Nanotechnologists at UT make orientation of magnetism adjustable in new materials

New material offers potential for data storage and spintronics applications

 

Nanotechnologists at the UT research institute MESA+ are now able to create materials in which they can influence and precisely control the orientation of the magnetism at will. An interlayer just 0.4 nanometres thick is the key to this success. 

The materials present a range of interesting possibilities, such as a new way of creating computer memory as well as spintronics applications – a new form of electronics that works on the basis of magnetism instead of electricity. The research was published today in the leading scientific journal Nature Materials. 

Nanotechnologists at the University of Twente are specialized in creating new materials. Thanks to the top-level facilities at the MESA+ NanoLab they are able to combine materials as they wish, with the ability to control the material composition down to atom level. In particular, they specialize in creating materials composed of extremely thin layers, sometimes just one atom thick. 

Computer memory

 

In research published today in the scientific journal Nature Materials, they show their ability to create new materials within which they can precisely and locally control the orientation of the magnetism. This opens the way to new possibilities of creating computer memory. Moreover, this method of creating materials is interesting for spintronics, a new form of electronics that does not utilize the movement of charges but instead the magnetic properties of a material. This not only makes electronics very fast and efficient, but also allows them to be produced in extremely small dimensions.  

Interlayer

 

In the course of this research the scientists stacked up various thin layers of Perovskite materials. By placing an extremely thin interlayer of just 0.4 nanometres between the layers (a nanometre is a million times smaller than a millimetre), it becomes possible to influence the orientation of the magnetism in the individual Perovskite layers as desired, whereby the orientation of the magnetism in the bottom layer, for instance, is perpendicular to that of the layer above. By varying the location where the interlayer is applied, it becomes possible to select the local orientation of the magnetism anywhere in the material. This is an essential property for new forms of computer memory and for spintronics applications. This effect was already known for much thicker layers, but never before had researchers demonstrated that the orientation of the magnetism can be controlled so precisely with extremely thin layers, too. 

Research

 

The research has been conducted by scientists of the MESA+ research groupInorganic Materials Science in collaboration with colleagues from other institutes, including the University of Antwerp (Belgium), the University of British Columbia (Canada) and TU Wien (Vienna, Austria). Within the research project, the Twente-based researchers were responsible for coordination and for creating the materials. The colleague researchers from Antwerp visualized the materials and were able to image even the smallest atoms in the material. The Canadian researchers created a magnetic cross-section of the material, while the Austrian researchers handled the theoretical calculations.

The research is published under the title ‘Controlled lateral anistropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling’ by Z. Liao, M. Huijben, Z. Zhong, N. Gauquelin, S. Macke, R. J. Green, S. Van Aert, J. Verbeeck, G. Van Tendeloo, K. Held, G. A. Sawatzky, G. Koster and G. Rijnders.

MESA+ Institute for Nanotechnology

Nanoscale cavity strongly links quantum particles

Scientists have created a crystal structure that boosts the interaction between tiny bursts of light and individual electrons, an advance that could be a significant step toward establishing quantum networks in the future.

Today’s networks use electronic circuits to store information and optical fibers to carry it, and quantum networks may benefit from a similar framework. Such networks would transmit qubits – quantum versions of ordinary bits – from place to place and would offer unbreakable security for the transmitted information. But researchers must first develop ways for qubits that are better at storing information to interact with individual packets of light called photons that are better at transporting it, a task achieved in conventional networks by electro-optic modulators that use electronic signals to modulate properties of light.

Now, researchers in the group of Edo Waks, a fellow at JQI and an Associate Professor in the Department of Electrical and Computer Engineering at the University of Maryland, have struck upon an interface between photons and single electrons that makes progress toward such a device. By pinning a photon and an electron together in a small space, the electron can quickly change the quantum properties of the photon and vice versa. The research was reported online Feb. 8 in the journal Nature Nanotechnology.

“Our platform has two major advantages over previous work,” says Shuo Sun, a graduate student at JQI and the first author of the paper. “The first is that the electronic qubit is integrated on a chip, which makes the approach very scalable. The second is that the interactions between light and matter are fast. They happen in only a trillionth of a second – 1,000 times faster than previous studies.”

CONSTRUCTING AN INTERFACE

The new interface utilizes a well-studied structure known as a photonic crystal to guide and trap light. These crystals are built from microscopic assemblies of thin semiconductor layers and a grid of carefully drilled holes. By choosing the size and location of the holes, researchers can control the properties of the light traveling through the crystal, even creating a small cavity where photons can get trapped and bounce around.

