(a) Constant-current image of an Fe3-array, an Fe and a Ho atom (Vs=−5 mV and Istab=0.5 nA) recorded with a spin-polarized tip. The height of the Fe3-array (≈200 pm) changes randomly while recording the image due to switching of its magnetization. The white scale bar has a width of 1 nm and the colour scale on the left side indicates the measured apparent height in a range of −32 to 195 pm. (b) Time trace of the magnetic telegraph noise of the Fe3-array (colours red and blue represent the two magnetization states up and down) and of a Ho atom recorded with the same magnetic tip (orange line) (Vs=5 mV, Istab=1 nA and B=0.2 T). The magnetic tip is sensitive to the out-of-plane magnetization component. (c) Time trace of the Ho atom from b shown in a more narrow ΔZ range. (d) Thirty-nine time traces on 13 different Ho atoms (colour gradient from yellow to orange) recorded with spin-polarized tips and various parameters Istab (0.5 to 50 nA), Vs (3 to 10 mV), B (−0.2 to 0.2 T) and recording times t (200 to 1,600 s) (see Supplementary Note 3 for a list of parameter combinations). The telegraph noise of the Fe3-cluster (blue) from b serves as a reference.