Domain dynamics in strained metal films with perpendicular magnetic anisotropy

There is currently growing interest in trying to manipulate magnetic properties, including the direction of magnetization in magnetic thin films and nanostructures, using not just magnetic fields or electric currents, but electric fields. The practical reason for this is that electric fields are expected to dissipate less energy in switching the magnetization than either magnetic fields or electric currents, and this could have an important effect on new generations of electronic devices that incorporate nanomagnets, such as magnetic random access memories. Even if used simply in conjunction with existing methods, electric field-induced effects could reduce the power density required in a device and lessen the amount of energy that is lost as heat.

Sufficiently large electric fields can directly affect the tendency of the magnetization in a thin film to point in a particular direction, known as the magnetic anisotropy. The electric field distorts the outer electronic orbitals and couples to the atomic magnetic moments, enabling manipulation of the magnetic anisotropy and hence the magnetization direction. Multiferroic materials, in which magnetism can be controlled by electric fields (and vice versa), are not very useful in themselves because only one of them, bismuth ferrite, is multiferroic at room temperature and the magnetoelectric effect is generally rather small, but in combination with ferromagnetic thin films improved electric control may be achieved by "magnetic exchange" coupling between the two. However, it is a third method of electric field control that will be used in this research, namely, to combine piezoelectric and ferromagnetic materials and utilise the strain coupling between them. A voltage applied to the piezoelectric makes it expand and compress and thereby strains the magnetic film on top, altering its magnetic properties, including its anisotropy. From the point of view of basic research, there has been very little work so far to investigate the effect of strain on the magnetic properties of thin films with an orientation of the magnetization perpendicular to the plane, and in particular on the switching of the magnetization. This study will focus on such perpendicular films, which the Leeds group excels in making, preparing them in both single crystal and alloy form and using magnetic imaging to study the effect of strain on the magnetization dynamics.

Magnetic imaging enables a direct visualisation of the magnetization direction in a thin film, which is generally not uniform but split up into regions, or "domains" where the magnetization direction is different. In perpendicular films the magnetization in the domains may point up or down relative to the film plane. The polarization of light reflected from the film is rotated by an amount that depends on the magnetization direction (the magneto-optic Kerr effect), and this enables the domains to be pictured in an appropriately designed microscope. Such a microscope offers advantages over other forms of magnetic imaging in that it produces large-area images very quickly and requires no special sample preparation. In principle it can also be used to study changes to the domain pattern with a very high (sub-nanosecond) time resolution. These dynamical changes to the magnetization are useful to know when assessing the suitability of the material for electronic devices, which need to operate at high speed. In this project, nanosecond magnetic field pulses will modify the domain pattern, and taking images before and after each pulse will give an insight into the domain dynamics. This will be done as a function of voltage-induced strain from the piezoelectric, providing a route to understanding how electric fields can improve the efficiency of electronic device function.

Using electric fields to manipulate or switch the magnetization direction in thin magnetic films or nanomagnets via piezoelectric strain could lead to a new generation of spintronic devices that consume very little power. This project will pioneer the study of magnetization dynamics in strained thin magnetic films, leading to knowledge about how to engineer devices that possess both high operating speeds and optimum energy efficiency.

The new spintronic devices that this project could help deliver will feed first into applications in computing, information and communications technologies. Beneficiaries in the first instance will be integrated circuit designers and electronic engineers (and the companies they work for) who will be able to produce better-performing microelectronic components such as digital memories and data processors. The next set of beneficiaries will be those who design computer hard- and software, information and communications systems, who will make use of the new components to produce faster, more efficient ways of storing and processing data, and higher performance consumer electronics. The companies that manufacture such products, as well as the general public, will eventually benefit over a 10-20 year timescale. Low power miniature portable electronic devices are particularly exciting, with potential applications in medical implants and control/sensing in buildings and infrastructure. Consequently, this research could not only stimulate new electronic product development but also lead to improvements in health care and energy efficiency/safety in the built environment. Lastly, the PhD student working on parts of this project will learn industrially relevant technical skills in device fabrication, characterisation and advanced microscopy, which will benefit future employers in academia or industry.