D. V. Karpinsky

Magnetic and piezoelectric properties of the Bi1 − xLaxFeO3 system near the transition from the polar to antipolar phase

The crystal structure, piezoelectric and magnetic properties of the Bi1 − xLaxFeO3 solid-solution system near the structural transition between the rhombohedral and orthorhombic phases (0.15 ≤ x ≤ 0.2) have been investigated. The regions of existence of the polar rhombohedral and orthorhombic phases have been determined, and the sequence of structural transitions as a function of the lanthanum ion concentration and temperature has been studied. The maximum piezoelectric signal is found for the solid solution with the composition x = 0.16, which has a single-phase rhombohedral structure. The relation between the type of crystal structure distortions and the increase in the magnetization upon the concentration-driven structural transition from the polar to antipolar phase has been established.

Magnetic ordering in Ln0.7Sr0.3Mn0.85Sb0.15O3 (Ln = La, Nd, Sm, Eu)

The crystal structure and magnetic properties of stoichiometric compounds Ln0.7Sr0.3Mn0.85Sb0.15O3, in which manganese ions are in a trivalent state, have been investigated. No cooperative orbital ordering has been found in the Ln = La composition, but structural parameters of the Ln = Nd composition indicate Jahn-Teller distortions. It has been shown that, as the ionic radius of lanthanide (Ln) decreases, structural distortions of the MnO6 octahedron increase and the ferromagnetic component weakens due to a decrease in hybridization of manganese and oxygen orbitals. An increase in the covalent component of the chemical bond along with the orbital disorder promotes an increase in the role of positive superexchange interactions Mn3+-O-Mn3+.

Temperature evolution of the crystal structure of Bi1 − x Pr x FeO3 solid solutions

The crystal structure of solid solutions in the Bi1 − xPrxFeO3 system near the structural transition between the rhombohedral and orthorhombic phases (0.125 ≤ x ≤ 0.15) has been studied. The structural phase transitions induced by changes in the concentration of praseodymium ions and in the temperature have been investigated using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. It has been established that the sequence of phase transformations in the crystal structure of Bi1 − xPrxFeO3 solid solutions with variations in the temperature differs significantly from the evolution of the crystal structure of the BiFeO3 compounds with the substitution of other rare-earth elements for bismuth ions. The regions of the existence of the single-phase structural state and regions of the coexistence of the structural phases have been determined in the investigation of the crystal structure of the Bi1 − xPrxFeO3 solid solutions. A three-phase structural state has been revealed for the solid solution with x = 0.125 at temperatures near 400°C. The specific features of the structural phase transitions of the compounds in the vicinity of the morphotropic phase boundary have been determined by analyzing the obtained results. It has been found that the solid solutions based on bismuth ferrite demonstrate a significant improvement in their physical properties.

Crystal structure and magnetic properties of the LaCo0.5Fe0.5O3 perovskite

The magnetic and crystal structures of the LaCo0.5Fe0.5O3 perovskite are investigated. It is established that the unit cell of this compound at room temperature is characterized by rhombohedral distortions. As the temperature decreases, the compound undergoes a structural phase transition from the rhombohedral phase to the orthorhombic phase in the temperature range 200–300 K. The LaCo0.5Fe0.5O3 perovskite has an antiferromagnetic structure with the Gz spatial orientation of the antiferromagnetic vector. The magnetic properties of the LaCo0.5Fe0.5O3 perovskite are interpreted within a model according to which the ground state of Co3+ ions is a low-spin state and the existence of the weak ferromagnetic component is associated with the exchange interactions between the Fe3+ ions.

Crystal structure and piezoelectric and magnetic properties of Bi1–xSmxFeO3 solid solutions

The crystal structure and piezoelectric and magnetic properties have been studied in Bi1‒xSmxFeO3 solid solutions with the compositions near the morphotropic boundary between rhombohedral and orthorhombic (Rh–Orh) phases. The coexistence areas of rhombohedral and antipolar orthorhombic phases, as well as the evolution of structural phases at the interface, have been established. A maximum piezoelectric signal is found for the two-phase composition with the dominating rhombohedral phase, and an increase in the piezoresponse is caused by the decreasing structural stability of the sample. The evolution of magnetic properties in Bi1–xSmxFeO3 compounds has been elucidated depending on the substitutional ion concentration. The orthorhombic phase composites are the weak ferromagnetics with the residual magnetization of ~0.2 emu/g.

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.

