Magnetic phase transitions,soliton lattices,and electric polarization in <Emphasis Type="Italic">R</Emphasis>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

The magnetic phase transitions in RMn₂O₅ oxides were analyzed. It was shown that the transition from the paramagnetic phase to the incommensurate phase is described by not only the basic magnetic parameter but also an associated order parameter: electric polarization along the y axis of the crystal. It was established that the antiferromagnetic-incommensurate phase transformation is a second-order phase transition, accompanied by a decrease in electric polarization.     

Specificity of magnetoelectric effects in a new GdMnO<Subscript>3</Subscript> magnetic ferroelectric

A complex study of the magnetic, electric, magnetoelectric, and magnetoelastic properties of GdMnO₃ single crystals has been performed in the low-temperature region in strong pulsed magnetic fields up to 200 kOe. An anomaly of the dielectric constant along the a axis of a crystal has been found at 20 K, where a transition from an incommensurate modulated phase to a canted antiferromagnetic phase, as well as electric polarization along the a and b axes of the crystal induced by the magnetic field H‖ b (H_cr ～ 40 kOe), is observed. Upon cooling the crystal in an electric field, the magnetic-field-induced electric polarization changes its sign depending on the sign of the electric field. The occurrence of the electric polarization is accompanied by anisotropic magnetostriction, which points to a correlation between the magnetoelectric and magnetoelastic properties. Based on these results, it has been stated that GdMnO₃ belongs to a new family of magnetoelectric materials with the perovskite structure.     

Magnetic and ferroelectric ordering in BiMn<Subscript>2</Subscript>O<Subscript>5</Subscript> oxide

A group-theoretical analysis of the magnetic phase of BiMn₂O₅ oxide is performed using the space symmetry group of the compound. Using the projection operator method, we determine the basis functions of the irreducible representation of the space group, which are expressed in terms of the magnetic vector components. This representation can govern two phase transitions from the paramagnetic state to the antiferromagnetic phase with close temperatures and ordering of the spins of manganese ions in two crystallographic positions. It is found from renorm group analysis of these transitions that when these transitions occur as second- order transitions, the electric polarization does not appear in the system because spin fluctuations in this case elevate the symmetry of the system. Polarization appears when at least one of these transitions becomes a first-order transition as a result of spin fluctuations.     

Peculiarities of magnetic phase separation in anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The temperature and field dependences of the specific magnetic moment of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite have been measured. It is established, that the magnetic state of the sample studied is a cluster spin glass and it is the result of frustration of exchange Mn³⁺-O-Mn³⁺ interactions due to the redistribution of oxygen vacancies. The increase of the magnetic field leads to an increase in the degree of polarization of local spins of manganese. It is established using the magnetic criterion that a phase transition into the paramagnetic state for the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite is a thermodynamic second order phase transition. The causes and mechanism of the magnetic phase separation are discussed.     

Modulated magnetic structure in quasi-one-dimensional clinopyroxene NaFeGe<Subscript>2</Subscript>O<Subscript>6</Subscript>

The magnetic structure of the NaFeGe₂O₆ monoclinic compound has been experimentally investigated using the elastic scattering of neutrons. At a temperature of 1.6 K, an incommensurate magnetic structure has been observed in the form of an antiferromagnetic helix formed by a pairs of the spins of the Fe³⁺ ions with helical modulation in the ac plane of the crystal lattice. The wave vector of the magnetic structure has been determined and its temperature behavior has been studied. The analysis of the temperature dependences of the specific heat and susceptibility, as well as the isotherms of the field dependence of the magnetization, has revealed the existence of not only the order-disorder magnetic phase transition at the point T _N = 13 K, but also an additional magnetic phase transition at the point T _c = 11.5 K, which is assumingly an orientation phase transition.     

Pressure-induced electron spin transition in the paramagnetic phase of the GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> Heisenberg magnet

The HS → LS spin crossover effect (high-spin → low-spin transition) induced by high pressure in the range 45–53 GPa is observed in trivalent Fe³⁺ ions in the paramagnetic phase of a Gd⁵⁷Fe₃(BO₃)₄ gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K _β high-resolution x-ray emission spectroscopy (XES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.     

