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.

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.

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.

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.

A combination of experiments and numerical modeling was used to study the spatial evolution of the ferromagnetic phase transition in a thin film engineered to have a smooth gradient in exchange strength. Mean-field simulations predict, and experiments confirm that a 100 nm Ni x Cu 1−x alloy film with Ni concentration that varies by 9 % as a function of depth behaves predominantly as if comprised of a continuum of uncoupled ferromagnetic layers with continuously varying Curie temperatures. A mobile boundary separating ordered and disordered regions emerges as temperature is increased. We demonstrate continuous control of the boundary position with temperature, and reversible control of the magnetically ordered sample volume with magnetic field.

Magnetic properties of Y0.9Gd0.1Fe2D4.2 compound under continuous magnetic field up to 310 kOe J. Appl. Phys. 111, 07A934 (2012) Magnetic and magnetocaloric properties of the new rare-earth-transition-metal intermetallic compound Gd3Co29Ge4B10 J. Appl. Phys. 111, 07E333 (2012) The magnetocaloric effect in thermally cycled polycrystalline Ni-Mn-Ga J. Appl. Phys. 111, 07A933 (2012) Isothermal entropy changes in nanocomposite Co:Ni67Cu33 J. Appl. Phys. 111, 07A930 (2012) Giant magnetocaloric effect of Mn0.92Ba0.08As thin film grown on Al2O3(0001) substrate

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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.