LANL RESEARCH ON MAGNETIZING A SEMICONDUCTOR MATERIAL WITH LIGHT

Photo Credit:  U.S. Government/Wikimedia 

FROM:  LOS ALAMOS NATIONAL LABORATORY 

Flipping the Switch on Magnetism in Strontium Titanate

Semiconductor material can be magnetized with light, suggesting new technology opportunities

LOS ALAMOS, N.M., March 27, 2013—Interest in oxide-based semiconductor electronics has exploded in recent years, fueled largely by the ability to grow atomically precise layers of various oxide materials.

One of the most important materials in this burgeoning field is strontium titanate (SrTiO3), a nominally nonmagnetic wide-bandgap semiconductor, and researchers at Los Alamos National Laboratory have found a way to magnetize this material using light, an effect that persists for hours at a time.

“One doesn’t normally think of this material as being able to support magnetism. It’s supposed to be useful – but magnetically uninteresting – stuff. So when we started shining light on it and saw what appeared to be extremely long-lived magnetic signals – that persisted for hours even after we turned the light off – it came as quite a surprise,” said Scott Crooker, lead scientist on the project at Los Alamos.

Studies of strontium titanate’s electrical and optical properties abound, it’s not a new material – in fact it was marketed in the 1950s and ‘60s as a “faux diamond” product before cubic zirconium gained popularity. Though often used in industry for its robust dielectric properties, its potential magnetic properties were less well understood. A renewed interest in SrTiO3 was recently sparked by observations of an unexpected and emergent magnetization in strontium titanate-based structures.

“There’ve been tantalizing hints in recent years that there might be more to SrTiO3 than originally thought. When layered with other ‘nominally non-magnetic’ oxides, a handful of recent experiments around the world have shown not only superconductivity but also an unexpected magnetism. So that piqued our interest in this material,” Crooker said.

"This is really something completely new in oxide materials like these - the ability to write permanent magnetic patterns into an otherwise non-magnetic material. The challenge will be to properly understand how and why this works, and to increase the temperature at which it can be done. The exciting possibility is to potentially use this to store data in some way,” said collaborator Chris Leighton of the University of Minnesota.

In a paper published this week in Nature Materials, Crooker and collaborators illustrated a new aspect to the nature of magnetism in strontium titanate, reporting the observation of an optically induced and persistent magnetization in crystals of SrTiO3 when they are slightly oxygen-deficient.

Using samples prepared by collaborators in Leighton’s group, Crooker and Los Alamos colleagues William Rice and Joe Thompson used magnetic circular dichroism spectroscopy and also SQUID magnetometry to show that circularly polarized light can induce an extremely long-lived magnetic moment in SrTiO3 at zero applied magnetic field.

These signals appear below 18 Kelvin, persist for hours below 10 K, and can be controlled in both magnitude and sign via the circular polarization and wavelength of blue/green light in the range spanning 400-500 nm. As such, magnetic patterns can be “written” into SrTiO3, and subsequently read out, using light alone. These effects occur only in crystals containing oxygen vacancies, revealing a detailed interplay between magnetism, lattice defects and light in an archetypal complex oxide material.

This work was funded by Laboratory Directed Research and Development Exploratory Research at Los Alamos National Laboratory under the auspices of the US DOE Office of Science.

HERBEVORES AND FERTILIZER CAN INCREASE PLANT BIODIVERSITY

FROM:  NATIONAL SCIENCE FOUNDATION 
Herbivores + light = more plant biodiversity in fertilized grasslands
Research on six continents shows that it all comes down to the light
It all comes down to the light. At least in plant species diversity in fertilized grasslands.

Fertilizing by humans and plant-eating by herbivores can combine to benefit plant biodiversity--if enough light still reaches the ground, according to results of a study by ecologists Elizabeth Borer and Eric Seabloom of the University of Minnesota and colleagues.

The findings, published this week in the online edition of the journal Nature, are important in a world where humans are changing both where herbivores are found and the supply of plant nutrients such as nitrogen, phosphorus and potassium.

Enter the Nutrient Network

To conduct the study, Borer and Seabloom enlisted the help of the Nutrient Network, or NutNet, an experiment they and other researchers began as a way to understand how grasslands around the world respond to changing environments.

NutNet scientists at 40 sites set up plots with and without added fertilizer and with and without fences to keep out local herbivores such as deer, kangaroos, sheep or zebras.

The research took place in the United States, Canada, China, Australia, Switzerland, United Kingdom, South Africa, Tanzania, Germany and Argentina.

The scientists' hypothesis was that grassland plant species losses caused by eutrophication (overfertilization) could be offset by the increased light availability that results when taller plants are munched down by herbivores like deer and sheep.

This "trimming" by herbivores ultimately lets in more light, fueling increased plant growth.

The experiment, replicated in 40 grasslands on six continents, demonstrated that the researchers had it right.

New explanation for grassland plant biodiversity

"Global patterns of biodiversity have largely defied explanation due to many interacting, local driving forces," says Henry Gholz, a program director in the National Science Foundation's (NSF) Division of Environmental Biology, which funded coordination of the research, along with the many institutions involved.

"These results show that grassland biodiversity is likely largely determined by the offsetting influences of nutrition and grazing on light capture by plants," Gholz says.

In the study, the ecologists measured the amount of plant material, the light reaching the ground and the number of species of plants in the plots.

When the scientists compared results across the sites, they found that fertilizer both reduced the number of plant species in the plots and favored those that were faster-growing. Species less able to tolerate a lack of light in shady conditions were literally overshadowed by their faster-growing neighbors.

So there were fewer kinds of plants, but taller-growing ones.

An herbivore is an herbivore is an herbivore?

In both fertilized and unfertilized plots, removal of vegetation by herbivores increased the amount of light reaching the ground. The taller plants were eaten by the herbivores. Then plant species diversity increased.

The results were the case whether the grassland was in Minnesota, the United Kingdom or Tanzania, and whether the herbivores were rabbits, sheep or elephants.

"This suggests that these effects dovetail with changes in light availability at the ground level," says Borer. "That appears to be a big factor in maintaining or losing biodiversity in grasslands."

Light a key piece of the puzzle

In short, Borer says, "where we see a change in light, we see a change in biodiversity" for the better.

The findings offer important insights into how humans are affecting prairies, savannas, alpine meadows and other grasslands by adding fertilizers.

In showing how fertilization, grazing, light availability and biodiversity are linked, scientists are closer to understanding grassland ecosystems in a changing world.

-- Cheryl Dybas
Investigators
Elizabeth Borer
Related Institutions/Organizations
University of Minnesota-Twin Cities