V. V. Sikolenko

Inhomogeneous magnetic states in Fe and Cr substituted LaMnO3

Magnetization and neutron diffraction measurements have been performed on LaMn0.5Fe0.5O3+d and LaMn0.5Cr0.5O3+d perovskites. Neutron diffraction study revealed a G-type antiferromagnetic component as well as a ferromagnetic one for both stoichiometric compounds. Magnetic structure refinement for the LaMn0.5Fe0.5O3 compound was performed on the basis of the three different models: noncollinear, multiphasic and ferrimagnetic. The reliability factors obtained for the Fe doped LaMnO3 nearly coincide for the mentioned models and there are certain difficulties in preferring one of them. The magnetic moments calculated for these compounds have smaller values than the theoretically estimated ones, which can be explained by a magnetically frustrated phase within the samples. Magnetic properties can be well described using a superexchange mechanism. This model supposes Mn3+–O–Mn3+ ferromagnetic interactions when static Jahn–Teller distortions are removed, whereas Cr3+–O–Cr3+ and Fe3+–O–Fe3+ interactions are strongly antiferromagnetic.

Magnetic and structural phase transitions in La0.5Sr0.5CoO3−δ (0 ≤ δ < 0.3) cobaltites

The evolution of the crystal structure and the magnetic properties was investigated in the La0.5Sr0.5CoO(3-δ) (0 < δ < 0.3) system as a function of the oxygen deficit δ. Compounds with a low oxygen deficit (δ < 0.1) are shown to be predominantly ferromagnetic, while further increase (δ > 0.1) gradually changes the magnetic structure from ferromagnetic to G-type antiferromagnetic and causes a structural transition from rhombohedral to cubic symmetry. Resistivity and magnetoresistance at low temperature increase with increasing of oxygen vacancies. It is argued that oxygen reduction facilitates stabilization of the high spin state of Co(3+) ions. Antiferromagnetic interactions between cobalt ions in the high spin state are found to dominate in compounds with the oxygen deficit δ > 0.18.

Neutron and synchrotron X-ray powder study of copper(II) chloride complex with deuterated 1-ethyltetrazole

Abstract The structure of the copper(II) chloride complex with deuterated 1-ethyltetrazole has been investigated in the temperature range of 2–290 K using neutron and synchrotron X-ray powder diffraction. The compound was found to exhibit structural transformation at ca. 180 K, without change of space group and main structural motif. At higher temperatures, the complex reveals positional disorder of the ethyl group, whereas no disorder is observed at lower temperatures. Temperature dependence of the lattice parameters, obtained from synchrotron X-ray data, showed main lattice changes at the transformation, explained by structural features of the complex. From the magnetic measurements, the effect of the disorder on paramagnetic behaviour of the compound was found. Detailed structural data of the compound at 2 and 290 K, obtained from neutron powder diffraction data, are reported.

Neutron diffraction studies of the structure of substituted complex cobalt oxides

The effect of substitution of manganese and niobium for cobalt on the magnetic and crystal structure of doped cobaltites has been studied by the neutron diffraction method. The Pr0.5Sr0.5Co1 − xMnxO3 samples undergo, with increasing x, a series of transitions of the crystal structure from the monoclinic I2/a to orthorhombic Immm phase, and then, to the high-temperature tetragonal I4/mcm phase and low-temperature orthorhombic Fmmm phase. Undoped Pr0.5Sr0.5CoO3 is a ferromagnet, but the magnetic ordering is destroyed with increasing x. At high degrees of substitution of manganese, the A-type antiferromagnetic ordering is observed. Substitution of niobium for cobalt in La1 − xSrxCoO3 compositions leads to changes in the Co-O bond length and the unit cell volume, which is accompanied by a decrease in the ferromagnetic moment. The Co → Nb substitution prevents the formation of Co4+ and leads to the stabilization of Co3+ in a high-spin state.

Anomalous behavior of displacement correlation function and strain in lanthanum cobalt oxide analyzed both from X-ray powder diffraction and EXAFS data

A combined X-ray powder diffraction (XPD) and high-resolution extended X-ray absorption fine structure (EXAFS) at the Co and Ga K -edges study has been performed for LaCoO 3 and LaGaO 3 ceramics, the latter sample was used as a reference without spin transitions. Based on the X-ray diffraction data, we have found that isotropic atomic displacement parameters (ADP) or mean-squared displacement of the Co–O bond exhibit gradual growth below ~50 K, wherein the strain dependencies testify rapid increase below 150 K for the LaCoO 3 having rhombohedral structure. No similar features could be observed for LaGaO 3 sample. Above ~100 K the isotropic ADP of the Co–O bond indicate a gradual growth, whereas strain curves show distinct bend near the spin-state transition temperature at about 150 K. According to the EXAFS data, the correlated parallel mean squared relative displacement (MSRD || ) of Co–O and Ga–O bonds exhibit a gradual growth above 150 K; however, in the LaCoO 3 this parameter is notably bigger. It is supposed that at low temperature the cobalt ions are dominantly in low-spin (LS) state, while certain amount of Co 3+ ions located within the surface layer of the crystallines have high-spin state (HS). Temperature growth leads to a gradual transformation of the HS state of the cobalt ions into the highly-hybridized intermediate-spin (IS) state, while the cobalt ions located in the inner part of the crystallines remain LS configuration up to 150 K. Further temperature increase leads to a spin transition of the Co 3+ ions located within the crystallines from the LS state into the IS one.

Negative magnetization of Li2Ni2Mo3O12 including two spin subsystems, distorted honeycomb lattice and linear chain

We studied magnetism of a spin-1 insulating substance Li2Ni2Mo3O12. The spin system consists of distorted honeycomb lattices and linear chains of Ni2+ spins. A magnetic phase transition occurs at Tc = 8.0 K in the zero magnetic field. In low magnetic fields, the magnetization increases rapidly below Tc, decreases below 7 K and becomes negative at low temperatures. We determined the magnetic structure using neutron powder diffraction data. The honeycomb lattices and linear chains show antiferromagnetic and ferromagnetic long-range order, respectively. We discuss the origin of the negative magnetization.

Structural and magnetic properties of La0.5Ba0.5CoO3-δ cobaltites

The magnetic and structural properties of non-stoichiometric La0.5Ba0.5CoO3-δ cobaltites have been studied by neutron and synchrotron powder diffraction. The oxygen content decrease leads to transformation of the magnetic structure from ferromagnetic to G-type antiferromagnetic through the two-phase state. Both coexisting phases have a cubic crystal structure with different volume of the unit cell.

Neutron diffraction studies of the magnetic properties of Pr0.5Sr0.5Co1 − xMnxO3 solid solutions

The crystal structure and magnetic properties of a system of Pr0.5Sr0.5Co1 − xMnxO3 solid solutions were studied by neutron diffraction and magnetization measurements. It is shown that, at a low manganese concentration, the structure can be described by the I/2a monoclinic space group; with increasing substitution level x the structure becomes orthorhombic. For x > 0.9 the crystal structure is tetragonal at high temperatures and the symmetry is lowered to orthorhombic with lowering the temperature. The substitution of cobalt for manganese leads to the destruction of long-range ferromagnetic order near x ∼ 0.25. A transition from the high-temperature ferromagnetic phase to the A-type low-temperature antiferromagnetic phase is observed at x ∼ 0.93 in the temperature range 110–160 K.