It’s amazing the lengths physicists will go to get things done – from building telescopes on the tops of mountains to lowering neutrino detectors to the bottom of the sea and from firing satellites into space to colliding particles in tunnels. We’ve covered all those efforts in Physics World many times, but there’s one extreme activity that’s been off our radar – until now.
That is the new but little-known field of “speleophysics” – or “the physics of caves” – which we tackle in the cover feature of the October 2015 issue of Physics World magazine. For the small band of researchers who brave the journey underground, being a speleophysicist is almost the perfect job. Armed with helmets, ropes, torches and boots, they’re able to combine their love of physics with a fascination for the nether world – and experience the thrill (and danger) of caving, too.
Matt Covington didn’t sleep much the night before his big swim. Who could blame him? For two days, the wiry caver had, with the help of ropes and ladders, been making his way down through J2, a deep cave in the mountainous Mexican province of Oaxaca. Marcin Gala, an experienced caver from Poland, joined him on the descent, just as he had on multiple previous adventures.
The entrance to J2 is dramatic: it’s an enormous 100 m wide sinkhole, lined with stubborn plants and surrounded by tall trees. A small hole – about as tall as a person – leads from the bottom of the sinkhole deeper into the cave. Cables and ladders lead to a series of tight turns and drops. Because of the quick descent, once a caver enters J2, the Sun virtually vanishes. No-one knows how far down J2 goes.
The year was 2009 and Covington and Gala were in Mexico as part of a caving expedition dedicated to finding and exploring the untouched netherworld of the world’s deepest caves. “In mountaineering, you have Everest. In caving, there’s this push to find the deepest caves in the world,” says Covington, a physicist at the University of Arkansas. “The weird thing is, you never really know when you’ve found it.”
For this part of the mission, he and Gala were on their own. Already, they’d made their way through a tight spot called the Barbie Squeeze and rappelled down a series of striated cliffs known as the Jungle Series. By the third day, the duo were 1200 m below the cave entrance and putting on scuba suits outfitted with heavy oxygen tanks. They were preparing to swim through a sump – a dark underground passage filled with water – which was the only way to keep searching for J2’s secrets.
Gala was getting cold, so he plunged first into the clear water. Covington followed, weighed down by their supply bags and air tanks. They carefully made their submerged way along the floor of the sump, the bags snagging on jagged pillars of rock. About seven minutes into the dive, however, Covington became short of breath. Worrying that his air tank had malfunctioned, he reached for his back-up air – but again, his attempts to inhale were met with resistance. Panic lurked at the periphery. His wife was scheduled to arrive for a visit the next day. It was common knowledge among the team that there were no good rescue options in that sump or beyond, and the chances of being rescued alive from the bottom of J2 were slim to none. The key was not to let panic win.
Caving isn’t for everyone, but among cavers Covington is something of an exception. He’s both an explorer and a physicist, which means his research lands him in a small and curious area of endeavour that merges physics, chemistry and mathematics to quantify how karst forms, evolves and moves water from one place to another. (Karst is the general term for landforms that have been sculpted by dissolution of soluble rocks such as limestone. It may include caves, sinkholes, cliffs and fissures.) He’s part of a small group – including maybe two dozen, maybe only a handful, depending on whom you ask – that is taking the rigour of physics to the netherworld. He light-heartedly refers to the field as “speleophysics”.
A quest for karst
Cavers and scientists alike see caves as a sort of last frontier. “Caves are the last parts of Earth which are not yet explored,” says Franci Gabrovšek, a physicist at the Karst Research Institute in Postojna, Slovenia. “They are full of surprises, and when you progress to the next step, you never know what your next challenge will be. Of course, that’s general in science, not just in our field.” He thinks the field is growing. “There are still so many open questions in speleogenesis,” or cave formation, says Gabrovšek. “What are the dynamics of growth? How do cave channels self-organize to form the geometry that we observe in nature?” Scientists have a good understanding of the dissolution process, but “we don’t know the exact circumstances that govern it”, he says. Gabrovšek adds that he and others also want to get a better understanding of the relative roles of chemical and mechanical processes both at the beginning of a cave’s formation and during its later growth.