Elastic properties of a La<Subscript>0.5</Subscript>Pr<Subscript>0.2</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal

The temperature dependences of the velocity of longitudinal sound and internal friction in the ferromagnetic La_0.5Pr_0.2Ca_0.3MnO₃ single crystal with magnetic first-order phase transition were studied. It was found that the sound velocity decreases by ≈20% in transition from the ferromagnetic to paramagnetic state. In the paramagnetic region, the extended temperature hysteresis of the sound velocity and the internal friction was observed. It was shown that La_0.5Pr_0.2Ca_0.3MnO₃ has two paramagnetic phases with different sound velocities.     

Magnetic and electric properties of Eu<Subscript>0.62</Subscript>Bi<Subscript>0.38</Subscript>MnO<Subscript>3</Subscript> and Eu<Subscript>0.53</Subscript>Bi<Subscript>0.32</Subscript>Sr<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> crystals

Single crystals of Eu_0.62Bi_0.38MnO₃ and Eu_0.53Bi_0.32Sr_0.15MnO₃ solid solutions crystallizing in an orthorhombically distorted perovskite structure were prepared. At temperatures above 120 K, Eu_0.62Bi_0.38MnO₃ exhibits the properties of structural glass while remaining a dielectric at all temperatures. There is no long-range magnetic order in this compound. Eu_0.53Bi_0.32Sr_0.15MnO₃ behaves as a semiconductor above 120 K and exhibits a jump in conductivity at T = 175 K associated with a metal-insulator transition occurring within limited regions of the crystal. In these regions, there appears a ferromagnetic moment (due to double exchange mediated by charge carriers) and local electric polarization.     

Magnetoelectric effects in gadolinium iron borate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Magnetoelectric interactions have been investigated in a single crystal of gadolinium iron borate GdFe₃(BO₃)₄, whose macroscopic symmetry is characterized by the crystal class 32. Using the results of this study, the interplay of magnetic and electric orderings occurring in the system has been experimentally revealed and theoretically substantiated. The electric polarization and magnetostriction of this material that arise in spin-reorientation transitions induced by a magnetic field have been investigated experimentally. For H ‖ c and H ⊥ c, H-T phase diagrams have been constructed, and a strict correlation between the changes in the magnetoelectric and magnetoelastic properties in the observed phase transitions has been ascertained. A mechanism of specific noncollinear antiferroelectric ordering at the structural phase transition point was proposed to interpret the magnetoelectric behavior of the system within the framework of the symmetry approach in the entire temperature range. This ordering provides the conservation of the crystal class of the system when the temperature decreases to the antiferroelectric ordering point. The expressions that have been obtained for the magnetoelectric and magnetoelastic energy describe reasonably well the behavior of gadolinium iron borate observed experimentally.     

Magnetic and structural anomalies of Na<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>C<Subscript>60</Subscript> (n = 2, 3)

Sodium fullerides Na _n C60 (n = 2, 3) have been synthesized by a liquid phase reaction and investigated with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), electron paramagnetic resonance, and differential thermal analysis. XRD data indicate that the crystal structure of Na₂C₆₀ at 300 K is face centered cubic (FCC). A phase transition from primitive cubic to FCC crystal structure has been observed in this work in Na₂C₆₀ fulleride at 290 K. The transition is accompanied by the step-like change of paramagnetic susceptibility. The crystal structure of Na₃C₆₀ is more complicated than, and different from, what has been reported in the literature. A nearly seven-fold increase of paramagnetic susceptibility with increasing temperature has been observed in the Na₃C₆₀ fulleride at 240–260 K. In the same temperature range, a new line at about 255 ppm appears in the ²³Na NMR spectrum, indicating a significant increase of electron density near the Na nucleus. The observed effect can be explained by a metal-insulator transition caused by a structural transition.     

