Millikelvin magnetometry up to 30 T using a multi-cross micro-Hall sensor

We report a multi-cross micro-Hall sensor fabricated from an MBE-grown GaAs/AlGaAs heterostructure for high-sensitivity magnetometry. The device enables dc magnetization measurements at temperatures down to ∼15 mK and in dc magnetic fields up to 30 T. Bulk samples with dimensions ranging from 100 μm to 1 mm can be measured with a resolution better than 1 μT with a 1 μA excitation at 13.77 Hz. Test-of-performance measurements were carried out by probing the field-dependent magnetization plateaus of the triangular-lattice quantum magnet K₂Co₂(SeO₃)₃ at 20 mK, the antiferromagnetic transition of K₃GdTe₂O₉ near 220 mK via temperature-dependent magnetization, and the quantum oscillations in a ruthenium single crystal. A simple and reliable sample-mounting setup that ensures reproducible magnetic measurements is provided.