Investigation of a Spin Transition in a LaCoO 3 Single Crystal by the Method of X-Ray Magnetic Circular Dichroism at the Cobalt K - and L 2,3 -Edges

Spin transitions of cobalt ions in LaCoO3 single crystals have been studied by the method of X-ray magnetic circular dichroism (XMCD) at the K- and L2,3-edges of Co3+ ions. The orbital momentum of cobalt ions obtained for the K-edge at the 3d level in the region of the spin transition in the temperature range from 25 to 120 K increases by a factor of approximately 1.6, whereas the slope of the magnetization curve value in the same temperature range and magnetic field increases by a factor of more than 10. XMCD experiments at the cobalt L2,3-edges demonstrate gradual growth of the ratio of the orbital momentum to the spin one L/S from 0.48 to 0.53 in the temperature range from 60 K to 120 K.

Influence of Superexchange Interaction on the Ferromagnetic Properties of Manganites and Cobaltites

The role of superexchange interaction in the formation of the ferromagnetic state of cobaltites in the La0.5Sr0.5Co1–xMexO3 (Me = Cr, Ga, Fe) systems and manganites La0.7Sr0.3Mn0.85M0.15O3 (M–Nb, Mg) with a perovskite structure has been studied. It was found that the ferromagnetic state in cobaltites can be implemented in some compositions without the mixed-valence effect of cobalt ions. The initial compound (x = 0) is ferromagnetic (ТС = 247 K) with the saturation magnetization close to 2μB (at T = 30 K) per formular unit. It has been shown that the chemical substitution of cobalt by chromium reduces the spontaneous magnetization to 0.3μB (at х = 0.2), while the substitution of cobalt by iron ions (х = 0.2) does not alter the magnetization. The obtained data are interpreted in the model of positive superexchange interactions between cobalt and iron ions and negative ones between cobalt and chrome. It has been shown that the La0.7Sr0.3Mn0.85Nb0.15O3 composition is ferromagnetic with ТС=145K, with a magnetic moment of 3.1 μB/Mn at 10 K, and no evidence of a cooperative orbital ordering in the manganite compounds has been revealed. Partial chemical substitution of Nb5+ ions by Mg2+ ones leads to the formation of Mn4+ ions, while it does not strengthen the ferromagnetic state. The strengthening of the structural distortions reduces the ferromagnetic component. It is assumed that the ferromagnetic state is caused by a significant hybridization of eg-orbitals of the manganese and oxygen ions, which strengthens the positive component of the superexchange interactions.

Magnetic phase transformations and magnetotransport phenomena in La0.7Sr0.3Mn1 – x Co x O3 perovskite compounds

The compositions La0.7Sr0.3Mn1 – xCoxO3 (0.13 ≤ x ≤ 1) are studied by neutron diffraction, magnetometry, and measuring the magnetotransport properties. The substitution of cobalt ions for manganese ions is shown to decrease the magnetization and the Curie temperature from 270 K (x = 0.13) to 140 K (x = 0.33). As the cobalt ion content increases to x = 0.5, the Curie temperature increases to 190 K, the magnetization decreases, and the electrical resistivity increases. At x > 0.5, the temperature of transition into a paramagnetic state decreases to 68 K (x = 0.8) and then again increases to 225 K for the La0.7Sr0.3CoO3 composition. The magnetoresistive effect in the range 0.3 ≤ x ≤ 0.4 reaches 97% and decreases gradually with increasing temperature without anomalies near the Curie point. At x ≤ 0.2, the magnetoresistive effect increases near the Curie temperature. The composition at x = 0.6 is stoichiometric, and no coherent magnetic contribution to neutron scattering is detected. The magnetic properties near x ∼ 0.5 are assumed to be caused by partial ordering of Co3+ and Mn4+ ions, and the Co3+ ions can be in both low- and high-spin states. The magnetic interaction between Co3+ ions in a high-spin state and Mn4+ is predominantly ferromagnetic, and the ferromagnetic part of the exchange interactions is close to the ferromagnetic part. These data are used to plot a magnetic La0.7Sr0.3Mn1 – xCoxO3 phase diagram.

Phase transformations in multiferroics Bi1–xCaxFe1–xMnxO3

The crystal structure and the magnetic properties of multiferroics Bi1–xCaxFe1–xMnxO3 (x ≤ 0.22) have been studied. It has been found that the stoichiometric compositions undergo a crystal-structure transformation from the rhombohedral (space group R3c) polar phase (x ≤ 0.18) to the orthorhombic (space group Pnma) nonpolar phase (x ≥ 0.20) via a two-phase structural state. The polar phase is antiferromagnetic at x < 0.10 and exhibits a metamagnetic behavior. The polar (x ≥ 0.10) and nonpolar phases are weak ferromagnets at room temperature with a spontaneous magnetization close to 0.07 emu/g (x = 0.18 and 0.22). A decrease in temperature leads to the transition to a state close to an antiferromagnetic one.