Papers by Keyword: Charge Ordering

Abstract: Charge ordering of nonstoichiometric LuFe₂O_4+δ is discussed. Properties of triangular bilayer are investigated via mean field theory with Coulomb interaction between sites. It is shown that low temperature state of bilayer has dipole moment essentially under any doping parameter. Correlation between sites in neighbor layers is estimated and dimer partially disordered antiferroelectric phase is extended for the case of nonstoichiometric samples. Competition of correlation effect with bilayer interaction and external electric field is discussed. Phase diagrams for doped specimen are presented. The results of investigation are used to clarify a significant surface impact.

Abstract: Magnetic properties of polycrystalline Bi_0.5Ca_0.5MnO₃ are investigated by means of magnetization and electron spin resonance (ESR). The charge ordering (CO) and antiferromagnetic (AFM) transitions are determined from the temperature dependence of magnetization and ESR parameters. It is found that weak ferromagnetic (FM) spin correlations exist in the paramagnetic (PM) state. With temperature decrease, FM spin correlations are suppressed by gradually increased AFM spin correlations in the CO state and frozen in the long range AFM state at the AFM transition temperature TN. Our study suggests that the competition between FM and AFM spin correlations is accompanied by a change of spin-orbit coupling and spin-lattice interaction.

Abstract: In Sm_0.55Sr_0.45MnO₃ single crystals, grown by the floating-zone method with the cooling in oxygen, has been observed the spontaneous generation of voltage (SGV). Its maximum reaches 60 μV and occurs in the temperature range where simultaneous decay of the CE-type antiferromagnetic order and the charge order take place in some clusters. The SGV peak becomes lower by about 45% if magnetic field 14.2 kOe applied. Also we observed the SGV in La_0.75Ba_0.25MnO₃ single crystals. Maximum value of SGV occurs in a vicinity of the Curie temperature. The SGV value seems to be almost independent from the cooling or heating rate and crystallographic direction, but in La_0.75Ba_0.25MnO₃ it is smaller than in Sm_0.55Sr_0.45MnO₃ by an order of magnitude. La_0.75Ba_0.25MnO₃ compound contains ferromagnetic clusters, in which the electrical charges are localized according to gain in the s-d exchange energy. They are distributed in the paramagnetic lattice, impoverished of an electrical charge. It is shown that SGV stems from the presence of regions with different electrical charges in both samples.

Abstract: In the present study we focused on the nickelate system, which presents stable stripe order and allow a large domain of oxygen doping. More specifically, we studied the non-stoichiometric system La2NiO4 in which the oxygen excess tends to order. ESR technique has been used due to its sensibility to microscopic magnetic fluctuations. We present a systematic study of the charge and spin fluctuations in oxygen doped nickelates to get insight into the dynamic of the system using ESR. The intrinsic resonance signal of polycrystalline La2NiO4 - values 0.09 and 0.12 have been studied, in a large temperature range (10K < T < 300 K). The = 0.12 compound has been chosen as a starting point, since neutron diffraction studies in composition very close to this value have been well characterized the stripe order in this system, showing that the holes order at around TCO = 200K, while the spins order at TS = 110K. By analogy, the ESR results on this compound allowed us to interpret the results in the other compound.

Abstract: The effect of 16О → 18О isotope substitution on the properties of Pr0.5Ca0.5MnO3 manganites doped by Cr and Ru is studied. In these compounds, chromium and ruthenium favor (i) the suppression of a charge-ordered state and (ii) the formation of a ferromagnetic metallic phase. The 16О → 18О isotope substitution leads to the growth of the charge-ordering transition temperature (TCO), and to the lowering of ferromagnetic transition temperature (TFM) accompanied by a decrease in the content of ferromagnetic phase. The difference in the behavior of the Cr- and Ru-substituted samples is analyzed.

Abstract: It was found that below the room temperature both parent Nd2/3Ca1/3MnO3 and doped (Nd0.9Y0.1)2/3Ca1/3MnO3 compounds exhibit a sequence of phase transformations: charge ordering, structural transformation of O-O type within the orthorhombic structure, and three magnetic transitions. Three different types of long-range magnetic order co-exist in (Nd0.9Y0.1)2/3Ca1/3MnO3 at low temperatures (as it was earlier found in Nd2/3Ca1/3MnO3): the antiferromagnetic orderings of PCE and DE types existing below ~110 K and ~60 K, respectively, and the ferromagnetic one of B type existing below ~42 K. Charge ordering occurs at 290 K in the doped compound. Diluting of Nd subsystem by Y in the parent perovskite has opposite effects on the temperatures of magnetic and charge orderings: the temperatures of all magnetic transformations are reduced in the doped compound by 20-30 K, while the charge ordering one increases by 80 K. A relationship between evolution of the phase transformation temperatures and crystal and electronic structures of the compound are analyzed.