Magnetic ordering and magnetoresistive effect in La1−x Srx(Mn1−y Mey)O3 perovskites (Me = Nb, Mg)

Perovskites of composition La_1?x Sr_x(Mn_1?x/2Nb _x/2)O₃ and La_0.49Sr_0.51(Mn_1?y Nb_y)O₃ have been synthesized and investigated. The substitution of nonmagnetic niobium ions for manganese was shown to lead to a transition from the metallic into the insulating state due to a decrease in the number of dissimilar (different-valence) manganese atoms in the lattice. In spite of the high resistivity, the niobium-containing perovskites exhibit a large magnetoresistive effect and ferromagnetic ordering. Small additions of Nb⁵⁺ to La_0.49Sr_0.51MnO₃ stimulate the transition from the antiferromagnetic into the ferromagnetic state, whereas the substitution of Mg²⁺ for Mn stabilizes the antiferromagnetic state. It is supposed that the ferromagnetism in the insulating perovskites at hand is due to the positive superexchange of the Mn³⁺-O-Mn³⁺ type, and the magnetoresistive effect owes to the intergranular transfer of spin-polarized charge carriers and the suppression of magnetic nonuniformities by an applied magnetic field near T _C.     

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.     

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.     

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.     

Phase transitions in La1−x CaxMnO3−x/2 manganites

The crystal structure parameters and magnetic and electrical properties of La_1?x Ca_xMnO_3?x/2 reduced manganites with 0≤x≤0.5 are established. These investigations contribute to the understanding of magnetic interactions in manganites without Mn⁴⁺ ions. It is found that these manganites show a long-range antiferromagnetic order up to x=0.09 and transform into spin glasses at 0.09<x≤0.35. The compositions in the range 0.35<x≤0.5 show a strong increase in the spontaneous magnetization and critical point associated with the appearance of spontaneous magnetization and can therefore be viewed as inhomogenious ferromagnets. The magnetic and crystal structure peculiarities of La_0.5Ca_0.5MnO_2.75 are established by the neutron diffraction method. The strongly reduced samples show a large magnetoresistance below the point where the spontaneous magnetization develops. The magnetic phase diagram of La_1?x Ca_xMnO_3?x/2 is established by magnetization measurements. The magnetic behavior is interpreted assuming that the Mn³⁺-O-Mn³⁺ magnetic interaction is anisotropic (positive-negative) in the orbitally ordered phase and isotropic (positive) in the orbitally disordered phase. Introduction of the oxygen vacancies changes the magnetic interaction sign from positive to negative, thereby leading to a spin glass state in strongly reduced compounds. The results obtained reveal unusual features of strongly reduced manganites such as a large ferromagnetic component, a high magnetic ordering temperature, and a large magnetoresistance despite the absence of Mn³⁺-Mn⁴⁺ pairs. In order to explain these results, the oxygen vacancies are supposed to be ordered.     

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...     

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.