Effect of annealing on structural and magnetic properties of Pr2Co7 compounds

Using high-energy ball milling, nanostructured Pr2Co7 powders can be obtained from their arc-melted bulk alloys. The structure and the microstructure properties were studied by X-ray diffraction and transmission electron microscopy. The crystallization, phase components and magnetic properties of these alloys were investigated systematically. Structural and magnetic measurements reveal different phases for specific annealing temperature ranges. For samples annealed at T-a = 1023 K, the main phase is hexagonal of the Ce2Ni7 type structure whereas at T-a = 1323 K, the main one is rhombohedral of the Gd2Co7 type structure. The autocoherent diffraction domain size derived from Rietveld analysis, varying between 6 and 40 nm, are confirmed by transmission electron microscopy. The coercivity increases with annealing temperature reaching a maximum for T-a = 1073 K, the highest value is equal to 18 kOe at 293 K and 23 kOe at 10 K. This coercivity is attributed to the high magnetocrystalline anisotropy of the Pr2Co7 phase and its optimized nanoscale grain size. The remanence ratio M-r/M-max is equal to 0.64. Thermomagnetic measurements indicate that the intermetallic Curie temperature (TC) is about 600 K for hexagonal phase and 580 K for rhombohedral one. These properties are suitable for permanent magnet applications. (C) 2011 Elsevier B. V. All rights reserved.