Nd-based amorphous materials
It is becoming increasingly clear that a significant value of coercivity can be induced even in amorphous materials by either anisotropy (local anisotropy of rare earth elements, surface anisotropy, shape anisotropy) or controlled crystallization. Quite a large number of amorphous RE-TM alloys have been studied by Buschow, Croat and several others. A variety of magnetic structures depending on RE (Pr, Nd, Sm, Eu, Gd, Tb) and TM (Fe, Co, Ni) were found to exist in these materials. Among these Nd-Fe system attracted special attention due to the formation of number of metastable and equilibrium phases with different magnetic properties. Several studies to identify the phases and their intrinsic properties were carried out. Initially, in Nd-Fe system only one intermetallic equilibrium phase Nd2Fe17 was beleived to exist . Later on Nd5Fe17 and NdFe2 were included in the revised phase diagram. Hard magnetic properties of Nd-rich Nd-Fe melt-spun ribbons and as-cast alloys were attributed to the relaxed cluster structure and unknown hard magnetic phase (A1). Concurrently hard magnetic phase Nd2Fe14B (Ø) had been discovered and much efforts were deverted towards ternary alloy systems and to improve the magnetic properties of Ø phase. As a result it turned out that 2-3 at % Al substitution enhances the coercivity of Ø phase . The role of Al to increase the coercivity was difficult to expalin due to the formation of several intergranular ternary phases with unkwon crystal structure and magnetic properties. Thereafter, a large number of RE-Fe-M (Al, Si) crystalline ternary alloys were investigated. Grieb at al. identified two ternary phases i.e., d (tetragonal) and m (unknown structure) in Nd-Fe-Al system and proposed a possible ternary phase diagram for this system. By that time the science and technology of amorphous metals had very much developed and several advanced methods of preparation were explored to achieve very high cooling rates. Inoue et al. invesigated the effect of preparation on the structural and magnetic properties of Nd,Pr-Fe-Al,Si alloys in RE-rich range. Bulk amorphous alloys of several mm diameter could be easily prepared by suction casting techniques. The mechanism of the coercivity in these amorphous alloys remained unclear. The another interesting feature of these alloys is the absence of glass transition in DSC curves. Although the absence of glass transition does not challenge the amorphous structure of the alloys. The thermal annealing above 600 °C destroys the hard magnetic properties. Despite the interesting magnetic properties, leas concern has attention has been given to study the the crystallization and structural studies, which are of great importance to to understand the correlation between coercivity and structure. Presently, we have found that the partial substitution of Fe by Co improves the glass-forming ability by lowering the liquidus temperature. The aim of this project is to study the structural and magnetic properties of binary (Nd-Fe), ternary (Nd-Fe-Al), quatenary (Nd-Fe-Co-Al) alloys prepared by different methods. Special efforts will be paid to identify the different equilibrium phases by microscopic and magnetic characterization. The role of Nd, TM and Al in the GFA and magnetic properties will be discussed.
Recent results