Hybrid multiferroics for improved data storage and new wireless applications

To exploit and control magnetism, technology often relies on electromagnets, which limit hardware configurations due to their size and energy consumption. As an alternative, scientists like those in the Nanomagnetism and Spintronics Group at Aalto University are beginning to develop hybrid nanomaterials that are responsive to both electric and magnetic fields. These materials can save energy and space, and are opening doors to a wealth of new technological applications. Creating materials that can be influenced by both magnetic and electric forces - so called ‘multiferroics’ - is an emerging field that is currently being addressed mainly by academics, although there is notable interest from companies with interests in data storage, magnetic sensors and microwave components. Power savings are one incentive, but another exciting factor is the multi-functionality they could offer. The potential they hold could lead to the evolution of entirely new forms of information storage. “Some materials exhibit spontaneous electric or magnetic order. These materials are called ferroelectric and ferromagnetic, respectively,” explains professor Sebastiaan van Dijken, leader of the Nanomagnetism and Spintronics Group at Aalto University. “However, among these two ferroic materials there is little overlap. While it is possible to create materials that achieve this overlap in a laboratory setting, it is usually at very low temperatures and so is fairly useless in terms of the development of components for nanoelectronic devices.” However, there is a way around this problem. Professor van Dijken’s team has been creating hybrid structures using one stable ferroelectric material and one stable ferromagnetic material. Built at the nanoscale, thin films of each material are linked together by strong coupling at their interfaces, enabling them to be used as a single entity (although the ferroelectric and ferromagnetic properties of the compound remain physically separated). If the connection between the materials can be sustained, magnetic fields will not only influence the ferromagnetic film, but also its ferroelectric counterpart. Conversely, if a bias voltage is applied across these junctures, the properties of the magnetic segment of the structure can be manipulated using an electric field. Choosing appropriate materials, engineering robust interfaces and optimising strong interferroic coupling are all essential in ensuring the effectiveness of the entire structure. Achieving control over these hybrid materials is a highly desirable objective. Many mainstream technologies depend on the principle of manipulating the direction of magnetic order. For example, computer hard disk drives ‘write’ information magnetically, by realigning small sections of the drive to represent either a binary 1 or a 0. The way most information is stored relies on the application of magnetic fields. New types of memory are being developed, such as non-volatile magnetic RAM (MRAM) memory, which use electric current to write information. However, there are disadvantages to both of these mechanisms. Electromagnets are bulky components that make storage optimisation difficult, while manipulating data using an electric current uses large amounts of energy. By employing hybrid multiferroic materials, however, electric fields can be used to control magnetic devices, nullifying these drawbacks, says the team. The emission of microwaves could also be controlled and tuned by multiferroics. “Some high frequency electronic components are based on the principle of ferromagnetic resonance, and hence they could find great utility here,” said van Dijken. “Presently, wireless products like mobile phones operate using fixed frequencies, but by using these new materials, tuneable models capable of accessing different frequencies could be delivered.” The project has already delivered a robust coupling system, which bodes well for the future. “Both classes of materials we’re using are subdivided into different domains in which magnetisation or polarisation point in different directions,” explained van Dijken. “Normally, the material isn’t uniform in this respect. However, through coupling them, the ferroelectric domains can be powerfully imprinted into the ferromagnetic film. Because of this, the magnetic orientation and the ferroelectric polarisation of the hybrid become mutually aligned throughout. This emphasises the strength of the bond we’ve achieved between the two materials. It’s highly encouraging, and creates a variety of new opportunities for future research and practical development within our group.”