Showing papers by "Laura H. Lewis published in 2018"

Effects of Al content and annealing on the phases formation, lattice parameters, and magnetization of A l x F e 2 B 2 ( x = 1.0 , 1.1 , 1.2 ) alloys

Abstract: $\mathrm{AlF}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$ is a ferromagnet with the Curie temperature around 300 K and has the potential to be an outstanding rare-earth free candidate for magnetocaloric applications. However, samples prepared from the melt contain additional phases which affect the functional response of the $\mathrm{AlF}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$ phase. We report on the effects of Al content in samples with the initial (nominal) composition of $\mathrm{A}{\mathrm{l}}_{x}\mathrm{F}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$, where $x=1.0$, 1.1, and 1.2 prepared by arc-melting followed by suction casting and annealing. The as-cast $\mathrm{A}{\mathrm{l}}_{x}\mathrm{F}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$ alloys contain $\mathrm{AlF}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$ as well as additional phases, including the primary solidifying FeB and $\mathrm{A}{\mathrm{l}}_{13}\mathrm{F}{\mathrm{e}}_{4}$ compounds, which are ferromagnetic and paramagnetic, respectively, at 300 K. The presence of these phases makes it difficult to extract the intrinsic magnetic properties of $\mathrm{AlF}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$ phase. Annealing of $\mathrm{A}{\mathrm{l}}_{x}\mathrm{F}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$ alloys at 1040 \ifmmode^\circ\else\textdegree\fi{}C for 3 days allows for reaction of the FeB with $\mathrm{A}{\mathrm{l}}_{13}\mathrm{F}{\mathrm{e}}_{4}$ to form the $\mathrm{AlF}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$ phase, significantly reduces the amount of additional phases, and results in nearly pure $\mathrm{AlF}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$ phase as confirmed with XRD, magnetization, scanning electron microscopy, and electronic transport. The values of the magnetization, effective magnetic moment per Fe atom, specific heat capacity, electrical resistivity, and Seebeck coefficient for the $\mathrm{AlF}{\mathrm{e}}_{2}{\mathrm{B}}_{2}$ compound have been established.

Strain-tuning of the magnetocaloric transition temperature in model FeRh films

Abstract: The chemically ordered B2 phase of equiatomic FeRh is known to absorb or evolve a significant latent heat as it traverses its first-order phase transition in response to thermal, magnetic, and mechanical drivers. This attribute makes FeRh an ideal magnetocaloric material testbed for investigation of relationships between the crystalline lattice and the magnetic spins, which are especially experimentally accessible in thin films. In this work, epitaxial FeRh films of nominal 30 nm and 50 nm thicknesses with out-of-plane c-axis orientation were sputter-deposited at high temperature onto (0 0 1)-MgO or (0 0 0 1)-Al2O3 substrates and capped with Al, Au, Cr, or W after in situ annealing at 973 K to promote CsCl-type chemical order. In this manner a controlled strain state was invoked. Experimental results derived from laboratory and synchrotron x-ray diffraction combined with magnetometry indicate that the antiferromagnetic (AF)—ferromagnetic (FM) magnetostructural phase transformation in these films may be tuned over an ~50° range (373 K–425 K) through variation in the c/a ratio derived from lattice strain delivered by the substrate and the capping layers. These results supply fundamental information that might be used to engineer the magnetocaloric working material in new system designs by introducing targeted values of passive strain to the system.

Fe-incorporated Ti O 2 Nanotube Arrays: Electronic Structure and Magnetic Response

Abstract: Incorporating Fe atoms into the lattice is shown to significantly alter electronic and magnetic properties of $\mathrm{Ti}{\mathrm{O}}_{2}$ nanotubes synthesized by electrochemical anodization of Ti-Fe alloy sheets. The effects of Fe incorporation on the nanotube morphology, crystallinity, crystal structure, magnetic behavior and electronic structure were investigated with crystallographic and magnetic probes, including synchrotron-based spectroscopy. Results indicate that the iron cations predominately adopt the $\mathrm{F}{\mathrm{e}}^{3+}$ configuration, leading to a large increase of the electronic density of states at the Fermi energy. This increase is anticipated to provide enhanced catalytic action, for instance, in the degradation of water and of air pollutants. These results provide insight for tailoring the functionality of these nanostructures for energy-related applications.

Method of Tetratenite Production and System Therefor

Abstract: The invention provides method for making high coercivity magnetic materials based on FeNi alloys having a Llo phase structure, tetratenite, and provides a system for accelerating production of these materials. The FeNi alloy is made by preparing a melt comprising Fe, Ni, and optionally one or more elements selected from the group consisting of Ti, V, Al, B, C, Mo, Ir, and Nb; cooling the melt and applying extensional stress and a magnetic field. This is followed by heating and cooling to form the L10 structure.