ONE April night in 1820, the Danish experimentalist Hans Christian Ăersted made a remarkable discovery. By bringing an electrical wire near a compass lying on his workbench, he found its needle could be made to shiver and dance. Whether a lucky accident or an inspired bit of experimentation, that moment cemented Ăerstedâs reputation. What he had discovered was that electricity and magnetism, long thought to be entirely distinct phenomena, were in fact inextricably linked.
Two hundred years later, this connection powers our world. Moving magnets give rise to electric fields, driving generators in, for example, hydroelectric dams and wind turbines. Flowing electric currents in turn give rise to magnetic fields, such as those used in MRI scanners and particle accelerators like the Large Hadron Collider at CERN. But this symbiosis has its limits. Until recently, it was thought to be impossible to produce a single material that could possess a permanent magnetic field and electric field at the same time.
Then, one day in 1998, a researcher at Yale University named Nicola Spaldin asked a deceptively simple question. Why?
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âIt was a question that really no one was asking, or had thought to ask before,â says Spaldin. That moment marked a turning point in her career and launched a revolution in materials science, a decades-long pursuit of elusive wonder stuff with both properties. Today, the first examples of these so-called multiferroics could change technology for good. There might be no end to their power: from making better solar cells and boosting computational power to helping search for the universeâs missing matter (see âHeroic multiferroicsâ).
Spaldin, now at the Swiss Federal Institute of Technology in Zurich, was ideally suited to hunt such substances. âMy passion is really electrons,â she says. âI love thinking about them.â That boded well because understanding electrons is key to understanding why multiferroics are so valuable.
Virtually all the matter that we can see is made up of atoms. These, in turn, consist of electrons spinning around a nucleus formed of protons and neutrons. Despite their tiny size, electrons play a vital role in determining a materialâs electric and magnetic properties.
Letâs take magnetism first. All electrons have a quantum property called spin that can be thought of as an arrow that points in one of two directions. Most of the time, these arrows are oriented randomly, with no one direction dominating. In some materials, however, the arrows get in formation when they are exposed to an external magnetic field. If all the arrows are aligned the same way, the material starts generating a magnetic field of its own. Materials like iron, which are capable of becoming magnetised and retaining their magnetism even when the external field is removed, are said to be ferromagnetic.
âIt was a question that really no one was asking, or had thought to ask beforeâ
Ferromagnets are everywhere in our daily lives. A compass needle is one example, and your fridge is probably covered with dozens more, holding up your holiday snaps and reminders. Less well known to most of us, but also well-established, are ferroelectrics â materials that can produce electric fields, used today to power some types of computer chip.
Their superpower, just like ferromagnetism, starts with electrons. Briefly put, some materials have a mix of charged atoms built into their structure. If an electric field is applied to the material, these charges can permanently shift, and the separation of negative and positive charges generates a tiny electric field, called a dipole. When these dipoles line up in the same direction, they form what is called an electric polarisation. This means the material produces an electric field. Materials that can do this are said to be ferroelectric (see âFields of dreamsâ).
âFerroelectric behaviour was first seen in a laxative called Rochelle saltâ
The first observation of ferroelectric behaviour came in an unlikely substance: a laxative called Rochelle salt, developed by a French pharmacist in the 17th century. Its creator wouldnât reveal his recipe, but its ingredients werenât its only secret. In 1824, Scottish physicist David Brewster observed that Rochelle salt is pyroelectric, which means it produces a small voltage when heated or cooled. And in 1880, the Curie brothers â Jacques and Pierre â showed that it was also piezoelectric, generating voltage when it was squeezed, stretched or otherwise physically deformed. In 1899, Thomas Edison took advantage of Rochelle saltâs piezoelectricity to build a commercial version of his phonograph to play back sound recordings.
Those early findings suggested something strange was happening in the saltâs atoms. The situation became even more interesting in 1921, when a physicist at the University of Minnesota found that if Rochelle salt was immersed in an electric field, its electric charges line up. Even if the electric field is taken away, the charges stay put, and the salt produces its own electric field. By analogy with ferromagnetism, which had been known about for millennia, this property was called ferroelectricity.
