The magnetic properties of high-entropy alloys based on equimolar FeCoCrNi were investigated using vibrating sample magnetometry to determine their usefulness in high-temperature magnetic applications. Nuclear resonant inelastic x-ray scattering measurements were performed to evaluate the vibrational entropy of the 57 Fe atoms and to infer chemical order. The configurational and vibrational entropy of alloying are discussed as they apply to these high-entropy alloys.

The next step forward, transforming polymer nanocomposites from filled-polymer replacements to designed and engineered materials, necessitates the development of techniques to transform the random or ill-defined nanoscale morphologies into compositionally and spatially engineered hierarchal structures, paralleling that which underpins conventional continuous-fiber-reinforced composites. By exploiting an orthogonal magnetic susceptibility of montmorillonites (MMTs) from different deposits, a three-dimensional morphology composed of orthogonal alignment of aluminosilicate layers is generated from a mixture of montmorillonites and a uniaxial external magnetic field. Depending on the source, MMTs exhibit remnant magnetization arising from antiferro-and ferrimagnetic impurities and align with layers parallel or perpendicular to the field. Within a few minutes, application of static magnetic fields (1.2 or 11.7 T) induces stable alignment of organically modified MMT within an epoxy resin at room temperature. Structural relaxation is orders of magnitude slower, enabling the alignment to be captured during the subsequent cure. Thermal mechanical measurements demonstrate that morphology manipulation impacts the coefficient of thermal expansion (CTE), decreasing CTE the most in the direction of maximum MMT alignment. Knowledge of the detailed mechanism that leads to a change of the magnetic easy axis within layered silicates opens up opportunities to design novel synthetic layered silicates with unusual magnetic properties.

Nanocrystalline Ni 0.5 Zn 0.5 Fe 2 O 4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673-1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni-Zn ferrite films was measured in the frequency range of 2-15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315-355 K was observed in the film close to the critical grain size.

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.

C-coated FexCo1−x (x=0.50, 0.45, 0.40, 0.35, 0.30, 0.25) nanoparticles were produced using a rf plasma torch. The only C source was acetylene used as a carrier gas. Structural determination by x-ray diffraction indicated a single disordered bcc α-FeCo phase along with graphitic C for all compositions. A Scherrer analysis of the peak widths revealed particles to have an average diameter of 50 nm. A broad log-normal size distribution was found from transmission electron microscopy observations. Magnetic hysteresis loops have been measured to temperatures exceeding 1050 K and revealed relatively high room temperature coercivities (200–400 Oe), with a strong compositional variation similar to that observed in bulk alloys. Larger coercivities are consistent with particles near the monodomain size for these alloys. The temperature dependence of the magnetization revealed the effects of atomic ordering. The variation of the saturation magnetization as a function of temperature showed a discontinuity near the bulk order–disorder (α→α′) transformation temperature, as well as loss of magnetization at the α→γ structural phase transition temperature. Other features of M(T) near 500–550 °C are consistent with prior observations of a “550 °C structural anomaly” which has been observed in bulk alloys with less than perfect order.

A radio frequency (rf) plasma torch has been used to produce FeCo nanoparticles with a thin protective oxide coating from metal powder precursors. Structural characterization by conventional and synchrotron x-ray diffraction indicated a disordered bcc α-FeCo phase. High resolution transmission electron microscopy revealed spherical particles with several monolayer thick protective oxide coatings. Thermomagnetic measurements were carried out using a superconducting quantum interference device magnetometer and a vibrating sample magnetometer at temperatures between 5 and 1050 K. Antiferromagnetic (exchange bias) coupling was observed due to the presence of the oxide layer. Relatively high coercivities were observed (280 Oe at 5 K and 250 Oe at room temperature). Néel’s surface (interface) anisotropy model was employed to explain the origin of the observed coercivities. As produced powders were hot isostatically pressed at 1023 K and 22 ksi for 2 h. Dense structures were observed and compacted particles revealed coercivities as low as 25 Oe at room temperature.

In previously reported work FexCo1−x[C] (x=0.0, 0.2, 0.4, 0.5, 0.6, and 0.8 nominally) nanoparticles were prepared by a Kratschmer–Huffman carbon-arc method. Fe0.5Co0.5[C] exhibited the largest magnetizations heretofore observed in similarly produced nanoparticles. Here we present a more detailed study of the magnetic properties of Fe0.5Co0.5[C] nanocrystals. Magnetic hysteresis loops have been measured to temperatures exceeding 1050 K. This is attributed to rotational processes in monodomain particles and is shown to be sensitive to ordering of the particles. Low-field thermomagnetic data clearly show features which we attribute to the α→α′ disorder–order and α→γ phase transformations, respectively.

We report the results of an experimental study of a persistent coil made out of YBa2Cu3O7−δ coated conductors. The magnitude of the persistent current and the rate of decay were investigated. Two distinct modes of relaxation are evident—one is flux creep and the other, which is much faster, is of less obvious origin. Our conclusion is that the persistent current in such a coil can be large enough and decay slowly enough so that coated conductors can be used to make persistent coils for variety of applications.