Category Archives: Materials

Light is the most energy-efficient way of moving information. Yet, light shows one big limitation: it is difficult to store. As a matter of fact, data centers rely primarily on magnetic hard drives. However, in these hard drives, information is transferred at an energy cost that is nowadays exploding. Researchers of the Institute of Photonic Integration of the Eindhoven University of Technology (TU/e) have developed a ‘hybrid technology’ which shows the advantages of both light and magnetic hard drives. Ultra-short (femtosecond) light pulses allows data to be directly written in a magnetic memory in a fast and highly energy-efficient way. Moreover, as soon as the information is written (and stored), it moves forward leaving space to empty memory domains to be filled in with new data. This research, published in Nature Communications, promises to revolutionize the process of data storage in future photonic integrated circuits.

Data are stored in hard drives in the form of ‘bits’, tiny magnetic domains with a North and a South pole. The direction of these poles (‘magnetization’), determines whether the bits contain a digital 0 or a 1. Writing the data is achieved by ‘switching’ the direction of the magnetization of the associated bits.

Synthetic ferrimagnets

Conventionally, the switching occurs when an external magnetic field is applied, which would force the direction of the poles either up (1) or down (0). Alternatively, switching can be achieved via the application of a short (femtosecond) laser pulse, which is called all-optical switching, and results in a more efficient and much faster storage of data.

Mark Lalieu, PhD candidate at the Applied Physics Department of TU/e: ‘All-optical switching for data storage has been known for about a decade. When all-optical switching was first observed in ferromagnetic materials – amongst the most promising materials for magnetic memory devices – this research field gained a great boost’. However, the switching of the magnetization in these materials requires multiple laser pulses and, thus, long data writing times.

Storing data a thousand times faster

Lalieu, under the guidance of Reinoud Lavrijsen and Bert Koopmans, was able to achieve all-optical switching in synthetic ferrimagnets – a material system highly suitable for spintronic data applications – using single femtosecond laser pulses, thus exploiting the high velocity of data writing and reduced energy consumption.

So how does all-optical switching compare to modern magnetic storage technologies? Lalieu: “The switching of the magnetization direction using the single-pulse all-optical switching is in the order of picoseconds, which is about a 100 to 1000 times faster than what is possible with today’s technology. Moreover, as the optical information is stored in magnetic bits without the need of energy-costly electronics, it holds enormous potential for future use in photonic integrated circuits.”

‘On-the-fly’ data writing

In addition, Lalieu integrated all-optical switching with the so-called racetrack memory – a magnetic wire through which the data, in the form of magnetic bits, is efficiently transported using an electrical current. In this system, magnetic bits are continuously written using light, and immediately transported along the wire by the electrical current, leaving space to empty magnetic bits and, thus, new data to be stored.

Koopmans: “This ‘on the fly’ copying of information between light and magnetic racetracks, without any intermediate electronic steps, is like jumping out of a moving high-speed train to another one. From a ‘photonic Thalys’ to a ‘magnetic ICE’, without any intermediate stops. You will understand the enormous increase in speed and reduction in energy consumption that can be achieved in this way”.

What’s next? This research was performed on micrometric wires. In the future, smaller devices in the nanometer scale should be designed for better integration on chips. In addition, working towards the final integration of the photonic memory device, the Physics of Nanostructure group is currently also busy with the investigation on the read-out of the (magnetic) data, which can be done all-optically as well.

For the first time, researchers used benzene – a common hydrocarbon – to create a novel kind of molecular nanotube, which could lead to new nanocarbon-based semiconductor applications.

Researchers from the Department of Chemistry have been hard at work in their recently renovated lab in the University of Tokyo’s Graduate School of Science. The pristine environment and smart layout affords them ample opportunities for exciting experiments. Professor Hiroyuki Isobe and colleagues share an appreciation for “beautiful” molecular structures and created something that is not only beautiful but is also a first for chemistry.

Their phenine nanotube (pNT) is beautiful to see for its pleasing symmetry and simplicity, which is a stark contrast to its complex means of coming into being. Chemical synthesis of nanotubes is notoriously difficult and challenging, even more so if you wish to delicately control the structures in question to provide unique properties and functions.