”These photonic crystals can concentrate light in an extremely small volume, allowing devices to operate at the fundamental quantum limit where a single photon can make a big difference,” says Waks.

The results also rely on previous studies of how small, engineered nanocrystals called quantum dots can manipulate light. These tiny regions behave as artificial atoms and can also trap electrons in a tight space. Prior work from the JQI group showed that quantum dots could alter the properties of many photons and rapidly switch the direction of a beam of light.

The new experiment combines the light-trapping of photonic crystals with the electron-trapping of quantum dots. The group used a photonic crystal punctuated by holes just 72 nanometers wide, but left three holes undrilled in one region of the crystal. This created a defect in the regular grid of holes that acted like a cavity, and only those photons with only a certain energy could enter and leave.

Inside this cavity, embedded in layers of semiconductors, a quantum dot held one electron. The spin of that electron – a quantum property of the particle that is analogous to the motion of a spinning top – controlled what happened to photons injected into the cavity by a laser. If the spin pointed up, a photon entered the cavity and left it unchanged. But when the spin pointed down, any photon that entered the cavity came out with a reversed polarization – the direction that light’s electric field points. The interaction worked the opposite way, too: A single photon prepared with a certain polarization could flip the electron’s spin.

Both processes are examples of quantum switches, which modify the qubits stored by the electron and photon in a controlled way. Such switches will be the coin of the realm for proposed future quantum computers and quantum networks.

QUANTUM NETWORKING

Those networks could take advantage of the strengths that photons and electrons offer as qubits. In the future, for instance, electrons could be used to store and process quantum information at one location, while photons could shuttle that information between different parts of the network.

Such links could enable the distribution of entanglement, the enigmatic connection that groups of distantly separated qubits can share. And that entanglement could enable other tasks, such as performing distributed quantum computations, teleporting qubits over great distances or establishing secret keys that two parties could use to communicate securely.

Before that, though, Sun says that the light-matter interface that he and his colleagues have created must create entanglement between the electron and photon qubits, a process that will require more accurate measurements to definitively demonstrate.

“The ultimate goal will be integrating photon creation and routing onto the chip itself,” Sun says. “In that manner we might be able to create more complicated quantum devices and quantum circuits.”

In addition to Waks and Sun, the paper has two additional co-authors: Glenn Solomon, a JQI fellow, and Hyochul Kim, a post-doctoral researcher in the Department of Electrical and Computer Engineering at the University of Maryland.

Joint Quantum Institute

Darwin on a chip

UT researchers develop (r)evolutionary circuits

Researchers of the MESA+ Institute for Nanotechnology and the CTIT Institute for ICT Research at the University of Twente in The Netherlands have demonstrated working electronic circuits that have been produced in a radically new way, using methods that resemble Darwinian evolution. The size of these circuits is comparable to the size of their conventional counterparts, but they are much closer to natural networks like the human brain. The findings promise a new generation of powerful, energy-efficient electronics, and have been published in the leading British journal Nature Nanotechnology.

One of the greatest successes of the 20th century has been the development of digital computers. During the last decades these computers have become more and more powerful by integrating ever smaller components on silicon chips. However, it is becoming increasingly hard and extremely expensive to continue this miniaturisation. Current transistors consist of only a handful of atoms. It is a major challenge to produce chips in which the millions of transistors have the same characteristics, and thus to make the chips operate properly. Another drawback is that their energy consumption is reaching unacceptable levels. It is obvious that one has to look for alternative directions, and it is interesting to see what we can learn from nature. Natural evolution has led to powerful ‘computers’ like the human brain, which can solve complex problems in an energy-efficient way. Nature exploits complex networks that can execute many tasks in parallel.

Moving away from designed circuits

The approach of the researchers at the University of Twente is based on methods that resemble those found in Nature. They have used networks of gold nanoparticles for the execution of essential computational tasks. Contrary to conventional electronics, they have moved away from designed circuits. By using 'designless' systems, costly design mistakes are avoided. The computational power of their networks is enabled by applying artificial evolution. This evolution takes less than an hour, rather than millions of years. By applying electrical signals, one and the same network can be configured into 16 different logical gates. The evolutionary approach works around - or can even take advantage of - possible material defects that can be fatal in conventional electronics.

