EXTREMAG: an Exeter-based Time Resolved Magnetism Facility

Lead Research Organisation: University of Exeter
Department Name: Physics and Astronomy


Understanding the workings of our every day 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 10-9s. 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 10-13s.

In this project we will construct a new ultrafast laser facility 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. We will therefore analyze the polarization change of reflected ultrafast laser pulses 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 now share its expertise with external users of 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 will be 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.

Planned Impact

The EXTREMAG facility will study the high frequency properties of magnetic and spintronic materials and devices. New knowledge will be obtained of how the magnetic state evolves on sub-nanosecond timescales. Magneto-optical measurements allow the magnetization to be sensed directly and non-invasively, the focused optical probe may be raster-scanned over the sample to yield time resolved images, and femtosecond temporal resolution may be readily achieved. The developed measurement techniques could be used for metrology of partially built wafers within a manufacturing environment, checking local properties so that bad wafers may be stopped, with consequent cost-savings.

High frequency measurement is of growing importance across information technology (IT). The use of state of the art electrical and optical measurement apparatus will equip early career researchers (ECRs) for employment in academia or high-tech industry. Prospective users from Exeter and beyond are affiliated to 6 Centres for Doctoral Training (CDTs), which will benefit from expertise at the facility. For ECRs from outside Exeter, participation in research at external facilities, where time allocation is predetermined and finite, also develops planning and team working skills that are valued by employers.

Research in thin film and nanoscale magnetism is highly relevant to present and future IT, in non-volatile data storage, but also in data processing and communications. As bit rates increase, the response of magnetic components to different stimuli on sub-ns timescales becomes ever more important. EXTREMAG will provide understanding of high frequency processes, for which potential uses in technology are real and immediate. The response of nanostructured magnetic materials to pulsed magnetic fields is key to the operation of hard disk drives (HDDs). However, prospects for further scaling of HDD technology are limited because smaller magnetic grains that are thermally stable require infeasibly large write fields. Seagate has targeted heat-assisted magnetic recording (HAMR) for delivery of increased data density, which requires improved understanding of how storage media respond to pulsed optical excitation. Seagate already manufacture about 30% of the world supply of recording head transducers in Northern Ireland, providing thousands of jobs. Exeter and many prospective users of EXTREMAG already work closely with Seagate and other players in the recording industry, allowing economic benefit to be obtained from their basic academic research.

Magnetic random access memory (MRAM) written by spin transfer torque (STT) has been demonstrated, but reliability must be improved. Improved understanding of alternative mechanisms obtained at EXTREMAG will accelerate the development of MRAM as a product for use in computer memory. While there are currently no plans for manufacture within the UK, as MRAM finds more applications, mainstream integrated circuit manufacturers (e.g. Plessey in Plymouth) may wish to embed MRAM into other products. Seagate already possess know-how and capability relevant to MRAM manufacturer and could elect to enter this market. A reservoir of university researchers with relevant expertise will be important in attracting inward investment.

Memory, data storage, and wireless communications are key components of the Big Data revolution that has multiple perceived societal benefits, e.g in preemptive intervention within healthcare. The intention to record and process more information about our lives assumes that the underlying hardware has the necessary speed, capacity and energy efficiency. The development of technologies such as HDDs and MRAM will help deliver this vision. Many of EXTREMAG's users are active in outreach activity, with reach from schools to the national media, and so represent a conduit through which the excitement of fundamental science and its potential uses can be conveyed to a general audience.
Title Dataset associated with 'Scaling of the Dzyaloshinskii-Moriya interaction with magnetization in Pt/Co(Fe)B/Ir multilayers' 
Description Magnetic multilayers with perpendicular anisotropy and an interfacial Dzyaloshinskii-Moriya interaction contain chiral domain walls and skyrmions that are promising for applications. Here we measure the temperature dependence of the Dzyaloshinskii-Moriya interaction (DMI) in Pt/CoFeB/Ir and Pt/CoB/Ir multilayers by means of static domain imaging. First, the temperature dependences of saturation magnetization ($M_{\rm{S}}$), exchange stiffness ($A$) and intrinsic perpendicular anisotropy ($K_{\rm{u}}$) are determined. Then the demagnetized domain pattern in each multilayer is imaged by wide-field Kerr microscopy in the temperature range 9-290 K, and the characteristic domain period at each temperature is determined. We calculate the DMI constant $D$ from an analytical expression for the domain wall energy density that treats the multilayer as a uniform medium. Scaling laws for $K_{\rm{u}}$ and $D$ with the magnetization are established from the experiments. While the scaling of $K_{\rm{u}}$ is consistent with Callen-Callen theory, we find that the scaling of $D$ is similar to that of $A$ predicted theoretically ($\sim1.8$). 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
URL https://archive.researchdata.leeds.ac.uk/907/