Magnetics Letters, IEEE - new TOC

Front Cover

2013

Presents the cover/table of contents for this issue of the periodical.

A New Scalar Potential Modeling Technique for Magnetic Fields Due to Arbitrary Distributions of Electric Current

2013

A single-valued magnetic scalar potential is introduced for arbitrary distributions of electric current in free space. It is defined as a Laplacian potential within a specified region outside the current distribution and is determined from the free-space values of the normal component of the field intensity over the region boundary. Once this scalar potential is obtained, the resultant magnetic field in the presence of magnetic material bodies located in the region considered can be expressed in terms of only a scalar potential. This yields a field analysis method substantially more efficient than existing methods, where the field due to the given current distributions is usually computed by employing the Biot-Savart formulas and, thus, the resultant field outside the magnetic bodies remains expressed in terms of vector quantities.

Impact of Ta Diffusion on the Perpendicular Magnetic Anisotropy of Ta/CoFeB/MgO

2013

Effects of Ta diffusion on the perpendicular magnetic anisotropy (PMA) of Ta/Co_0.6Fe_0.2B_0.2 (12.5 Å)/MgO stacks were investigated. We found that Ta initially mixed with Co_0.6Fe _0.2B_0.2 and its diffusion became prominent above 300 °C. Initial diffusion of Ta into the Co_0.6 Fe_0.2B_0.2 was not detrimental to the PMA, but a significant PMA reduction occurred when Ta migrated to the Co _0.6Fe_0.2 B_0.2/MgO interface. This accounts for the fact that optimizing the PMA of Ta/CoFeB/MgO requires the tuning of both CoFeB thickness and annealing temperature.

Magnetic hardening from the suppression of domain walls by nonmagnetic particles

2013

Magnetic domain switching and hysteresis loops in a single crystal α-iron with and without nonmagnetic particles were simulated based on the Landau-Lifshitz-Gilbert equation of magnetization dynamics. Both the nonmagnetic particle and the 360° domain wall are nucleation sites of an antidirection domain during domain switching; however, the 360° Bloch domain wall is the easiest nucleation site. The nucleation occurs by splitting the 360° Bloch domain wall into two 180° Bloch domain walls. The existence of nonmagnetic particles could prevent the formation of 360° Bloch domain walls and cause magnetic hardening. Simulations demonstrate the impact of nonmagnetic particle sizes on magnetic domain switching and coercive field.

Redshift and Blueshift Regimes in Spin-Transfer-Torque Nano-Oscillator Based on Synthetic Antiferromagnetic Layer

2013

The spin-torque-driven in-plane precession mode of synthetic antiferromagnets (SAFs) is characterized by a frequency current dependence that changes its slope from negative redshift to positive blueshift upon increasing the applied magnetic field. Here, we show that the transition from redshift to blueshift is due to additional torques arising from the dynamic interlayer exchange coupling of the SAF. These torques depend on the precession amplitude that changes with current and field. We present an analysis of the corresponding energy surface that is able to predict such behavior for any spin-torque-driven excitation of a coupled two-layer system.

Resonance of Spin Waves and Domain-Wall Excitations in Ferromagnetic Stripes

2013

The interaction of a domain wall (DW) with a propagating spin wave (SW) in soft-magnetic nanostripes is studied analytically. The resonance of the propagating mode with an internal spin excitation of the DW (a Winter-like mode) is examined with relevance to the SW-driven translational motion of the DW. This frequency coincidence (a classical counterpart of the Fano resonance) is possible due to the width quantization of the magnetization waves in the stripe and it enables forward pushing of the DW by the SW. Frequencies of the internal excitations of the DW (resonance frequencies) and the corresponding values of the DW velocity are evaluated.

Role of Small Permeability in Gigahertz Ferrite Antenna Performance

2013

We demonstrate that even small relative permeability (μr<; 2) is effective enough in the improvement of gigahertz (GHz) antenna performance. Based on antenna performance simulation results, a 1.57 GHz small permeability (μr = 1.97) hexaferrite antenna (8 mm × 5 mm × 1.5 mm) was fabricated and characterized for antenna miniaturization factor, fractional bandwidth (FBW), and radiation efficiency (RE). Return loss and FBW were 13 dB at the resonance frequency fr of 1.57 GHz and 7% at voltage standing wave ratio of 2.5:1, respectively. On the other hand, antenna simulation results show that an alumina ( εr = 9.4) antenna with tan δ_ε of 0.01 and 0.05 resonates at fr of 1.65 GHz and has FBW of 4.1% and 6.0%. The hexaferrite antenna volume was 30% of the alumina dielectric antenna volume. The experimental RE of the hexaferrite antenna was 66% at 1.57 GHz, which is much higher than the simulated RE of 55.4% for the lossy dielectric antenna (εr = 11; tan δ_μ = 0; tan δ_ε = 0.05), even though ferrite has tan δ_μ = 0.05 and tan δ_ε = 0.008. The antenna performance simulation results confirmed that the RE of the ferrite antenna can be increased to 80% at tan δ_μ of 0.01. Both simulation and experimental results demonstrate that even small permeability of GHz hexaferrite greatly contributes to miniaturization, FBW, impedance matching, and RE.

Magneto-Optic Interferometric Switch With Resonator Configuration

2013

All-optical resonators are candidates for next-generation, integrated, optical switching devices for communication purposes. Their simplicity in design can allow for easy integration using standard CMOS fabrication processes. Other switch configurations, such as the Mach-Zhender and the Sagnac, have been implemented but are more complicated. Traditionally, optical resonators have been used as optical all-pass filters. However, by including a Faraday rotator in the resonating loop with an actuating field, an all-optical switch can be realized. Optical resonator switches have often been proposed and implemented utilizing the electro-optic effect. This study reveals that magneto-optic equivalents show promise as alternatives. Here, a magneto-optical switch in a resonator configuration is proposed and implemented with theoretical analysis using the Jones calculus technique.

Magnetic Field Distribution and Analytical Optimization of Foil Windings Conducting Sinusoidal Current

2013

Magnetic field distribution in field inductors at various frequencies is presented. Optimization of foil winding thickness to achieve the minimum winding resistance and the minimum winding loss of resonant power inductors is performed. The equation for optimum foil thickness is derived. The results obtained are required to design low loss inductors and high-efficiency resonant circuits.