• Journal of the Korean Magnetics Society
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• v.16 no.2
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• pp.149-156
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• 2006

The frequency of the recording current and the rotating speed of the recording media are Increase for the high densities in perpendicular magnetic recording system with high conductive pole tip head and soft magnetic under-layer. In the paper, the frequency Induced eddy current and velocity induced eddy currents are analyzed by non-linear 3-dimensional finite element analysis. It it turned out that the frequency induced eddy current decreases the amplitude of the recording fields, whereas the velocity induced eddy current only distorts the distribution of the recording fields in the recording media.

• Journal of the Korean Magnetics Society
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• v.5 no.5
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• pp.515-518
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• 1995

We have studied the magnetization in fields up to 1T at 5K, the saturation magnetization dependence on temperature and the temperature dependence of AC-susceptibility at very low fields (5mOe to 50mOe) of glassy $Fe_{91-x}Zr_{7}B_{2}Ni_{x}$ (x = 0, 5, 10, 15) alloys. The temperature dependence of the magnetization follows the predictions of spin wave excitations with long wavelengths. At zero Ni concentration there is a clear competition between ferromagnetic and antiferromagnetic interactions giving rise to spin-glass behaviour. The addition of Ni drastically modifies the magnetic properties: the antiferromagnetic exchange coupling is reduced and finally disappears, the spin wave stiffness increases from 39.5 to $87.3\;meV{\AA}^{2}$ and To increases from 230 K to 478 K. We develop a simple model to quantify the competing interactions and to relate the antiferromagnetically coupled Fe moments to the Ni concentration. We find that the initial susceptibility increases with increasing Ni content along with a decrease of the temperature dependence.

• Journal of the Korean Magnetics Society
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• v.12 no.2
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• pp.73-79
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• 2002

Compared with Newtonial fluids, magnetic fluids have effects on magnetic force. In this study, the purpose is to research the heat transfer characteristic of magnetic fluids which have metalic and fluid characteristics as the external pipe is being cooled and internal pipe is heated. This study found the experimental results from the study of the variety of natural convection for magnetic fluids and the characteristics of the heat transfer by using numerical analysis according to the strength and direction of the magnetic fields from being imposed from the outside. Natural convection of magnetic fluids was controlled by the impressed magnetic fields, and the result of mean nusselt number was calculated. If the impressed magnetic field is in the direction of gravity or the strength of impressed magnetic field is more than -14 mT in the opposite direction, the heat transfer is more than that without the impressed magnetic field. If the strength of impressed magnetic field is less than -14 mT in the opposite direction, it is smaller than that without the impressed magnetic field. Especially, when the strength of the magnetic field is -14 mT, the heat transfer was at the minimum.

• Journal of the Korean Magnetics Society
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• v.26 no.6
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• pp.196-200
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• 2016

In this study the mechanisms of magnetization reversal and magnetic interaction effects on activation volumes for Sr-ferrite with different particle sizes are investigated. The activation volumes of C2 sample are larger than those of C3 sample in the range of low magnetic fields. But the fields above the coercivity of sample C2, the activation volumes of both samples are decreased linearly with increasing the applied magnetic field. These phenomena can be explained by the strengths of two critical fields representing the reverse domain nucleation field and the domain wall pinning field as well as the strength of dipolar interaction.

• Journal of Magnetics
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• v.16 no.2
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• pp.83-87
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• 2011

The vortex-driven magnetization process of micron-sized, exchange-coupled square elements with composition of $Ni_{80}Fe_{20}$ (12 nm)/$Ir_{20}Mn_{80}$ (5 nm) is investigated. The exchange-bias is introduced by field-cooling through the blocking temperature (TB) of the system, whereby Landau-shaped vortex states of the $Ni_{80}Fe_{20}$ layer are imprinted into the $Ir_{20}Mn_{80}$. In the case of zero-field cooling, the exchange-coupling at the ferromagnetic/antiferromagnetic interface significantly enhances the vortex stability by increasing the nucleation and annihilation fields, while reducing coercivity and remanence. For the field-cooled elements, the hysteresis loops are shifted along the cooling field axis. The loop shift is attributed to the imprinting of displaced vortex state of $Ni_{80}Fe_{20}$ into $Ir_{20}Mn_{80}$, which leads to asymmetric effective local pinning fields at the interface. The asymmetry of the hysteresis loop and the strength of the exchange-bias field can be tuned by varying the strength of cooling field. Micromagnetic modeling reproduces the experimentally observed vortex-driven magnetization process if the local pinning fields induced by exchange-coupling of the ferromagnetic and antiferromagnetic layers are taken into account.