Weathered and worn This Alpine karst landscape conceals a deep network of caves. (Courtesy: Matt Covington)
There’s also a practical concern at stake in speleophysics: karst and caves play a critical role in the water cycle. Roughly 20% of the fresh water supply in the US – and about half of that in Europe – comes from karst aquifers, where water can flow rapidly through labyrinthine networks of channels like water pipes through a city. The water’s movement can be fickle, changing by the season or during times of flood. Understanding how aquifers shuttle water from one point to another deepens our understanding of the water cycle, and may suggest solutions should something go wrong. Right now, “you put a pollutant in somewhere, and you don’t know where that pollutant will come out”, says Gabrovšek. “The caves control where the water’s going to go and how quickly it will get from one place to another,” Covington adds.
Wolfgang Dreybrodt, a pioneer in the field and now an emeritus professor of experimental physics at the University of Bremen, Germany, thinks that applying the laws of physics to caves can bring together an interdisciplinary understanding of a complex natural phenomenon. “Caves are usually described by geologists or geographers who don’t have a mathematical education,” he says. Rigorous models have the potential to stitch descriptive theories from other disciplines into a coherent whole. But he also says the field calls for geologists who have better training in physics – and physicists with a more thorough knowledge of geology.
“Engineers, physicists and chemists involved in karst must be ready to learn about geology to at least the extent which enables them to have a good idea of what they are dealing with,” he said in a 2011 interview with the journal Acta Carsologica (40 225). In a way, his advice mirrors the approach needed to tackle a cave like J2 – co-operation is the only way forwards.
Secrets of the caves
Caves, by their very nature, are always hiding something. What that something is, however, depends largely on time and place.
Thousands of years ago, caves were believed to harbour dragons or other beasts, gods and goddesses, or passages to the underworld. In Indonesia, caves are the canvasses for the oldest known art in the world. Seers from ancient Greek cults immersed themselves in caves to commune with the immortals and emerge with prophecies. The wonder has changed flavour in more recent times but it remains intact. Now, cavers search for unknown and unmapped subterranean chambers. Millions of tourists annually flock to show caves such as Mammoth Cave in Kentucky – the largest known cave system in the world – for the experience of simply being somewhere deep, somewhere dark, surrounded by a strange wonderland of natural sculpture.
Deep down Matt Covington during his 2009 descent of the J2 cave system in Mexico. (Courtesy: Marcin Gala)
People have been describing caves and cave features for centuries. By the 1830s scientists began hammering out the basic recipe for karstification. It’s deceptively simple. Flowing water absorbs carbon dioxide from soil air and becomes carbonic acid, which Dreybrodt calls “the motor of erosion”. Carbonic acid is weak, but strong enough to dissolve limestone. As small channels grow wider, more water flows and more limestone dissolves, thanks to a feedback loop that allows more water through, which dissolves more limestone, and so on. The channels open up, the cave grows.
Scientists began to quantify that characterization of speleogenesis in equations in the mid-20th century. But new problems quickly emerged. A 1958 quantitative model published by Peter K Weyl of the Shell Development Company, for example, accounted for how water absorbs carbon dioxide and erodes limestone. On the one hand, that model was a welcome step forwards in the field, moving previous descriptive models into the realm of physics. On the other, that rigorous treatment led to a paradox.
According to the model, water flowing over limestone becomes saturated with calcium ions, which means it can’t dissolve any more limestone, which means deep caves shouldn’t exist at all. Scientists remained in the dark about explaining the existence of caves until the 1980s, when Dreybrodt and other researchers created 1D models showing how the evolution of caves depends on a nonlinear erosion behaviour that arises in the interaction between the water and the limestone. (But even that solution has been challenged: in 2010, speleophysicists in Poland and the US used a 2D model and avoided nonlinearity altogether.)
