Taek Dong Lee

Current-induced domain wall motion in a nanowire with perpendicular magnetic anisotropy

We theoretically study the current-induced magnetic domain wall motion in a metallic nanowire with perpendicular magnetic anisotropy. The anisotropy can reduce the critical current density of the domain wall motion. We explain the reduction mechanism and identify the maximal reduction conditions. This result facilitates both fundamental studies and device applications of the current-induced domain wall motion.

Compositional change of MgO barrier and interface in CoFeB∕MgO∕CoFeB tunnel junction after annealing

Recent experiments have demonstrated high tunneling magnetoresistance (TMR) ratios in magnetic tunnel junctions (MTJs) with the MgO barrier. The CoFeB∕MgO∕CoFeB junctions showed better properties than the CoFe∕MgO∕CoFe junctions because the MgO layer had a good crystalline structure with (001) texture and smooth and sharp interface between CoFeB∕MgO. In this work, the compositional changes in the MgO barrier and at the interface of CoFeB∕MgO∕CoFeB after the CoFeB crystallization were studied in annealed MTJs. X-ray photoelectron spectroscopy depth profiles were utilized for the as-deposited and 340°C annealed specimens. Transmission electron microscope analyses showed that the MgO barrier had (100) texture on CoFeB in the junctions and CoFeB was crystallized in the annealed junctions. B in the bottom CoFeB layer diffused into the MgO barrier and B–oxide was formed at the interface of CoFeB∕MgO∕CoFeB after the CoFeB crystallization. The B behavior will be discussed.

The effect of heat treatments and Si contents on B2 ordering reaction in high-silicon steels

The silicon content was increased up to 6.5% (by weight, unless specified otherwise) to reduce the power loss of the silicon steels. These steels were prepared by the conventional casting method or by spray forming and were investigated with the aid of light optical microscopy (LOM) and transmission electron microscopy (TEM). The difference in the casting method did not result in any difference in suppressing the B2 ordering. The D03 phase was observed only in the as-cast 6.5%Si steel. It was almost impossible to suppress the B2 ordered phase keeping the silicon level as high as 6.5% even after the heat treatment at 1000°C for 24 h or after hot rolling. It was necessary to change the Si level and control the cooling rate to suppress the ordering reaction, especially, in cooling after heat treatment. The silicon level of 5.87% was observed to be a critical value in suppressing the B2 ordering reaction.

Effect of shape anisotropy on threshold current density for current-induced domain wall motion

Using micromagnetic simulations, the authors investigate the effects of the shape anisotropy of nanowires on the threshold current density (JC) in the adiabatic limit. It is observed that the type of domain wall significantly affects the dependence of JC on the shape anisotropy. For a transverse wall, JC is proportional to the shape anisotropy, but slightly deviates from the theoretical prediction [G. Tatara and H. Kohno, Phys. Rev. Lett. 92, 086601 (2004)] possibly due to antivortex formation. For a vortex wall, JC is almost independent of the shape anisotropy, as experimentally observed [A. Yamaguchi et al., Jpn. J. Appl. Phys. Part 1 45, 3850 (2006)]. Interestingly, JC and the velocity of the vortex wall at finite temperatures are in good agreement with the experimental values even when the nonadiabatic spin torque is not considered.

Effect of top Ru deposition pressure on magnetic and microstructural properties of CoCrPt-SiO2 media in two-step Ru layer

Two-step-deposited Ru layers of 20 nm have been employed as an underlayer for CoCrPt–SiO2 perpendicular magnetic recording media. The bottom Ru layer of 10 nm is deposited at 3 mTorr for good texture of hcp (0002) and the top Ru layer of 10 nm is deposited at various conditions. Sputtering power and Ar pressure are variables for the top Ru layer deposition. As the sputtering pressure of the top Ru layer increases, coercivity (Hc) of the CoCrPt–SiO2 layer increases and then saturates or slightly decreases with a further increase of sputter pressure. The Hc increase is mainly due to exchange decoupling among grains of CoCrPt. The c-axis alignment of the Ru layer by the two-step deposition method is much improved compared to that of a single-step-deposited Ru layer when the sputtering pressure is higher than 3 mTorr. The grain size of Ru layer is smaller and the grains are well separated by void boundaries when the top Ru sputtering pressure is high. There is a one-to-one relationship between small Ru grains...

