arXiv subjects

S. Joseph Poon

Publications and source records attributed to S. Joseph Poon.

At least 19 recordsLinked to original sources

arXiv2025

Reexamining Machine Learning Models on Predicting Thermoelectric Properties

Thermoelectric materials can generate clean energy by transforming waste heat into electricity. The effectiveness of thermoelectric materials is measured by the dimensionless figure of merit, ZT. The quest for high ZT materials has drawn extensive research experimentally and theoretically. However, due to the vast material space, finding high ZT materials is time-consuming and costly. To improve the efficiency of discovering new thermoelectric materials, recent studies have employed machine learning with databases to search for high ZT candidates. In this work, we examine the effects of adding various physical concepts on the performance of machine learning models in predicting TE properties. The objective is to improve the model ability to capture the underlying physics in designing TE materials. These concepts include short range order and crystal structure class. Results show some improvements in accuracy. However, the current models do not distinguish between dilute alloys and concentrated alloys, rendering them inadequate in predicting doping effects. To better capture the electronic band structure effect from doping, we included various dopant properties as features. This increases the prediction accuracy in doped materials. Furthermore, we used a genetic algorithm to rank features for various thermoelectric properties to provide physical insight into key parameters in designing thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2024

High Strength Refractory AlHfNbTiV B2 High Entropy Alloys with High Fracture Strains

We demonstrate the development of a series of refractory high-entropy alloys containing aluminum AlRHEAs in the ordered BCC-B2 phase by varying the aluminum content within 10 to 25 atomic percent, with the goal of high strength and good ductility synergy. The AlRHEAs obtained are found to show promising potential for high-temperature applications. The incorporation of Al lowers the density and promotes the long-range atomic ordering, which in turn stabilizes the B2 formation, and strengthens the material but usually deteriorates ductility. Several B2 AlRHEAs that contain a combination of Ti, Hf, Nb, and V with moderate to high Poisson ratios are investigated for high strength and ductility. Furthermore, through statistical analysis, we identify a valley around the valence electron concentration VEC of 6 where low ductility is prominently observed. Machine-learning models are employed to screen the vast compositional space of AlRHEA alloys to predict B2 formation and toughness indicated by the yield strength and fracture strain. High prediction accuracies are achieved. As the Al content decreases, the B2 atomic ordering decreases, compression yield strengths decrease from 1500 MPa to 1200 MPa, and compression fracture strains increase from 0.06 to over 0.5. Notably, Al10Hf20Nb22Ti33V15 retains a compression yield strength exceeding 800 MPa up to 700 C, tensile yield strength of 1100 MPa, and fracture strain of 0.083. Our findings on enhancing ductility in pure B2 alloys pave the way for further research on Al-RHEA superalloys, striving to achieve high strength and ductility, reduced density, and improved oxidation resistance.

cond-mat.mtrl-sci↗

arXiv2023

Phase Change Induced Magnetic Switching through Metal-insulator Transition in VO2/TbFeCo Films

The ability to manipulate spins in magnetic materials is essential in designing spintronics devices. One method for magnetic switching is through strain. In VO2 on TiO2 thin films, while VO2 remains rutile across the metal-insulator transition, the in-plane lattice area expands going from low temperature insulating phase to high temperature conducting phase. In a VO2/TbFeCo bilayer, the expansion of the VO2 lattice area exerts tension on the amorphous TbFeCo layer. Through the strain effect, magnetic properties, including the magnetic anisotropy and magnetization, of TbFeCo can be changed. In this work, the changes in magnetic properties of TbFeCo on VO2/TiO2(011) are demonstrated using anomalous Hall effect measurements. Across the metal-insulator transition, TbFeCo loses perpendicular magnetic anisotropy, and the magnetization in TbFeCo turns from out-of-plane to in-plane. Using atomistic simulations, we confirm these tunable magnetic properties originating from the metal-insulator transition of VO2. This study provides the groundwork for controlling magnetic properties through a phase transition.

cond-mat.mtrl-sci↗

arXiv2023

Machine Learning-Based Classification, Interpretation, and Prediction of High-Entropy-Alloy Intermetallic Phases

