domains and electro-chemo-mechanical behavior. A key finding is that intercalation compounds satisfying specific compatibility conditions (e.g., $\lambda_{2} = 1$ or $|\det \mathbf{U} - 1| = 0)$ show lower elastic energy barriers, require smaller driving forces, and display narrower voltage hysteresis loops. Furthermore, we show that twinned domains act as conduits for fast Li-diffusion. These results establish quantitative design guidelines for intercalation compounds, which focuses on tailoring lattice deformations (rather than suppressing them) and reducing energy barriers to mitigate structural degradation and enhance the electrochemical performance of battery electrodes.

cond-mat.mtrl-sci↗

arXiv2025

Demonstration of Electron-Mediated Voltage-Controlled Exchange Coupling in Perpendicular Magnetic Tunnel Junctions

Electron-mediated voltage control of exchange coupling (EM-VCEC) has been proposed as a mechanism for magnetization switching via modulation of spin-dependent electron reflection. However, its experimental verification has been challenging due to the coexistence of slower, voltage-induced ionic effects. Here, we fabricate magnetic tunnel junction (MTJ) devices that enable nanosecond timescale voltage application. Our results reveal rapid exchange coupling modulation on the nanosecond timescale, consistent with an electronic origin. The observed enhancement and saturation at low temperatures further rule out ionic migration, conclusively confirming the electronic nature of the mechanism. These results establish EM-VCEC as a viable mechanism for fast and energy-efficient voltage-driven magnetic switching.

physics.app-ph↗

arXiv2024

Energy Efficient Stochastic Signal Manipulation in Superparamagnetic Tunnel Junctions via Voltage-Controlled Exchange Coupling

Superparamagnetic tunnel junctions (sMTJs) are emerging as promising components for stochastic units in neuromorphic computing, owing to their tunable random switching behavior. Conventional MTJ control methods, such as spin-transfer torque (STT) and spin-orbit torque (SOT), often require substantial power. Here, we introduce the voltage-controlled exchange coupling (VCEC) mechanism, enabling switching between antiparallel and parallel states in sMTJs with an ultralow power consumption of only 40 nW, approximately two orders of magnitude lower than conventional STT-based sMTJs. This mechanism yields a sigmoid-shaped output response, making it ideally suited for neuromorphic computing applications. Furthermore, we validate the feasibility of integrating VCEC with the SOT current control, offering an additional dimension for magnetic state manipulation. This work marks the first practical demonstration of VCEC effect in sMTJs, highlighting its potential as a low-power control solution for probabilistic bits in advanced computing systems.

physics.app-ph↗

arXiv2024

Orbital torque switching of room temperature two-dimensional van der Waals ferromagnet Fe3GaTe2

Efficiently manipulating the magnetization of van der Waals ferromagnets has attracted considerable interest in developing room-temperature two-dimensional material-based memory and logic devices. Here, taking advantage of the unique properties of the van der Waals ferromagnet as well as promising characteristics of the orbital Hall effect, we demonstrate the room-temperature magnetization switching of van der Waals ferromagnet Fe3GaTe2 through the orbital torque generated by the orbital Hall material, Titanium (Ti). The switching current density is estimated to be around 1.6 x 10^6 A/cm^2, comparable to that achieved in Fe3GaTe2 using spin-orbit torque from spin Hall materials. The efficient magnetization switching arises from the combined effects of the large orbital Hall conductivity of Ti and the strong spin-orbit correlation of the Fe3GaTe2, as confirmed through theoretical calculations. Our findings advance the understanding of orbital torque switching and pave the way for exploring material-based orbitronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Orbital torque switching in perpendicularly magnetized materials

The orbital Hall effect in light materials has attracted considerable attention for developing novel orbitronic devices. Here we investigate the orbital torque efficiency and demonstrate the switching of the perpendicularly magnetized materials through the orbital Hall material (OHM), i.e., Zirconium (Zr). The orbital torque efficiency of approximately 0.78 is achieved in the Zr OHM with the perpendicularly magnetized [Co/Pt]3 sample, which significantly surpasses that of the perpendicularly magnetized CoFeB/Gd/CoFeB sample (approximately 0.04). Such notable difference is attributed to the different spin-orbit correlation strength between the [Co/Pt]3 sample and the CoFeB/Gd/CoFeB sample, which has been confirmed through the theoretical calculations. Furthermore, the full magnetization switching of the [Co/Pt]3 sample with a switching current density of approximately 2.6x106 A/cm2 has been realized through Zr, which even outperforms that of the W spin Hall material. Our finding provides a guideline to understand orbital torque efficiency and paves the way to develop energy-efficient orbitronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Coupling Diffusion and Finite Deformation in Phase Transformation Materials

