Effect of oxygen content on the magnetic state of La0.5Ca0.5MnO3−γ perovskites

The magnetization and magnetoresistance are studied in La_0.5Ca_0.5MnO_3−γ as a function of oxygen content. It is found that as the oxygen content is decreased, this compound undergoes a sequence of transitions from an antiferromagnetic to a ferromagnetic state (γ⩾0.04), from the ferromagnetic to a spin-glass state (γ ⩾0.14), and from the spin glass to an inhomogeneous ferromagnetic state (γ⩾0.25). Strongly reduced samples (γ⩾0.25) show a large magnetoresistance despite the absence of Mn³⁺–Mn⁴⁺ pairs. It is suggested that the oxygen vacancies in the strongly reduced samples (γ⩾0.25) are long-range ordered. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 583–587 (10 November 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Structural,Magnetic, and Thermodynamic Properties of Ordered and Disordered Cobaltite Gd0.1Sr0.9CoO3 – δ

Journal of Experimental and Theoretical Physics - The effect of cationic and anionic orderings on the crystal structure and magnetic properties of substituted rare-earth cobaltites Gd0.1Sr0.9CoO3...     

High-pressure Synthesis and Properties of Manganates with Ilmenite Structure

A²⁺Mn⁴⁺0₃(A  Ni, Co, Mg, Cu) manganates with ilmenite structure have been obtained by solid phase synthesis at high pressure and temperature. The synthesis conditions for single phase samples have been determined, and investigations of the magnetic, electric, magnetooptic, and resonance properties were carried out.     

Phase transitions and magnetic-transport phenomena in the system La2/3Ba1/3(Mn1−x Cox)O3

A study is performed of the crystalline structure, magnetization, and magnetotransport properties of the system La_2/3Ba_1/3(Mn_1−x Co_x)O₃ with perovskite structure. It is shown that cubic solid solutions exist over the entire range of cobalt concentrations 0⩽x⩽1. Compositions with x⩽0.2 are ferromagnets with maximum resistance near T _C . Compositions with 0.2<x<0.4 manifest properties of inhomogeneous ferromagnets. Measurements of magnetic properties indicate the absence of long-range magnetic order in compositions with 0.5⩽x⩽0.9, which are probably spin glasses. The spontaneous magnetization of cobaltate (2μ _B per formula unit) corresponds to ferromagnetic ordering of the moments of the Co³⁺ and Co⁴⁺ ions found in the intermediate spin state. It is conjectured that the magnetoresistance consists of an extrinsic and an intrinsic contribution. The first arises as a result of intergrain transport of spin-polarized charge carriers, and the second, as a result of magnetic ordering near T _C . The magnetoresistance is essentially independent of the spontaneous magnetization and decreases abruptly as the cobalt concentration is increased with a corresponding transition from long-range to short-range magnetic order. Zh. éksp. Teor. Fiz. 116, 604–610 (August 1999)     

Properties of some perovskites synthesized under pressure

Interstitial solid solutions with the perovskite – like structure were synthesized by solid phase reactions at high pressure and temperature. The synthesis conditions of single phased samples, their stability on heating in air were determined. The magnetic and electrical properties were measured.     

Crystal Structure Phase Transitions in the Perovskites A(Cu1−xMnx)Mn4O12(A = Na+, Ca2+, Y3+, La3+)

X-Ray diffraction and neutronographical studies on the crystal structure of the perovskitelike compounds A(Cu_1−xMn_x)Mn₄O₁₂ have been undertaken. It has been shown that the samples with 50% of Mn³⁺ in the octahedral sites have a cubic unit cell, while the samples with higher Mn³⁺ composition may have a monoclinic or rhombohedral unit cell. The temperature increase raises the symmetry to the cubic one. The concentration ranges with different symmetry type have been determined. The lowering of the symmetry of the unit cell is due to the cooperative Jahn-Teller effect.     

Magnetic properties of La0.70Sr0.30MnO2.85 anion-deficient manganite under hydrostatic pressure

The magnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite are studied experimentally under hydrostatic pressure. The results show that, in the whole pressure range under investigation (0–1 GPa), the sample is a spin glass with a smeared phase transition to the paramagnetic state. It is found that the spin glass state arises from the frustration of the exchange coupling of the ferromagnetic clusters embedded in the antiferromagnetic matrix. The fraction of the sample volume occupied by the ferromagnetic phase is found to be V _fer ～ 13%. Under hydrostatic pressure, the freezing temperature T _f of the magnetic moments of the ferromagnetic clusters increases at a rate of 4.30 K/GPa and the magnetic ordering temperature T _MO increases at a rate of 12.90 K/GPa. In addition, the ferromagnetic part of the sample increases by ΔV _fer ～ 5%. The enhancement of the ferromagnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite under hydrostatic pressure is explained by the redistribution of oxygen vacancies and a decrease in the unit-cell parameters.     

