Hyon-Seok Song

Observation of the intrinsic Gilbert damping constant in Co/Ni multilayers independent of the stack number with perpendicular anisotropy

We investigate the intrinsic Gilbert damping constant in perpendicular magnetic anisotropy Co/Ni multilayer system by means of an all-optical method. We find that the intrinsic Gilbert damping constant does not depend on the stack number and the perpendicular magnetic anisotropy when the magnetic field is high enough. In contrast, the extrinsic Gilbert damping is strongly correlated with the inhomogeneous anisotropy distribution in the low-field regime, as observed in magneto-optical images. Intriguingly, the extrinsic Gilbert damping is consistently reduced with decreasing length scale in the measurements, providing a concrete means to determine the intrinsic Gilbert damping.

Relationship between Gilbert damping and magneto-crystalline anisotropy in a Ti-buffered Co/Ni multilayer system

The relationship between Gilbert damping and magneto-crystalline anisotropy is studied here using an all-optical method in a perpendicular Co/Ni multilayer system by varying the Ti-buffer thickness. As the Ti-buffer thickness increases, the magneto-crystalline anisotropy is enhanced. The time-resolved Kerr signal of each sample is well described by its own intrinsic Gilbert damping constant in a wide range of the external magnetic field. Interestingly, we find that Gilbert damping constants increase linearly from 0.021 to 0.036 when the magneto-crystalline anisotropy of the samples varies from 2.4 to 3.4 Merg/cm3. Such a linear relationship implies that the spin-orbit interaction is the main source of the damping process through spin-lattice relaxation in our system.

Ultrafast magnetization relaxation of L10-ordered Fe50Pt50 alloy thin film

We have investigated the ultrafast magnetization dynamics of L10-ordered Fe50Pt50 thin film by means of a time-resolved magneto-optical Kerr effect measurement. We have found a high Gilbert damping value of α∼0.26, together with a very high precession frequency of f∼85 GHz and the shortest relaxation characteristic time of τ∼6.5 ps ever reported. We believe that L10-ordered FePt film with the unique property of a very high precession frequency and the shortest relaxation time will be very useful for the realization of picosecond spin switching.

Thermoelectric Signal Enhancement by Reconciling the Spin Seebeck and Anomalous Nernst Effects in Ferromagnet/Non-magnet Multilayers

The utilization of ferromagnetic (FM) materials in thermoelectric devices allows one to have a simpler structure and/or independent control of electric and thermal conductivities, which may further remove obstacles for this technology to be realized. The thermoelectricity in FM/non-magnet (NM) heterostructures using an optical heating source is studied as a function of NM materials and a number of multilayers. It is observed that the overall thermoelectric signal in those structures which is contributed by spin Seebeck effect and anomalous Nernst effect (ANE) is enhanced by a proper selection of NM materials with a spin Hall angle that matches to the sign of the ANE. Moreover, by an increase of the number of multilayer, the thermoelectric voltage is enlarged further and the device resistance is reduced, simultaneously. The experimental observation of the improvement of thermoelectric properties may pave the way for the realization of magnetic-(or spin-) based thermoelectric devices.

Thermal spin injection and accumulation in CoFe/MgO/ n- type Ge contacts

Understanding the interplay between spin and heat is a fundamental and intriguing subject. Here we report thermal spin injection and accumulation in CoFe/MgO/n-type Ge contacts with an asymmetry of tunnel spin polarization. Using local heating of electrodes by laser beam or electrical current, the thermally-induced spin accumulation is observed for both polarities of the temperature gradient across the tunnel contact. We observe that the magnitude of thermally injected spin signal scales linearly with the power of local heating of electrodes, and its sign is reversed as we invert the temperature gradient. A large Hanle magnetothermopower (HMTP) of about 7.0% and the Seebeck spin tunneling coefficient of larger than 0.74 meV K−1 are obtained at room temperature.

Dissipative soliton dynamics in a discrete magnetic nano-dot chain

Soliton dynamics is studied in a discrete magnetic nano-dot chain by means of micromagnetic simulations together with an analytic model equation. A soliton under a dissipative system is driven by an applied field. The field-driven dissipative soliton enhances its mobility nonlinearly, as the characteristic frequency and the intrinsic Gilbert damping decrease. During the propagation, the soliton emits spin waves which act as an extrinsic damping channel. The characteristic frequency, the maximum velocity, and the localization length of the soliton are found to be proportional to the threshold field, the threshold velocity, and the initial mobility, respectively.

Gilbert damping and critical real-space trajectory of L10-ordered FePt films investigated by magnetic-field-induction and all-optical methods

The magnetization dynamics of perpendicularly magnetized FePt films is studied using both magnetic-field-induction and all-optical methods. A critically damped trajectory was observed in this system, where the precession ended within subnanoseconds after a single large oscillation. Using the Landau?Lifshitz?Gilbert (LLG) calculation with an experimental configuration, the effective anisotropy and damping constant were obtained. A damping constant of approximately 0.2 was determined after both a magnetic field and a laser pulse were used. The laser-induced real-space trajectory was well explained by the modified LLG calculation taking into account the demagnetization and time-dependent anisotropy.

Two-dimensional nanoplates of Bi2Te3 and Bi2Se3 with reduced thermal stability

Free-standing thin nanoplates of Bi2Te3 and Bi2Se3 were synthesized by solvothermal method. It was demonstrated that the thickness of the nanoplates can be controlled by introducing a controlled amount of polyvinylpyrrolidone (PVP) in the synthesis reaction. PVP bonds to the polar basal planes of hexagonal crystal structure of Bi2Te3 and Bi2Se3, and they suppress the growth (speed) of the hexagonal crystals in the c-axis direction. Highly anisotropic growth yielded the formation of 2-dimensional nanostructures of nanoplates. The plates were examined directly with transmission electron microscopy (TEM) with in-situ heating. These crystalline nanoplates with extremely high width to thickness ratios were found to exhibit much lower thermal stability compared to the bulk counterpart or the conventional nanoparticles as represented by the reduced melting temperature. The melting temperature of a nanoplate decreased by more than 100°C compared to the melting temperature of the bulk material. While it is widely ...

Intrinsic and extrinsic Gilbert damping in exchange-biased IrMn/Cu/CoFe trilayer films

We studied the intrinsic and extrinsic Gilbert damping constants measured with an all-optical method in an exchange-biased IrMn/Cu/CoFe trilayer system with various Cu-spacer layer thicknesses. To understand the effect of the exchange bias on damping, the strength of the exchange bias was controlled by the Cu-spacer layer. When the external magnetic field is high enough compared with the exchange fields, the intrinsic Gilbert damping constant is independent of the exchange bias field. On the other hand, under a low external magnetic field, the extrinsic Gilbert damping is dominant because of the magnetic inhomogeneities, and is strongly correlated with the exchange bias field.

Speed and stability of magnetic chiral motion in a chain of asymmetric thin nanodots

The speed and stability of magnetic chiral motion are numerically investigated in a chain of asymmetric thin nanodots. The chirality of the magnetization rotation in an asymmetric nanodot plays a significant role in the velocity at low critical field, and there exists a stable operating magnetic field at the intermediate level, irrespective of the arrangement of asymmetric nanodots. Additionally, with induced in-plane anisotropy, we find that the chiral motion yields more stability with a lower critical field at room temperature. We ascribe the shift of the energy barrier as a major contribution to the thermal stability, high speed, and low critical field of chiral motion.