9th Metallic Nanostructure Group Seminar
February 23, 2006, 9:00 am
7th floor seminar room, Sengen
Microstructure and Magnetic Properties of CoW Thin Film
N. Abe
Hard disk drives (HDDs) are most important technology for data storage. Since its recording density is higher than other technique. In the near future, however, achievable data density will be restricted by fundamental physical phenomena, such as bit instability due to thermal agitation of magnetic moment. In order to solve that problem, both much larger magnetocrystalline anisotropy and enhancement of magnetic isolation between ferromagnetic grains are required. CoW system has larger magnetcrystalline anisotropy then CoCr system, and it is expected that the magnetically induced phase separation type material. The CoW system, therefore, is considered to be a promising candidate as a base alloy system for high-density recording media.

Atom probe, microstructure and spin polarization of Heusler alloys
S. Vijay Karthik
Half-metallic ferromagnetic materials, which exhibits high spin polarization, have gained immense scientific interest in the field of spintronics to realize spin-dependent phenomenon for various applications like magnetic random access memory (MRAM), hard disk drives (HDD). Recently, Galanakis et al had found the theoretical possibility of obtaining new half-metallic systems on the full-Heusler quaternary alloys such as X2Y1-xY’xZ, (X1-xX’x)2YZ and X2YZ1-xZ’x. The microstructure, magnetic properties and spin polarization of quaternary Co2Cr1-xFexAl, Co2Cr1-xVxAl and Co2Fe1-xVxAl alloys have been investigated. The ordered L21 structure becomes more stable with increasing vanadium concentration (x ≥ 0.35) explained by strain relaxation theory. On the contrary, Co2CrAl and Co2Cr1-xFexAl exhibits a two-phase region of A2 and B2/L21 structure from TEM analysis. There is no other experimental evidence in explaining the undergoing mechanism of the observed phase separation. 3D Atom probe analysis of Co2CrAl alloy was carried out and it was observed to have Co-Al rich matrix (B2 structure) and Cr-rich phase (A2 structure) thereby confirming the presence of spinodal decomposition. The spin polarization (P) of quaternary alloys was measured by Point Contact Andreev Reflection (PCAR) technique. For Co2Cr1-xVxAl, the spin polarization decreases from ~ 0.55 to ~ 0.34 with increase in the concentration of vanadium from x = 0.0 to x = 0.5 and then increases to ~ 0.50 for the composition x = 1.0. The spin polarization decreases from ~ 0.55 to ~ 0.25 with increasing concentration of Fe from x = 0.0 to 1.0 for Co2Cr1-xFexAl. Possible reasons for this reduced spin polarization of these alloys are discussed.

Spin polarization study of disordered and ordered ferromagnetic and half-metallic materials
A. Rajani Kanth
Bulk metallic glass composites are gaining much importance because of their improved properties such as high strength and improved ductility. This is achieved by in-situ formation of crystallites or ex-situ reinforcement. Recently, Kuendig et al. experimentally demonstrated the formation of two amorphous phases in a multicomponent metallic glass. This observation opens a new field of composites composed of two glassy phases that may exhibit some interesting properties. The two phase amorphous structure has been mostly observed in alloy systems containing elements with a positive heat of mixing. On the contrary, the present work focuses on the possibility of achieving two phase amorphous structure with elements having negative heat of mixing as predicted by thermodynamic models. In my presentation, I will discuss thermodynamics of several alloy systems that exhibit a miscibility gap despite having negative heat of mixing among the constituents.