Physical properties of hexagonal high-entropy alloys

Lužnik, Janez (avtor), Dolinšek, Janez (mentor)

In 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.

• Jezik: Angleški jezik
• Ključne besede: hexagonal high-entropy alloys, rare-earth elements, spin-glass, geometrically frustrated magnetism, disordered ferromagnet
• Vrsta gradiva: Doktorsko delo/naloga
• Tipologija: 2.08 - Doktorska disertacija
• Organizacija: FMF - Fakulteta za matematiko in fiziko
• Leto izida: 2018
• PID: 20.500.12556/RUL-105426
• COBISS.SI-ID: 3266148
• Datum objave v RUL: 29.11.2018

Sekundarni jezik

• Jezik: Slovenski jezik
• Naslov: Fizikalne lastnosti heksagonalnih visokoentropijskih spojin

Izvleček

V doktorskem delu je predstavljena študija kompleksnega magnetizma treh heksagonalnih visokoentropijskih zlitin (ang. High-entropy alloys - HEA) iz redkih zemelj: Ho-Dy-Y-Gd-Tb (označenega kot HEA-Y), Ho-Dy-Lu-Gd-Tb (HEA-Lu) and Ho-Dy-Ce-Gd-Tb (HEA-Ce). HEA-Y in HEA-Lu zlitini sta primera idealne visokoentropijske zlitine, ki je stabilizirana z entropijo mešanja pri katerikoli temperaturi z naključnim mešanjem elementov na heksagonalnem najtesnejšem skladu. Vključitev cerija znatno zmanjša ˝idealnost˝ zlitine, saj povzroči nastanek dvofazne strukture, v kateri se znotraj heksagonalnega najgostejšega sklada pojavljajo romboedrični precipitati. HEA-Y in HEA-Lu kažeta bogat in zapleten (H,T) fazni diagram. Ta je posledica tekmovanja med periodičnim potencialom (izhaja iz strukture elektronskih pasov, ki povzročajo periodično magnetno urejanje), naključnim lokalnim potencialom (povzroča naključno zamrznitev spinov v strukturo tipa spinskega stekla), Zeemanovo interakcijo z zunanjim magnetim poljem (spine ureja v smeri polja) ter termičnimi fluktuacijami, ki nasprotujejo spinskemu urejanju. V (H,T) faznih diagramih HEA-Y in HEA-Lu med sobno temperaturo in 2 K ločimo tri značilna območja. V zgornji temperaturnem območju I (približno med 300 in 75 K za HEA-Y ter 300 in 60 K za HEA-Lu) termične fluktuacije izpovprečijo efekt pripetja spinov zaradi prisotnosti lokalnega naključnega potenciala in spinski sistem se obnaša kot čist sistem kompozicijsko-povprečenih spinov. Pri Néelovi temperaturi (T_N^(HEA-Y)=180 K in T_N^(HEA-Lu)=174 K) spinski sistem doživi termodinamski fazni prehod v urejeno helikoidalno antiferomagnetno stanje dolgega dosega. V območju II (med 75 in 20 K za HEA-Y ter med 60 in 20 K za HEA-Lu) se periodični spinski red dolgega dosega podre. Prevladovati začne naključno urejanje spinov v lokalnem naključnem potencialu. V nizkotemperaturnem ombočju III (pod 20 K tako za HEA-Y kot za HEA-Lu) spini postopno zamrznejo v posebno fazo spinskega stekla, ki je značilna za heksagonalne HEA spojine redkih zemelj. Ta faza ima podobne lastnosti kot spinska stekla ter geometrijsko frustrirani urejeni spinski sistemi in je posledica štirih različnih spinov v heksagonalnem skladu (Gd, Tb, Dy in Ho), ki med seboj močno interagirajo in so šibko razredčeni z nemagnetnimi atomi (Y ali Lu.) Fazna diagrama HEA-Y in HEA-Lu pri T=2 K kažeta tudi metamagnetni prehod prvega reda, ki ga inducira magnetno polje. Cerij v zlitini s štirimi redkimi zemljami (Gd, Tb, Dy in Ho) spremeni magnetno urejanje in s tem povezan (H,T) fazni diagram. Magnetne strukture dolgega reda se ne tvorijo več. Namesto tega magnetna struktura razpade v feromagnetno polarizirane spinske domene različnih velikosti in orientacij. Magnetno stanje HEA-Ce lahko zato opišemo kot neurejen feromagnet.

• Ključne besede: heksagonalne visokoentropijske zlitine, redke zemlje, spinsko steklo, geometrijsko frustriran magnetizem, neurejen feromagnet