Specific features of the magnetic field-induced orientational transition in EuMnO<Subscript>3</Subscript>

It is found that the spin-flop transition in EuMnO₃ manganite induced by a magnetic field H parallel to the b axis is accompanied not only by a magnetization jump and magnetostriction anomalies but also by the appearance of electric polarization in the vicinity of the transition field H_cr ～ 200 kOe. This phenomenon can be associated with the occurrence of magnetically inhomogeneous (modulated) states in the vicinity of H_cr. In these states, the system loses the center of symmetry, which allows for the appearance of the polarization. The formation of such states in a magnetic field is caused by the general tendency of the occurrence of magnetically inhomogeneous (incommensurate) structures in the RMnO₃ series due to the frustration of exchange interactions with decreasing ionic radius of the rare-earth ion R starting with R = Eu.     

Incommensurate helical magnetic order in LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> and NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> quasi-one-dimensional compounds

The results of the measurements of the ^{6, 7}Li and ²³Na nuclear magnetic resonance (NMR) and ^{63, 65}Cu nuclear quadrupole resonance in LiCu₂O₂ and NaCu₂O₂ quasi-one-dimensional compounds with a spin chains in the paramagnetic and magnetically ordered states are presented. The shape of the NMR line below T _c = 24 and 13 K for LiCu₂O₂ and NaCu₂O₂, respectively, is characteristic of the incommensurate static modulation of the local magnetic field matching with the incommensurate spiral modulation of the magnetic moments. The differences in the shape of the NMR spectra of ²³Na and ⁷Li are discussed in terms of the features of the crystal structure of LiCu₂O₂ and NaCu₂O₂.     

Paramagnetic-ferromagnetic and insulator-metal phase transitions in La<Subscript>0.88</Subscript>MnO<Subscript>2.95</Subscript>

We present the results a study of structure by neutron diffraction and data on the magnetic properties (linear and nonlinear (second and third order) susceptibilities) of polycrystalline La_0.88MnO_2.95. This compound exhibits an insulator-metal (IM) phase transition at T _IM ≈ 253 K (above the Curie temperature, T _C ≈ 244 K) and reveals colossal magnetoresistance. The crystal structure is found to be rhombohedral, and the space group is R3c. Analysis of magnetic properties shows that at T* ≈ 258 K > T _C , isolated paramagnetic clusters occur in the paramagnetic matrix; their concentration increases upon cooling. We observed no noticeable differences between the temperature evolution of the clustered state of this manganite with its insulator-metal transition and in the insulator La_0.88MnO_2.91. Possible scenarios of the paramagnet-ferromagnet and I-M transitions in a self-organized clustered structure are discussed.     

New magnetic states in copper metaborate CuB<Subscript>2</Subscript>O<Subscript>4</Subscript>

The static and resonance properties of copper metaborate CuB₂O₄ were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.     

Magnetic ordering in Fe<Subscript>1.087</Subscript>Te under pressure

The crystal and magnetic structures of Fe_1.087Te have been studied by neutron powder diffraction in the temperature range from 1.7 to 80 K at pressures of  ≈0.4 and ≈1.2 GPa. No symmetry change of the tetragonal paramagnetic ambient pressure phase (space group P4/nmm) was observed for temperatures above 60 K and pressures up to  ≈1.2 GPa. A novel pressure-induced phase of Fe_1.087Te having orthorhombic symmetry (space group Pmmn) and incommensurate antiferromagneticbicollinear order was observed in the temperature range from 50 to 60 K at  ≈1.2 GPa. The known monoclinic ambient pressure phase of Fe_1.087Te (space group P2 ₁/n) with commensurate antiferromagnetic order was found to be stable up to at least  ≈1.2 GPa at low temperature.     

Colossal magnetodielectric effect in SmFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The colossal (more than threefold) decrease in the dielectric constant ɛ in the easy-plane SmFe₃(BO₃)₄ ferroborate in a magnetic field of ∼5 kOe applied in the basal ab plane of the crystal has been found. A close relation of this effect to anomalies in the field dependence of the electric polarization has been established. It has been shown that this magnetodielectric effect is due to the contribution to ɛ from the electric susceptibility, which is related to the rotation of spins in the ab plane, arises in the region of the antiferromagnetic ordering T < T _N = 33 K, and is suppressed by the magnetic field. A theoretical model describing the main features of the behavior of ɛ and electric polarization in the magnetic field has been proposed, taking into account the additional anisotropy in the basal plane induced by the magnetoelastic stresses.     

