- Single pulse all-optical toggle switching of magnetization without gadolinium in the ferrimagnet Mn2RuxGa
Chandrima Banerjee, Niclas Teichert, Katarzyna Siewierska, Zsolt Gercsi, Gwenael Atcheson, Plamen Stamenov, Michael Coey and Jean Besbas, Nature Comunications, 7th September 2020.
- Exchange-driven all-optical magnetic switching in compensated 3d ferrimagnets
C. S. Davies, G. Bonfiglio, K. Rode, J.Besbas, P.Stamenov, C. Banerjee, J. M. D. Coey, A. V. Kimmel and A Kirilyuk, Physical Review Research, 2 032044 (R).
- Collective magnetic response of CeO2 nanoparticles, M. Coey, K. Ackland, M. Venkatesan and S. Sen, Nature Physics (2016) doi.10.1038/nphys3676
- Mesoscopic structure formation in condensed matter due to vacuum fluctuations, S. Sen, K. S. Gupta and J. M. D. Coey, Phys. Rev B 92 155115 (2015)
New Magnets Flip in a Flash!
Chandrima Banerjee and Jean Besbas recently discovered that 100 fs laser pulses could switch the magnetization of thin films of our favourite ferrimagnetic, the Heusler half-metal Mn2RuxGa (MRG). When struck by a sequence of laser pulses, MRG abruptly flips its direction with each one. So far, this single-pulse all-optical toggle switching had only ever been seen in amorphous Gd(Fe,Co)3. Finding it in a useful spintronic compound opens new possibilities. Our paper in Nature Communications shows that demagnetization, by fast exchange of angular momentum between the two manganese sublattices, takes less than a picosecond, and that the compensation temperature has to be above ambient to get the fleeting parallel alignment of sublattice moments needed for them re-magnetize in reversed directions. Low-energy, ultra-fast magnetic switches open the prospect for real-time multiplexing of optical communications that could increase the capacity of the global fibre-optic network by an order of magnitude – without laying a single kilometre of new fibre. We followed up the work with colleagues in Nijmegen, who use a free-electron laser.
Chandrima Banerjee and Jean Besbas
Single-pulse all-optical switching in Mn2Ru1.0Ga.
Fluid Flow with no Walls
A paper in Nature by a Strasbourg team spearheaded by Perter Dunne, including Michael Coey relates how, by using magnetic confinement it is possible to pump, pinch and mix liquids without them ever touching a solid surface. Peter graduated from the our Group in 2011 with a PhD on Near-electrode Effects in Magneto-electrochemistry. He developed an earlier idea of partial magnetic confinement and showed how it is possible to conduct all the functions of microfluidics in aqueous channels as narrow as 10 µm when they are stabilized at the centre of a quadrupole field produced by permanent magnets acting on a ferrofluid. Blood in particular can be pumped much more gently than by normal methods, which may be useful in future for heart transplants. We work together with the Strasbourg group in the MaMi Marie-Curie training network, which supports Tim Butcher and Sruthy Poulose
Trinity Physicists Capture Magnetism from Empty Space
Theoretical physicists have long believed that empty space is not a formless void. The vacuum is thought to be seething with zero-point energy, the inevitable quantum residue of every sort of electromagnetic radiation. Nobody has ever managed to find a way to tap this limitless store of energy, and the signs that it even exists are slender. Direct evidence is mainly based on a tiny shift of a few parts per billion in an energy level of hydrogen, discovered 70 years ago by Willis Lamb in the USA, and a minute force between metal plates when they are only a few nanometers apart in vacuum predicted around the same time by Dutchman Hendrik Casimir.
Now, a team of scientists working in Trinity College Dublin's Centre for Research on Advanced Nanostructures and Nanodevices (CRANN) have discovered a new and unexpected manifestation of this elusive energy. In a study of cerium dioxide nanoparticles – mainly used in catalytic converters that control toxic exhaust emissions from automobiles – Professor Michael Coey, his former PhD student Karl Ackland and Dr Munuswamy Venkatesan came across a strange magnetic effect. Quite unlike the behavior of normal magnets like iron, the effect did not vary at all with temperature. Stranger still, the magnetism only appeared when the particles were clumped together. Separating them into smaller clumps by diluting with nonmagnetic nanoparticles destroyed the magnetism.
In a paper published on-line today in Nature Physics, the team that was completed by Professor Siddhartha Sen, a quantum field theorist renown for his brilliant lectures in Trinity's Mathematics Department, reported their findings, and came up with an astonishing explanation. Electrons in the clumps of tiny particles were responding to the vacuum electromagnetic field. Sen and Coey had recently predicted that such behavior might be possible in systems with an enormous surface area – a milligram speck of the cerium dioxide nanoparticles has as much surface area as an entire sheet of newspaper. Furthermore they predicted that when the particles were separated out into regions smaller that the wavelength of the light associated with them, the effect would disappear. This is exactly what was observed.
As with any fundamental discovery in science, it is difficult to predict where this could lead. Others will want to test the results. The theory shows that effects can only be expected when there is a huge surface to volume ratio, as in the thin layers of interfacial water attached to biomolecules. Sen is already beginning to apply the ideas to protein folding. The zero point energy may never power our cars, but it might be shaping our lives.