We present a multiscale theoretical framework to investigate the interplay between diffusion and finite lattice deformation in phase transformation materials. In this framework, we use the Cauchy-Born Rule and the Principle of Virtual Power to derive a thermodynamically consistent theory coupling the diffusion of a guest species (Cahn-Hilliard type) with the finite deformation of host lattices (nonlinear gradient elasticity). We adapt this theory to intercalation materials--specifically Li$_{1-2}$Mn$_2$O$_4$--to investigate the delicate interplay between Li-diffusion and the cubic-to-tetragonal deformation of lattices. Our computations reveal fundamental insights into the microstructural evolution pathways under dynamic discharge conditions, and provide quantitative insights into the nucleation and growth of twinned microstructures during intercalation. Additionally, our results identify regions of stress concentrations (e.g., at phase boundaries, particle surfaces) that arise from lattice misfit and accumulate in the electrode with repeated cycling. These findings suggest a potential mechanism for structural decay in Li$_2$Mn$_2$O$_4$. More generally, we establish a theoretical framework that can be used to investigate microstructural evolution pathways, across multiple length scales, in first-order phase transformation materials.

cond-mat.mtrl-sci↗

arXiv2023

Magnetization Dynamics in Synthetic Antiferromagnets with Perpendicular Magnetic Anisotropy

Understanding the rich physics of magnetization dynamics in perpendicular synthetic antiferromagnets (p-SAFs) is crucial for developing next-generation spintronic devices. In this work, we systematically investigate the magnetization dynamics in p-SAFs combining time-resolved magneto-optical Kerr effect (TR-MOKE) measurements with theoretical modeling. These model analyses, based on a Landau-Lifshitz-Gilbert approach incorporating exchange coupling, provide details about the magnetization dynamic characteristics including the amplitudes, directions, and phases of the precession of p-SAFs under varying magnetic fields. These model-predicted characteristics are in excellent quantitative agreement with TR-MOKE measurements on an asymmetric p-SAF. We further reveal the damping mechanisms of two procession modes co-existing in the p-SAF and successfully identify individual contributions from different sources, including Gilbert damping of each ferromagnetic layer, spin pumping, and inhomogeneous broadening. Such a comprehensive understanding of magnetization dynamics in p-SAFs, obtained by integrating high-fidelity TR-MOKE measurements and theoretical modeling, can guide the design of p-SAF-based architectures for spintronic applications.

cond-mat.mes-hall↗

arXiv2023

Observation and enhancement of room temperature bilinear magnetoelectric resistance in sputtered topological semimetal Pt3Sn

Topological semimetal materials have become a research hotspot due to their intrinsic strong spin-orbit coupling which leads to large charge-to-spin conversion efficiency and novel transport behaviors. In this work, we have observed a bilinear magnetoelectric resistance (BMER) of up to 0.1 nm2A-1Oe-1 in a singlelayer of sputtered semimetal Pt3Sn at room temperature. Different from previous observations, the value of BMER in sputtered Pt3Sn does not change out-of-plane due to the polycrystalline nature of Pt3Sn. The observation of BMER provides strong evidence of the existence of spin-momentum locking in the sputtered polycrystalline Pt3Sn. By adding an adjacent CoFeB magnetic layer, the BMER value of this bilayer system is doubled compared to the single Pt3Sn layer. This work broadens the material system in BMER study, which paves the way for the characterization of topological states and applications for spin memory and logic devices.

cond-mat.mtrl-sci↗

arXiv2023

Inverse orbital Hall effect and orbitronic terahertz emission observed in the materials with weak spin-orbit coupling

The Orbital Hall effect, which originates from materials with weak spin-orbit coupling, has attracted considerable interest for spin-orbitronic applications. Here, we demonstrate the inverse effect of the orbital Hall effect and observe orbitronic terahertz emission in the Ti and Mn materials. Through spin-orbit transition in the ferromagnetic layer, the generated orbital current can be converted to charge current in the Ti and Mn layers via the inverse orbital Hall effect. Furthermore, the inserted W layer provides an additional conversion of the orbital-charge current in the Ti and Mn layers, significantly enhancing the orbitronic terahertz emission. Moreover, the orbitronic terahertz emission can be manipulated by cooperating with the inverse orbital Hall effect and the inverse spin Hall effect in the different sample configurations. Our results not only discover the physical mechanism of condensed matter physics but also pave the way for designing promising spin-orbitronic devices and terahertz emitters.

cond-mat.mes-hall↗

arXiv2023

Robust negative longitudinal magnetoresistance and spin-orbit torque in sputtered Pt3Sn topological semimetal