Researchers are now developing 3D models of speleogenesis and more powerful tools to collect data. Postojna in Slovenia hosts a giant show cave – complete with a mile-long “cave train” that whisks tourists into the belly of the karst – in which Gabrovšek has recently set up sensors to monitor air flow, temperature, humidity and carbon-dioxide levels, to build a cave climate model. Covington has similarly set up sensors in a cave in Arkansas more than a mile long, creating a field laboratory where he can keep tabs on the cave climate and send his students for research projects.
He’s also something of a hacker. “We’ve been doing 3D scans inside of caves that get reconstructed in real time,” he says, equipped with only an XBox Kinect and a laptop. “The XBox Kinect is basically a video game controller, but in reality it’s a very inexpensive 3D scanner.”
From hobby to research
Not surprisingly, “most of us who are doing this somehow started caving as a hobby”, says Gabrovšek. “You try to join your hobby with your profession, and I think speleophysics is one of those areas where you can do this.” Figuring out how to merge the two, like finding the next clear passage in an uncharted cave, isn’t always easy.
Danger physics Matt Covington during his 2009 descent of the J2 cave system in Mexico. (Courtesy: Marcin Gala)
Dreybrodt, who explored abandoned mines and canyons during his childhood in the German Democratic Republic, trained as an experimental physicist and in 1974 he joined the faculty at the newly founded University of Bremen. But Dreybrodt’s mood quickly soured due to the educational climate, which was wracked by student protests and even bullying in his lectures, which he says was supported by the Social Democratic Party, the leading political establishment in Bremen. In his 2011 interview, he described the university at that time as “an incredible realm of ignorance and intolerance”. As a means of escape from the hostile intellectual environment, he turned to the caves in Germany’s Harz mountains and began reading up on speleogenesis.
Those excursions gave him solace – and a rising curiosity. Dreybrodt describes his time in caves with reverence and awe. “I was fascinated by the realm of darkness in caves from which fantastic shapes emerge in the faint light on the helmet. How could such a variety of structures arise by dissolution of limestone and later by precipitation of calcite?” he said. More recently, Dreybrodt told Physics World “Going to caves for some people is a deep, spiritual thing, which is not easy to explain…If you go in there and see all those caves, then you want to understand why. It has to do with pure inspiration and something emotional and spiritual and philosophical.”
He tackled research projects aimed at understanding what he’d witnessed first-hand in the caves. Those included a mathematical explanation for the growth of stalagmites (the ones that point up), as well as that 1D model that uses nonlinearity to show how deep caves can evolve. In 2011, in the Journal of Hydrology (409 20), he and Gabrovšek published a model of how pollution can spread through a karst aquifer.
Gabrovšek says that when he began looking at graduate schools, nearly 20 years ago, there were only a few physicists studying caves in earnest. “The Internet had just started,” he says, “and I had a hard time finding a supervisor who could guide me to a doctoral thesis.” He’d just finished an undergraduate degree in physics and was spending almost every weekend in caves, which helped inspire the questions he wanted to answer in his research. His quest led him to Dreybrodt, who agreed to take him on as a student – and the two went on to become close collaborators.
Covington, too, travelled a circuitous route before he arrived at speleophysics. He was finishing his PhD in astrophysics at the University of California, Santa Cruz, when he attended a lecture that helped send him on a different path. The speaker began describing how the statistical technique of Markov Chain Monte Carlo can be applied to theories of galaxy formation. But Covington began to wonder if physics equations could be applied to caves – and discovered an entire body of research. After he finished his PhD, he secured a fellowship from the National Science Foundation in the US to study with Gabrovšek in Slovenia. Another collaboration was born.
“I felt like when I first started working in this field, there was almost a whole playground of things to work with,” he says. And he kept caving, of course. He’s noticed that the experience of mapping or studying a cave differs noticeably from the experience of pure exploration. “When you’re mapping, you move through a cave in a different way,” he says. “If you’re the one who’s sketching, drawing the features of the cave, you experience the cave at a slow pace and absorb a lot of information about the cave that you wouldn’t necessarily observe if you were just going through.”
