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
- A new way to control the magnetic properties of rare earth elementsThe special properties of rare earth magnetic materials are due to the electrons in the 4f shell. Until now, the magnetic properties of 4f electrons were considered almost impossible to control. Now, a team from HZB, Freie Universität Berlin and other institutions has shown for the first time that laser pulses can influence 4f electrons- and thus change their magnetic properties. The discovery, which was made through experiments at EuXFEL and FLASH, opens up a new way to data storage with rare earth elements.
- BESSY II shows how solid-state batteries degradeSolid-state batteries have several advantages: they can store more energy and are safer than batteries with liquid electrolytes. However, they do not last as long and their capacity decreases with each charge cycle. But it doesn't have to stay that way: Researchers are already on the trail of the causes. In the journal ACS Energy Letters, a team from HZB and Justus-Liebig-Universität, Giessen, presents a new method for precisely monitoring electrochemical reactions during the operation of a solid-state battery using photoelectron spectroscopy at BESSY II. The results help to improve battery materials and design.
- HZB magazine lichtblick - the new issue is out!In his search for the perfect catalyst, HZB researcher Robert Seidel is now getting a tailwind – thanks to a ERC Consolidator Grant. In the cover story, we explain why the X-ray source BESSY II plays an important role for his research.