The Strain Manipulation of Nanoscale Magnetic Structures
Lead Research Organisation:
University of Nottingham
Department Name: Sch of Physics & Astronomy
Abstract
Many of the components in modern technological devices such as computers, communications devices (e.g. mobile phones) and sensors are made on a very small scale from magnetic materials. For example, modern computer hard drives and magnetic random access memory (MRAM) contain magnetic elements that are a few tens of nanometres in size. In such devices the direction of the magnetisation of the magnetic elements is used to store information. Controlling the direction of magnetisation is achieved by using electrical current to generate a magnetic field locally or by passing an electrical current through the device using an effect called spin transfer torque . These techniques have disadvantages arising from the energy dissipated in applying electrical currents, the limits on miniaturisation (due to the need to integrate the components which generate the field with other magnetic devices) and the difficulty in addressing individual elements due to stray magnetic fields. A solution to these problems would be to create devices in which the magnetic state is controlled by applying electrical voltages. In this project I will do this by adopting a novel approach, combining the magnetic material with piezoelectric material in hybrid devices. Piezoelectric material has the property that it will physically expand or contract when an electrical voltage is applied to it. This can be used to transfer strain to the magnetic material. Certain magnetic materials have large magnetostrictive properties, which means that if they are strained then the magnetisation direction will rotate. For example, I will study the magnetostrictive transition metal alloys FeCo, FePd and FePt. I will study the magnetic properties of these materials in the bulk and on the nanoscale using modern characterisation techniques such as Superconducting Quantum Interference Device (SQUID) magnetometry and Magnetic Force Microscopy (MFM), and I will use state of the art growth and fabrication techniques (e.g. sputter deposition and electron beam lithography) to fabricate devices a few tens of nanometres in size. By conducting electrical transport experiments at GHz frequencies (comparable to the frequencies used in modern computing technology) I aim to demonstrate ultra-fast switching of the magnetic state of the devices by applying ultra-fast (picosecond) voltage pulses. The nanoscale devices will also be used to study the fundamental physics of phenomena such spin transfer torque . Another class of devices that I will study are nano-electro-mechanical systems (NEMS) which consist of nanoscale oscillating beams and cantilevers. Such devices have potential applications as highly sensitive weighing scales and are also interesting for more fundamental studies of the overlap between quantum and classical physics. The use of magnetostrictive ferromagnetic materials to fabricate NEMS will offer new means to detect and drive the mechanical oscillations.This proposal presents exciting opportunities to study fundamental physical phenomena in new material systems and promises to produce knowledge of new phenomena and new functionalities in nanoscale devices. The results of this work will contribute to the design of future computing, communications and sensor technologies.
Publications
Beardsley R
(2017)
Effect of lithographically-induced strain relaxation on the magnetic domain configuration in microfabricated epitaxially grown Fe81Ga19
in Scientific Reports
Beardsley RP
(2017)
Deterministic control of magnetic vortex wall chirality by electric field.
in Scientific reports
Casiraghi A
(2011)
Fast switching of magnetization in the ferromagnetic semiconductor (Ga,Mn)(As,P) using nonequilibrium phonon pulses
in Applied Physics Letters
Casiraghi A
(2012)
Piezoelectric strain induced variation of the magnetic anisotropy in a high Curie temperature (Ga,Mn)As sample
in Applied Physics Letters
Cavill S
(2013)
Electrical control of magnetic reversal processes in magnetostrictive structures
in Applied Physics Letters
Danilov A
(2018)
Optically excited spin pumping mediating collective magnetization dynamics in a spin valve structure
in Physical Review B
| Description | Key Findings of EPSRC grant EP/H003487/1 "The strain manipulation of nanoscale magnetic structures" The main aim of this project was to investigate how mechanical strain can be used to manipulate the magnetisation in nanoscale structures. By using voltages to induce strain in hybrid piezoelectric/ferromagnetic devices, a possible route to low energy control of magnetisation was investigated, with potential applications in information storage and communications technologies. Materials development: An important component of the devices to be investigated was the ferromagnetic film possessing a large magnetostriction. We investigated Galfenol (an alloy of Fe and Ga) for this purpose. It was discovered recently that alloying Fe with Ga increases the magnetostriction by over an order of magnitude, up to a Ga concentration of 19%. The magnetostriction is enhanced mainly along the [100] crystal direction, so single crystal samples are important to observe significant effects. We investigated the development of single crystal thin films of Fe81Ga19 to determine if films with magnetostriction comparable to bulk samples could be achieved. Using the techniques of molecular beam epitaxy and sputter deposition to deposit Fe81Ga19 films onto closely