Still alive Matt Covington in 2009 still wearing his scuba gear after diving through the sump leading to the J2 cave system in Mexico. (Courtesy: Marcin Gala)
He also knows the benefits of slowing down. That night in 2009, finding it hard to breathe in a flooded channel 1200 m beneath J2’s entrance, Covington remembered to open a valve on his rescue air. The air flowed, and he was relieved. He paused there, at the bottom of the lake, buried deep in the cave, to gather his wits. Down there, in such an extremely wild environment, panic was the enemy. Then he picked his way again, and two minutes later emerged at the other side. He didn’t know whether the mishap was due to faulty equipment or stress, but in some ways it didn’t matter. He knew that on the way back, staying calm would be his top priority.
In July this year Covington returned to Slovenia to investigate a cave he’d begun to explore on a previous visit, but he couldn’t say whether his trip was more research- or exploration-related. As he explains, “It’s not always 100% clear which one I’m doing at any given time.”
Around the world: how has immigration shaped the global physics community? (Courtesy: iStockphoto/Joel Carillet)
In December 1938 Enrico Fermi travelled to Stockholm, where he was presented with that year’s Nobel Prize for Physics for his insights into the atomic nucleus. But after the ceremony, Fermi did not return to his native Italy. Instead, he joined his wife and young children on a voyage to the US. Fermi went on to make major contributions to physics in that country – including playing crucial roles in developing nuclear weapons and nuclear energy.
Fermi is just one of many Nobel laureates who became immigrants, by which I mean he settled permanently in a country different to that in which he was born. He made the move in 1938 because his wife Laura was Jewish and Italy had just passed racial laws that restricted the civil rights of Jews. Another Nobel-prize-winning physicist to flee fascism was Albert Einstein, who moved to the US in 1933 after Germany passed similar laws.
And it wasn’t only established physicists who got caught up in this migration. Arno Penzias, who shared the 1978 prize for discovering the cosmic microwave background, was part of the Kindertransport that rescued thousands of Jewish children from Europe in 1938–1940. Penzias and his brother were taken from Germany to Britain, where they were reunited with their parents and then eventually ended up in the US. We are left to wonder whether physics in postwar Europe would have been much richer if these and other talented scientists had not been forced to leave.
Physicists change country for a variety of reasons. Einstein and Penzias fled persecution and possible death, whereas others, such as Marie Curie or Abdus Salam, moved to improve their career prospects. In the run-up to the 2015 physics Nobel – which will be awarded at 11:45 CEST on Tuesday 6 October – we will be looking at how Nobel-prize-winning physicists have been moving around the globe over the past century.
Tomorrow we will unveil a series of Physics World infographics that illustrates the patterns of migration. The story begins with Curie herself, who emigrated from Poland to France in her mid-20s, arriving as a “foreign student” in today’s parlance. Despite her stunning scientific achievements in France – she remains the only person to win Nobel prizes in both physics (1903) and chemistry (1911) – Curie was vilified by some parts of French society for being a foreigner. Xenophobia notwithstanding, she resisted persistent calls to return to Poland and has become one of the most well-known French scientists of all time.
The story ends, at least for now, with Shuji Nakamura, who shared last year’s prize for his development of blue light-emitting diodes. He left his native Japan in 1999 after doing his prize-winning research to join the University of California, Santa Barbara.
Check back tomorrow to explore our great new infographics, in which we reveal which countries have been the big winners when it comes to attracting top physicists. And you’ll also be able to find out which countries have lost the most talent, and why.
A new type of compact infrared laser promises to make it easier to identify specific molecules at very low concentrations within complex chemical samples. That is the claim of physicists in Germany and Spain, who have created the high-power, ultrashort-pulsed, broadband laser. They add that the device shows particular promise for spotting molecules within exhaled breath that are indicative of certain kinds of disease.
Molecular spectroscopy, or “molecular fingerprinting”, involves shining a laser beam spanning a certain portion of the electromagnetic spectrum through a liquid or gas and then comparing the beam before and after it travels through the sample – the specific wavelengths absorbed revealing the composition and structure of molecules within the sample. Most molecular vibrations can be stimulated by mid-infrared radiation (2–25 μm), and therefore laser light covering this part of the spectrum is very useful for molecular fingerprinting.
