Reduced exchange interactions in thin perpendicularly magnetized magnetic tunnel junction free layers and spin-transfer reversal mechanisms*

Perpendicularly magnetized magnetic tunnel junctions (pMTJs) are being widely developed for spin-transfer torque magnetic random-access memories for data storage and embedded memories. The magnetic properties of the pMTJ free layer affect the dynamic properties of the free layer, such as the spin-torque switching efficiency and the switching speed, and therefore the device performance.
The magnetic exchange constant is one of the most important yet hard to measure properties of the free layer, as the layer is very thin and embedded in a pMTJ layer stack that includes a magnetic reference layer coupled to a synthetic antiferromagnet. The exchange constant sets the length scale for spatial variations of the magnetization. In perpendicularly magnetized tunnel junction devices, the exchange constant can also set the energy barrier for thermally activated reversal [1].
I will present the results of a systematic experimental study of the exchange constant of CoFeB free layers with perpendicular anisotropy. We have used vibrating sample magnetometry to determine the temperature dependence of the magnetic moment, and therefore the magnon population density [2] of the free layer. In particular, I will show that when a thin W insertion layer is introduced into a 2.3 nm thick CoFeB free layer to increase the perpendicular magnetic anisotropy of the layer, the exchange constant of the free layer is significantly reduced.
I will also present a micromagnetic study that shows that the spin-transfer reversal nucleates at the center of the element and completed by domain wall motion in elements larger than a critical size set by the exchange constant, magnetization and perpendicular magnetic anisotropy.
[1] G. D. Chaves et al., Phys. Rev. Applied 4, 024010 (2015)
[2] J. Beik Mohammadi et al., ACS Appl. Electron. Mater., 20191102025-2029 (2019)