Typical carbon nanotubes are famous for their perfect graphite structures without defects, but they vary widely in length and diameter. Isobe and his team wanted a single type of nanotube, a novel form with controlled defects within its nanometer-sized cylindrical structure allowing for additional molecules to add properties and functions.

The researchers’ novel process of synthesis starts with benzene, a hexagonal ring of six carbon atoms. They use reactions to combine six of these benzenes to make a larger hexagonal ring called a cyclo-meta-phenylene (CMP). Platinum atoms are then used which allow four CMPs to form an open-ended cube. When the platinum is removed, the cube springs into a thick circle and this is furnished with bridging molecules on both ends enabling the tube shape.

It sounds complicated, but amazingly, this complex process successfully bonds the benzenes in the right way over 90 percent of the time. The key also lies in the symmetry of the molecule, which simplifies the process to assemble as many as 40 benzenes. These benzenes, also called phenines, are used as panels to form the nanometer-sized cylinder. The result is a novel nanotube structure with intentional periodic defects. Theoretical investigations show these defects imbue the nanotube with semiconductor characters.

“A crystal of pNT is also interesting: The pNT molecules are aligned and packed in a lattice rich with pores and voids,” Isobe explains. “These nanopores can encapsulate various substances which imbue the pNT crystal with properties useful in electronic applications. One molecule we successfully embedded into pNT was a large carbon molecule called fullerene (C70).”

“A team lead by Kroto/Curl/Smalley discovered fullerenes in 1985. It is said that Sir Harold Kroto fell in love with the beautiful molecule,” continues Isobe. “We feel the same way about pNT. We were shocked to see the molecular structure from crystallographic analysis. A perfect cylindrical structure with fourfold symmetry emerges from our chemical synthesis.”

“After a few decades since the discovery, this beautiful molecule, fullerene, has found various utilities and applications,” adds Isobe. “We hope that the beauty of our molecule is also pointing to unique properties and useful functions waiting to be discovered.”

UNSW researchers at the Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) have shown for the first time that they can build atomic precision qubits in a 3D device – another major step towards a universal quantum computer.

The team of researchers, led by 2018 Australian of the Year and Director of CQC2T Professor Michelle Simmons, have demonstrated that they can extend their atomic qubit fabrication technique to multiple layers of a silicon crystal – achieving a critical component of the 3D chip architecture that they introduced to the world in 2015. This new research was published today in Nature Nanotechnology.

The group is the first to demonstrate the feasibility of an architecture that uses atomic-scale qubits aligned to control lines – which are essentially very narrow wires – inside a 3D design.

What’s more, the team was able to align the different layers in their 3D device with nanometer precision – and showed they could read out qubit states single shot, i.e. within one single measurement, with very high fidelity.

“This 3D device architecture is a significant advancement for atomic qubits in silicon,” says Professor Simmons. “To be able to constantly correct for errors in quantum calculations – an important milestone in our field – you have to be able to control many qubits in parallel.

“The only way to do this is to use a 3D architecture, so in 2015 we developed and patented a vertical crisscross architecture. However, there were still a series of challenges related to the fabrication of this multi-layered device. With this result we have now shown that engineering our approach in 3D is possible in the way we envisioned it a few years ago.”

In this paper, the team has demonstrated how to build a second control plane or layer on top of the first layer of qubits.

“It’s a highly complicated process, but in very simple terms, we built the first plane, and then optimized a technique to grow the second layer without impacting the structures in first layer,” explains CQC2T researcher and co-author, Dr Joris Keizer.

“In the past, critics would say that that’s not possible because the surface of the second layer gets very rough, and you wouldn’t be able to use our precision technique anymore – however, in this paper, we have shown that we can do it, contrary to expectations.”

The team also demonstrated that they can then align these multiple layers with nanometer precision.

“If you write something on the first silicon layer and then put a silicon layer on top, you still need to identify your location to align components on both layers. We have shown a technique that can achieve alignment within under 5 nanometers, which is quite extraordinary,” Dr Keizer says.

Lastly, the researchers were able to measure the qubit output of the 3D device with what’s called single shot – i.e. with one single, accurate measurement, rather than having to rely on averaging out millions of experiments. “This will further help us scale up faster,” Dr Keizer explains.

