La1.5Sr0.5NiO4 is a well-known dielectric with colossal dielectric constant but a superparamagnet-like material with tiny magnetization. Mixing La1.5Sr0.5NiO4 with high permittivity ferrimagnets or ferromagnets is expected to modify the dielectric-magnetic balance and enhance the magnetic loss in the microwave absorption capability. In this work, dielectric La1.5Sr0.5NiO4 and soft ferromagnetic La0.7Sr0.3MnO3 nanoparticles were prepared by a combination method of solid-state reaction and highenergy ball milling. The microwave absorption properties of (100-x) La1.5Sr0.5NiO4/xLa(0.7)Sr(0.3)MnO(3) nanocomposites mixed in paraffin were studied. The optimal microwave absorption is obtained for x = 4 with the reflection loss (RL) reaches down to -28.5 dB. The impedance (Z) matching resonance shifts to lower frequencies with increasing x, in contrast to what observed in (100-x) La1.5Sr0.5NiO4/xNiFe(2)O(4). The results suggest that beside the modification of Z by the dielectric-magnetic balancing, the coupling among the nanoparticles plays a major role in governing the microwave absorption properties of the composites. We also verify that the phase matching resonance that produces a zero total reflection from the absorber can be greatly improved when the sample is attached on a metal plate. (C) 2019 Elsevier B.V. All rights reserved.

One of many challenges for niobium (Nb) based superconducting devices is the improvement over the surface morphology and superconducting properties as well as the reduction of defects. We employed a novel deposition technique, i.e. biased target ion beam deposition technique (BTIBD) to prepare Nb thin films with controlled crystallinity and surface morphology. We found that the target current (ITarget) and the target bias (VTarget) were critical to the crystallinity and surface morphology of Nb films. The high target current (ITarget >500 mA and VTarget = 400 V bias) during the deposition degraded the Nb crystallinity, and subsequently reduced the critical temperature for superconductivity (Tc). VTarget was critical to the surface morphology, i.e. grain size and shape and the surface roughness. The optimized growth condition yielded very smooth film with RMS roughness of 0.4 nm that was an order of magnitude smoother than that of Nb films by sputtering process. The critical temperature for superconductivity was also close to the value of the bulk Nb. The quality of Nb film was evident in the presence of a very thin proximity layer (~ 0.8 nm). The experimental results demonstrated that the preparation of smooth Nb films with adequate superconductivity by BTIBD could serve as a base electrode for the in-situ magnetic layer or insulating layer for superconducting electronic devices.

DC current induced magnetization reversal and magnetization oscillation was observed in 500 nm large size Co90Fe10/Cu/Ni80Fe20 pillars. A perpendicular external field enhanced the coercive field separation between the reference layer (Co90Fe10) and free layer (Ni80Fe20) in the pseudo spin valve, allowing a large window of external magnetic field for exploring the free-layer reversal. The magnetization precession was manifested in terms of the multiple peaks on the differential resistance curves. Depending on the bias current and applied field, the regions of magnetic switching and magnetization precession on a dynamical stability diagram has been discussed in details. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight for synchronization of inhomogenieties in large sized device. The ability to manipulate spin-dynamics on large size devices could prove useful for increasing the output power of the spin-transfer nano-oscillators (STNOs).

We have investigated the structure, magnetic, and electrical properties, and the resistance switching effect (RSE) of La 2 NiO 4+δ compounds with different excess oxygen levels of δ = 0.004 - 0.031. Our results showed the structure changes from tetragonal I4/mmm to orthorhombic Fmmm when reducing δ value from 0.011 to 0.006. The M(H) curves measured at several temperatures exhibit a coexistence of the weak ferromagnetic and antiferromagnetic interactions. Two shoulders on M(T) curves related to the spin and charge ordering have also been observed. These ordering temperatures depend strongly on δ value and they shift toward lower temperature with decreasing δ. All the samples exhibit a semiconductor behavior and their temperature dependences of the resistivity obey the variable-range-hoping model. Interestingly, the typical current-voltage characteristics of the samples show a hysteresis, which suggests an existence of the RSE. We have observed two different states of the resistance corresponding to high (R H ) and low (R L ) values. The resistance change ratio of the samples defined as 100% x [R H - R L ]/R L is found to be about 18%-57%, which depends on the value of δ in the samples.

