M. V. Bushinsky

Morphotropic phase boundary, weak ferromagnetism, and strong piezoelectric effect in Bi1 − xCaxFeO3 − x/2 compounds

The crystal structure and magnetic and piezoelectric properties of the Bi1 − xCaxFeO3 − x/2 system (x ≤ 0.2) are studied. The crystal-structure transformations occur in the following sequence: rhombohedral (R3c) polar phase (x ≤ 0.06) → modulated polar phase (0.07 ≤ x ≤ 0.1) → modulated antipolar phase (0.11 ≤ x ≤ 0.14). The modulated polar and antipolar phases are weakly ferromagnetic with a spontaneous magnetization of 0.25 G cm3/g (x = 0.09). In the polar weak ferromagnet with x = 0.09, a uniform piezoelec- tric response 2.5 times stronger than in the initial BiFeO3 compound is detected by the piezoelectric force microscopy.

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+.

Role of superexchange interactions in the ferromagnetism of manganites

Compound La0.7Sr0.3Mn0.85Nb0.15O3, in which manganese ions are in an oxidation state close to 3+, are studied by neutron diffraction and magnetic measurements. This compound is shown to be a ferromagnet with TC = 145 K and a magnetic moment of 3.1 μB/Mn at T = 10 K. No signs of cooperative orbital ordering are detected. When Mg2+ ions substitute for some Nb5+ ions, Mn4+ ions appear but ferromagnetism is not enhanced. An increase in the structural distortions leads to a decrease in the ferromagnetic component. The ferromagnetic state is assumed to be caused by substantial hybridization of the eg orbitals of manganese and oxygen, which increases the positive part of the superexchange interactions.

Conditions favoring the polar weak ferromagnetic state in BiFeO3-type multiferroics

The crystal structure and magnetic properties of Bi1 − xAxFeO3 − x/2 (A = Ca, Sr, Pb, Ba), Bi1 − xAx(Fe1 − xTix)O3, and Bi1 − xAx(Fe1 − x/2Nbx/2)O3 solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar phase. In the case of A = Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in the polar and nonpolar phases differ only slightly.

First-order magnetic phase transition in layered Sr3YCo4O10.5 + δ-type cobaltites

In layered Sr3YCo4O10.5 + 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.

Magnetic properties of manganites doped with gallium, iron, and chromium ions

The magnetization and the crystal structure of the La0.7Sr0.3Mn1 − xMxO3 (M = Ga, Fe, Cr; x ≤ 0.3) systems are studied. The substitution of gallium and chromium is shown to cause phase separation into antiferromagnetic and ferromagnetic phases, whereas the substitution of iron for manganese stabilizes a spinglass state. The ferromagnetic phase in the chromium-substituted compositions is much more stable than that in the case of substitution by iron ions or diamagnetic gallium ions. The magnetic properties are explained in terms of the model of superexchange interactions and the localization of most eg electrons of manganese. The stabilization of ferromagnetism in the chromium-substituted compositions can be caused by the fact that the positive and negative contributions to the superexchange interaction between Mn3+ and Cr3+ ions are close to each other but the antiferromagnetic part of the exchange is predominant. Moreover, some chromium ions are in the tetravalent state, which maintains the optimum doping conditions.

Magnetic Structure and Magnetotransport Properties of La 0.7 Sr 0.3 Mn 1 – x Ni x O 3

La0.7Sr0.3Mn1 – xNixO3 (0.12 ≤ x ≤ 0.35) compositions have been studied using neutron diffraction, magnetometry, and measurements of magnetotransport properties. At temperatures of 5–300 K, these compounds were found to have a rhombohedral crystal structure. The substitution of nickel for manganese has been shown to result in a decrease in the Curie temperature from 278 K (х = 0.12) to 60 K (х = 0.3); in this case, the spontaneous magnetization of the compositions decreases to zero (x = 0.33). The magnetoresistive effect for the semimetals with 0.12 ≤ x < 0.18 increases near the Curie temperature, whereas the magnetoresistance of semiconducting compositions with х ≥ 0.2 progressively decreases as the temperature increases. For compositions with х ≥ 0.25, an antiferromagnetic G-type component has been found by neutron diffraction, the Neel temperature of which reaches 260 K (at х = 0.35). The study of the La1–ySryMn0.65Ni0.35O3 (y ≤ 0.3) system showed that the content of ferromagnetic component decreases with increasing Sr content. It has been inferred that the antiferromagnetism of the compositions with х > 0.25 is due to the strong negative exchange interactions Ni2+–О–Ni2+ and Mn4+–О–Mn4+ and the absence of ionic order. The obtained data have been used to construct the magnetic phase diagram of the La0.7Sr0.3Mn1–xNixO3 (0.12 ≤ x ≤ 0.35) system.

Causes of the Metamagnetism in a Disordered EuMn0.5Co0.5O3 Perovskite

The magnetic properties of the EuMn0.5Co0.5O3 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 TC = 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.

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.