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A boost for magnetic tunnel junctions!

The magnetic and electrical properties of magnetic tunnel junctions could be significantly improved by inserting layers of a refractory metal (tungsten) in the stacks. This breakthrough opens up new perspectives for MRAM magnetic memories composed of such junctions, which are entering into bulk production.
Published on 5 February 2018

Magnetic tunnel junctions are the basic elements of new magnetic memories known as MRAM (Magnetic Random Access Memory). These are now in the process of entering into bulk production at the main microelectronic foundries (Samsung, TSMC, Global Foundries, etc.).

However, progress still needs to be made in moving towards higher-density memories (i.e. a few gigabits) with very low access times (less than 3 nanoseconds) and the capability of operating at high temperatures (150°C for automobiles). In particular, scientists at the Inac are seeking to increase the magnetic anisotropy of magnetic electrodes that determine the conservation time of written information (retention of memory) and the amplitude of the magnetoresistance tunnel, which is the physical phenomenon underlying magnetic tunnel junctions. They have just discovered another path to improvement, this time linked to the process of manufacturing.

At the heart of a magnetic tunnel junction is a stack of thin Ta/FeCoB/MgO/FeCoB/Ta layers, in which FeCoB refers to magnetic electrodes of a thickness of one to two nanometers and MgO is the magnetic tunnel barrier. The FeCoB electrodes are initially amorphous and the MgO tunnel barrier is polycrystalline. This stack must be annealed after deposition to crystallize the MgO and FeCoB. The higher the annealing temperature, the better the crystallization and the stronger the magnetoresistance tunnel. Nonetheless, the annealing temperature is usually limited to about 300°C by the diffusion of tantalum in FeCoB.

The Inac team has shown that by inserting layers of tungsten (a refractory metal with a melting temperature of 3,422°C) into the stack, the annealing temperature can be raised to 570°C. The magnetoresistance tunnel is thus increased by about 30%.

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