In modern permanent magnet research, implementation of an exchange-coupling interaction in spring magnets that are composed of more than two kinds of magnetic materials (e.g., Sm-Co@Fe) with core-shell architecture is a major direction to afford an enhanced magnetic performance far beyond those of the single-phased counterparts. Many efforts have been made to synthesize exchange-spring nanomagnets with conventional physical techniques such as electroless deposition, however, an ideal realization of the exchange-coupling effect has been hampered by a variety of restrictions. Moreover, even the use of a strong acidic/basic solution for electroless coatings causes fast oxidation of the metallic magnets and at worst, phase decomposition into a non-magnetic phase because metal magnets are vulnerable to harsh process conditions. Therefore, the great challenge remains hitherto to prepare binary-phased spring magnets possessing an ideal exchange-coupling interaction. Herein, we report a streamlined spring magnet preparation based on NH_4F-assisted direct displacement. By turning a byproduct (i.e., CaO) obtained during Sm-Co preparation process into an initiator for Fe-coating under neutral solution condition, uniformly nanoscale Fe layer was successfully deposited onto the surface of a Sm-Co magnet, while Ca-based byproduct was selectively rinsed out by NH_4F, which is employed as Ca~(2+) scavenger. This is the first report about the direct Fe-plating process on rare-earth magnet, which enabled it without a use of catalysts or such strong acidic/basic solution.
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