Cobalt ferrites exhibit widely varied magnetic behaviour due to the presence of a miscibility gap leading to the formation of periodic self-assembled nanostructures via spinodal decomposition. Periodicity and amplitude of the compositional fluctuations can be controlled by thermodynamic and kinetic processing parameters which allows for careful tuning of the magnetic properties. Although reports have shown evidence of spinodal decomposition, there is a lack of detailed characterization of the magnetic interactions and reversal mechanisms in these materials. In this work we use high-resolution first order reversal curves (FORC) measurements to understand the underlying magnetic processes occurring in a cobalt ferrite with a nominal composition of Co1.8Fe1.2O4 before (calcined) and after spinodal decomposition (annealed). Additionally, FORC measurements with preconditioning fields were conducted to separate the interaction signatures at low coercive fields by biasing the sample in positive and negative mean fields. Microstructural characterization using TEM combined with EDS showed uniform chemistry in the calcined sample and the presence of Fe rich and Co rich regions in the annealed sample, due to spinodal decomposition. Signs of positive exchange interactions were observed in both calcined and annealed samples. This work presents the first detailed magnetic characterization of magnetic interactions in a nanostructured cobalt ferrite, and provides an example of magnetic characterization of nanostructured ferrites using FORC.

Many cryogenic applications critically depend upon accurate temperature measurement for success. In certain applications, including aerospace missions, temperature sensor components are procured years in advance of the mission’s launch date in order to accommodate installation and ground-based testing. Once installed there is little opportunity for recalibration, so the stability of the sensor stored at room temperature over extended time periods becomes vitally important. One of the most common cryogenic temperature sensor types used in aerospace applications is the Cernox Resistance Temperature Sensor manufactured by Lake Shore Cryotronics, Inc., and has been commercially available since 1993. For 29 years a set of 35 sensors manufactured from two of the initial production wafers has been stored at room temperature in ambient atmosphere. These sensors have been periodically recalibrated over the 1.4 K to 325 K or 4 K to 325 K temperature range as appropriate to provide an estimate of the long-term stability of these sensors when stored at room temperature. Data for each temperature sensor were analyzed in terms of equivalent temperature shift between each subsequent calibration and its initial calibration in 1992. The data show that the overall 29-year average stability for devices from these two Cernox wafers roughly follows a stability of ±10 mK for temperatures below 10 K and 0.07% of temperature for temperatures above 10 K. Data for an additional nine Cernox devices from an actual JWST production build lot showed they all exhibited stabilities better than ±8 mK for T < 10 K and (ΔT/T) < ±0.075% for T > 10 K.

Magnetic ceramics are important for numerous technologically relevant applications with a detailed understanding of structure, property, and processing inter-relationships playing a critical role in tailoring magnetic properties. Spinel ferrites are a particularly interesting class of magnetic ceramics of chemical formula AB2O4, with applications including biomedical hyperthermia and high frequency electrical power conversion. In this contribution, we seek to investigate a unique class of Co-ferrites in which spinodal decomposition can produce a ferrite nanocomposite with chemistry and stress state fluctuating within the interior of crystalline grains on the nm-scale, resulting in corresponding fluctuations of intrinsic magnetic properties as well as exchange and magnetostatic interactions. Structural and magnetic characterization of spinel ferrite samples are carried out (1) in the as-milled state prior to thermal processing, (2) after chemical and structural homogenization with a thermal calcination step, and (3) in the spinodal decomposed state following a subsequent annealing treatment within the Co-ferrite miscibility gap. Of note is the formation of a wasp-waisted hysteresis loop which emerges for the spinodal decomposed Co-ferrite sample, indicative of more complex magnetization reversal processes at relatively large applied fields than for homogeneous Co-ferrite samples of similar particle size and identical nominal chemistry. First order reversal curve (FORC) analysis is applied to further characterize the magnetization response, and a conventional interpretation of observed features in the FORC contrast is presented to discuss potential dominant magnetization mechanisms. The work described here represents the first application of FORC to spinodal decomposed magnetic ceramics and provides a strong foundation for future investigations seeking to quantitatively describe the impacts of nm-scale chemical, structural, and magnetic fluctuations on magnetization processes in ferrite spinel nanocomposite systems.

