arXiv subjects

Myung-Hwa Jung

Publications and source records attributed to Myung-Hwa Jung.

27 records · Page 2Linked to original sources

arXiv2014

Voltage controlled propagating spin waves on a perpendicularly magnetized nanowire

We numerically and analytically investigate the voltage controlled spin wave (SW) propagations in a nanowire with locally manipulated perpendicular magnetic anisotropy (PMA) by applying an electric field. It is shown that the velocity and wavelength of the propagating SWs are tailored by the modified PMA, which can be locally controlled by the external electric field. First, we observe a phase shift when the propagating SWs pass through the area of locally modified PMA. By introducing phase control of the SWs, we finally propose a three terminal SW device. Constructive/destructive interferences of two propagating SWs are controlled at the detecting area by the voltage controlled phase shift.

cond-mat.mes-hall↗

arXiv2013

Role of the Non-Collinear Polarizer Layer in Spin Transfer Torque Switching Processes

We have recently reported that the spin transfer torque switching current density is very sensitive to not only the junction sizes but also the exchange stiffness constants of the free layer according to the micromagnetic simulations. The results are very complicate and far from the macro-spin model because of the non-coherent spin switching processes. The dependence of the switching current density on the junction sizes and the exchange stiffness constants becomes systematic when we employ the non-collinear polarizer layer. It is found that the non-collinear polarizer layer enhances the coherency of the spin dynamics by breaking symmetric spin configurations.

cond-mat.mtrl-sci↗

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↗

arXiv2011

Hole mediated ferromagnetism in Cu-doped ZnO thin films

We report the successful synthesis of ZnO:Cu thin films doped with holes, resulting in room temperature ferromagnetism. Hole doping is achieved by As-diffusion from the GaAs substrate into ZnO films, assisted by thermal annealing. The As-diffusion is probed with the help of x-ray absorption spectra collected at the As K-edge which show enhanced signature of diffusion in the annealed samples. Introduction of holes, due to the As doping, in ZnO films is further evidenced by the Cu L3,2-edge spectra. XMCD and magnetic measurements show that the ferromagnetic interaction between doped Cu ions is enhanced after hole doping.

cond-mat.mes-hall↗

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

In-situ fabrication of cobalt-doped SrFe2As2 thin films by using pulsed laser deposition with excimer laser

The remarkably high superconducting transition temperature and upper critical field of iron(Fe)-based layered superconductors, despite ferromagnetic material base, open the prospect for superconducting electronics. However, success in superconducting electronics has been limited because of difficulties in fabricating high-quality thin films. We report the growth of high-quality c-axis-oriented cobalt(Co)-doped SrFe2As2 thin films with bulk superconductivity by using an in-situ pulsed laser deposition technique with a 248-nm-wavelength KrF excimer laser and an arsenic(As)-rich phase target. The temperature and field dependences of the magnetization showing strong diamagnetism and transport critical current density with superior Jc-H performance are reported. These results provide necessary information for practical applications of Fe-based superconductors.

cond-mat.supr-con↗

arXiv2009

Concurrent Magnetic and Metal-Insulator Transitions in (Eu,Sm)B_6 Single Crystals

The effects of magnetic doping on a EuB_6 single crystal were investigated based on magnetic and transport measurements. A modest 5% Sm substitution for Eu changes the magnetic and transport properties dramatically and gives rise to concurrent antiferromagnetic and metal-insulator transitions (MIT) from ferromagnetic MIT for EuB6. Magnetic doping simultaneously changes the itinerant carrier density and the magnetic interactions. We discuss the origin of the concurrent magnetic MIT in (Eu,Sm)B_6.

cond-mat.dis-nn↗

arXiv2005

Comparison of $H_{c2}(T,θ)$ in Mg$_{1-x}$Al$_x$B$_2$ single crystals with the dirty-limit two-gap theory

We studied the temperature and the angular dependences of the upper critical field ($H_{c2}(T,θ$)) of Mg$_{1-x}$Al$_x$B$_2$ single crystals ($x = 0.12$ and 0.21) and compared with the dirty-limit two-gap theory. We found that $H_{c2}(T,θ)$'s were well described in a unified way by this theory. The obtained values of the parameters indicated that as the Al concentration was increased, anisotropic impurity scattering increased, making the $σ$ bands less anisotropic. Accordingly, the temperature dependence of the anisotropy ratio of $H_{c2}$ ($γ_H$) systematically decreased, and for $x=0.21$, $γ_H$ was nearly constant. Our results imply that Mg$_{1-x}$Al$_x$B$_2$ single crystals are in dirty-limit and that two-gap nature survives until $x = 0.21$.

cond-mat.supr-con↗