13th Magnetic Materials Center Seminar
April 13, 2006, 9:00 am
7th floor seminar room, Sengen
Spin Polarization of Co2FeSi Heusler Alloy
Z. Gercsi
Spintronics is a new emerging field of electronic devices, which are based on the conception of using the spin orientation of electron instead of its charge. Heusler alloys are the most promising candidates for such applications, since they are predicted to have 100% polarized electrons above the Fermi energy (see for instance in [1]). These alloys generally are X2YZ-ternary compounds with ordered L21 structure, where X and Y are transition metals and Z is a main group element.
Co2FeSi is in the focus of recent research because it exhibits the highest magnetic moment (6μB at 5K) and Curie temperature (1100K) of Heusler alloys [2]. However some studies dealing with magnetic properties and structural investigations were recently published from the present compound, spin polarization value has not yet been reported. In my speech, I will present the results of structural investigation and spin polarization measurements carried out by point contact Andrew reflection of Co2FeSi thin film. The obtained results, P˜50%, are consistent with TMR results.
[1] R. A. d. Groot, F. M. Muller, P. G. v. Engen, and K. H. J. Buschow, Phys. Rev. Lett. 50, 2024 (1983)
[2] S. Wurmehl, G. H. Fecher, H. C. Kandpal, V. Ksenofontov, C. Felser, H. J. Li, and J. Morais, Phys. Rev. B 72, 184434 (2005)

TEM characterization of Alumina nanorods and investigations on structure and magnetoresistance of metallic compounds
W. Li
In this presentation I will make a introduction to my thesis work. It can be divided into three sections:
1. TEM characterization of alumina nanorods
One-dimensional nanorods of cubic aluminum oxide (gamma-Al2O3) have been synthesized by dc arc discharging an anode of Fe and Al powders pellet. It is seen that the diameters of the gamma-Al2O3 nanorods are relatively uniform. We proposed that the growth of a gamma-Al2O3 nanorod follows the Vapor – Liquid -Solid (VLS) process. The twinned nanorods are usually enclosed by {100} and {111} facets and their growth direction is changed from <110> into <111>.Our experimental results directly show that the ordered arrangement of stacking faults in gamma-alumina leads to a transformation from gamma-alumina to delta-alumina.
2. TEM investigation of a new phase in Fe-Ge system
A new phase in the Ge rich part of the Fe-Ge system has been studied by employing the HRTEM, X-ray diffraction and EDS analysis. The new phase can be described as Fe2Ge3 with a space group of P -4c 2. It contains high density of wavy antiphase domain boundaries, which lead to the split of certain diffraction spots in the SAED patterns. The average displacement vector is determined .
3. GMR effect in MnCoSi compound
Due to its fragility, there has been no report on the electronic properties of MnCoSi compound. We synthesized MnCoSi compound with a TiNiSi structure by sintering and subsequent annealing. Magnetism and magneto resistance measurements indicate a magnetic transition from antiferromagnetic to ferromagnetic with increasing temperature. Such a transition will cause an increase of resistivity. A strong external field could lead to the phase transition at lower temperatures, which leads to a magnetoresistance larger than -10 % in the temperature range of 200-350K. The magnetoresistance can be interpreted in terms of the superzone gap.