Apple is investing $500 million in the mining company MP Materials to secure a domestic supply of rare earth magnets for its devices.

The $400 million investment will create a domestic supply chain for materials used in fighter jets and missiles.

Neha Mukherjee, a rare earths analyst with Benchmark Mineral Intelligence, said in a research note that the Pentagon's 10-year promise to guarantee a minimum price for the key elements of neodymium a...