Magnetically Driven Spinodal Decomposition as the Origin of Magnetic Softness of CoFeNi‐Based Ferromagnetic High‐Entropy Alloys

Many high-entropy alloys (HEAs) based on ferromagnetic (FM) 3d elements Co, Fe, and Ni are soft ferromagnets, possessing vanishingly small hysteresis loops. Their magnetic softness originates from specific nanostructure that is formed by spinodal decomposition, where nanodomains enriched in the FM elements Co, Fe, and Ni alternate with nanodomains enriched in either the nonmagnetic elements such as Cu or the antiferromagnetic 3d elements Cr and Mn. In nonmagnetic HEAs composed of the elements with markedly different atomic radii, spinodal decomposition is driven by the local volumes misfit of the two decomposed phases, each one gathering elements of similar atomic radii to minimize the lattice distortion energy. By a detailed analysis of the FM GaCoCrFeNi nanostructured HEA, where all five elements possess practically identical atomic radii, we argue that spinodal decomposition in the CoFeNi-based FM HEAs does not occur due to the local volumes misfit, but is magnetically driven via short-range FM order within the CoFeNi-rich clusters at temperatures far above the long-range FM transition, which reduces the magnetic Gibbs free energy <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msup><mml:mi>G</mml:mi> <mml:mrow><mml:mi>m</mml:mi> <mml:mi>a</mml:mi> <mml:mi>g</mml:mi></mml:mrow> </mml:msup> </mml:mrow> <mml:annotation>$G^{mag}$</mml:annotation></mml:semantics> </mml:math> sufficiently to stabilize the nanostructured state. The related FM AlCoCrFeNi nanostructured HEA behaves identically, supporting the magnetic origin of spinodal decomposition.