Woncheol Lee

Definition of Magnetic Exchange Length

The magnetostatic exchange length is an important parameter in magnetics as it measures the relative strength of exchange and self-magnetostatic energies. Its use can be found in areas of magnetics including micromagnetics, soft and hard magnetic materials, and information storage. The exchange length is of primary importance because it governs the width of the transition between magnetic domains. Unfortunately, there is some confusion in the literature between the magnetostatic exchange length and a similar distance concerning magnetization reversal mechanisms in particles known as the characteristic length. This confusion is aggravated by the common usage of two different systems of units, SI and cgs. This paper attempts to clarify the situation and recommends equations in both systems of units.

Soft M-type hexaferrite for very high frequency miniature antenna applications

We have studied soft M-type BaFe9.6Co1.2Ti1.2O19 hexaferrite for T-DMB (terrestrial digital media broadcasting) antenna applications. The effect of magneto-dynamic properties on antenna size, bandwidth, and radiation efficiency was investigated. The soft M-type BaFe9.6Co1.2Ti1.2O19 hexaferrite (Co/Ti-substituted BaM) was synthesized by a combination of ball-milling and two-step sintering processes. Permeability and loss tan δµ of the Co/Ti-substituted BaM were measured to be 4.5 and 0.039 at 200 MHz, respectively. The Wheeler cap method and network analyzer were used to evaluate antenna radiation efficiency and measure return losses, respectively. Our experimental results show that the low-loss Co/Ti-substituted BaM is an excellent soft magnetic material for VHF (very high frequency: 30–300 MHz) miniature antenna applications.

Control of magnetic loss tangent of hexaferrite for advanced radio frequency antenna applications

We demonstrate that magnetic loss tangent of M-type hexaferrite can be controlled by a small dc magnetic field, thereby improving radio frequency (RF) antenna radiation efficiency and realizing antenna miniaturization. Magnetic loss tangent (tan δμ) of the M-type BaFe9.6Co1.2Ti1.2O19 hexaferrite at 200 MHz decreased significantly from 11.8% to less than 1% as the applied dc magnetic field increased from 0 to 400 Oe. This is because the contribution of domain wall motion to permeability dispersion is decreased, and the ferromagnetic resonance frequency increases with the magnetic field. Antenna simulation results showed that radiation efficiency of the designed ferrite helical antenna increased from −22.9 to −9.2 dB with dc magnetic field of 400 Oe. Therefore, the small dc magnetic field played a key role in reduction of tan δμ of hexaferrite and improvement of antenna performance in the RF range.

Role of Small Permeability in Gigahertz Ferrite Antenna Performance

We demonstrate that even small relative permeability (μ<i>r</i><; 2) is effective enough in the improvement of gigahertz (GHz) antenna performance. Based on antenna performance simulation results, a 1.57 GHz small permeability (μ<i>r</i> = 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 <i>fr</i> 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 ( ε<i>r</i> = 9.4) antenna with tan δ_ε of 0.01 and 0.05 resonates at <i>fr</i> 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 (ε<i>r</i> = 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.

Exchange coupled SrFe12O19/Fe-Co core/shell particles with different shell thickness

SrFe12O19/Fe-Co core/shell particles with different shell thickness were synthesized by polyol reduction of Fe and Co ions at 180°C with SrFe12O19 particles dispersed in solvent. The core/shell structure is formed by magnetic self-assembly due to the remanent magnetization of SrFe12O19 particles. Within a limited concentration range, the shell thickness could be controlled by regulating the concentration of Fe and Co ions. Core/shell structured SrFe12O19/Fe-Co particles showed more effective exchange coupling effects between hard and soft phases than physically mixed SrFe12O19 and Fe-Co particles.

Control of IPMSM drive system for drum washing machine

The drum washing machine needs high torque than the other types of washing machine because of the large load torque variation. Therefore, in this paper, we applied a interior permanent magnet synchronous motor (IPMSM) to the drum washing machine system. However, IPMSM operate at lower output torque than the other permanent magnet synchronous motor (PMSM) when the motor control is performed by the conventional control method. This paper suggests adaptive control method for IPMSM to obtain better torque performance and apply the control method in actual experiments of the drum washing machine system.

