Fig. 2.

Details of the resonant slice and associated point spread function (PSF). (A) Three-dimensional representation corresponding to the conditions B_res = ω₀/γ = 2.697 T and B_ext = 2.482 T. The center of the tip apex is assumed to be at the origin of the coordinate system. The resonant slice is the hemispherical “shell” outlined in red, representing the region of space for which B₀(r) lies within B_res ± Δω_rf,peak/γ (here 2.697 ± 0.014 T). Regions to the left and right of the tip (shaded red) contribute most to the signal because this is where the lateral gradient G(r) is largest. (B) Cross-sections of the point spread function at the 4 tip-sample spacings used in the imaging experiment. The PSF was calculated by using Eq. 2, assuming A = 1. The color scale reflects the force variance per spin (zN = zeptonewton = 10⁻²¹ N). The size of the tip apex (r_a = 100 nm) is indicated by a dotted circle. At z = 24 nm, the PSF lobe thickness reaches a minimum of ≈4.8 nm (FWHM). The small left–right asymmetry is due to a slight (1.7°) tilt of the sample plane with respect to the magnetic tip.