Tag Archives: iron oxide

Creating multiferroic material at room temperature

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A Sept. 23, 2016 news item on ScienceDaily describes some research from Cornell University (US),

Multiferroics — materials that exhibit both magnetic and electric order — are of interest for next-generation computing but difficult to create because the conditions conducive to each of those states are usually mutually exclusive. And in most multiferroics found to date, their respective properties emerge only at extremely low temperatures.

Two years ago, researchers in the labs of Darrell Schlom, the Herbert Fisk Johnson Professor of Industrial Chemistry in the Department of Materials Science and Engineering, and Dan Ralph, the F.R. Newman Professor in the College of Arts and Sciences, in collaboration with professor Ramamoorthy Ramesh at UC Berkeley, published a paper announcing a breakthrough in multiferroics involving the only known material in which magnetism can be controlled by applying an electric field at room temperature: the multiferroic bismuth ferrite.

Schlom’s group has partnered with David Muller and Craig Fennie, professors of applied and engineering physics, to take that research a step further: The researchers have combined two non-multiferroic materials, using the best attributes of both to create a new room-temperature multiferroic.

Their paper, “Atomically engineered ferroic layers yield a room-temperature magnetoelectric multiferroic,” was published — along with a companion News & Views piece — Sept. 22 [2016] in Nature. …

A Sept. 22, 2016 Cornell University news release by Tom Fleischman, which originated the news item, details more about the work (Note: A link has been removed),

The group engineered thin films of hexagonal lutetium iron oxide (LuFeO3), a material known to be a robust ferroelectric but not strongly magnetic. The LuFeO3 consists of alternating single monolayers of lutetium oxide and iron oxide, and differs from a strong ferrimagnetic oxide (LuFe2O4), which consists of alternating monolayers of lutetium oxide with double monolayers of iron oxide.

The researchers found, however, that they could combine these two materials at the atomic-scale to create a new compound that was not only multiferroic but had better properties that either of the individual constituents. In particular, they found they need to add just one extra monolayer of iron oxide to every 10 atomic repeats of the LuFeO3 to dramatically change the properties of the system.

That precision engineering was done via molecular-beam epitaxy (MBE), a specialty of the Schlom lab. A technique Schlom likens to “atomic spray painting,” MBE let the researchers design and assemble the two different materials in layers, a single atom at a time.

The combination of the two materials produced a strongly ferrimagnetic layer near room temperature. They then tested the new material at the Lawrence Berkeley National Laboratory (LBNL) Advanced Light Source in collaboration with co-author Ramesh to show that the ferrimagnetic atoms followed the alignment of their ferroelectric neighbors when switched by an electric field.

“It was when our collaborators at LBNL demonstrated electrical control of magnetism in the material that we made that things got super exciting,” Schlom said. “Room-temperature multiferroics are exceedingly rare and only multiferroics that enable electrical control of magnetism are relevant to applications.”

In electronics devices, the advantages of multiferroics include their reversible polarization in response to low-power electric fields – as opposed to heat-generating and power-sapping electrical currents – and their ability to hold their polarized state without the need for continuous power. High-performance memory chips make use of ferroelectric or ferromagnetic materials.

“Our work shows that an entirely different mechanism is active in this new material,” Schlom said, “giving us hope for even better – higher-temperature and stronger – multiferroics for the future.”

Collaborators hailed from the University of Illinois at Urbana-Champaign, the National Institute of Standards and Technology, the University of Michigan and Penn State University.

Here is a link and a citation to the paper and to a companion piece,

Atomically engineered ferroic layers yield a room-temperature magnetoelectric multiferroic by Julia A. Mundy, Charles M. Brooks, Megan E. Holtz, Jarrett A. Moyer, Hena Das, Alejandro F. Rébola, John T. Heron, James D. Clarkson, Steven M. Disseler, Zhiqi Liu, Alan Farhan, Rainer Held, Robert Hovden, Elliot Padgett, Qingyun Mao, Hanjong Paik, Rajiv Misra, Lena F. Kourkoutis, Elke Arenholz, Andreas Scholl, Julie A. Borchers, William D. Ratcliff, Ramamoorthy Ramesh, Craig J. Fennie, Peter Schiffer et al. Nature 537, 523–527 (22 September 2016) doi:10.1038/nature19343 Published online 21 September 2016

Condensed-matter physics: Multitasking materials from atomic templates by Manfred Fiebig. Nature 537, 499–500  (22 September 2016) doi:10.1038/537499a Published online 21 September 2016

Both the paper and its companion piece are behind a paywall.

