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MgO Overlayer Thickness Dependence of Perpendicular Magnetic Anisotropy in CoFeB Thin Films
D. D. Lam,F. Bonell,S. Miwa,Y. Shiota,K. Yakushiji,H. Kubota,T. Nozaki,A. Fukushima,S. Yuasa,Y. Suzuki 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.10
The dependence of the magnetization and perpendicular magnetic anisotropy (PMA) of Ta/CoFeB/MgO/Ta on the thicknesses of the CoFeB layer and the MgO overlayer was investigated by vibrating sample magnetometer. The magnetization is found to be small for samples with a thin MgO overlayer. The PMA strongly depends on the MgO overlayer (hereafter, MgO) thickness and its maximum value of 1.74 erg/cm2 is achieved for a 1.0 nm thick MgO overlayer with annealing at 300 ◦C. The volume anisotropy of the CoFeB layer is found to be independent of the MgO thickness,which suggest that only the interface anisotropy depends on the MgO thickness. The possible mechanisms that may influence the interface magnetization and anisotropy are discussed herein.
D. D. Lam,F. Bonell,S. Miwa,Y. Shiota,K. Yakushiji,H. Kubota,T. Nozaki,A. Fukushima,S. Yuasa,Y. Suzuki 한국자기학회 2013 Journal of Magnetics Vol.18 No.1
The perpendicular magnetic anisotropy of sputtered CoFeB thin films covered by MgO was investigated by vibrating sample magnetometry. Three different CoxFe80-xB₂? alloys were studied. Under out-of plane magnetic field, the saturation field was found to increase with increasing the Co content. The magnetization and interface anisotropy energy were obtained for all samples. Both showed a marked dependence on the MgO overlayer thickness. In addition, their variations were found to be non-monotonous as a function of the Co concentration.
BUILDUP AND RELEASE OF MAGNETIC TWIST DURING THE X3.4 SOLAR FLARE OF 2006 DECEMBER 13
Inoue, S.,Shiota, D.,Yamamoto, T. T.,Pandey, V. S.,Magara, T.,Choe, G. S. IOP Publishing 2012 The Astrophysical journal Vol.760 No.1
<P>We analyze the temporal evolution of the three-dimensional magnetic structure of the flaring active region (AR) NOAA 10930 by using the nonlinear force-free fields extrapolated from the photospheric vector magnetic fields observed by the Solar Optical Telescope on board Hinode. This AR consisted mainly of two types of twisted magnetic field lines: one has a strong negative (left-handed) twist due to the counterclockwise motion of the positive sunspot and is rooted in the regions of both polarities in the sunspot at a considerable distance from the polarity inversion line (PIL). In the flare phase, dramatic magnetic reconnection occurs in those negatively twisted lines in which the absolute value of the twist is greater than a half-turn. The other type consists of both positively and negatively twisted field lines formed relatively close to the PIL between two sunspots. A strong Ca II image began to brighten in this region of mixed polarity, in which the positively twisted field lines were found to be injected within one day across the pre-existing negatively twisted region, along which strong currents were embedded. Consequently, the central region near the PIL contains a mix of differently twisted field lines and the strong currents may play a prominent role in flare onset.</P>
TWIST AND CONNECTIVITY OF MAGNETIC FIELD LINES IN THE SOLAR ACTIVE REGION NOAA 10930
Inoue, S.,Kusano, K.,Magara, T.,Shiota, D.,Yamamoto, T. T. IOP Publishing 2011 The Astrophysical journal Vol.738 No.2
<P>Twist and connectivity of magnetic field lines in the flare-productive active region NOAA 10930 are investigated in terms of the vector magnetograms observed by the Solar Optical Telescope on board the Hinode satellite and the nonlinear force-free field (NLFFF) extrapolation. First, we show that the footpoints of magnetic field lines reconstructed by the NLFFF correspond well to the conjugate pair of highly sheared flare ribbons on the Ca II images, which were observed by Hinode as an X3.4 class flare on 2006 December 13. This demonstrates that the NLFFF extrapolation may be used to analyze the magnetic field connectivity. Second, we find that the twist of magnetic field lines anchored on the flare ribbons increased as the ribbons moved away from the magnetic polarity inversion line in the early phase of the flare. This suggests that magnetic reconnection might commence from a region located below the most strongly twisted field. Third, we reveal that the magnetic flux twisted more than a half turn and gradually increased during the last one day prior to the onset of the flare, and that it quickly decreased for two hours after the flare. This is consistent with the store-and-release scenario of magnetic helicity. However, within this active region, only a small fraction of the flux was twisted by more than one full turn and the field lines that reconnected first were twisted less than one turn. These results imply that the kink mode instability could hardly occur, at least before the onset of flare. Based on our results, we discuss the trigger process of solar flares.</P>