A phenomenological theory of the martensitic fcc-hcp transformation is proposed and applied to the illustrative example of cobalt. The fcc and hcp structures are shown to result from different ordering mechanisms from a disordered polytypic structure and to be intrinsically faulted. The three, fcc, hcp, and disordered polytype, structures are inserted in the framework of the segregation process which leads to the formation of close-packed structures from the melt. The essential features reported for the fcc-hcp transformation in cobalt are explained within the preceding model, namely, the asymmetry of the interphase region, the phonon spectrum, the ␦-shape of its specific heat anomaly, and the existence of an intermediate modulated structure. The property of the transformation enthalpy to be different on heating and cooling is related to the different degree of order of the hcp and fcc structures. The partial dislocation mechanism currently assumed for the transformation is deduced from the secondary shear strains involved at the transformation.

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The ferroelectric order and magnetic field induced effects observed in the spiral phase of MnWO4 are described theoretically. It is demonstrated explicitly that the Dzyaloshinskii-Moriya antisymmetric interactions contribute to the correlation between spins and electric dipoles in the incommensurate and commensurate ferroelectric phases of magnetic multiferroics. However, other single-site symmetric interactions are shown to be involved in the magnetoelectric process, suggesting the possible existence of an electric polarization originating from purely symmetric effects.

In a recent paper [L. Cser, G. Krexner, and Gy. Török, Europhys. Lett. 54, 747 (2001)]] the use of thermal neutrons with wavelengths close to interatomic distances in condensed matter was proposed to obtain holographic images on an atomic scale. Two experimental methods were considered which either put the radiation source inside and the detector outside the object or vice versa. The second approach, called the inside-detector concept, requires strongly neutron-absorbing isotopes acting as pointlike detectors in the sample. In the present work, we demonstrate the feasibility of this technique by recording a holographic image of a lead nuclei in a Pb(Cd) single crystal.

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