Magnetic and half-metallic properties of the full-Heusler alloys Co2TiX(X=Al,Ga;Si,Ge,Sn;Sb)

The electronic structure and magnetic properties of the full-Heusler alloys Co2TiX (X element from groups III, IV and V) were studied by first principle calculations. Previous calculations found Co2TiAl and Co2TiSn not to be half metallic. In this paper, however, it will be shown that the alloys with X=Al,Si,Ge,Sn are half metallic and ferromagnetic. The effect of atomic disorder in the Ti–Al sublattices reduces the half metallicity of the Co2TiAl Heusler alloy.

Interdiffusion in antiferromagnetic/ferromagnetic exchange coupled NiFe/IrMn/CoFe multilayer

Auger electron spectroscopy (AES) and Rutherford backscattering spectroscopy (RBS) analysis were carried out in order to study the extent of interdiffusion during thermal treatment of the pinned electrode (Ta/NiFe/Cu/NiFe/IrMn/CoFe) of the magnetic tunneling junction. From the concentration profile results from RBS and AES, a significant amount of Mn–CoFe interdiffusion was observed when the sample was annealed at 200 °C–400 °C under vacuum. The multilayer was completely intermixed at 400 °C, losing the exchange bias interaction between the IrMn and CoFe layers. It was demonstrated that the migration of Mn was enhanced by the preferential oxidation of Mn on the surface. In fact, when a thin layer of Ta for oxidation protection was deposited on top of the electrode, the Mn diffusion was minimal up to 300 °C. Our experiment suggests that in actual magnetic tunneling junctions, the Mn diffusion to the insulation layer could be enhanced by the presence of the free oxygen radicals in the insulation layer produ...

Non-colloidal sedimentation compared with Kynch theory

Instantaneous concentration profiles of the batch sedimentation of non-colloidal hard spheres were measured for various initial suspension concentrations from 0.04 to 0.55 using the magnetic resonance imaging (MRI) technique. Measured profiles commonly had two distinct interfaces, the upper one between the clarified fluid and the settling suspension, and the lower one between the settling suspension and the sediment. The upper interface was found to keep spreading due to polydispersity, especially for suspensions with low initial concentration. It was observed that the lower interface pattern is highly dependent on the initial concentration. If the pattern is considered to be spread due to the broadening effects of hydrodynamic diffusion and polydispersity, its overall behavior seems to be consistent with Kynchs prediction. For all initial concentrations the sediment was incompressible with packing concentration 0.60±0.01. Three transition concentrations separating four regions of different profile patterns were determined by fitting the experimental results to the Richardson-Zaki formula. The validity of the first (0.16) and third (0.46) transition concentrations was cross-checked by various criteria based on the analysis of the experimental results. The second transition concentration (0.33) determined from the flux curve analysis could not be confirmed experimentally.

Study of the effect of natural oxidation and thermal annealing on microstructures of AlOx in the magnetic tunnel junction by high-resolution transmission electron microscopy

In a magnetic tunnel junction, the formation of an insulator is sensitive and critical to the stable performance and reproducibility of the junction. The oxidation path and the microstructural change with time of the insulator in natural oxidation have been studied by the high-resolution transmission electron microscopy. It has been observed that the oxidation path is primarily through the grain boundary at an early stage of oxidation and then through the grains at a later stage. The morphology of the oxide layer was rugged and modulated. There also occurred an isotropic volume expansion with increased oxidation. It was observed that the ferromagnetic Co layer below an insulator was partially oxidized because of the preferred grain boundary oxidation. When this multilayer was annealed, the locally oxidized Co layer was reduced and the metallic layer formed as a continuous film type, thereby improving the interface.

Attempt frequency of magnetization in nanomagnets with thin-film geometry

Solving the stochastic Landau-Lifshitz-Gilbert equation numerically, we investigate the effect of the potential landscape on the attempt frequency of magnetization in nanomagnets with the thin-film geometry. Numerical estimates of the attempt frequency are analyzed in comparison with theoretical predictions from the Fokker-Planck equation for the Neel-Brown model. It is found that for a nanomagnet with the thin-film geometry, theoretically predicted values for the universal case are in excellent agreement with numerical estimates.