Magnetic-resonance and thermophysical studies of the magnetic phase diagram for a GdFe<Subscript>2.1</Subscript>Ga<Subscript>0.9</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The antiferromagnetic resonance, heat capacity, magnetic properties, and magnetic phase diagram of a GdFe₃(BO₃)₄ crystal in which some of the iron ions were substituted by diamagnetic gallium ions have been investigated. It has been found that the Neél temperature upon diamagnetic substitution decreased to 17 K compared to 38 K in the unsubstituted crystal. The effective exchange and anisotropy fields for GdFe_2.1Ga_0.9(BO₃)₄ have been estimated from the field dependences of magnetization and resonance measurements. The magnetic phase diagram of the crystal has been constructed from magnetic and resonance measurements. In GdFe_2.1Ga_0.9(BO₃)₄, there is no spontaneous reorientation and, in the absence of a magnetic field, the crystal remains an easy-axis one in the entire domain of magnetic ordering. The critical field of the reorientation transition to an induced easy-plane state in a magnetic field along the trigonal axis has been found to increase compared to that in the unsubstituted crystal.     

Modulated magnetic structure of weak rhombohedral ferromagnets α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga and FeBO<Subscript>3</Subscript>:Mg

The effect of diamagnetic impurities on the stability of the homogeneous magnetic state of rhombohedral antiferromagnets with weak ferromagnetism (α-Fe₂O₃:Ga and FeBO₃:Mg) is studied experimentally. It is shown that the application of an external magnetic field in the basal plane in the crystals under study in a certain temperature range induces a magnetic superstructure along the hard magnetization axis, which can be presented in the form of a ripplon phase with the azimuth of the local ferromagnetism vector oscillating about the direction of the field. The preferred orientation of the discovered modulated structures relative to crystallographic directions in the basal plane of α-Fe₂O₃:Ga and FeBO₃:Mg is studied, and the dependence of the spatial period of the superstructure on the applied magnetic field and temperature is analyzed. The magnetic-field-induced transition of the studied crystals from a homogeneous to an inhomogeneous magnetic state is described phenomenologically on the basis of the thermodynamic potential with gradient terms. In the discussion of physical reasons for magnetic order parameter modulation in weak ferromagnetic doped with diamagnetic ions, preference is given to the mechanism associated with the emergence of uniaxial magnetic centers with a random distribution of azimuths of easy axes in the basal plane of the crystal in the vicinity of impurities. A model describing the formation of a modulated magnetic state in α-Fe₂O₃:Ga and FeBO₃:Mg is proposed, according to which the competition between magnetoanisotropic and Zeeman interactions in the inhomogeneous magnetic phase of these crystals leads to periodic deviations in the direction of the local ferromagnetism vector from the direction of magnetization.     

High-frequency properties of KNiPO<Subscript>4</Subscript> in alternating magnetic and electric fields

The dependences of the antiferromagnetic resonance frequencies on the constant magnetic field H and constant electric field E are calculated for a KNiPO₄ crystal with spontaneous electric polarization and antiferromagnetic order. It is demonstrated that the KNiPO₄ crystal is characterized by an exchange-enhanced effect of the electric field E on the antiferromagnetic resonance frequencies. This effect is not revealed in the magnetoelectric materials studied earlier. It is established that oscillations of both magnetization and electric polarization exhibit resonance response at antiferromagnetic resonance frequencies. The expressions for these responses in alternating magnetic and electric fields are presented.     

Acoustical investigations of a La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> single crystal

The acoustical, resistive, and magnetic properties of a La_0.75Sr_0.25MnO₃ lanthanum manganite single crystal are investigated in the temperature range involving the second-order magnetic phase transition. The acoustical measurements are performed by the pulse-echo method in the frequency range 14–90 MHz. It is found that, as the temperature decreases, the velocity of a longitudinal acoustic wave propagating along the [111] axis in the single crystal drastically increases at temperatures below the critical point of the magnetic phase transition. No dispersion of the acoustic velocity is revealed. A sharp increase in the acoustic velocity is accompanied by the appearance of an acoustical absorption peak. The observed effects are discussed with due regard for the interaction of acoustic waves with the magnetic moments of the manganese ions.