Jungbum Yoon

Direct observation of the Dzyaloshinskii-Moriya interaction in a Pt/Co/Ni film.

The interfacial Dzyaloshinskii-Moriya interaction in an in-plane anisotropic Pt(4  nm)/Co(1.6  nm)/Ni(1.6  nm) film has been directly observed by Brillouin spectroscopy. It is manifested as the asymmetry of the measured magnon dispersion relation, from which the Dzyaloshinskii-Moriya interaction constant has been evaluated. Linewidth measurements reveal that the lifetime of the magnons is asymmetric with respect to their counter-propagating directions. The lifetime asymmetry is dependent on the magnon frequency, being more pronounced, the higher the frequency. Analytical calculations of the magnon dispersion relation and linewidth agree well with experiments.

Magnetic anisotropy of vertically aligned α-Fe2O3 nanowire array

Vertically aligned Fe2O3 nanowire arrays were synthesized by the thermal oxidation of Fe foil. They consist of single-crystalline rhombohedral α-Fe2O3 nanocrystals grown along the [112¯0] direction. Most of the nanowires have a sharp tip (diameter=50nm) and beltlike base. The magnetization measurement reveals a Morin temperature of 125K and unique magnetic anisotropy due to the easy axis of the magnetocrystalline anisotropy perpendicular to the nanowire axis.

Magnetodielectric coupling in core/shell BaTiO3∕γ-Fe2O3 nanoparticles

We report an intriguing magnetodielectric coupling in BaTiO3∕γ-Fe2O3 dielectric core/ferrimagnetic shell nanoparticles. The dielectric constant steeply increases with magnetic field, and the frequency dependent magnetodielectric curve shows a resonancelike peak at high temperatures, while it decreases smoothly with field and no peak appears in the frequency dependent magnetodielectric curve at low temperatures. We attribute the observed magnetodielectric coupling to the Maxwell-Wagner effect combined with magnetoresistance at high temperatures and to possible spin-lattice coupling and its modification near interfaces at low temperatures.

Transport and magnetic properties of Cr-, Fe-, Cu-doped topological insulators

We report a new class of three-dimensional topological insulators of transition metal doped Bi2Se3, TrxBi2Se3, and Bi2-xTrxSe3 (Tr = Cr, Fe, Cu) with x = 0.15, for the intercalation and substitution cases. With varying the doping atoms, a great variety of properties are observed in a single crystal form. The Cu-intercalated crystal shows superconducting behavior below 3 K, where the diamagnetic signal is found. The increase of carrier density at low temperature is likely to be responsible for the superconductivity. The magnetically doped case of the Fe-substituted Bi2Se3 exhibits dominance of ferromagnetic interactions, whereas the Cr-substituted Bi2Se3 favors antiferromagnetic interactions. From these results, we learn that the peculiar transport and magnetic properties of topological insulator Bi2Se3 are easily tunable by chemical doping elements, which change the Fermi energy and manipulate the charge carriers.

Synchronous precessional motion of multiple domain walls in a ferromagnetic nanowire by perpendicular field pulses

Magnetic storage and logic devices based on magnetic domain wall motion rely on the precise and synchronous displacement of multiple domain walls. The conventional approach using magnetic fields does not allow for the synchronous motion of multiple domains. As an alternative method, synchronous current-induced domain wall motion was studied, but the required high-current densities prevent widespread use in devices. Here we demonstrate a radically different approach: we use out-of-plane magnetic field pulses to move in-plane domains, thus combining field-induced magnetization dynamics with the ability to move neighbouring domain walls in the same direction. Micromagnetic simulations suggest that synchronous permanent displacement of multiple magnetic walls can be achieved by using transverse domain walls with identical chirality combined with regular pinning sites and an asymmetric pulse. By performing scanning transmission X-ray microscopy, we are able to experimentally demonstrate in-plane magnetized domain wall motion due to out-of-plane magnetic field pulses.

