I. O. Troyanchuk

Phase transitions in La1−xCaxMnO3−x/2 manganites

The crystal structure parameters and magnetic and electrical properties of La1−xCaxMnO3−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 La0.5Ca0.5MnO2.75 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 La1−xCaxMnO3−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.

Magnetic phase diagram of the manganites Bi1−xSrxMnO3

An experimental study of the crystal structure and the magnetic and elastic properties of the manganites Bi1−xSrxMnO3 is carried out. The following phase transformations are found: ferromagnet (x<0.15)–spin glass (0.15⩽x⩽0.25)–charge-ordered antiferromagnet (0.35⩽x⩽0.8). The ferromagnetic state corresponds to ordering of the orbitals of the Mn3+ ions. It is assumed that the orbitally disordered phase is not realized in the Bi1−xSrxMnO3 system in the concentration interval 0.15⩽x⩽0.35. Samples with 0.25⩽x⩽0.8 undergo a first-order transition of the crystal structure, attributed to ordering of the Mn3+ and Mn4+ ions in the ratios 1:1 (x⩽0.6) and 1:3 (x⩾0.7). The antiferromagnetic charge-ordered and spin glass phases coexist in samples with 0.25<x<0.35, possibly because of the martensitic character of the charge order–disorder phase transformation. A hypothetical magnetic phase diagram is constructed.

Inhomogeneous magnetic states in the Nd(Mn1−xCrx)O3 system

The crystal structure and magnetic properties of the Nd(Mn−xCrx)O3 system (x≤0.85) have been studied. Substitution of chromium for manganese was shown to induce a transition from the antiferromagnetic to ferromagnetic state (x≈0.2) and a decrease in the critical temperature followed, conversely, by an increase in the Néel temperature and decay of spontaneous magnetization. At low temperatures, the magnetization was found to behave anomalously as a result of magnetic interaction between the ferromagnetic and antiferromagnetic phases. The formation of the ferromagnetic phase is attributed to destruction of cooperative static orbital ordering, while the coexistence of different magnetic phases is most probably due to internal chemical inhomogeneity of the solid solutions.

Structural and magnetic phase transformations in the BiFeO3-CaFe0.5Nb0.5O3 system

The crystal structure and magnetic properties of the Bi1 − xCaxFe1 − x/2Nbx/2O3 system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinskiĭ-Moriya interaction. In CaFe0.5Nb0.5O3, the Fe3+ and Nb5+ ions are partially ordered and the unit cell is monoclinic (space group P21/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.

Magnetic properties of the Nd0.9MnOx cation-deficient manganites

This paper reports on the results of x-ray diffraction and magnetic studies of manganites in the Nd0.9MnOx system with an oxygen content varying in the range 2.84 < x < 2.93. A sample with an oxygen content x = 2.84 undergoes a first-order phase transition at a temperature close to T = 1050 K, whereas an anomaly observed in the elastic properties for a sample with x = 2.93 indicates a phase transition near T ≈ 500 K. It is assumed that these transitions are governed by cooperative ordering of Mn eg orbitals of the same type as in stoichiometric NdMnO3. The manganite at an oxygen content x = 2.85 is an antiferromagnet with a Néel temperature TN = 85 K, whereas the magnetic properties of the manganites at x = 2.90 and 2.93 suggest that an antiferromagnetic component coexists with a ferromagnetic component. The magnetic interaction between the ferromagnetic and antiferromagnetic components manifests itself in the fact that the magnetic moment becomes opposite in direction to the external magnetic field. The properties of the samples are consistent with the hypothesis that part of the neodymium ions (up to 5%) can be substituted for manganese ions.

Magnetic structure and properties of the Nd0.6Ca0.4Mn0.5Cr0.5O3 manganite

The effect of high chromium concentrations on the charge and orbital ordering in manganites is studied using neutron diffraction and magnetic measurements. It is found that the Nd0.6Ca0.4Mn0.5Cr0.5O3 manganite exhibits a CE-type antiferromagnetic ordering with a weak ferromagnetic component below 160 K. In the Nd0.6Ca0.4Mn0.5Cr0.5O3 manganite, the Mn and Cr ions form antiferromagnetic zigzag chains that are likewise antiferromagnetically coupled with one another in the basal plane and are arranged along the b axis of the orthorhombic structure with oppositely directed spins. An applied magnetic field of 5 T does not change the magnetic structure.

Spin state of cobalt ions in NdCoO3

The crystal structure of NdCoO3 is studied by neutron diffraction. It is shown that static cooperative Jahn—Teller distortions are absent up to the temperature of transition to the metallic state. The results are discussed within the model considering the transition of cobalt ions from the low-spin to the intermediate-spin state by gradual thermal excitation without a clearly pronounced critical behavior.

Magnetic properties of anion-deficient La1−xBaxMnO3−x/2 (0≤x≤0.30) manganites

An experimental investigation is performed of the crystal structure and magnetic and electrical properties of anion-deficient compositions of La1−x3+Bax2+Mn3+O3−x2/2− (0≤x≤0.30) which do not contain manganese ions of different vacancies. It is found that all reduced samples are single-phase perovskites with O-orthorhombic (x=0, 0.05), rhombohedral (x= 0.10, 0.15, 0.20, 0.25), and cubic (x=0.30) symmetry of the unit cell. It is observed that systems of the compounds being treated experience a transition from a weakly ferromagnetic (x=0) to a nonuniform ferromagnetic (0≤x≤0.10) state. An increase in the degree of nonstoichiometry with respect to oxygen leads to the emergence of the antiferromagnetic orbitally disordered phase. For compounds with x>0.20, clearly defined properties are observed that are characteristic of cluster spin glass with the freezing temperature of magnetic moments TF∼45 K. The maximal amount of the ferromagnetic component is registered for x=0.15. All of the reduced samples are semiconductors. As the substitution level increases, the electrical resistivity (at room temperature) first decreases in magnitude (0≤x≤0.15) and then increases (x>0.15). The magnetoresistance of all reduced samples increases gradually upon transition to a magnetically ordered state and reaches its maximal value at the liquid nitrogen temperature. A hypothetical magnetic phase diagram is constructed for the system of anion-deficient compositions of La1−x3+Bax2+Mn3+O3−x2/2−(0≤x≤0.30) being treated. The investigation results contribute to understanding the nature of 180-degree indirect superexchange interactions between ions of trivalent manganese. It is assumed that the Mn3+-O-Mn3+ superexchange interactions are negative in the orbitally disordered phase in the case of pentahedral coordination of Mn3+ ions.

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

A study is performed of the crystalline structure, magnetization, and magnetotransport properties of the system La2/3Ba1/3(Mn1−xCox)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 TC. 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 (2μB 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 TC. 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.

Spin state and magnetic interaction of cobalt ions in niobium-doped cobaltites

The magnetic properties and electrical conductivity of La1−xSrxCo1−x/2Nbx/2O3 solid solutions with trivalent cobalt ions are studied. These solid solutions are found to be spin glasses with Tf ∼ 25 K. The ferromagnetic component is most pronounced in the composition with x = 0.15. The electrical conductivity decreases with increasing strontium content. The results obtained are interpreted within a model according to which cobalt ions located in the vicinity of strontium ions reside in an intermediate-spin state and the Co3+-O-Co3+ super-exchange interaction is ferromagnetic because of the local dynamic orbital correlations.