M. Markina

Competition between helimagnetism and commensurate quantum spin correlations in LiCu2O2

Neutron diffraction and bulk measurements are used to determine the nature of the low-temperature ordered state in LiCu2O2, a S=1/2 spin-chain compound with competing interactions. The spin structure is found to be helimagnetic, with a propagation vector (0.5,zeta,0), zeta=0.174. The nearest-neighbor exchange constant and frustration ratio are estimated to be J(1)=5.8 meV and J(2)/J(1)=0.29, respectively. For idealized quantum spin chains, these parameter values would signify a gapped spin-liquid ground state with commensurate spin correlations. The observed temperature dependence of the magnetic propagation vector in LiCu2O2 is attributed to a competition between incommensurate helimagnetism in the classical spin model and commensurability in the quantum case. It is also proposed that long-range ordering in LiCu2O2 is facilitated by intrinsic nonstoichiometry.

Spin Waves and Magnetic Interactions in LiCu2O2

The quasi-one-dimensional helimagnet LiCu{sub 2}O{sub 2} was studied by single crystal inelastic neutron scattering. The dispersion relation of spin wave excitations was measured in the vicinity of the principal magnetic Bragg reflection. A spin wave theoretical analysis of the data yields an estimate of the relevant exchange constants and explains the mechanism of geometric frustration that leads to helimagnetism. It is found that the simple antiferromagnetic J{sub 1}-J{sub 2} model that was previously proposed is inadequate for LiCu{sub 2}O{sub 2}. The experimental findings are generally in a qualitative agreement with first principles calculations of [A. A. Gippius, E. N. Morozova, A. S. Moskvin, A. V. Zalessky, A. A. Bush, M. Baenitz, H. Rosner, and S.-L. Drechsler, Phys. Rev. B 70, 020406 (2004)], though certain important discrepancies remain to be explained.

Synthesis and crystal structure of the Sr2Al1.07Mn0.93O5 brownmillerite

A new brownmillerite-type compound Sr2Al1.07Mn0.93O5 was synthesized. The crystal structure was determined using electron diffraction and high resolution transmission electron microscopy and refined from X-ray powder diffraction data (space group Imma, a = 5.4358(1) A, b = 15.6230(4) A, c = 5.6075(1) A, RI = 0.036, RP = 0.023). The structure is characterized by a disordered distribution of the tetrahedral chains in L and R configuration and a partial occupation of the octahedral position by the Mn3+ and Al3+ cations. The relationships between the crystal structures of Sr2Al1.07Mn0.93O5 and its A2B′MnO5 analogues (A = Ca, Sr, B′ = Al, Ga) and the structural reasons for the different types of tetrahedral chain ordering in brownmillerites are discussed. The temperature dependences of the magnetic susceptibility and specific heat reveal that the compound is antiferromagnetically ordered below TN = 105 K.

Synthesis and crystal structure of the Sr_{2}Al_{1.07}Mn_{0.93}O_{5} brownmillerite

A new brownmillerite-type compound Sr2Al1.07Mn0.93O5 was synthesized. The crystal structure was determined using electron diffraction and high resolution transmission electron microscopy and refined from X-ray powder diffraction data (space group Imma, a = 5.4358(1) A, b = 15.6230(4) A, c = 5.6075(1) A, RI = 0.036, RP = 0.023). The structure is characterized by a disordered distribution of the tetrahedral chains in L and R configuration and a partial occupation of the octahedral position by the Mn3+ and Al3+ cations. The relationships between the crystal structures of Sr2Al1.07Mn0.93O5 and its A2B′MnO5 analogues (A = Ca, Sr, B′ = Al, Ga) and the structural reasons for the different types of tetrahedral chain ordering in brownmillerites are discussed. The temperature dependences of the magnetic susceptibility and specific heat reveal that the compound is antiferromagnetically ordered below TN = 105 K.

Magnetic phase diagram and first-principles study ofPb3TeCo3V2O14

An antiferromagnetic ordering in Pb3TeCo3V2O14 takes place through formation of short range correlation regime with T* ~ 10.5 K and succession of second order phase transition at TN1 = 8.9 K and first order phase transition at TN2 = 6.3 K. An external magnetic field rapidly destroys magnetic structure at T < TN2 and influences the magnetic order at TN2 < T < TN1 resulting in complex magnetic phase diagram of Pb3TeCo3V2O14 as derived from magnetization and specific heat measurements. The first principles calculations indicate that in variance with layered crystal structure the magnetic subsystem of Pb3TeCo3V2O14 is quasi-one-dimensional and highly unusual consisting of weakly coupled triangular tubes.

