'Colossal' Magnetic Effect Under Pressure

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The structure models for F-type and A-type magnetic ordering in manganite in response to pressure. The arrows inside orbitals indicate the spin direction of d electrons.

(PhysOrg.com) -- Millions of people today carry around pocket-sized music players capable of holding thousands of songs, thanks to the discovery 20 years ago of a phenomenon known as the “giant magnetoresistance effect,” which made it possible to pack more data onto smaller and smaller hard drives. Now scientists are on the trail of another phenomenon, called the “colossal magnetoresistance effect” (CMR) which is up to a thousand times more powerful and could trigger another revolution in computing technology.

Understanding, and ultimately controlling, this effect and the intricate coupling between electrical conductivity and magnetism in these materials remains a challenge, however, because of competing interactions in manganites, the materials in which CMR was discovered. In the June 12, 2009, issue of the journal Physical Review Letters, a team of researchers report new progress in using high pressure techniques to unravel the subtleties of this coupling.
To study the magnetic properties of manganites, a form of manganese oxide, the research team, led by Yang Ding of the Carnegie Institution’s High Pressure Synergetic Center (HPSync), applied techniques called x-ray magnetic circular dichroism (XMCD) and angular-dispersive diffraction at the Advanced Photon Source (APS) of Argonne National Laboratory in Illinois. High pressure XMCD is a newly developed technique that uses high-brilliance circularly polarized x-rays to probe the magnetic state of a material under pressures of many hundreds of thousands of atmospheres inside a diamond anvil cell.
The discovery of CMR in manganite compounds has already made manganites invaluable components in technological applications. An example is magnetic tunneling junctions in soon-to-be marketed magnetic random access memory (MRAM), where the tunneling of electrical current between two thin layers of manganite material separated by an electrical insulator depends on the relative orientation of magnetization in the manganite layers. Unlike conventional RAM, MRAM could yield instant-on computers. However, no current theories can fully explain the rich physics, including CMR effects, seen in manganites.
“The challenge is that there are competing interactions in manganites among the electrons that determine magnetic properties,” said Ding. “And the properties are also affected by external stimuli, such as, temperature, pressure, magnetic field, and chemical doping.”
“Pressure has a unique ability to tune the electron interactions in a clean and theoretically transparent manner,” he added. “It is a direct and effective means for manipulating the behavior of electrons and could provide valuable information on the magnetic and electronic properties of manganite systems. But of all the effects, pressure effects have been the least explored.”

The researchers found that when a manganite was subjected to conditions above 230,000 times atmospheric pressure it underwent a transition in which its magnetic ordering changed from a ferromagnetic type (electron spins aligned) to an antiferromagnetic type (electron spins opposed). This transition was accompanied by a non-uniform structural distortion called the Jahn-Teller effect.
“It is quite interesting to observe that uniform compression leads to a non-uniform structural change in a manganite, which was not predicted by theory,” said Ding, “Working with Michel van Veenendaal’s theoretical group at APS, we found that the predominant effect of pressure on this material is to increase the strength of an interaction known as superexchange relative to another known as the double exchange interaction. A consequence of this is that the overall ferromagnetic interactions in the system occur in a plane (two dimensions) rather than in three dimensions, which produces a non-uniform redistribution of electrons. This leads to the structural distortion.”
Another intriguing response of manganite to high pressure revealed by the experiments is that the magnetic transition did not occur throughout the sample at the same time. Instead, it spread incrementally.
“The results imply that even at ambient conditions, the manganite might already have two separate magnetic phases at the nanometer scale, with pressure favoring the growth of the antiferro-magnetic phase at the expense of the ferromagnetic phase,” said coauthor Daniel Haskel, a physicist at Argonne’s APS. “Manipulating phase separation at the nanoscale level is at the very core of nanotechnology and manganites provide an excellent playground to pursue this objective”.
“This work not only displays another interesting emergent phenomenon arising from the interplay between charge, spin, orbital and lattice in a strongly correlated electron system,” commented coauthor Dr. Ho-kwang Mao of Carnegie’s Geophysical Laboratory, Director of HPSync,” but it also manifests the role of pressure in magnetism studies of dense matter.”
More information: Pressure-induced magnetic transition in manganite (La0.75Ca0.25MnO3) Yang Ding, Daniel Haskel, Yuan-Chieh Tseng, Eiji Kaneshita, Michel van Veenendaal, John Mitchell, Stanislav V. Sinogeikin, Vitali Prakapenka, and Ho-kwang Mao, Physical Review Letters, June 2009.

