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 Néel, 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 bottleneck 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)