Towards commercialisation

Professor Simmons says that this research is a major milestone in the field.

“We are working systematically towards a large-scale architecture that will lead us to the eventual commercialisation of the technology.

“This is an important development in the field of quantum computing, but it’s also quite exciting for SQC,” says Professor Simmons, who is also the founder and a director of SQC.

Since May 2017, Australia’s first quantum computing company, Silicon Quantum Computing Pty Limited (SQC), has been working to create and commercialise a quantum computer based on a suite of intellectual property developed at CQC2T and its own proprietary intellectual property.

“While we are still at least a decade away from a large-scale quantum computer, the work of CQC2T remains at the forefront of innovation in this space. Concrete results such as these reaffirm our strong position internationally,” she concludes.

Over a long period, industrial companies followed up at a distance the development of GaN-based solutions mainly managed by R&D institutes and laboratories. Today the context has changed.
Under the updated of its annual report, Power GaN: Epitaxy, Devices, Applications and Technology Trends, Yole Développement (Yole) identified, a lot of power electronics & compound semiconductor companies including leading players such as Infineon Technologies, STMicroelectronics… strongly engaged in significant projects of development. Some of them already introduce in their portfolio a GaN product. But it is not the majority. So what is the status of GaN technologies? Can we affirm a clear adoption of GaN products? What would be the main applications?… Business dream or reality, the power GaN industry has been deeply analyzed by the Power & Wireless team from Yole. The analysts propose you today to discover a snapshot of this industry.

Today, it is crystal-clear that, from theoretical point of view, GaN offers fantastic technical advantages over traditional Si MOSFETs; the technology is very appealing, and more and more players are entering; moreover the lowering of prices could make GaN devices a good competitor of the currently used Si-based power switching transistors.

“Nevertheless the technical panorama is not clear yet; every manufacturer presents its solution on die design and packaging integration. This brings to a strong competition which will accelerate technical innovations in terms of integration and better performances,” says Elena Barbarini, PhD, Head of Department Semiconductors Devices at System Plus Consulting.

Even though the current GaN power market remains tiny compared to US$32.8 billion silicon power market, GaN devices are penetrating confidently into different applications.

The biggest segment in the power GaN market is still power supply applications, i.e. fast charging for cellphones. This year, Navitas and Exagan introduced 45W fast-charging power adaptors with an integrated GaN solution. Then, LiDAR applications are high-end solutions that take full benefit of high-frequency switching in GaN power devices.

And what about the EV/HEV market? What is the status of GaN solutions in a market segment step by step dominated by SiC technology replacing Si IGBTs in main inverters? Therefore, Yole announces a US$450 million SiC market in 2023 in its Power SiC report.

“The accumulation of the market growth in various applicative markets, especially the power supply market segment which is the most important in that case, confirms our first scenario,” comments Ana Villamor, PhD, Technology & Market Analyst at Yole.“Under this Base Case scenario, GaN market is expected to grow steadily. At Yole, we announce a GaN market to grow with 55% CAGR between 2017 and 2023”.

However, this analysis is not the only way to see the tomorrow’s industry. Yole’s Power & Wireless team went further in their investigations. Is there any killer application that could cause the GaN power device market to explode? Yes possibly, Yole’s analysts said. As matter of fact, several industrial players confirm that the leading smartphones manufacturer, Apple could consider the GaN technology for its wireless charging solution.

“It goes without saying that the potential adoption of GaN by Apple or another smartphone giant would completely change the market’s dynamics and finally provide a breath of life to the GaN power device industry,” comments Ezgi Dogmus, PhD, Technology & Market Analyst and part of the Yole’s Power & Wireless team. “Indeed we imagine that after a company like Apple adopts GaN, numerous other companies would follow on the commercial electronics market.”

What could be the added-value of GaN technology? Various players, such as EPC and Transphorm, have already obtained automotive qualification in preparation for GaN’s potential ramp-up. In addition BMW i Ventures’s investment in GaN Systems clearly demonstrates the automotive industry’s interest in GaN solutions for EV/HEV technology… Globally, Yole’s second scenario, named Bull Case Scenario is much more aggressive, conditioned by the adoption of GaN wireless charging solution by leading consumer manufacturers.

