I don't think 60/40 solder is magnetic, at least I don't feel it attracted by magnets.
A way to test for very faint magnetic interaction is to put a small bit of the tested material to float on water then to keep a strong magnet nearby (almost any small blob of material can float on water because of the superficial tension, or by improvising a tiny boat/raft for bigger lumps).
Some apparently non magnetic metals are in fact diamagnetic (Landau diamagnetism), but I didn't tested for solder.
Last weekend I revisited an old NMR project that I put on hold many years ago (that is why I am so curios about your CW NMR). I've seen solder blobs on some other DYI NMR machine (e.g.
https://youtu.be/Bf3r9Db0TSs?t=269 at minute 4:29 is the reading coil for a FT NMR, with lots of solder around, but he is sweeping the magnetic field intensity, not the excitation frequency).
The FT NMR project I was trying back then was aiming to get 3D MRI (Magnetic Resonance Imaging) in the Earth's magnetic field. The best found online at that time was the 3D image of a fruit and vague hints about its internal structure, with a complete 3D scan for an object the size of an apple being ready in "just" a day or two
. Can not find that DIY project any more, but the results were similar with the 3D demo from this educational machine
http://www.magritek.com/wp-content/uploads/2013/12/Magritek-MRI-2011-web.pdf (note that that 3D image took hours to scan, and the resolution is very low, so pretty unusable).
To read the NMR signal there was a sharp band-pass filter to reduce the out of band noise, and a few amplification stages. All I've seen use to had filter + low noise pre-amplifier, thinking here it will be very unlikely to see the NMR signal with just an oscilloscope.
Didn't know about the trick with the marginal oscillator, that was very interesting to learn, thank you.
Another thing that I found fascinating this weekend was to learn about how NMR can help to identify organic molecular structure (yep, never dive into that before). Judging by the other related project of yours, the gas chromatograph
https://www.eevblog.com/forum/projects/gas-chromatograph-diy-done!/ , I guess this is what you are after, organic molecule identification by NMR, is it?
These are the videos that I liked so much (linking them here so I could find them in the future, after I'll forget
), from the 'Knowbee' channel: