About

Prof. Dr. Axel Hoffmann is a Principal Investigator at the Materials Science & Engineering department at Illinois. His research focuses on spintronics, exploring phenomena such as spin orbit torque effects in ultrathin collinear and noncollinear Ir-Mn alloys, and emulating 2D materials with magnons. His work involves understanding and manipulating spin currents, chiral phonons, and magnon-magnon coupling in various magnetic and oxide materials. Axel Hoffmann has been recognized as a 2025 Highly Cited Researcher and has been selected as the I-MRSEC director, highlighting his significant contributions to the field of materials science and spintronics.

Research topics

• Quantum mechanics
• Physics
• Computer Science
• Condensed matter physics
• Materials science
• Engineering
• Artificial Intelligence
• Nanotechnology
• Electrical engineering
• Aerospace engineering
• Electronic engineering
• Optoelectronics
• Engineering physics
• Acoustics

Selected publications

• Magnetostriction and temperature dependent Gilbert damping in boron doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Fe</mml:mi><mml:mn>80</mml:mn></mml:msub><mml:msub><mml:mi>Ga</mml:mi><mml:mn>20</mml:mn></mml:msub></mml:mrow></mml:math> thin films

  Physical Review Materials · 2025-07-24 · 1 citations

  articleSenior author

  Magnetic thin films with strong magnetoelastic coupling and low Gilbert damping are key materials for many magnetoelectric devices. Here, we investigated the effects of boron doping concentration on magnetostriction and temperature dependent Gilbert damping in magnetron sputtered ${({\mathrm{Fe}}_{80}{\mathrm{Ga}}_{20})}_{1\ensuremath{-}x}{\mathrm{B}}_{x}$ films. A crystalline to amorphous structural transition was observed for a boron content near 8% and coincided with a decrease in coercivity from 76 Oe to 3 Oe. A 10% doping concentration is optimal for achieving both large magnetostriction of 48.8 ppm and low Gilbert damping of $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. The temperature dependence of the damping shows an increase at low temperatures with a peak around 40 K, and we associate the relative increase $\mathrm{\ensuremath{\Delta}}\ensuremath{\alpha}/{\ensuremath{\alpha}}_{\mathrm{RT}}$ with magnetoelastic contributions to the damping, which has a maximum of 55.7% at 8% boron. An increase in the inhomogeneous linewidth broadening was observed in the structural transition regime at about 8% boron concentration. This study suggests that incorporation of glass forming elements, in this case boron, into ${\mathrm{Fe}}_{80}{\mathrm{Ga}}_{20}$ is a practical pathway for simultaneously achieving enhanced magnetoelastic coupling and reduced Gilbert damping.

  PublisherDOI

• Spin-Torque-Driven Non-uniform Dynamics of an Antivortex Core in Truncated Astroid Shaped Nanomagnets

  ArXiv.org · 2025-04-05

  preprintOpen access

  Spin textures that are not readily available in the domain structures of continuous magnetic thin films can be stabilized when patterned to micro/nano scales due to the dominant effect of dipolar magnetic interactions. Fabrication of such devices enables a thorough study of their RF dynamics excited by highly concentrated spin-polarized/pure-spin currents. For this purpose, in this study, we have employed a truncated astroid geometry to achieve stable magnetic antivortex core nucleation/annihilation which was detectable using the anisotropic magnetoresistance (AMR) at various temperatures. Furthermore, by depositing a soft magnetic thin film (20 nm thick permalloy) capped with a heavy-metal 2nm Pt layer, we were able to probe the spin orbit torque induced excitations accompanied by self-torque due to half-antivortex cores reminiscent of an isolated-antivortex, yielding GHz frequency oscillations with high quality factors (~50000). The observed RF oscillations can be attributed to a non-uniform domain wall oscillation mode close to the stable-antivortex core nucleation site as seen in micromagnetic simulations. This fundamental study of antivortex core response to spin currents is crucial for the assessment of their potential applications in high frequency spintronic devices such as reservoir computers.

