[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-09] Developments and prospects of paleomagnetism and rock-magnetism utilizing magnetic microscopes

*Hirokuni Oda¹, Jun Kawai², Naoto Fukuyo¹, Akihiro Tanimoto^4,1, Isao Yosano^3,1, Miki Kawabata² (1.Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 2.Kanazawa Institute of Technology, 3.Kochi University, 4.Ibaraki University)

Keywords:Magnetic Microscope, scanning SQUID microscope, Quantum diamond microscope, liquid helium

We have been developing scanning SQUID microscope (SSM) for paleomagnetic studies from 2015 [1-2]. We could untangle paleoenvironments with the precise analyses of paleomagnetic information recoded in geological archives using the SSM. Higher sensitivity and higher spatial resolution measurements are suitable for analyzing remanent magnetization of geological archives, which requires the reduction of the sensor-to-sample distance (lift-off). During JpGU2023, we presented the results of an improved mounting method of SQUID chip aiming at reducing the lift-off and stabilizing the contact resistance. The new mounting method was achieved by dividing the sapphire rod into upper and lower parts. Aluminum wires were bonded before wiring, which were connected electronically to gold wires with with conductive paste. While operating with the existing low drift FLL circuit, the noise in the magnetic field was about 4pT/√Hz@1Hz with a sensitivity of 660nT/V. The resistance including the wires were stable and the contact resistance was quite low. Just after the improvement, line current scans provided a lift-off value of 187 μm, which was further reduced to a minimum accomplishment of 136 μm using a 12 μm thick film to protect the surface of a thin section. The discrepancy of the magnetic field values from the theoretical values for the line current were 2%. By reducing the lift-off from 187 μm to 136 μm, sensitivity is expected to increase 2.6 times for a magnetic dipole as well as the increase of spatial resolution.
Despite the successful improvements of SQUID microscope system, the price of liquid helium has been increasing and its supply is unstable due to the worldwide shortage of liquid helium. This suggests that continuous and stable analyses using the SQUID microscope is not easy. On the other hand, other types of magnetic microscopes such as Quantum Diamond Microscopes were developed and available as commercial products. Considering these situations, we review the developments of paleomagnetism and rock-magnetism utilizing a scanning SQUID microscope and the possibilities. Also, we discuss continuous operation of SQUID microscope in this situation, and further discuss the possibility of a high sensitivity magnetic microscope that can be operated at room temperature.