The role of Fe and Cu additions on the structural, thermal and magnetic properties of amorphous Al-Ce-Fe-Cu alloys

Abstract

Iron and copper are immiscible elements, but can be combined in the presence of a third element such as aluminium or cerium. We varied the additions of immiscible Fe and Cu (ratio y) in the stoichiometric Al20.5Ce41.5(FeyCu1−y)38 alloys in order to trace their impact on the structure, microstructure, thermal properties and magnetism. By powder X-ray diffraction (PXRD), we found that Fe-rich alloys (0.87≤y≤1) do not completely hinder crystallisation and yield nanocrystalline precipitates. Cu-rich alloys completely vitrify for 0≤y≤0.74 and exhibit a characteristic broad hump in PXRD patterns. DSC shows that Cu-rich alloys exhibit a noticeably lower crystallisation temperature Tx of about 510K, liquidus temperature Tl at approximately 900K and a more pronounced drop Δ of the heat flux at the glass transition temperature Tg. These criteria indicate that Cu-rich alloys are better glass formers. Transmission electron microscopy (TEM) shows that the specimens with y=0 and y=0.34 are composed of an amorphous matrix with minor impurities of CeO2 nanocrystals, which can only be observed via transmission electron microscopy. However, these nanocrystals do not interfere with the formation of the glassy phase and are therefore inert. Magnetic measurements show that Fe-rich alloys possess higher magnetisation at maximum field (Mmax H), peaking at 3.3 and 3emu/g for y=1 and y=0.87, respectively. From ferromagnetism in the latter two alloys, the behaviour turns to paramagnetism for 0≤y≤0.74. Cryogenic measurements on SQUID show that the sample with y=0 displays two different temperature regions at 2–50K and 100–300K. The first one exhibits nearly pure paramagnetic behaviour with Θ=−0.7, while the second one shows a highly negative Weiss constant of Θ=−80.6.