RUB researchers present a new switching principle for magnetic fields

An international team of researchers from Germany and the Netherlands has developed a new material for storage media. For the first time they enable the switching of so called spin currents at room temperature in a vertical magnetic field. This increases the storage density distinctly. The novel switches can be used, for example, as read heads in future hard discs or as bits in non-volatile random access memory devices (MRAM). The research group from the Ruhr-Universität Bochum, the Helmholtz-Zentrum Berlin and Nijmegen are reporting their results in Nature Communications.

"For the first time, the principle of the so called exchange-spring-magnet was suggested by Prof. Eckart Kneller, an electrical engineer from Bochum, at the Beginning of the 1990s", says Prof. Dr. Hartmut Zabel, chair of experimental physics/solid-state physics of the RUB. The basic work has been quoted more than a thousand times since then and has been spread scientifically and technically all over the world. 20 years later the effect was casted into a novel layer system, which has particularly sensitive properties for the magnetic sensor and storage technology. Proven was this in the measuring chamber ALICE at the electron-synchrotron BESSY II of the Helmholtz-Zentrum Berlin, which has been built and is being operated by the Ruhr-Universität Bochum.

While ferromagnets are familiar, for instance as refrigerator-magnets, ferrimagnetic materials are much more exotic. The best known example is magnetite, which has already been used in China as compass needle over thousand years ago. Ferrimagnets are made of two interleaved materials with different magnetic moment. The vertical arranged magnetic domains of the novel switches are made of such ferrimagnetic materials, separated by an ultra-thin tantalum layer. "The particular feature is the fact that the switching point can be fine tuned after production and can be used and readjusted arbitrarily often ", explains Prof. Zabel. The researchers screened the switches with high-precision magnetic X-ray spectroscopy and scattering (X-ray dichroism) and vetted the functionality in all details.

For this purpose they used inter-metallic compounds of transition metals and rare earth metals. One of both layers within the switch is composed of a GdFe alloy, the other layer is made of a DyCo alloy. The first one is magnetically soft, the other one magnetically hard. "If the layers are piled one upon the other, then both characteristics act in combination similar to a Damascene knife, as well the cutting edge as the hardness", explains Zabel.

If there is a direct contact between both layers, the magnetically soft GdFe layer loses its feature and adapts to the magnetically hard DyCo layer. However, the magnetically soft features are necessary for high sensitivity and for switching at already low magnetic fields. Therefore, the direct contact is prevented by inserting an extremely thin tantalum layer. Thereby the magnetically soft features of GdFe are re-established without being completely independent of DyCo. Both layers are comparable with a boat (GdFe), which is moored with its short end (tantalum) on a pale (DyCo). If the end is too short the boat hasn't enough freedom, if the end is too long, it can't be controlled. Thereby the thickness of the tantalum layer is the key to the functionality of the switch. If the thickness of the layer is chosen well, the switching point of the magnetically soft layer can be swayed and adjusted to the right value within a high magnetic field.

F. Radu, R. Abrudan, I. Radu, D. Schmitz, H. Zabel: Perpendicular exchange bias in ferrimagnetic spin valves. Nature Communications, 2012. DOI: 10.1038/ncomms1728

Prof. Dr. Hartmut Zabel, chair of experimental physic/solid-state physics of the Ruhr-Universität Bochum, +49 234 32 23649, hartmut.zabel@rub.de

Editorial journalist: Jens Wylkop