Boffins pioneer electron spin data storage

Taking spintronics for a spin

Ohio State University boffins have built a device that stores data based on electron spin which uses less energy and is cooler than existing DRAM.

Electrons can be forced to orient their magnetisation or spin in parallel (on) or anti-parallel (off) alignment by using an external magnetic field. The alignment, called spin up or spin down, can be used to filter electrons as they pass through a gate because the spin states give the electrons different resistance levels. This can be detected such that spin up and spin down can represent binary ones and zeroes.

This field of research is known as spintronics (spin transport electronics) and involves a spin-detecting system and injecting or moving spin-polarised electrons from one material to another. The spin direction is manipulated between the injection and detection points by the external magnetic field. Passing a current through a ferromagnetic material generates a spin-polarised current of electrons. This is done in GMR (giant magneto-resistive) heads with two ferromagnetic layers separated by a spacer layer.

Grandis is working on STT-RAM (Spin-Transfer Torque RAM) which is a spintronics application. Hynix has licensed IP from Grandis. Hitachi, Renesas and Fujitsu also have STT-RAM initiatives, as does IBM with its racetrack memory idea.

The Ohio researchers, led by Arthur Epstein, director of the Institute for Magnetic and Electron Polymers at the university, produced a device, a spin-valve, combining a conventional epitaxial iron magnet film with a thin non-magnetic polymer semiconductor, vanadium tetracyanoethylene, to form a hybrid magnetic tunnel junction. This functioned as a spin injector/detector.

Spin-polarised electrons were injected from the ferromagnetic film into the organic semiconductor and a magnetic field oriented their spin up or down. They were then fed back into the conventional magnetic layer and their resistance detected. Data was read from the spin-valve depending upon whether the resistance was high or low.

Researcher Jung-Woo Yoo said: "Our main achievement is that we applied this polymer-based magnet semiconductor as a spin polarizer - meaning we could save data (spin up and down) on it using a tiny magnetic field - and a spin detector - meaning we could read the data back. Now we are closer to constructing a device from all-organic material.” That would mean getting rid of the ferromagnetic material.

Spin valves need less energy than conventional DRAM to operate and generate less heat. The researchers also say the electrons in spin memory devices are able to store two bits of information instead of one. All this could lead to much smaller devices needing much smaller batteries. Epstein, the head boffin, said: "If we had a lighter weight spintronic device which operates itself at a lower energy cost, and if we could make it on a flexible polymer display, soldiers and other users could just roll it up and carry it."

Woo thinks a commercial manufacturing process is feasible: "Any place that makes computer chips could do this. Plus, in this case, we made the device at room temperature, and the process is very eco-friendly." Ah, there's a green spin to this story. ®