About

Our research focuses on the fundamentals of magnetic nanoparticles that have very uniform sizes, as well as possible applications in magnetic storage and logic, permanent magnets, high frequency composites, and biomedicine. Monodisperse nanoparticles are synthesized by chemical methods and used as building blocks for self-assembly into arrays and for nanomasking pattern transfer into thin films. The collective magnetic behavior of the arrays has been studied using electron holography and Lorentz microscopy to image domains, and using polarized small angle neutron scattering to investigate the magnetization length scales within nanoparticles and their assemblies. We have demonstrated the ability of conductive atomic force microscopy to detect the state of a magnetic tunnel junction nanopillar, and switch it using a spin-polarized current.

Research topics

• Physics
• Materials science
• Condensed matter physics
• Nanotechnology
• Optics
• Computer Science
• Chemistry
• Nuclear magnetic resonance
• Metallurgy
• Composite material
• Chemical physics
• Acoustics

Selected publications

• Magnetic Tunnel Junction-Based Stochastic Logic Gates

  IEEE Magnetics Letters · 2026-01-01

  articleSenior author

  Magnetic tunnel junctions with voltage-controllable telegraphing between the parallel and antiparallel states were combined with hybrid circuits to create probabilistic bit modules. The time-dependent behavior was measured both when these devices were free running and when they were coupled together to form stochastic logic gates. Two modules were used to create a nearly deterministic NOT gate. A stochastic AND gate was demonstrated using a combination of three modules, and a statistical preference for microstates consistent with the AND gate truth table was observed. A metric is proposed for quantifying the performance through pairwise comparison of microstate probabilities.

  PublisherDOI

• Atomic-scale observation of vacancy ordering in magnetite nanoparticles

  Ultramicroscopy · 2026-02-17

  article
  PublisherDOI

• Magnetization Dynamics in Magnetic Tunnel Junction Artificial Spin Ice - Dataset

  Zenodo (CERN European Organization for Nuclear Research) · 2025-11-06

  datasetOpen accessSenior author

  This dataset contains experimental and simulation data for superparamagnetic tunnel junctions arranged in artificial spin ice arrays. Experimental tunnel magnetoresistance (TMR) data for individual devices in the arrays were collected with an RHK R9 conductive atomic force microscope. TMR hysteresis loops are provided for the two types of behavior observed in the array, termed X and Y devices. Micromagnetic simulations explore the effects of nearest-neighbor and next-nearest-neighbor interactions on a central Y device, including canting, and provide insight into the underlying magnetostatic interactions.

  PublisherDOI

• The role of faceting and elongation on the magnetic anisotropy of magnetite Fe₃O₄ nanocrystals

  Scientific Reports · 2023 · 65 citations

  • Materials science
  • Condensed matter physics
  • Nanotechnology

  Fe₃O₄ nanoparticles are one of the most promising candidates for biomedical applications such as magnetic hyperthermia and theranostics due to their bio-compatibility, structural stability and good magnetic properties. However, much is unknown about the nanoscale origins of the observed magnetic properties of particles due to the dominance of surface and finite size effects. Here we have developed an atomistic spin model of elongated magnetite nanocrystals to specifically address the role of faceting and elongation on the magnetic shape anisotropy. We find that for faceted particles simple analytical formulae overestimate the magnetic shape anisotropy and that the underlying cubic anisotropy makes a significant contribution to the energy barrier for moderately elongated particles. Our results enable a better estimation of the effective magnetic anisotropy of highly crystalline magnetite nanoparticles and is a step towards quantitative prediction of the heating effects of magnetic nanoparticles.

