First-order magnetic phase transition in layered Sr3YCo4O10.5 + δ-type cobaltites

JETP Letters - In layered Sr3YCo4O10.5 + δ-type cobaltites with different oxygen contents, we have observed a first order magnetic phase transition from the high-temperature...     

Successive structural phase transitions in Pr<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript> in the range 10–1120 K

The crystal and magnetic structures of the Pr_0.5Sr_0.5CoO₃ metallic ferromagnet have been studied using neutron diffraction and synchrotron radiation. Successive structural transitions with the reduction of the crystal symmetry from cubic (space group Pm3m) to rhombohedral (\(R\bar 3c\), ～800 K), orthorhombic (Imma, ～300 K) and, then, to triclinic at ～120 K are detected during cooling from 1120 K. The transition from the orthorhombic system to a phase with a lower symmetry is characterized by a sharp change in the anisotropy of the unit cell, which indicates the partial ordering of the e _g orbitals of cobalt. The accompanying change in the interatomic distances and valence angles give rise to an anomaly in the temperature dependence of the magnetic susceptibility at T ≈ 120 K. The ordered magnetic moment μ_Co ≈ 2μ_B corresponds to the assumption of the intermediate spin state of Co³⁺ ions and the mixture of low- and intermediate-spin states of Co⁴⁺ ions.     

Crystal structure and properties of Bi1 ? x Ca x FeO3 and Bi1 ? x Ca x FeO1 ? x Ti x O3 solid solutions

Systems of solid solutions Bi_{1 − x} Ca _x FeO₃ (I) and Bi_{1 − x} CaxFeO_{1 − x} Ti _x O₃ (II) have been obtained and their properties have been studied using neutronography and M?ssbauer spectroscopy; magnetization and permittivity have been measured. It is shown that an increase in the concentration of calcium ions in system I for x > 0.2 changes rhombohedral distortions of a unit cell (space group R3c) to tetragonal distortions (space group I4/mcm), while rhombohedral distortions in system II for x > 0.2 are replaced by orthorhombic distortions (space group Pnma). Analysis of the crystal structure has revealed strong anisotropy in vibrations of Bi and O ions in system I compounds. Systems I and II are characterized by G-type antiferromagnetic ordering of magnetic moments. In system II, spontaneous magnetization associated with the Dzyaloshinskii-Moriya interaction has been detected. According to M?ssbauer spectroscopy data, iron ions in compounds I and II are in the trivalent state and charge compensation occurs due to the formation of oxygen vacancies. The permittivity increases with the calcium concentration. Original Russian Text ? I.O. Troyanchuk, D.V. Karpinsky, M.V. Bushinskii, O. Prokhnenko, M. Kopcevicz, R. Szymczak, J. Pietosa, 2008, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2008, Vol. 134, No. 1, pp. 105–112.     

Magnetic phase transitions in Nd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Emphasis Type="Italic">x</Emphasis>MnO<Subscript>3</Subscript> manganites

The magnetic properties of manganites of the Nd_1?xCa_xMnO₃ system with x≤0.15 have been studied. It is shown that, in the 0.06≤x≤0.1 interval, the results can be interpreted using a model according to which the concentrational transition from a weakly ferromagnetic (WFM) state (x=0) to a ferromagnetic (FM) state (x>0.15) proceeds via a mixture of the exchange-coupled FM and WFM phases. In the vicinity of T=9 K, samples with 0.06≤x≤0.1 exhibit a spontaneous magnetic phase transition involving reorientation of the magnetization vectors of the WFM and the exchange-coupled FM phases. In the temperature interval between 5 and 20 K, a sample with the composition Nd^0.92Ca^0.08MnO^2.98 exhibits metamagnetic behavior. Magnetic phase diagrams in the H?T and T?x coordinates are presented. The appearance of the spin-reorientation transitions is explained in terms of the magnetic analog of the Jahn-Teller effect with allowance for the fact that, according to the neutron diffraction data, the magnetic moments of neodymium ions in the FM phase are parallel to the magnetic moments of manganese ions.     

Magnetic structure of the manganites heavily doped by Fe and Cr ions

Powder neutron diffraction and magnetic studies have been performed for NdMn_0.5Fe_0.5O₃ and NdMn_0.5Cr_0.5O₃ manganites. In NdMn_0.5Cr_0.5O₃, magnetic structure has been revealed consisting of ferromagnetic and G-type antiferromagnetic components as result of a 3d-ions magnetic moments ordering. Magnetic moments of Nd-ions are parallel to the ferromagnetic component. In NdMn_0.5Fe_0.5O₃ only the antiferromagnetic G-type structure has been found whereas Nd-sublattice was not ordered. In the both compounds, magnetic moments of 3d-ions are significantly less than one can expect, what is interpreted in terms of intrinsic chemical inhomogeneity. Magnetic phase diagrams have been constructed for the Nd(Mn_1−xM_x)O₃ (M=Fe, Cr) systems, interpreted assuming superexchange interactions Mn³⁺–O–Cr³⁺ to be positive, Mn³⁺–O–Fe³⁺ negative and taking into account a disordered arrangement of Mn and Cr ions in the crystal structure sublattice as well as interplay between Jahn–Teller effect and superexchange interactions.     