lattice matched GaAs(001) substrates, we showed that single crystal thin films of 20nm thickness could be produced with magnetostriction as large as in the best measured bulk single crystal samples[Parkes et al., Scientific Reports 3, 2220, (2013)]. Non-volatile switching: The Fe81Ga19 thin films described in the previous section possessed a biaxial magnetic anisotropy in the plane. Thus, there are two stable magnetisation directions that can be set which are non-volatile at room temperature in the absence of any perturbation (e.g. magnetic field, electrical current, heat source etc). We exploited this property, in combination with the large magnetostriction to demonstrate a method to switch the magnetisation direction using voltage induced strain in a hybrid piezoelectric/ferromagnet device structure[Parkes et al., Applied Physics Letters 101, 072402 (2012)]. Our work demonstrated the potential for the inverse magnetostriction effect to be used for low energy information storage devices. Control of domain patterns: The non-volatile switching described in the previous section was demonstrated in devices several tens of micrometres in size. The magnetic switching occurred by the motion of magnetic domain walls between random pinning sites. Much more desirable for applications would be the ability to position and move domain walls reversibly and deterministically by design. By fabricating devices of order ten micrometres, in which the geometry resulted in a balance between the magnetostatic, magnetocrystalline and magnetoelastic energy terms, we demonstrated how voltage induced strain could be used to manipulate ordered magnetic domain patterns and magnetic reversal curves[Cavill et al., Applied Physics Letters 102, 032405 (2013)]. In devices or order 1 micrometre we found that strain relaxation near to the edges of lithographically defined structures affects the magnetic domain pattern significantly (Beardsley et al., Scientific Reports 7, 42107 (2017) Magnetisation dynamics: The dynamical modes of magnetisation precession are a function of the magnetic anisotropy. Therefore, it should be possible to use voltage induced strain to change the precession frequency of micro- and nano- structured devices fabricated from magnetostrictive material. We demonstrated how the magnetic anisotropy could be used to significantly modify the magnetic domain pattern in micron sized disc structures fabricated from a Fe81Ga19 thin film[Parkes et al., Applied Physics Letters 105, 062405 (2014)]. Our micromagnetic calculations predict that this should result in significant changes in the precession frequencies in such devices. Experimental work is under way to observe these effects. Such devices have the potential to serve as spin torque oscillators in microwave communications devices. Strain-induced magnetisation dynamics: The ability to induce precession of the magnetisation by a sub-nanosecond strain pulse could pave the way to using magnetostrictive materials in information storage and communications technologies. In collaboration with Dortmund Technical University we demonstrated a method to deliver a sub-nanosecond strain pulse to a Fe81Ga19 thin film, by firing a femtosecond laser pulse onto the back side of the GaAs substrate. Kerr effect magnetometry was used to detect the resulting precession of the magnetisation which was observed to persist for several nanoseconds and several tens of oscillation periods[Jäger et al., Applied Physics Letters 103, 032409 (2013)]. The same technique was used in collaboration with the Acoustics Group at the University of Nottingham to induce switching of the magnetisation direction in a sample fabricated from a thin film of the ferromagnetic semiconductor (Ga,Mn)As. Current-induced domain wall motion: The use of an electrical current to induce the motion of magnetic domain walls has been the subject of intense research over the past two decades and is used in several information storage concepts currently under development (e.g. IBMs racetrack memory and the domain wall magnetic random access memory). Manipulation of the internal angle of a magnetic domain wall can be achieved by controlling the magnetic anisotropy with voltage-induced strain. Numerical calculations by the PI predict that this can be used to significantly tune the magnitude and direction of magnetic domain wall motion induced by electrical currents in samples consisting of heavy metal layers sandwiched with ferromagnetic layers [Rushforth, Applied Physics Letters 104, 162408 (2014)]. Field driven domain wall motion: Magnetic domain wall motion driven by magnetic fields in the low velocity creep regime were investigated in collaboration with researchers at the University of Leeds. Significant variations of the domain wall velocity were achieved in response to a voltage-induced strain acting on Pt/Co/Pt trilayers possessing perpendicular magnetisation orientation [Shepley et al, Scientific Reports 5, 7921 (2015)]. Strain-induced domain wall motion: For low energy memory concepts it would be desirable to move magnetic domain walls deterministically between desired positions by the use of voltage-induced strain alone (i.e. in the absence of magnetic fields or electrical currents). In collaboration with researchers at Nottingham the PI has designed a method for achieving this functionality. The design is the subject of a patent application (PCT/GB2016/051721) and a project to develop a proof of concept device is under way, funded through the Impact Acceleration Fund EP/K503800/1. Control of vortex domain