Searcharxiv⌕ Search

arXiv subjects

Chun-Yeol You

Publications and source records attributed to Chun-Yeol You.

29 records · Page 2Linked to original sources
arXiv2012

Dependence of the Switching Current Density on the Junction Sizes in Spin Transfer Torque

We investigate the dependence of switching current density on the junction sizes in the spin transfer torque nanopillar structures by using micromagnetic simulations. While the macro spin model predicts weak dependence of switching current density on the junction sizes, we find that the switching current density is a sensitive function of the junction sizes. It can be explained with the complicated spin configurations and dynamics during the switching process. The detail spin configurations and dynamics are determined by spin wave excitation with the finite wave vector, which is related with the exchange coupling energy and junction shape.

cond-mat.mtrl-sci↗
arXiv2012

Dependence of the Spin Transfer Torque Switching Current Density on the Exchange Stiffness Constant

We investigate the dependence of the switching current density on the exchange stiffness constant in the spin transfer torque magnetic tunneling junction structure with micromagnetic simulations. Since the widely accepted analytic expression of the switching current density is based on the macro-spin model, there is no dependence of the exchange stiffness constant. When the switching is occurred, however, the spin configuration forms C-, S-type, or complicated domain structures. Since the spin configuration is determined by the shape anisotropy and the exchange stiffness constant, the switching current density is very sensitive on their variations. It implies that there are more rooms for the optimization of the switching current density with by controlling the exchange stiffness constant, which is determined by composition and the detail fabrication processes.

cond-mat.mtrl-sci↗
arXiv2012

Micromagnetic Simulations for Spin Transfer Torque in Magnetic Multilayers

We investigate the spin transfer torque (STT) in the magnetic multilayer structures with micromagnetic simulations. We implement the STT contribution for the magnetic multilayer structures in addition to the Landau-Lifshitz-Gilbert (LLG) micromagnetic simulators. Not only the Sloncewski STT term, the zero, first, and second order field- like terms are also considered, and the effects of the Oersted field by the current are addressed. We determine the switching current densities of the free layer with the exchange biased synthetic ferrimagnetic reference layers for various cases.

cond-mat.mtrl-sci↗
arXiv2010

Spin Transfer Torque and Tunneling Magnetoresistance Dependences on the Finite Bias Voltages and Insulator Barrier Energy

We investigate the dependence of perpendicular and parallel spin transfer torque (STT) and tunneling magnetoresistance (TMR) on the insulator barrier energy in the magnetic tunnel junction (MTJ). We employed single orbit tight binding model combined with the Keldysh non-equilibrium Green's function method in order to calculate the perpendicular and parallel STT, and TMR in MTJ with the finite bias voltages. The dependences of STT and TMR on the insulator barrier energy are calculated for the semi-infinite half metallic ferromagnetic electrodes. We find that perfect linear relation between the parallel STT and the tunneling current for the wide range of the insulator barrier energy. Furthermore, the TMR also depends on the insulator barrier energy, which contradicts to the Julliere's simple model.

cond-mat.mtrl-sci↗
arXiv2010

Effect of spin transfer torque on the magnetic domain wall ferromagnetic resonance frequency in the nanowires

We investigate the influence of the domain wall ferromagnetic resonance frequency on the spin transfer torque in a ferromagnetic nanowire. By employing micromagnetic simulations with the spin transfer torque, we find that the domain wall resonance frequency decreases with increasing spin polarized current density, when there is no change in the resonance frequency of the domain itself. Surprisingly, the variation of the resonance frequency is remarkable (> 1.6 GHz) with the spin transfer torque even though the domain wall is pinned. Since the presented domain wall ferromagnetic resonance study has been performed for the pinned domain wall, the contributions of extrinsic defects are excluded. It is strong advantages of the present study, since the effects of extrinsic pinning sites are inevitable in the imaging or transport measurements.

cond-mat.mtrl-sci↗
arXiv2010

Micromagnetic analysis of magnetic noise in ferromagnetic nanowires

We investigate the magnetic thermal noise in magnetic nanowires with and without a domain wall by employing micromagnetic simulations. The magnetic thermal noise due to random thermal fluctuation fields gives important physical quantities related with the magnetic susceptibility. We find that the resonance frequency of a domain wall is distinguishable from one of a magnetic domain itself. For the single domain without a domain wall, the resonance frequency is well described by the Kittel's formula considering a ferromagnetic specimen as a simple ellipsoid with demagnetizing factors for various wire widths and thicknesses. However, additional resonance frequencies from the magnetic domain wall show the different dependences of the wire width and thickness. It implies that the spins inside the domain wall have different effective fields and the spin dynamics.

cond-mat.mtrl-sci↗
arXiv2009

Critical current density for spin transfer torque switching with composite free layer structure

Critical current density of composite free layer (CFL) in magnetic tunneling junction is investigated. CFL consists of two exchange coupled ferromagnetic layers, where the coupling is parallel or anti-parallel. Instability condition of the CFL under the spin transfer torque, which is related with critical current density, is obtained by analytic spin wave excitation model and confirmed by macro-spin Landau-Lifshitz-Gilbert equation. The critical current densities for the coupled two identical layers are investigated with various coupling strengths, and spin transfer torque efficiencies.

cond-mat.mtrl-sci↗
arXiv2009

Analytic expression of the temperature increment in a spin transfer torque nanopillar structure

The temperature increment due to the Joule heating in a nanopillar spin transfer torque system is investigated. We obtain a time dependent analytic solution of the heat conduction equation in nanopillar geometry by using the Green's function method after some simplifications of the problem. While Holm's equation is applicable only to steady states in metallic systems, our solution describes the time dependence and is also applicable to a nanopillar-shaped magnetic tunneling junction with an insulator barrier layer. The validity of the analytic solution is confirmed by numerical finite element method simulations and by the comparison with Holm's equation.

cond-mat.other↗
arXiv2008

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.

cond-mat.mtrl-sci↗
arXiv2003

Domain-specific magnetization reversals on a permalloy square ring array

We present domain-specific magnetization reversals extracted from soft x-ray resonant magnetic scattering measurement 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.

cond-mat.mtrl-sci↗