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Device surface properties are critical for its performance such as channel electron density, leakage current, subthreshold swing (SS), and noise in gallium nitride high-electron-mobility transistors (HEMTs). In this article, the improved surface property of InAlN/GaN HEMTs with forming gas (FG, 5% H2, and 95% N2) annealing is demonstrated. X-ray photoelectron spectra (XPS) show that the number of Ga–O bonds decreases while that of the Ga–N bonds increases, an indication of the surface native oxide removal after FG annealing. Compared with N2 annealing, an increase of two-dimensional electron gas (2DEG) electron density with FG annealing is determined by both energy band simulation and capacitance–voltage measurement. Transmission line measurement (TLM) shows that N2 annealing offers a lower ohmic contact resistance ( ${R}_{\text {C}}$ ) while FG annealing features a lower sheet resistance ( ${R}_{\text {sheet}}$ ). Herein, a FG/N2 two-step ohmic contact annealing is developed to achieve a SS of 110 mV/dec, a transconductance ( ${g}_{\text {m}}$ ) peak of 415 mS/mm, a record low drain-induced barrier lowing (DIBL) of 65 mV/V, and a record high power gain cutoff frequency ( ${f}_{\text {max}}$ ) of 270 GHz on a 50-nm InAlN/GaN HEMT on Si.

We have theoretically and experimentally investigated magnetic resonance modes, i.e., optical mode (OM) and acoustic mode (AM), in two coupled ferromagnetic (FM) layers. We calculate the magnetization dynamics of the system by correlating each FM layer via exchange coupling (EC)-induced effective fields. Experimentally, we use CoFe/Ta/NiFe(Cu) trilayers in which the EC is tailored by varying the Ta spacer layer thickness and the saturation magnetization in the (NiFe)1– x Cu x alloy layer. The good agreement between theory and experiment clearly demonstrates that: 1) the resonance frequency of the OM can be significantly higher than that of the individual FM layer; 2) the intensity of the OM can be significantly enhanced by increasing the contrast in the saturation magnetization in two FM layers; and 3) the intensity of the OM, unlike the FMR of a single FM layer, increases with increasing applied magnetic field or frequency. By studying the EC as a function of spacer layer thickness and the saturation magnetization, we further verify that the Neel “orange-peel” coupling is a dominating mechanism in these trilayers, giving rise to coupling between FM layers.

We demonstrate simultaneous detection of current-driven dampinglike and fieldlike spin–orbit torques in heavy metal/ferromagnetic metal bilayers by measuring all three magnetization components $m_{x},m_{y}$ , and $m_{z} $ using a vector-resolved magnetooptic Kerr effect (MOKE) technique based on quadrant detection. We investigate the magnitude and direction of spin–orbit torques in a series of platinum/permalloy samples, finding good agreement with results obtained via polar and quadratic MOKE measurements without quadrant detection.

A systematic study of the electrodynamic coupling between the ferromagnetic resonance (FMR) and the cavity mode of a microwave cavity is presented. The nature of the FMR and cavity modes, described by their resonance position and line width, is measured and compared with a classical model based on the Landau–Lifshitz–Gilbert equation and an RLC circuit, which explicitly implement Faraday’s and Ampere’s laws. The relationship between the coupling gap, which can be accessed experimentally, and the coupling strength, which is a fundamental property of the FMR/cavity system, is discussed on the basis of this model. A straightforward method is experimentally developed to control the coupling gap. The capability of using such a tunable cavity-FMR coupling in combination with spin pumping could provide us with new technologies that utilize the light-matter interaction to control spin current in cavity spintronics.

We experimentally measured the fieldlike spin–orbit torque (SOT) by the second-order planar Hall effect, as well as the dampinglike SOT by the polar magnetooptic Kerr effect in NiFe(1.5 nm)/AuCu( $x$ nm) bilayers with $2~\textrm {nm} \le x \le 8$ nm. We found that the spin diffusion length in the Au–Cu alloy is $\sim 5$ nm, which is shorter than that of pure Au and Cu. When the thickness of the AuCu layer is above the spin diffusion length, the spin Hall angle of the Au–Cu alloy is $\sim 1.3\pm 0.2$ %, which is much larger than that of pure Cu and Au. The results imply that large spin Hall angle materials can be explored in the nonmagnetic alloys of constituent elements having weak or negligible spin–orbit coupling.

