Figure 1. Simplified sketch of the tip-particle magnetic layout in a dipole-dipole model, indicating the different distance terms (not at scale).

Magnetic dipoles (black arrows) are modeled as located in the center of a SP NP having diameter d_m and in the center of a sphere with diameter d_t approximating the tip apex, while d₀ represents the thickness of a non magnetic layer on the particle. Therefore in this scheme the dipole-dipole distance is easily obtained adding z₀ = d_m/2+d_t/2+d₀ to the physical distance s.