Imaging and treating artherosclerosis with a nanoparticle

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For anyone concerned about atherosclerosis (build up of plaque in the arteries) and who doesn’t need immediate assistance, this is encouraging news. A March 14, 2016 news item on ScienceDaily announces research into a nanoparticle that could both image and treat the condition,

Atherosclerosis, a disease in which plaque builds up inside arteries, is a prolific and invisible killer, but it may soon lose its ability to hide in the body and wreak havoc. Scientists have now developed a nanoparticle that functionally mimics nature’s own high-density lipoprotein (HDL). The nanoparticle can simultaneously light up and treat atherosclerotic plaques that clog arteries. Therapy with this approach could someday help prevent deadly heart attacks and strokes.

A March 13, 2016 American Chemical Society (ACS) news release (also on EurekAlert), which originated the news item, expands on the theme,

The researchers present their work today [March 13, 2016] at the 251st National Meeting & Exposition of the American Chemical Society (ACS). …

“Other researchers have shown that if you isolate HDL components from donated blood, reconstitute them and inject them into animals, there seems to be a therapeutic effect,” says Shanta Dhar, Ph.D. “However, with donors’ blood, there is the chance of immunological rejection. This technology also suffers scale-up challenges. Our motivation was to avoid immunogenic factors by making a synthetic nanoparticle which can functionally mimic HDL. At the same time, we wanted a way to locate the synthetic particles.”

Current detection strategies often fail to identify dangerous plaques, which can clog arteries over time or break off from arterial walls and block blood flow, causing a heart attack or stroke. Magnetic resonance imaging (MRI) offers a potential approach for plaque visualization, but requires the use of a contrast agent to show the atherosclerotic plaques clearly. But the potential for harmful immune reactions still exists with the use of donor-derived HDL.

Beyond imaging, there is a therapeutic aspect of using HDL. HDL is widely known as “good” cholesterol because of its ability to pull low-density lipoprotein, or “bad” cholesterol, out of plaques. This process shrinks the plaques, making them less likely to clog arteries or break apart.

To simultaneously identify and treat atherosclerosis without triggering an immune response, Dhar and Bhabatosh Banik, Ph.D., a postdoctoral fellow in her lab, created an MRI-active HDL mimic. The researchers, who are at the University of Georgia, Athens, had previously built synthetic HDL particles lacking a contrast agent. These particles lowered levels of total cholesterol and triglycerides in mice.

“The key challenge, then, was designing the contrast agent,” Banik says. “It took time to figure out the optimal lipophilicity and solubility.” The contrast agent, iron oxide, needs to be encapsulated in the synthetic lipoparticle’s hydrophobic core to provide the brightest possible signal. Eventually, the researchers hit on the right chemical combination — iron oxide with a fatty surface coating — for optimal particle encapsulation. They successfully visualized the contrast agent using MRI in cell studies.

The researchers are applying their synthetic nanoparticle to distinguish between unstable plaques and stationary ones. To do this, Dhar targeted the new MRI-active HDL mimics to macrophages, which are white blood cells that, along with lipids and cholesterol, make up atherosclerotic plaques.

The researchers targeted macrophages by decorating the nanoparticles’ surfaces with a molecule that selectively binds to macrophages. The team observed that the nanoparticles were engulfed by these white blood cells. “Then, when the macrophages ruptured, which is a sign of an unstable plaque, the cells spit out the nanoparticles, causing the MRI signal to change in a detectable fashion,” Banik says.

Dhar says her lab is now using MRI to study how well the particles light up and treat plaques in animals, and she hopes to begin clinical trials within two years. [emphasis mine]

Good luck to the researchers!

Rust shocks scientists

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Researchers at the Vienna University of Technology have made a surprising discovery about a well established atomic structure on magnetite surfaces (rust), according to a Dec. 4, 2014 news item on ScienceDaily,

Magnetite (or Fe3O4) is an elaborate kind of rust — a regular lattice of oxygen and iron atoms. But this material plays an increasingly important role as a catalyst, in electronic devices and in medical applications.

Scientists at the Vienna University of Technology have now shown that the atomic structure of the magnetite surface, which everybody had assumed to be well-established, has in fact been wrong all along. The properties of magnetite are governed by missing iron atoms in the sub-surface layer. “It turns out that the surface of Fe3O4 is not Fe3O4 at all, but rather Fe11O16,” says Professor Ulrike Diebold, head of the metal-oxide-research group at TU Wien (Vienna). The new findings have now been published in the journal Science.

A Dec. 5, 2014 Vienna University of Technology press release, which despite the date appears to have originated the news item, describes the process which resulted in the researchers changing how they thought about the surface chemistry and physics they were examining,

Perhaps the most surprising property of the magnetite surface is that single atoms placed on the surface, for instance gold or palladium, stay perfectly in place instead of balling up and forming a nanoparticle. This effect makes the surface an extremely efficient catalyst for chemical reactions – but nobody had ever been able to tell why magnetite behaves that way. “Also, Fe3O4-based electronics never function quite as well as they should”, says Gareth Parkinson (TU Wien). “Because materials interact with their environment through the surface, it’s really important to understand the structure of the surface and why it forms.”