The design of high-entropy alloys (HEA) with desired properties is challenging due to their large compositional space. While various machine learning (ML) models can predict specific HEA solid-solution phases (SS), predicting high-entropy intermetallic phases (IM) is underdeveloped due to limited datasets and inadequate ML features. This paper introduces feature engineering-assisted ML models that achieve detailed phase classification and high accuracy. By combining phase-diagram-based and physics-based features, it is found that the ML models trained on the Random Forest (RF) and Support Vector Machine (SVM) regressors, are able to classify individual SS and common IM (Sigma, Laves, Heusler, and refractory B2 phases) with accuracies ranging from 80 - 94%. The machine-learned features also enable the interpretation of IM formation. Furthermore, the efficacies of the RF, SVM, and neural network (NN) models are critically evaluated. The phase classification accuracies are found to decrease upon utilizing the NN model to train the datasets. The accuracy of the model prediction is validated by synthesizing 86 new alloys. This approach provides a practical and robust framework for guiding HEA phase design, particularly for technologically significant IM phases.

cond-mat.mtrl-sci↗

arXiv2023

Measurement of the Dzyaloshinskii-Moriya Interaction in Mn4N Films that Host Skyrmions

Mn4N thin film is one of the potential magnetic mediums for spintronic devices due to its ferrimagnetism with low magnetization, large perpendicular magnetic anisotropy (PMA), thermal stability, and large domain wall velocity. A recent experiment confirmed the existence of tunable magnetic skyrmions in MgO/Mn4N/CuxPt1-x(x=0,0.5,0.9,0.95), and density functional theory (DFT) calculation provided a large theoretical value of the interfacial Dzyaloshinskii-Moriya Interaction (iDMI) of Mn4N/Pt, which is consistent with the predicted chemical trend of DMI in transition metal/Pt films. So far, measured DMI has not been reported in Mn4N which is needed in order to support the predicted large DMI value. This paper reports the average DMI of MgO/Mn4N(17nm)/CuxPt1-x(3nm), extracted from the anomalous Hall effect with various tilted angles, based on magnetic droplet theory with DMI effects. The DMI decreases from 0.267 mJ/m2 to 0.011 mJ/m2 with non-linear tendencies as Cu concentration in the CuxPt1-x capping layer increases from 0 to 1, demonstrating the control of DMI through CuxPt1-x capping layer. Furthermore, a solid solution model is developed, based on X-ray photoelectron spectroscopy (XPS) compositional depth profile, to analyze the possible effects on DMI from the mixing layers at the surface of Mn4N. After taking into account the mixing layers, the large DMI in Mn4N film with Pt capping is consistent with the predicted DMI.

cond-mat.mtrl-sci↗

arXiv2022

Conventional Half-Heusler Alloys Advance State-of-the-Art Thermoelectric Properties

Half-Heusler (HH) phases have garnered much attention as thermally stable and non-toxic thermoelectric materials for power conversion. The most studied alloys to date utilize Hf, Zr, and Ti as the base components. These alloys can achieve a moderate dimensionless figure of merit, ZT, near 1. Recent studies have advanced the thermoelectric performance of HH alloys by employing nanostructures and novel compositions to achieve larger ZT, reaching as high as 1.5. Herein, we report that traditional alloying techniques applied to the conventional HfZr-based half-Heusler alloys can also lead to exceptional ZT. Specifically, we present the well-studied p-type Hf0.3Zr0.7CoSn0.3Sb0.7, previously reported to have a ZT~0.8, resonantly doped with less than 1 at. % metallic Al on the Sn/Sb site, touting a remarkable ZT near 1.5 at 980 K. This is achieved through a significant increase in power factor, by ~65%, and a notable but smaller decrease in thermal conductivity, by ~13%, at high temperatures. These favorable thermoelectric properties are discussed in terms of a local anomaly in the density of states near the Fermi energy designed to enhance the Seebeck coefficient, as revealed by first-principles calculations, as well as the emergence of a highly heterogeneous grain structure that can scatter phonons across different length scales, effectively suppressing the thermal conductivity. Consequently, the effective mass is significantly enhanced from ~ 7 to 10me within a single parabolic band model, consistent with the result from first-principles calculations. The discovery of high ZT in a commonly studied half-Heusler alloy through a conventional and non-complex approach opens a new path for further discoveries in similar types of alloys. Furthermore, it is reasonable to believe that the study will reinvigorate effort in exploring high thermoelectric performance in conventional alloy systems.

cond-mat.mtrl-sci↗

arXiv2022

Ultrafast Switching in Synthetic Antiferromagnet with Bilayer Rare-Earth Transition-Metal Ferrimagnets