Pressure-induced modifications of crystal and magnetic structure of oxygen deficient La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3-d</Subscript> manganites

The crystal and magnetic structures of the oxygen deficient manganites La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) have been studied by means of powder neutron diffraction over the 0–5.2 GPa pressure and 10–290 K temperature ranges. La_0.7Sr_0.3MnO_2.85 exhibits a coexistence of rhombohedral and tetragonal (I4/mcm) crystal structures and below T_g ~ 50 K a spin glass state is formed. La_0.7Sr_0.3MnO_2.80 exhibits a tetragonal (I4/mcm) crystal structure. Below T_g ~ 50 K a phase separated magnetic state is formed, involving coexistence of C-type AFM domains with spin glass domains. In both compounds the crystal structure and magnetic states remain stable upon compression. The factors leading to the formation of different magnetic states in La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) and their specific high pressure behavior, contrasting with that of the stoichiometric A_0.5Ba_0.5MnO₃ (A = Nd, Sm) compounds showing pressure-induced suppression of the spin glass state and the appearance of the FM state, are analysed.     

Structural investigation of anion-deficient manganites La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3 − δ</Subscript>

The crystal structure of the anion-deficient manganites La_0.7Sr_0.3MnO_{3 ? δ} (δ = 0, 0.10, 0.15, 0.20) is investigated using high-resolution neutron diffraction. At room temperature, the crystal structure of the stoichiometric manganite La_0.7Sr_0.3MnO₃ and the anion-deficient manganite La_0.7Sr_0.3MnO_2.9 is satisfactorily described in rhombohedral space group R \(\bar 3\) c. The anion-deficient manganite La_0.7Sr_0.3MnO_2.85 is characterized by two perovskite phases with space groups R \(\bar 3\) c and 14/mcm. The crystal structure of the La_0.7Sr_0.3MnO_2.8 manganite corresponds to the structure of a perovskite phase with space group I4/mcm. It is established that the phase separation in the crystal structure of the La_0.7Sr_0.3MnO_{3 ? δ} manganites at a temperature of 293 K is associated with a nonuniform distribution of oxygen vacancies.     

Magnetic transformations in the Sr0.78Y0.22Co1 ? xFexO3 ? γ system with a perovskite structure

The Sr_0.78Y_0.22Co_{1 − x} Fe _x O_{3 − γ} cobaltite system is studied by neutron and X-ray diffraction and by measuring the magnetization and elastic properties. The crystal structure of the composition with x = 0 is described in terms of the monoclinic space group A2/m with the unit cell 2 a _p × 4a _p × 2 a _p , and the crystal structure of the composition with x = 0.12 is orthorhombic (space group Imma). The crystal structure of these compounds is characterized by alternating CoO₆ and CoO_4.5 layers. The magnetic structure is a G-type antiferromagnetic structure. The magnetic moments in the CoO₆ layers are significantly higher than those in the CoO_4.5 layers. In the compound with x = 0, magnetic measurements reveal a small ferromagnetic component (0.2 μ_B/Co) below T _N ≈ 350 K. Near T _N, a phase transformation occurs and lowers the crystallographic symmetry. Doping with iron ions suppresses the ferromagnetic component and sharply increases the average magnetic moments in both layers. The spontaneous magnetization is assumed to result from noncollinear magnetic moments, which can be caused by the competition of exchange interactions of different signs and magnetic anisotropy. The compounds with x = 0.5 and 1.0 are cubic (space group Pm3m) and are characterized by a G-type collinear antiferromagnetic structure. Original Russian Text ? I.O. Troyanchuk, D.V. Karpinsky, V.M. Dobryanskiĭ, A.N. Chobot, G.M. Chobot, A.P. Sazonov, 2009, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2009, Vol. 135, No. 3, pp. 490–497.     

Structural stability and magnetic properties of Bi1−xLa(Pr)xFeO3 solid solutions

In this work, X-ray diffraction data taken on Bi_1−xLa_xFeO₃ solid solutions are used to verify the following structural phase transitions: “polar rhombohedral-antipolar orthorhombic” at x≈0.16 and “commensurate-incommensurate” within the orthorhombic phase at x≈0.18. In contrast, in the Bi_1−xPr_xFeO₃ series, the polar rhombohedral phase transforms into an antipolar orthorhombic one at x≥0.13. The polar rhombohedral phase near the morphotropic phase boundary exhibits an isothermal transformation into an antipolar orthorhombic phase, though the transformation occurs much faster in the case of La-doped compounds. The incommensurate structural phase was not detected in Bi_1−xPr_xFeO₃ solid solutions. The ternary structural phase diagram is constructed for (Bi,La,Pr)FeO₃ systems. In addition, the polar rhombohedral phase exhibits a magnetic field-induced transition from the modulated antiferromagnetic state into a homogeneous weak ferromagnetic state whereas the antipolar phase is a weak ferromagnetic state in the absence of an external field.     

First-order magnetic phase transition in layered Sr<Subscript>3</Subscript>YCo<Subscript>4</Subscript>O<Subscript>10.5 + δ</Subscript>-type cobaltites

In layered Sr₃YCo₄O_{10.5 + δ}-type cobaltites with different oxygen contents, we have observed a first order magnetic phase transition from the high-temperature “ferromagnetic” state to the low-temperature antiferromagnetic state. The transition can be induced by an applied magnetic field. It is accompanied by a significant hysteresis in the magnetic field (∼10 T) and temperature (∼10 K). A decrease and an increase in the yttrium content lead to a purely “ferromagnetic” and antiferromagnetic behavior, respectively.