Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and Forschungszentrum Jülich (FZJ) together with a colleague at the French Centre National de la Recherche Scientifique (CNRS) in Strasbourg have found a new way to electrically read out the orientation of magnetic vortices in nanodisks. Their method relies on measuring characteristic microwaves emanating from the vortices. The new knowledge about these signals could be used in the construction of extremely small components for novel memory technology or wireless data transmission. The results of the study appear in the current edition of the scientific journal ("Spin-torque-induced dynamics at fine-split frequencies in nano-oscillators with two stacked vortices"). 
Shown are four examples of the double-vortex states investigated with opposite vorticity. In each disk, the flat yellow arrows represent the sense of the circulation of the in-plane magnetization component. The red and green arrows indicate the polarity of the vortex core. The tiny core, with a diameter of only a few nanometers, is surprisingly important for the magnetization dynamics when a current flows through the double-vortex pillars. (Image: HZDR/FZJ) The internal spin configuration of magnetic nanodisks has been at the center of scientific attention for several years. "Spin" refers to the rotational momentum of charged particles that enables, among other things, iron to be magnetized, for example. The ferromagnetism of iron arises from the parallel alignment of the spins of all the electrons. However, in very thin and small disks of ferromagnetic material, the nanodisks, spins are also known to form magnetic vortices. Since researchers discovered these complex structures, they have been trying to use their properties to facilitate extremely compact and energy-efficient data storage. These nanodisk devices could, for instance, be employed in future smart phones or laptops, if the stored information can be read out successfully. In nanodisks, the spins - and thus the magnetic moment of the electrons - are arranged as if ordinary bar magnets were lined up in a circle. However, at the core of the disk, this order does not work anymore, and instead the small magnets align themselves out of the plane of the disk, either in an upward or downward direction. "With these two properties, the sense of circulation of the in-plane magnetization and the magnetic orientation of the core, information can be stored", says Dr. Attila Kákay, a former researcher of Forschungszentrum Jülich who recently moved to Dresden. "This means that we can store as much as two bits of information using a single vortex. Two vortices stacked on top of each other can already store four memory bits corresponding to 16 different states." Core of Nanodisks Too Small to Be Read Out Conventionally This type of magnetic nanopillar with two stacked vortices is just 50 nanometers in height and has a diameter of 150 nanometers - almost a thousand times thinner than a human hair. But while the circulation of the vortex and the magnetic orientation of the core can be quite easily affected by currents and magnetic fields, the small size of the nanodisks was previously an obstacle to reading out the information. "The magnetic orientation in the core, the so-called polarity, could not be read reliably because the core was simply too tiny", says Dr. Kákay. However, during the course of experiments in Jülich, the researchers found a solution: microwaves. These electrical alternating voltage signals are emitted by the stacked vortices when connected to a direct current. The characteristic frequencies of the microwaves are now used by the physicists to determine the core polarity and sense of circulation of the vortices. "This principle is similar to that of playing a wooden flute: here too, each position of the fingers over the holes clearly corresponds to a particular musical note, a vibration frequency," explains Dr. Alina Deac, head of the Helmholtz Young Investigator Group for spintronics at the HZDR. With this new method, the scientists from Dresden, Jülich and Strasbourg have been able to design new nanoscale components that are not only capable of storing information within magnetic vortices, but also enable it to be reliably read out electrically. Using this principle, in the future far more data could be stored in ever smaller memory chips that could have many applications in modern electrical engineering. In addition, the frequency of the AC voltage can even reach the gigahertz range, which makes ultra-fast wireless transfer of information possible, for example, for use in mobile communications and wireless network services.
Shown are four examples of the double-vortex states investigated with opposite vorticity. In each disk, the flat yellow arrows represent the sense of the circulation of the in-plane magnetization component. The red and green arrows indicate the polarity of the vortex core. The tiny core, with a diameter of only a few nanometers, is surprisingly important for the magnetization dynamics when a current flows through the double-vortex pillars. (Image: HZDR/FZJ) The internal spin configuration of magnetic nanodisks has been at the center of scientific attention for several years. "Spin" refers to the rotational momentum of charged particles that enables, among other things, iron to be magnetized, for example. The ferromagnetism of iron arises from the parallel alignment of the spins of all the electrons. However, in very thin and small disks of ferromagnetic material, the nanodisks, spins are also known to form magnetic vortices. Since researchers discovered these complex structures, they have been trying to use their properties to facilitate extremely compact and energy-efficient data storage. These nanodisk devices could, for instance, be employed in future smart phones or laptops, if the stored information can be read out successfully. In nanodisks, the spins - and thus the magnetic moment of the electrons - are arranged as if ordinary bar magnets were lined up in a circle. However, at the core of the disk, this order does not work anymore, and instead the small magnets align themselves out of the plane of the disk, either in an upward or downward direction. "With these two properties, the sense of circulation of the in-plane magnetization and the magnetic orientation of the core, information can be stored", says Dr. Attila Kákay, a former researcher of Forschungszentrum Jülich who recently moved to Dresden. "This means that we can store as much as two bits of information using a single vortex. Two vortices stacked on top of each other can already store four memory bits corresponding to 16 different states." Core of Nanodisks Too Small to Be Read Out Conventionally This type of magnetic nanopillar with two stacked vortices is just 50 nanometers in height and has a diameter of 150 nanometers - almost a thousand times thinner than a human hair. But while the circulation of the vortex and the magnetic orientation of the core can be quite easily affected by currents and magnetic fields, the small size of the nanodisks was previously an obstacle to reading out the information. "The magnetic orientation in the core, the so-called polarity, could not be read reliably because the core was simply too tiny", says Dr. Kákay. However, during the course of experiments in Jülich, the researchers found a solution: microwaves. These electrical alternating voltage signals are emitted by the stacked vortices when connected to a direct current. The characteristic frequencies of the microwaves are now used by the physicists to determine the core polarity and sense of circulation of the vortices. "This principle is similar to that of playing a wooden flute: here too, each position of the fingers over the holes clearly corresponds to a particular musical note, a vibration frequency," explains Dr. Alina Deac, head of the Helmholtz Young Investigator Group for spintronics at the HZDR. With this new method, the scientists from Dresden, Jülich and Strasbourg have been able to design new nanoscale components that are not only capable of storing information within magnetic vortices, but also enable it to be reliably read out electrically. Using this principle, in the future far more data could be stored in ever smaller memory chips that could have many applications in modern electrical engineering. In addition, the frequency of the AC voltage can even reach the gigahertz range, which makes ultra-fast wireless transfer of information possible, for example, for use in mobile communications and wireless network services.