In spintronics, it is important to be able to manipulate magnetization rapidly and reliably. Several methods can control magnetization, such as by applying current pulses or magnetic fields. An applied current can reverse magnetization with nanosecond speed through the spin torque effect. For faster switching, subpicosecond switching with femtoseconds laser pulse has been achieved in amorphous rare-earth transition-metal ferrimagnets. In this study, we employed atomistic simulations to investigate ultrafast switching in a synthetic antiferromagnet with bilayer amorphous FeGd ferrimagnets. Using a two-temperature model, we demonstrated ultrafast switching in this synthetic antiferromagnet without external magnetic fields. Furthermore, we showed that if we initially stabilize a skyrmion in this heterostructure, the ultrafast laser can switch the skyrmion state using the same mechanism. Furthermore, this bilayer design allows the control of each ferrimagnetic layer individually and opens the possibility for a magnetic tunnel junction.

cond-mat.mes-hall↗

arXiv2022

Interfacial Mixing Effect in a Promising Skyrmionic Material: Ferrimagnetic Mn$_4$N

Interfacial mixing of elements is a well-known phenomenon found in thin film deposition. For thin-film magnetic heterostructures, interfacial compositional inhomogeneities can have drastic effects on the resulting functionalities. As such, care must be taken to characterize the compositional and magnetic properties of thin films intended for device use. Recently, ferrimagnetic Mn$_4$N thin films have drawn considerable interest due to exhibiting perpendicular magnetic anisotropy, high domain-wall mobility, and good thermal stability. In this study, we employed X-ray photoelectron spectroscopy (XPS) and polarized neutron reflectometry (PNR) measurements to investigate the interfaces of an epitaxially-grown MgO/Mn$_4$N/Pt trilayer deposited at 450 $^{\circ}$C. XPS revealed the thickness of elemental mixing regions of near 5 nm at both interfaces. Using PNR, we found that these interfaces exhibit essentially zero net magnetization at room temperature. Despite the high-temperature deposition at 450 $^{\circ}$C, the thickness of mixing regions is comparable to those observed in magnetic films deposited at room temperature. Micromagnetic simulations show that this interfacial mixing should not deter the robust formation of small skyrmions, consistent with a recent experiment. The results obtained are encouraging in terms of the potential of integrating thermally stable Mn$_4$N into future spintronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Tunable Magnetic Skyrmions in Ferrimagnetic Mn$_4$N

Thin films of ferrimagnetic Mn$_4$N are candidate materials to host magnetic skyrmions that have demonstrated thermal stability up to 450$^\circ$C. However, there are no experimental reports observing skyrmions in this system. Here, we discuss the results of sputter grown 15-17~nm Mn$_4$N thin films on MgO substrate capped with Pt$_{1-x}$Cu$_{x}$ layers. Vibrating sample magnetometry measurement of out-of-plane hysteresis loops confirmed that magnetic properties are insensitive to the cap layer composition. Imaging based on magnetic force microscopy measurements observed 300 to 50~nm sized skyrmions, as the Cu concentration was increased from $x$ = 0 to 0.9. We performed density functional theory calculations and found that the interfacial Dzyaloshinskii-Moriya interactions (iDMI) follow a trend: Mn$_4$N/MgO(001) $<$ Cu/Mn$_4$N(001) $<$ Pt/Mn$_4$N(001). We infer from these calculations that $x$ in Pt$_{1-x}$Cu$_{x}$ capping layer can serve as a robust tuning knob to tailor the iDMI and control the skyrmion size. This work provides guidance to achieve smaller Néel-type skyrmions in Mn$_4$N thin films, which is an important step forward for building thermally stable skyrmionic devices.

cond-mat.mes-hall↗

arXiv2021

Navigating the Complex Compositional Landscape of High-Entropy Alloys

High-entropy alloys, which exist in the high-dimensional composition space, provide enormous unique opportunities for realizing unprecedented structural and functional properties. A fundamental challenge, however, lies in how to predict the specific alloy phases and desirable properties accurately. This review article provides an overview of the data-driven methods published to date to tackle this exponentially hard problem of designing high-entropy alloys. Various utilizations of empirical parameters, first-principles and thermodynamic calculations, statistical methods, and machine learning are described. In an alternative method, the effectiveness of using phenomenological features and data-inspired adaptive features in the prediction of the high-entropy solid solution phases and intermetallic alloy composites is demonstrated. The prospect of high-entropy alloys as a new class of functional materials with improved properties is featured in light of entropic effects. The successes, challenges, and limitations of the current high-entropy alloys design are discussed, and some plausible future directions are presented.