Beryllium has long been predicted by first principle theory as the best p-type dopant for GaN and AlN. But experimental validation of these theories has not, until now, borne out the original predictions. A key challenge is the dopant-induced strain leading to Be rejection from substitutional sites in favor of interstitial sites, leading to self-compensation. More flexible growth methods like metal modulated epitaxy (MME) that can operate at substantially lower temperatures than traditional approaches, can more effectively place Be into the proper substitutional lattice sites. MME grown Be-doped AlN shows substantial p-type conductivity with hole concentrations in the range of 2.3 × 1015 -3.1 × 1018 cm-3 at room temperature. While others have achieved sizable carrier concentrations near surfaces via carbon doping or Si implantation, this is the only known demonstration of substantial bulk p-type doping in AlN and is a nearly 1000 times higher carrier concentration than the best previously demonstrated bulk electron concentrations in AlN. The acceptor activation energy is found to be ≈37 meV, ≈8 times lower than predicted in literature but on par with similar results for MME p-type GaN. Preliminary results suggest that the films are highly compensated. A p-AlN:Be/i-GaN:Be/n-GaN:Ge pin diode is demonstrated with substantial rectification.

Ultrawide bandgap Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> N MESFETs with refractory Tungsten Schottky and Ohmic contacts are studied in 300-675 K environments. Variable-temperature dc electrical transport reveals large ON-state drain current densities for an AlGaN device: 209 mA/mm at 300 K and 156 mA/mm at 675 K in the ON-state (25% reduction). Drain and gate currents are only weakly temperature-dependent, suggesting potential for engineering temperature invariant operation. The ON-/ OFF-ratio is limited by OFF-state leakage through the gate, which is attributed to damage from sputter deposition. Future work using refractory metals with larger work functions that are deposited by electron beam deposition is proposed.

Nanomaterials have attracted attention from a variety of fields. For example, magnetic nanowires (MNWs) can be used to identify and/or distinguish objects using their magnetic signatures: first-ord...

To understand the interplay between lattice structure and spin degrees of freedom, a bulk-Gd lattice as a hexagonal close-packed structure was generated and populated with paramagnetic Gd3+ ions with random projections (MSGd(i), i = 1, 245, or 443). Isotropic interactions of every site with their 12 nearest neighbors were taken into account. On the basis of the Monte Carlo Metropolis algorithm, the variation with temperature and magnetic field strength was obtained for the following physical quantities: magnetization M(T,H), the product of the magnetic susceptibility and temperature χmol∗T, magnetic specific heat C(T,H), entropy variation ΔS(T,ΔH), and statistics of spin projections MSGd(i,T). We also show the resulting interatomic exchange JGd−Gd, the reorientation of the spin temperature, the behavior of the Curie temperature versus a magnetic field, and magnetocaloric properties of bulk Gd.

A new record‐high room‐temperature electron Hall mobility (μRT = 194 cm2 V−1 s−1 at n ≈ 8 × 1015 cm−3) for β‐Ga2O3 is demonstrated in the unintentionally doped thin film grown on (010) semi‐insulating substrate via metal‐organic chemical vapor deposition (MOCVD). A peak electron mobility of ≈9500 cm2 V−1 s−1 is achieved at 45 K. Further investigation on the transport properties indicates the existence of sheet charges near the epilayer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epilayer and the interface, originating from both surface contamination and growth environment. The pregrowth hydrofluoric acid cleaning of the substrate leads to an obvious decrease in Si impurity both at the interface and in the epilayer. In addition, the effect of the MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a β‐Ga2O3 film with very low bulk charge concentration, even a reduced sheet charge density plays an important role in the charge transport properties.

Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)-based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox-potential-driven. Remarkably, X-ray scattering shows that despite their large 2-nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.