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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. Adeagbo11Physics 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.

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