According to the market research, in this context, the GaN power business could reach around US$423 million by 2023, with 93% CAGR between 2017 and 2023.

A research study on low noise and high-performance transistors led by Suprem Das, assistant professor of industrial and manufacturing systems engineering, in collaboration with researchers at Purdue University, was recently published by Physical Review Applied.

The study has demonstrated micro/nano-scale transistors made of two-dimensional atomic thin materials that show high performance and low noise. The devices are less than one-hundredth of the diameter of a single human hair and could be key to innovating electronics and precision sensing.

Many researchers worldwide are focusing attention on building the next generation of transistors from atomic scale “exotic” 2D materials such as molybdenum di-selenide. These materials are promising because they show high-performance transistor-action that may, in the future, replace today’s silicon electronics. However, very few of them are looking at yet another important aspect: the inherent electronic noise in this new class of materials. Electronic noise is ubiquitous to all devices and circuits and only worsens when the material becomes atomic thin.

A recent study conducted by Das’ research team has systematically shown that if one can control the layer thickness between 10 and 15-atomic thin in a transistor, the device will not only show high performance — such as turning the switch “on” — but also experience very low electronic noise. This unique finding is essential to building several enabling technologies in electronics and sensing using a number of emerging 2D materials. This research is a comprehensive effort of a previous finding, where Das’ team conducted the first study on noise in MoSe2 transistors.

In microelectronic devices, the bandgap is a major factor determining the electrical conductivity of the underlying materials. Substances with large bandgaps are generally insulators that do not conduct electricity well, and those with smaller bandgaps are semiconductors. A more recent class of semiconductors with ultrawide bandgaps (UWB) are capable of operating at much higher temperatures and powers than conventional small-bandgap silicon-based chips made with mature bandgap materials like silicon carbide (SiC) and gallium nitride (GaN).

In the Journal of Applied Physics, from AIP Publishing, researchers at the University of Florida, the U.S. Naval Research Laboratory and Korea University provide a detailed perspective on the properties, capabilities, current limitations and future developments for one of the most promising UWB compounds, gallium oxide (Ga2O3).

Gallium oxide possesses an extremely wide bandgap of 4.8 electron volts (eV) that dwarfs silicon’s 1.1 eV and exceeds the 3.3 eV exhibited by SiC and GaN. The difference gives Ga2O3 the ability to withstand a larger electric field than silicon, SiC and GaN can without breaking down. Furthermore, Ga2O3 handles the same amount of voltage over a shorter distance. This makes it invaluable for producing smaller, more efficient high-power transistors.

“Gallium oxide offers semiconductor manufacturers a highly applicable substrate for microelectronic devices,” said Stephen Pearton, professor of materials science and engineering at the University of Florida and an author on the paper. “The compound appears ideal for use in power distribution systems that charge electric cars or converters that move electricity into the power grid from alternative energy sources such as wind turbines.”

Pearton and his colleagues also looked at the potential for Ga2O3 as a base for metal-oxide-semiconductor field-effect transistors, better known as MOSFETs. “Traditionally, these tiny electronic switches are made from silicon for use in laptops, smart phones and other electronics,” Pearton said. “For systems like electric car charging stations, we need MOSFETs that can operate at higher power levels than silicon-based devices and that’s where gallium oxide might be the solution.”

To achieve these advanced MOSFETs, the authors determined that improved gate dielectrics are needed, along with thermal management approaches that will more effectively extract heat from the devices. Pearton concluded that Ga2O3 will not replace SiC and GaN as the as the next primary semiconductor materials after silicon, but more likely will play a role in extending the range of powers and voltages accessible to ultrawide bandgap systems.

“The most promising application might be as high-voltage rectifiers in power conditioning and distribution systems such as electric cars and photovoltaic solar systems,” he said.

MIT researchers have invented a way to fabricate nanoscale 3-D objects of nearly any shape. They can also pattern the objects with a variety of useful materials, including metals, quantum dots, and DNA.

