17th Magnetic Materials Center Seminar
June 1, 2006, 9:00 am
7th floor seminar room, Sengen
Structure and Magnetic Property of Fe-Co and SmCo5 Alloys
Hayato Takeshita
The recording density of hard disk drives increase by more than 6orders of magnitude during the past 50 yeres.It has been achieved by simply scaling the dimentions of the bits recorded in the storage layer .However,this scaling is limited by the superparamagnetism.Therefore,two essential parameters of future recording media are a large uniaxial magnetic anisotropy energy (MAE) Ku and a large saturation magnetization Ms. Tetragonal Fe-Co alloys and SmCo5 alloy can use new recording media. The MAE of Fe-Co alloy is too low to be of interest as recording media. However, if the cubic symmetry is broken, the MAE increases by several orders of magnitude [1][2]. My research is make a tetragonal Fe-Co alloys of large Ku and Ms. On the other hand, SmCo5 alloy is well-known magnetic material with large MAE. The Cu underlayer is used for many study of Sm-Co alloy. Recently, it was found that the Ru layer can decrease the surface roughness and reduce the grain size of Cu underlayer [3]. The Ru interlayer promotes perpendicular orientation of c-axis in the SmCo5 crystallite and results in the perpendicular magnetic anisotropy. My research proposal are two plans. One is make a tetragonal Fe-Co alloys and analyze structure and magnetic property. Other one is analyze reason that Ru layer can decrease the surface roughness and reduce the grain size of Cu underlayer.
[1] Till Burkert, Lars Nordstrom, Olle Eriksson, and Olle Heinonen, Phys. Rev. Lett. 93, 027203 (2004)
[2] Gabriella Andersson, Till Burkert, Peter Warnicke, et al., Phys. Rev. Lett. 96, 037205 (2006)
[3] Issamu Kato, Shigeto Takei, Xiaoxi Liu, and Akimitsu Morisako, IEEE, in print

Analysis of MgO barrier and interface of ferromagnetic and barrier layers
Nakatani Tomoya
Ferromagnetic tunneling junctions (MTJs) composed of ferromagnetic / insulator barrier / ferromagnetic stacking structures show high magetoresistance called tunneling magnetoresistance (TMR), which is being studied energetically due to its potential of a innovative memory material. These days, it was reported that MTJs using MgO single crystal barriers show high TMR using MBEs [1], [2]. In these two reports and other following researches of MTJs using MgO barrier, the barrier was fabricated from MgO sources or targets, not by oxidation of Mg metal film. In case of using oxidized MgO in chamber, only a little TMR has reported [3]. On the other hand, Al2O3 barrier is fabricated by oxidation using by plasma or radical methods and so on, in order to obtain more stoichiometric and impurity-free barrier layers in comparison with those fabricated by RF sputtering of Al2O3 target. As the reason, there are two possibilities [4], one is that cracks or pinholes are formed in barrier layer because of a difference between density of Mg and that of MgO, and another is that oxidation is not developed below the surface of Mg. However, it has not been cleared up to now. In order to solve this problem, it is important to understand the reason and mechanism first. As my research proposal, I will talk about a probability of a research to analyze this problem using by Laser Assisted Atom Probe and element mapping by STEM-EELS with comparing to the electric properties, that is, TMR and spin polarization. I think this analysis may be significant to know not only the problem of oxidation of Mg but also a nature of the interface of ferromagnetic and insulator layers, which is said to play an important role to determine the value of TMR.
[1] S. Yuasa et al., Nature Mater. 3, 868, 2004
[2] S.S. Parkin et al., Nature Mater. 3, 862, 2004
[3] L. Gabillet et al., JMMM 272-276, 2004, e1525
[4] K. Tsunekawa et al., J. Mag. Soc. Jpn. Vol. 29, No.9, p.856, 2005 (in Japanese)

Reserch Proposals of Metallic glasses and Laser Pulse Atom Probe
Y.Chen
The presentation is divided into 2 parts. 1, Metallic glasses : BMGs is formed by supercooling the liquid state of certain metallic alloys and have potentially superior mechanical properties to crystalline materials. The generation mechanism of shear band is the main strengthening mechanism under the indentation pressure. In many cases the evidence of liquid droplets was visibled on the fractured surface, reflecting that higher energy during fracture contributed to local melting. And a drastic reduction of Tg induced by shear stress in bulk metallic glass was obtained. 2, Laser Pulse Atom Probe: Nanosecond laser pulses atom probe was designed by G.L.Kellogg and T.T.Tsong in 1980’s. Up to the present LA TAP has undergone many improvements making it a well established nanoanalysis tool in materials science. Different attempts have been made in order to overcome atom probe drawbacks. The main concern was to extend the field of technique to poor conductive materials. B. Gault and co-worker design a femtosecond LA TAP. I compare it with the nanosecond LPAP for informative.