Methods for controlling (by magnetic field, changing temperature, and optical pumping) the properties of nanoregions forming due to self-organization during phase separation in Eu0.8Ce0.2Mn2O5 multiferroics are studied. Such nanoregions are superlattices (semiconductor heterostructures). The dynamically equilibrium states of superlattices (their long-lived ground states) are formed during cycling under magnetic field. Such states of superlattices are electrically neutral and consist of ferromagnetic layers. A set of ferromagnetic resonances from superlattices layers make it possible to distinguish the states of superlattices. The optical pumping of various powers allows controlling both the magnetic and electrical properties of superlattices.

The effect of the rare-earth ion Er3+, which has a large orbital contribution to the magnetic moment, were studied to phase transitions and phase transformations of 2D nanoregions of phase separation in the ErMn2O5 multiferroic. These nanoregions are the semiconductor heterostructures (superlattices) and are formed due to self-organization processes in the ErMn2O5 matrix. Significant effect of Er3+ ions, the moments of which are rigidly oriented along the c axis of the crystal, on the magnetic dynamics, heat capacity and multiferroic properties of layers superlattises was found at a wide temperature range 5-300 K in ErMn2O5 multiferroics. Keywords: multiferroic, 2D phase separation nanoregions, semiconductor-heterostructures, superlattices, phase transitions, phase transformations.

The study of the magnetic dynamics of some magnetic crystals with 3d ions having different symmetries reveals phase separation domains. They turn out to be similar to phase separation domains in RMn2O5 multiferroics with charge ordering. This apparently occurs because flexoelastic magnetoelectric inhomogeneities with an increased electron density appear in domain walls of the studied crystals; this electron density changes the charge composition of ions in such walls. In this case, change in the valences of 3d ions at which their charge ordering occurs becomes energetically favorable. As a result, phase separation similar to that formed in RMn2O5 multiferroics appears in the domain walls of these crystals.

The effect of rare-earth ions Er3+, which has a large orbital contribution to the magnetic moment, on the phase transitions and states of the phase separation nanoregions has been studied in multiferroic ErMn2O5. These nanoregions are semiconductor heterostructures formed due to self-organization processes in the ErMn2O5 matrix. A significant effect of Er3+ ions, the moments of which are rigidly oriented along the c axis of the crystal, on the magnetic dynamics, heat capacity, and multiferroic properties of superlattices layers in a wide temperature range of 5 divided by 300 K has been revealed.

The study of the magnetic dynamics of some magnetic crystals with 3 d ions having different symmetries reveals phase separation domains. They turn out to be similar to phase separation domains in RMn 2 O 5 multiferroics with charge ordering. This apparently occurs because flexoelastic magnetoelectric inhomogeneities with an increased electron density appear in domain walls of the studied crystals; this electron density changes the charge composition of ions in such walls. In this case, change in the valences of 3 d ions at which their charge ordering occurs becomes energetically favorable. As a result, phase separation similar to that formed in RMn 2 O 5 multiferroics appears in the domain walls of these crystals.

The effect of rare-earth ions (R = Er$_3^+$, Tb$_3^+$, and Ce$_{3.75}^+$) on the dielectric properties and the electric polarization induced by local polar phase separation domains in solid solutions of R$_{0.8}$Ce$_{0.2}$Mn$_2$O$_5$ (R = Er, Tb) multiferroics has been studied. These parameters are found to qualitatively differ from those of initial RMn$_2$O$_5$ (R = Er, Tb) crystals studied before. It is shown that the properties of the polar phase separation domains that form in a subsystem of Mn3+ and Mn4+ ions due to a finite probability of tunneling electrons between these ions with different valences are substantially dependent on the values of crystal fields in which these domains exist. A combined influence of Er$_3^+$, Tb$_3^+$, and Ce$_{3.75}^+$ ions is found to substantially change the crystal field in R$_{0.8}$Ce$_{0.2}$ Mn$_2$O$_5$ (R = Er, Tb) as compared to RMn$_2$O$_5$ (R = Er, Tb).

