Invited Paper MI+2D-WeM10
Compositional Tuning of Magnetic Exchange Interactions and Interpretation of the Pressure Dependence of the Magnetic Curie Temperature in High Entropy Alloys.

Wednesday, October 23, 2019, 11:00 am, Room A210

Magnetocaloric effect (MCE) materials are of interest in a more efficient technology than conventional gas compression refrigeration.MCE cooling is environmentally friendly since ozone depleting refrigerants are not used. Critical rare earths metals (REs) and compounds have large MCE response and working temperatures near room temperature. However, their scarcity, high price and corrosion limit their use. Recently, transition metal based high entropy alloys (HEAs) are studied for MCE applications due to convenient tunability of Curie temperatures,use of inexpensive components and tuning of the breadth of the magnetic phase transformation by distributing pair-wise magnetic exchange interations on a single fcc crystalline lattice. I will present our understanding of Curie temperature, T_c, engineering in metals with direct exchange interactions as rooted in the famous Bethe-Slater curve semi-empirically derived from considerations of the chemical bond and the constraints of the Pauli exclusion principle.

The Bethe-Slater curve predicts the dependence of the magnetic exchange interactions on D/d where D is the transition metal interatomic spacing and d is the spatial extend of the magnetic d-orbitals. The Bethe-Slater curveguides alloy design to optimize T_c‘s through distribution of exchange interactions in MCE HEAs_. I will present results for the composition and pressure dependence of the Curie temperature along with Mossbauer spectra, for which the average hyperfine field is proportional to an average pairwise exchange interaction and by inference T_c. Within this formalism, we consider J(D/d), i.e. the exchange interaction(s) as a function of D/d the variable for which the Bethe-Slater curve is parameterized. The P-dependence of T_cwill be interpreted for FeCoNiMnCu 5-component HEAs with a room temperature T_c.