Coming just a century after Ăerstedâs demonstration, this Rochelle salt experiment deepened the known connections between electricity and magnetism. Given this relationship, you might think that getting ferroelectricity and ferromagnetism into the same material would be easy. But no such luck. âWhen you have magnetic materials, theyâre almost by definition not ferroelectric,â says materials scientist Manfred Fiebig at the Swiss Federal Institute of Technology.
Mutually exclusive
The logic is fairly simple: magnetism only occurs because electrons, in order to align their spins, must be free to move between atoms. For a ferroelectric material to create an electric field, charges must be free to move when an external field is applied â but then stay in place. âIt is not a trivial thing. You want to relate two different kinds of physical phenomenon. One with currents, one with stationary charges. How do you create materials that have both of these properties?â asks Ramamoorthy Ramesh at the University of California, Berkeley. âThese two things are in some sense pointing in opposite directions.â
But that didnât stop scientists from looking for examples. In the 1950s, Soviet physicists developed a synthetic material that had flickers of promising properties when cooled to below 0°C, but these vanished at room temperature, limiting their usefulness. In 1965, Swiss physicists overcame some of these difficulties, but the fragility of their material meant that industry wouldnât bite.
The next three decades brought a steady trickle of experimental attempts to mix magnetic and ferroelectric ingredients, but multiferroics remained mostly out of reach, difficult to make and harder to use.
This is where Spaldin comes in. When she left Yale for a new position at the University of California, Santa Barbara, she took a bold tack. She abandoned her original research plans and instead dedicated herself to hunting multiferroics full-time. Then, in 2000, she published an electrifying paper that changed everything. It was titled, simply: ââ
Her short, sharp analysis of the necessary properties of such materials was inspirational. One researcher she inspired was Ramesh, then working on the other side of the country. He had been conducting experiments on a synthetic compound called bismuth ferrite, and the weird results he was seeing seemed to match the signature of Spaldinâs multiferroics. So he picked up the phone.
âSpaldin changed her plans and started to hunt multiferroics full-timeâ
âI remember it very clearly,â says Spaldin. âHe was very Californian. He didnât know me very well, but he just asked, âWhat do you think is the electric polarisation of bismuth ferrite?â
That unconventional opening line launched a collaboration: Spaldin with the theory and big vision, Ramesh with the materials-making background. As it turned out, bismuth ferrite was the perfect candidate. On a microscopic level, it consists of a lattice of bismuth atoms interspersed with charged ions of iron and oxygen. The structure of the bismuth atoms provides the ferroelectricity, and the wiggling electrons in the iron ions supply the magnetic boost. But it isnât enough to simply have these two on their own, says Ramesh. The oxygen atoms play a crucial role too, creating the stable geometry that allows both properties to emerge. âYou have to have all of them together in a certain way,â he says.
When a plan comes together
In 2003, Spaldin and Ramesh reported on their first observation of multiferroicity in this substance. For the first time in history they had an example of this material that lent itself to applications. It had the necessary superpowers and it maintained its properties at room temperature. The group also showed that it was ideally suited to uses in computing, especially memory (see âBoosting memoryâ).
The revelation that practical multiferroics existed sparked a revolution. Before 2003, the related terms âmultiferroicâ or âelectro-ferromagneticâ were mentioned in a few hundred papers. Since 2003, they have shown up more than 32,000 times. The field exploded beyond the reach of Spaldin, as labs around the world took up the challenge to make and explore their own multiferroics.
âIt was exactly like a Bollywood movie, with a lot of fight scenes, and people crying, and dance sequences, things like that, but translated into physics,â jokes Ramesh. Since then, he and Spaldin and other physicists have been locked in an ongoing race to extract the next surprising, serendipitous revelation from a class of materials that just keeps on giving.
In some ways, though, computer memory turned out to be the low-hanging fruit. In the past few years, researchers have realised that these materials have so much more to offer, including in endeavours as diverse as medicine and cosmology (see âHeroic multiferroicsâ). âThe applications have exploded beyond what we ever imagined,â says Spaldin. In fact, she notes that perhaps the biggest surprise to emerge from the past two decades is how many uses have been found for multiferroic materials that have nothing to do with the coupling of magnetic and electric behaviours. âIn many applications, weâre finding that the multiferroicity itself isnât as interesting as something else that came along with it,â says Fiebig. For example, many multiferroics have a structure that makes them exceptional harvesters of solar energy. In principle at least, that means they should have conversion rates far greater than todayâs silicon-based top performers.
