AlNiCo magnet is an alloy made of Aluminum, Nickel, and Cobalt, it’s one of the most popular types of magnet used in electric guitar pickups and there are in fact several different types:
Alnico II (alnico 2), Alnico III (alnico 3), Alnico IV (alnico 4), Alnico V (alnico 5), Alnico VIII (alnico 8).
The different numbers are a classification based on chemical composition, but you can basically think of each as a different type of magnet with slightly different properties.
They all sound different and Alnico 2, 3, and 4 are generally used in more classic sounding pickups while Alnico 5 magnet and 8 are generally used in more modern sounding pickups.
Let’s see through all the types and the differences in tone and output between them：
Alnico 2 magnet (II)
Alnico 2 is the second weakest type of Alnico magnet. This means that the magnetic pull is not very strong so the pickups won’t sound particularly dirty compared to the likes of Alnico 5 and 8 especially.
The tone of Alnico 2 magnets is often described as “vintage” rather than modern due to its lower output.Alnico 2 is well known for sounding quite warm and mellow with a good amount of mid-range to give it a fuller sound.
It doesn’t have a huge amount of treble response though so isn’t the clearest sounding.
Tone = warm and loose
Output = low
Alnico 3 magnet (III)
Alnico 3 is the weakest type of Alnico magnet meaning it sounds the quietest through a clean amp, and the least distorted through a driven amp.
Alnico 3 magnets are not as common as Alnico 2 or 5 magnets but were used in the original mid-’50s Fender Stratocasters.
Interestingly, Alnico 3 doesn’t actually contain any cobalt at all, so really should probably be named AL-NI, rather than AL-NI-CO.
Alnico 3 tends to sound quite dark compared to the other Alnico magnets, however it does still have a decent amount of top-end clarity.
Tone = dark and glassy
Output = low
Alnico 4 magnet (IV)
Alnico 4 is stronger than Alnico 2 and 3, but weaker than Alnico 5 and 8. Similarly to Alnico 3, Alnico 4 is not a particularly popular type of magnet, but it is used in several guitars including the Seymour Duncan Saturday Night Special.
Alnico 4 is often described as having the most balanced EQ.
Tone = balanced
Output = low-moderate
Alnico 5 (V)
Alnico 5 magnet is very popular and is the second strongest type of Alnico magnet. It is often used in more modern electric guitars as it has a brighter tone compared to Alnico 2, 3 and 4. This provides it with more clarity and definition which is useful when using distorted amp settings.
Alnico 5 has a higher output compared to Alnico 2, 3 and 4 meaning it distorts more easily and sounds louder through a clean amp. Hence Alnico 5 is often described as sounding “hotter”.
Tone = punchy and bright
Output = moderate
Alnico 8 (VIII)
Alnico 8 is the strongest type of Alnico magnet. Alnico 8 magnets are typically reserved for high-output pickups and sound very aggressive in comparison to the other types of Alnico as they distort much more easily so are the closest to ceramic magnets out of all the different types of Alnico.
Tone = bright and aggressive
Output = high