NiZn spinel ferrite nanofibers with diameters in 50-100 nm were fabricated by electrospinning followed by thermal treatment. Deploying a rotating collector, nanofibers can achieve certain degree of alignment. The morphology phase contents, static, and dynamic magnetic properties of ferrite fibers were experimentally studied. Measured magnetic hysteresis loops indicate that the NiZn ferrite nanofibers are anisotropic with easy magnetization axis along the fiber alignment direction. The relationship between ferromagnetic resonant frequencies of nanofibers and bias magnetic fields was determined and can be described using a modified Kittel's formula developed for polycrystalline ferrites.

Large permeability, low core loss, and high-saturation magnetization are the important parameters for the applications of soft magnetic materials in high-frequency power electronics. By introducing Cr as oxygen-retardant agent in FeHfB melt-spun ribbons and annealing samples in an ultralow partial oxygen pressure environment, we have successfully prevented oxidation of Fe in forming FeCrHfO composites with exchange coupled Fe nanograins. A coercivity of 0.56 Oe and a saturation magnetic flux density of 1.33 T have been achieved in exchange coupled samples. The samples show a high permeability of 250 at 10 MHz.

A series of NiZn-ferrite/Co2Z-hexaferrite composite ferrites were successfully synthesized by the solid-state reaction method for their potential application as magneto-dielectric antenna substrate materials. It is found that, with the increasing amount of NiZn ferrite (x) from 55 wt% to 70 wt%, the real permeability μ' at low frequencies is improved from ~10 to ~19, while the real permittivity e' varies slightly with value around 14. The sample with wt % has almost equal values of μ' and e', large miniaturization factor about 15, as well as sufficiently low dielectric and magnetic loss (1-50 MHz). This composite ferrite is very useful to the design of miniaturized antenna with good impedance-matched to free space.

Soft magnetic nanogranular Fe70Co30-SiO2 thin films were fabricated on Si substrates using radio frequency (rf) magnetron sputtering in an argon ambient. The electrical, magnetic and high-frequency properties of these thin films were studied in details. SiO2 chips were placed in the erosion race-track of the alloy Fe70Co30 (at.%) target and change the number of chips result in the different film composition. The optimal films with desired properties of low coercivity, Hc ~ 2.5 Oe, high saturation magnetization, 4π Ms ~ 11.6 kGs and high electrical resistivity ρ ~ 1600 μΩcm were obtained. Permeability spectra measured shows that its natural ferromagnetic resonant (FMR) frequency (fr) and initial permeability were about 4.05 GHz and 120, respectively. The tested results shown that the film thickness had an important effect on films properties, it can convenient adjust the fr and damping factor of films. With the increase of film thickness from 50 nm to 1200 nm, the fr of films ranged from 2.86 GHz to 4.05 GHz. The excellent soft magnetic properties were ascribed to exchange coupling among magnetic particles.

Various amounts of NiZn spinel ferrite (Ni0.5Zn0.5Fe2O4) were introduced into Co2Z hexagonal ferrite (Ba3Co2Fe24O41 ) by a solid-state reaction method. The phases, microstructures, and magnetic and dielectric properties have been investigated. The results show that the grain size is obviously changed with introducing of NiZn ferrite, and the saturation magnetization increases from 45 emu/g to 55 emu/g. Compared with undoped Co2Z ferrite, the samples doped 15 wt % and 20 wt % NiZn ferrite exhibit much stable μ' and e' spectrums with frequency. The quality factor is improved to about 80 and the dielectric loss is reduced to 0.005 at 100 MHz.

Magnetic nanoparticles (MNPs) have been studied widely as a powerful diagnostic probe and therapeutic agent for biomedical applications. In recent years, they are also found to be sensitive to magnetoresistive (MR) devices and MNP-MR biochips are predicted to be more affordable, portable and sensitive than the conventional optical detection methods. In this MNP-MR biochip design, MNP probes are required to have high magnetic moment, high susceptibility, and be target-specific. This review summarizes recent advances in chemical syntheses and functionalization of MNPs with controlled magnetic properties for sensitive MR detection and for bio-sensing applications.