A combination of dielectric and magnetic materials is expected to demonstrate strong capability of absorbing microwave radiation. In this work, nanocomposite samples made of La1.5Sr0.5NiO4 (LSNO), which has a large dielectric constant, and NiFe2O4 (NFO), which is a typical soft ferrimagnet with high permeability, are prepared and used for measurements in the radar range of 4-18 GHz. The radar absorption capability is characterized by the reflection loss (RL) based on the method proposed by Nicolson - Ross - Weir (hence called NRW method) and the transmission line theory. Lowest RL values of -29.7 dB and -28.5 dB are found for the NFO concentration x = 8 and 30, respectively. Importantly, we observed that the absorption peak shifts to higher frequencies with higher NFO contents, likely due to either the ferromagnetic resonance mechanism or a direct result of the dielectric-magnetic balancing. By attaching a metal (Al) plate to the back of the samples, we are able to observe the phase matching resonance where the reflection signal S-11 is almost vanished in the low frequency regime. (C) 2016 Elsevier B.V. All rights reserved.

Microwave and radar absorbing materials (MAM and RAM) are widely used for reducing electromagnetic interference (EMI) for electronic equipment and devices, in electromagnetic anechoic technique, and especially in radar stealth technology. Ferromagnetic and ferrimagnetic nanoparticles have been known to have a strong microwave absorbing capability. To study magnetic MAMs and RAMs, we have prepared La 0.7 Sr 0.3 MnO 3 ferromagnetic and several (Co,Ni)Fe 2 O 4 ferrimagnetic nanoparticle powders using high-energy ball milling technique, which is capable of producing nanoparticles in reasonably larger scale comparing to conventional chemical methods. The magnetic properties of the nanoparticle powders are strongly dependent on the preparation conditions. The milling process produces damages and defects not only on the surface, but also the crystal structure inside the particles, that cause an undesired strong reduction of saturation magnetization ( M s ) and an increase of coercivity ( H c ). A suitable post-milling heat treatment is able to heal the particles and recover most of their saturation magnetization and magnetic softness. The nanoparticles are then mixed with paraffin for microwave and radar absorption measurements.

This work demonstrated two different kinds of magneto-transport behaviors in epitaxial L10 MnAl film as a function of temperature. The magneto-resistance ratio (MR) was negative and exhibited evident enhancement in the resistivity at coercive fields above 175 K. The MR enhancement was attributed to the increase in the magnetic domain walls based on the quantitative correlation between the domain density and the resistivity. Below 175 K, the MR was positive and showed a quadratic dependence on the external magnetic field, which implied that the MR was dominated by Lorentz effects.

Iron nanoparticles (FeNPs) have been successfully prepared by high-energy ball milling in air for various milling times from 1 h to 32 h. Their structure, particle size, elemental composition, magnetic, and inductive heating properties were investigated by means of x-ray diffraction (XRD) analysis, field-emission scanning electron microscopy, energy-dispersive x-ray (EDX) spectroscopy, vibrating-sample magnetometry, and magnetic induction heating, respectively. XRD analysis showed that the average crystallite size decreased to 11 nm after 10 h of milling, then remained almost unchanged for longer milling times. Coexistence of iron (Fe) and iron oxide (FeO) phases was detected after 12 h of milling. EDX analysis also confirmed the occurrence of oxidation, which can be reconciled with the corresponding decrease and increase in saturation magnetization (M (s)) with milling time when exposed to oxygen and when annealed under H-2 ambient due to oxygen reduction. The time-dependent magnetic and inductive heating responses of the FeNPs were investigated for prospective application in magnetic hyperthermia. The effect of varying the alternating-current (AC) magnetic field strength on the saturation heating temperature and specific loss power of FeNP-containing ferrofluid with concentration of 4 mg/mL was also studied and is discussed.

This work demonstrated two different kinds of magneto-transport behaviors in epitaxial L10 MnAl film as a function of temperature. The magneto-resistance ratio (MR) was negative and exhibited evident enhancement in the resistivity at coercive fields above 175 K. The MR enhancement was attributed to the increase in the magnetic domain walls based on the quantitative correlation between the domain density and the resistivity. Below 175 K, the MR was positive and showed a quadratic dependence on the external magnetic field, which implied that the MR was dominated by Lorentz effects. (C) 2016 AIP Publishing LLC.

A nanopowder of iron was prepared using a high-energy ball milling method, which is capable of producing nanoparticles at a reasonably larger scale compared to conventional chemical methods. Analyses using x-ray diffraction and magnetic measurements indicate that the iron nanoparticles are a single phase of a body-centered cubic structure and have quite stable magnetic characteristics in the air. The iron nanoparticles were then mixed with paraffin and pressed into flat square plates for free-space microwave transmission and reflection measurements in the 4–8 GHz range. Without an Al backing plate, the Fe nanoparticles seem to only weakly absorb microwave radiation. The reflected signal S 11 drops to zero and a very large negative value of reflection loss (RL) are observed for Al-backed samples, suggesting the existence of a phase matching resonance near frequency f ∼ 6 GHz.

