FYI, Alnico, whether 2, 3, or 5 (common types for pickup polepieces and speaker magnets) is simply an alloy, and is magnetized for the purpose of use in pickups or speakers, etc. Good thing too, because shipping magnets in any quantity must be a real nuisance. Its non-magnetic nature in native form means that pickup manufacturers use a magnetizer prior to installing them. Stew-Mac will sell you Alnico polepieces and has instructions on their website for how to magnetize them, using rare earth magnets. Since rare earth magnets are ALSO a nuisance (now let's see, did I or did I not have my credit card and ATM card in my pocket when I was leaning over the bench?), manufacturers tend to use electromagnets for "charging" the polepieces. Unlike rare earth (cobalt samarium) magnets, once those babies are off, they are OFF.
Because the slugs/polepieces have to be magnetized, that means they can be magnetized to varying strengths/gauss-levels. I would imagine that anything which is intended to use in close proximity to strings, with the express purpose of keeping those strings moving, would be designed to have very modest magnetic "tug". the use of Alnico is really more for the long term retention of magnetic qualities, I imagine, as well as machineability (prior to magnetization).