Jae-Hyeok Lee

Excited eigenmodes in magnetic vortex states of soft magnetic half-spheres and spherical caps

We studied the magnetization dynamics of excitation modes in special geometrical confinements of soft magnetic half-spheres and spherical caps in magnetic vortex states using finite-element micromagnetic numerical calculations. We found additional fine features of the zeroth- and first-order gyrotropic modes and asymmetric m = +1 and m = −1 azimuthal spin-wave modes, which detailed information is unobtainable from two-dimensional mesh-cell based numerical calculations. Moreover, we examined the perpendicular bias field dependence of the excited eigenmodes, which data provide for an efficient means of control over the excited modes. Such numerical calculations offer additional details or new underlying physics on dynamic features in arbitrary-shape magnetic nano-elements such as half-spheres and spherical caps in magnetic vortex states.

Vortex-chirality-dependent standing spin-wave modes in soft magnetic nanotubes

Spin-wave (SW) modes excited in cylindrical nanotubes of finite length were investigated using finite-element micromagnetic simulations. From the simulation results along with the relevant analytical interpretation, we found unique dynamic modes representative of a variety of standing SW modes. Those modes are controllable not only according to the geometric confinements of given nanotubes but also by the relative configuration of the vortex-chirality at both ends of the nanotubes. The asymmetric (symmetric) spin-wave dispersion originates from nonreciprocal (reciprocal) spin-wave propagations from the parallel (antiparallel) configuration of vortex chiralities at both ends of the nanotubes. Using a simple analytical model, we estimated the quantized dispersions of the excited modes that agree with the simulation results. This work facilitates further understanding of discrete standing SW modes in three-dimensional curvilinear nano-elements, such as cylindrical nanotubes, and opens up a broader and deeper...

Single-crystalline Gd-doped BiFeO3 nanowires: R3c-to-Pn21a phase transition and enhancement in high-coercivity ferromagnetism

We fabricated single-crystalline, Gd-doped BiFeO3 (BFO) nanowires using a hydrothermal technique. X-ray diffraction (XRD) data combined with their Rietveld refinements and high-resolution transmission electron microscopy (HRTEM) revealed pure single-phase crystalline Bi1−xGdxFeO3 (x = 0, 0.05, 0.10) nanowires of 40–60 nm diameter and their structural transformation from the rhombohedral R3c (for x = 0 and 0.05) to the orthorhombic Pn21a crystal structure (for x = 0.10). The addition of Gd3+ ions to the pure-phase BFO leads to remarkable changes in the structural and magnetic properties, and these effects are caused by differences in the ionic-radii and magnetic moment between the Bi3+ and Gd3+ ions. According to the observed magnetization-field (M–H) and magnetization-temperature (M–T) curves, with increasing Gd3+ concentration, the saturation magnetization (MS), squareness (Mr/MS), coercivity (HC), exchange-bias field (HEB) and magnetocrystalline anisotropy (K) increased markedly, by MS = 1.26 emu g−1 (640%), Mr/MS = 0.19 (20.5%), HC = 7788 Oe (4560%), HEB = 501 Oe (880%) and K = 1.62 × 105 erg cm−3 (3500%), for x = 0.10 relative to the data for x = 0. In such Gd-doped BFO nanowire samples, spin-canted Dzyaloshinskii–Moriya interaction, remarkable enhancements in the magnetocrystalline anisotropy as well as uncompensated surface ferromagnetic spin states in the antiferromagnetic core regions were also found. Such remarkable enhancements in Gd-doped BFO nanowires might offer a variety of spintronic applications.

Stress-induced magnetic properties of PLD-grown high-quality ultrathin YIG films

Yttrium iron garnet (YIG:Y3Fe5O12) thin films were grown on (111) gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates using pulsed-laser deposition under several different deposition and annealing conditions. X-ray diffraction measurements revealed that the crystallographical orientation of the YIG films is pseudomorphic to and the same as that of the GGG substrate, with a slight rhombohedral distortion along the surface normal. Furthermore, X-ray reciprocal space mapping evidenced that in-situ annealed YIG films during film growth are under compressive strain, whereas ex-situ annealed films have two different regions under compressive and tensile strain. The saturation magnetization ( 4 π M S) of the films was found to vary, according to the deposition conditions, within the range of 1350 to 1740 G, with a very low coercivity of H C < 5 Oe. From ferromagnetic resonance (FMR) measurements, we estimated the effective saturation magnetization ( 4 π M e f f) to be 1810 to 2530 G, which are larger than that of single crystalline bulk YIG (∼1750 G). Such high values of 4 π M e f f are attributable to the negative anisotropy field ( H U) that increases in size with increasing compressive in-plane strain induced in YIG films. The damping constant ( α G) of the grown YIG films was found to be quite sensitive to the strain employed. The lowest value of α G obtained was 2.8 × 10−4 for the case of negligible strain. These results suggest a means of tailoring H U and α G in the grown YIG films by the engineering of strain for applications in spintronics and magneto-optical devices.

Magnetization reversal mechanism and coercivity enhancement in three-dimensional granular Nd-Fe-B magnets studied by micromagnetic simulations

We studied the mechanism of magnetization reversals and coercivity enhancements in three-dimensional (3D) granular Nd-Fe-B permanent magnets using finite-element micromagnetic simulations. The magnetization reversals in the hard magnets consisting of hard-phase grains separated by relatively soft-phase grain boundaries were analyzed with reference to the simulation results for the magnetic field-dependent distributions of the local magnetizations. The saturation magnetization of the grain-boundary phase plays a crucial role in the transition between nucleation- and domain-wall-propagation-controlled reversal processes. The smaller the saturation magnetization of the grain-boundary phase is, the more preferable is the nucleation-controlled process, which results in a larger coercivity. The exchange stiffness of the grain-boundary phase determines the preferred paths of domain-wall propagations, whether inward into grains or along the grain boundaries for relatively small and large exchange stiffness, respe...