18th Magnetic Materials Center Seminar
June 5, 2006, 16:00
7th floor seminar room, Sengen
Development of new quaternary Heusler alloy systems for Spintronic applications
S. Vijay Karthik
Half-metallic ferromagnetic materials possess a gap in the minority band (viz. semiconducting), whereas exhibits metallic behavior in the majority band, in other words, the electrons at the Fermi level have 100% spin polarization. Currently, the most promising materials which exhibit high spin polarization and high Curie temperature are believed to be C1b type half-Heusler and L21 type full-Heusler alloys. Galanakis et al studied the quaternary Heusler alloys and found the possibility of obtaining new half-metallic systems. In this presentation, the microstructure, 3DAP analysis, and spin polarization of the polycrystalline bulk quaternary Co2Cr1-xFexAl, Co2Fe1-xVxAl and Co2Cr1-xVxAl alloys shall be discussed. Also, a brief overview on the possibility of obtaining high spin polarization and Curie temperature required for spintronic applications from the Heusler alloy systems such as Co2FeZ (where Z = Al, Ga, Ge, Sb, In), Co2MnSb, CoMnSb, CoFeSb and zinc-blend CrSb shall be studied.

Effect of structural disorder on Spin polarization in half-metals
A. Rajanikanth
Spin polarization of half-metals is strongly effected by the structural disorder. From the theoretical calculations by Picozzi et al, a site disorder of 8% will completely destroy the half-metallicity in Co2MnSi and Co2MnGe. In order to see the effect of structural disorder on spin-polarization, thin films of L21 and A2 structural ordering were grown on SiO2 substrate and spin polarization is measured by point contact Andreev reflection method. Using Nb as a superconducting tip a spin polarization of 58% for L21 ordered and 35% for A2 ordered thin films of Co2MnGe were obtained. Similarly a spin polarization of 58% for L21 ordered Co2MnSi and 52% for A2 ordered thin films were obtained. Our recent measurements on the spin polarization of Co2FeSi both in the bulk and thin films have shown a spin polarization of only 49%, because of the weak L21 ordering. With the substitution of Si with Al by Co2FeSi0.5Al0.5, the spin polarization value increases drastically to 61%. The possible reasons for the increase in spin polarization will be discussed.

NdFeB multilayers for highest energy product
Zsolt Gercsi
Exchange coupled hard / soft nanocomposite magnets were predicted to exhibit remarkably high maximum energy product (BH)max over 120 MGOe. In such composite structures, a magnetically soft phase with high saturation magnetization (Ms) is added to the hard magnetic one, which has lower Ms value. If the thickness of the soft phase is smaller than 2dh (dh- exchange length of hard phase), the n the Ms value of composite material can be improved, meanwhile the Hc value of hard phase is maintained. Nevertheless, it is not easy to achieve such a high energy product. So far, maximum energy products of around 50-60 MGOe were reported. The main reasons for this difference are the size of nanograins, their distribution and alignment. In order to achieve high maximum energy product, Br/Bs ratio must be maximized by introducing texture into the structure and also the particle size must be reduced into the single domain region. Another important criterion is to minimize the exchange coupling between single domain particles, which averages out the magnetocrystalline anisotropy. In this talk, my research progress on exchange coupled NdFeB / FeCo thin films prepared by dc magnetron / rf sputtering techniques will be presented.