Münster 1999 – scientific programme
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M: Metallphysik
M 2: Struktur und Umwandlungen I
M 2.4: Talk
Monday, March 22, 1999, 11:00–11:15, S 8
Premartensitic phonon softening at a free Ni2MnGa single crystal surface: an x-ray study — •G. Landmesser1, U. Klemradt1, T.R. Finlayson2, R.L. Johnson3, and J. Peisl1 — 1Sektion Physik der Ludwig-Maximilians-Universit"at M"unchen — 2Department of Physics, Monash University — 3II. Institut f"ur Experimentalphysik, Universit"at Hamburg
Interest in martensitic systems showing only a weakly first-order transition with premonitory phonon mode softening has increased steadily in recent years. The Heusler alloy Ni2MnGa is a model system for this class of materials with a transition heralded by strong softening of the [ζ ζ 0 ] TA2 bulk phonon mode at ζ=1/3
[1] . For the composition investigated here, the softening precedes a transition into an intermediate phase, which is followed by the martensitic phase. Classical soft mode theory is not applicable to the bulk transition because of the incomplete phonon softening. However, it was suggested recently that the enhanced softening of associated surface phonons could initiate the phase transformation
[2] . To test experimentally whether the free surface of a crystal can act as a preferred nucleation site, we have used intense synchrotron radiation at HASYLAB to measure diffuse scattering from phonon softening. A grazing incidence geometry guaranteed tunable scattering depths ranging from nanometers to microns. A first data evaluation indicates that the surface does not simply mirror the bulk behaviour. The data will be discussed critically with regard to the role of surface nucleation. Support by DFG PE 127 and NATO Linkage Grant 951422 is gratefully acknowledged.
[1] A. Zheludev et al., Phys.Rev.B, 51(17), pp. 11310, (1995)
[2] V.V. Kokorin, Phase Transitions, 54, pp. 143-149, (1995)