Random anisotropy in R2CuO4 (R=Nd, Pr, Sm, Gd, Eu) quasi-2D antiferromagnets

The mechanisms of random anisotropy produced by an an effective coupling between rare-earth ion moments and orbital momenta of Cu₂₊ ions through spin fluctuations is studied in R₂CuO₄ crystals. The effective random-anisotropy fields are estimated from an analysis of experimental data for R₂CuO₄ crystals (R=Eu, Pr, Gd). Fiz. Tverd. Tela (St. Petersburg) 41, 1259–1263 (July 1999)     

Nonlinear magnetic susceptibility and microwave spin dynamics of R2CuO4 quasi-2D antiferromagnets (R=Eu, Pr, Gd)

An experimental study of nonlinear ac magnetic susceptibility and microwave spin dynamics of R₂CuO₄ quasi-2D Heisenberg antiferromagnets (R=Eu, Pr, Gd) has been carried out. The data obtained can be accounted for if one assumes the existence of a random-field (RF) state in the Eu₂CuO₄ and Pr₂CuO₄ tetragonal crystals within the 77⩽T⩽350 K range covered. If this is so, 3D antiferromagnetic order persists only within limited regions, while in CuO₂ layers there are 2D Heisenberg antiferromagnetic spin fluctuations with large correlation lengths. In the Gd₂CuO₄ crystal there exists, besides uniform 3D antiferromagnetic long-range order with weak ferromagnetism, an admixture of an RF-type state. Fiz. Tverd. Tela (St. Petersburg) 41, 1437–1444 (August 1999)     

Thermal vibrations and the structure of quasi-two-dimensional R 2CuO4 crystals (R=La, Pr, Nd, Sm, Eu, and Gd)

Thermal vibrations of ions in R ₂CuO₄ crystals (R=La, Pr, Nd, Sm, Eu, Gd) were studied by x-ray diffractometry. A comparative analysis of thermal displacements of the copper and rare-earth ions permitted a conclusion as to the main interactions responsible for the structural state of the CuO₂ sheets and of a crystal as a whole. The structural properties were found to correlate with the magnitude of the ionic radius and with the ground state of the rare-earth ions.     

Spin dynamics and phase transitions in quasi-2D antiferromagnets <Emphasis Type="Italic">R</Emphasis><Subscript>2</Subscript>CuO<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis>=Pr,Nd, Sm,Eu, Gd)

The spin and lattice dynamics of the R₂CuO₄ quasi-2D antiferromagnetic crystals (R=Pr, Nd, Sm, Eu, Gd) were studied in the millimeter-range electromagnetic wave band. Strong variations of the absorption coefficient were observed to occur at temperatures T≈T₀. Absorption lines of electrical nature due to lattice dynamics were also revealed near the T₀ temperatures. The observed anomalies are assumed to originate from phase transitions at T≈T₀, which entail changes in the structural and magnetic properties.     

Transport properties and mangetoresistance of (RE,Sr)2NiO4 (RE=Pr,Nd, Sm and Eu)

We report on the crystal growth and the measurements of resistivity, magnetoresistance and photoemission spectroscopy of (RE,Sr)₂NiO₄ in the heavily Sr-doped region. Although the in-plane resistivity of the heavily Sr-doped crystals was rather small at room temperature, it increased for several orders of magnitude with decreasing temperature. A large negative magnetoresistance was observed for NdSrNiO₄ at low temperature, while only small magnetoresistance was observed for the single crystals of RE=Pr, Sm and Eu. Photoemission spectroscopy measurements revealed a significant influence of the RE 4f states to the electronic structure at relatively low binding energies in PrSrNiO₄ and NdSrNiO₄.     

Spin reorientations in R2Fe14-xCoxB systems (R = Pr,Nd and Er)

The effect of cobalt substitution on spin reorientation phenomena in R₂Fe_14-xCo_xB systems (R = Pr, Nd, Er) was studied by means of bulk magnetometry in the temperature range 4.2–1100 K. It was established that in the Nd-based system the introduction of cobalt resulted not only in a shift of the low temperature spin reorientation (cone to axis) but also triggered, for x ⩾ 10, an appearance of a second spin reorientation (axis to plane) at high temperature. In the Pr-based system, for x ⩾ 9.5, a high temperature spin reorientation (axis to plane) was also observed. Its dependence on Co content was determined. For the Er-based system, an increase of the spin reorientation temperature (plane to axis) was observed as more cobalt was introduced into that system. It was also established that the tetragonal single-phase materials in this system exist only up to x = 5. Temperature-composition diagrams are presented, indicating types of spin arrangements observed in the investigated systems.     

X-ray diffraction study of ion thermal vibrations in R2CuO4 (R=Pr and Gd) crystals

An X-ray diffraction study is reported of the symmetry in the spatial distribution of thermal vibrations of Cu₂₊, Gd³⁺, and Pr³⁺ ions in Gd₂CuO₄ crystals. An analysis of the pattern of the angular thermal-vibration amplitude distribution obtained experimentally at different temperatures allows a conclusion about the character of the local Jahn-Teller effect for the Cu²⁺ ions, structural phase transitions, and the orbital ground state of the Cu²⁺ ions.     

Raman study of orbital mediated multiphonons in RMnO3 (R = Pr,Sm,Eu,Tb,Y)

We have studied RMnO₃ manganites (R = Pr, Sm, Eu, Tb, Y) Raman excitations in the 200–2800 cm^-1 range as a function of temperature. Combinations of phonon energies are observed up to the fourth order, indicating the presence of electron-phonon coupling. In comparison to Γ-point phonon combinations, double phonon excitations appear to be blue shifted in large size rare earth ion compounds. The phonon combination intensities decrease rapidly with their increasing order, confirming other studies which conclude that the electron-phonon coupling is not as strong as supposed in the localized limit. Moreover, different intensity order dependences are observed between the phonon combination and the so-called Jahn-Teller mode. These effects are better described in the orbiton-phonon coupling scheme.     

IR active phonons in single crystals RFe3(BO3)4 (R = Pr,Nd, and Sm)

We have studied the reflection and transmission spectra of multiferroics RFe₃(BO₃)₄ (R = Pr, Nd, and Sm) at room temperature. Simulation of reflection spectra by the dispersion-analysis method have allowed us to determine the frequencies and oscillator strengths of the majority of IR active A ₂ and E vibrational modes.     

Structural phase transition in substituted La2CuO4 and Pr2CuO4−y

The structural phase transition from orthorhombic (T) phase to tetragonal (T′) phase in substituted La_2−x R _x CuO₄ (R = Pr, Nd, Sm, Eu and Gd) and T′ to T-phase in Pr_2−x M _x CuO_4−y (M = Sr, Ca) has been studied by X-ray diffraction technique. The T-phase of La₂CuO₄ is transferred to T′ phase abruptly atx=0.8, 0.4, 0.4, 0.3 and 0.4 respectively for substitution of Pr, Nd, Sm, Eu and Gd for La in La₂CuO₄ without evidence of the T* phase. The T′ structure of Pr₂CuO₄ (x = 0.0) gets transformed to the T* structure at 30% Ca doping (x=0.6) and then to the T structure at 50% Ca doping (x=1.0), while for Sr-contentx=0.0, 0.4 and 1.0 it shows T′, T* and T structure respectively.