Long-range and short-range magnetic order in new compound NaVGe<Subscript>2</Subscript>O<Subscript>6</Subscript>

New metal oxide pyroxene compound NaVGe₂O₆ containing isolated edge-sharing VO₆ (S=1) chains undergoes transition into a long-range antiferromagnetic state at T _N =16 K. The broad maximum in the temperature dependence of magnetic susceptibility at T _M =26 K indicates the low-dimensional character of the magnetic subsystem. Even though the antiferromagnetic ordering is accompanied by a sharp peak of specific heat, significant magnetic entropy is released above the Néel temperature.     

Interrelation of anisotropy of magnetic properties and magnetodielectric effect in a Cu<Subscript>3</Subscript>B<Subscript>2</Subscript>O<Subscript>6</Subscript> single crystal

The effect of an external magnetic field on permittivity has been studied in a Cu₃B₂O₆ single crystal with a layered structure in the direction perpendicular to layers (bc-planes). It has been found that the appreciable magnetodielectric effect in the temperature range below the Néel temperature (≈10 K) takes place only at one magnetic field orientation H and one crystallographic direction, i.e., H || b. Such “selectivity” of the magnetodielectric effect correlates with the anisotropic behavior of magnetic properties of the crystal.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

Peculiarities of the dielectric,elastic, and anelastic properties of the PbFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ferroelectromagnet at the antiferromagnetic phase transition

Peculiarities in the behavior of the dielectric, elastic, and anelastic properties of the PbFe_1/2Nb_1/2O₃ ceramic ferroelectromagnet have been found and investigated near the antiferromagnetic phase transition (Néel temperature T _N = 160 K). It is established that the transition to the antiferromagnetic phase leads to a decrease in the permittivity and elastic compliance. The anomaly of permittivity found near T _N indicates the presence of magnetoelectric interaction in the magnetically ordered phase.     

Specific heat of La(Fe<Subscript>0.873</Subscript>Co<Subscript>0.007</Subscript>Al<Subscript>0.12</Subscript>)<Subscript>13</Subscript> compound in antiferromagnetic and ferromagnetic states

The low-temperature specific heat C _p of La(Fe_0.873Co_0.007Al_0.12)₁₃ compound has been measured in two states: (i) antiferromagnetic (AFM) with a Néel temperature of T _N = 192 K and (ii) ferromagnetic (FM). The FM order appears at T = 4.2 K in a sample exposed to an external magnetic field with induction B _C ≥ 2.5 T and is retained for a long time in a zero field at temperatures up to T*_C = 23 K. The coefficient γ_FM in the low-temperature specific heat C = γT + βT ³ in the FM state differs quite insignificantly from that (γ_AFM) in the AFM state. Contributions to the low-temperature specific heat, which are related to a change in the elastic and magnetoelastic energy caused by magnetostrictive deformations, are considered.     

A phenomenological theory for polarization flop in spiral multiferroic TbMnO<Subscript>3</Subscript>

A phenomenological Landau theory has been used to explain magnetic field-driven polarization flop in TbMnO ₃. The Néel wall-like magnetic structure in spiral multiferroics induces a space-dependent internal magnetic field which exerts a torque on spins to rotate bc-spiral to ab-spiral. The external magnetic field is argued to be competing with easy axis anisotropy and the system stabilizes when anisotropy is minimum. With the help of Landau free energy with DM magnetoelectric coupling and a general ansatz for magnetization, the phenomenon of polarization flop has been explained. Relation between T_flop and critical magnetic field has been established and found to be in good agreement with the experiment. This could be an indication that anisotropy of the system is temperature- and magnetic field-dependent.     

Magnetic and magnetoelectric properties of the Tb<Subscript>0.75</Subscript>Ho<Subscript>0.25</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> ferroborate

Single crystals of the Tb_0.75Ho_0.25Fe₃(BO₃)₄ ferroborate have been grown by the group method from a solution–melt based on bismuth trimolybdate. The magnetic and magnetoelectric properties of the ferroborate single crystals have been investigated in the temperature range from 4.2 to 300 K and in magnetic fields up to 9 T. Magnetically, this material is an antiferromagnet with the Néel temperature T _N = 38.8 K and easy-axis anisotropy. The magnitude of the magnetoelectric polarization has been found to be more than 1.5–2.0 times greater than the sum of the polarizations induced by the magnetic field for the ferroborates TbFe₃(BO₃)₄ and HoFe₃(BO₃)₄ taken in the corresponding shares.     

Features of the low-frequency polarization response in the region of the ferroelectric phase transition in multiferroic TbMnO<Subscript>3</Subscript>

The unusual behavior of the low-frequency (10 Hz–1 MHz) permittivity in single crystals of ferroelectric multiferroic TbMnO₃ has been experimentally and theoretically studied in detail in the region of the narrow temperature peak of the permittivity, associated with the ferroelectric phase transition (T_C ~ 27.4 K). It has been found that the ε_c^′(ω, T) maximum sharply decreases with increasing measured field frequency, while the temperature position of the maximum is independent of frequency. It has been shown that the observed features of the polarization response can be satisfactorily described within the Landau–Khalatnikov polarization relaxation theory.     

