a) Light collected by the CCD camera showing a stationary wave produced by the interference between the SPP wave excited by the tip (located above the top right corner of the image) and the one back reflected by a large defect on the surface of the Au/Fe/MgO(001) sample. b) Intensity profile (dots) and sinusoidal fitting (line) along the dashed line shown in panel a. From the period of the stationary wave, we obtain that a SPP with wavelength λ_SPP = 896 ± 1.5 nm is excited by light impinging on the tip with λ_exc = 880 nm.