Ki-Yeon Kim

Morphology Control of Surfactant-Assisted Graphene Oxide Films at the Liquid–Gas Interface

Control of a two-dimensional (2D) structure of assembled graphene oxide (GO) sheets is highly desirable for fundamental research and potential applications of graphene devices. We show that an alkylamine surfactant, i.e., octadecylamine (ODA), Langmuir monolayer can be utilized as a template for adsorbing highly hydrophilic GO sheets in an aqueous subphase at the liquid-gas interface. The densely packed 2-D monolayer of such complex films was obtained on arbitrary substrates by applying Langmuir-Schaefer or Langmuir-Blodgett technique. Morphology control of GO sheets was also achieved upon compression by tuning the amount of spread ODA molecules. We found that ODA surfactant monolayers prevent GO sheets from sliding, resulting in formation of wrinkling rather than overlapping at the liquid-gas interface during the compression. The morphology structures did not change after a graphitization procedure of chemical hydrazine reduction and thermal annealing treatments. Since morphologies of graphene films are closely correlated to the performance of graphene-based materials, the technique employed in this study can provide a route for applications requiring wrinkled graphenes, ranging from nanoelectronic devices to energy storage materials, such as supercapacitors and fuel cell electrodes.

Exchange bias and compositional depth profiles of annealed NiFe∕FeMn∕CoFe trilayers

We investigate the exchange bias fields and compositional depth profiles of the NiFe (bottom)/FeMn∕CoFe (top) trilayers after a thermal treatment at different annealing temperatures. Interestingly, the magnetic hysteresis measurement revealed that the NiFe∕FeMn∕CoFe trilayers exhibit a contrasting variation of the exchange bias fields at the two interfaces in a completely different way to each other. High angle x-ray diffraction indicates that there is no distinguishable effect of a thermal treatment on the NiFe (111) and FeMn (111) peaks. The Ni 2p and Mn 2p x-ray photoelectron spectroscopy (XPS) spectrums near these two interfaces along with the XPS compositional depth profiles are measured. We find the asymmetric depth profiles of the Fe and Mn atoms throughout the FeMn layer and the preferential Mn diffusion into the NiFe layer compared to the CoFe layer. We believe that in situ applied fields during sample growth and ex situ cooling fields after sample growth have a different effect on the exchange b...

Ferromagnetic Resonance Study of Annealed NiFe/FeMn/CoFe Trilayers

The effect of a thermal treatment on the exchange field in 19-nm NiFe(bottom)/15-nm FeMn/19-nm CoFe(top) trilayers has been investigated by employing a vibrating sample magnetometer (VSM) and a ferromagnetic resonance (FMR) spectrometer. FMR spectra as a function of applied dc field reveal that there are two distinct resonance peaks corresponding to each ferromagnetic layer. It is found that exchange fields determined from the in-plane angular dependence of the resonance field are in accord with that determined through the magnetic hysteresis loops for the NiFe/FeMn interface rather than the CoFe/FeMn one. A FMR linewidth broadening as a function of the annealing temperatures is attributed to the interdiffusion between the two magnetic interfaces across a FeMn layer.

Uncompensated spins in exchange-biased BiFeO3/γ-Fe2O3 core/shell-like thin films

We have investigated the uncompensated spins in exchange-biased BiFeO3/γ-Fe2O3 core/shell-like thin films. For both in-plane and out-of-plane directions of the films, clear exchange bias appears and coercive field asymmetrically changes with respect to the annealing temperature and the cycling of magnetic field. These results can be explained by the different characters of two types of uncompensated spins. The uncompensated spins at antiferromagnetic domain-walls (rough interface) are weakly (strongly) affected by the external magnetic field. The control of uncompensated spins at domain-walls is crucial in order to realize exchange bias at room temperature along the out-of-plane direction of BiFeO3/γ-Fe2O3 core/shell-like thin films.

