Tunable magnetocaloric effect in transition metal alloys.

The magnetocaloric effect (MCE) is a thermodynamic phenomenon being exploited for the next generation of highly efficient refrigeration technologies. The MCE enables a refrigerant's temperature to change with applied magnetic fields1,2,3,4,5,6, which allows for a simple refrigeration cycle. As in adiabatic demagnetization, which is used to reach microKelvin temperatures in solids, changes in a material's magnetic entropy through an adiabatic process results in a change in the lattice entropy, and thus a temperature change. Since the discovery of the giant MCE in certain Gd silicates7,8, MCE research has led to the potential for magnetic refrigeration in room temperature applications with efficiencies of up to 60% of the Carnot limit9. While the magnetocaloric effect is generally large for rare earth metals, geopolitical issues and their related price fluctuations have catalyzed the exploration of transition metal systems with potential for magnetocaloric applications10,11. Further, though magnetocaloric materials exhibiting first order transitions are scientifically en vogue because they have higher entropy changes than most second order materials, they suffer major disadvantages for commercial viability. In particular, materials with coupled magneto-structural transitions suffer from cracking and fatigue, which severely limits their useful lifetime. Together, these points suggest it is important to explore transition metal systems displaying second order transitions for technological applications, since these systems offer supply chain and cost stability, and superior mechanical properties such as ductility, corrosion resistance, machinability, all of which ease manufacturing and bolster product longevity. The so-called “high entropy alloys” (HEAs) are a class of emergent transition metal alloys that hold great potential for advanced manufacturing, and which may impact magnetocalorics. These are near equimolar alloys with high entropy of mixing when in a random solid solution. These materials have been pursued for high hardness and resistance to wear and corrosion, which are attractive properties for a myriad of applications12,13. Fe1Co1Ni1Cr1Pdx is an example HEA system with tunable magnetic properties. The addition of Pd was shown to change the critical temperature (Tc) from 300 to 500 K for x = 1 to 2, along with enhancement of the saturation magnetization14; previous works have used high percentages of Pd as a stabilizing agent in similar high strength bulk alloys15. In this work, we explore the magnetic properties of the Fe1Co1Ni1Cr1Pdx alloy family, as-rolled and after annealing, with low molar fractions of Pd. We find improvement in the magnetocaloric properties throughout the system. A detailed statistical analysis of the universal scaling behavior suggest this originates predominately from the homogenization of the alloy with both Pd and heat treatment. While the use of Pd is disadvantageous because of its cost, we find benefit from as little as 3 atomic percent of this model additive. An important implication of this work is that other FCC metals, e.g., Cu or Ag, may have similar potential to tune the magnetism of this system, which would significantly reduce the materials cost, thereby bolstering commerical potential.