Provided by Carnegie Institution

Star Trek Gadget? 'Tractor Beam' For Cells Developed

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http://www.sciencedaily.com/releases/2007/10/071030095128.htm

ScienceDaily (Oct. 31, 2007) — In a feat that seems like something out of a microscopic version of Star Trek, MIT researchers have found a way to use a “tractor beam” of light to pick up, hold, and move around individual cells and other objects on the surface of a microchip.
The new technology could become an important tool for both biological research and materials research, say Matthew J. Lang and David C. Appleyard, whose work is being published in the journal Lab on a Chip.
The idea of using light beams as tweezers to manipulate cells and tiny objects has been around for at least 30 years. But the MIT researchers have found a way to combine this powerful tool for moving, controlling and measuring objects with the highly versatile world of microchip design and manufacturing.
Optical tweezers, as the technology is known, represent “one of the world's smallest microtools,” says Lang. “Now, we're applying it to building [things] on a chip.”
Says Appleyard, “We've shown that you could merge everything people are doing with optical trapping with all the exciting things you can do on a silicon wafer…There could be lots of uses at the biology-and-electronics interface.”
For example, he said, many people are studying how neurons communicate by depositing them on microchips where electrical circuits etched into the chips monitor their electrical behavior. “They randomly put cells down on a surface, and hope one lands on [or near] a [sensor] so its activity can be measured. With [our technology], you can put the cell right down next to the sensors.” Not only can motions be precisely controlled with the device, but it can also provide very precise measurements of a cell's position.
Optical tweezers use the tiny force of a beam of light from a laser to push around and control tiny objects, from cells to plastic beads. They usually work on a glass surface mounted inside a microscope so that the effects can be observed.
But silicon chips are opaque to light, so applying this technique to them not an obvious move, the researchers say, since the optical tweezers use light beams that have to travel through the material to reach the working surface. The key to making it work in a chip is that silicon is transparent to infrared wavelengths of light - which can be easily produced by lasers, and used instead of the visible light beams.
To develop the system, Lang and Appleyard weren't sure what thickness and surface texture of wafers, the thin silicon slices used to manufacture microchips, would work best, and the devices are expensive and usually available only in quantity. “Being at MIT, where there is such a strength in microfabrication, I was able to get wafers that had been thrown out,” Appleyard says. “I posted signs saying, 'I'm looking for your broken wafers'.”
After testing different samples to determine which worked best, they were able to order a set that were just right for the work. They then tested the system with a variety of cells and tiny beads, including some that were large by the standards of optical tweezer work. They were able to manipulate a square with a hollow center that was 20 micrometers, or millionths of a meter, across - allowing them to demonstrate that even larger objects could be moved and rotated. Other test objects had dimensions of only a few nanometers, or billionths of a meter. Virtually all living cells come in sizes that fall within that nanometer-to-micrometers range and are thus subject to being manipulated by the system.
As a demonstration of the system's versatility, Appleyard says, they set it up to collect and hold 16 tiny living E. coli cells at once on a microchip, forming them into the letters MIT.
Lang is an assistant professor in the Department of Biological Engineering and the Department of Mechanical Engineering. Appleyard is a graduate student in Biological Engineering.
The work was supported by the Biotechnology Training Program of the National Institutes of Health, the W.M. Keck Foundation, and MIT's Lincoln Laboratory.
Adapted from materials provided by Massachusetts Institute Of Technology.

Fausto Intilla
www.oloscience.com

Computer Memory May Leap With Solution To Chemical Mystery

Source:

http://www.sciencedaily.com/releases/2007/10/071019123619.htm

ScienceDaily (Oct. 22, 2007) — A Florida State University researcher has helped solve a scientific mystery that stumped chemists for nearly seven decades. In so doing, his team's findings may lead to the development of more-powerful computer memories and lasers.

Naresh S. Dalal, the Dirac Professor of Chemistry and Biochemistry at FSU, recently collaborated with three colleagues, Jorge Lasave, Sergio Koval and Ricardo Migoni, all of the Universidad Nacional de Rosario in Argentina, to determine why a certain type of crystal known as ammonium dihydrogen phosphate, or ADP, behaves the way it does.
"ADP was discovered in 1938," Dalal said. "It was observed to have some unusual electrical properties that weren't fully understood -- and for nearly 70 years, scientists have been perplexed by these properties. Using the supercomputer at SCRI (FSU's Supercomputer Computations Research Institute), we were able to perform in-depth computational analyses that explained for the very first time what causes ADP to have these unusual properties."
ADP, like many crystals, exhibits an electrical phenomenon known as ferroelectricity. Ferroelectric materials are analogous to magnets in that they maintain a positively charged and a negatively charged pole below a certain temperature that is characteristic for each compound.
"Ferroelectric materials can stay in a given state of charge for a long time -- they retain their charge after the external electrical source is removed," Dalal said. "This has made ADP and other materials like it very useful for storing and transmitting data.
ADP is commonly used in computer memory devices, fiber optic technology, lasers and other electro-optic applications."
What researchers found perplexing about ADP was that it often displays a very different electrical phase -- one known as antiferroelectricity.
"With antiferroelectricity, one layer of molecules in a crystal has a plus and a minus pole, but in the next layer, the charges are reversed," Dalal said. "You see this reversal of charges, layer by layer, throughout the crystal."
Using the supercomputer at SCRI enabled Dalal and his colleagues to perform numerous highly complex calculations that couldn't be duplicated in a laboratory environment. For example, they were able to theoretically alter the angles of ADP's ammonium ions and then measure the effects on the crystal's electrical charge. That approach ultimately led to their solution to the seven-decade mystery.
"We found that the position of the ammonium ions in the compound, as well as the presence of stresses or defects in the crystal, determine whether it behaves in a ferroelectric or antiferroelectric manner," Dalal said.
The team's research is important for two main reasons, Dalal said: "First, this allows us to further understand how to design new materials with both ferroelectric and antiferroelectric properties. Doing so could open new doors for computer memory technology -- and possibly play a role in the development of quantum computers.
"Second, our research opens up new ways of testing materials," Dalal said. "Using supercomputers, we can quickly perform tests to see how materials would react under a variety of conditions. Many such tests can't even be performed in the lab."
A paper "Origin of Antiferroelectricity in NH4H2PO4 from First Principles,"describing Dalal, Lasave and Migoni's findings was published recently in Physical Review Letters.
Adapted from materials provided by Florida State University.

