The magnetic domain patterns of the pure double-hcp (dhcp) phase and hcp-mixed-dhcp phase of Co-1.2%Fe alloy single crystals were investigated to clarify the nucleation and growth process of the hcp phase from the dhcp phase in the external magnetic field. The thin hcp phases appeared to be parallel to the c-plane in the pure dhcp phase at phase transformation. The domain observation failed to detect the nucleation site of the dhcp-to-hcp phase transformation. The nucleation site and growing process of the hcp phase are discussed with respect to the effect of the shear stress induced by magnetostriction. The calculated maximum shear stress induced by magnetostriction was found to be on the same order of the resolved shear stress of Co, and it could be realized in the domain walls but not in the domains. This shear stress in the domain walls might be the motivating force in generating nucleation sites of hcp phase.
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