Double barrier magnetic tunnel junctions with

Write/Read mode select layer

P.-Y. Clément, C. Baraduc, M. Chshiev, B. Diény, L. Vila and C. Ducruet

Grenoble, France

International Memory Workshop (IMW), 2014

Motivation Regular Single Junction MTJ:

Critical Current (IC0) or Critical Current Density (JC0): The minimum current required to make the MTJ free layer instable enough to cause a switching of its magnetic orientation

Motivation Challenges: Read Disturbance: Inadequate margin between the read current and the critical current can cause a read operation to actually switch the free layer of the MTJ. High Write Current (Power): High critical current forces write current to be even higher, making STT-MRAM write operation a power hungry task.

Motivation Challenges: Typically, write voltages of the order of 0.4-0.5V The read voltage must be adjusted accordingly to a sufficiently low

value to avoid any risk of write during read

Typically read voltage is in the range of 0.1- 0.12V. Such low voltage actually slows down the readout

Ideally, the read voltage should be around 0.2 to 0.3V

Double Barrier MTJ (DMTJ)

The storage layer is sandwiched between two polarizing layers

Each polarizing layer is separated from the storage layer by a tunnel barrier

The magnetization of one of the polarizing layer is always pinned in a fixed direction (Reference Layer)

Whereas the other one, called mode select layer, can be switched parallel or antiparallel to the first one depending whether the magnetic tunnel junction (MTJ) is in read or respectively write mode

Double Barrier MTJ (DMTJ)

In the parallel configuration of the polarizing layers, the STT efficiency is minimized allowing to read at relatively high voltage leading to fast readout without risk of write disturb during read.

In the antiparallel configuration of the polarizing layer, the STT efficiency is maximized allowing writing at lower current density.

DMTJ STT-RAM with read/write mode select layer

DMTJ STT-RAM with read/write mode select layer

Principle of operation: The storage layer magnetization is switched by STT whereas the

magnetization of the read/write mode select layer is switched by magnetic field.

The storage layer is preferably a synthetic antiferromagnetic layer so that its magnetization is almost insensitive to the magnetic field pulse but sensitive to the STT contributions from the reference layer and mode select layer.

The mode select layer is relatively thick so that it is itself weakly sensitive to the STT from the storage layer

Write Mode 1 Oe (Oersted) ≈ 79.5774715 Amperes per meter

Vc1 = - 0.55 V = Switching from antiparallel to parallel state = Write 0 Vc2 = 0.39 V = Switching from parallel to antiparallel state = Write 1

Read Mode 1 Oe (Oersted) ≈ 79.5774715 Amperes per meter

No storage layer switching is observed regardless of the applied current amplitude and for any value of the applied field

Only a continuous small variation of resistance is observed corresponding to bias voltage, which is expected

Conclusion

It is possible to choose the write/read mode of the DMTJ STT-MRAM memory.

Write mode guarantees low current densities to switch the storage layer, whereas read mode allows significantly increasing the reading voltage without any risk of writing.

Such dual MTJ structures can allow developing faster memories by increasing the readout speed of the storage layer.

The gain in speed will be significant for memories that are seldom written, so that the extra switching time between read and write configuration is largely compensated by the time saved by a faster reading.

Thank you