30th Magnetic Materials Center Seminar
November 20, 2006, 10:30
7th floor seminar room, Sengen
Microstructure studies on sintered NdFeB magnet
W. Li
Although NdFeB magnets present excellent hard magnetic properties, the coercivity is still far from the theoretical value. The discrepancy between the experimental and the theoretical values for the nucleation field is generally attributed to the microstructure of permanent magnets. In order to increase the coercivity, numerous elements have been added into the NdFeB magnets. It has been proved that addition of Cu, Al and Dy can improve the coercivity substantially. Cu and Al can not enhance the anisotropic field of Nd2Fe14B, therefore the enhancement of coervity should be caused by the change of microstructure. In this presentation I will report our investigation in the microstructure of NdFeB magnet with Cu, Al or Dy addition by means of TEM and atom probe.

Co2Fe1-xCrxSi and Co2FeSi1-xAlx quarternary Heusler alloys: Theory and experiment
Z. Gercsi
The Heusler alloys are traditionally considered to be ideal local magnetic moment systems. This means their valence electron configuration is a good measure to calculate the overall magnetization of an alloy from this family.
Therefore ab-initio calculations using the LDA (linear density approximation) theory can successfully explain the magnetic and half metallic behaviors of most Heusler alloys.
Nevertheless, the LDA model fails to explain the 6 Bohr magneton magnetization of the Co2FeSi alloy. Interestingly, if the electron-electron Coulomb interaction (U) is also taken it account in the theory (LDA+U) using the Hubbard model, this contradiction might be resolved.
A moderate (1.9-2.3eV) exchange constant not only explains the 6 Bohr magneton found experimentally in the highly ordered L21 phase but it can also give reasonable explanation, why the experimentally found spin polarization and magnetization deteriorate with B2 and A2 type of disorders.

If this corrected model is applied on the quarternary Co2Fe1-xCrxSi and Co2FeSi1-xAlx alloys, one can explain several experimentally observed phenomena such as high TMR of the Co2FeSi0.5Al0.5 composition even at B2 disordered state (Tezuka et. al.). In this case, the ab-initio calculations predict 100% spin polarization at B2 disordered state, with Fermi level lying in the middle of the band gap.
Another important conclusion from the calculations is that the partly disordered structure is energetically more favorable than the long range ordered one in the case of Co2FeSi0.5Al0.5, which can be the reason for the lack of L21 order in the single crystal thin film and for the existence of very high density of antiphase boundaries in the bulk sample.
The role of Cr addition will also be discussed in this talk.