Nanocomposites and nanomaterials
Metastable states and physical properties of Co-Cr-Fe-Mn-Ni high-entropy alloy thin films
Kushnerov O.I., Ryabtsev S.I., Bashev V.F.  
Department of Experimental Physics, Oles Honchar Dnipro National University. Gagarin ave., 72, Dnipro, 49010, Ukraine. 
E-mail: kushnrv@gmail.com

About 17 years ago, the first papers on the study of high-entropy alloys (HEAs) were published. The HEAs are composed of at least 5 major components with equiatomic or nearly equiatomic concentrations which are between 5 and 35 at.%. Choosing a number of components and their concentration allows one to achieve an increased entropy of mixing, which remains not only in the melt but after solidification. Because of the high entropy, usually simple substitutional solid solutions with BCC or FCC crystal lattices are formed during the solidification of such alloys. The HEAs are characterized by many useful characteristics, such as hardness, wear resistance, resistance to oxidation, corrosion, and ionizing radiation. At the same time, the majority of HEAs were investigated in the as-quenched or homogenized state, whereas much less attention was paid to the study of thin films of high-entropy alloys.
In this study, Co19Cr18Fe22Mn21Ni20 HEA thin films have been synthesized by ion plasma sputtering of mosaic targets consists of pure metals. The deposition process was carried out at room temperature with Ar atmosphere. The as-deposited HEA film thickness was estimated to be ~ 110 nm. Single diffuse halo is observed on the XRD patterns of the as-deposited films, that confirms their amorphous structure. Some of the thin films were annealed at 600ºC in a vacuum furnace. After the heat treatment, the Co19Cr18Fe22Mn21Ni20 film transforms from an amorphous form into a crystallized FCC solid solution structure with the lattice parameter a=0.3613 nm. The cubic B2 phase of FeCo with a lattice parameter of 0.2857 nm is also formed in the annealed films. As a result of oxidation by the small amount of oxygen in the vacuum chamber, a dispersed phase of cubic FCC manganese oxide MnO is also formed after annealing.
The magnetic properties of the films were measured by a vibrating sample magnetometer. The as-deposited HEA film exhibits the soft magnetic properties and the coercivity of 400 A/m. After annealing the value of coercivity increase to 8760 A/m. So the annealed films can be considered as hard magnetic materials. The temperature dependencies of electrical resistivity, measured by the four-point technique showed that the films in the initial amorphous state are characterized by a very low value of the temperature coefficient of resistance (average value ~3.5·10-5 1/K, while after crystallization it increases up to 5.1·10-4 1/K.