26th Magnetic Materials Center Seminar
October 23, 2006, 10:30
7th floor seminar room, Sengen
Investigation of the strcture, magnetic property and spin polarization in bulk Co2FeSi and Co2FeAlxSi1-x
T. Nakatani
A Heusler alloy Co2FeSi shows the highest Curie temperature and magnetization and is theoretically predicted to be a half-metal. I investigated this alloy and Co2FeAl_xSi_1-x (bulk) in which Si is substitutedwith Al from a point of view structure and properties (magnetization and spin polarization).
By the substitution of Si with Al, the lattice constant of Co2FeAlxSi1-x increased linearly with increasing Al (Vegard's law). On the other hand two ordering parameters which indicate the degree of order were about constant,which showed the 10% of Co site is occupied by improper atoms (Fe, Si, Al) and 20% of Fe site is occupied by Si or Al. TEM showed the size of APB changes with the concentration of Al. Saturation magnetization of Co2FeSi at 10K is lower than theoretical value. This is due to the disordering. The highest spin polarization measured by PCAR in the series was 0.6 for Co2FeAl0.5Si0.5.
In my seminar, except for this I will introduce some off-stoichiomeric CoFeAlSi alloys from a viewpoint of degree of order and value of spin polarization and a trial of annealing condition to get higher ordering.

Engineering the band gap in Heusler alloys. & The role of bulk and tunneling spin polarization in GMR and TMR devices
A. Rajanikanth
Galanakis has theoretically predicted the half-metallicity and the effect of substitution of Al with Sn in Co2MnAl, assuming an L21 phase. In this work, we found the induction melted Co2MnAl0.5Sn0.5 alloy is in the two phase state with dendrite microstructure, as Al-rich B2 dendrite and Sn-rich L21 matrix phases. A dendrite microstructure forms due to the phase separation during solidification. Co2MnAl form B2 phase and Co2MnSn form L21 phase at equilibrium. The spin polarization P measured by point contact Andreev reflection technique were 0.56 for B2-Co2MnAl, 0.6 for L21-Co2MnSn and interestingly the two phase alloy 0.63 B2+L21-Co2MnAl0.5Sn0.5 has higher spin polarization than the single phase alloys. Our results have shown that, unlike other Co-Mn based Heusler alloys, Co2MnAl1-xSnx can sustain high spin polarization even with the presence of disordered phases, which suggests the possibility of a higher spin polarization if single phase L21 is achieved.
As a future work the role of bulk and tunneling spin polarization and the possible ways of materials engineering will be discussed. Some new proposals for the TMR and GMR will be discussed.

Half metals in spintronics
K. Inomata
Half metallic ferromagnets are key materials for spintronics, and have receiving growing interest, especially for Co-based full Heusler alloys since 2003. Half metals can provide high density MRAMs, ultrahigh density HDDs, high sensitivity sensors, semiconductor spintronics due to the spin injection into semiconductors and nonvolatile logic etc. It is hard, however, to realize the half metallic properties, especially at RT. This may be due to some causes such as site disordering of L21 structure for full-Heusler alloys and interface structures not like bulk, which are not explored sufficiently yet. Very recently we have proposed new full-Heusler alloys, Co2FeSi1-xAlx with a high spin polarization. In this seminar the latest results for TMR in tunnel junctions using Co2FeSi0.5Al0.5 electrodes ( x = 0.5 ) are introduced.