Scientists at John Hopkins University have created the first half-metallic ferromagnet, which may help in an effort to revolutionise the way computer memory works.
The JHU development was realised with scientists from Brown University, IBM and the US Naval Research Laboratory.
‘Currently, computers use a technology called dynamic random access memory, or DRAM, and data is lost if the power supply is cut off,’ said Chia-Ling Chien, professor of physics in the Krieger School of Arts and Sciences at JHU. ‘The new technology under development is called magnetic random access memory or MRAM, and it will be able to retain data even when power is lost.’
‘Half-metallic materials will also allow scientists and engineers to make superior magnetic sensors,’ said Gang Xiao, professor of physics at Brown University. Xiao’s lab, with help from IBM, developed the single crystal films of the new material. ‘With these sensors, more information can be stored in hard drives of computers.’
The secret to the potential of MRAM and other related technologies lies in harnessing spin, a characteristic of electrons that conventional electronic circuits do not use.
According to Chien the spin of each electron behaves like a tiny magnet with north and south poles. Scientists describe an electron’s spin, and the orientation of the magnet, as ‘up’ or ‘down.’
They use the percentage of electrons in a metal with spin up and the percentage with spin down to determine ‘spin polarisation.’ Copper, for example, has zero spin polarisation. Common magnets have 40 percent spin polarisation.
All the electrons in a 100 percent spin-polarised material have the same spin orientation — either all up or all down. By definition, that means one of the two possibilities for electron spin (called the spin band) is absent, leaving only one spin band. In a normal metallic compound, by contrast, both spin bands are present. Scientists call a 100 percent spin-polarised material ‘half-metallic.’
Xiao and colleagues at Brown and IBM used a technique called chemical vapour deposition to grow single-crystal films of chromium dioxide. Using a superconductor, scientists at JHU measured the spin polarisation of the chromium dioxide films and found that they were at least 96 percent spin polarised.
Researchers are already working to incorporate the chromium dioxide films into a structure at the heart of MRAM technology known as a magnetic tunnel junction.
The structure consists of an insulator sandwiched between two electrodes. By controlling the orientation of the magnetisation of each electrode, scientists can make the junction switch between high resistance to electricity and low resistance.
Using the new half-metallic ferromagnet for the electrodes should make it possible to increase the high-resistance configuration to an insulating configuration.
Scientists hope one day to allow computers to use magnetic tunnel junctions in the same way that they use tiny capacitors in current memory systems.
‘The difference, though, would be that the capacitors in current memory systems leak and have to be electronically refreshed periodically to prevent data loss,’ said Chien. ‘Because MRAM relies on magnetic orientations, loss of power would not mean loss of the data it stores.’