“It’s a way of putting nearly any kind of material into a 3-D pattern with nanoscale precision,” says Edward Boyden, an associate professor of biological engineering and of brain and cognitive sciences at MIT.

Using the new technique, the researchers can create any shape and structure they want by patterning a polymer scaffold with a laser. After attaching other useful materials to the scaffold, they shrink it, generating structures one thousandth the volume of the original.

These tiny structures could have applications in many fields, from optics to medicine to robotics, the researchers say. The technique uses equipment that many biology and materials science labs already have, making it widely accessible for researchers who want to try it.

Boyden, who is also a member of MIT’s Media Lab, McGovern Institute for Brain Research, and Koch Institute for Integrative Cancer Research, is one of the senior authors of the paper, which appears in the Dec. 13 issue of Science. The other senior author is Adam Marblestone, a Media Lab research affiliate, and the paper’s lead authors are graduate students Daniel Oran and Samuel Rodriques.

Implosion fabrication

Existing techniques for creating nanostructures are limited in what they can accomplish. Etching patterns onto a surface with light can produce 2-D nanostructures but doesn’t work for 3-D structures. It is possible to make 3-D nanostructures by gradually adding layers on top of each other, but this process is slow and challenging. And, while methods exist that can directly 3-D print nanoscale objects, they are restricted to specialized materials like polymers and plastics, which lack the functional properties necessary for many applications. Furthermore, they can only generate self-supporting structures. (The technique can yield a solid pyramid, for example, but not a linked chain or a hollow sphere.)

To overcome these limitations, Boyden and his students decided to adapt a technique that his lab developed a few years ago for high-resolution imaging of brain tissue. This technique, known as expansion microscopy, involves embedding tissue into a hydrogel and then expanding it, allowing for high resolution imaging with a regular microscope. Hundreds of research groups in biology and medicine are now using expansion microscopy, since it enables 3-D visualization of cells and tissues with ordinary hardware.

By reversing this process, the researchers found that they could create large-scale objects embedded in expanded hydrogels and then shrink them to the nanoscale, an approach that they call “implosion fabrication.”

As they did for expansion microscopy, the researchers used a very absorbent material made of polyacrylate, commonly found in diapers, as the scaffold for their nanofabrication process. The scaffold is bathed in a solution that contains molecules of fluorescein, which attach to the scaffold when they are activated by laser light.

Using two-photon microscopy, which allows for precise targeting of points deep within a structure, the researchers attach fluorescein molecules to specific locations within the gel. The fluorescein molecules act as anchors that can bind to other types of molecules that the researchers add.

“You attach the anchors where you want with light, and later you can attach whatever you want to the anchors,” Boyden says. “It could be a quantum dot, it could be a piece of DNA, it could be a gold nanoparticle.”

“It’s a bit like film photography — a latent image is formed by exposing a sensitive material in a gel to light. Then, you can develop that latent image into a real image by attaching another material, silver, afterwards. In this way implosion fabrication can create all sorts of structures, including gradients, unconnected structures, and multimaterial patterns,” Oran says.

Once the desired molecules are attached in the right locations, the researchers shrink the entire structure by adding an acid. The acid blocks the negative charges in the polyacrylate gel so that they no longer repel each other, causing the gel to contract. Using this technique, the researchers can shrink the objects 10-fold in each dimension (for an overall 1,000-fold reduction in volume). This ability to shrink not only allows for increased resolution, but also makes it possible to assemble materials in a low-density scaffold. This enables easy access for modification, and later the material becomes a dense solid when it is shrunk.

“People have been trying to invent better equipment to make smaller nanomaterials for years, but we realized that if you just use existing systems and embed your materials in this gel, you can shrink them down to the nanoscale, without distorting the patterns,” Rodriques says.

Currently, the researchers can create objects that are around 1 cubic millimeter, patterned with a resolution of 50 nanometers. There is a tradeoff between size and resolution: If the researchers want to make larger objects, about 1 cubic centimeter, they can achieve a resolution of about 500 nanometers. However, that resolution could be improved with further refinement of the process, the researchers say.

