Effect of magnetic ordering on the lattice dynamics of FCC Fe1-xPdx

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Abstract
Neutron inelastic scattering measurements have been carried out on two single crystals of the thermally quenched gamma phase (FCC) alloy Fe1-xPdx with x=0.28 and 0.37. The x=0.28 sample (crystal 2) exhibits a face centred cubic to face centred tetragonal (FCC-FCT) transition at Tm approximately 265K. The x=0.37 sample (crystal 1) does not undergo a structural transition. For both crystals the ( zeta zeta 0) TA1 phonon branch, which corresponds to the elastic constant (C11-C12)/2, decreases in energy around the Gamma point as the magnetisation increases. The amount of softening of the elastic constant, Delta (C11-C12)/2, of crystal 2 is much larger than that of crystal 1 and has the same temperature dependence as the square of the magnetisation, M(T)2. The temperature dependence of (C11-C12)/2 implies that this phonon softening is the cause of the FCC-FCT transition. The other branches display very little if any anomalous behaviour. A large phonon broadening has been observed in the soft phonon branch. Discussions of the broadening and the results of magnon measurements are also presented.