A simple theoretical analysis indicates that high permeability is possible in nanowire arrays with the magnetocrystalline anisotropy comparable to the demagnetization energy and its easy axis perpendicular to the nanowire. Both conditions can be satisfied in Co nanowire arrays. With proper fabrication conditions, we have fabricated Co nanowire arrays with crystalline easy axis perpendicular to the nanowire. For Co nanowire arrays with diameters of 50 nm and inter-pore distances of 90 nm, the real part of the permeability is about 3.5 and the loss tangent is about 0.045 below 4.5 GHz. We have also shown that by magnetizing the sample to different remanent states, its ferromagnetic resonance frequency can be tuned between 4.5 and 6.5 GHz.

Carbon doped TiO2 nanocrystal powder samples with various C contents were synthesized by a low-temperature sol-gel method, and a large lattice C contents of 12.7% was obtained. We demonstrated that the C ions were homogenously doped into the TiO2 matrix at the form of substitutional C (Cs-o) and interstitial C (CI), without precipitation of TiC and other impurity phase. Our results show that C-doped TiO2 is ferromagnetic with the Curie temperature well above room temperature. The magnetization is mainly related to the content of Cs-o, suggesting that the intrinsic ferromagnetism originates from the Ti-C system in the TiO2 environment.

In the first part of this paper, we report a systematic study on the structural evolution under rapid thermal annealing and the corresponding transport properties in magnetic tunnel junctions (MTJs) with a crystalline MgO barrier. The results clearly indicate that high tunneling magnetic resistance can be achieved by annealing MTJs at a very short time, and it is directly related to the formation of (001) crystalline structures. In the second part, we report the spin dynamics in tunneling structure through direct electrical detection. A surprisingly large voltage generation in F/I/N and F/I/F junctions was observed, which is contradictory to the prediction from the standard spin-pumping theory. We proposed a theoretical formalism to study spin-pumping effects in ferromagnetic multilayer structures. The formalism can yield a remarkably clean physical picture of the spin and charge pumping in tunneling structures. The calculated values are consistent with experimental results.

The authors present various new phenomena in magnetic tunneling junction (MTJ). Since the interface and barrier structure play a significant role in determining the physical phenomena, they have employed various techniques to characterize this structure. For obtaining the barrier information, they have performed electron holography studies which directly show the energy profile with atomic resolution. The interdiffusion between different layers has been studied using high-resolution transmission electron microscopy, elemental mapping, and time-of-flight secondary ion mass spectrometry. With this structural information, the authors showed the existence of bulklike contribution in MTJ and determined the characteristic length. In addition, they also observed a conductance minimum anomaly which is much different from the normally observed zero bias anomaly. Large inversed TMR has also been observed at bias as high as 0.4 V in MTJs with an Al/sub 2/O/sub 3/-ZrO barrier. The authors have modified the Brinkman model by incorporating the voltage dependent density of states of the ferromagnetic electrodes, which successfully explains the bias dependence of the tunneling magnetoresistance (TMR), conductance minimum anomaly, and inversed TMR.

An inductive-based detection method of ferromagnetic resonance spectrum is demonstrated. For a single microscopic (~40 nm × 40 μm × 60 μm) magnetic dot, which is much smaller than the microwave wavelength, the change in the microwave transmission signal is as large as 15 dB when its magnetic permeability changes. This detection is several orders more sensitive than the conventional transmission line method. Further understanding and development of this concept may lead to a microscopic microwave permeability sensor.

We have systematically investigated the microstructural effects including grain size, precipitation, and structural order parameter on the high temperature magnetic and mechanical properties of FeCo-based commercial alloys. At high temperatures the equilibrium nonmagnetic precipitates significantly deteriorate the soft magnetic properties. Poor mechanical properties are mainly due to the nature of the ordered structure of FeCo alloys. Based on this understanding we have designed new magnetic composites by reinforcing FeCo alloys with high strength fibers. The magnetic and mechanical properties can thus be improved independently through optimizing the magnetic matrix and fiber network, respectively. These new magnetic composites show excellent soft magnetic and mechanical properties. In particularly, negligible creep has been observed at 600/spl deg/C.

