Complex magnetism of Ho-Dy-Y-Gd-Tb hexagonal high-entropy alloy

Rare earth based equimolar Ho-Dy-Y-Gd-Tb hexagonal high-entropy alloy (HEA) is a prototype of an ideal HEA, stabilized by the entropy of mixing at any temperature with random mixing of elements on the hexagonal close-packed lattice. In order to determine intrinsic properties of an ideal HEA characterized by the enormous chemical (substitutional) disorder on a weakly distorted simple lattice, we have performed measurements of its magnetic and electrical response and the specific heat. The results show that the Ho-Dy-Y-Gd-Tb hexagonal HEA exhibits a rich and complex magnetic field-temperature (H, T) phase diagram, as a result of competition among 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 diagram between room temperature and 2 K. Within the upper temperature region I (roughly between 300 and 75 K), 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 Neel temperature T-N = 180K that is a compositional average of the Neel temperatures of pure Tb, Dy, and Ho metals. Region II (between 75 and 20 K) 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), 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 yttrium atoms.