• Journal of Magnetics
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• v.7 no.3
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• pp.80-93
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• 2002

The role of magnetic anisotropy of the antiferromagnetic layer on the magnetization process of exchange coupled polycrystalline ferromagnetidantiferromagnetic bilayers is discussed. In order to elucidate the magnetic torque response of Ni-Fe/Mn-Ir bilayers, the single spin ensemble model is newly introduced, taking into account the two-dimensionally random distribution of the magnetic anisotropy axes of the antiferromagnetic grains. The mechanism of the reversible inducement of the exchange anisotropy along desirable directions by field cooling procedure is successfully explained with the new model. Unidirectional anisotropy constant, J$k$, of polycrystalline Ni-Fe/Mn-Ir and Co-Fe/Mn-Ir bilayers is investigated as functions of the chemical composition of both the ferromagnetic layer and the antiferromagnetic layer. The effects of microstructure and surface modification of the antiferromagnetic layer on JK are also discussed. As a notable result, an extra large value of J$k$, which exceeds 0.5 erg/cm$^2$, is obtained for $Co_{70}Fe_{30}Mn_{75}Ir_{25}$ bilayer with the ultra-thin (50${\AA}$∼100${\AA}$) Mn-Ir layer. The exchange anisotropy of $Co_{70}Fe_{30}$ 40 ${\AA}/Mn_{75}Ir_{25}$ 100 ${\AA}$ bilayer is stable for thermal annealing up to $400{^{\circ}C}$, which is sufficiently high for the application of spin valve magnetoresistive devices.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2008.04a
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• pp.735-741
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• 2008

Identification of asymmetry and anisotropy of rotor system is important for diagnosis of rotating machinery. Directional frequency response functions (dFRFs) are known to be a powerful tool in effectively detecting the presence of asymmetry or anisotropy. In this paper, an input noise effect of dFRFs for rotors is estimated, when both asymmetry and anisotropy are present. The normalized random errors of the dFRFs are calculated to verify the validity of the method, which is demonstrated by numerical simulation with a simple rotor model.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1998.03a
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• pp.242-245
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• 1998

Effects of the anisotrpy due to the initial textures in the plane sheet on plane strain punch stretching has been investigated. In this study, the anisotropy from textures in the sheet is incoporated into the finite element process model by combining the theory of crstal plasticity. Three different textures such as random texture, plane strain compression texture and cube texture are considered. Variations of puch loads as well as thickness distributions of the sheets with three different initial textures are investigated.

• Journal of Magnetics
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• v.16 no.4
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• pp.403-407
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• 2011

Metal-insulator type, nano-granular soft magnetic films have been reviewed from the viewpoint of high frequency magnetic materials. The formation of nano-granular structure is related to the magnitude of heat of formation of intergranule materials. Variation of the ratio of granule phase to intergranule phase in the film is found to produce various characteristics in the magnetic properties of the film. The HRTEM observation reveals that neighboring granules in the film with above 60 at.% Co, contact at considerable points and the films show soft magnetic properties which are explainable in terms of the random anisotropy model for nano-crystalline materials. Addition of Ni group elements in Co-O based films enhances their anisotropy field up to 400 Oe and they exhibit excellent frequency response of permeability. Also, large electromagnetic noise suppression effect is demonstrated as one of their potential applications.

• Korean Journal of Materials Research
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• v.13 no.7
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• pp.447-452
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• 2003

Effects of annealing and thickness of Co layer in Co/IrMn bilayers on the magnetic properties have been investigated. The highest interfacial exchange coupling energy($J_{K}$ = 0.12 erg/$\textrm{cm}^2$) was obtained for 10 nm Co layer thickness. Exchange bias field is inversely proportional to the magnetization, the thickness of the pinned layer, and the grain size of antiferromagnetic layer. Also it is related to the interfacial exchange energy difference, which is expected to depend on the surface roughness. These results almost agree with the random-field model of exchange anisotropy proposed by Malozemoff. Exchange bias field decreased slowly with increasing annealing temperature up to X$300^{\circ}C$. However, exchange bias field increased above $300^{\circ}C$.