• Journal of Magnetics
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• v.14 no.2
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• pp.53-61
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• 2009

Herein, different concepts for domain wall propagation based on currents and fields that could potentially be used in magnetic data storage devices based on domains and domain walls are reviewed. By direct imaging, we show that vortex and transverse walls can be displaced using currents due to the spin transfer torque effect. For the case of field-induced wall motion, particular attention is paid to the influence of localized fields and local heating on the depinning and propagation of domain walls. Using an Au nanowire adjacent to a permalloy structure with a domain wall, the depinning field of the wall, when current pulses are injected into the Au nanowire, was studied. The current pulse drastically modified the depinning field, which depended on the interplay between the externally applied field direction and polarity of the current, leading subsequently to an Oersted field and heating of the permalloy at the interface with the Au wire. Placing the domain wall at various distances from the Au wire and studying different wall propagation directions, the range of Joule heating and Oersted field was determined; both effects could be separated. Approaches beyond conventional field- and current-induced wall displacement are briefly discussed.

• Journal of Magnetics
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• v.11 no.2
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• pp.61-65
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• 2006

The physical origin of complex dynamic domain configuration in nonequilibrium magnetic systems with mesoscopic length scales has been studied. An increasing complexity in the spatial feature of the evolution is found to accompany the increasing reversal speed, when a ferromagnetic element is driven by progressively faster switching fields applied antiparallel to the initial magnetization direction. As reversal rates approach the characteristic precession frequencies of spin fluctuations, the thermal energy can boost the magnetization into local configurations which are completely different from those experienced during quasistatic reversal. The sensitive dependence of the spatial pattern on switching speed can be understood in terms of a dynamic exchange interaction of thermally excited spins; the coherent modulation of the spins is strongly dependent on the rise time of switching pulses.

• Journal of Magnetics
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• v.15 no.2
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• pp.78-84
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• 2010

A 3-D.O.F. spherical PM motor has 3 degrees of freedom in its motion by tilting and rotating of a shaft, which can be applied in a range of fields. The back-EMF is proportional to the field flux and angular velocity. The back-EMF constant in conventional rotating machine has a uniform value. However, in a spherical PM motor, the back-EMF constant of the coils varies according to the tilting conditions regardless of whether the angular speed is constant. Consideration of the back-EMF constant is useful for designing 3-D.O.F. spherical PM motors. In this study, the back-EMF constant of the spherical PM motor was considered carefully.

• Journal of Magnetics
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• v.1 no.2
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• pp.69-74
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• 1996

Justeresos loops of melt-spun Nd13Fe77B10 cooled down at remanent state were measured at 4.2 K and 250 K. The hysteresis loops were analysed on the basis of the Stoner-Wohlfarth (S-W) model, the inter-grain exchange coupled single domain (SD) model and micromagnetism. The coercivity higher than that predicted from the S-W model and the striking shift of the thin minor loop along the H-axix observed at the fields of Hmax=4MA/m at 4.2 K indicated new evidences for the inter-grain exchange interaction . The S-W model failed in explaining the high iHc and the shift of the thin minor loop. The exchange coupled SD model was found to explain the experimental results qualitatively without difficulties associated with the S-W model. The micromagnetic calculations using a finite element technique simulated the experimental results fairly well quntitatively.

• Journal of the Korean Magnetics Society
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• v.7 no.3
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• pp.159-165
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• 1997

Recently, many researchers are interested in the analysis of the magnetic fields considering the vector relation between the magnetic flux density and the magnetic field intensity. In this paper, Finite Element Method with two dimensional tensor permeability is adopted to calculate the magnetic field quantities in the system having the rotational magnetic field. In addition, the method of analyzing the magnetic field considering the harmonic components is introduced. The validity of the proposed method is proved compared with the experimental results.