Synthesis and crystal structure of novel CaRMnSnO6(R = La, Pr, Nd, Sm–Dy) double perovskites

The CaRMnSnO6 (R = La, Pr, Nd, Sm–Dy) double perovskites were prepared using a solid state reaction in evacuated sealed silica tubes at 1050–1100 °C. All compounds belong to the GdFeO3-type structure (orthorhombic a : b : c ≈ √2ap : 2ap : √2ap unit cell, space group Pnma, (a+b−b−) tilt system). The crystal structures were investigated by electron diffraction and high resolution electron microscopy and were refined from X-ray powder diffraction data for the R = La, Sm, Dy representatives (La: RI = 0.030, RP = 0.033; Sm: RI = 0.030, RP = 0.010; Dy: RI = 0.033, RP = 0.010). A disordered arrangement of the Ca and R cations over the A positions and Mn and Sn cations over the B positions was found in all cases. The small difference in charge and size between the Mn3+ and Sn4+ cations, the small degree of Jahn–Teller deformation of the MnO6 octahedra, and the ac-plane orbital ordering type were proposed as possible reasons for the cation disorder on the B sublattice. The compounds demonstrate a spin glass behavior in the temperature range from 25 to 300 K.

A2MnXO4 Family (A = Li, Na, Ag; X = Si, Ge): Structural and Magnetic Properties

Four new manganese germanates and silicates, A2MnGeO4 (A = Li, Na) and A2MnSiO4 (A = Na, Ag), were prepared, and their crystal structures were determined using the X-ray Rietveld method. All of them contain all components in tetrahedral coordination. Li2MnGeO4 is orthorhombic (Pmn21) layered, isostructural with Li2CdGeO4, and the three other compounds are monoclinic (Pn) cristobalite-related frameworks. As in other stuffed cristobalites of various symmetry (Pn A2MXO4, Pna21 and Pbca AMO2), average bond angles on bridging oxygens (here, Mn-O-X) increase with increasing A/X and/or A/M radius ratios, indicating the trend to the ideal cubic (Fd3̅m) structure typified by CsAlO2. The sublattices of the magnetic Mn2+ ions in both structure types under study (Pmn21 and Pn) are essentially the same; namely, they are pseudocubic eutaxy with 12 nearest neighbors. The magnetic properties of the four new phases plus Li2MnSiO4 were characterized by carrying out magnetic susceptibility, specific heat, magnetization, and electron spin resonance measurements and also by performing energy-mapping analysis to evaluate their spin exchange constants. Ag2MnSiO4 remains paramagnetic down to 2 K, but A2MnXO4 (A = Li, Na; X = Si, Ge) undergo a three-dimensional antiferromagnetic ordering. All five phases exhibit short-range AFM ordering correlations, hence showing them to be low-dimensional magnets and a magnetic field induced spin-reorientation transition at T < TN for all AFM phases. We constructed the magnetic phase diagrams for A2MnXO4 (A = Li, Na; X = Si, Ge) on the basis of the thermodynamic data in magnetic fields up to 9 T. The magnetic properties of all five phases experimentally determined are well explained by their spin exchange constants evaluated by performing energy-mapping analysis.

Long-range magnetic order in LixNa1−xCu2O2

Single crystals of LixNa1 − xCu2O2 solid solutions have been grown from the melt and crystal lattice parameters have been studied. It is found that the solid solution of this system exists in the region x ≤ 0.25. Specific heat and magnetic susceptibility are investigated on monocrystalline samples with x = 0.07, 0.14, and 0.21. Long-range magnetic order in these compounds is formed at T = 13.8, 4.1, and 14.8 K, respectively. It is found that the Néel temperature TN in these compounds increases with Li content.Single crystals of Li{sub x}Na{sub 1-x}Cu{sub 2}O{sub 2} solid solutions have been grown from the melt and crystal lattice parameters have been studied. It is found that the solid solution of this system exists in the region x {<=} 0.25. Specific heat and magnetic susceptibility are investigated on monocrystalline samples with x = 0.07, 0.14, and 0.21. Long-range magnetic order in these compounds is formed at T = 13.8, 4.1, and 14.8 K, respectively. It is found that the Neel temperature T{sub N} in these compounds increases with Li content.

Long-range magnetic order in Li x Na1−x Cu2O2

Single crystals of LixNa1 − xCu2O2 solid solutions have been grown from the melt and crystal lattice parameters have been studied. It is found that the solid solution of this system exists in the region x ≤ 0.25. Specific heat and magnetic susceptibility are investigated on monocrystalline samples with x = 0.07, 0.14, and 0.21. Long-range magnetic order in these compounds is formed at T = 13.8, 4.1, and 14.8 K, respectively. It is found that the Néel temperature TN in these compounds increases with Li content.Single crystals of Li{sub x}Na{sub 1-x}Cu{sub 2}O{sub 2} solid solutions have been grown from the melt and crystal lattice parameters have been studied. It is found that the solid solution of this system exists in the region x {<=} 0.25. Specific heat and magnetic susceptibility are investigated on monocrystalline samples with x = 0.07, 0.14, and 0.21. Long-range magnetic order in these compounds is formed at T = 13.8, 4.1, and 14.8 K, respectively. It is found that the Neel temperature T{sub N} in these compounds increases with Li content.