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What is the chemical formula of hexagonal ferrites?

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The chemical formula for hexagonal ferrites is $M (F e_{12} O_{19})$ , where $M$ typically represents barium (Ba), strontium (Sr), or lead (Pb).1

These materials are a class of magnetic ceramics characterized by a complex crystal structure with a unique vertical axis, known as the c axis. Because this axis serves as the easy axis of magnetization and the direction of magnetization cannot be easily altered, hexagonal ferrites are classified as "hard" magnetic materials.1