Annealing Effect on Exchange Bias in NiFe/FeMn/CoFe Trilayer Thin Films

We investigated the exchange bias fields at the NiFe/FeMn and FeMn/CoFe interfaces in 18.9-nm NiFe/15.0-nm FeMn/17.6-nm CoFe trilayer thin films as the annealing temperature was varied from room temperature to 250 o C in a vacuum for 1 hour in a magnetic field of 150 Oe. Interestingly, magnetic hysteresis (M-H) measurements showed that NiFe/FeMn/CoFe trilayer thin films exhibited a completely contrasting variation of the exchange bias fields at both the NiFe/FeMn and FeMn/CoFe interfaces with annealing temperatures. Highangle X-ray diffraction (XRD) measurements indicated the absence of any discernible effect of thermal treatment on the NiFe(111) and FeMn(111) peaks. The compositional depth profile obtained from X-ray photoelectron spectroscopy (XPS) results presented the asymmetric compositional depth profiles of the Mn and Fe atoms throughout the FeMn layer. We contend that this asymmetric compositional depth profile and the preferential Mn diffusion into the NiFe layer, compared to that into the CoFe layer, are conclusive experimental evidence of the contrasting variation of the exchange bias fields at two interfaces having a common polycrystalline FeMn(111) layer.

Effects of the Addition of Permalloy Powder on the High-Frequency Magnetic Properties of Fe-Based Amorphous Powder Cores

The fabrication of Fe-based soft magnetic amorphous cores by powder metallurgical process using various consolidation techniques is discussed. In this study, FeSiB amorphous powder cores with an addition of permalloy powder were prepared by cold pressing. The effects of the permalloy powder on the high-frequency magnetic properties such as permeability, coercivity and eddy current loss were reported. According to the cross-sectional SEM micrograph of the compacted core, the permalloy powders were plastically deformed by cold pressing and this can be helpful for compaction of amorphous powder cores.

Oscillatory magnetic anisotropy in Fe/Cr/Fe trilayers

We report our experimental finding that there exists an oscillation of magnetic anisotropy with varying Cr sublayer thickness along the direction normal to the easy axis in Fe/Cr/Fe trilayers, where the easy axis is induced along a certain direction in the film plane by applying an external field during film deposition. Interestingly enough, the squareness of the hysteresis loop measured along the direction transverse to the easy axis exhibits an oscillatory behavior with a period of 5–6 A Cr sublayer thickness. We also witness that the magnetic anisotropy measured by a torque magnetometer shows an oscillating behavior with the same periodicity. Domain evolution pattern investigated using a time-resolved magneto-optical microscope reveals that domain configuration correspondingly oscillates between simple 180° type and complex non-180° type with varying Cr thickness.

Azimuthal angular dependent hysteresis loops of Fe50Mn50/Ni81Fe19 bilayers grown under a magnetic field

The azimuthal angular dependence of the vectorial hysteresis loops in the Fe50Mn50(AF)/Ni81Fe19(F) bilayer grown under a magnetic field was investigated using a combination of vectorial magneto-optic Kerr effect and model calculation. From a comparison of the experimental and calculation results, it is found that the AF easy axis is not parallel with but rotated by about 20° away from the applied magnetic field during the sample growth. Moreover, the transverse loop at the AF easy axis does not vanish but displays an open full circle (i.e., magnetization changes sign between decreasing and increasing field branches for the full hysteresis measurement). Our model calculation reveals that they are reminiscent of the non-collinear uniaxial and unidirectional anisotropies. Specifically, the angular dependence of the transverse hysteresis is well reproduced with our model calculation taking non-collinear magnetic anisotropies into account. Coercivity determined from the longitudinal loops, on the other hand, is found to be nonzero and comparatively large at all azimuthal angles. This is in stark contrast with previous results regarding FeMn/NiFe bilayers field-cooled after sample growth. Neither domain wall nor incoherent magnetic rotation in the F layer is likely to be responsible for this coercivity discrepancy between theory and experiments. Apart from the uniaxial F and unidirectional AF-F anisotropies, we suggest that the F rotatable anisotropy equivalent of 40% to 60% of the interfacial coupling energy should be taken into account to properly address the coercivity enhancement in the FeMn/NiFe bilayer grown under a magnetic field.