Contrary to topological insulators, topological semimetals possess a nontrivial chiral anomaly that leads to negative magnetoresistance and are hosts to both conductive bulk states and topological surface states with intriguing transport properties for spintronics. Here, we fabricate highly-ordered metallic Pt3Sn and Pt3SnxFe1-x thin films via sputtering technology. Systematic angular dependence (both in-plane and out-of-plane) study of magnetoresistance presents surprisingly robust quadratic and linear negative longitudinal magnetoresistance features for Pt3Sn and Pt3SnxFe1-x, respectively. We attribute the anomalous negative longitudinal magnetoresistance to the type-II Dirac semimetal phase (pristine Pt3Sn) and/or the formation of tunable Weyl semimetal phases through symmetry breaking processes, such as magnetic-atom doping, as confirmed by first-principles calculations. Furthermore, Pt3Sn and Pt3SnxFe1-x show the promising performance for facilitating the development of advanced spin-orbit torque devices. These results extend our understanding of chiral anomaly of topological semimetals and can pave the way for exploring novel topological materials for spintronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Room temperature spin-orbit torque efficiency in sputtered low-temperature superconductor delta-TaN

In the course of searching for promising topological materials for applications in future topological electronics, we evaluated spin-orbit torques (SOTs) in high-quality sputtered $δ-$TaN/Co20Fe60B20 devices through spin-torque ferromagnetic resonance ST-FMR and spin pumping measurements. From the ST-FMR characterization we observed a significant linewidth modulation in the magnetic Co20Fe60B20 layer attributed to the charge-to-spin conversion generated from the $δ-$TaN layer. Remarkably, the spin-torque efficiency determined from ST-FMR and spin pumping measurements is as large as $Θ =$ 0.034 and 0.031, respectively. These values are over two times larger than for $α-$Ta, but almost five times lower than for $β-$Ta, which can be attributed to the low room temperature electrical resistivity $\sim 74μΩ$ cm in $δ-$TaN. A large spin diffusion length of at least $\sim8$ nm is estimated, which is comparable to the spin diffusion length in pure Ta. Comprehensive experimental analysis, together with density functional theory calculations, indicates that the origin of the pronounced SOT effect in $δ-$TaN can be mostly related to a significant contribution from the Berry curvature associated with the presence of a topically nontrivial electronic band structure in the vicinity of the Fermi level (EF). Through additional detailed theoretical analysis, we also found that an isostructural allotrope of the superconducting $δ-$TaN phase, the simple hexagonal structure, $θ-$TaN, has larger Berry curvature, and that, together with expected reasonable charge conductivity, it can also be a promising candidate for exploring a generation of spin-orbit torque magnetic random access memory as cheap, temperature stable, and highly efficient spin current sources.

cond-mat.mes-hall↗

arXiv2022

Designing shape-memory-like microstructures in intercalation materials

During the reversible insertion of ions, lattices in intercalation materials undergo structural transformations. These lattice transformations generate misfit strains and volume changes that, in turn, contribute to the structural decay of intercalation materials and limit their reversible cycling. In this paper, we draw on insights from shape-memory alloys, another class of phase transformation materials, that also undergo large lattice transformations but do so with negligible macroscopic volume changes and internal stresses. We develop a theoretical framework to predict structural transformations in intercalation compounds and establish crystallographic design rules necessary for forming shape-memory-like microstructures in intercalation materials. We use our framework to systematically screen open-source structural databases comprising n > 5000 pairs of intercalation compounds. We identify candidate compounds, such as Li$_x$Mn$_2$O$_4$ (Spinel), Li$_x$Ti$_2$(PO$_4$)$_3$ (NASICON), that approximately satisfy the crystallographic design rules and can be precisely doped to form shape-memory-like microstructures. Throughout, we compare our analytical results with experimental measurements of intercalation compounds. We find a direct correlation between structural transformations, microstructures, and increased capacity retention in these materials. These results, more generally, show that crystallographic designing of intercalation materials could be a novel route to discovering compounds that do not decay with continuous usage.

cond-mat.mtrl-sci↗

arXiv2022

Element Doping Enhanced Charge-to-Spin Conversion Efficiency in Amorphous PtSn4 Dirac Semimetal

Topological semimetals (TSs) are promising candidates for low-power spin-orbit torque (SOT) devices due to their large charge-to-spin conversion efficiency. Here, we investigated the charge-to-spin conversion efficiency of amorphous PtSn4 (5 nm)/CoFeB (2.5-12.5 nm) layered structures prepared by a magnetron sputtering method at room temperature. The charge-to-spin ratio of PtSn4/CoFeB bilayers was 0.08, characterized by a spin torque ferromagnetic resonance (ST-FMR) technique. This ratio can further increase to 0.14 by inducing dopants, like Al and CoSi, into PtSn4. The dopants can also decrease (Al doping) or increase (CoSi doping) the resistivity of PtSn4. The work proposed a way to enhance the spin-orbit coupling (SOC) in amorphous TSs with dopants.

