The Effect of Surface Condition on the Magnetic Susceptibility of Nitinol

The magnetic susceptibility of fully austenitic nitinol in the electropolished condition is determined to be χ_g = 2.87*10^-6 cgs. Susceptibility increases by more than 6% due to the formation of a nascent oxide layer, even after subsequent mechanical polishing. Thicker, thermally-produced oxide layers are shown to increase the susceptibility 100 times or more. This physically translates to nitinol wire being capable of supporting its own weight with external magnetic fields representing that of an ordinary magnet. This has significant ramifications to devices used in and near MRI fields and potentially other areas. Detailed examination of the surface shows that thermal oxidation results in five distinct layers, including a pure nickel subsurface layer determined to be the primary cause of the ferromagnetic behavior of nitinol. The susceptibility of Martensite and the R-phase are also reported.