Annealing-induced high ordering and coercivity in novel L10 CoPt-based nanocomposite magnets

Abstract

A novel class of quaternary intermetallic alloys based on CoPt is investigated in view of their interesting magnetic properties induced by the presence of hard magnetic L10 phase. A Co48 Pt28 Ag6 B18 alloy has been prepared by rapid solidification from the melt and subjected to various isothermal annealing procedures. The structure and magnetism of both as-cast and annealed samples as well as the phase evolution with temperature are investigated by means of thermal analysis, X-ray, and selected area electron diffraction, scanning and high-resolution electron microscopy, and magnetic measurements. The X-ray diffraction (XRD) analysis shows that both the as-cast alloy and the sample annealed at 400° C (673 K) have a nanocrystalline structure where fcc CoPt phase predominates. Annealing at 473° C promotes the formation of L10 phase triggered by the disorder-order phase transformation as documented in the differential scanning calorimetry results. The sample annealed at 670° C (943 K) shows full formation of L10 CoPt as revealed by XRD. Magnetic measurements showed coercivity values ten times increased compared to the as-cast state. This confirms the full formation of L10 CoPt in the annealed samples. Moreover, detailed atomic resolution HREM images and SAED patterns show the occurrence of the rarely seen (003) superlattice peaks, which translated into a high ordering of the L10 CoPt superlattice. Such results spur more interest in finding novel classes of nanocomposite magnets based on L10 phase.