Physical properties of hexagonal high-entropy alloysLužnik, Janez (Avtor) Dolinšek, Janez (Mentor) hexagonal high-entropy alloysrare-earth elementsspin-glassgeometrically frustrated magnetismdisordered ferromagnetIn this Thesis, a study of complex magnetism of three rare-earth (RE) based hexagonal high-entropy alloys (HEAs) is presented: Ho-Dy-Y-Gd-Tb (denoted as HEA-Y), Ho-Dy-Lu-Gd-Tb (HEA-Lu) and Ho-Dy-Ce-Gd-Tb (HEA-Ce). HEA-Y and HEA-Lu alloys are prototypes of an ideal HEA, stabilized by the entropy of mixing at any temperature with random mixing of elements on a hexagonal close-packed lattice (HCP). The introduction of Ce degrades "ideality" of the HEA significantly by producing a two-phase structure with precipitates of a rhombohedral phase within the HCP matrix. The results show that HEA-Y and HEA-Lu show rich and complex magnetic field-temperature (H,T) phase diagram, as a result of competition between the periodic potential arising from the electronic band structure that favors periodic magnetic ordering, the disorder-induced local random potential that favors spin glass-type spin freezing in random directions, the Zeeman interaction with the external field that favors spin alignment along the field direction, and the thermal agitation that opposes any spin ordering. Three characteristic temperature regions were identified in the (H,T) phase diagrams of HEA-Y and HEA-Lu between room temperature and 2 K. Within the upper temperature region I (roughly between 300 and 75 K for HEA-Y and 300 and 60 K for HEA-Lu), thermal fluctuations average out the effect of local random pinning potential and the spin system behaves as a pure system of compositionally averaged spins, undergoing a thermodynamic phase transition to a long-range ordered helical antiferromagnetic state at the Néel temperature (T_N^(HEA-Y)=180 K and T_N^(HEA-Lu)=174 K). Region II (between 75 and 20 K for HEA-Y and 60 and 20 K for HEA-Lu) is an intermediate region where the long-range periodic spin order “melts” and the random ordering of spins in the local random potential starts to prevail. Within the low-temperature region III (below 20 K for both alloys), the spins gradually freeze in a spin glass configuration. The spin glass phase appears to be specific to the rare earths containing hexagonal HEAs, sharing properties of site-disordered spin glasses and geometrically frustrated (site-ordered) spin systems, as a consequence of strongly interacting large abundant spins of four magnitudes (those of Gd, Tb, Dy, and Ho) on the hexagonal lattice, being weakly diluted by nonmagnetic atoms (Y or Lu). The magnetic field-temperature (H,T) phase diagrams of HEA-Y and HEA-Lu also show a 1st-order field-induced metamagnetic transition at T=2 K. Alloying the Ce light-RE element with the same four heavy RE elements (Gd, Tb, Dy, and Ho) has changed the magnetic ordering and the associated (H,T) phase diagram of the HEA-Ce profoundly. Long-range-ordered periodic magnetic structures no more form, but the magnetic structure breaks into ferromagnetically (FM) polarized spin domains distributed in size and orientation, so that the magnetic state of the HEA-Ce can be described as a disordered ferromagnet.20182018-11-29 07:45:01Doktorsko delo/naloga105426sl