Expanded access to the Exeter time resolved magnetism (EXTREMAG) facility

Understanding the workings of our everyday world requires us to examine its constituent components under increased magnification until the underlying fundamental processes are observed. In many cases it is sufficient to examine objects that are tens or hundreds of atoms in length, at what is known as the nanoscale. Nanotechnology seeks to tame this Lilliputian world, creating artificial structures that deliver a specific function, leading to the construction of macroscopic materials and devices with useful properties. However, we must remember that the nanoscale world moves at very high speed, with many processes occurring in times of less than a single nanosecond (ns), or 10e-9 s. Our computers have central processor units that run at GHz clock speeds, while fiber optic cables transfer data at rates of Gbits per second. This means that each logical operation within the computer, or the sending of each bit (a 0 or 1 in binary code) of information, occurs in less than 1 ns. The same is true of devices for storage of information such as the hard disk drives (HDDs) that are found within our computers and the server farms for cloud computing, where data is written or retrieved on sub-ns timescales.

The human eye is capable of observing changes that occur on timescales of about one tenth of a second, so different methods are required to observe changes within the nanoworld. Sometimes the rotating blades of a helicopter may appear stationary on a TV screen. The TV presents a series of still images at a rate of about one hundred per second, which is sufficient to convince the eye that the subject is moving continuously. However, if by coincidence the helicopter blades are in the same position each time an image is acquired, the blades will instead appear stationary. This is the principle of stroboscopic imaging of repetitive phenomena, whereby the motion of the subject is deliberately synchronized with the acquisition of images by the camera. By slightly shifting the relative time at which the pictures are taken, the subject can be frozen at any point in its cycle of motion. The shutter speed of a conventional camera is too slow to capture sub-ns changes, so it is instead necessary to leave the shutter open and use a flash-gun that is synchronized with the subject. Today an ultrafast laser is able to generate a beam of pulses that each has duration of less than 100 femtosconds (fs), or 10e-13 s.

The Exeter time resolved magnetism (EXTREMAG) facility uses ultrafast laser sources to probe and image the sub-ns dynamics of magnetic and spintronic systems. While conventional electronics control the movement of electrons by means of electric fields acting upon the electron charge, the electron also has a magnetic moment, as if the charged particle were "spinning" about an axis through its centre. Spintronics seeks to control the movement of the electron via its magnetic moment. The magnetization of a magnetic material causes a small but measurable change in the polarization of light reflected from its surface. Analysis of the polarization changes of reflected ultrafast laser pulses can therefore be used to obtain time resolved images of magnetization in magnetic and spintronic materials and devices.

The University of Exeter has a long track record in the use of time-resolved magneto-optical imaging and will share its expertise with external users through the EXTREMAG facility. The users have a very broad range of interests ranging from switching of magnetic moments to store information in the next generation of HDDs, to understanding the formation and dynamics of objects such as vortices and skyrmions that may form within the magnetization, to manipulating the magnetic state through interaction with superconducting materials. While the research is of a fundamental nature, it will power the future development of information technology that will have a profound impact on the way we live and work.