Critical slowing down in the single crystalline relaxor SBN75

The dielectric susceptibility of strontium-barium niobate, Sr_0.75Ba_0.25Nb₂O₆, reveals strong relaxor behaviour. It is shown that the relationship between the frequency of the ac probing field and the temperature, at which the real part of the susceptibility passes through a maximum, satisfactorily follows the activated dynamic scaling law predicted for the random-field Ising model. Applicability of a power law as suggested by the dynamic scaling theory is discussed.     

Herstellung und untersuchung von holographischen gittern für den infraroten spektralbereich

The production of high efficient holographic gratings in photoresist for the spectral range between 9 μm and 11 μm is reported. Gratings with 136 mm^-1 and 171 mm^-1 were produced and tested in regard of the diffraction efficiency and of the distribution of the energy among the orders of diffraction. The 171 mm^-1-gratings showed an absolute diffraction efficiency 0.92 in the case of S-polarization in accordance with the theoretical results.     

Linear optical anisotropy and domain structures at two successive antiferromagnetic phase transitions in α-MnS

Two successive antiferromagnetic phase transitions of α-MnS are confirmed to occur at T_C1 = 152.7 K and T_C2 = 129.5 K, respectively, by microscopic observations of T domain formation and rearrangement, linear birefringence (LB), optical absorption and linear dichroism (LD) measurements. Stress experiments and X-ray data reveal a rhombohedral lattice contraction below T_C1, which switches into a trigonal elongation below T_C2. The second-order transition at T_C1 (β = 0.40) is proposed to yield the familiar fcc type-2 single axis spin order of NiO and MnO, whereas a multispin axis structure seems to be achieved via a first-order transition below T_C2. Model calculations of the exchange striction essentially agree with the observed lattice distortions, whereas neutron diffraction data are less conclusive with respect to the proposed spin structure transformation. The interdependence of LB and LD is shown via dispersion relations and a Kramers-Kronig analysis involving the d-d transitions of Mn²⁺ in the visible region.     

Memory effects in the random-field Ising antiferromagnet Fe0.9Mg0.1Cl2

In this paper we present magneto-optical studies, viz., domain imaging by Faraday contrast, on the diluted Ising antiferromagnet Fe_0.9Mg_0.1Cl₂. The metamagnetic domain structure observed within the mixed antiferro-paramagnetic phase below the tricritical temperature (T_t) in applied fields H_c1H_aH_c2 depends on time and history. Memory of position and shape of antiferromagnetic domains, which first grow from the more magnetized phase by field decreasing, is preserved even after cycling through H_a = 0 as a consequence of random-field induced nano-domain structures. The extremely slow relaxation of magnetization in the conjectured spin-glass phase, just above H_c2, is discussed as a possible origin for the memory effect in this region.     

Farbzentren und Kolloide in abschreckend kondensierten Schichten von NaJ,CaF2 und SrCl2

By a simultaneous evaporation of NaI with excess alkali or alkaline earth metal onto a cooled substrate (T _K =20–150 K) highly disordered thin films are produced, which contain electron excess centers up to a concentration of more than 10²⁰ cm^?3. A complex optical absorption band peaking at 600 and 530 nm is found. The 600 nm band is mainly due to unperturbed F centers, whereas the 530 nm band is ascribed to perturbed F centers, whose symmetry is lowered to C_3v by a nearby Frenkel defect. Another optical transition of this F* center appears at about 600 nm in the dichroic spectrum, which is induced by polarized illumination at 20 K. The transition energies of the F* center are calculated by means of a semi-continuum model. A photochemical transformation of F* and F centers into one another is possible at 20 K. AboveT _K =220 K the excess metal is forming colloidal centers. Two absorption peaks at about 600 and 400 nm, respectively, are ascribed to different colloids, which are either embedded in the crystalline material (600 nm) or only adsorbed to the crystallites (400 nm). By annealing up to 350 K a particle growth and transformation take place. Amorphous quenched films of CaF₂ and SrCl₂ with excess alkaline earth metal reveal mainly colloidal absorption (except a weak F band in CaF₂). The colloids disappear by annealing above 150 K. This is believed to be closely connected with the crystallisation of the films.