Because no existing lasing media are able to emit light across a broad range of mid-infrared (MIR) wavelengths, current devices operating in this part of the spectrum use nonlinear crystals to shift shorter-wavelength near-infrared (NIR) radiation to longer wavelengths. However, these crystals have significant limitations, and practical fingerprinting systems would benefit from a different approach.
In the latest work, Ioachim Pupeza of the Max Planck Institute of Quantum Optics near Munich and colleagues have created a system that makes use of a different type of nonlinear crystal. NIR radiation is first created in a novel high-power, diode-pumped femtosecond laser “oscillator”, in which a thin disc made from a ytterbium-doped material forms the active medium. The light from the oscillator is compressed into pulses lasting just 20 fs (2 × 10–14 s) and is then converted into the MIR using a nonlinear crystal made from lithium–gallium-sulphide. The current prototype device occupies an area of about 2 m2.
Crucial crystal
“Apart from developing the oscillator, the choice of the nonlinear medium was crucial,” explains Pupeza. “It was not clear that any crystal could be found that fulfils the necessary requirements of low absorption and high damage threshold.”
Pupeza says that the new system combines a number of features that make it useful for molecular fingerprinting – including its power. As Pupeza points out, many molecules studied using the technique exist in minuscule concentrations. Exhaled human breath, for example, containing organic compounds that are present at the level of just a few parts per billion. An intense light source is therefore needed to have a decent chance of detecting such molecules. The average power of the pulse train in the current work is 0.1 W.
The new system also spans a broad range of wavelengths (6.8–16.4 μm), which allows a large number of individual absorption lines to be recorded for any given type of molecule.
Clear identification
This means that a molecule can be more clearly identified against the very high background noise. “It’s the same with one’s fingerprints,” explains Pupeza. “To precisely identify a person, an entire fingerprint is much more useful than just a tiny fraction of it.”
Another useful characteristic of the new laser is its spatial coherence. This increases the distance the beam can travel through a sample without undergoing significant losses, which boosts its sensitivity to low-concentration molecules. In addition, the beam has phase coherence, which means that the electric field of its ultrashort pulses – each less than two wavelengths long – is identical from one pulse to the next.
Phase coherence increases the amount of information that can be extracted from the sample because the phase of the light will change as it interacts with the sample molecules. In addition, the laser’s MIR beam can be combined with a part of the original NIR beam. This allows the laser’s output to be measured using NIR detectors, which have far lower noise levels than their MIR equivalents.
Designed for hospitals
The group has designed its new device so that it can be used for one application in particular: detecting molecular markers of disease. The air we breathe out is believed to contain very small traces of molecules specific to certain types of disease, including some forms of cancer, and Pupeza says that the new laser could help scientists to better understand the cellular processes underpinning those diseases. He also says that such a compact device, being potentially easy to install in hospitals and clinics, could “facilitate a standardized collection of disease-specific molecular fingerprints”.
The researchers are currently working to increase the bandwidth of their device so that it covers the full 2–25 μm band that is relevant to molecular fingerprinting. Other potential applications include detecting explosives or monitoring air quality.
Søren Rud Keiding, a chemist at Aarhus University in Denmark, describes the latest work as “an impressive example of the development of new few-cycle infrared and terahertz sources with extreme brightness”, adding that such sources have “spurred a renewed interest in molecular spectroscopy”. But he notes that the new device is not without precedent. It is, he says, a more powerful version of an ultrashort-pulse source developed by Alfred Leitenstorfer and colleagues at the Technical University of Munich in 2000.
As well as being incredibly fascinating, quantum mechanics is also set to revolutionize the fields of information theory and computation. There exist certain problems that could be solved using a quantum computer in many fewer steps than is possible with a classical computer. One of the key explanations of why this is the case is Shor’s factoring algorithm, which was formulated by the US mathematician Peter Shor in 1994. In this short video, Shor introduces his eponymous mathematical concept.
If you enjoyed this video explainer, then check out more from our 100 Second Science series.
