All-optical switching of magnetization in Tb/Co-multilayer based electrodes

This work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of Tb/Co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. Toggling of the magnetization of the Tb/Co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm2.

Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. In particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards THz frequencies.

Our results highlight a first evidence of helicity-independent all-optical switching in a [Co/Tb]5 multilayered-based system coupled to CoFeB layers with both ps- and fs-long single laser pulses. We also explored the magneto-optical properties of the multilayers and its thermal stability upon different annealing temperatures. The magneto optical response and the perpendicular magnetic anisotropy of our system was achieved even after annealing at 250 °C. The laser pulse duration and fluence dependence for the CoFeB/[Tb/Co]5 electrodes was also explored using single 60 fs and 5 ps laser pulses with fluences below 4.0 mJ/cm2. Images obtained for a laser pulse duration D = 5 ps and 60 fs, shows a clear reverse of the magnetization is observed for F = 3.5 mJ/cm2. Our all-optical switching electrode FeCoB/Ta/[Tb/Co]5 was integrated into a perpendicularly magnetized tunnel junction. Electrical evaluation of nanopatterned AOS-MTJ showed TMR ratios up to 36 % depending on the diameter of the junctions and on the number of repetitions of the [Tb/Co] bilayers. The full structure of the junctions consist of: Ta(30Å)/FeCoB(11Å)/MgO(23Å)/FeCoB(13Å)/Ta(2Å)/ [Tb(9.5Å)/Co(12.5Å)]5. The TMR distribution of hundreds of MTJ with different diameters have a minimum resistance of 6 kΩ. As far as we know, we are presenting the first study that reports the helicity-independent all-optical switching in a Co/Tb multilayered-based system.

These results are highly promising for the development of p-MTJs in which the storage layer is all-optically addressed using single-shot laser pulses, thus facilitating writing frequencies that could be advanced towards the THz scale.

Team: MRAM
Collaboration: Radboud University, Nijmegen
Funding: European Union’s Horizon 2020, FET-Open Grant Agreement No. 713481 (Project SPICE)
Further reading: Single-shot all-optical switching of magnetization in Tb/Co multilayer based electrodes, L. Avilés-Félix, A. Olivier, G. Li, C. Davies, L. Álvaro-Gómez, M. Rubio-Roy, S. Auffret, A. Kirilyuk, A. Kimel, Th. Rasing, L. D. Buda-Prejbeanu, R. C . Sousa, B. Dieny & I. L. Prejbeanu, Sci. Rep. 10, 5211 (2022). DOI: 10.1038/s41598-020-62104-w
Contact: Ricardo Sousa, Lucian Prejbeanu.

Figure: a) Coercive field mapping of the [Tb/Co]5 multilayers indicating the combinations of thicknesses which present all-optical switching. b) and c) Background substracted Kerr microscopy images showing the reversal of the oop magnetization of the FeCoB/Ta/[Tb/Co]5 after a series of laser pulses of b) ps and c) fs-long duration.

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