Causes of the Metamagnetism in a Disordered EuMn<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>O<Subscript>3</Subscript> Perovskite

The magnetic properties of the EuMn_0.5Co_0.5O₃ perovskite synthesized under various conditions are studied in fields up to 140 kOe. The sample synthesized at T = 1500°C is shown to exhibit a metamagnetic phase transition, which is irreversible below T = 40 K, and the sample synthesized at T = 1200°C demonstrates the field dependence of magnetization that is typical of a ferromagnet. Both samples have T_C = 123 K and approximately the same magnetization in high magnetic fields. The metamagnetism is assumed to be related to a transition from a noncollinear ferromagnetic phase to a collinear phase, and the presence of clusters with ordered Co²⁺ and Mn⁴⁺ ions leads to ferromagnetism. The noncollinear phase is formed due to the competition between positive Co²⁺–Mn⁴⁺ and negative Mn⁴⁺–Mn⁴⁺ and Co²⁺–Co²⁺ interactions, which make almost the same contributions, and to the existence of a high magnetic anisotropy.     

Oxygen-deficiency and high-pressure effects on the magnetic and crystal structures of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3−<Emphasis Type="Italic">d</Emphasis></Subscript> managanites

The magnetic and crystal structures of anion-deficient La_0.7Sr_0.3MnO_3?d manganites (d = 0.15 and 0.20) are studied by neutron diffraction in the range of high pressures 0–5 GPa and temperatures 10–300 K. It is found that a spin-glass state forms in La_0.7Sr_0.3MnO_2.85 below T _g ～ 50 K, while magnetic phase separation is observed in La_0.7Sr_0.3MnO_2.80, which is characterized by the coexistence of AFM domains of the C type with spin-glass domains. As distinct from the stoichiometric A_0.5Ba_0.5MnO₃ manganites (A = Nd, Sm), in which the high-pressure effect suppresses the spin-glass state and gives rise to ferromagnetism, the spin-glass state in La_0.7Sr_0.3MnO_2.85 is stable under pressure. The bulk modulus of La_0.7Sr_0.3MnO_2.85 is considerably smaller than that for the stoichiometric La_0.7Sr_0.3MnO₃ compound. The causes of the formation of different types of the magnetic structure in La_0.7Sr_0.3MnO_3?d (d = 0.15 and 0.20) and different high-pressure effects on the magnetic structure of stoichiometric and anion-deficient manganites are analyzed.     

Crystal structure phase separation in anion-deficient La0.70Sr0.30MnO3 − δ manganite system

The crystal structure of anion-deficient La_0.70Sr_0.30MnO_{3 ? δ} manganite powders (δ = 0, 0.10, 0.15, 0.20) has been investigated at room temperature by the neutron diffraction method using a high-resolution Fourier diffractometer. The structure data have been refined by the Rietveld method. The crystal structures of the stoichiometric La_0.70Sr_0.30MnO₃ and anion-deficient La_0.70Sr_0.30MnO_2.90 manganites are satisfactorily described by the rhombohedral space group R $\bar 3$ c. A small amount of the non-perovskite MnO phase (space group Fm $\bar 3$ m) has been found for the anion-deficient La_0.70Sr_0.30MnO_2.90 sample. It has been found that the anion-deficient La_0.70Sr_0.30MnO_2.85 sample consists of two perovskite phases described by space groups R $\bar 3$ c and I4/mcm and the MnO phase (space group Fm $\bar 3$ m). The crystal structure of the anion-deficient La_0.70Sr_0.30MnO_2.80 sample is described by one perovskite phase with space group I4/mcm. The volume fraction of the MnO phase is ?1% for all anion-deficient samples. Oxygen vacancies in the anion-deficient La_0.70Sr_0.30MnO_{3 ? δ} manganite system stimulate the structure phase separation at 293 K.     

Phase transformations in perovskites TbBaCo2−x FexO5 + γ

The transport, magnetic, and elastic properties of TbBaCo_2?x Fe_xO_{5 + γ} are investigated. It is shown that these compounds exhibit first-order metal-insulator and antiferromagnet-weak ferromagnet transitions in the orthorhombic phase (x < 0.12), while these transitions are not observed in the tetragonal phase (x > 0.12). In the concentration range corresponding to the orthorhombic phase, doping with iron stabilizes the weakly ferromagnetic phase. However, the tetragonal phase is antiferromagnetic. Oxygen vacancies are assumed to be ordered in the orthorhombic case and disordered in the tetragonal phase. An analysis of Young’s modulus, magnetostriction, and effects of pressure and substitution of the O¹⁸ oxygen isotopes for O¹⁶ indicates a weak correlation between magnetic transformations and the crystal lattice.     

