AlNiCo stands for aluminum nickel cobalt. The rest of the magnet contains mostly iron (over 50%) and typically copper. These five alnico bar magnets all look the same but are very different in strength and tone. Here are the common bar magnets usedin humbuckers and P90s, explained in order of grading (which seems to follow no order whatsoever):
Alnico II has a nice vintage sound to it. Treble frequencies are very much in the forefront with this magnet. In terms of strength, it’s right in the middle of these five magnets.
Alnico III has nice, sweet tone. Great for a neck pickup. Excellent in a guitar with a maple top. I use it in my Clear Tone neck humbucker and I mix it with other magnets in some of my P90s. It really has no business being called “alnico” since it actually has no cobalt in it. It is the second to weakest in this bunch.
Alnico IV has this really cool vowel vocal sound to it and holds the bottom end quite well. I’m really getting into it. Currently, it’s in two of my neck pickups but I’m looking to try a higher output bridge pickup with this magnet. Alnico IV is the only one in the bunch without copper and is the weakest in this group.
Alnico V is really the most popular magnet here. This is the most versatile magnet of them all. Prominent treble response and a strong bass response. For the longest time, this magnet was the strongest of the group.
Alnico VIII is the strongest magnet here and has a tone similar to alnico V but really holds the bottom end quite nicely. If someone tells me that their guitar is dropped down a full step or more, this is the magnet I recommend.
I hope that this helps educate you so that you can build your signature sound.