structures The dynamics of magnetic vortex cores in confined magnetic nanostructures is of great interest because the gyrotropic mode has applications in spin torque driven magnetic microwave oscillators, and also provides a means to flip the direction of the core for use in magnetic storage devices. Using micromagnetic calculations we proposed and investigated a new means of stimulating magnetization reversal of the vortex core by applying a time-varying strain gradient to planar structures of the magnetostrictive material Fe81Ga19 (Galfenol), coupled to an underlying piezoelectric layer. The simulations revealed that the vortex core state can be deterministically reversed by electric field control of the time-dependent strain-induced anisotropy. This work was carried out in collaborations with researchers at York University [Ostler et al., Physical Review Letters 115, 06202 (2015)] Future research projects: The materials, devices and methods developed within this project, and the knowledge gained in the control of magnetisation using strain have been achieved in collaboration with researchers at the Universities of Cambridge, Dortmund, Glasgow, Leeds, Prague, Seoul, Sheffield, York and at Diamond Light Source. Several research proposals are in preparation or have been submitted with some of these collaborating organisations. Working with the Technology Transfer Office at the University of Nottingham we are also seeking business investment in the development of an information storage technology. |
| Exploitation Route | The materials, devices and methods developed within this project, and the knowledge gained in the control of magnetisation using strain have been achieved in collaboration with researchers at the Universities of Cambridge, Dortmund, Glasgow, Leeds, Prague, Seoul, Sheffield, York and at Diamond Light Source. Several research proposals are in preparation or have been submitted with some of these collaborating organisations. Working with the Technology Transfer Office at the University of Nottingham we are also seeking business investment in the development of an information storage technology. |
| Sectors | Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Energy,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology |
| Description | In collaboration with researchers at Nottingham the PI has designed a method for moving and manipulating the structure of magnetic domain walls using voltage-induced strain. The method has potential to be used in low energy information storage technologies The design is the subject of a patent application (PCT/GB2016/051721 ) and a project to develop a proof of concept device is under way, funded through the Impact Acceleration Fund EP/K503800/1. The project has provided training to one post-doctoral research, two postgraduate research students and several undergraduate project students in techniques including materials growth, device fabrication, magnetometry, magnetotransport, structural characterisation and x-ray synchrotron techniques. |
| First Year Of Impact | 2015 |
| Sector | Digital/Communication/Information Technologies (including Software),Education,Electronics |
| Impact Types | Societal,Economic |
| Description | EPSRC Impact Acceleration Account |
| Amount | £56,285 (GBP) |
| Organisation | Engineering and Physical Sciences Research Council (EPSRC) |
| Sector | Public |
| Country | United Kingdom |
| Start | 01/2016 |
| End | 03/2017 |
| Description | Impact Acceleration Account |
| Amount | £50,858 (GBP) |
| Funding ID | EP/K503800/1 |
| Organisation | Engineering and Physical Sciences Research Council (EPSRC) |
| Sector | Public |
| Country | United Kingdom |
| Start | 01/2014 |
| End | 03/2015 |
| Title | MAGNETIC STORAGE DEVICES AND METHODS |
| Description | The disclosure relates to information storage using magnetic domains or magnetic domain walls, and relates to methods of moving, creating, detecting and transforming the structure of magnetic domain walls using mechanical strain induced by electric fields. Applications include information storage and processing devices with high energy efficiency and small dimensions. Example embodiments include a magnetic storage device (100) comprising: an electroactive element (103) comprising a piezoelectric or electrostrictive element having first and second electrodes (101) arranged to apply an electric field across the electroactive element (103) to induce a strain; and a magnetic wire (102) having a plurality of magnetic domains separated by domain walls, the magnetic wire (102) aligned along an axis and mechanically coupled to the electroactive element (103); wherein the first and second electrodes (101) are arranged such that the electric field is aligned in a direction having a component orthogonal to the axis (104) of the magnetic wire (102), a magnitude of the electric field having a gradient along the axis (104) of the magnetic wire (102) to cause movement of the magnetic domains along the axis (104) of the magnetic wire (102). |
| IP Reference | WO2016198886 |
| Protection | Patent application published |
| Year Protection Granted | 2016 |
| Licensed | No |
| Impact | Publications in preparation. |
| Description | Publication in International Innovation magazine |
| Form Of Engagement Activity | A magazine, newsletter or online publication |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Policymakers/politicians |
| Results and Impact | None yet |
| Year(s) Of Engagement Activity | 2013 |
| URL | http://www.research-europe.com/magazine/NANOTECHNOLOGY/EX15/index.html |