High entropy alloy (HEA) or multiprincipal alloy have minimum of five key elements with
concentration between 5-35 at.%. Owing to their unique excellent mechanical and functional
properties they find applications in the areas of hydrogen storage, diffusion barriers in electronics,
electro-magnetic shielding materials, thermo-electric materials etc. Having elements like Fe, Co,
Ni as constituents in the HEA composition help improve magnetic properties, while elements like
V, Cr are detrimental to magnetism as these elements quite often couple antiferromagnetically with
Fe, Co and Ni elements. It is claimed in the literature that addition of non-magnetic Cu to HEA
improves saturation magnetization (MS) value due to formation of Cu rich clusters owing to a high
positive mixing enthalpy of Fe-Cu and Co-Cu. Further the magnetic properties of HEAs are
sensitive to synthesis process. It is envisaged that magnetic HEAs potentially can replace some of
the conventional magnetic alloys.
In the present work the effect of elemental composition, synthesis route on microstructure
and magnetic properties of equiatomic HEAs has been investigated. Initially attempts were made
to understand ternary AlNiCo medium entropy alloy and then role of magnetic and non-magnetic
elemental additions viz., Cu, Fe and CuFe to AlNiCo have been investigated. These compositions
were synthesized by vacuum arc melting (VAM) and mechanical alloying (MA) routes to study
the effect of synthesis process on their magnetic properties. Also, we have explored the possibility
of improving the magnetic properties by substituting Mn in place of in AlNiCoCuFe HEA. From
the analysis of temperature and field dependence of magnetization data it is identified AlNiCo as
weak itinerant ferromagnetic alloy, while MA AlNiCo shows presence of ferromagnetic clusters.
Addition of Cu to VAM AlNiCo enhanced the coercivity (HC) as a result of segregation effect
owing to the positive enthalpy of mixing of Cu and constituents. On the other hand, addition of Fe
improves MS and appears to exhibit soft magnetic character. Present studies show that equiatomic
HEAs have complex magnetic properties which depends on the nature of exchange interaction
among the constituent elements. Our studies indicate that mechanically alloyed Cu containing
HEAs are more homogenous compared to VAM alloys and hence result in lower coercivity values.
First-principles density functional theory calculations from the literature revealed that the anti-
ferromagnetic coupling of Mn atoms is suppressed especially in the CoFeMnNiAl HEA because
Al changes the Fermi level and itinerant electron-spin coupling that lead to ferromagnetism. Our
experimental results on AlNiCoCuFeMn corroborate well with these theoretical predictions.