Electron transition in intercalated disulfide CuCrS<Subscript>2</Subscript>

Electrical, resonant, and magnetic properties of intercalated copper chromium disulfide CuCrS₂ are studied. It is established that CuCrS₂ is an antiferromagnetic semiconductor with Néel temperature T_N=40.7 K and an effective magnetic moment of 4.3µ_B. Anomalies in the electrical, magnetic, and resonant properties of CuCrS₂ are found at T_c=110 K, which suggest an electron transition accompanied by alteration of the valences of the 3d-metal ions.     

Frustrated antiferromagnetism in the Sr<Subscript>2</Subscript>YRuO<Subscript>6</Subscript> double perovskite

The spins of Ru⁵⁺ ions in Sr₂YRuO₆ form a face-centered cubic lattice with antiferromagnetic nearest neighbor interaction J≈25 meV. The antiferromagnetic structure of the first type experimentally observed below the Néel temperature T _N =26 K corresponds to four frustrated spins of 12 nearest neighbors. In the Heisenberg model in the spin-wave approximation, the frustrations already cause instability of the antiferromagnetic state at T=0 K. This state is stabilized by weak anisotropy D or exchange interaction I with the next-nearest neighbors. Low D/J～I/J～10^?3 values correspond to the experimental T _N and sublattice magnetic moment values.     

Valence and magnetic states of impurity iron ions in the La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript> perovskite

The local magnetic and valence states of impurity iron ions in the rhombohedral La_0.75Sr_0.25Co_0.98 ⁵⁷Fe_0.02O₃ perovskite were studied using Mössbauer spectroscopy in the temperature range 87–293 K. The Mössbauer spectra are described by a single doublet at 215–293 K. The spectra contained a paramagnetic and a ferromagnetic component at 180–212 K and only a broad ferromagnetic sextet at T < 180 K. The results of the studies showed that, over the temperature range 87–295 K, the iron ions are in a single (tetrahedral) state with a valence of +3. In the temperature range 180–212 K, two magnetic states of Fe³⁺ ions were observed, one of which is in magnetically ordered microregions and the other, in paramagnetic microregions; these states are due to atomic heterogeneity. In the magnetically ordered microregions in the temperature range 87–212 K, the magnetic state of the iron ions is described well by a single state with an average spin S = 1.4 ± 0.2 and a magnetic moment μ(Fe) = 2.6 ± 0.4μ _B .     

Peculiarities of antiferromagnetic ordering in orthorhombic LiMnO<Subscript>2</Subscript>

Data on the antiferromagnetic ordering in orthorhombic lithium manganite LiMnO₂ are obtained from magnetic-susceptibility, calorimetry, and nuclear magnetic resonance studies. The minimal hysteresis and the absence of jumps in the temperature dependences of the sublattice magnetization M(T) and the magnetic susceptibility near T _N indicate that the ordering occurs through a continuous second-order phase transition. Within the critical temperature range, the M(T?T _N) variation is satisfactorily described by a power-law dependence with a critical exponent β = 0.25(4), which is substantially smaller than that predicted for 3D magnetic systems with isotropic Heisenberg exchange. The band structure of orthorhombic LiMnO₂ is calculated using the LMTO-ASA method. Taking into account the spin states of manganese ions, an adequate pattern is obtained for the density-of-states distribution with an energy gap near the Fermi level (～0.7 eV), which is in agreement with the measured electrical parameters of lithium manganite. The calculations demonstrate that the exchange interactions between Mn³⁺ ions leading to antiferromagnetic ordering are significantly anisotropic. It is found that small paramagnetic regions persist in the manganite below the Néel temperature, and it is concluded that the reason for this is partial structural disordering of LiMnO₂. As a result, a certain fraction of the manganese positions is occupied by lithium ions (Li_Mn) and vise versa (Mn_Li). These defects are not involved in the formation of the ordered magnetic structure and compose a paramagnetic fraction.     

On the possible coexistence of spiral and collinear structures in antiferromagnetic KFe(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The static and resonance properties of a quasi-two-dimensional antiferromagnet phase on a distorted triangular lattice of KFe(MoO₄)₂ have been experimentally studied. Magnetization curves exhibit features corresponding to the spin-flop transition in a collinear biaxial antiferromagnet and simultaneously show a magnetization plateau characteristic of a triangular spin structure. The magnetic resonance spectra also display absorption lines corresponding to the spin structures of both types. The experimental data are described in terms of a model comprising alternating weakly bound magnetic layers, in which the main two exchange integrals have different values. Below the Néel temperature (T _N =2.5 K), some of these layers possess a collinear antiferromagnetic structure, while the other layers have a triangular or spiral structure.     