The effect of rare-earth ions (R = Er 3+ , Tb 3+ ) with strong spin-orbit coupling on the dielectric properties and the electric polarization induced by local polar phase separation domains in RMn 2 O 5 multiferroics has been studied. These parameters is found to be qualitatively distinguished from those studied before in GdMn 2 O 5 , in which Gd 3+ ion in the ground 8 S 7/2 state is weakly bounded with the lattice. It is shown that the properties of the polar phase separation domains, which form in the subsystem of Mn 3+ and Mn 4+ ions, are substantially dependent on the values of crystal fields, in which these domains exist.

The effect of rare-earth ions (R = Er^{3+}, Tb^{3+}, Ce^{3.75+}) on the dielectric properties and electric polarization induced by the local polar phase separation domains in solid solutions of multiferroics R_{0.8}Ce_{0.2}Mn_2O_5 (R = Er, Tb) is studied. A qualitative difference between these parameters and the previously studied in initial RMn_2O_5 crystals (R = Er, Tb) was found. It has been shown that the properties of the polar phase separation domains that arise in the subsystem of Mn^{3+} and Mn^{4+} ions due to the finite probability of tunneling of electrons between these ions substantially depend on the magnitudes of the crystal fields in which these domains are located. It was shown that the combined effect of E^{r3+}, Tb^{3+}, and Ce^{3.75+} ions significantly changes the crystal field in R_{0.8}Ce_{0.2}Mn_2O_5 (R = Er, Tb) compared to RMn_2O_5 (R = Er, Tb).

The effect of rare-earth ions (R = Er^{3+}, Tb^{3+}) with a strong spin-orbit coupling on the dielectric properties and electric polarization induced by the local polar phase separation domains in RMn_2O_5 multiferroics is studied. A qualitative difference between these parameters and those studied previously in GdMn_2O_5 was discovered, in which the Gd^{3+} ion in the ^8S_{7/2} ground state is weakly coupled to the lattice. It has been shown that the properties of the polar phase separation that arise in the subsystem of Mn^{3+} and Mn^{4+} ions due to the finite probability of electron tunneling between these ions of different valencies substantially depend on the magnitudes of the crystal fields in which these domains are located. Authors on English: B.Kh. Khannanov, E.I. Golovenchits, V.A. Sanina

The effect of optical pumping and applied magnetic field on the characteristics of ferromagnetic layers in one-dimensional superlattices is studied. At low enough temperatures, these layers correspond to phase separation domains in RMn(2)O(5)and R(0.8)Ce(0.2)Mn(2)O(5)multiferroics. The formation of such domains occurs owing to the charge ordering of Mn(3+)and Mn(4+)ions and to the finite probability fore(g)electrons to tunnel between these pairs of ions. The volume occupied by such superlattices is rather small, and they can be treated as isolated ferromagnetic semiconductor heterostructures, spontaneously formed in the host crystal. The sequences of ferromagnetic resonances related to the superlattice layers in Eu(0.8)Ce(0.2)Mn(2)O(5)are studied. The characteristics of these resonances give information on the properties of such layers. For the first time, it is demonstrated that the optical pumping gives rise to a new metastable state of superlattices, which can be recovered by the magnetic field cycling to the state existing before the optical pumping. It is found that the superlattices recovered by the magnetic field exist up to temperatures higher than those in as-grown crystals.

The permittivity, conductivity, electric polarization, and features of high-resolution X-ray diffraction scattering of a relaxor ferroelectric PbCo1/3Nb2/3O3 have been investigated in the temperature range 5–350 K. Continuous correlated temperature changes in dielectric properties and electric polarization have been revealed, which were not typical of relaxor ferroelectrics. These changes can be attributed to local polar domains, which were induced in the original crystal matrix. In such domains, the charges (valences) of Co and Nb ions were continuously changed.