Singapore/TCD Team Discover a New Magnetic Interaction
The big data revolution relies on vast stores of digital information that are growing at an explosive pace. Most of us never spare a thought about where all the data we download from the Cloud onto our hand-held devices really comes from. In fact, it is all stored in minute magnetic dots written in ultra-thin layers only a few nanometers thick that cover the surface of millions of saucer-sized spinning discs. These hard discs are stacked by the thousand in racks in 'server farms' distributed across the planet. Goggle and Facebook each have one in Ireland, Amazon and Microsoft each have one in Singapore.
In recent years, technology has been perfected for growing uniform magnetic layers only 10 – 100 atoms thick and combining then into complex stacks; these nanostructures are the foundation of 'spin electronics'; the 'spin' here refers to the resemblance between the electron and a spinning ball of electric charge. It is the spin that makes the electron a tiny magnet. By analogy with the solar system, where spinning planets orbit the Sun, the atom is composed spinning electrons that orbit the nucleus. Spin and orbital motion each generates its own type of magnetism.
Two adjacent magnetic layers in a thin film stack couple together when they close enough to exchange electrons with each other. The electrons carry across their spin, and the directions of magnetization of the two layers are aligned. This coupling is broken if the two magnetic layers are separated by an insulating spacer that is more than a few atoms thick. The insulator is almost impenetrable for the free electrons.
Now a team led by Professors Venky Venkatesan and Ariando at the National University of Singapore have made a startling discovery, which they report this week in Nature Communications in their paper entitled 'Long-range magnetic coupling across a polar insulating layer'. By choosing a special type of insulator that has its opposite surfaces covered with positive or negative electric charge, Weiming Lv (now a Professor at Harbin Institute of Technology) found that the range of the magnetic coupling jumps from about one nanometer to more than ten, and its strength oscillates with spacer thickness. No electrons could ever make their way across such an impenetrable layer, so how can the two magnetic layers be coupled? Here Visiting Professor Michael Coey, from Trinity College Dublin came up with a suggestion. Instead of spin magnetism being carried across directly by messenger electrons, it is the orbital magnetism that is passed along from atom to the next across the insulator. The atomic electrons are engaged in a dance, each twirling their partners on the neighbouring atoms until the orbital motion reaches the other side.
Discoveries in magnetism have a habit of turning out to be useful, though it may take years for the right application to become apparent. The French physicist Louis Né el, discovered antiferromagnetism in the course of his thesis work in the 1930s, but he could think of no practical use for antiferromagnets in his 1970 Nobel Prize acceptance speech. Yet by 1990, antiferromagnetic layers had become indispensable components of the thin film stacks used in spin electronics. The NUS team point out that the frequency of the orbital excitations lies in the terahertz frequency range, currently a bottlekneck for progress in the big data revolution, which is demanding ever-faster data transmission rates. Nowadays it should not take 60 years to find an application for new discoveries in magnetism.
Lv, Weiming et al. Nature Communications, 10.1038/NCOMMS11015 (2016)
Magnetic switching without a Magnetic Field
Magnetic and electric switches underpin the information revolution. The magnetic switches are tiny patches of magnetic material that store the information; they can be magnetized in one of two opposite directions 'up' or 'down' to represent the binary digits '0' and '1'. Everything we download daily onto our computers or mobile phones is stored magnetically on millions of spinning discs located in data centres scattered across the world.
Unfortunately, writing and erasing all this information currently needs a magnetic field to switch the magnetic bits. These fields are produced by passing current pulses through minute coils, consuming vast amounts of energy in the process. Now a team in the Magnetism and Spin Electronics Group at Trinity College Dublin have devised an elegant scheme for magnetic switching that does away with any need for a magnetic field.
Two PhD students, Yong Chang Lau from Malaysia and Davide Betto from Italy, working with senior researcher Karsten Rode and physics professors Michael Coey and Plamen Stamenov publish their results in the prestigious journal Nature Nanotechnology this week.
Their device consists of a stack of five metal layers, each of them a few nanometers thick. At the bottom of the stack is a layer made of platinum, and above it is the iron-based magnetic storage layer just six atoms thick. Platinum is a favourite of researchers in spin electronics (also known as spintronics), the technology that makes use of the property that each electron is a tiny magnet. Passing a current through the platinum separates the electrons with their magnetism pointing in opposite directions at the top and bottom surfaces. Those pumped into the storage layer try to switch its magnetic direction, thanks to an effect known a 'spin-orbit torque' that follows from Einstein's theory of relativity. But like a pencil balanced on its point, the magnetism of the storage layer doesn't know which way to fall. The team designed the rest of the stack to solve that dilemma by acting like a nanoscale permanent magnet that creates the small field necessary to make the switching determinate, at zero cost in energy.
The Group now plan to demonstrate a full memory cell, and an ultra-fast oscillator based on spin-orbit torque and layers of a novel magnetic alloy they developed recently. The thin film device stacks will be grown in a new SFI-funded thin film facility in the AMBER Centre at Trinity's CRANN Institute for nanoscience. These new spintronic devices have potential to deliver the breakthrough needed to sustain the information revolution for another 25 years.