cond-mat.mtrl-sci↗

arXiv2019

Multi-Principal-Element Approach to High-Performance Thermoelectric Materials

High-entropy alloys are characterized by high configurational entropy. Since the discovery of high-entropy alloys (HEA) in 2004, entropy engineering has provided a promising direction for exploiting composition, lattice disorder, band structure, and microstructure effects to advance thermoelectric performance. This review discusses the impact of entropy on thermoelectric properties and looks back at the role of multi-principal-element alloys, a weaker version of HEA, on the development of compositionally complex thermoelectric alloys in achieving high thermoelectric performance. The experimental and theoretical efforts in a wide range of material systems such as TAGS, LAST, half-Heusler, liquid-like copper chalcogenides, SnTe, and CuInTe2 chalcopyrites provide insights into the entropy engineering approach and also promise an emerging paradigm of high-entropy thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2019

Enhanced Figure of Merit in Bismuth-Antimony Fine-Grained Alloys at Cryogenic Temperatures

Thermoelectric (TE) materials research plays a vital role in heat-to-electrical energy conversion and refrigeration applications. Bismuth-antimony (Bi-Sb) alloy is a promising material for thermoelectric cooling. Herein, a high figure of merit, ZT, near 0.6 at cryogenic temperatures (100-150K) has been achieved in melt-spun n-type Bi85Sb15 bulk samples consisting of micron-size grains. The achieved ZT is nearly 50 percents higher than polycrystalline averaged single crystal ZT of ~0.4, and it is also significantly higher than ZT of less than ~0.3 measured below 150K in Bi-Te alloys commonly used for cryogenic cooling applications. The improved thermoelectric properties can be attributed to the fine-grained microstructure achieved from rapid solidification, which not only significantly reduced the thermal conductivity but also mitigated a segregation effect. A record low thermal conductivity of ~1.5 W m-1 K-1 near 100 K was measured using the hot disk method. The thermoelectric properties for this intriguing semimetal-semiconductor alloy system were analyzed within a two-band effective mass model. The study revealed a gradual narrowing of the band gap at increasing temperature in Bi-Sb alloy for the first time. Magneto-thermoelectric effects of this Bi-Sb alloy further improved the TE properties, leading to ZT of about 0.7. The magneto-TE effect was further demonstrated in a combined NdFeB/BiSb/NdFeB system. The compactness of the BiSb-magnet system with high ZT enables the utilization of magneto-TE effect in thermoelectric cooling applications.

cond-mat.mtrl-sci↗

arXiv2019

Amorphous Ferrimagnets: an Ideal Host for Ultra-Small Skyrmions at Room Temperature

Recently, magnetic skyrmion has emerged as an active topic of fundamental study and applications in magnetic materials research. Magnetic skyrmions are vortex-like spin excitations with topological protection and therefore are more robust to pinning compared with magnetic domain walls. We employ atomistic simulations to create room-temperature ultra-small Neel skyrmions in amorphous ferrimagnet. The fast propagation and low-dissipation dynamics of ultra-small ferrimagnetic skyrmions make them attractive for utilization as an alternative to domain walls in spin-based memory and logic devices.

cond-mat.mtrl-sci↗

arXiv2019

Robust Formation of Ultrasmall Room-Temperature Neél Skyrmions in Amorphous Ferrimagnets from Atomistic Simulations

Neél skyrmions originate from interfacial Dzyaloshinskii Moriya interaction (DMI). Recent studies have explored using thin-film ferromagnets and ferrimagnets to host Neél skyrmions for spintronic applications. However, it is unclear if ultrasmall (10 nm or less) skyrmions can ever be stabilized at room temperature for practical use in high density parallel racetrack memories. While thicker films can improve stability, DMI decays rapidly away from the interface. As such, spins far away from the interface would experience near-zero DMI, raising question on whether or not unrealistically large DMI is needed to stabilize skyrmions, and whether skyrmions will also collapse away from the interface. To address these questions, we have employed atomistic stochastic Landau-Lifshitz-Gilbert simulations to investigate skyrmions in amorphous ferrimagnetic GdCo. It is revealed that a significant reduction in DMI below that of Pt is sufficient to stabilize ultrasmall skyrmions even in films as thick as 15 nm. Moreover, skyrmions are found to retain a uniform columnar shape across the film thickness despite the decaying DMI. Our results show that increasing thickness and reducing DMI in GdCo can further reduce the size of skyrmions at room temperature, which is crucial to improve the density and energy efficiency in skyrmion based devices.

cond-mat.mes-hall↗

arXiv2019

Half-Heusler Compounds: Promising Materials For Mid-To-High Temperature Thermoelectric Conversion

