Individual atoms of a ferromagnetic material possess magnetic moments, but its extremely large magnetisation cannot be explained merely by the direct aligning action of an external field. The crucial microscopic mechanism is:
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Question

Individual atoms of a ferromagnetic material possess magnetic moments, but its extremely large magnetisation cannot be explained merely by the direct aligning action of an external field. The crucial microscopic mechanism is:

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G1 Physics · Accepted Answer

Ferromagnetism is a cooperative quantum phenomenon. The exchange interaction between neighbouring atomic magnetic moments can energetically favour parallel alignment. This produces regions of strong spontaneous magnetisation called magnetic domains. Therefore the large magnetisation of ferromagnets is fundamentally associated with exchange interaction, not merely the weak direct torque of the external field.