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

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, μ n = 2.21μ B per atom, in the clusters if compared with that in the individual particles (e.g., μ n = 2.30μ B in 8-20nm particles). An extended milling for 100h, 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 H c with a decrease of temperature from 300 to 12 K, with H c = 7.2kA/m at 295 K and H c = 150kA/m at 12 K. A similar temperature dependence of H c , from 11.9kA/m at 295 K to 191.7kA/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.