Fe50Co50 nanoparticles were prepared by mechanical alloying method in air and subsequently annealing at various temperatures (773 K, 873 K, and 973 K). X-ray diffraction and selective area electron diffraction measurements on the powder sample milled for 10 h showed the Fe50Co50 coexisting with a minor secondary phase of Fe3O4. It was found that the Fe3O4 phase decreased gradually with increasing annealing temperature and disappeared at 973 K. Moreover, the saturation magnetization of annealed samples was found not only depend on the annealing temperature but also the milling time. Finally, we showed that the temperature dependence of the saturation magnetization of sample after annealing at 973 K could be well descried by the Bloch's law. (C) 2015 Elsevier B.V. All rights reserved.

Chromium dioxide (CrO2) is a half metal that is of interest for spintronic devices. It has not been synthesized through traditional physical vapor deposition (PVD) techniques because of its thermodynamic instability in low oxygen pressures. Epitaxial thin films of Ru doped tetragonal rutile CrO2 were synthesized by a PVD technique. The as-deposited RuxCr1−xO2 was ferrimagnetic with the saturation magnetization moment showing a strong dependence on the Ru concentration. Curie temperature as high as 241 K has been obtained for ∼23 at. % Ru. The Ru substitution increased the electrical conductivity by increasing the minority spin concentration. The spin polarization was found to be as high as 70% for 9 at. % Ru and decreased to ∼60% with Ru concentrations up to ∼44 at. %, which is determined by the Fermi velocities of the majority and minority spins. First principle calculations were performed to understand the effect of Ru content on the properties of CrO2. The PVD processes of Ru doped CrO2 could lead to the practical applications of the high spin polarization of CrO2 in spintronic devices.

La1.5Sr0.5NiO4 is well known as a dielectric material that has a colossal permittivity ( up to 10(7)) and a weak paramagnet at room temperature. The permeability is about 1.005, which is just slightly larger than that of air. The weak magnetic moment together with the huge imbalance between permittivity and permeability seemed to negate La1.5Sr0.5NiO4 as a promising candidate for electromagnetic absorption due to the lack of magnetic losses. However, we have found that La1.5Sr0.5NiO4 nanopowder indeed has a reasonably strong microwave absorption capability in the range of 4-18 GHz. Apparently, impedance matching (vertical bar Z vertical bar = Z(0) = 377 Omega) is found to be responsible for the absorption resonance that shifts to lower frequencies with increasing the absorber's thickness. To improve magnetic losses, as well as to balance out the dielectric and magnetic components, CoFe2O4 nanoparticles are gradually added to the La1.5Sr0.5NiO4/CoFe2O4 composites. The influence of adding magnetic nanoparticles on reflection loss, resonance frequency, and matching effects will be discussed.

Fe-Co alloy powders were prepared by mechanical alloying of the elemental Fe and Co powders in air and subsequently annealed at various temperatures. Structural and magnetic characteristics of the annealed powders were studied in detail as a function of annealing temperatures by using an X-ray diffractometer (XRD), a field emission scanning electron microscope, a vibrating sample magnetometer, and a physical property measurement system. The XRD results showed an existence of the nanocrystals with sizes of 15-50 nm. The magnetic studies indicated a strong increase of magnetization and a sharp decrease of coercivity as annealing temperature increased. Both the effect of the oxidation on the magnetic properties as well as magnetization stability of the annealed samples will be discussed.

La2-xSrxNiO4 compounds are well known dielectric materials that have colossal permittivities (epsilon(R) > 10(7)). In the present work, the powder of La1.5Sr0.5NiO4 ultrafine particles was prepared by a combinatorial method of solid-state reaction and high-energy ball milling. Magnetic measurements, M(H), show a very small magnetization and paramagnetic characteristics at room temperature. Flat layers of,La2-xSrxNiO4/paraffin mixture of different thicknesses (t) exhibits strong microwave absorption resonances in the 4-18 GHz range. The reflection loss (RL) decreases with t and reaches down to 36.7 dB for t= 3.0 mm. The impedance matching (vertical bar Z vertical bar= Z(0) = 377 Omega), rather than the phase matching mechanism, is found responsible for the resonance for 1.5 mm <= t <= 3.0 mm. Further increase in the thickness leads to vertical bar Z vertical bar > Z(0) at all frequencies and a reduced absorption. The influence of non-metal backing is also discussed. The obtained low RL suggests that La1.5Sr0.5NiO4 particles could be a potential filler for high performance radar absorbing material. (C) 2014 Elsevier B.V. All rights reserved.