Structure defects, phase transitions, magnetic resonance and magneto-transport properties of La0.6–xEuxSr0.3Mn1.1O3–δ ceramics

Abstract

Structure and its defects, magnetic resonance and magneto-transport properties of La0.6–xEuxSr0.3Mn1.1O3–δ magnetoresistive ceramics were investigated by x-ray diffraction, thermogravimetric, resistance, magnetic, 55Mn NMR and magnetoresistance methods. It was found that isovalent substitution of lanthanum by europium A-cation of a smaller ionic radius increases the structural imperfection and leads to a symmetry change from the rhombohedrally distorted perovskite structure of R3¯c symmetry to the pseudocubic type. It was shown that the real structure contains anionic and cationic vacancies, the concentrations of which increases with the Eu concentration and the sintering temperature tann. A decrease in the temperature of the metal–semiconductor Tms and ferromagnetic–paramagnetic TC phase transitions as well as an increase in the resistivity ρ and the activation energy Ea with increasing x are due to an increase in vacancy concentration, which weakens the high-frequency electron double exchange Mn3+ ↔ O2– ↔ Mn4+. The crystal structure of the compositions x = 0 and 0.1 contains nanostructured planar clusters, causing anomalous magnetic hysteresis at T = 77 K. Broad asymmetric 55Mn NMR spectra confirm high-frequency electron double exchange Mn3+(3d4) ↔ O2–(2p6) ↔ Mn4+(3d3) and indicate inhomogeneity of the manganese environment due to the surrounding ions and vacancies. The effective local fields of the hyperfine interaction HHF at 55Mn nuclei have been calculated by decomposing asymmetric NMR spectra into three Gaussian components. The constructed experimental phase diagram of the magnetic and conducting states of the La0.6–xEuxSr0.3Mn1.1O3–δ ceramics revealed strong correlation between the composition, structural imperfection, phase state, and magnetotransport properties of rare-earth manganites.

Expansion of the multifunctionality in off-stoichiometric manganites using post-annealing and high pressure: physical and electrochemical studies

“…2,23,[25][26][27][28] In addition, a change in the content of Mn in the A-and Bsites by the substitution of other ions, creation of cation vacancies V (c) B or introduction of overstoichiometric manganese also strongly influences the magneto-transport properties of manganites. 2,23,25,26,[28][29][30][31][32] Introducing excess Mn in the A-and/ or B-positions has a number of advantages since it brings to the completeness of the B-sublattice due to the existence of V (c) B vacancies and significantly improves the magneto-resistance effect without lowering T C ; [33][34][35][36] increases the metal-insulator temperature; 28 enhances transport properties and the FM metallic state due to the appearance of Mn 2+ ions in the A-sites, which have a half-filled conduction band crossing the Fermi level, and causes the multiple DE. 23 It also increases the T C and MCE, for example, in La 0.67 Ca 0.33 Mn 1+d O 3 nanopowders with ÀDS M = 2.94 J kg À1 K À1 (5 T) at T C = 240 K for d = 0 and ÀDS M = 2.90 J kg À1 K À1 (5 T) at T C = 248 K for d = 0.05; 37 in La 0.8Àx Ag 0.2 Mn 1+x O 3 bulk with ÀDS M = 2.40 J kg À1 K À1 (1 T) at T C = 300 K for x = 0, 38 and ÀDS M = 2.46 J kg À1 K À1 (1 T) at T C = 271 K for x = 0.1, 3 and in La 0.8Àx Ag 0.2 Mn 1+x O 3 nanopowders with ÀDS M = 0.96 J kg À1 K À1 (2 T) at T C = 306 K for x = 0, 39 and ÀDS M = 2.03 J kg À1 K À1 (1 T) at T C = 308 K for x = 0.2.…”

“…23 It also increases the T C and MCE, for example, in La 0.67 Ca 0.33 Mn 1+d O 3 nanopowders with ÀDS M = 2.94 J kg À1 K À1 (5 T) at T C = 240 K for d = 0 and ÀDS M = 2.90 J kg À1 K À1 (5 T) at T C = 248 K for d = 0.05; 37 in La 0.8Àx Ag 0.2 Mn 1+x O 3 bulk with ÀDS M = 2.40 J kg À1 K À1 (1 T) at T C = 300 K for x = 0, 38 and ÀDS M = 2.46 J kg À1 K À1 (1 T) at T C = 271 K for x = 0.1, 3 and in La 0.8Àx Ag 0.2 Mn 1+x O 3 nanopowders with ÀDS M = 0.96 J kg À1 K À1 (2 T) at T C = 306 K for x = 0, 39 and ÀDS M = 2.03 J kg À1 K À1 (1 T) at T C = 308 K for x = 0.2. 22 The concentration of point defects of the vacancy type can be controlled by an annealing process 24,36 that influences the internal chemical pressure modifying structural properties and, as a result, functional properties. Additionally, as is shown, 40 the external high hydrostatic pressure affects oppositely the structural parameters of manganites relative to the chemical pressure.…”

“…For this purpose, off-stoichiometric La 0.9 Mn 1.1 O 3Àd ceramic samples have been selected and synthesized due to the following reasons: (i) high reproducibility of the samples, 3,32,38 (ii) Mn-containing perovskites are effective catalysts, 18 (iii) this offstoichiometric compound 0.9 : 1.1 : 3Àd demonstrates the nonzero magnetization in contrast to the classical antiferromagnetic (AFM) one 1 : 1 : 3, 41 (iv) they have one of the most stable perovskite crystal structure with a tolerance factor t of 0.946 closed to 1, 42 (v) the presence of cation vacancies at A-positions leads to the appearance of an additional magnetism from Mn 2+ ions, 2,23,[25][26][27][28] and (vi) the overstoichiometric manganese on B-sites improves the magneto-transport and magnetocaloric properties. 3,[33][34][35][36]40 Thus, the structural, microstructural, magnetic, magnetocaloric, and electrochemical properties of La 0.9 Mn 1.1 O 3 ceramics depending on post-annealing and/or high hydrostatic pressure have been studied systematically.…”

“…The concentration of point defects of the vacancy type can be controlled by an annealing process 24,36 that influences the internal chemical pressure modifying structural properties and, as a result, functional properties. Additionally, as is shown, 40 the external high hydrostatic pressure affects oppositely the structural parameters of manganites relative to the chemical pressure.…”