System Bits: May 13

Bringing hyperbolic metamaterials closer to reality
Purdue Researchers have taken a step toward practical applications for hyperbolic metamaterials, which are ultra-thin crystalline films that could bring optical advances for microscopes, quantum computers and high-performance solar cells.

Optical metamaterials harness clouds of electrons called surface plasmons to manipulate and control light. However, some of the plasmonic components under development rely on the use of metals such as gold and silver, which are incompatible with the complementary metal–oxide–semiconductor (CMOS) manufacturing process used to construct integrated circuits and do not transmit light efficiently, the researchers said.

However, they’ve now shown how to create “superlattice” crystals from layers of the metal titanium nitride and aluminum scandium nitride, a dielectric, or insulator. Superlattices are crystals that can be grown continuously by adding new layers, a requirement for practical application.

The Purdue team created the superlattices using a method called epitaxy, “growing” the layers inside a vacuum chamber with a technique known as magnetron sputtering. It is difficult to use the technique to create structures that have sharply defined, ultra-thin and ultra-smooth layers of two different materials.

“Hyperbolic metamaterials” could bring optical advances including powerful microscopes, quantum computers and high-performance solar cells. The graphic at left depicts a metamaterial’s “hyperbolic dispersion” of light. At center is a high-resolution transmission electron microscope image showing the interface of titanium nitride and aluminum scandium nitride in a “superlattice” that is promising for potential applications. At right are two images created using a method called fast Fourier transform to see individual layers in the material. (Source: Purdue University)

The list of possible applications for metamaterials includes a “planar hyperlens” that could make optical microscopes 10 times more powerful and able to see objects as small as DNA, advanced sensors, more efficient solar collectors, and quantum computing.

The magnetism of a single atom
An EPFL-led research collaboration has shown, in what they say is the first time, the maximum theoretical limit of energy needed to control the magnetization of a single atom that could have positive implications for the future of magnetic research and technology.

Magnetic devices like hard drives, magnetic random access memories, molecular magnets, and quantum computers depend on the manipulation of magnetic properties, they explained. In an atom, magnetism arises from the spin and orbital momentum of its electrons. ‘Magnetic anisotropy’ describes how an atom’s magnetic properties depend on the orientation of the electrons’ orbits relative to the structure of a material. It also provides directionality and stability to magnetization.

A research team led by EPFL combined various experimental and computational methods to measure the energy needed to change the magnetic anisotropy of a single Cobalt atom. They believe their methodology and findings can impact a range of fields from fundamental studies of single atom and single molecule magnetism to the design of spintronic device architectures.

Magnetism is used widely in technologies from hard drives to magnetic resonance, and even in quantum computer designs. In theory, every atom or molecule has the potential to be magnetic, since this depends on the movement of its electrons. Electrons move in two ways: Spin, which can loosely be thought as spinning around themselves, and orbit, which refers to an electron’s movement around the nucleus of its atom. The spin and orbital motion gives rise to the magnetization, similar to an electric current circulating in a coil and producing a magnetic field. The spinning direction of the electrons therefore defines the direction of the magnetization in a material.

The magnetic properties of a material have a certain ‘preference’ or ‘stubbornness’ towards a specific direction. This phenomenon is referred to as ‘magnetic anisotropy’, and is described as the “directional dependence” of a material’s magnetism. Changing this ‘preference’ requires a certain amount of energy. The total energy corresponding to a material’s magnetic anisotropy is a fundamental constraint to the downscaling of magnetic devices like MRAMs, computer hard drives and even quantum computers, which use different electron spin states as distinct information units, or ‘qubits’.

The team of EPFL, ETH Zurich, Paul Scherrer Institute, and IBM Almaden Research Center developed a method to determine the maximum possible magnetic anisotropy for a single Cobalt atom. Cobalt, which is classed as a ‘transition metal’, is widely used in the fabrication of permanent magnets as well as in magnetic recording materials for data storage applications.