Berlin 2005 – scientific programme

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MA: Magnetismus

MA 9: FV-internes Symposium "Heusler-Legierungen" (Organizers: B. Hillebrands, H. Zabel)

MA 9.7: Invited Talk

Friday, March 4, 2005, 18:15–18:45, TU EMH225

Anomalous electronic and vibrational effects in magnetic shape memory alloys — •Peter Entel¹, Alexey T. Zayak², and Waheed A. Adeagbo¹ — ¹Physics Section, University of Duisburg-Essen, Duisburg Campus, 47048 Duisburg, Germany — ²Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854-8019, USA

First-principles calculations are used to address the problem of anomalous electronic and vibrational effects in magnetic Heusler compounds. The ferromagnetic shape memory systems Ni₂Mn(Ga, Ge, Al) undergo a structural transformation from the high-temperature L2₁ structure to some low-temperature tetragonal or modulated structure. This can be related to Fermi surface nesting caused by a peculiar combination of electronic states of Ni and, for example, Ga. The resulting tetrahedral-like coordination of the transition metal ions causes also the lattice instability, where the acoustic modes tend to soften while the optical modes disperse in a way which is significantly different from that in stable ferromagnetic Co₂Mn(Ga, Ge). Critical valence electron numbers per atom can still be used to characterize the unstable and stable magnetic alloys. However, in a more detailed descripion the anharmonic behavior of the unstable magnetic systems is related to the Jahn-Teller splitting and an amplification of anharmonicity by competing ferromagnetic and antiferromagnetic interactions. We discuss the close relationship of anharmonic trends observed in ferroelectrics.