PhD

Van der Waals ferromagnets and heterostructures for spin-orbitronics

 

Laboratory : LPCNO, Nanomagnetism group, INSA Toulouse, France

https://lpcno.insa-toulouse.fr/research-groups/nanomagnetism

Supervisors : Benjamin Lassagne, Thomas Blon ; lassagne@insa-toulouse.fr, thomas.blon@insa-toulouse.fr

 

Recently discovered 2D van der Waals (vdW) ferromagnet materials such as CrI₃, CrBr₃, Cr₂Ge₂Te₆ or Fe_xGeTe_y, etc are extremely intriguing materials. Indeed, in the monolayer state, they exhibit ferromagnetic state contradicting the Mermin-Wagner [1] theorem stating that magnetism cannot exist in two-dimensions. Furthermore, it is remarkable that, by adjusting their number of layers, their magnetic properties can be easily modified, for example from a ferromagnetic to an antiferromagnetic magnetic state. Additionally, the weak interlayer bonding makes these materials compatible with a wide variety of substrates and enables the building of van der Waals heterostructures combining different materials exhibiting new properties. Thus, those materials constitute a very promising playground for spintronics. However, to date, the critical Curie temperatures are limited to low temperatures for the materials currently studied, therefore strong efforts are done to discover and stabilize new 2D vdW ferromagnets (2D FM).

 

The Nanomagnetism group of the LPCNO has developed a dedicated laboratory for the elaboration, observation and integration of van der Waals materials, from ambient conditions to air-free environments and low temperatures. We recently designed extremely sensitive graphene-based Hall effect magnetic sensors based on hexagonal Boron Nitride/graphene/hexagonal Boron Nitride (hBN) vdW heterostructures. We also developed a comprehensive and novel model for the detailed understanding of this device [2].

In this context, the aim of the PhD position we propose is to focus on the elaboration of 2D FM/hBN/graphene/hBN heterostructures to directly probe the magnetic response of flakes of 2D FM with the highly sensitive Hall response of graphene. This will be done by combining magneto-transport measurements at various temperatures with local magneto-optical measurements on the same sample in a dedicated low temperature cryostat with 3D vectorial magnetic field.

A second aspect of the PhD position is to combine the targeted 2D FM with topological insulators (TI). Our collaborators of the LAAS laboratory in Toulouse master the molecular beam epitaxy growth of BiSb and BiSbTeSe topological insulators [3,4]. Here the aim for the PhD position is to deposit 2D FM vdW directly on such TI to explore charge-spin conversion and spin-orbit torques in these heterostructures. The final objective is to address spin-orbitronics in full van der Waals FM/TI heterostructures, notably with the adding of graphene at the interface to preserve the topological surface states [5]. 

In summary, the proposed PhD concerns (i) the transfer of 2D van der Waal ferromagnets on graphene-based Hall-effect sensors, (ii) the measurement of the magnetic properties of ferromagnetic 2D materials in these 2D FM/hBN/graphene/hBN heterostructures, and (iii) the transfer of 2D FM on TI for the spin-orbit torque manipulation of 2D FM.

 

Specific Requirements:

Master 2 or engineering degree, specializing in physics of matter, nano-physics and/or nanotechnology. The student should have a definite attraction for experimentation.

 

References

[1] N. D. Mermin and H. Wagner, Phys. Rev. Lett. 17 1133 (1966)

[2] Performance of graphene Hall sensors: role of bias current, disorder and Fermi velocity, L. Petit, T. Fournier, G. Ballon, C. Robert, D. Lagarde, T. Blon, B. Lassagne, Phys. Rev. App. (2024) under review, http://arxiv.org/abs/2403.11342

[3] D. Sadek, et al., ACS Appl. Mater. Interfaces 13 36492 (2021)

[4] D. Sadek, et al., Cryst. Growth Des. 22 5081 (2022)

[5] T. Guillet, et al., Graphene intercalation in topological insulator/ferromagnet heterostructures for efficient spin-orbit torques, to be published (2024)