Powerful and energy-efficient

It is the first time that scientists have succeeded in this way in realizing robust electronics with dimensions that can compete with commercial technology. According to prof. Wilfred van der Wiel, the realized circuits currently still have limited computing power. “But with this research we have delivered proof of principle: demonstrated that our approach works in practice. By scaling up the system, real added value will be produced in the future. Take for example the efforts to recognize patterns, such as with face recognition. This is very difficult for a regular computer, while humans and possibly also our circuits can do this much better."  Another important advantage may be that this type of circuitry uses much less energy, both in the production, and during use. The researchers anticipate a wide range of applications, for example in portable electronics and in the medical world.

University of Twente

Argonne-UChicago researchers work to annihilate nanoscale defects in semiconductors

Researchers from the University of Chicago and Argonne use the supercomputing resources at the Argonne Leadership Computing Facility to predict the path molecules must follow to find defect-free states. They designed a process that delivers industry-standard nanocircuitry that can be scaled down to smaller densities without defects.
Courtesy of Argonne National Laboratory

Target dates are critical when the semiconductor industry adds small, enhanced features to consumer devices by integrating advanced materials onto the surfaces of computer chips. Missing a target means postponing a device’s release, which could cost a company millions of dollars or the loss of competitiveness and an entire industry.

But meeting target dates can be challenging because the final integrated devices, which include billions of transistors, must be flawless—less than one defect per 100 square centimeters.

Researchers at the University of Chicago and Argonne National Laboratory, led by Profs. Juan de Pablo and Paul Nealey, may have found a way for the semiconductor industry to hit miniaturization targets on time and without defects.

To make microchips, de Pablo and Nealey’s technique includes creating patterns on semiconductor surfaces that allow block copolymer molecules to self-assemble into specific shapes, but thinner and at much higher densities than those of the original pattern. The researchers can then use a lithography technique to create nano-trenches where conducting wire materials can be deposited.

This is a stark contrast to the industry practice of using homo-polymers in complex “photoresist” formulations, where researchers have “hit a wall,” unable to make the material smaller.

Before they could develop their new fabrication method, however, de Pablo and Nealey needed to understand exactly how block copolymers self-assemble when coated onto a patterned surface—their concern being that certain constraints cause copolymer nanostructures to assemble into undesired metastable states. To reach the level of perfection demanded to fabricate high-precision nanocircuitry, the team had to eliminate some of these metastable states.

Using the Argonne Leadership Computing Facility, UChicago and Argonne researchers have found a way miniaturize microchip components using a technique producing zero defects. This advance will allow semiconductor manufacturers to meet miniaturization target dates to produce smaller components with added functionality for consumer devices.
Courtesy of Argonne National Laboratory

To imagine how block copolymers assemble, it may be helpful to picture an energy landscape consisting of mountains and valleys, in which some valleys are deeper than others. The system prefers defect-free stability, which can be characterized by the deepest (low-energy) valleys, if they can be found. However, systems can get trapped inside higher (medium-energy) valleys, called metastable states, which have more defects.

To move from a metastable to stable state, block copolymer molecules must find ways to climb over the mountains and find lower energy valleys.

“Molecules in these metastable states are comfortable, and they can remain in that state for extraordinarily long periods of time,” said de Pablo.

“In order to escape such states and attain a perfect arrangement, they need to start rearranging themselves in a manner that allows the system to climb over local energy barriers, before reaching a lower energy minimum. What we have done in this work is predict the path these molecules must follow to find defect-free states and designed a process that delivers industry-standard nanocircuitry that can be scaled down to smaller densities without defects.”

Supported by a DOE leadership computing grant, de Pablo and his team used the Mira and Fusion supercomputers at the Argonne Leadership Computing Facility. The team generated molecular simulations of self-assembling block polymers along with sophisticated sampling algorithms to calculate where barriers to structural rearrangement would arise in the material. 

After all the calculations were done, the researchers could precisely predict the pathways of molecular rearrangement that block copolymers must take to move from a metastable to stable state. They could also experiment with temperatures, solvents and applied fields to further manipulate and decrease the barriers between these states.

To test these calculations, de Pablo and Nealey partnered with IMEC, an international consortium located in Belgium. Their commercial-grade fabrication and characterization instruments helped the researchers perform experiments under conditions that are not available in academic laboratories.

An individual defect measures only a handful of nanometers; “finding a defect in a 100-square centimeter area is like finding a needle in hay stack, and there are only a few places in the world where one has access to the necessary equipment to do so,” said de Pablo.

