Earth's inner core grows by approximately one millimeter per year as molten iron crystallizes onto its surface, releasing energy that drives convection in the outer core and generates the planet's magnetic field.

https://spacedaily.com/j-the-earths-inner-core-grows-by-roughly-a-millimetre-a-year-as-molten-iron-freezes-onto-it-and-that-slow-crystallisation-releases-the-energy-that-stirs-the-outer-core-and-keeps-the-planets-magnetic-f/