A study of crystal structure, elastic, and magnetic properties of low-doped Nd 1−x Ca x MnO 3 (x 0.15) perovskites has been carried out. The ferromagnetic component is shown to increase under hole doping and, simultaneously, the temperature of the orbital order-disorder phase transition decreases. The mechanism of the concentrational transition from a weak ferromagnetic state (x = 0) to a ferromagnetic one (x > 0.15) is discussed using a two-phase model, according to which the samples consist of weak ferromagnetic and ferromagnetic phases exchange coupled at their boundary. It is found that interaction between different magnetic phases leads to spin reorientation which takes place for 0.06 x 0.1 compounds around T eff ∼ 9 K. In the temperature range from 5 to 20 K, metamagnetic behaviour is revealed for the Nd 0.92 Ca 0.08 MnO 2.98 sample. H versus T as well as T versus x magnetic phase diagrams, which are characterized by the missing of a canted phase, are proposed. The appearance of orientational transitions is explained on the basis of a magnetic analogue of the Jahn-Teller effect taking into account that the magnetic moments of Nd ions are ordered parallel to the moments of Mn ions in the ferromagnetic phase, and opposite to the direction of the weak ferromagnetic vector at T > T eff in the weak ferromagnetic phase.
We report Raman light-scattering and optical conductivity measurements on a single crystal of La1.775Sr0.225NiO4 which exhibits incommensurate charge-stripe order. The extra phonon peaks induced by stripe order can be understood in terms of the energies of phonons that occur at the charge-order wave vector Q(c). A strong Fano antiresonance for a Ni-O bond-stretching mode provides clear evidence for finite dynamical conductivity within the charge stripes.
Transmission spectra of synthetic and natural hematite (α-Fe2O3) crystals are measured at temperatures 10, 25, and 300 K in the wavelength range 500–1100 nm, and the absorption spectra are computed. Pure exciton and exciton–magnon d–d transition bands are revealed, the corresponding wavelengths at 10 K being λ0=1020 nm and λ1=965 nm respectively. The half-widths and oscillator forces are g0=84 cm−1, f0=4×10−9, g1=60 cm−1, f1=1.4×10−7 for 10 K, g0=85 cm−1, f0=5×10−9, g1=110 cm−1, f1=2.1×10−7 for 25 K. The mechanisms of band formation for weakly allowed d–d transitions in hematite are analyzed.
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