Non-Magnetic Drill Collars Functions & Selection
Non-magnetic drill collar is made from specially treated steel alloys so that it does not affect magnetic surveying instruments. Their purpose is to isolate directional survey instruments from magnetic distortion due to the steel drill string.

Non Magnetic Drill Collar Function

The magnetic survey instrument is influenced by the presence of a magnetic field around the tool: ferromagnetic material in the formation, drill pipe, drill collar, or other bottom hole assemblies.
https://www.drillingmanual.com/non-magnetic-drill-collars-functions-selection/