Better optics

The MIT team is now exploring potential applications for this technology, and they anticipate that some of the earliest applications might be in optics — for example, making specialized lenses that could be used to study the fundamental properties of light. This technique might also allow for the fabrication of smaller, better lenses for applications such as cell phone cameras, microscopes, or endoscopes, the researchers say. Farther in the future, the researchers say that this approach could be used to build nanoscale electronics or robots.

“There are all kinds of things you can do with this,” Boyden says. “Democratizing nanofabrication could open up frontiers we can’t yet imagine.”

Many research labs are already stocked with the equipment required for this kind of fabrication. “With a laser you can already find in many biology labs, you can scan a pattern, then deposit metals, semiconductors, or DNA, and then shrink it down,” Boyden says.

Billions of tiny transistors supply the processing power in modern smartphones, controlling the flow of electrons with rapid on-and-off switching.

But continual progress in packing more transistors into smaller devices is pushing toward the physical limits of conventional materials. Common inefficiencies in transistor materials cause energy loss that results in heat buildup and shorter battery life, so researchers are in hot pursuit of alternative materials that allow devices to operate more efficiently at lower power.

Now, an experiment conducted at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has demonstrated, for the first time, electronic switching in an exotic, ultrathin material that can carry a charge with nearly zero loss at room temperature. Researchers demonstrated this switching when subjecting the material to a low-current electric field.

The team, which was led by researchers at Monash University in Australia and included Berkeley Lab scientists, grew the material from scratch and studied it with X-rays at the Advanced Light Source (ALS), a facility at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

The material, known as sodium bismuthide (Na3Bi), is one of two materials that is known to be a “topological Dirac semimetal,” meaning it has unique electronic properties that can be tuned to behave in different ways – in some cases more like a conventional material and in other cases more like a topological material. Its topological properties were first confirmed in earlier experiments at the ALS.

Topological materials are considered promising candidates for next-generation transistors, and for other electronics and computing applications, because of their potential to reduce energy loss and power consumption in devices. These properties can exist at room temperature – an important distinction from superconductors that require extreme chilling – and can persist even when the materials have structural defects and are subject to stress.

Materials with topological properties are the focus of intense research by the global scientific community (see a related article), and in 2016 the Nobel Prize in physics was awarded for theories related to topological properties in materials.

The ease in switching the material studied at the ALS from an electrically conducting state to an insulating, or non-conducting state, bode well for its future transistor applications, said Sung-Kwan Mo, a staff scientist at the ALS who participated in the latest study. The study is detailed in the Dec. 10 edition of the journal Nature.

Another key aspect of the latest study is that the team from Monash University found a way to grow it extremely thin, down to a single layer arranged in a honeycomb pattern of sodium and bismuth atoms, and to control the thickness of each layer they create.

“If you want to make a device, you want to make it thin,” Mo said. “This study proves that it can be done for Na3Bi, and its electrical properties can easily be controlled with low voltage. We are a step closer to a topological transistor.”

Michael Fuhrer, a physicist at Monash University who participated in the study, said, “This discovery is a step in the direction of topological transistors that could transform the world of computation.”

He added, “Ultra-low energy topological electronics are a potential answer to the increasing challenge of energy wasted in modern computing. Information and communications technology already consumes 8 percent of global electricity, and that’s doubling every decade.”

In the latest study, researchers grew the material samples, measuring several millimeters on a side, on a silicon wafer under ultrahigh vacuum at the ALS Beamline 10.0.1 using a process known as molecular beam epitaxy. The beamline allows researchers to grow samples and then conduct experiments under the same vacuum conditions in order to prevent contamination.

This beamline is specialized for an X-ray technique known as angle-resolved photoemission spectroscopy, or ARPES, which provide information about how electrons travel in materials. In typical topological materials, electrons flow around the edges of the material, while the rest of the material serves as an insulator that prevents this flow.

Some X-ray experiments on similar samples were also performed at the Australian Synchrotron to demonstrate the ultrathin Na3Bi was free-standing and did not chemically interact with the silicon wafer it was grown on. Researchers had also studied samples with a scanning tunneling microscope at Monash University that helped to confirm other measurements.

“In these edge paths, electrons can only travel in one direction,” said Mark Edmonds, a physicist at Monash University who led the study. “And this means there can be no ‘back-scattering,’ which is what causes electrical resistance in conventional electrical conductors.”