Magnetodielectric effect in BaTiO3−LaMnO3 composites

The structural, dielectric, magnetic, and magnetodielectric properties were systematically investigated for (1−x)BaTiO3−(x)LaMnO3 composites in wide temperatures, frequencies, and mole percent (x). Dielectric anomaly around 410 K and magnetic hysteresis at 293 K were clearly observed, which might imply the coexistence of ferroelectricity and ferromagnetism at room temperature. The values of magnetodielectric effect were well scaled with M2, i.e., ([e(H)−e(0)]/e(0))∼M2, and were increased with x. The observed magnetodielectric effect were discussed in conjunction with the formation of a magnetoresistive (La,Ba)MnO3 phase.

Magnetic properties of gapless semiconductors: PbPdO2 and PbPd0.9Co0.1O2

PbPdO2 is a new class of gapless semiconductors, which is extremely sensitive to external influences such as temperature, magnetic field, and carrier doping, because of their peculiar band structure. With varying temperature, a broad transition from a high-temperature metallic behavior to a low-temperature insulating behavior was observed at TMI=100 K in the electrical resistivity, which is related to the thermally assisted excitation near the Fermi level due to its gapless band structure. By doping 10% Co for Pd in PbPdO2, the number of hole charge carriers was increased by ten times, and the transition temperature was increased to TMI=150 K. When applying a magnetic field, a ferromagnetic component was found at low temperatures in the magnetization curves of both materials, in addition to diamagnetic background signals for PbPdO2 and paramagnetic background signals for PbPd0.9Co0.1O2. In the low temperature regime, the slope of magnetoresistance is negative, while it is changed into positive with a quad...

Spin orbit torques and Dzyaloshinskii-Moriya interaction in dual-interfaced Co-Ni multilayers

We study the spin orbit torque (SOT) and Dzyaloshinskii-Moriya interaction (DMI) in the dual-interfaced Co-Ni perpendicular multilayers. Through the combination of top and bottom layer materials (Pt, Ta, MgO and Cu), SOT and DMI are efficiently manipulated due to an enhancement or cancellation of the top and bottom contributions. However, SOT is found to originate mostly from the bulk of a heavy metal (HM), while DMI is more of interfacial origin. In addition, we find that the direction of the domain wall (DW) motion can be either along or against the electron flow depending on the DW tilting angle when there is a large DMI. Such an abnormal DW motion induces a large assist field required for hysteretic magnetization reversal. Our results provide insight into the role of DMI in SOT driven magnetization switching, and demonstrate the feasibility of achieving desirable SOT and DMI for spintronic devices.

Fluctuation conductivity of single-crystalline BaFe1.8Co0.2As2 in the critical region

The magnetofluctuation conductivity, called excess conductivity, originated from the forming of the superconducting droplet near to the mean-field transition temperature, was measured for the optimally doped BaFe1.8Co0.2As2 single crystals with a critical temperature, Tc, of 24.6 K. This measurement of the excess conductivity for magnetic fields up to 9 T was compared with the thermodynamic scaling theory in the critical region, in which not only the Gaussian fluctuation but also fourth order terms of the order parameter are included. An analysis of the excess conductivity showed that the superconductivity followed three-dimensional scaling rather than two-dimensional scaling even though the sample had a layered structure.

In-plane angular dependence of the spin-wave nonreciprocity of an ultrathin film with Dzyaloshinskii-Moriya interaction

The nonreciprocal propagation of spin waves in an ultrathin Pt/Co/Ni film has been measured by Brillouin light scattering. The frequency nonreciprocity, due to the interfacial Dzyaloshinskii-Moriya interaction (DMI), has a sinusoidal dependence on the in-plane angle between the magnon wavevector and the applied magnetic field. The results, which are in good agreement with analytical predictions reported earlier, yield a value of the DMI constant which is the same as that obtained previously from a study of the magnon dispersion relations. We have demonstrated that our magnon-dynamics based method can experimentally ascertain the DMI constant of multilayer thin films.