Siwar Zouari

Structural and magnetic properties of alkali-substituted praseodymium manganites Pr1−xAxMnO3 (A=Na, K)

Abstract Structural and magnetic properties of Pr 1− x A x MnO 3 (A=Na, K) are studied. Solid state reactions with alkali metal excess allowed preparation of samples with x up to 0.14 for K and 0.19 for Na. All members of this series are orthorhombic at room temperature, space group Pbnm , with orthorhombic distortion decreasing with increasing alkali metal content. Magnetic studies show a transition from antiferromagnetism in pure PrMnO 3 to canted magnetic arrangement for ∼12% Mn 4+ to ferromagnetism in the range 26–38% Mn 4+ . Curie temperatures strongly depend on substitution: they initially increase with doping level up to an optimum x ≈0.15, corresponding to 30% Mn 4+ , and seem to decrease for higher sodium doping level.

New model for the magnetic structure of the marokite-type oxide CaMn2O4

Abstract This paper reports a neutron powder diffraction (NPD) study of polycrystalline marokite-type oxide CaMn 2 O 4 as a function of temperature in the range 1.5–280 K, together with magnetization measurements up to 8 T and 800 K. The orthorhombic room-temperature structure (space group Pbcm ) is confirmed. Manganese atoms are located in distorted octahedral sites with four short and two long distances at respectively 1.90–1.95 and 2.35–2.44 A, in agreement with the Jahn–Teller character of Mn 3+ ions. Below T N =220 K, this compound orders antiferromagnetically. The magnetic unit cell requires a doubling of the crystallographic a -axis. The magnetic structure has been determined by Rietveld refinement and group theory considerations. The most probable spin arrangement is found to belong to the Γ 2 irreducible representation in the Pbca space group.

Structural and magnetic properties of the CaFe2 − xMnxO4 solid solution

Substitution of Mn for Fe in the orthorhombic compound CaFe2O4 has been achieved by solid-state reactions at 1100 °C in air up to the composition Ca(Fe1.6Mn0.4)O4. The evolution of cell and atomic parameters was determined from X-ray powder data using the Rietveld method. The results show that the presence of the Jahn–Teller ion Mn3+ distorts the Fe1 octahedral site and that Mn most probably occupies the Fe2 site. With increasing Mn content, the magnetism evolves from antiferromagnetic to ferrimagnetic, with Curie temperature (TC) values (194–209 K) well above the Neel temperature of CaFe2O4 (160 K). A correlation between the evolution of TC and the deviation of the Fe2–O–Fe2 angle from 90° is established.

Effect of Fe doping on structural and magnetic properties of Pr1.2Sr1.8Mn2O7 bilayered manganese oxides.

Abstract The structural and magnetic properties of iron doped Pr 1.2 Sr 1.8 Mn 2− x Fe x O 7 bilayered manganese oxides have been investigated. Powder X-ray diffraction characterisations at room temperature showed that all our samples are single phase and crystallize in the tetragonal symmetry with I 4/ mmm space group. Fe doping leads to a weakening of the ferromagnetic (FM) ordering at low temperature. This effect can be explained by the interruption by the Fe ions of the double-exchange interaction between the Mn ions.