Phase transformations in Pr1–x Sr x CoO3

The elastic properties and crystal structure of the Pr_1?x Sr _x CoO₃ system are studied. Two types of crystal structure transitions are found. For the composition x = 0.5, the monoclinic phase transforms to a rhombohedral one in the high-temperature transition (T ≈ 310 K), while the unit cell symmetry remains monoclinic though the unit cell parameters change drastically in the low-temperature transformation (T ≈ 110 K). It is suggested that the high-temperature transition is caused by the dimensional effect, while the low-temperature transition is associated with the presence of praseodymium ions actively involved in chemical bonding.     

Crystal structure and magnetic properties of Ba-ordered manganites Ln0.70Ba0.30MnO3−δ (Ln = Pr, Nd)

The structure and magnetic properties of the Ba-ordered state in solid solutions of manganites Ln_0.70Ba_0.30MnO_3?δ (Ln = Pr, Nd) with a cation ratio Ln³⁺/Ba²⁺ ? 1 are studied experimentally. The samples are obtained by two-stage synthesis. The initial stoichiometric Ba-disordered solid solutions Ln_0.70Ba_0.30MnO₃ synthesized in air according to traditional ceramic technology are characterized by the orthorhombic (Imma, Z = 4) perovskite-like unit cell and are ferromagnets with Curie temperatures T _C ≈ 173 and ≈ 143 K for Pr and Nd, respectively. The average size <D> of a crystalline in the initial samples is 5 μm. It is found that annealing of the initial samples in a vacuum of P[O₂] = 10^?4 Pa leads to their separation into three phases: (1) the anion-deficient ordered LnBaMn₂O₅ phase described by a tetragonal (P4/mmm, Z = 2) perovskite-like unit cell, as well as the phases (2) Ln₂O₃ (P $\bar 3$ m1, Z = 1) and (3) MnO (Fm $\bar 3$ m, Z = 2). Reduction leads to the formation of a nanocomposite with an average crystallite size <D> = 100 nm. Anion-deficient Ba-ordered phases of LnBaMn₂O₅ exhibit ferrimagnetic properties with Néel temperatures T _N ≈ 113 and ≈123 K for Pr and Nd, respectively. Annealing of anion-deficient samples in air at a moderate temperature of T = 800°C does not change the average size of the nanocrystallite, but noticeably alters their phase composition. Stoichiometric nanocomposites consist of two perovskite-like phases: (1) the Ba-deficient ordered stoichiometric phase LnBaMn₂O₆, which is described by a tetragonal (P4/mmm, Z = 2) unit cell and has the Curie temperatures T _C ≈ 313 (Pr) and ≈303 K (Nd), and (2) the Ba-disordered superstoichiometric phase Ln_0.90Ba_0.10MnO_3+δ, which is described by an orthorhombic (Imma, Z = 4) unit cell and has Curie temperatures T _C ≈ 138 (Pr) and ≈123 K (Nd). The two magnetic phases of the Ba-ordered nanocomposite are exchange-coupled. For the low-temperature magnetic phase, a temperature hysteresis is observed at ΔT ≈ 22 K in a field of 10 Oe and at ΔT ≈ 5 K in a field of 1 kOe. It is shown that states with different degrees of ordering of cations in the A sublattice can be obtained employing different technological conditions of treatment. The significant changes in the magnetic properties of Ba-ordered nanocomposites are explained on the basis of chemical phase separation taking into account the effect of compression, which is a consequence of the action of chemical (cation ordering) and external (surface tension) pressures.     

Magnetic properties of multiferroics Bi1 ? xAxFeO3 ? x/2 (A = Ca, Sr, Pb, Ba)

The magnetic properties of Bi_{1 − x} A _x FeO_{3 − x/2} (A is an alkaline-earth ion) solid solutions have been studied in magnetic fields of up to 140 kOe. The ferroelectric phase (space group R3c) transforms into the nonpolar pseudocubic phase at x ≈ 0.2. It is demonstrated that the substitution of alkaline-earth ions for bismuth ions (x ≥ 0.1) leads to a complete destruction of the modulated antiferromagnetic structure and the appearance of a weakly ferromagnetic state within the R3c state. In the pseudocubic phase, the spontaneous magnetization is absent; however, the field dependence of the magnetization is nonlinear and depends on the magnetic prehistory.