YIG nanostripes: width W = 500 nm, thickness T = 100 nm, length L = 100 μm, Bsat = 1700 G; x,y,z as in Figure 1. (a and b) EPR spectrum at X (9.7 Hz) and Q (34 GHz) band resp., showing, in blue, YIG SWR and, in red, the shifted EPR of g = 2.00 electron spins placed at xopt = 150 nm above the YIG nanostripe center (x = 0) (1 G linewidth for all here; SWR oscillator strength not calculated). (c) 1D eigenenergies of the spin waves along z axis; here z* = 300+z; (d) z component of the dipolar magnetic field of the YIG nanostripe as a function of x (black), as well as its gradient along x (red) multiplied here by 100 for clarity. (e and f) Total effective Zeeman splitting at X band (dot line), expressed in Gauss (thus divided by (g μ_B), assuming g =.00), as well as its two contributions: Bdz (to first order in blue), and Bdx (in red to second order), at xopt(e) and at xopt +/−10 nm (f).