Speromagnetism and asperomagnetism as the ground states of the Tb-Dy-Ho-Er-Tm “ideal” high-entropy alloy

Abstract

We address the nature of the collective magnetic state in an ideal high-entropy alloy (HEA), representing a magnetically concentrated system with all lattice sites occupied by localized magnetic moments and containing randomness and frustration due to chemical disorder. Being a “metallic glass on a topologically ordered lattice”, HEAs possess simultaneously the properties of an ordered crystal and an amorphous glass. The influence of this crystal-glass duality on the collective magnetic state was studied experimentally on a hexagonal Tb-Dy-Ho-Er-Tm (TDHET) HEA, composed of rare-earth (RE) elements with zero pair mixing enthalpies that assure completely random mixing of the elements and very similar atomic radii that minimize lattice distortions, representing a prototype of an ideal HEA. The TDHET HEA is characterized by probability distributions of the atomic moments P(μ), the exchange interactions P(J), the magnetocrystalline anisotropy P(D), and the dipolar interactions P(Hd). Based on the measurements of the static and dynamic magnetization, the magnetization M(H) curves, the thermoremanent magnetization, the specific heat and the magnetoresistance, we found that the collective magnetic state of the TDHET is temperature-dependent, forming a speromagnetic (SPM) state in the temperature range between about 140 and 30 K and an asperomagnetic (ASPM) state below 20 K. In the intermediate temperature range between 30 and 20 K, a spin glass (SG) state is formed, representing a transition state between the speromagnetic and asperomagnetic states. The observed temperature evolution of the magnetic ground state in the TDHET HEA upon cooling in the sequence SPM→SG→ASPM is a result of temperature-dependent, competing magnetic interactions. The distribution of the exchange interactions P(J) shifts continuously on the J axis from the high-temperature SPM-type with the average interaction biased towards a net negative value, J‾<0, through the SG-type with J‾=0, to the low-temperature ASPM-type with J‾>0. This shift is a band-structure effect, closely linked with the crystallinity of the spin system, which the TDHET HEA shares with the topologically ordered crystals. The probability distributions P(μ), P(J), P(D) and P(Hd) are, on the other hand, a consequence of chemical disorder, a property that the TDHET HEA shares with the amorphous magnets. Both features, the topologically ordered lattice and the amorphous-type chemical disorder essentially determine the magnetic state of an ideal, RE-based HEA.