Sam Brown

Spin torque switching of perpendicular Ta∣CoFeB∣MgO-based magnetic tunnel junctions

Spin torque switching is investigated in perpendicular magnetic tunnel junctions using Ta∣CoFeB∣MgO free layers and a synthetic antiferromagnet reference layer. We show that the Ta∣CoFeB interface makes a key contribution to the perpendicular anisotropy. The quasistatic phase diagram for switching under applied field and voltage is reported. Low switching voltages, Vc 50 ns=290 mV are obtained, in the range required for spin torque magnetic random access memory. Switching down to 1 ns is reported, with a rise in switching speed from increased overdrive that is eight times greater than for comparable in-plane devices, consistent with expectations from a single-domain model.

Spin torque switching of 20 nm magnetic tunnel junctions with perpendicular anisotropy

Spin-transfer torque magnetic random access memory (STT-MRAM) is one of the most promising emerging non-volatile memory technologies. MRAM has so far been demonstrated with a unique combination of density, speed, and non-volatility in a single chip, however, without the capability to replace any single mainstream memory. In this paper, we demonstrate the basic physics of spin torque switching in 20 nm diameter magnetic tunnel junctions with perpendicular magnetic anisotropy materials. This deep scaling capability clearly indicates the STT MRAM device itself may be suitable for integration at much higher densities than previously proven.

Demonstration of Ultralow Bit Error Rates for Spin-Torque Magnetic Random-Access Memory With Perpendicular Magnetic Anisotropy

Bit error rates below 10-11 are reported for a 4-kb magnetic random access memory chip, which utilizes spin transfer torque writing on magnetic tunnel junctions with perpendicular magnetic anisotropy. Tests were performed at wafer level, and error-free operation was achieved with 10 ns write pulses for all nondefective bits during a 66-h test. Yield in the 4-kb array was limited to 99% by the presence of defective cells. Test results for both a 4-kb array and individual devices are consistent and predict practically error-free operation at room temperature.

Development of perpendicularly magnetized Ta|CoFeB|MgO-based tunnel junctions at IBM (invited)

The discovery of perpendicular magnetic anisotropy (PMA) in Ta|CoFeB|MgO and the subsequent development of perpendicularly magnetized tunnel junctions at IBM is reviewed. The fast-turn-around method used for screening materials for interface PMA by measuring the moment/area and anisotropy field of in-plane materials as a function of CoFeB thickness is presented, including the data as a function of seed-layer material which led to the discovery of PMA in Ta|CoFeB|MgO. Magnetic and electrical data are reported for the first PMA magnetic tunnel junction we made using this material. By inserting a thin Fe layer at the Ta|CoFeB interface, a substantial increase in the PMA energy density was obtained. Pure Fe layers (which required the use of a TaMg seed) greatly improved the thermal stability, allowing annealing up to 400  °C.

Recent Advances in Spin Torque MRAM

The switching current of Spin Torque Magnetic Random Access Memory (MRAM) can be reduced significantly by using perpendicularly magnetized materials. The Ta|CoFeB|MgO system provides both high tunneling magnetoresistance and perpendicular anisotropy. Using this materials system we have demonstrated basic write functionality in fully integrated Spin Torque MRAM arrays. Here, we further demonstrate device scaling down to 20 nm diameter, opening up the possibility of ultra-dense Spin Torque MRAM.

Materials investigation for thermally-assisted magnetic random access memory robust against 400 °C temperatures

Magnetic materials are investigated in order to enable a new type of Thermally Assisted Magnetic Random Access Memory (TAS-MRAM). A TAS-MRAM materials stack that is robust against the 400 °C process temperatures required for embedded integration with complementary metal oxide silicon processes is demonstrated. In unpatterned sheet film stacks, a stable resistance-area product, tunneling magnetoresistance (MR) > 100%, and temperature-dependent exchange bias of 1500 Oe after 400 °C anneal are shown for this stack. It is further shown that by doping the sense and storage layers with Ta using thin laminations of Ta/CoFeB, the moment of each layer can be reduced by more than 40% without a major reduction in MR. In patterned nanopillar devices, it is shown that by reducing the moment of the sense and storage layers with laminations of Ta, and by adding a second MgO barrier, the resistance versus applied field loop quality is maintained, while the read field is reduced by more than 40% and devices survive 108 wr...