Millimeter Sized Ferromagnetic Fe-clusters: Formation by Mechanical Attrition, Microstructure and Magnetic Properties

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Ferromagnetic Fe-clusters of millimeter size are fabricated by surface deformation and recombination of spherical Fe-particles (1–2 μm in diameter) by mechanical attrition in a pure argon. The morphology changed from spherical particles to clusters during a non-interrupted attrition for 50 h. The clusters are separated apart and have an ellipsoidal shape with a sharp size distribution. The particles in cluster are interconnected through a structurally disordered surface structure. A strong dipole-dipole (DD) interaction between the particles designs a reduced effective magnetic moment, μn=2.21μB per atom, in the clusters if compared with that in the individual particles (e.g., μn=2.30μB in 8–20 nm particles). An extended milling for 100 h, with 5 mass% addition of a volatile hydrocarbon liquid, recovers the disordered structure with a resolved and refined structure of 8–20 nm particles. The sample, which consists of single magnetic domain particles, exhibits a monotonically increasing value of coercivity Hc with a decrease of temperature from 300 to 12 K, with Hc=7.2 kA/m at 295 K and Hc=150 kA/m at 12 K. A similar temperature dependence of Hc, from 11.9 kA/m at 295 K to 191.7 kA/m at 12 K, appears in the clusters because of intracluster DD interactions. The results are discussed in relation to microstructure and modified magnetic anisotropies of the particles.

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