Strange Metallicity and Magnetic Order in the CoNi(Cr/V) Medium-Entropy Alloy System

Abstract

CoNiCr is a prototypical example of topical multi-principle element alloys with superior cryogenic and high-temperature mechanical strength, corrosion, oxidation resistance, and yet-to-be-explored magnetic and electronic functionalities. The remarkable properties of this transition metal ternary system are not only due to atomic radii, electronic configurational mismatch, and atomic volume misfit but are also dependent on the debated magnetically driven chemical short-range order. The current study focuses on the electric and magnetic properties of the single-phase face-centered cubic CoNi(Cr/V) system in which V is introduced to the system at the expense of Cr to fine-tune the volume misfit in the system. All the samples exhibited ultra-small magnetic moments due to the complex magnetic interactions of the constituent elements. The electric transport measurements revealed a strange metallicity evidenced through the observation of the linear temperature dependence of the resistivity. Our findings support the recent theoretical studies on the magnetically driven chemical short-range order of the CoNiCr system.

Electrochemical Hydrogen Evolution Reaction Evaluation of CoNi(Cr/V) Medium-Entropy Alloys in an Acidic Environment

“…We believe that the superior electrocatalytic activity of CoNi­(Cr/V) MEA originates not only from the metallic nature, i.e., having good electric conductivity, but also from the unique short-range atomic ordering and high lattice distortion in its atomic structure. Note that such short-range order is mostly proposed theoretical works and a debated topic within the scientific community; however, the number of experimental works is gradually increasing . This aspect is subjected to further discussion in the DFT section, where the results suggest that the undercoordinated surface atoms synergistically promote hydrogen adsorption and efficient charge transfer for HER.…”

“…Note that such short-range order is mostly proposed theoretical works and a debated topic within the scientific community; however, the number of experimental works is gradually increasing. 34 This aspect is subjected to further discussion in the DFT section, where the results suggest that the undercoordinated surface atoms synergistically promote hydrogen adsorption and efficient charge transfer for HER. In fact, the large difference between the overpotentials of CoNiCr and CoNiV compared to CoNi(Cr/V) provides straightforward evidence that Cr and V play a role in promoting the catalytic activity of Co−Ni-based alloys, which will be further explained through post-XPS analysis in the coming section.…”

“…The CoNiCr system has been one of the most studied systems in terms of its mechanical properties, as it exhibits extremely large yield strength values both at room temperature and cryogenic temperatures. 34 Recently, it has been suggested that the extreme strength of CoNiCr is linked to magnetically driven chemical short-range order. In this regard, the antiferromagnetic magnetic coupling between (Co/ Ni) and Cr has crucial significance.…”

“…We used X-ray photoelectron spectroscopy, electron microscopy analysis, and density functional theory calculations to interpret and understand the experimental electrochemistry findings. 34 The MEA ingots were then homogenized in vacuum for 24 h at 1200 °C, further cold rolled into sheets with a thickness of 1.3 mm, followed by solutionizing at 1100 °C for 30 min, and then quenched in water to maintain a single-phase face-centered cubic (FCC) structure. For electrochemical analysis, electrodes of 1 cm 2 area were machined from the solutionized sheets using an electric discharge machine (EDM), and their weights were recorded to an average of 160 mg each.…”

“…Hence, in this study, we concentrate on the investigation of equiatomic CoNi-based MEAs (CoNiCr, CoNiV, and CoNi (Cr/V)) as potential electrocatalysts for HER in acidic media. The CoNiCr system has been one of the most studied systems in terms of its mechanical properties, as it exhibits extremely large yield strength values both at room temperature and cryogenic temperatures . Recently, it has been suggested that the extreme strength of CoNiCr is linked to magnetically driven chemical short-range order.…”