publications

Published

2026

1. The 4-2 Rule: The (Proposed) Answer to Heusler Alloys, Stability, and (Ordering) Everything

   Ridwan Nahar, Riley Nold, Naomi Derksen, Ka Ming Law, Babajide Akintunde, Michael Zengel, Thomas Roden, Justin Lewis, Harshil Avlani, Sujan Budhathoki, Patrick R. LeClair, and Adam J. Hauser

   Journal of Alloys and Compounds, Jul 2026

   DOI

2025

1. Bilinear and Biquadratic Interlayer Exchange Coupling in Asymmetric Co65Fe35 Films with Interrupting Ru Pseudolayer

   Upama Karki, Bhuwan Nepal, Naomi Derksen, Anupama Karki, Riley Nold, Michael Zengel, Adam Hauser, Xin Wang, Gary J. Mankey, Tim Mewes, and Jamileh Beik Mohammadi

   Oct 2025

   Abs DOI

   This study investigates the bilinear and biquadratic coupling strength of asymmetric CoFe/Ru/CoFe structures via quasi-static and dynamics magnetization characterization. We report the bilinear and biquadratic exchange coupling mechanisms, revealing distinct ferromagnetic (FM) and antiferromagnetic (AFM) coupling regimes, emphasizing the role of spacer layer thickness in influencing the coupling behavior. Magnetic frustration in the AFM coupling regime is observed, confirming the coexistence of bilinear (J) and biquadratic (J) exchange coupling constants.

2. Coherent Control of Photocurrents in Germanium for Gapless and Broadband Terahertz Emission

   Daniel Anders, Michael Zengel, Rodrigo Sandoval Rodriguez, Kalle Bräumer, Sangam Chatterjee, and Markus Stein

   Aug 2025

   Abs DOI

   Readily implemented broadband THz sources extend the reach of THz in diagnostic and sensing applications. Coherently controlled photocurrents in pure, cost-effective bulk Germanium and Germanium quantum wells render such broadband and gapless terahertz emission, overcoming the bandwidth limitations of traditional III-V and II-VI semiconductor-based THz emitters. Precise phase control between the fundamental and its second harmonic governs the THz field dynamics. The presented scheme, which utilizes low-energy 100 fs optical pulses centered around 1550 nm, is readily accessible through affordable ultrafast fiber laser technology and frequency doubling.

3. Atomic Ordering and Electronic Structure of A2YSn Heusler Alloys: A First-Principles Study

   Michael Zengel, Riley Nold, Thomas Roden, Ka Ming Law, Ridwan Nahar, Justin Lewis, and Adam J. Hauser

   Apr 2025

   Abs DOI

   First-principles calculations are performed for the full Heusler L21 and inverse Heusler XA phases of the A2YSn series, with A spanning the 3d block of the transition metals (A = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn). Yttrium is chosen as the B-site element in order to investigate the role of 4d B-site orbitals on the magnetic character of the system as a whole. Formation energy calculations indicate a transition from XA to L21 dominant phase preference crossing from a V to a Cr A-site element choice, with L21 remaining the dominant phase across the rest of the series. Magnetic data reveal a weak ferrimagnetic yttrium sublattice forming in a similar region of A-site valency as the phase preference transition, indicating a possible structural-magnetic coupling. Total and projected density of states calculations reveal a rich electronic structure with unique hybridization between the A-site and yttrium atoms, particularly for the L21 phase. A near half-metallic gap is found to form in the L21 phase with increasing A-site valency, culminating in a 97.71 % spin polarization for Cr2YSn. Phase stability conclusions are also compared to data from the Open Quantum Materials Database, providing important context for the role of density functional theory (DFT) in informing material synthesis studies. These results provide new insights into the role of element choice in Heusler phase stability and electronic structure, which is critical to identifying novel materials who properties are robust against phase competition.

2024

1. Phase and D-d Hybridization Control via Electron Count for Material Property Control in the X2FeAl Material Class

   Ka Ming Law, Ridwan Nahar, Riley Nold, Michael Zengel, Justin Lewis, and Adam J. Hauser

   Apr 2024

   Abs DOI

   First-principles calculations are performed for full (L21) and inverse (XA) Heusler compounds X2FeAl, where X comprise a range of 3d (Sc, Ti, V, Cr), 4d (Y, Zr, Nb, Mo), and 5d (Hf, Ta, W) early and middle column transition metal elements. The formation energy difference between full and inverse phase and the degree of d-d orbital hybridization with increasing total valence electron count are shown to drive the magnetic properties (total and atomic magnetic moments, spin polarization) and electronic properties (band structure and projected density of states) of the material system. Specifically, X-site atomic magnetic moments take on increasingly Fe character with increasing valence electron count, in both full and inverse Heusler phases. This can be explained by changes on the degree of d-d hybridization between X- and Fe-site d orbitals. Synchronized energy shifts in the PDOS of the X- and Y-sites (Fe) across each of the full and inverse Heusler series provide us insight to controlling spin polarization via composition. This work demonstrates the need to holistically study the thermodynamic phase stability, magnetic moments, and spin polarization of Heusler alloys, in the framework of anti-site disorder and in a wider compositional context. The end goal of this study is to benefit the mapping of experimental results in search of a specific property, by providing a methodology for extrapolating properties based on experimental or theoretical results.

2. Dataset on Density Functional Theory Investigation of Ternary Heusler Alloys

   Ridwan Nahar, Ka Ming Law, Thomas Roden, Michael Zengel, Justin Lewis, Sujan Budhathoki, Riley Nold, Harshil Avlani, Babajide Akintunde, Naomi Derksen, and Adam J. Hauser

   Feb 2024

   Abs DOI

   This paper contains data and results from Density Functional Theory (DFT) investigation of 423 distinct X2YZ ternary full Heusler alloys, where X and Y represent elements from the D-block of the periodic table and Z signifies element from main group. The study encompasses both “regular” and “inverse” Heusler phases of these alloys for a total of 846 potential materials. For each specific alloy and each phase, a range of information is provided including total energy, formation energy, lattice constant, total and site-specific magnetic moments, spin polarization as well as total and projected density of electronic states. The aim of creating this dataset is to provide fundamental theoretical insights into ternary X2YZ Heusler alloys for further theoretical and experimental analysis.