• Geophysics and Geophysical Exploration
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• v.8 no.1
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• pp.41-49
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• 2005

We have developed a random heterogeneous velocity model with bimodal distribution in methane hydrate-bearing Bones. The P-wave well-log data have a von Karman type autocorrelation function and non-Gaussian distribution. The velocity histogram has two peaks separated by several hundred metres per second. A random heterogeneous medium with bimodal distribution is generated by mapping of a medium with a Gaussian probability distribution, yielded by the normal spectral-based generation method. By using an ellipsoidal autocorrelation function, the random medium also incorporates anisotropy of autocorrelation lengths. A simulated P-wave velocity log reproduces well the features of the field data. This model is applied to two simulations of elastic wane propagation. Synthetic reflection sections with source signals in two different frequency bands imply that the velocity fluctuation of the random model with bimodal distribution causes the frequency dependence of the Bottom Simulating Reflector (BSR) by affecting wave field scattering. A synthetic cross-well section suggests that the strong attenuation observed in field data might be caused by the extrinsic attenuation in scattering. We conclude that random heterogeneity with bimodal distribution is a key issue in modelling hydrate-bearing Bones, and that it can explain the frequency dependence and scattering observed in seismic sections in such areas.

• Journal of the Korean Magnetics Society
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• v.24 no.5
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• pp.140-145
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• 2014

We analyzed the MnIr thickness dependence of torque signals measured in exchange coupled CoFe/MnIr ($t_{AF}$) bilayers. The measured torque signals were compared with calculated ones by Stoner-Wohlfarth model. The exchange coupling anisotropy $J_c$ was considered for the model calculation between ferromagnetic (F) and antiferromagnetic (AF) layers with uniaxial anisotropy constant of $K_F$ and $K_{AF}$, respectively. The rotational losses were appeared in the range of $0.5t_c$ < $t_{AF}$ < $t_c$ ($=J_c/K_{AF}$) by the unpinned AF layer. While, the unidirectional anisotropy ($J_k$) was caused by the pinned AF layer at $t_{AF}$ > $t_c$. The critical thickness of MnIr layer was $t_c$ = 3.4 nm in CoFe/MnIr bilayers. The rotational losses behavior as shown in $t_{AF}$ = 3 nm sample were explained by the random orientation of the easy axis of AF grains. The unidirectional anisotropy obtained from torque signal of $t_{AF}$ = 10 nm sample was $J_k=0.63J_c$. Thus, the unidirectional anisotropy can be enhanced up to $J_k=J_c$ by aligning the AF easy axis.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.9
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• pp.2181-2193
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• 1995

The effective elasticity and conduction of composite materials containing arbitrarily oriented multiple phases has been analyzed using the concept of orientation-dependent average fields and concentration factors. The analysis provided closed form expressions for the effective stiffnesses and conductivities. Under the prescribed boundary conditions, the concentration factors were evaluated by the equivalent inclusion principle, through which the interaction between various phases is approximated by the Mori-Tanaka mean-field approximation. SiC particulate(SiC$_{p}$) reinforce aluminum(Al) matrix composites were fabricated and their elastic constants and electrical conductivities were measured together with a careful study of their microstructure. The measured properties showed a systematic anisotropy and this behavior could be attributed to the preferred orientation of SiC$_{p}$. The theoretical model developed was applied to the computation of the anisotropic properties of these composites. Both two-phase and three-phase composites were considered based on the microstructural information. The SiC$_{p}$ was modeled as an ellipsoid with planar random orientation distribution in the extruded Al/SiC$_{p}$ composites. The effect of extraneous phase such as intermetallic compounds was also investigated.tigated.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.7
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• pp.785-792
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• 2012

Foam structure is usually hard to model due to the complexity of the geometry of cells. So, many simplified models to represent complicated foam structures have been proposed, but most of them are not actually describe the random feature of the cell structure well. So, in this study, two dimensional isotropic and anisotropic closed cell structures of the foam were modeled using the concept of Voronoi cells. The elasto-plastic deformation behavior under compressive loads was investigated by finitie element analysis, and the results were compared with ideal honeycomb structure. In addition, the effect of anisotropy of Voronoi cell structures of the foam on Young's modulus and yield stress under compressive loads was studied.

• Journal of Magnetics
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• v.14 no.3
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• pp.104-107
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• 2009

The effects of the magnetic property variation on current-induced magnetization switching in magnetic tunnel junction with perpendicular magnetic anistoropy (PMA) and the soft magnetic polarization-enhancement layers (PELs) inserted between the layers with PMA and the MgO layer was studied. A micromatnetic model was used to estimate the switching time of the free layer by different applied current densities, with changing saturation magnetization ($M_s$) of the PELs, interlayer exchange coupling between PMA layers and PELs. The switching time could be significantly reduced at low current densities, by increasing $M_s$ of PELs and decreasing interlayer exchange coupling.