In this case, researchers found that the ultrathin material became fully conductive when subjected to the electric field, and could also be switched to become an insulator across the entire material when subjected to a slightly higher electric field.

Mo said that the electrically driven switching is an important step to realizing applications for materials – some other research efforts have pursued mechanisms like chemical doping or mechanical strain that are more challenging to control and to perform the switching operation.

The research team is pursuing other samples that can be switched on and off in a similar way to guide the development of a new generation of ultralow-energy electronics, Edmonds said.

The pattern of arrangement of atoms in a crystal, called the crystal lattice, can have a huge effect on the properties of solid materials. Controlling and harnessing these properties is a challenge that promises rewards in applications such as novel sensors and new solid-state devices. An international research collaboration, including researchers from Osaka University, has reported the induction of an interesting type of magnetic order, called helimagnetism, in a cobalt oxide material by expanding its lattice structure. Their findings were published in Physical Review Materials.

This is a schematic illustration of the helimagnetic-ferromagnetic transition driven by the lattice expansion/compression in the cubic perovskite Sr1-xBaxCoO3. Credit: S. Ishiwata and H. Sakai

Magnetic behavior results from the order of the magnetic moments of the many individual atoms in a material. In helimagnetism, instead of the magnetic moments being aligned–as they are in permanent magnets, producing ferromagnetism–the moments arrange themselves in a helical pattern. This behavior is generally only observed in complicated lattice structures where different types of magnetic interactions compete with each other, therefore the report of induced helimagnetism in a simple cubic cobalt oxide structure, is highly significant.

“We have shown emergent helical spin order in a cubic perovskite-type material, which we achieved simply by expanding the lattice size,” study first author Hideaki Sakai says. “We were able to control the size of the lattice expansion by using a high-pressure technique to grow a series of single crystals with particular chemical compositions. Changing the amount of different ions in our materials provided us with sufficient control to investigate the magnetic properties.”

Systematically replacing strontium ions in the structure with larger barium ions caused the lattice to continually expand until the regular ferromagnetic magnetic order present at room temperature was disrupted, resulting in helimagnetism. These experimental findings were successfully supported by calculations.

“The fact that we were able to largely reproduce our findings by first principles calculations verifies that the magnetic interactions in the materials are highly sensitive to the lattice constant,” Sakai says. “The more we can understand about the magnetic behavior of crystalline materials, the closer we move towards translating their properties into useful functions. We hope that our findings will pave the way for novel sensor applications.

The control of magnetic order simply by changing the lattice chemistry, as demonstrated by this research, provides a foundation for investigating the properties of many other crystalline materials.

A team of scientists from Arizona State University’s School of Molecular Sciences and Germany have published in Science Advances online today an explanation of how a particular phase-change memory (PCM) material can work one thousand times faster than current flash computer memory, while being significantly more durable with respect to the number of daily read-writes.

PCMs are a form of computer random-access memory (RAM) that store data by altering the state of the matter of the “bits”, (millions of which make up the device) between liquid, glass and crystal states. PCM technology has the potential to provide inexpensive, high-speed, high-density, high-volume, nonvolatile storage on an unprecedented scale.

The basic idea and material were invented by Stanford Ovshinsky, long ago, in1975, but applications have lingered due to lack of clarity about how the material can execute the phase changes on such short time scales and technical problems related to controlling the changes with necessary precision. Now high tech companies like Samsung, IBM and Intel are racing to perfect it.

The semi-metallic material under current study is an alloy of germanium, antimony and tellurium in the ratio of 1:2:4. In this work the team probes the microscopic dynamics in the liquid state of this PCM using quasi-elastic neutron scattering (QENS) for clues as to what might make the phase changes so sharp and reproducible.

On command, the structure of each microscopic bit of this PCM material can be made to change from glass to crystal or from crystal back to glass (through the liquid intermediate) on the time scale of a thousandth of a millionth of a second just by a controlled heat or light pulse, the former now being preferred. In the amorphous or disordered phase, the material has high electrical resistance, the “off” state; in the crystalline or ordered phase, its resistance is reduced 1000 fold or more to give the “on” state.

