Magnetic Switching and Dancing Atoms

Two more of my news stories appear in the most recent issue of Materials Today – you can read them in the Magazine here: http://digital.materialstoday.com/ (May 2013, Volume 16, Number 5). And I’ve reproduced them here too:

Quantum leap in magnetic switching

A new high-speed route to magnetic switching, 1000 times faster than that currently used in magnetic memory technologies,has been developed by researchers at the U.S. Department of Energy’s Ames Laboratory [Li et al., Nature 496 (2013) 69].

Silicon transistor technology has become cheaper, denser and more efficient in the last decade, but fundamental physical limits are being reached. The newest logic and magnetic memory devices, such as hard drives, have begun to approach a maximum switching speed in the gigahertz range (10^9 Hz).

To extend this to higher speeds, new and alternative approaches to device engineering are needed. A group from the Ames Laboratory have looked towards magnetic switching, which is used to encode information in hard drives, magnetic RAM and other computing devices. Their work has used all-optical quantum methods to control magnetism in these devices, and has helped open the door to the terahertz (10^12 Hz) regime and beyond. Currently, switching in ferromagnetic materials uses a combination of magnetic field and continuous laser light. Photoexcitation by the laser causes vibrations within the material, allowing data to be ‘read’. An external magnetic field can ‘write’ to a material, by inducing magnetic flips. So together, they can be used to encode information in the material.

But the maximum achievable speed of this technique is limited by how long it takes to vibrate, or heat, the atoms within the lattice. The Ames team, under Jigang Wang, used short laser pulses to create ultra-fast changes in the magnetic structure of a
colossal magnetoresistive (CMR) material. CMR materials are highly responsive to the external magnetic fields used to write data into memory, but do not require heat to trigger magnetic switching. Using this technique, the material switches from anti-ferromagnetic to ferromagnetic, within a femtosecond.

The team collaborated with researchers from Iowa State University and the University of Crete who confirmed their observations using modelling and theoretical calculations. Devices produced from colossal magnetoresistive material show great promise for use in next-generation memory and logic devices. Wang and his team hope that their all-optical quantum methods will help create devices that can read and write information faster, while consuming less power.

Dancing silicon atoms in a graphene sheet

Scientists at Oak Ridge National Laboratory (ORNL) may have opened the door to atomic-scale tuning of material properties by trapping clusters of silicon atoms within a graphene nanopore [Lee et al., Nat. Commun. 4 (2013) 1650].

An atomic ballet of jumping silicon atoms were the subject of a recent paper in Nature Communications. Silicon has formed the cornerstone of the electronics industry for decades, and so silicon clusters – small groups of bound silicon atoms – have recently come under scrutiny as a potential building block for nanoscale electronic devices. But identification of silicon clusters has primarily been a theoretical pursuit, with a small number of experimen- tal techniques providing some limited information and indirect observations.

A group from ORNL have now directly imaged silicon clusters using a scanning transmission electron microscope (STEM), by pinning the cluster in a nanopore within a sheet of graphene. Recently, nanopore technology has emerged as a powerful tool for DNA sequencing – using an electric field it is possible to force DNA through a gra- phene pore. Its motion can then be used to calculate the diameter and length of the molecule. But this work from Lee et al. is the first to use this technology to trap sili- con clusters. ORNL’s cluster composed of an array of six silicon atoms, the structure of which changed reversibly approximately every 10 s. Under the influence of the electron beam, the atoms move around within the confines of the nanopore.

The group found that an oscillatory motion occurs in the trapped Si6 cluster – a single silicon atom was seen to jump back and forth between two different positions within the cluster. This corresponded to the con- formal change in the structure, and this was confirmed by theoretical modelling of the system. The ability to analyse the structure of small clusters will provide insight into the influence of different atomic configurations on a material’s electronic and optical properties, and may allow for atomic-scale control of these and other properties. In turn, it’s hoped that this discovery could lead to a new generation of atomically-tuned electronic and optoelectronic devices.