  DOI

• Magnetostatic coupling effects on reversal dynamics

  Journal of Physics D Applied Physics · 2022-03-30 · 1 citations

  articleSenior authorCorresponding

  Abstract The effects of magnetostatic coupling on switching dynamics are investigated for assemblies of patterned disc-shaped magnetic elements using mumax 3 micromagnetic simulations. The arrangements of coupled dots were designed using information about the switching fields and reversal dynamics of isolated dots, as well as the magnitude of the magnetic stray fields they generate. The magnetization dynamics for individual dots was examined during a reversal cascade down a linear chain of dots. The magnetization angle fluctuated much more when neighboring dots have opposite magnetization directions, consistent with a lower energy barrier for reversal. The data were analyzed to differentiate thermal and interaction field effects. While many systems of interacting nanomagnets have been analyzed in terms of empirical models, the dynamical energy barrier approach offers a methodology with a more detailed and physically intuitive way to study both simple systems like the chain and more complex assemblies such as artificial spin ice.

  PublisherDOI

• Angle-dependent switching in a magnetic tunnel junction containing a synthetic antiferromagnet

  Applied Physics Letters · 2022-05-23 · 1 citations

  articleSenior author

  The angle dependence of field-induced switching was investigated in magnetic tunnel junctions with in-plane magnetization and a pinned synthetic antiferromagnet reference layer. The 60 × 90 nm2 elliptical nanopillars had sharp single switches when the field was applied along the major axis of the ellipse, but even with small (20°) deviations, reversal occurred through an intermediate state. The results are interpreted with a model that includes the external applied field and the effective fields due to shape anisotropy and the fringe field of the synthetic antiferromagnet and used to extract the magnetization direction at various points in the magnetoresistance loop. The implications for faster spintronic probabilistic computing devices are discussed.

  PublisherDOI

• STEM Analysis of Vacancies in Magnetite Nanoparticles

  Microscopy and Microanalysis · 2022-07-22 · 1 citations

  article

  An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

  PublisherDOI

• Bipolar Electric-Field Switching of Perpendicular Magnetic Tunnel Junctions through Voltage-Controlled Exchange Coupling

  Nano Letters · 2022 · 45 citations

  • Computer Science
  • Condensed matter physics
  • Materials science

  , is 1 order of magnitude lower than that of the best-reported spin-transfer torque devices. Theoretical results suggest that the electric field induces a ferromagnetic-antiferromagnetic exchange coupling transition of the synthetic antiferromagnetic free layer and generates a fieldlike interlayer exchange coupling torque, which causes the bidirectional magnetization switching of p-MTJs. These results could eliminate the major obstacle in the development of spin memory devices beyond their embedded applications.

  PublisherDOI

• Magnetic Nanoparticles

  2021-01-01

  book-chapter1st authorCorresponding
  PublisherDOI

• Magnetic Nanoparticles

  2021-01-01 · 1 citations

  book-chapter1st authorCorresponding
  PublisherDOI

Recent grants

• Magnetic Nanoparticle Interactions: From Magnetostatics to Exchange

  NSF · $300k · 2008–2012

• NIRT: Single Particle Per Bit Magnetic Information Storage

  NSF · $1.0M · 2005–2010

• Magnetic Control and Optical Imaging of Nanoparticles for Biosensing

  NSF · $300k · 2009–2013

• Magnetic Nanostructures through Metallic Dewetting

  NSF · $352k · 2014–2018

• Superparamagnetic Tunnel Junctions for Logic Devices

  NSF · $360k · 2017–2020

Frequent coauthors

• Dorothy Farrell

  American Association of Colleges of Pharmacy

  51 shared

• Madhur Sachan

  Carnegie Mellon University

  41 shared

• Yuhang Cheng

  The University of Texas at Arlington

  27 shared

• J. A. Borchers

  National Institute of Standards and Technology

  26 shared

• R. W. McCallum

  25 shared

• Keith D. Humfeld

  24 shared

• Y. Ijiri

  Oberlin College

  22 shared

• Sanford A. Asher

  University of Pittsburgh

  21 shared

Labs

• Majetich Research GroupPI

  Not provided

Awards & honors

• Carnegie Science Award (2010)
• NSF National Young Investigator Award (1992)
• Fellow, Institute for Electrical and Electronic Engineers
• Fellow, American Physical Society