Theoretical study of magnetic susceptibility of orbitally unquenched compound KCoF3

The effect of residual orbital magnetic moment of Co²⁺ in KCoF₃ on the magnetic susceptibility has been studied. For the calculation for both the ordered state and the paramagnetic state we have applied the correlated effective field approximation developed by Lines. An excellent value of the Néel temperature is obtained and, except near the Néel temperature, the calculated susceptibility agrees well with the experimental results over the whole temperature range.     

Magnetic structure and magnetic excitations in the two-dimensional spin system of the Cu<Subscript>3</Subscript>B<Subscript>2</Subscript>O<Subscript>6</Subscript> compound

This paper reports on the results of measurements of the magnetic susceptibility, heat capacity, neutron scattering, muon spin relaxation, and electron paramagnetic resonance in Cu₃B₂O₆ for the study of the ground state of the spin system of this compound. The results obtained suggest that, at a temperature of 10 K, the spin subsystem of the crystal, which consists of single spins and clusters of pairs and fours of spins interacting with one another, undergoes a transition to a state representing a superposition of the singlet (for clusters) and magnetically ordered (for single spins) states.     

Low-temperature specific heat of GdCu,GdCu<Subscript>2</Subscript>, and GdCu<Subscript>5</Subscript> antiferromagnets

It is found that the expansion of the low-temperature specific heat C _p of the antiferromagnetic metallic compounds GdCu, GdCu₂, and GdCu₅ contains a large term that is proportional to the square of temperature (δT ²). The value of δ is inversely proportional to the Néel temperature T _N. The GdCu₂ compound exhibits a strong dependence of the specific heat anomaly at T _N on an external magnetic field. The results obtained are compared with the data for other metal antiferromagnets, for example, the CuMnSb Heusler alloy.     

Long-range magnetic order in quasi-one-dimensional NaCrSi<Subscript>2</Subscript>O<Subscript>6</Subscript> and NaCrGe<Subscript>2</Subscript>O<Subscript>6</Subscript> metal oxides

Formation of a long-range magnetic order is observed at low temperatures in NaCrSi₂O₆ and NaCrGe₂O₆ quasi-one-dimensional metal oxide compounds with a pyroxene structure. The first of these compounds, NaCrSi₂O₆, is an antiferromagnet with the Néel temperature T _N =3 K, while the second, NaCrGe₂O₆, is a ferromagnet with the Curie temperature T _C =6 K. From the measurements of magnetization and specific heat of these compounds, the main parameters of their magnetic subsystems are determined. In NaCrSi₂O₆, a spin-flip transition is observed. A change in the type of magnetic order that accompanies the replacement of Si by Ge can be attributed to a change in the parameters of the competing direct antiferromagnetic Cr-Cr and indirect ferromagnetic Cr-O-Cr interactions in isolated chains of CrO₆ octahedra.     

Effect of a structural disorder on the magnetic properties of the sodium–cobalt tellurate Na<Subscript>3.70</Subscript>Co<Subscript>1.15</Subscript>TeO<Subscript>6</Subscript>

A new layered oxide, sodium–cobalt tellurate Na_3.70Co_1.15TeO₆, is synthesized and structurally characterized, and its static and dynamic magnetic properties are studied. This compound has a new monoclinic structure type with quasi-one-dimensional cation ordering in magnetically active layers. This compound is antiferromagnetically ordered at a Néel temperature T _N ~ 3.3 K, and the temperature and field dependences of magnetization suggest competing antiferromagnetic and ferromagnetic interactions. EPR spectroscopy reveals complex spin dynamics when temperature changes and the presence of two different paramagnetic centers, which is attributed to the existence of two structurally nonequivalent (regular and antisite) positions for magnetic Co²⁺ ions.     

Magnetic properties of single crystals of ludwigites Cu<Subscript>2</Subscript><Emphasis Type="Italic">M</Emphasis>BO<Subscript>5</Subscript> (<Emphasis Type="Italic">M</Emphasis> = Fe<Superscript>3+</Superscript>, Ga<Superscript>3+</Superscript>)

High-quality single crystals of ludwigites Cu₂ MBO₅ (M = Fe³⁺, Ga³⁺) have been grown, and the magnetic, resonance, and Mössbauer studies have been performed. It is established that the Cu₂FeBO₅ and Cu₂GaBO₅ compounds are antiferromagnets with Néel temperatures of 32 and 3.4 K, respectively. A model of the magnetic structure of the compounds is proposed. It is shown that the magnetic properties of the ludwigites are substantially dependent on the degree of ion distribution over crystallographic positions.     

Spin fluctuations in the stacked-triangular antiferromagnet YMnO<Subscript>3</Subscript>

The spectrum of spin fluctuations in the stacked-triangular antiferromagnet YMnO₃ was studied above the Néel temperature using both unpolarized and polarized inelastic neutron scattering. We find an in-plane and an out-of-plane excitation. The in-plane mode has two components just above T _N : a resolution-limited central peak and a Debye-like contribution. The quasi-elastic fluctuations have a line width that increases with q as Dq ^z and the dynamical exponent z = 2.3. The out-of-plane fluctuations have a gap at the magnetic zone center and do not show any appreciable q dependence at small wave vectors.