Fausto Intilla

www.oloscience.com

Physicists Discover 'Super Crystals' In A Semiconductor

Source:

http://www.sciencedaily.com/releases/2007/08/070816094653.htm

Science Daily — University of Arizona physicists have discovered that "super crystals" -- crystals which are hundreds to thousands times larger than conventional crystals -- exist in certain organic semiconducting solids.

Pure super-crystalline organic semiconductors will conduct electricity much differently than conventional solids. Super-crystalline semiconductors, for example, could create splashes of current on electrical contacts, even in a uniform electric field, say UA physicist Andrei Lebed and graduate student Si Wu.
Most people understand how liquids freeze as solid crystals when temperatures become cold enough, like water droplets crystallizing into snowflakes or molten glass hardening into solid glass. Snowflakes are homogenous solids formed by a repeating, three-dimensional pattern of molecules that have fixed distances between the repeating molecular units. Solid glass approaches a perfect crystalline pattern, too, after a few hundred years, Lebed said.
Latter 20th-century physicists realized that at low enough temperatures, most liquids that exist in nature become energetically unstable as they solidify. Scientists discovered solids that don't have the commonly known, regular crystalline and glass phases - things like liquid crystals, quasi-crystals and charge-density waves. Charge-density waves are systems that display interesting physics, such as metals becoming insulators.
Understanding the physical nature of a solid phase is one of the most important problems in condensed matter physics, both from a fundamental point of view and from an applications point of view, Lebed and Wu said.
Leading American and Soviet physicists first predicted more than 25 years ago that some organic metals should be made up of "super crystals," Lebed said. Nobel laureate Robert Schrieffer, physicist Lev Gor'kov, who is a pioneer in superconductivity, and other members of the National Academy of Sciences were among the first to predict super crystals.
In super crystals, not only do the patterns of atoms or molecules repeat, there is also a periodically repeating super-structure of plane traps for electrons, Lebed said. "The distance between these plane traps, which are called soliton walls, are typically hundreds to thousands times greater than the distances between the organic molecules." (See the accompanying graphic, above, that illustrates this concept.)
U.S., Soviet and Japanese scientists, including Lebed, collaborated in research to discover the soliton wall superlattices, or super crystals, in organic metals. "Unfortunately, so far no one has discovered super crystals in organic metals," Lebed said.
Lebed and Wu are among the solid state theorists who collaborate with experimentalists in studying other materials that might possibly be super-crystalline.
"Our hopes for a discovery of a long awaited super-crystalline phase were raised after we started to analyze experimental data of James Brooks' group," Lebed said.
Brooks directs condensed matter experiments at the National High Magnetic Field Laboratory in Tallahassee, Fla. Three years ago, physicists there discovered a mysterious solid-state phase in a semiconductor made up of very complicated organic molecules, molecules of perylene (Per) and maleonitriledithiolate (mnt), in high magnetic fields.
"When Wu and I, who are theorists, analyzed the experimental data, what we found was a complete surprise to us," Lebed said. "Our theoretical calculations showed that the only way to explain the appearance of a mysterious high magnetic field state was to suggest that it appears inside a super-crystalline phase."
Lebed and Wu published their study in the July 13 issue of Physical Review Letters.
Future experiments are needed to confirm the theoretical discovery, Lebed added.
If experiments do confirm Lebed's and Wu's results, the novel, exotic solid phase in organic semiconductors promises important technological applications. Such semiconductors will conduct electricity in novels ways. Another striking feature of the super-crystalline semiconductor is that its period and electronic properties might be tuned by changing the strength of the external magnetic field, Lebed said.
Note: This story has been adapted from a news release issued by University of Arizona.

Fausto Intilla

www.oloscience.com