Very often, the properties of metal oxides depend on oxygen vacancies in the topmost atomic layers. Depending on the environment, some oxygen atoms on the surface may be missing. This can dramatically influence the electronic properties of the material. “Everyone in our community thinks about missing oxygen atoms. That is why it took us quite a while to figure out that it is in fact missing iron atoms that do the trick”, says Gareth Parkinson.

It’s not the oxygen, it’s the metal

Developing this new understanding, further the scientists proposed a new theory (from the press release),

Instead of a fixed structure of metal atoms with built-in oxygen atoms, one rather has to think of iron-oxides as a well-defined oxygen structure with little metal atoms hiding inside. Directly below the outermost atomic layer, the crystal structure is rearranged and some iron atoms are absent.

It is precisely above such places of missing iron atoms that other metal atoms attach. These iron-vacancy-sites are regularly spaced, and so there is always some well-defined distance between gold or palladium atoms attaching to the surface. This explains why magnetite surfaces prevent these atoms from forming clusters.

The idea of completely re-thinking the crystal structure of magnetite was rather bold, and therefore the scientists analysed their theory very carefully. Quantum simulations were carried out on large supercomputers to show that the proposed structure was indeed physically reasonable. After that, electron diffraction measurements were done together with researchers at the University of Erlangen-Nuremberg, Germany.

“By scattering slow electrons at surfaces, one can measure how well the actual crystal structure of the material agrees with a theoretical model”, says Ulrike Diebold. This agreement is quantified by the so-called “R-value”. “For very well-known structures, one may achieve an R-value of 0.1 or 0.15. For magnetite, nobody had ever managed to get anything better than 0.3, and people said it just could not be done.” But the new magnetite structure with missing iron atoms agrees very well with the experimental data, yielding an R-value of 0.125.

This new theory may apply to more than one metal oxide (from the press release),

Metal oxides are widely known to be technologically important but extremely complicated to describe. “Our results show that there is no need to despair. Metal oxides can be modelled quite accurately after all, but maybe not in the way one might expect at first glance”, says Gareth Parkinson. The scientists expect that their findings do not just apply to iron oxide but also to oxides of cobalt, manganese or nickel. Re-thinking their crystal structures could possibly boost iron-oxide research in many areas and lead to applications in chemical catalysis, electronics or medicine.

Here’s a link to and a citation for the research paper,

Subsurface cation vacancy stabilization of the magnetite (001) surface by R. Bliem, E. McDermott, P. Ferstl, M. Setvin, O. Gamba, J. Pavelec, M. A. Schneider, M. Schmid, U. Diebold, P. Blaha, L. Hammer, and G. S. Parkinson. Science 5 December 2014: Vol. 346 no. 6214 pp. 1215-1218 DOI: 10.1126/science.1260556

This paper is behind a paywall.

Insomniac iron oxide (rust) electrons and environmentally friendly semiconductors

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The Sept. 7, 2012 news item by Lynn Yarris for physorg.com highlights some research on rust being conducted (pun intended) at Lawrence Berkeley National Laboratory (Berkeley Labs).

Rust – iron oxide – is a poor conductor of electricity, which is why an electronic device with a rusted battery usually won’t work. Despite this poor conductivity, an electron transferred to a particle of rust will use thermal energy to continually move or “hop” from one atom of iron to the next. Electron mobility in iron oxide can hold huge significance for a broad range of environment- and energy-related reactions, including reactions pertaining to uranium in groundwater and reactions pertaining to low-cost solar energy devices.  …

“We believe this work is the starting point for a new area of time-resolved geochemistry that seeks to understand chemical reaction mechanisms by making various kinds of movies that depict in real time how atoms and electrons move during reactions,” says Benjamin Gilbert, a geochemist with Berkeley Lab’s Earth Sciences Division and a co-founder of the Berkeley Nanogeoscience Center who led this research. “Using ultrafast pump-probe X-ray spectroscopy, we were able to measure the rates at which electrons are transported through spontaneous iron-to-iron hops in redox-active iron oxides. Our results showed that the rates depend on the structure of the iron oxide and confirmed that certain aspects of the current model of electron hopping in iron oxides are correct.”

The news item provides a wealth of detail about electron hopping and iron oxide but I was most intrigued by future applications,

Katz [Jordan Katz, the lead author, now with Denison University]  is excited about the application of these results to finding ways to use iron oxide for solar energy collection and conversion.

“Iron oxide is a semiconductor that is abundant, stable and environmentally friendly, and its properties are optimal for absorption of sunlight,” he says. “To use iron oxide for solar energy collection and conversion, however, it is critical to understand how electrons are transferred within the material, which when used in a conventional design is not highly conductive. Experiments such as this will help us to design new systems with novel nanostructured architectures that promote desired redox reactions, and suppress deleterious reactions in order to increase the efficiency of our device.”

I find rust quite attractive although, admittedly, very irritating at times. I have never before considered the possibility it might prove useful nor had I realized that it never rests (sleeps).