Half-Heusler compounds (space group Fm3m) has garnered increasing attention in recent years in the thermoelectric community. Three decades ago, refractory RNiSn half-Heusler compounds (R represents refractory metals such as Hf, Zr, Ti) were found to be narrow-gap semiconductors with large Seebeck coefficients in 100s of micro-volt per Kelvin. Today, half-Heusler (HH) compounds have emerged as promising thermoelectric materials in the intermediate temperature range (400-800oC). HH materials are endowed with good thermal stability and scalability. Thermoelectric n-p modules based on HH materials demonstrate conversion efficiency near 10% and power density output near 9 W/cm2. The objective of this article is to present a historical account of the research and development of thermoelectric half-Heusler compounds. Particularly, there have been notable achievements since 2012 thanks to the emergence of new approaches. As a result, ZT has risen from ~1 to 1.5. The various advances made since the early 1990s to the present are recounted by categorizing half-Heusler materials into three generations (Gen): Gen-1 Gen-2, and Gen-3 HH materials.

cond-mat.mtrl-sci↗

arXiv2019

High Entropy Alloys Mined From Phase Diagrams

High entropy alloys (HEA) show promise as a new type of high-performance structural material. Their vast degrees of freedom provide for extensive opportunities to design alloys with tailored properties. However, the compositional complexities of HEAs present great challenges for alloy design. Current approaches have shown limited reliability in accounting for the compositional regions of single solid solution and composite phases. We present a phenomenological method, analyzing binary phase diagrams to predict HEA phase formation on the hypothesis that the structural stability of HEAs is encoded within. Accordingly, we introduce a small number of phase-diagram inspired parameters and employ machine learning to partition the formation region of 500+ reported HEA compositions. The model achieved a single phase HEA prediction rate >80 %. To validate our method, we demonstrated the capability of this method in predicting HEA solid solution phases with and without intermetallics in 30 randomly selected complex compositions, with a success rate of 77 %. The presented efficient search approach with high predictive capability can be exploited to complement computation-intense methods in providing a powerful platform for the design of high entropy alloys.

cond-mat.dis-nn↗

arXiv2018

Recent Advances in Thermoelectric Performance of Half-Heusler Compounds

Half-Heusler phases (space group F43m, C1b) have recently captured much attention as promising thermoelectric materials for heat-to-electric power conversion in the mid-to-high temperature range. The most studied ones are the RNiSn-type half-Heusler compounds, where R represents refractory metals Hf, Zr, and Ti. These compounds have shown a high-power factor and high-power density, as well as good material stability and scalability. Due to their high thermal conductivity, however, the dimensionless figure of merit (zT) of these materials has stagnated near 1 for a long time. Since 2013, the verifiable ZT of half-Heusler compounds has risen from 1 to near 1.5 for both n- and p-type compounds in the temperature range of 500 to 900 degrees C. In this brief review, we summarize recent advances as well as approaches in achieving the high ZT reported. In particular, we discuss the less-exploited strain-relief effect and dopant resonant state effect studied by the author and his collaborators in more detail. Finally, we point out directions for further development. Keywords: half-Heusler compounds; figure of merit; power density; lattice disorder; dopant resonant states

cond-mat.mtrl-sci↗

arXiv2018

Skyrmion Formation Induced by Antiferromagnetic-enhanced Interfacial Dzyaloshinskii Moriya Interaction

Neél skyrmions originate from interfacial Dzyaloshinskii Moriya interaction (DMI). Recent studies have explored using ferromagnet to host Neél skyrmions for device applications. However, challenges remain to reduce the size of skyrmion to near 10 nm. Amorphous rare-earth-transitional-metal ferrimagnets are attractive alternative materials to obtain ultrasmall skyrmions at room temperature. Their intrinsic perpendicular magnetic anisotropy and tunable magnetization provides a favorable environment for skyrmion stability. In this work, we employ atomistic stochastic Landau-Liftshitz-Gilbert (LLG) algorithm to investigate skyrmions in GdFe within the interfacial DMI model. Despite the rapid decay of DMI away from the interface, small skyrmions of near 10 nm are found in thick ~ 5 nm amorphous GdFe film at 300K. We have also considered three scenarios for the sign of DMI between Gd-Fe pair. It is revealed that antiferromagnetic coupling in the ferrimagnet plays an important role in enhancing the effect of interfacial DMI and to stabilize skyrmion. These results show that ferrimagnets and antiferromagnets with intrinsic antiferromagnetic couplings are appealing materials to host small skyrmions at room temperature, which is crucial to improve density and energy efficiency in skyrmion based devices.

cond-mat.mes-hall↗