Disordered ferromagnetic state in the Ce-Gd-Tb-Dy-Ho hexagonal high-entropy alloy

• Published in: Journal of alloys and compounds Vol. 742; pp. 877 - 886
• Main Authors: Vrtnik, S., Lužnik, J., Koželj, P., Jelen, A., Luzar, J., Jagličić, Z., Meden, A., Feuerbacher, M., Dolinšek, J.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 25-04-2018; Elsevier BV
• Subjects: Alloying elements; Alloys; Cerium; Crystal structure; Disordered systems; Dysprosium base alloys; Electrical transport; Enthalpy; Entropy; Ferromagnetism; Gadolinium; Heat capacity; High entropy alloys; Magnetic fields; Magnetic measurements; Magnetic properties; Magnetic structure; Magnetism; Phase transitions; Precipitates; Rare earth alloys and compounds; Rare earth elements; Solid phases; Solid solutions; Terbium; Trace elements; Unconventional superconductivity; High-entropy alloys; Heat capacity; Disordered systems; Magnetic measurements; Rare earth alloys and compounds; Electrical transport
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2018.01.331

Rare-earth (RE) based hexagonal high-entropy alloys (HEAs) containing elements from the heavy half of the RE series (from Gd to Lu) are considered as prototypes of an ideal HEA, stabilized by the entropy of mixing with completely random distribution of the elements on an almost undistorted hexagonal close-packed (hcp) lattice. Here we present a study of the Ce-Gd-Tb-Dy-Ho hexagonal HEA (abbreviated as HEA-Ce), where a light-RE element Ce is alloyed with four heavy-RE elements. Since the binary mixing enthalpies of Ce with these elements are all zero, random mixing of the elements and an ideal solid solution can also be expected. Contrary to the expectations, a two-phase structure forms in the HEA-Ce, consisting of the majority hcp matrix and the rhombohedral precipitates that occupy a significant fraction of the sample's volume, with both phases having very similar composition. The “ideality” of the HEA-Ce solid solution is very likely compromised by the fact that the crystal structure of Ce is different from the structures of other elements. By performing measurements of the magnetic properties, the specific heat and the electrical resistivity in a magnetic field, we have determined the magnetic state of the HEA-Ce. Long-range-ordered periodic magnetic structures do not form (like they do in the hexagonal HEAs containing heavy-RE elements only), but the magnetic structure breaks up into ferromagnetically (FM) polarized spin domains distributed in size that orient randomly in zero field. The magnetically ordered state of the HEA-Ce can be described as a disordered FM state with a 2nd-order thermodynamic FM phase transition at TC= 140 K. The introduction of Ce did not yield any of the phenomena that are exceptional for the Ce-containing alloys and compounds (mixed valence, heavy-fermion, unconventional superconductivity).
•Ce-Gd-Tb-Dy-Ho is the first Ce-containing hexagonal high-entropy alloy (HEA).•Cerium compromises “ideality” of the rare-earth based solid solution.•Two-phase structure consisting of a hcp matrix and rhombohedral precipitates forms.•A disordered ferromagnetic state forms in the Ce-Gd-Tb-Dy-Ho HEA.