cond-mat.mtrl-sci↗

arXiv2021

Determination of spin-orbit torques by thickness-dependent spin-orbit torque FMR measurement

Current induced spin-orbit torques (SOTs) in Fe/Pt bilayers have been investigated utilizing the spin-orbit torque ferromagnetic resonance (SOT-FMR) measurement. Characterization of thin films with different thicknesses indicates existence of a sizable field-like spin-orbit torque competing with the Oersted field induced torque (Oersted torque). The field-like torque is neglected in the standard SOT-FMR method and the presence of a strong field-like torque makes estimation of the spin Hall angle (SHA) problematic. Also, it is challenging to differentiate the field-like torque from the Oersted torque in a radiofrequency measurement. Based on the thickness dependence of field-like torque, anti-damping torque, and Oersted torque, the thickness-dependent SOT-FMR measurement is proposed as a more reliable, self-calibrated approach for characterization of spin-orbit torques.

physics.app-ph↗

arXiv2021

Bipolar electric field switching of perpendicular magnetic tunnel junctions through voltage controlled exchange coupling

Perpendicular magnetic tunnel junctions (p-MTJs) switched utilizing bipolar electric fields have extensive applications in energy-efficient memory and logic devices. Voltage-controlled magnetic anisotropy linearly lowers the energy barrier of ferromagnetic layer via electric field effect and efficiently switches p-MTJs only with a unipolar behavior. Here we demonstrate a bipolar electric field effect switching of 100-nm p-MTJs with a synthetic antiferromagnetic free layer through voltage-controlled exchange coupling (VCEC). The switching current density, ~1.1x10^5 A/cm^2, is one order of magnitude lower than that of the best-reported spin-transfer torque devices. Theoretical results suggest that electric field induces a ferromagnetic-antiferromagnetic exchange coupling transition of the synthetic antiferromagnetic free layer and generates a field-like interlayer exchange coupling torque, which cause the bidirectional magnetization switching of p-MTJs. A preliminary benchmarking simulation estimates that VCEC dissipates an order of magnitude lower writing energy compared to spin-transfer torque at the 15-nm technology node. These results could eliminate the major obstacle in the development of spin memory devices beyond their embedded applications.

physics.app-ph↗

arXiv2021

Thermal Contribution in the Electrical Switching Experiments with Heavy Metal / Antiferromagnet Structures

We examine the thermal origin of the detected "saw-tooth" shaped Hall resistance (Rxy) signals in the spin-orbit torque switching experiment for antiferromagnetic MnN. Compared with the results of the heavy metal / antiferromagnet bilayers (MnN/Ta), the qualitatively same "saw-tooth" shaped signals also appear in the samples with the heavy metal layer alone (either Ta or Pt) without MnN layer. In addition, The Rxy signal changes oppositely in the devices with Ta and Pt, due to the opposite temperature coefficient of resistivity (TCR) of the two materials. All those results are consistent with the "localized Joule heating" mechanism in devices with Hall crosses geometry. Moreover, by utilizing a structure with separated writing current paths and Hall cross area, the quadratic relationship between delta-Rxy and the writing current's amplitude is observed, which provides quantitative evidence of the thermal contribution. These results reveal the dominant thermal artifact in the widely used Hall crosses geometry for Neel vector probing, and also provide a strategy to semi-quantitatively evaluate the thermal effect, which can shed light on a more conclusive experiment design.

cond-mat.mes-hall↗

arXiv2021

Film strains enhance the reversible cycling of intercalation electrodes

A key cause of chemo-mechanical degradation in battery electrodes is that they undergo abrupt phase transformation during the charging/discharging cycle. This phase transformation is accompanied by lattice misfit strains that nucleate microcracks, induce fracture and, in extreme cases, amorphize the intercalation electrode. In this work, we propose a strategy to prevent the chemo-mechanical degradation of intercalation electrodes: we show that by engineering suitable film strains we can regulate the phase transformations in thin-film intercalation electrodes and circumvent the large volume changes. We test this strategy using a combination of theory and experiment: we first analytically derive the effect of film strain on the electrochemical response of a thin-film intercalation electrode and next apply our analytical model to a representative example (LixV2O5 with multiple phase transformations). We then test our theoretical predictions experimentally. Specifically, we electrochemically cycle thin-film V2O5 electrodes with different film strains and measure their structure, voltage, and stress responses. Our findings show that tensile film strains lower the voltage for phase transformations in thin-film V2O5 electrodes and facilitate their reversible cycling across a wider voltage window without chemo-mechanical degradation. These results suggest that film strain engineering is an alternative approach to preventing chemo-mechanical degradation in intercalation electrodes. Beyond thin-film electrodes, our findings from this study are applicable to the study of stress-induced phase transformations in particle-based electrodes and the thin surface layers forming on cathode particles.

cond-mat.mtrl-sci↗