Magnetic and transport properties of the charge-ordered manganite Pr2/3Ca1/3MnO3

The transport properties, ac susceptibility χ, and EPR spectra have been studied for the insulator polycrystalline manganite Pr_2/3Ca_1/3MnO₃ undergoing the orbital (O) and charge (C) orderings below T _CO ～ 250 K that lead to antiferromagnetic (AF) ordering below T _N ≈ 155 K. Above T _C ～ 110 K, the χ′(T) dependence indicates that the sample contains a phase undergoing a second-order transition from the paramagnetic to ferromagnetic state. In the vicinity of T _C , the colossal magnetoresistance effect is observed. The EPR spectrum characteristics are sensitive to the development of the O/C and AF orderings, and they show peculiarities at T _CO and T _N . Between T _CO and T _N , the temperature dependence of the g factor exhibits a characteristic point that can be related to the appearance of the electric polarization revealed in manganites of this class.     

Phase transformations and magnetotransport properties of the Pr0.5Sr0.5Co1 ? xMnxO3 system

The structural, magnetic, and magnetotransport properties of Pr_0.5Sr_0.5Co_{1 − x} Mn _x O₃ (x < 0.65) perovskites are studied by magnetization and electrical conductivity measurements in magnetic fields up to 14 T and by neutron diffraction. In the manganese concentration range x < 0.5 and T = 300 K, the crystal structure is described by monoclinic space group I2/a; at x > 0.5, it is described by orthorhombic space group Imma. When the temperature decreases, a structural transformation without changing the symmetry takes place in all compounds. This transformation is caused by an active role of the inner shells of the praseodymium ion in chemical bond formation. The substitution of manganese for cobalt breaks a long-range ferromagnetic order near x ≈ 0.25, and a metal-dielectric transition occurs at x ≈ 0.15. The negative magnetoresistance is found to be maximal near a critical manganese concentration, where a long-range magnetic order is broken; it reaches 95% in a field of 14 T at T = 10 K for x = 0.2. An unusual dielectric magnetic state with a small spontaneous magnetic moment and a sharp transition into a paramagnetic state at T > 200 K is revealed in the concentration range 0.30 ≤ x ≤ 0.65 in spite of the absence of coherent magnetic neutron scattering. A model is proposed to explain the behavior of the magnetic properties in this phase.     

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.     

Magnetic interaction in Mg, Ti, Nb doped manganites

An effect of Mn substitution with Me=Mg²⁺, Ti⁴⁺, Nb⁵⁺ in manganites has been investigated by preparing La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃ series. It was established that substitution of manganese with magnesium up to x = 0.16 leads to a collapse of a long-range ferromagnetic order whereas La_0.7Sr_0.3(Mn ^{3 +} _0.85Nb ^{5 +} _0.15)O₃ is ferromagnet with T _C = 123 K and exhibits a large magnetoresistance below Curie point despite an absence of four-valent manganese. Hypothetical magnetic phase diagrams are constructed for La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃. Our results show that Mn³⁺-O-Mn³⁺ exchange interaction is ferromagnetic in the orbitally disordered manganites as well as an increase of Mn⁴⁺ content above 50% from a total amount of manganese ions leads to formation of a spin glass state due to a competition between antiferromagnetic Mn⁴⁺-O-Mn⁴⁺ and ferromagnetic Mn³⁺-O-Mn⁴⁺(Mn³⁺) superexchange interactions. Received 24 January 2002 Published online 9 July 2002     

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.     

Low-temperature structural anomalies in Pr<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript>

The magnetic and crystal structures of the Pr_0.5Sr_0.5CoO₃ metallic ferromagnet have been studied by the neutron diffraction technique. It is demonstrated that below 150 K, the compound is mesoscopically separated into two crystalline phases with different spatial symmetries and with different directions of the magnetic anisotropy. The phase separation exists down to 1.5 K, and at temperatures below 90 K, the low-symmetry phase occupies about 80% of the sample volume. The main structural difference between the phases is the configuration of oxygen atoms around praseodymium and, to a certain extent, around cobalt. The ferromagnetic structure with the magnetic moment lying in the basal plane of the structure (μ_Co ≈ 1.7 μ _B at 1.5 K) arises at 234 K, whereas the component directed along the long axis of the unit cell appears at 130 K. The formation of the new structural phase and change in the orientation of the magnetic moment give rise to the anomalies of the physical and magnetic characteristics of this compound observed earlier at temperatures about 120 K.