Nav: Home

Supercomputing, experiment combine for first look at magnetism of real nanoparticle

February 02, 2017

Barely wider than a strand of human DNA, magnetic nanoparticles -- such as those made from iron and platinum atoms -- are promising materials for next-generation recording and storage devices like hard drives. Building these devices from nanoparticles should increase storage capacity and density, but understanding how magnetism works at the level of individual atoms is critical to getting the best performance.

However, magnetism at the atomic scale is extremely difficult to observe experimentally, even with the best microscopes and imaging technologies.

That's why researchers working with magnetic nanoparticles at the University of California, Los Angeles (UCLA), and the US Department of Energy's (DOE's) Lawrence Berkeley National Laboratory (Berkeley Lab) approached computational scientists at DOE's Oak Ridge National Laboratory (ORNL) to help solve a unique problem: to model magnetism at the atomic level using experimental data from a real nanoparticle.

"These types of calculations have been done for ideal particles with ideal crystal structures but not for real particles," said Markus Eisenbach, a computational scientist at the Oak Ridge Leadership Computing Facility (OLCF), a DOE Office of Science User Facility located at ORNL.

Eisenbach develops quantum mechanical electronic structure simulations that predict magnetic properties in materials. Working with Paul Kent, a computational materials scientist at ORNL's Center for Nanophase Materials Sciences, the team collaborated with researchers at UCLA and Berkeley Lab's Molecular Foundry to combine world-class experimental data with world-class computing to do something new--simulate magnetism atom by atom in a real nanoparticle.

Using the new data from the research teams on the West Coast, Eisenbach and Kent were able to precisely model the measured atomic structure, including defects, from a unique iron-platinum (FePt) nanoparticle and simulate its magnetic properties on the 27-petaflop Titan supercomputer at the OLCF.

Electronic structure codes take atomic and chemical structure and solve for the corresponding magnetic properties. However, these structures are typically derived from many 2-D electron microscopy or x-ray crystallography images averaged together, resulting in a representative, but not true, 3-D structure.

"In this case, researchers were able to get the precise 3-D structure for a real particle," Eisenbach said. "The UCLA group has developed a new experimental technique where they can tell where the atoms are--the coordinates--and the chemical resolution, or what they are -- iron or platinum."

The results were published on February 2 in Nature.

New and Improved Data

Using a state-of-the-art electron microscope at Berkeley Lab's Molecular Foundry, the Berkley Lab and UCLA teams measured multiple 2-D images from a single FePt nanoparticle at different orientations. UCLA researchers then used GENFIRE, a reconstruction algorithm they developed, to align 2-D images and reconstruct the 3-D atomic positions with cutting-edge precision. The nanoparticle they imaged was synthesized at the University of Buffalo.

"Our technique is called atomic electron tomography (AET) and enables the reconstruction of 3?D atomic structure in materials with 22-picometer precision," said Jianwei (John) Miao of UCLA. A picometer is one-trillionth of a meter. "Like a CT scan, you take multiple images from samples and reconstruct them into a 3-D image."

However, a CT scan is on the order of millimeters for medical diagnoses, whereas the UCLA team's AET technique is measuring atom locations on the order of hundreds of picometers, or the space between atoms.

The UCLA team also developed algorithms to trace the positions of about 6,500 iron and 16,500 platinum atoms, revealing 3-D chemical disorder and other defects at the atomic level.

"We find that the atomic structure is much more complicated than people thought," Miao said. "There were a lot of defects and imperfections in this iron-platinum nanoparticle."

One of the defining characteristics of the FePt nanoparticle is the grouping of iron and platinum atoms into regions or "grains" divided by boundaries. Researchers wanted to understand how magnetism would differ across boundaries given that the ratio and order of iron and platinum atoms changes from grain to grain. Ultimately, magnetism from grain to grain could influence the performance of a magnetic storage device.

"The computational challenge was to demonstrate how magnetism is ordered in the real particle and understand how it changes between boundaries of differently ordered grains," Eisenbach said.

A Supercomputing Milestone

Magnetism at the atomic level is driven by quantum mechanics -- a fact that has shaken up classical physics calculations and called for increasingly complex, first-principle calculations, or calculations working forward from fundamental physics equations rather than relying on assumptions that reduce computational workload.

For magnetic recording and storage devices, researchers are particularly interested in magnetic anisotropy, or what direction magnetism favors in an atom.

"If the anisotropy is too weak, a bit written to the nanoparticle might flip at room temperature," Kent said.

To solve for magnetic anisotropy, Eisenbach and Kent used two computational codes to compare and validate results.

To simulate a supercell of about 1,300 atoms from strongly magnetic regions of the 23,000-atom nanoparticle, they used the Linear Scaling Multiple Scattering (LSMS) code, a first-principles density functional theory code developed at ORNL.

"The LSMS code was developed for large magnetic systems and can tackle lots of atoms," Kent said.

As principal investigator on 2017, 2016, and previous INCITE program awards, Eisenbach has scaled the LSMS code to Titan for a range of magnetic materials projects, and the in-house code has been optimized for Titan's accelerated architecture, speeding up calculations more than 8 times on the machine's GPUs. Exceptionally capable of crunching large magnetic systems quickly, the LSMS code received an Association for Computing Machinery Gordon Bell Prize in high-performance computing achievement in 1998 and 2009, and developments continue to enhance the code for new architectures.

