MRAM – Magnetoresistive RAM

MRAM does not store information via electric charge like DRAM or flash, but via the magnetic orientation of electron spins – a fundamentally different physical storage mechanism. The basis is the tunnel magnetoresistance effect (TMR), a quantum-mechanical phenomenon: two ferromagnetic layers, separated by an insulating layer only a few atomic layers thick, allow different amounts of electric current to tunnel through depending on their relative magnetization direction. Electrons preferentially tunnel through the insulating layer when their spin orientation matches the majority spin orientation in both ferromagnetic layers.

If both layers are magnetized in parallel (same orientation), most electrons find a matching state to tunnel through, and the cell's electrical resistance is low. In the antiparallel orientation (opposite magnetization), electrons must reverse their spin while tunneling, which is quantum-mechanically far less likely – resistance rises noticeably, typically by 100 to 200 percent compared to the parallel state (the so-called TMR ratio). These two clearly distinguishable resistance states encode the bits “0” and “1” and can be read out non-destructively simply by applying a small read voltage.

The memory cell – the magnetic tunnel junction (MTJ) – consists of three central layers, typically deposited by sputtering (see the PVD chapter) in a single vacuum chamber without breaking vacuum, in order to avoid oxidation and contamination at the sensitive interfaces:

A reference layer (fixed layer or pinned layer) made of a ferromagnetic alloy, whose magnetization direction is fixed by coupling to an additional antiferromagnetic layer and stabilized against external stray fields. Above it, a wafer-thin tunnel barrier, in modern MTJs almost without exception made of crystalline magnesium oxide (MgO) only one to two nanometers thick – the crystal structure of the MgO is critical for a high TMR ratio, since it favors spin-selective tunneling probability. And finally a free layer, also ferromagnetic, whose magnetization direction can be deliberately switched for writing. Only the free layer changes state during a write operation; the reference layer serves as a stable comparison point against which resistance is measured during readout.

MTJ in the parallel and antiparallel state