Magnetic properties and crystal structure of elemental cobalt powder modified by high-energy ball milling

Abstract

Elemental cobalt powder was modified by high-energy ball milling for 0–15 h, and the results obtained by X-ray diffraction (XRD) revealed a phase transformation. Initially, the unmilled powder consisted of two phases: hexagonal close-packed (hcp) and face centered cubic (fcc). From a milling time of 0–5 h, the fcc phase underwent a phase transformation to hcp, and increased milling times promoted a transformation back to the fcc phase. Moreover, the crystallite size decreased from 88.9 nm to 16.7 nm for the samples at 0 and 15 h, respectively. Conversely, the microstrain increased from 0.02% to 0.489% for the same times. On the other hand, the mean particle size increased from 6.8 μm to 68.6 μm for the first 5 h, and this effect is related to the cold welding step in the milling process and cobalt ductility. The magnetic properties of the samples were analyzed by vibrating sample magnetometry (VSM) at room temperature. The hysteresis loops exhibited an increase in saturation polarization from 1.18 to 1.33 T (145.5–163 A m2/kg) for the first hour. On the other hand, the coercivity field and magnetic anisotropy presented a progressive reduction as the milling time increased, and this effect was attributed to particle size growth and the main content of the fcc phase. Magnetic–thermogravimetric analysis revealed that the Curie temperature was ∼1391 K, which was associated with the fcc phase. In addition, other events between 700 and 900 K were identified. These events were associated with the TC of the hcp phase. Keywords: High-energy ball milling, Cobalt, Allotropic transformation, Magnetic properties, Magnetocrystalline anisotropy, Curie temperature