We have developed an experimental technique to electrically characterize ferromagnetic resonance (FMR) with a large precession cone angle by using magnetic tunnel junctions on a coplanar waveguide. We observed FMR of a Ni80Fe20 layer as the tunneling resistance change capturing the angle between the Ni80Fe20 precession magnetization and a fixed magnetization in a reference electrode. The resonant frequency shifts with increasing microwave power as a result of the large precession cone angle that reduces the effective anisotropy field. We confirmed the validity of our results by the numerical simulations based on the Landau- Lifshitz-Gilbert equation.

Comparison of the extraordinary Hall effect (EHE) on top and bottom NiO-pinning Co/Cu/Co spin valves (SVs), has been made in order to investigate the relationship Rs~rhon between the extraordinary Hall coefficient Rs and the resistivity rho. Bottom and top spin valves have stronger and weaker specular reflection effects which mainly originate from smoother and rougher Co/NiO interface, respectively. The scaling law Rs~rho2, which implies that the side-jump mechanism is the dominant scattering process, works fairly well (n=1.93plusmn0.02) in the bottom NiO-pinning spin valve, while it deviates much (n=1.32plusmn0.04) in the top one

Ferromagnetic resonance (FMR) is a powerful tool in probing the local magnetic properties in composite magnetic systems. Reactive sputter deposited Fe-R-O (R = Hf, Nd, and Dy) films were systematically studied using in-plane FMR. The as-deposited films render broad FMR spectra, with multiple peaks becoming obvious on annealing. The overall local saturation magnetization M/sub s/ at the atomic sites, extracted from in-plane FMR data, was compared with grain size, as measured by X-ray diffraction. M/sub s/ values decreased with decreasing grain size, being as low as 0.6 T for 5-nm grains. These values are lower than the bulk value calculated based on the Fe concentration determined by compositional analysis. The deviation increases with decreasing grain size. The linear dependence on the inverse of the grain size indicates a strong surface effect. The results were independent of the R element and, therefore, of the nature of the oxide.

The authors report a simple method to fabricate cobalt nanoparticles with homogeneous spherical shape and narrow size distribution, which can be easily scaled up for large quantity production. The method is based on conventional chemical reduction method by using NaBH/sub 4/ as areducing reagent; however, the process is modified so that the particle nucleation and growth are controlled by immediately transferring the formed particles out of the reaction site. In addition, the chemical reaction is carried out in a nonaqueous solution to avoid possible formation of oxides as well as borides.

The giant magnetoresistance (GMR) in granular Co-Ag, Co-Cu, and Fe-Ag systems, as well as in the metastable alloy state, is studied. The absolute values of the resistivity at zero field ( rho /sub 0/), the magnetic resistivity ( rho /sub m/), and GMR are determined. The resistivity of an alloy is found to be significantly larger than that of a granular solid. As the composition (x) of Co or Fe is increased, rho /sub m/ shows a broad maximum at x=20 to 30%. Among the three systems, Co-Ag has the smallest rho (0), the largest rho /sub m/, and hence the largest GMR. Both rho (0) and rho /sub m/ at 5 K in all three systems show a similar dependence on the annealing temperature (T/sub A/). They decrease with increasing T/sub A/, having the largest values in the as-deposited samples, regardless of the sample being alloy or granular. Because rho /sub 0/ and rho /sub m/ decrease with T/sub A/ at different rates, there may or may not be a peak in the GMR at certain value of T/sub A/. >

Measurements on the thermal conductivity and thermoelectric power of granular Co/sub 20/Ag/sub 80/ solids annealed at various temperatures are reported. Thermal conductivity is dominated by its electronic contribution, and the Wiedemann-Franz law to found to be satisfied between 2 K and 300 K, which is mainly ascribed to dominant large-angle elastic scattering processes. The thermoelectric power is negative, and is considerably reduced by the annealing process. Giant magnetothermal conductivity and giant magnetothermopower are found to be correlated with the giant magnetoresistance. >