Mixed methyl‐(2,4‐dinitrophenyl)‐aminopropanoate: 2‐methyl‐4‐nitroaniline crystal−a new nonlinear optical material

Single crystals of mixed methyl‐(2,4‐dinitrophenyl)‐aminopropanoate (MAP): 2‐methyl‐4‐nitroaniline (MNA) have been grown from solution using a 40:60 mixture of ethanol and ethyl acetate. Formation of an equimolar complex is confirmed by x‐ray diffraction studies. The complex crystallizes in the monoclinic space group P21. The lattice parameters are a=6.9196(±0.005) A; b=7.673(±0.008) A; c=18.554(±0.001) A; and β=92.547(±0.006)0. b axis is the unique twofold axis and there are two molecules per unit cell. The molecules stack along the b axis with MAP atop MNA atop MAP. Details of the molecular orientation are given. The Raman spectra also indicate that the parent molecules are not disturbed in the process of crystallization. The second harmonic generation efficiency of the new material for 1064 nm laser radiation is observed to be the same as that of MAP but lower than that of MNA. The latter is attributed to the absorption at 532 nm in the mixed crystal. The melting point determined by differential scanni...

Magnetization and Intrinsic Coercivity for τ-phase Mn 54 Al 46 /α-phase Fe 65 Co 35 Composite

We have synthesized ferromagnetic τ-phase Mn 54 Al 46 /α-phase Fe 65 Co 35 composite by annealing a mixture of paramagnetic e-phase Mn 54 Al 46 and ferromagnetic α-phase Fe 65 Co 35 particles at 650℃. The volume fraction ( f h ) of hard τ-phase Mn 54 Al 46 of the composite was varied from 0 to 1. During the annealing, magnetic phase transformation occurred from paramagnetic e-phase to ferromagnetic τ-phase Mn 54 Al 46 . The magnetization and coercivity of the composite monotonically decreased and increased, respectively, as the f h increased. These results are in good agreement with our proposed composition dependent coercivity and modified magnetization equations.

Miniature Hexaferrite Axial-Mode Helical Antenna for Unmanned Aerial Vehicle Applications

We demonstrated the size reduction of axial-mode helical antenna based on Co2Z hexaferrite-glass composite. Axial-mode helical antenna was employed to provide reliable communication for unmanned aerial vehicle (UAV) applications. The conventional axial-mode helical antenna uses an air core or low dielectric material, resulting in large antenna size. To increase the miniaturization factor n = (μrεr)0.5, a Co2Z hexaferrite-glass composite was used as an antenna core. The 3-dimensional finite element method (FEM) simulation was performed to design a hexaferrite helical antenna and confirm the axial-mode operation at 2.44 GHz with gain of 2.0 dBi. The designed hexaferrite helical antenna showed 82% of volume reduction and good impedance matching compared to the air-core antenna. The axial-mode hexaferrite antenna was fabricated based on the designed structure and characterized in an anechoic chamber. The maximum gain of 0.541 dBi was measured with the pitch angle of 10° at 2.39 GHz. Also, a two-element axial-mode antenna array was designed based on the miniature hexaferrite antenna to further improve antenna gain. Maximum gain of 4.5 dBi at 2.43 GHz was simulated for the antenna array. Therefore, high gain and miniature antenna can be achieved with the combination Co2Z hexaferrite-glass composite and antenna design technology.

Ex situ synthesis of magnetically exchange coupled SrFe12O19/Fe-Co composites

Magnetically exchange coupled SrFe12O19/Fe-Co composites with different mass percentage of Fe-Co were synthesized through an ex situ process. The morphology, magnetic properties, and crystallization of SrFe12O19/Fe-Co composites were investigated. Lower mass percentage of Fe-Co presented an even distribution of Fe-Co nanoparticles on the surface of SrFe12O19, and effective magnetic exchange coupling between Fe-Co and SrFe12O19. Higher mass percentage of Fe-Co leads to an agglomeration of Fe-Co nanoparticles on SrFe12O19 surface, and a weak magnetic exchange coupling between Fe-Co and SrFe12O19. This ex situ process proposed a new method to synthesize magnetically exchange coupled SrFe12O19/Fe-Co core/shell composites with precise control of the magnetic properties. This method can also be potentially used for other hard/soft magnetic composite synthesis.