Why are neodymium magnets ground into powder rather than cast? According to Wikipedia, neodymium magnets are first cast into ingots, then milled into powder and sintered into final shapes.
Why not just cast them? Why bother milling? It seems like an extra step for no reason.
https://en.wikipedia.org/wiki/Neodymium_magnet
 A: First watch MAGNETS: How Do They Work? by MinutePhysics and Veritasium.
It is a good explanation, but one point we care about was simplified away. Magnetic domains are tied to crystal structure. Crystal structure creates easy directions.
For example, an iron crystal has either a body centered cubic or face centered cubic structure. Each atom has nearest neighbors in various different directions. Nearest neighbor atoms tend to align N pole to S pole, like bar magnets. This can happen in any of the nearest neighbor directions. These are called "easy" directions because it is easy to magnetize an iron crystal in these directions.

Image from https://www.sciencedirect.com/topics/engineering/delta-iron
Typically iron is made out of many microscopic crystals oriented in random directions. You can magnetize iron, making the magnetic field of each domain point in the same direction. But what you really get is the magnetic field of each domain pointing in the easy direction closest to the desired direction. Having domains point more or less in the right direction make iron a weaker magnet.
This is OK for iron. It gets magnetized enough for ordinary purposes. But sometimes you need the strongest magnet you can get. This would be a Neodymium magnet. When these are made, no expense is spared to being out their full strength.
See How Neodymium Magnets are Made
The key point is they are reduced to a fine powder, so fine that each grain is a single domain. The grains are lined up so their easy directions are parallel. Then they are sintered - heated to tack them together into a single solid. As All About Magnetization Direction explains, orienting the easy directions makes the magnet 4 or 5 x stronger. (See Temperature and Neodymium Magnets for an explanation of magnetic strength.)
Sintering has drawbacks. The material isn't nearly as strong as solid metal. It is fragile, breakable, and hard to machine. People use Neodymium magnets when they care more about magnetic strength than anything else.