Magnetic spin-waves in a solid. (Illustration: Christoph Hohmann / NIM) Since magnetic vortices are microscopic and easy to move, computer components may need 10,000 times less electricity than today with this technology and store much larger amounts of data. Recent research results showed that the unique electromagnetic properties of skyrmions could also be used for the construction of efficient and very small microwave receivers and transmitters. Conductors, semiconductors and insulators The production of computer chips requires insulating, semiconducting and conducting materials. Today, magnetic vortex structures are available for all these three classes of materials. An important advantage is that these vortices respond easily to alternating fields so that information can be processed at high rates. Now a team of physicists at the TU München, the University of Cologne and the École Polytechnique Fédérale de Lausanne (Switzerland) has examined the dynamic behavior of the three materials. With the results of their measurements, the team developed a theoretical description of behavior valid for all three material classes. “With this theory, we have laid an important foundation for further developments,” says Professor Dirk Grundler, Chair of Physics of Functional Multilayers at the TU München. “In the future, we will therefore be able to identify materials with the specific properties we need for functional devices.” Extremely compact frequency devices The typical resonance frequencies of the skyrmions are in the microwave range – the frequency range of mobile phones, Wi-Fi and many types of microelectronic remote controls. Thanks to the robustness of the magnetic vortices and their ease of excitability, skyrmion-materials could be the basis for highly efficient microwave transmitters and receivers. 
An oscillating field around the mean (signal) and the outer conductors (ground) induces spin waves in the probe. (Image: TUM) While the wavelength of electromagnetic microwaves typically lies in the range of centimeters, the wave lengths of the magnetic spin waves, so-called magnons, are 10,000 times shorter. “In the area of microelectronics, much more compact or even entirely new devices could be developed from magnetic nanomaterials such as the skyrmion-materials,” says Grundler. In addition to the material itself, its shape also significantly influences the electromagnetic properties of the device. Here, too, the researchers' newly developed theory is very useful. It can predict which form produces the best properties for which material. “Chiral magnetic materials promise a lot of new functionalities with an interesting interplay of electronic and magnetic properties,” says Dr. Markus Garst, a physicist at the Institute for Theoretical Physics at the University of Cologne. “But for all applications, it is essential to predict the possibilities and limitations of various materials. We have come a big step closer to achieving this goal.”
A large-surface light-emitting plastic film developed by VTT is based on OLED technology. (Image: Juha Sarkkinen) This type of light-emitting plastic film and processing in ambient atmosphere has not been created before on this scale. Traditional printing methods such as gravure and screen printing, enabling very large production volumes, are used for manufacturing OLED light surfaces. Production is, therefore, possible in facilities such as traditional printing houses. Manufactured using the gravure and screen printing methods, OLED light surfaces are around 0.2 mm thick, and include electrodes and polymer layers measuring up to a few hundred nanometres, in which the light emission occurs. This phenomenon is called electroluminescence; it entails an organic semiconductor emitting light in an electric field. The luminosity of OLED (lm/W) amounts up to around one third of an LED's luminosity. It has one advantage: OLED emits light throughout its entire surface, whereas LED is a spotlight technology. At this point, VTT's plastic OLED film will only emit light for around a year, since light-emitting polymer materials are susceptible to oxygen and moisture. In the future, the film's lifespan will increase as the development of screen protectors continues and the film's application possibilities grow. "The plastic film is optimally suited to advertising campaigns, in which large light-emitting surfaces can be used to draw significantly more attention than can be gained through mere printed graphics or e-ink-type black-and-white displays that do not emit light," says Head of Research Area Raimo Korhonen from VTT. It is also possible to use OLED light as a transmitter in wireless data transfer, which opens up new possibilities for utilising printed light surfaces in Internet of Things applications.