A. Ya. Shapiro

Controlled oxygen removal in the high-Tc YBa2(Cu1−xFex)3O7−y system and its influence on superconductivity and local structure

Abstract The YBa 2 (Cu 0.99 57 Fe 0.01 ) 3 O 7− y system with a precisely determined oxygen content in the range 0.03≤ y ≤0.80 has bee n obtained for the first time. It is shown that 1% substitution of paramagnetic Fe atoms for Cu does not change the overall dependence of T c on y . A “shelf”-type region at T c ≅60 K is observed on the T c = f ( y ) curve at 0.2 y y =0.5, indicating that the individual YBa 2 Cu 3 O 6.5 phase is probably not superconductive. From the 57 Fe Mossbauer spectra it is shown that at least 90% of the Fe atoms are situated in Cu(1) sites, but iron does not occupy two-fold coordinated sites even in the samples with large oxygen deficiency. About 70% of the Fe atoms are in the twin domains, and about 20% in the domain boundaries. In the domains Fe atoms mainly occupy Cu(1) sites with plane square and five-fold pyramidal oxygen coordinations. The pyramidal environment of Fe atoms in the domains is structurally unstable and easily gives up one oxygen atom at oxygen removal, transforming into the planar square. Every removable oxygen atom belongs to two Fe atoms at Cu(1) sites.

Magnetic ordered states in the oxygen-deficient YBa2(Cu1-xFex)3Oy<6.5 system

Abstract The magnetically ordered states in the oxygen-deficient YBa 2 (Cu 1- x Fe x ) 3 O y system (0.01≤ x ≤0.30) have been studied by Mossbauer spectroscopy. Magnetic ordering of Fe atoms is found at Cu(1) sites as well as at Cu(2) sites. At small iron concentrations x ≤0.05 the Cu(1) and Cu(2) sublattices are not coupled magnetically and two magnetic phase transitions are observed: the low-temperature transition for the Cu(1) sublattice at T m1 ≈20 K and the high-temperature transition for the Cu(2) sublattice at T m2 ≈400 K. With increasing iron concentration at x ≥0.10 a strong coupling between the Cu(1) and Cu(2) sublattices appears and the whole matrix is ordered antiferromagnetically with a single magnetic ordering point T m2 ≈460 K. The phase diagrams showing the regions of existence of different magnetic phases versus iron concentration are plotted for the oxygen-saturated and oxygen-reduced YBa 2 (Cu 1- x Fe x ) 3 O y systems.

Multiferroicity in the generic easy-plane triangular lattice antiferromagnet RbFe(MoO4)(2)

RbFe(MoO4)(2) is a quasi-two-dimensional (quasi-2D) triangular lattice antiferromagnet (TLA) that displays a zero-field magnetically driven multiferroic phase with a chiral spin structure. By inelastic neutron scattering, we determine quantitatively the spin Hamiltonian. We show that the easy-plane anisotropy is nearly 1/3 of the dominant spin exchange, making RbFe(MoO4)(2) an excellent system for studying the physics of the model 2D easy-plane TLA. Our measurements demonstrate magnetic-field-induced fluctuations in this material to stabilize the generic finite-field phases of the 2D XY TLA. We further explain how Dzyaloshinskii-Moriya interactions can generate ferroelectricity only in the zero-field phase. Our conclusion is that multiferroicity in RbFe(MoO4)(2), and its absence at high fields, results from the generic properties of the 2D XY TLA.

Magnetic phase diagram, critical behavior, and two-dimensional to three-dimensional crossover in the triangular lattice antiferromagnet RbFe"MoO4…2

We have studied the magnetic and thermodynamic properties as well as the NMR spectra of the Heisenberg antiferromagnet RbFe(MoO4)2. The observed temperature dependence of the order parameter, the critical indices and the overall magnetic H-T phase diagram are all in a good agreement with the theoretical predictions for a 2D XY model. The temperature dependence of the specific heat at low temperature demonstrates a crossover from a T^2 law characteristic of a two-dimensional antiferromagnet to a three-dimensional T^3 law.

Chiral and collinear ordering in a distorted triangular antiferromagnet

Magnetization, specific heat, and neutron diffraction measurements are used to map out the entire magnetic phase diagram of KFe(MoO4)2. This stacked triangular antiferromagnet is structurally similar to the famous multiferroic system RbFe(MoO4)2. Because of an additional small crystallographic distortion, it contains two sets of inequivalent distorted magnetic triangular lattices. As a result, the spin network breaks down into two intercalated yet almost independent magnetic subsystems. One is a collinear antiferromagnet that shows a simple spin-flop behavior in applied magnetic fields. The other is a helimagnet that instead goes through a series of exotic commensurate-incommensurate phase transformations. In the various phases one observes either true three-dimensional or unconventional quasi-two-dimensional ordering.

On the possible coexistence of spiral and collinear structures in antiferromagnetic KFe(MoO4)2

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 (TN=2.5 K), some of these layers possess a collinear antiferromagnetic structure, while the other layers have a triangular or spiral structure.

Structural properties and Mössbauer spectroscopy of the high TcYBa2Cu3−xFexOy compounds

New results on the Mossbauer spectroscopy of the YBa2(Cu1−xFex)3Oy system are reported and the availible reliable experimental data are briefly summarized.

High-field magnetic resonance of spinons and magnons in the triangular lattice S=1/2 antiferromagnet Cs2CuCl4

The electron spin resonance doublet indicating the width of the two spinon continuum in a spin-1/2 triangular-lattice Heisenberg antiferromagnet Cs2CuCl4 was studied in high magnetic field. The doublet was found to collapse in a magnetic field of a half of the saturation field. The collapse of the doublet occurs via vanishing of the high frequency component in a qualitative agreement with the theoretical prediction for the S=1/2 chain. The field of the collapse is, however, much lower than expected for the S=1/2 chain. This is proposed to be due to the destruction of frustration of interchain exchange bonds in a magnetic field, which restores the 2D character of this spin system. In the saturated phase the mode with the Larmor frequency and a much weaker mode downshifted for 119~GHz are observed. The weak mode is of exchange origin, it demonstrates a positive frequency shift at heating corresponding to the repulsion of magnons in the saturated phase.

Magnetically ordered states in the YBa2(Cu1−xFex)3O7+δ system with 0.15 < x < 0.30

Abstract In the YBa2(Cu1−xFex)3O7+δ system with 0.15 ≤ x ≤ 0.30 two magnetic phase transitions have been observed at temperatures Tm1 ≈ 30K and Tm2 ≈ 420 K. At these temperatures Fe atoms at different structural sites undergo a transition from a magnetically ordered to the paramagnetic state. The transition at Tm1 is associated with Fe atoms occupying Cu1 sites with oxygen coordination k = 4 and k = 5. A spin glass type ordering of the Fe magnetic moments appears below the Tm1 point. The transition at T = Tm2 is directly related to the magnetic state of the Cu2 sublattice and concerns some iron clusters. The magnetic order in those clusters is determined by exchange interactions of Fe atoms at Cu1 and Cu2 sites. A phase diagram “magnetism-superconductivity” vs. iron concentration is presented for the oxygen saturated YBa2(Cu1−xFex)3O7+δ system.

87Rb NMR study of the magnetic structure of the quasi-two-dimensional antiferromagnet RbFe(MoO4)2 on a triangular lattice

Abstract87Rb nuclear magnetic resonance was experimentally studied in a quasi-two-dimensional Heisenberg antiferromagnet RbFe(MoO4)2. Dipole fields at the 87Rb nuclei were found over a wide range of temperatures and static magnetic fields. Magnetic structures in the ordered phase were determined at various magnetic fields.