India’s first dedicated astronomy satellite has been launched by the Indian Space Research Organisation (ISRO). The mission took off yesterday on a Polar Satellite Launch Vehicle from ISRO’s Satish Dhawan Space Centre, located in Sriharikota, Andhra Pradesh. Dubbed ASTROSAT, the Rs3.7bn ($70m) mission will study black holes, neutron stars and active galactic nuclei over a wide wavelength range from visible to hard X-rays.
Weighing around 1600 kg, ASTROSAT will operate for five years in a near-equatorial orbit 650 km above the Earth’s surface. It contains a 750 kg payload featuring a suite of five instruments, including imagers and detectors. ASTROSAT will use these instruments for a range of studies, including surveying the skies in the hard X-ray and ultraviolet bands, as well as monitoring the sky for new transients and studying X-ray binaries, active galactic nuclei and clusters of galaxies.
Unmatched capability
Sandip Trivedi is director of the Tata Institute of Fundamental Research (TIFR), which led the construction of three of the probe’s instruments – LAXPC, SXT and CZT. “These will give us a capability in X-ray astronomy that is unmatched, in many ways, globally,” says Trivedi. “We look forward to exciting science results coming out in the near future.”
ASTROSAT will help to replace some of the missing capacity in space-based X-ray observations, particularly following the demise of NASA’s Rossi X-ray Timing Explorer satellite, which was turned off in 2012 after 16 years of operation. “There are currently very few space-based missions able to provide data on transient and variable objects on a regular basis,” David Burrows, an astronomer from Penn State University in the US, told physicsworld.com. “I think that ASTROSAT will be able to contribute significantly to this effort.”
New era
The craft will be controlled by a ground station at ISRO’s Satellite Centre in Bangalore, with data download possible during every visible pass over the city. The satellite is capable of gathering 420 gigabits of data every day. TIFR scientist K P Singh, who is lead scientist for the SXT instrument, says that the mission will “usher a new era in, not just for X-ray astronomy”. “We hope that this will galvanize the astronomy community to use this observatory nationally and globally for the next 5–10 years,” he says. ASTROSAT’s instruments will now be tested for two months before full operation begins.
Light was the theme in the UK’s second city last Friday when I and my colleague James Dacey attended Lightfest at the Library of Birmingham. Organized by Aston University and funded by the European Commission, the festival was a celebration of light in science, art, technology and culture during the International Year of Light (IYL2015).
Outreach activities like these often attract a self-selecting group of people already interested in science, which is great but means that others miss out. The beauty of holding the event at Birmingham’s splendid new library was that it was enjoyed by people who might have had no idea about IYL2015 or who would never have gone out of their way to attend a science event.
That’s because the Library of Birmingham is not just about books; it’s also a great public building where people come to meet up, study, eat or just enjoy the panoramic views of the city from the sunny, roof-top terrace (complete with fragrant herb garden).
In addition to a series of public lectures, there were lots of demonstrations put on by electronic-engineering students from Aston University’s Institute of Photonic Technologies, including a “laser harp”, an LED light cube and a clever gadget that lights up when you receive an e-mail or social-media post. (I’m sure that could make a great geeky product some day.) Local astronomers were doing a spot of solar-watching from the balcony, while film-makers screened a selection of short light-related films.
The festival was officially opened by Julia King, a former chief executive of the Institute of Physics, which publishes Physics World, who has spent the last nine years as vice-chancellor of Aston University. In the photograph above, King (right) was on hand to symbolically accept Maxwell’s Torch.
Created by Mike Stoane Lighting in Edinburgh with support from the Institute, the torch marks the 150th anniversary of James Clerk Maxwell’s famous equations of electromagnetism, which show that electrical and magnetic disturbances travel at the speed of light. The torch has been touring Scotland and this was its first appearance south of the border.
If you want to find out more about IYL2015, don’t forget our collection of 10 of the best features from Physics World on the science and applications of light, which you can access here free of charge. We’ll also be posting some videos from Lightfest on to this website – so stay tuned.
A tiny, 83-page book about some of the basic principles of physics has been a surprise hit in Italy – becoming the single bestselling book of any kind to be published in the country this year.
