Chun Yeol You

Annealing effects on the magnetic dead layer and saturation magnetization in unit structures relevant to a synthetic ferrimagnetic free structure

The changes in the magnetic dead layer (MDL) and saturation magnetization of the CoFeB layers are investigated as a function of the annealing temperature for four different unit structures, that are relevant to the synthetic ferrimagnetic free structure in MgO-based magnetic tunnel junctions. The MDL results for these unit structures are then converted into those for the constituent interfaces of the free structure. Most of the changes in the MDL thickness occur during annealing at a low temperature of 150u2009°C while those in the saturation magnetization occur at a high annealing temperature of 350u2009°C. These results for the MDL and saturation magnetization are critically tested by using the synthetic ferrimagnetic free structures with various thickness asymmetries. The observed switching properties of these tested structures are in good agreement with those expected from the results for the MDL and saturation magnetization, confirming the accuracy of the present results. The accuracy of the saturation magne...

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.

Spin wave quantization in continuous film with stripe domains

We investigated the spin wave dynamics of CoFeSiB film, which has a stripe domain structure at a low magnetic field region (<1u2002kOe). We measured the spin wave excitation spectra by employing Brillouin light scattering. Abnormal field dependence and dispersion relations were observed, and they are similar to spin wave quantization in laterally confined magnetic structures such as arrays of magnetic nanowires. The observed spin wave excitation spectra must be interpreted with spin wave quantization such as Damon–Eshbach mode separation. It was found that the spin wave quantization is related to the stripe magnetic domain structure in continuous film. The physical origin of the quantization is the partial reflection of the propagating spin wave at the periodic stripe domain boundaries.

Structural and Magnetic Properties of Amorphous and Nanocrystalline CoFeSiB Thin Films

We have studied the structural, magnetic, and transport properties of CoFeSiB films with various Co compositions. Here, we focus on two amorphous Co₇₄Fe₄Si₁₄B₈ and nanocrystalline Co₇₈Fe₂Si₁₂B₈ thin films. Our results show that the amorphous film is a typical soft magnetic material, while the nanocrystalline film has a large saturation field. We conjecture that in the nanocrystalline film, the super-paramagnetism of nanocrystalline phase or antiferromagnetic exchange at the boundary between the amorphous and nanocrystalline phases causes the large saturation field.

Domain-specific magnetization reversals on a Permalloy square ring array

We present domain-specific magnetization reversals extracted from soft x-ray resonant magnetic scattering measurements on a Permalloy square ring array. The extracted domain-specific hysteresis loops reveal that the magnetization of the domain parallel to the field is strongly pinned, while those of other domains rotate continuously. In comparison with the micromagnetic simulation, the hysteresis loop on the pinned domain indicates a possibility of the coexistence of the square rings with the vortex and onion states.

Solvent-dependent self-assembly of two dimensional layered perovskite (C6H5CH2CH2NH3)2MCl4 (M = Cu, Mn) thin films in ambient humidity

Two dimensional layered organic-inorganic halide perovskites offer a wide variety of novel functionality such as solar cell and optoelectronics and magnetism. Self-assembly of these materials using solution process (ex. spin coating) makes crystalline thin films synthesized at ambient environment. However, flexibility of organic layer also poses a structure stability issue in perovskite thin films against environment factors (ex. moisture). In this study, we investigate the effect of solvents and moisture on structure and property in the (C6H5(CH2)2NH3)2(Cu, Mn)Cl4 (Cu-PEA, Mn-PEA) perovskite thin films spin-coated on Si wafer using three solvents (H2O, MeOH, MeOHu2009+u2009H2O). A combination of x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) show that relative humidity (RH) has a profound effect on perovskite thin films during sample synthesis and storage, depending on the kind of solvent used. The ones prepared using water (Cu-PEA:H2O, Mn-PEA:H2O) show quite different behavior from the other cases. According to time-dependent XRD, reversible crystalline-amorphous transition takes place depending on RH in the former cases, whereas the latter cases relatively remain stable. It also turns out from XAS that Mn-PEA thin films prepared with solvents such as MeOH and MeOHu2009+u2009H2O are disordered to the depth of about 4u2009nm from surface.

Perpendicular Magnetic Anisotropy and Interfacial Dzyaloshinskii-Moriya Interaction in Pt/CoFeSiB Structures

Magnetic materials exhibiting perpendicular magnetic anisotropy (PMA) have an important role in the development of high-density magnetic random-access memory and domain-wall devices. Exotic physical phenomena result from spin-orbit coupling, such as PMA and the interfacial Dzyaloshinskii-Moriya interaction (iDMI), at interfaces between nonmagnetic (NM) and ferromagnetic (FM) materials. We report on the NM Pt thickness dependence of PMA and iDMI in Ta/Pt/CoFeSiB/Ta films and the MgO thickness dependence of PMA in Ta/Pt/CoFeSiB/MgO/Ta films. We selected amorphous FM CoFeSiB because of its lower saturation magnetization (560 emu/cm3) than that of Co or CoFeB, which may be beneficial for lowering the current density for switching. All samples were deposited by dc magnetron sputtering and annealed at 300 °C for 1 h. The Ta(3)/Pt(5)/CoFeSiB(1.5)/Ta(5) (nm) film exhibited PMA in the as-deposited state as well as after heat treatment. This structure possessed an iDMI energy density of 0.386 mJ/m2.

Spin transfer torque in nanopillar spin-valve with [CoFe/Pd]2 double free-layers

We studied spin transfer torque in nanofabricated pillars with |CoFe/Pd]2 double free-layers. This sample showed various spin transfer switching properties as a function of external magnetic fields. We report the first observation of two-step switching and the inversion of f Ic APrarrP and Ic APrarrP characterized by spin directions of three magnetic layers. Telegraph noise confirms the inversion in single domain model.

Structural and magnetic properties of amorphous and nanocrystalline CoFeSiB thin films

We have studied the structural, magnetic, and transport properties of CoFeSiB films with various Co compositions. Here, we focus on two amorphous Co 74 Fe 4 Si 14 B 8 and nanocrystalline Co 78 Fe 2 Si 12 B 8 thin films. Our results show that the amorphous film is a typical soft magnetic material, while the nanocrystalline film has a large saturation field. We conjecture that in the nanocrystalline film, the superparamagnetism of nanocrystalline phase or antiferromagnetic exchange at the boundary between the amorphous and nanocrystalline phases causes the large saturation field.

Step-induced slanted perpendicular magnetic anisotropy in 4°-miscut Si(111)/Cu/Au/Co/Au system

We performed magneto-optic Kerr effect and Brillouin light-scattering measurements on a 4°-miscut Si(111)/Cu/Au(2ML)/Co(5ML)/Au system to understand the correlation we observed between slanted angle and magnetic anisotropy of the ultrathin Co layer. We find that the easy axis orientation is neither perpendicular nor in-plane with respect to the sample surface. We also find that the direction of incline is strongly correlated with the step direction, and that there is no such slanted magnetic anisotropy axis in films grown on a nonmiscut Si substrate. Therefore, we conclude that the slanted magnetic anisotropy axis is caused by the steps in the 4°-miscut Si(111) substrate.