  PublisherOA PDFDOI

• Magnetostriction and Temperature Dependent Gilbert Damping in Boron Doped Fe$_{80}$Ga$_{20}$ Thin Films

  ArXiv.org · 2025-05-16

  preprintOpen accessSenior author

  Magnetic thin films with strong magnetoelastic coupling and low Gilbert damping are key materials for many magnetoelectric devices. Here, we investigated the effects of boron doping concentration on magnetostriction and temperature dependent Gilbert damping in magnetron sputtered (Fe$_{80}$Ga$_{20}$)$_{1-x}$B$_{x}$ films. A crystalline to amorphous structural transition was observed for a boron content near 8% and coincided with a decrease in coercivity from 76 Oe to 3 Oe. A 10% doping concentration is optimal for achieving both large magnetostriction of 48.8 ppm and low Gilbert damping of $6 \times 10^{-3}$. The temperature dependence of the damping shows an increase at low temperatures with a peak around 40 K and we associate the relative increase $Δα/α_{RT}$ with magnetoelastic contributions to the damping, which has a maximum of 55.7% at 8% boron. An increase in the inhomogeneous linewidth broadening was observed in the structural transition regime at about 8% boron concentration. This study suggests that incorporation of glass forming elements, in this case boron, into Fe$_{80}$Ga$_{20}$ is a practical pathway for simultaneously achieving enhanced magnetoelastic coupling and reduced Gilbert damping.

  PublisherOA PDFDOI

• Single-shot electrical detection of short-wavelength magnon pulse transmission in a magnonic thin-film waveguide

  npj Spintronics · 2025-04-01 · 2 citations

  articleOpen access

  Abstract The advance of magnon spintronics requires understanding of time-domain magnon pulse transmission in order to develop high-speed information processing protocols. In this work, we demonstrate single-shot electrical detection of narrow-band magnon pulse transmission in a yttrium iron garnet thin-film delay line. The high signal-to-background ratio of magnon transmission band allows us to directly probe the magnon transmission electrically using a fast oscilloscope and to study its spectral evolution using Fast Fourier Transform (FFT) of the time-domain transmitted signal. At elevated input power, we show a magnon transmission reduction and a spectral distortion, which can be understood by the nonlinear magnon excitation in the transmission band defined by the antenna geometry. In addition, we also find that the higher- (lower-) frequency magnon spectral component exhibits a lower (higher) magnon group velocity, showing a dispersion agreeing with the Damon-Eshbach dependence. Our results provide important guidance of magnon pulse engineering for their applications in spin wave computing and coherent magnon information processing.

  PublisherDOI

• Single-shot magnon interference in a magnon-superconducting-resonator hybrid circuit

  Nature Communications · 2025-04-17 · 10 citations

  articleOpen access

  Magnon interference is a hallmark of coherent magnon interactions. In this work, we demonstrate single-shot magnon interference using up to four magnon pulses in two remotely coupled yttrium iron garnet spheres mediated by a coplanar superconducting resonator. By exciting one YIG sphere with injected microwave pulses, we achieve coherent energy exchange between the two spheres, facilitating their interference processes, including Rabi-like oscillation with a single pulse, constructive and destructive interference with two pulses, and interference peak sharpening with up to four pulses-analogous to diffraction grating in optical interference. The resulting interference patterns can be precisely controlled by changing the frequency detuning and time delay of the magnon pulses. The demonstration of time-domain coherent control of remote magnon interference opens new pathways for advancing coherent information processing through multi-operation, circuit-integrated hybrid magnonic networks.

  PublisherOA PDFDOI

• Magnetic Skyrmions in Synthetic Ferri- and Antiferromagnets

  2024-05-05

  article1st authorCorresponding

  Magnetic skyrmions have generated a lot of interest due to their distinct physical properties derived from their unique topology, which can be characterized by their winding number or topological charge. At the same time synthetic ferri- and antiferromagnets provide an opportunity for coupling two magnetic skyrmions with opposite topological charges resulting in novel behaviors. This presentation will discuss two possible physical representations of these systems. One is given by multilayers where the antiferromagnetic coupling comes from interlayer exchange coupling through a non-magnetic layer. Micromagnetic simulations showed how the dynamics of these systems depends sensitively on the interplay between exchange and dipolar coupling. Another system are inversion-broken multilayers of rare earth and transition metal systems. Here temperature allows to control the magnetization of the individual skyrmions, and in fact temperature can result also in a change from out-of-plane to in-plane anisotropy and a concomitant spin-reorientation transition. Repeated cycling through this transition shows that the topology of the spin textures is preserved during this continuous deformation.

