Visualizing isospin magnetic textures in rhombohedral tetralayer graphene using a newly developed milli-Kelvin SQUID-on-Tip Microscope

I briefly review recent advancements in scanning SQUID-on-Tip (SOT) microscopy that enable magnetic imaging down to the milli-Kelvin regime. We developed two new SOT microscopes integrated into dilution refrigerators: a cryogen-free system operating at 150 mK[1] and a helium-based system reaching 20 mK. I briefly address design challenges and demonstrate current capabilities. I then describe in detail our recent transport and SOT measurements in rhombohedral tetralayer graphene[2]. Our main observations are: (1) a distinct phase shift in quantum oscillations, which serves as a direct signature of a strong valley-Zeeman effect; (2) an isotropic spin-polarized ferromagnetic phase; and (3) an anisotropic spin-valley–locked phase. We quantify the isotropy in (2) and (3) by performing spatial measurements in the presence of in-plane fields, and place lower bounds on the intrinsic Kane-Mele type spin-orbit coupling and the intervalley Hund’s exchange interaction.

[1] H. Zhou, N. Auerbach et al., “Scanning SQUID-on-tip microscope in a top-loading cryogen-free dilution refrigerator,” Review of Scientific Instruments, vol. 94, no. 5, May 2023, doi: 10.1063/5.0142073/2891379.

[2] N. Auerbach, S. Dutta et al., “Visualizing isospin magnetic texture and intervalley exchange interaction in rhombohedral tetralayer graphene,” Mar. 2025, Accessed: May 10, 2025. [Online]. Available: https://arxiv.org/pdf/2503.14146