Figure 6

Experimental results obtained on powdered aluminium samples by (a) varying k (using a fixed pulse width, τ _π/2 = 11 μs and changing the pulse power, with the k = 1 pulse power calibrated with a 1 M Al(NO₃)₃ solution) in the pulse sequence shown in Fig. 2 with N = 8 and Δ = 80 μs. The intensities of the on-resonance positions in the spectra are plotted. Error bars are calculated following the methodology described in the caption to Fig. 5. Where errors are not visible, they fall within the marker size. (b,c) SEM images of the two samples. The second peak at higher k-value for the smaller particles is likely a consequence of the n = 2 overtone signal, which is due to the fact that the particle size is <δ/4. As a result, signals from regions experiencing a readout pulse slightly larger and smaller than π do not cancel each other completely.