Antiferromagnet-ferromagnet phase transition in lightly doped manganites

Magnetic and structural phase diagrams of the La₀.₈₈MnOx, La₁₋xSrx(Mn₁₋x/₂Nbx/₂)O₃, Nd₁₋xCaxMnO₃, and Bi₁₋xCaxMnO₃ series constructed on the basis of x-ray, neutron powder diffraction, Young’s modulus, magnetization and resistivity measurements are presented. It is shown that the main factor controlling the antiferromagnet–ferromagnet phase transition in the manganites is a type of an orbital state. The results are discussed in the framework of structurally driven magnetic phase separation model

Electromechanical and magnetic properties of BiFeO3-LaFeO3-CaTiO3 ceramics near the rhombohedral-orthorhombic phase boundary

BiFeO3-LaFeO3-CaTiO3 ceramics have been studied by X-ray diffraction, magnetization measurements, and piezoresponse force microscopy (PFM). The compositional ranges of the polar, antipolar, and non-polar phases have been estimated. PFM measurements testify gradual decrease of piezoelectric response in Bi(0.85-x)La(0.15)CaxFe(1-x)Ti(x)O(3) system with Ca/Ti content increase, except a narrow concentration region near polar-antipolar phase boundary where piezoelectric signal shows maximum value. It is found that increase of dopant concentration leads to apparent decrease of the off-center Bi-O displacement and, consequently, causes a reduction of piezoelectric response. It is concluded that notable remanent magnetization in polar and non-polar structural phases is a result of the Dzyaloshinsky-Moria interaction. (C) 2013 AIP Publishing LL

Pressure effect on magnetic susceptibility of LaCoO3_3

The effect of pressure on magnetic properties of LaCoO$_3$ is studied experimentally and theoretically. The pressure dependence of magnetic susceptibility $\chi$ of LaCoO$_3$ is obtained by precise measurements of $\chi$ as a function of the hydrostatic pressure $P$ up to 2 kbar in the temperature range from 78 K to 300 K. A pronounced magnitude of the pressure effect is found to be negative in sign and strongly temperature dependent. The obtained experimental data are analysed by using a two-level model and DFT+U calculations of the electronic structure of LaCoO$_3$. In particular, the fixed spin moment method was employed to obtain a volume dependence of the total energy difference $\Delta$ between the low spin and the intermediate spin states of LaCoO$_3$. Analysis of the obtained experimental $\chi(P)$ dependence within the two-level model, as well as our DFT+U calculations, have revealed the anomalous large decrease in the energy difference $\Delta$ with increasing of the unit cell volume. This effect, taking into account a thermal expansion, can be responsible for the temperatures dependence of $\Delta$, predicting its vanishing near room temperature.Comment: 7 pages, 9 figure

First Principles Investigation of Ferromagnetism and Ferroelectricity in Bismuth Manganite

We present results of local spin density approximation (LSDA) pseudopotential calculations for the perovskite structure oxide, bismuth manganite (BiMnO3). The origin of the differences between bismuth manganite and other perovskite manganites is determined by first calculating total energies and band structures of the high symmetry cubic phase, then sequentially lowering the magnetic and structural symmetry. Our results indicate that covalent bonding between bismuth cations and oxygen anions stabilizes different magnetic and structural phases compared with the rare earth manganites. This is consistent with recent experimental results showing enhancement of charge ordering in doped bismuth manganite

МАГНИТНЫЕ ФАЗОВЫЕ ПРЕВРАЩЕНИЯ В ПЕРОВСКИТАХ La1–x Srx Mn0,5Ni0,5O3 (0 ≤ x ≤ 0,2)

The La1–x Srx Ni0.5Mn0.5O3 (0 ≤ x ≤ 0.2) perovskites have been studied by the methods of neutron powder diffraction and magnetization and by magnetoresistance measurement. It was shown that Ni and Mn ions are partially ordered in spite of the presence of Ni3+ ions arising from La3+ with Sr2+ substitution. The magnetic structure changes from ferromagnetic (x = 0) to antiferromagnetic (x ≥ 0.1); however, the transition temperature into the paramagnetic state is constant. Magnetoresistance in the ferromagnetic phase is large and gradually decreases with increasing temperature and Sr2+ content. The results are discussed in terms of close in value ferromagnetic and antiferromagnetic parts of Ni2+–O–Mn4+ magnetic interactions and the enforcement of 3d-O2p orbital hybridization in the magnetic field. Перовскиты La1–x Srx Ni0,5Mn0,5O3 (0 ≤ x ≤ 0,2) были исследованы методами дифракции нейтронов, измерения намагниченности и магниторезистивного эффекта. Показано, что ионы никеля и марганца частично упорядочены во всех составах, несмотря на замещение ионов La3+ на Sr2+ и повышение средней валентности ионов никеля. Магнитная структура изменяется от ферромагнитной (х = 0) к антиферромагнитной (х ≥ 0,1), однако температура перехода в парамагнитное состояние не меняется. Магнитосопротивление в ферромагнитной фазе большое и уменьшается с ростом температуры и увеличением отношения Ni3+/Ni2+. Результаты обсуждаются в модели, согласно которой ферро- магнитная и антиферромагнитная части обменных взаимодействий Ni2+–O–Mn4+ близки по величине, тогда как для Ni3+–O–Mn4+ сверхобменное взаимодействие антиферромагнитно.

Intermediate structural state in Bi1−xPrxFeO3 ceramics at the rhombohedral–orthorhombic phase boundary

Crystal structure of the Bi1−xPrxFeO3 ceramics of the compositions corresponding to the threshold concentrations separating the polar rhombohedral (R3c) and anti-polar orthorhombic (Pbam) phases has been investigated with X-ray diffraction, transmission electron microscopy and differential scanning calorimetry measurements performed in a broad temperature range. The structural study specifies the peculiarities of the temperature-driven transition into the non-polar orthorhombic (Pnma) phase depending on the structural state of the compounds at room temperature. The crystal structure analysis reveals the revival of the anti-polar orthorhombic phase upon the temperature-induced transition, thus assuming that it can be considered as the bridge phase between the polar rhombohedral and the non-polar orthorhombic phases.publishe