The book has now been translated into English, entitled Seven Brief Lessons on Physics, and its author – the Italian-born theoretical physicist Carlo Rovelli – dropped by the Physics World offices in Bristol yesterday en route to giving a sold-out lecture about the book as part of the city’s Festival of Ideas.
In the interview above, Rovelli explains what the book’s about, how he managed to condense big physics ideas into such a short space – and why its success was absolutely not what he expected.
When he’s not writing popular-science books, Rovelli is based at the University of Marseilles in France, where he carries out research into loop quantum gravity, which he once tackled for Physics World.
If you want to find out more about the book, check out Penguin’s rather splendid interactive website.
At 2.39 a.m. local time last Monday, I was rudely reminded that I was in a tectonically active region by a magnitude-6.3 earthquake. At the time, I was staying overnight in Santiago, with two flights down and one to go on my way to the ALMA site in the Atacama Desert further north.
A new kind of device that can store information using the magnetic permeability of a material has been created by physicists in the US. The devices could be much more stable than conventional magnetic data-storage devices, which can be corrupted by stray magnetic fields. The devices are also less susceptible to radiation damage, and the inventors claim that the storage method could be adapted to create high-density memory devices.
Magnetic memory chips, magnetic strips on credit cards and hard-disk drives (HDDs) all store information in the magnetization of materials. In this approach, a binary bit (0 or 1) is written to the device by magnetizing a very small region of it. The information can then be read out later by measuring its magnetization.
Corruption problem
But there is one big problem with storing data magnetically, which is that the information can be corrupted by external magnetic fields and by thermal fluctuations. In the case of data stored on credit cards, this has been solved by introducing radio-frequency (RF) chips – but this introduces its own problem. Unlike magnetic bits, which must be read at very short distances, RF chips can be read some distance away – and that can cause security concerns.
The new memory, which could solve both of these problems, has been created by Alan Edelstein and colleagues at the US Army Research Laboratory, along with scientists at Corning, the University of Nebraska and the Naval Research Laboratory. In the new devices, data are stored by changing the magnetic permeability of a tiny region of a magnetic material.
Permeability is a measure of the magnetization of a material that results from the application of a magnetic field. Being an intrinsic property of a material, it is much less susceptible to change by external agents such as stray magnetic fields.
Reading and writing
Edelstein and colleagues have taken an important step towards a practical permeability-based memory by coming up with a way of writing the data and then reading them back. They made several prototype devices using high-permeability alloys of iron and nickel. A thin layer of the alloy was deposited onto a non-magnetic substrate and then lithography techniques were used to create circular regions of alloy just 300 nm across that are the individual data bits.
Each alloy bit has an amorphous atomic structure, which gives it a very high magnetic permeability. However, when a bit is heated by a laser for about 100 μs, it becomes crystalline after cooling, thus giving it a much lower permeability. So, by firing a laser at specific bits, the team was able to write information to the device.
Data were then read from the device by measuring the permeability of the individual bits using a magnetic tunnel junction (MTJ) sensor, which works on the same principles as the read heads in HDDs. This involves applying a magnetic field to all of the bits and then scanning the MTJ across the bits to measure their magnetizations. The measurement confirmed that the bits that had been heated with the laser had much lower permeability than the bits that had not been heated.
Permeability in space
The team also investigated how the devices respond to gamma radiation, and found that their ability to store data is unaffected by relatively high doses of radiation. This could be important for creating memories for use on spacecraft. Current onboard memories are susceptible to radiation damage and must therefore be shielded, which adds to the weight and size of the components.
While the current writing process is limited by the wavelength of the laser, the team points out that technology developed for other heat-assisted memories could be adapted to create bits that are 20 nm across or smaller. Permeability also offers a way to get round an effect called superparamagnetism, which causes the magnetization of very small magnetic bits to randomly flip direction. While superparamagnetism scrambles information stored in the magnetization, it would not affect data stored in the permeability.
The current devices can only be written to once, which makes them suitable for applications such as credit-card strips. However, the team is now working on a re-writable memory based on magnetic permeability.