“Manufacturers have long been exploring the feasibility of using block copolymer assembly to reach the small critical dimensions that are demanded by modern computing and higher data storage densities,” de Pablo said. “Their biggest challenge involved evaluating defects; by following the strategies we have outlined, that challenge is greatly diminished.”

John Neuffer, president and CEO of the Semiconductor Industry Association, said industry is relentlessly focused on designing and building chips that are smaller, more powerful and more energy-efficient.

“The key to unlocking the next generation of semiconductor innovation is research,” he said. “SIA commends the work done by Argonne National Laboratory and the University of Chicago, as well as other critical scientific research being done across the United States.”

De Pablo, Nealey and their team will continue their investigations with a wider class of materials, increasing the complexity of patterns and characterizing materials in greater detail while also developing methods based on self-assembly for fabrication of three-dimensional structures.

Their long-term goal, with support from the DOE’s Office of Science, is to arrive at an understanding of directed self-assembly of polymeric molecules that will enable creation of wide classes of materials with exquisite control over their nanostructure and functionality for applications in energy harvesting, storage and transport.

University of Chicago

Sensors slip into the brain, then dissolve when the job is done

A team of neurosurgeons and engineers has developed wireless brain sensors that monitor intracranial pressure and temperature and then are absorbed by the body, negating the need for surgery to remove the devices.

Such implants, developed by scientists at Washington University School of Medicine in St. Louis and engineers at the University of Illinois at Urbana-Champaign, potentially could be used to monitor patients with traumatic brain injuries, but the researchers believe they can build similar absorbable sensors to monitor activity in organ systems throughout the body. Their findings are published online Jan. 18 in the journal Nature.

"Electronic devices and their biomedical applications are advancing rapidly," said co-first author Rory K. J. Murphy, MD, a neurosurgery resident at Washington University School of Medicine and Barnes-Jewish Hospital in St. Louis. "But a major hurdle has been that implants placed in the body often trigger an immune response, which can be problematic for patients. The benefit of these new devices is that they dissolve over time, so you don't have something in the body for a long time period, increasing the risk of infection, chronic inflammation and even erosion through the skin or the organ in which it's placed. Plus, using resorbable devices negates the need for surgery to retrieve them, which further lessens the risk of infection and further complications."

Murphy is most interested in monitoring pressure and temperature in the brains of patients with traumatic brain injury.

About 50,000 people die of such injuries annually in the United States. When patients with such injuries arrive in the hospital, doctors must be able to accurately measure intracranial pressure in the brain and inside the skull because an increase in pressure can lead to further brain injury, and there is no way to reliably estimate pressure levels from brain scans or clinical features in patients.

"However, the devices commonly used today are based on technology from the 1980s," Murphy explained. "They're large, they're unwieldy, and they have wires that connect to monitors in the intensive care unit. They give accurate readings, and they help, but there are ways to make them better."

Murphy collaborated with engineers in the laboratory of John A. Rogers, PhD, a professor of materials science and engineering at the University of Illinois, to build new sensors. The devices are made mainly of polylactic-co-glycolic acid (PLGA) and silicone, and they can transmit accurate pressure and temperature readings, as well as other information.

"With advanced materials and device designs, we demonstrated that it is possible to create electronic implants that offer high performance and clinically relevant operation in hardware that completely resorbs into the body after the relevant functions are no longer needed," Rogers said. "This type of bio-electric medicine has great potential in many areas of clinical care."

The researchers tested the sensors in baths of saline solution that caused them to dissolve after a few days. Next, they tested the devices in the brains of laboratory rats.

Having shown that the sensors are accurate and that they dissolve in the solution and in the brains of rats, the researchers now are planning to test the technology in patients.

"In terms of the major challenges involving size and scale, we've already crossed some key bridges," said co-senior author Wilson Z. Ray, MD, assistant professor of neurological and orthopaedic surgery at Washington University.

In patients with traumatic brain injuries, neurosurgeons attempt to decrease the pressure inside the skull using medications. If pressure cannot be reduced sufficiently, patients often undergo surgery. The new devices could be placed into the brain at multiple locations during such operations.

"The ultimate strategy is to have a device that you can place in the brain -- or in other organs in the body -- that is entirely implanted, intimately connected with the organ you want to monitor and can transmit signals wirelessly to provide information on the health of that organ, allowing doctors to intervene if necessary to prevent bigger problems," Murphy said. "And then after the critical period that you actually want to monitor, it will dissolve away and disappear."

Washington University School of Medicine