Better and more efficient multiferroics are surely still out there. And, no doubt, somewhere beyond them are entirely new classes of material with as-yet-undreamed-of combinations of natural properties. Perhaps all it will take for us to root them out is for someone like Spaldin to start asking the right questions.
Heroic multiferroics
Cancer detection and brain mapping
From the signal-sending of neurons to the ion channels of cells, your body is positively tingling with electrical activity. âIf you have access to electricity at the molecular level, then you can actually control cells, treat diseases and even control biological processes,â says Sakhrat Khizroev, a physicist and inventor at the University of Miami in Florida looking for medical applications for a new class of wonder materials called multiferroics (see main story).
The potential is vast. Multiferroics might reduce the need for invasive techniques by being made into nanobots designed to swim through blood vessels and deliver life-saving drugs. They would be guided by magnetic fields outside the body and able to interact with tissues through their electric properties.
For his part, Khizroev has developed multiferroic nanoparticles to spot signs of cancer. The idea is that once inside the body, the multiferroics flag cancerous cells in a way that can be detected through nuclear magnetic resonance imaging. Studies suggest they can outperform traditional, cobalt-based nanoparticles.
Ultimately, he has other targets in mind, including the brain. âThe brain is more energy-efficient than any computer working today,â he says. His vision is to use multiferroic particles to map the organâs network of neurons, and then develop a computer based on that map.
Unpicking string theory
String theory is one of the most popular (and controversial) candidates for a theory of everything, a unified mathematical framework capable of describing the entirety of physics. Among its predictions is that everything in the universe is made of unbelievably tiny strings, whose vibrations correspond to the subatomic particles we see in our daily lives.
In the 1970s, physicist Tom Kibble described how such strings might arise in the early universe, in the moments shortly after the big bang. Gaining insight into those conditions appears impossibly difficult, but Kibble identified a number of mathematical symmetries that the early universe should obey. If someone could only find something with those symmetries, they might be able to model those primordial conditions.
Forty years later, Nicola Spaldin, while at the University of California, Santa Barbara, proposed a multiferroic called yttrium manganite as the answer. The equations that describe the material as it changes polarisation, wrote Spaldin, match the conditions Kibble laid out to such an extent that it is the âcrystallographic equivalent of cosmic stringsâ. She suggests that the material may enable physicists to simulate the conditions that prevailed billions of years in the past.
Finding dark matter
Around 85 per cent of the matter in the universe is invisible. We know this so-called dark matter is out there because of its gravitational effects, but nobody has yet spotted it directly. Sinéad Griffin at the Lawrence Berkeley National Lab in California is one of the many physicists looking to change that.
Her idea is simple. As the Earth whooshes through a big cloud of dark matter, that directional motion might give rise to a kind of invisible wind. Such a wind would impart energy that ordinary matter could pick up, providing evidence of dark matterâs existence and possibly its composition.
âA smoking gun for a dark matter experiment would be getting this directionality,â says Griffin. âIf you have a target that can pick this up, itâs enough.â
Conveniently, the energy range that multiferroics are sensitive to is exactly right for picking up the likely constituents of dark matter. Most dark matter detectors are huge vats of inert liquids, tucked away deep underground, that are looking for high-energy particles. But they have largely seen a lot of nothing. The detectors that Griffin envisions would pick up even fainter signals, making them an exciting option.
Boosting memory
Computer hard drives are essentially made of tiny magnets whose polarity can be used to store binary information. Magnets point one way for a â1â or flipped over for a â0â. Right now, computers use an electric current, applied directly through a wire, to flip the magnets when necessary.
Multiferroics suggest another way. Earlier this year, Ramamoorthy Rameshâs group at the University of California, Berkeley, unveiled a multiferroic electrical component that also stores information as 0s and 1s. But unlike ordinary hardware, it doesnât need current from a wire. Instead, these switches can be flipped by applying an external electric field. That may not seem like a big deal, until you consider it would take 10 to 30 times less energy per bit of information stored to do it this way.
Given the explosive growth of power-hungry technologies like the internet of things, self-driving cars and artificial intelligence, it is small wonder that companies like Intel are actively pursuing such devices.
Article amended on 16 December 2019
We corrected the examples of uses of moving magnets giving rise to electric fields.
