Hyeok-Cheol Choi

Ratchet effect of the domain wall by asymmetric magnetostatic potentials

We investigate a ratchet effect of magnetic domain wall motion in a ferromagnetic nanowire under AC magnetic fields using micromagnetic simulation. The ratchet effect for a transverse domain wall is achieved using an asymmetric magnetic potential generated by stray fields from non-contact trapezoidal ferromagnetic stubs near the straight nanowire. The ratchet phenomenon has been examined with various combinations of amplitude and frequency of the driving AC field. Interestingly, we find that the domain wall propagates along a preferential direction by the diode-like ratchet effect under AC field. The propagation of the domain wall strongly depends on the profile of the asymmetrical magnetic potentials and the driving AC field characteristics.

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.

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.

Condition of the ratchet effect of a magnetic domain wall motion under an asymmetric potential energy

We have investigated the ratchet effect of magnetic domain wall (DW) motion in a straight ferromagnetic nanowire under ac magnetic field by means of micromagnetic simulation. A structure-stable DW ratchet effect along the ferromagnetic nanowire is observed utilizing an asymmetric potential produced by a nonuniform magnetostatic stray field from an array of a periodic non-contact trapezoidal stubs. A diode-like consecutive operation process for a transverse DW motion is examined with variation of the ac field frequency and amplitude, where the necessary conditions for the DW ratchet effect are systematically examined. We have also obtained the empirical relation between a DW velocity of the ratchet effect and the ac field frequency and amplitude.

The effect of annealing temperature and Ta layer on the electric conductivity of Au thin film deposited by the magnetron sputtering

We fabricated thin films of Au and Ta/Au with thicknesses of 30 nm and 5 nm/30nm, respectively on Si(100) or Si(111) substrates using a dc magnetron sputtering system. Grain sizes, roughness and conductivity for Au thin films are measured as a function of the annealing temperatures. We observed that the grain size of samples enlarged and the surface became rougher with increasing annealing temperature. The grain size and roughness were improved in the structure of Si/Ta/Au than Si/Au. Furthermore, the Si(100) substrate was more effective for decreasing the resistance for Ta/Au system than Si(111) substrate. We confirm that by inserting a Ta buffer layer in Si(100)/Au, surface roughness was reduced and by adjusting the annealing temperature the grain size were enlarged. Consequently, the Au thin-film has improved conductivity.

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.

Study of the Perpendicular Magnetic Anisotropy and Exchange Bias in [Pd/Co] 5 /FeMn Superlattices

We investigate the exchange bias effect in superlattice structures which are representative system of the perpendicular magnetic anisotropy. We fabricate Si//FeMn structures, and study the exchange bias variations by measuring hysteresis loop variations with thickness of FeMn layer. In order to optimize the perpendicular magnetic anisotropy, we fix the thickness of Pd with 1.1 nm and investigate the dependence of the perpendicular magnetic anisotropy on the ferromagnetic Co layer thickness. As results, we find that the biggest coercivity in 0.3 nm of Co layer without FeMn layer. The biggest exchange bias field is found for 0.3 nm of Co layer when we change the Co thickness with fixed FeMn thickness. When we vary thickness of FeMn layer, the biggest coercivity is found for 5 nm of FeMn layer. No exchange bias is observed when the FeMn layer is thinner than 3 nm, and the exchange bias field increases with FeMn layer thickness continuously up to 15 nm.