Dustin D. Belyea

Thermomagnetic analysis of FeCoCrxNi alloys: Magnetic entropy of high-entropy alloys

The equimolar alloy FeCoCrNi, a high-entropy alloy, forms in the face-centered-cubic crystal structure and has a ferromagnetic Curie temperature of 130 K. In this study, we explore the effects of Cr concentration, cold-rolling, and subsequent heat treatments on the magnetic properties of FeCoCrxNi alloys. Cr reductions result in an increase of the Curie temperature, and may be used to tune the TC over a very large temperature range. The magnetic entropy change for a change in applied field of 2T is ΔSm = −0.35 J/(kg K) for cold-rolled FeCoCrNi. Cold-rolling results in a broadening of ΔSm, where subsequent heat treatment at 1073 K sharpens the magnetic entropy curve. In all of the alloys, we find that upon heating (after cold-rolling) there is a re-entrant magnetic moment near 730 K. This feature is much less pronounced in the as-cast samples (without cold-rolling) and in the Cr-rich samples, and is no longer observed after annealing at 1073 K. Possible origins of this behavior are discussed.

“Treasure maps” for magnetic high-entropy-alloys from theory and experiment

The critical temperature and saturation magnetization for four- and five-component FCC transition metal alloys are predicted using a formalism that combines density functional theory and a magnetic mean-field model. Our theoretical results are in excellent agreement with experimental data presented in both this work and in the literature. The generality and power of this approach allow us to computationally design alloys with well-defined magnetic properties. Among other alloys, the method is applied to CoCrFeNiPd alloys, which have attracted attention recently for potential magnetic applications. The computational framework is able to predict the experimentally measured TC and to explore the dominant mechanisms for alloying trends with Pd. A wide range of ferromagnetic properties and Curie temperatures near room temperature in hitherto unexplored alloys is predicted in which Pd is replaced in varying degrees by, e.g., Ag, Au, and Cu.

Tunable magnetocaloric effect in transition metal alloys.

The unpredictability of geopolitical tensions and resulting supply chain and pricing instabilities make it imperative to explore rare earth free magnetic materials. As such, we have investigated fully transition metal based “high entropy alloys” in the context of the magnetocaloric effect. We find the NiFeCoCrPdx family exhibits a second order magnetic phase transition whose critical temperature is tunable from 100 K to well above room temperature. The system notably displays changes in the functionality of the magnetic entropy change depending on x, which leads to nearly 40% enhancement of the refrigerant capacity. A detailed statistical analysis of the universal scaling behavior provides direct evidence that heat treatment and Pd additions reduce the distribution of exchange energies in the system, leading to a more magnetically homogeneous alloy. The general implications of this work are that the parent NiFeCoCr compound can be tuned dramatically with FCC metal additives. Together with their relatively lower cost, their superior mechanical properties that aid manufacturability and their relative chemical inertness that aids product longevity, NiFeCoCr-based materials could ultimately lead to commercially viable magnetic refrigerants.

The impact of barrier height distributions in tunnel junctions

We demonstrate that including continuous and discrete tunnel barrier height distributions in otherwise traditional tunneling formalisms enables straightforward modeling of several phenomena important to tunneling. Random barrier height inhomogeneities significantly impact the tunneling conductance, as evidenced by ideal tunneling models extracting faulty barrier parameters, with the incurred errors strongly dependent on the variance. Thermal smearing is addressed by transferring the energy distribution from the electrons to the barrier potential energy, thereby enabling zero-temperature tunneling models to model temperature dependent tunneling. For discrete tunneling channels, a secondary, impuritylike channel is shown to dominate the net conductance at surprisingly low impurity levels, implying that the observation of intrinsically large barrier heights is highly unlikely with transport measurements. Finally, spin-filter tunneling is modeled with independent tunneling channels whose barrier heights are l...

Magnetocaloric effect in nanoscale thin films and heterostructures

This review focuses on the magnetocaloric effect with special attention to nanoscale thin films and heterostructures. The authors outline the general phenomenon of the magnetocaloric effect and discuss how using materials in reduced dimensions can impact this emerging area. The authors note works of significance to date and highlight general features emanating from the community. They provide important details related to sample fabrication, relevant metrology, and discuss advanced data analyses, all of which are done in a tutorial fashion. Finally, the authors provide an outlook for the application of nanoscience to magnetocalorics.

Effects of preparation conditions on the magnetocaloric properties of Gd thin films

The impact of the deposition temperature and chamber gettering on Ta(5 nm)/Gd(30 nm)/Ta(5 nm) thin films magnetocaloric effect properties was investigated. Increasing the deposition temperature generally improves the entropy peak (magnitude, full width at half max, and temperature of the peak) but also leads to significant oxidation. Gettering the chamber prior to deposition not only reduced this oxidation issue but also increased the relative cooling power of films grown at elevated temperatures by as much as 33% over ungettered samples.

Impact of interfacial roughness on spin filter tunneling

The impact of interface roughness on spin filter tunneling is considered at low biases as functions of temperature and barrier parameters. Roughness reduces the maximum achievable spin polarization, which results from tunneling “hot spots” (thin regions of the barrier) having intrinsically reduced spin filtering efficiency. Surveying a range of experimentally reasonable roughness and mean barrier thickness values allows us to conclude that roughness values greater than 10% of the mean barrier thickness have an adverse impact on the spin polarization. Atomic-scale roughness may thus be critical for achieving 100% spin polarization in spin filter tunnel junctions at low biases.

Magnetocaloric effect in epitaxial La0.56Sr0.44MnO3 alloy and digital heterostructures

This work investigates the magnetocaloric effect of two epitaxial manganite heterostructures, one being a single layer La0.56Sr0.44MnO3 alloy with randomly distributed La and Sr cations, the other a digitally synthesized superlattice of LaMnO3 and SrMnO3 fabricated to be compositionally identical to the alloy. The magnetic entropy change and relative cooling power were larger for the alloy than the superlattice, though both are suppressed relative to bulk materials. These results indicate that disorder of the A-site cation species in the perovskite structure may play a crucial role in defining the magnetocaloric effect in complex oxide materials.