These elements are arranged in two dimensional layers between activating electrodes, which can be stacked to give a three dimension array with particularly high active site density making it possible for the PCM device to function many times faster than conventional flash memory, while using less power.

“The amorphous phases of this kind of material can be regarded as “semi-metallic glasses”,” explains Shuai Wei, who at the time was conducting postdoctoral research in SMS Regents’ Professor Austen Angell’s lab, as a Humboldt Foundation Fellowship recipient.

“Contrary to the strategy in the research field of “metallic glasses”, where people have made efforts for decades to slow down the crystallization in order to obtain the bulk glass, here we want those semi-metallic glasses to crystallize as fast as possible in the liquid, but to stay as stable as possible when in the glass state. I think now we have a promising new understanding of how this is achieved in the PCMs under study.”

A Deviation from the expected

Over a century ago, Einstein wrote in his Ph.D. thesis that the diffusion of particles undergoing Brownian motion could be understood if the frictional force retarding the motion of a particle was that derived by Stokes for a round ball falling through a jar of honey. The simple equation: D (diffusivity) = kBT/6??r where T is the temperature, ? is the viscosity and r is the particle radius, implies that the product D?/T should be constant as T changes, and the surprising thing is that this seems to be true not only for Brownian motion, but also for simple molecular liquids whose molecular motion is known to be anything but that of a ball falling through honey!

“We don’t have any good explanation of why it works so well, even in the highly viscous supercooled state of molecular liquids until approaching the glass transition temperature, but we do know that there are a few interesting liquids in which it fails badly even above the melting point,” observes Angell.

“One of them is liquid tellurium, a key element of the PCM materials. Another is water which is famous for its anomalies, and a third is germanium, a second of the three elements of the GST type of PCM. Now we are adding a fourth, the GST liquid itself..!!! thanks to the neutron scattering studies proposed and executed by Shuai Wei and his German colleagues, Zach Evenson (Technical University of Munich, Germany) and Moritz Stolpe (Saarland University, Germany) on samples prepared by Shuai with the help of Pierre Lucas (University of Arizona).”

Another feature in common for this small group of liquids is the existence of a maximum in liquid density which is famous for the case of water. A density maximum closely followed, during cooling, by a metal-to semiconductor transition is also seen in the stable liquid state of arsenic telluride, (As2Te3), which is first cousin to the antimony telluride (Sb2Te3 ) component of the PCMs all of which lie on the “Ovshinsky” line connecting antimony telluride (Sb2Te3 ) to germanium telluride (GeTe) in the three component phase diagram. Can it be that the underlying physics of these liquids has a common basis?

It is the suggestion of Wei and coauthors that when germanium, antimony and tellurium are mixed together in the ratio of 1:2:4, (or others along Ovshinsky’s “magic” line) both the density maxima and the associated metal to non-metal transitions are pushed below the melting point and, concomitantly, the transition becomes much sharper than in other chalcogenide mixtures.

Then, as in the much-studied case of supercooled water, the fluctuations associated with the response function extrema should give rise to extremely rapid crystallization kinetics. In all cases, the high temperature state (now the metallic state), is the denser.

“This would explain a lot,” enthuses Angell “Above the transition the liquid is very fluid and crystallization is extremely rapid, while below the transition the liquid stiffens up quickly and retains the amorphous, low-conductivity state down to room temperature. In nanoscopic “bits”, it then remains indefinitely stable until instructed by a computer-programmed heat pulse to rise instantly to a temperature where, on a nano-second time scale, it flash crystallizes to the conducting state, the “on” state.

Lindsay Greer at Cambridge University has made the same argument couched in terms of a “fragile-to-strong” liquid transition”.

A second slightly larger heat pulse can take the “bit” instantaneously above its melting point and then, with no further heat input and close contact with a cold substrate, it quenches at a rate sufficient to avoid crystallization and is trapped in the semi-conducting state, the “off” state.

“The high resolution of the neutron time of flight-spectrometer from the Technical University of Munich was necessary to see the details of the atomic movements. Neutron scattering at the Heinz Maier-Leibnitz Zentrum in Garching is the ideal method to make these movements visible,” states Zach Evenson.