  PublisherDOI

• Unconventional Spin-Orbit Torques Due to Reduced Crystal Symmetries

  IEEE Transactions on Magnetics · 2024-10-14 · 2 citations

  articleOpen accessSenior author

  Spin-orbit torques have emerged as a powerful mechanism for manipulating magnetic moments in spintronic devices, offering a pathway to more efficient and scalable memory and logic technologies. While conventional spin-orbit torques generated in heavy metals and topological insulators have been extensively studied, recent advancements in unconventional spin-orbit torques demonstrated out-of-plane spin polarizations that could effectively switch perpendicular magnetizations without the need for additional external in-plane magnetic fields, promising significant implications for the development of energy-efficient and compact spintronic devices. Unconventional spin-orbit torques are usually found in materials with low symmetries, such as transition metal dichalcogenides, topological insulators, and 2-D materials. Here, we provide a brief overview of unconventional spin-orbit torques and present two example material systems: CrPt3 and MoTe2, both exhibiting strong spin-orbit coupling and phase-dependent spin-orbit torques, and focus on their unique origins and potential applications. We discuss the roles of magnetic and crystallographic orders in generating unconventional spin-orbit torques, highlighting how these factors contribute to the observed anisotropic and directional dependencies.

  PublisherDOI

• Spin-Polarized Antiferromagnetic Metals

  Annual Review of Condensed Matter Physics · 2024-10-14 · 23 citations

  articleOpen access

  Spin-polarized antiferromagnets have recently gained significant interest because they combine the advantages of both ferromagnets (spin polarization) and antiferromagnets (absence of net magnetization) for spintronics applications. In particular, spin-polarized antiferromagnetic metals can be useful as active spintronics materials because of their high electrical and thermal conductivities and their ability to host strong interactions between charge transport and magnetic spin textures. We review spin and charge transport phenomena in spin-polarized antiferromagnetic metals in which the interplay of metallic conductivity and spin-split bands offers novel practical applications and new fundamental insights into antiferromagnetism. We focus on three types of antiferromagnets: canted antiferromagnets, noncollinear antiferromagnets, and collinear altermagnets. We also discuss how the investigation of spin-polarized antiferromagnetic metals can open doors to future research directions.

  PublisherDOI

• Spin-polarized antiferromagnetic metals

  arXiv (Cornell University) · 2024-08-28

  preprintOpen access

  Spin-polarized antiferromagnets have recently gained significant interest because they combine the advantages of both ferromagnets (spin polarization) and antiferromagnets (absence of net magnetization) for spintronics applications. In particular, spin-polarized antiferromagnetic metals can be useful as active spintronics materials because of their high electrical and thermal conductivities and their ability to host strong interactions between charge transport and magnetic spin textures. We review spin and charge transport phenomena in spin-polarized antiferromagnetic metals, in which the interplay of metallic conductivity and spin-split bands offers novel practical applications and new fundamental insights into antiferromagnetism. We focus on three types of antiferromagnets: canted antiferromagnets, noncollinear antiferromagnets, and collinear altermagnets. We also discuss how the investigation of spin-polarized antiferromagnetic metals can open doors to future research directions.

  PublisherOA PDFDOI

• Inverse chirality-induced spin selectivity effect in chiral assemblies of π-conjugated polymers

  Nature Materials · 2024-03-15 · 61 citations

  articleOpen access
  PublisherOA PDFDOI

Frequent coauthors

• John E. Pearson

  University of Otago

  212 shared

• M. Benjamin Jungfleisch

  184 shared

• Wei Zhang

  112 shared

• J. B. Ketterson

  Northwestern University

  108 shared

• V. Novosad

  Argonne National Laboratory

  97 shared

• Wonbae Bang

  85 shared

• Joseph Sklenar

  84 shared

• S. G. E. te Velthuis

  Argonne National Laboratory

  66 shared

Labs

• Hoffmann Research GroupPI

  Materials Science & Engineering

Education

• Ph.D., Physics Department

  University of California - San Diego

  1999

• Diplom, Physik

  RWTH Aachen University

  1994

Awards & honors

• Fellow of the American Physical Society
• Fellow of the American Vacuum Society
• Fellow of IEEE