Working with Renat Sabirianov at the University of Nebraska at Omaha, the team also ran VASP, a simulation package that is better suited for smaller atom counts, to simulate regions of about 32 atoms.

"With both approaches, we were able to confirm that the local VASP results were consistent with the LSMS results, so we have a high confidence in the simulations," Eisenbach said.

Computer simulations revealed that grain boundaries have a strong effect on magnetism. "We found that the magnetic anisotropy energy suddenly transitions at the grain boundaries. These magnetic properties are very important," Miao said.

In the future, researchers hope that advances in computing and simulation will make a full-particle simulation possible -- as first-principles calculations are currently too intensive to solve small-scale magnetism for regions larger than a few thousand atoms.

Also, future simulations like these could show how different fabrication processes, such as the temperature at which nanoparticles are formed, influence magnetism and performance.

"There's a hope going forward that one would be able to use these techniques to look at nanoparticle growth and understand how to optimize growth for performance," Kent said.
-end-
Related Publication: Y. Yang, C-C. Chen, M.C. Scott, C. Ophus, R. Xu, A. Pryor, L. Wu, F. Sun, W. Theis, J. Zhou, M. Eisenbach, P.R.C. Kent, R. F. Sabirianov, H. Zeng, P. Ericus, and J. Miao, "Deciphering Chemical Order/Disorder and Material Properties at the Single-atom Level." Nature 542 (2017), doi: 10.1038/nature21042.

Oak Ridge National Laboratory is supported by the US Department of Energy's Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

ORNL's Center for Nanophase Materials Sciences and Berkeley Lab's Molecular Foundry are DOE Office of Science User Facilities.

DOE/Oak Ridge National Laboratory

Related Nanoparticle Articles:

Nanoparticle chomps away plaques that cause heart attacks
Michigan State University and Stanford University scientists have invented a nanoparticle that eats away -- from the inside out -- portions of plaques that cause heart attacks.
Nanoparticle orientation offers a way to enhance drug delivery
MIT engineers have shown that they can enhance the performance of drug-delivery nanoparticles by controlling an inherent trait of chemical structures, known as chirality -- the 'handedness' of the structure.
Modeling a model nanoparticle
New research from the University of Pittsburgh Swanson School of Engineering introduces the first universal adsorption model that accounts for detailed nanoparticle structural characteristics, metal composition and different adsorbates, making it possible to not only predict adsorption behavior on any metal nanoparticles but screen their stability, as well.
Nanoparticle therapy targets lymph node metastases
Metastasis, in which cancer cells break free from the primary tumor and form tumors at other sites, worsens the prognosis for many cancer patients.
Nanoparticle computing takes a giant step forward
Inspired by how cellular membranes process biological information, we developed a platform for constructing nanoparticle circuits on a supported lipid bilayer.
Nanoparticle breakthrough in the fight against cancer
A recent study, affiliated with South Korea's Ulsan National Institute of Science and Technology (UNIST) has introduced a novel targeted drug delivery system in the fight against cancer.
Ultra-sensitive sensor with gold nanoparticle array
Scientists from the University of Bath (UK) and Northwestern University (USA) have developed a new type of sensor platform using a gold nanoparticle array, which is 100 times more sensitive than current similar sensors.
Illuminating nanoparticle growth with X-rays
Ultrabright X-rays at NSLS-II reveal key details of catalyst growth for more efficient hydrogen fuel cells.
Chemists create new quasicrystal material from nanoparticle building blocks
Brown University researchers have discovered a new type of quasicrystal, a class of materials whose existence was thought to be impossible until the 1980s.
New nanoparticle superstructures made from pyramid-shaped building blocks
In research that may help bridge the divide between the nano and the macro, Brown University chemists have used pyramid-shaped nanoparticles to create what might be the most complex macroscale superstructure ever assembled.
More Nanoparticle News and Nanoparticle Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Listen Again: Reinvention
Change is hard, but it's also an opportunity to discover and reimagine what you thought you knew. From our economy, to music, to even ourselves–this hour TED speakers explore the power of reinvention. Guests include OK Go lead singer Damian Kulash Jr., former college gymnastics coach Valorie Kondos Field, Stockton Mayor Michael Tubbs, and entrepreneur Nick Hanauer.
Now Playing: Science for the People

#562 Superbug to Bedside
By now we're all good and scared about antibiotic resistance, one of the many things coming to get us all. But there's good news, sort of. News antibiotics are coming out! How do they get tested? What does that kind of a trial look like and how does it happen? Host Bethany Brookeshire talks with Matt McCarthy, author of "Superbugs: The Race to Stop an Epidemic", about the ins and outs of testing a new antibiotic in the hospital.
Now Playing: Radiolab

Dispatch 6: Strange Times
Covid has disrupted the most basic routines of our days and nights. But in the middle of a conversation about how to fight the virus, we find a place impervious to the stalled plans and frenetic demands of the outside world. It's a very different kind of front line, where urgent work means moving slow, and time is marked out in tiny pre-planned steps. Then, on a walk through the woods, we consider how the tempo of our lives affects our minds and discover how the beats of biology shape our bodies. This episode was produced with help from Molly Webster and Tracie Hunte. Support Radiolab today at Radiolab.org/donate.