New options for spintronic devices: Switching between 1 and 0 with low voltage
A thin magnetic FeRh film is grown onto a ferroelastic BTO substrate with two different crystal domains a and c. At 0 Volt ferromagnetic domains (red-blue pattern) are observed above BTO a-domains, whereas above c-domains the net magnetization is zero. At 50 Volt all BTO domains are converted into c-domains, which switches off ferromagnetic domains in FeRh. © HZB
Scientists from Paris and Helmholtz-Zentrum Berlin have been able to switch ferromagnetic domains on and off with low voltage in a structure made of two different ferroic materials. The switching works slightly above room temperature. Their results, which are published online in Scientific Reports, might inspire future applications in low-power spintronics, for instance for fast and efficient data storage.
Their sample consisted of two different ferroic layers: on a ferroelastic BaTiO3 (BTO) substrate a thin film of ferromagnetic FeRh was grown. Last year, they observed already that a small voltage across the BTO could change magnetic order in the ferromagnetic FeRh film via a strong magnetoelectric coupling between both layers.
Now, they could see much larger effects. “We could switch ferromagnetic states in the FeRh film completely on and off with a low voltage applied to the underlaying BTO”, reports Sergio Valencia, the HZB scientist who led the study. With XPEEM imaging at BESSY II they observed the transition between different magnetic orders in the FeRh layer, driven by an electrical field applied across the BTO substrate.
Electric fields, strain, magnetic order and temperature
It works because a low voltage on the BTO substrate deforms its crystal structure via a ferroelastic effect, creating a strain. This strain is transferred to the FeRh film grown on top of the BTO and influences its magnetic order. As physicist Valencia puts it: “By the strain on the BTO substrate we can increase the transition temperature of FeRh, a characteristic temperature which separates antiferromagnetic order from ferromagnetic order. Below this temperature, FeRh is antiferromagnetic (net magnetic moment is zero), above it becomes ferromagnetic. Normally this transition temperature for FeRh is around 90°C, but under strain (through the voltage applied to the BTO substrate) it is shown to rise to ca. 120 °C. To demonstrate this effect, the experiment was conducted at 115 °C, a temperature at which in absence of strain FeRh was observed to be ferromagnetic. When the voltage was applied to the BTO substrate, the strain transferred from BTO to the FeRh increased the temperature needed to have a ferromagnetic order and the FeRh became antiferromagnetic.
Switiching near room temperature
“This is quite relevant. Here we have a structure showing switching effects between two different magnetic states close to room temperature. This is precisely what you need in order to develop room temperature working devices. Moreover, to switch between these two states we use electric fields instead of magnetic fields which consumes less energy. In the near future we aim at doping the FeRh film with palladium to get effects even closer to room temperature.” Valencia says.
To the article: Scientific Reports doi:10.1038/srep10026
Local electrical control of magnetic order and orientation by ferroelastic domain arrangements just above room temperature, L. C. Phillips, R. O. Cherifi, V. Ivanovskaya, A. Zobelli, I. C. Infante, E. Jacquet, N. Guiblin, A. A. Ünal, F. Kronast, B. Dkhil, A. Barthélémy, M. Bibes and S. Valencia
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.
- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.