AbstractElectric polarization in ErCrO_3 single crystals has been investigated in the temperature range of 5‒370 K. Ferroelectric ordering has not been found in any of the directions. However, electric polarization induced by restricted polar domains of structural origin has been observed. These domains are formed in the crystal matrix near impurity Bi^3+ ions partially substituting Er^3+ ions during the growth of single crystals by the method of spontaneous crystallization using solvent Bi_2O_3. The restricted polar domains form the superparaelectric state. Hysteresis loops with remanent polarization, both along the c axis and in the [110] directions, have been observed below some temperatures T _fr (in the frozen superparaelectric state). The polarization exists up to certain temperatures, which depend on the applied electric field orientation with respect to the crystal axes and exceed significantly temperature T _N of magnetic ordering. These temperatures correspond to the condition kT _fr ≈ E _A for activation barriers at the boundaries of the restricted polar domains.

The dynamics of internal magnetic correlations in Tb 0.95 Bi 0.05 MnO 3 multiferroic in the temperature range of 10–290 K has been studied. Separation into two phases with different relaxations of the polarization of muons in the basic matrix of the crystal and the phase separation regions has been detected for the first time both in a magnetically disordered state at T < T N = 40 K and in a paramagnetic state in a transverse magnetic field of 290 G in a temperature range of 80–150 K. A muon ferromagnetic complex (Mn 3+ -Mu-Mn 4+ ), where the hyperfine interaction in a muonium depolarizes a muon in a time less than 10 −8 s, is formed at T < 40 K in phase separation regions containing pairs of Mn 3+ and Mn 4+ ions, as well as electrons that recharge them. In the matrix of the original crystal containing only Mn 3+ ions, a muonium is formed with a broken hyperfine bond. In this case, muons are depolarized at a high rate because of their interactions with the local magnetic fields of a cycloid. At temperatures of 80–150 K, one phase in the phase separation regions constitutes approximately 50% and is characterized by long relaxation times about 10 µs (described by the Gaussian relaxation function). The other phase is formed by Mn 3+ -Mn 3+ correlations in the short-range magnetic order regions in the matrix of the original crystal, which are weakly sensitive to a magnetic field of 290 G.

The electric polarization induced by local polar domains of two types (phase separation domains of magnetic nature and structure-distorted domains) has been observed in a YCrO3 single crystal. These domains form a superparaelectric state. Below some temperatures, in the frozen superparaelectric state, the pyrocurrent maxima and the hysteresis loops with remanent polarization are observed as along axis c so in directions [110]. The polarization exists to the temperatures depending on the orientation of electric field with respect to the crystal axes. The sources of formation of such local domains are analyzed and their properties are studied.

AbstractThe electric polarization induced by local polar domains of two types (phase separation domains of magnetic nature and structure-distorted domains) has been observed in a YCrO_3 single crystal. These domains form a superparaelectric state. Below some temperatures, in the frozen superparaelectric state, the pyrocurrent maxima and the hysteresis loops with remanent polarization are observed as along axis c so in directions [110]. The polarization exists to the temperatures depending on the orientation of electric field with respect to the crystal axes. The sources of formation of such local domains are analyzed and their properties are studied.

AbstractA comparative study of the dielectric properties and electric polarization of multiferroics GdMn_2O_5 and Gd_0.8Ce_0.2MnO_5 has been carried out in the temperature range 5–330 K. The polarization properties in the ferroelectric state that forms due to a charge ordering and exchange striction have been studied at T ≤ T _C = 30 K. The properties of the restricted polar phase separation domains formed in the crystals containing ions Mn^3+ and Mn^4+ have been studied, too. These domains exhibit the electric polarization in the temperature range from 5 K to some temperatures T _f ≫ T _C. Such a high-temperature polarization is due to the frozen superparaelectric state of the restricted polar domains.