Order Parameters and Phase Diagrams of Multiferroics

The symmetry properties, order parameters, and magnetoelectric phase diagrams of multiferroics are discussed. After brief reviews of Ni$_3$V$_2$O$_8$, TbMnO$_3$, and RbFe(MoO$_4$)$_2$, we present a detailed analysis of RMn$_2$O$_5$ (with R=Y, Ho, Dy, Er, Tb, Tm).Comment: 23 pages, 7 figures, 3 tables. For FOCUS ON MULTIFERROICS (IOP

Order Parameters and Phase Diagram of Multiferroic \u3cem\u3eR\u3c/em\u3eMn\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e

The generic magnetic phase diagram of multiferroic RMn2O5 (with R = Y, Ho, Tb, Er, Tm), which allows different sequences of ordered magnetic structures for different R’s and different control parameters, is described using order parameters which explicitly incorporate the magnetic symmetry. A phenomenological magnetoelectric coupling is used to explain why some of these magnetic phases are also ferroelectric. Several new experiments, which can test this theory, are proposed

Order Parameters and Phase Diagram of Multiferroic RMn2_2O5_5

The generic magnetic phase diagram of multiferroic RMn$_2$O$_5$ (with R=Y, Ho, Tb, Er, Tm), which allows different sequences of ordered magnetic structures for different R's and different control parameters, is described using order parameters which explicitly incorporate the magnetic symmetry. A phenomenological magneto-electric coupling is used to explain why some of these magnetic phases are also ferroelectric. Several new experiments, which can test this theory, are proposed

Study of RFe12−xMox (R =Y, Ho) compounds by neutron powder diffraction, ac susceptibility and magnetization

Neutron powder diffraction, magnetization and ac susceptibility measurements were performed on compounds of the series RFe12−xMox (R D Y and Ho, x D 1, 2, 3). The influence of the Mo content on both structural and magnetic properties is discussed. Comparison with published data for different Mo concentrations is made. It is found that the effect of Mo substitution for Fe on structural and magnetic properties can be described by two regimes separated by a critical Mo content around x=2.Portuguese-French JNICT-CNRS collaboration

Strain-driven non-collinear magnetic ordering in orthorhombic epitaxial YMnO3 thin films

We show that using epitaxial strain and chemical pressure in orthorhombic YMnO3 and Co-substituted (YMn0.95Co0.05O3) thin films, a ferromagnetic response can be gradually introduced and tuned. These results, together with the measured anisotropy of the magnetic response, indicate that the unexpected observation of ferromagnetism in orthorhombic o-RMnO3 (R= Y, Ho, Tb, etc) films originates from strain-driven breaking of the fully compensated magnetic ordering by pushing magnetic moments away from the antiferromagnetic [010] axis. We show that the resulting canting angle and the subsequent ferromagnetic response, gradually increase (up to ~ 1.2\degree) by compression of the unit cell. We will discuss the relevance of these findings, in connection to the magnetoelectric response of orthorhombic manganites.Comment: Text + Figs Accepted in J. Appl. Phy

Optical Spectroscopy of Orbital and Magnetic Excitations in Vanadates and Cuprates

Within the scope of this thesis the low-energy excitations of undoped Mott insulators RVO(3) with R = Y, Ho, and Ce, (Sr,Ca)CuO(2) and La(8)Cu(7)O(19) have been investigated by means of optical spectroscopy. The compounds RVO(3) with R = rare-earth ion recently have attracted a lot of interest because of their unusual structural, orbital, and magnetic properties. The compounds undergo a series of temperature-induced phase transitions accompanied by a change of orbital and magnetic order. Furthermore, it has been proposed that YVO(3) represents the first realization of a one-dimensional orbital liquid and an orbital Peierls phase, with a transition to an orbitally ordered phase at lower temperatures. In this thesis, we present the optical conductivity sig(w) of RVO(3) with R = Y, Ho, and Ce for energies from 0.1 to 1.6 eV as a function of temperature (10-300 K) and polarization of the incident light parallel to the crystallographic axes (sig_a, sig_b, sig_c). Our main experimental result is the observation of two absorption features at 0.55 eV in sig_a and 0.4 eV in sig_c which are assigned to collective orbital excitations, in contrast to conventional local crystal-field transitions. Altogether our results strongly suggest that in RVO(3) with R = Y, Ho, and Ce the orbital exchange interactions play a decisive role. In a second study we have investigated the magnetic excitations of low-dimensional quantum magnets, namely the spin chain (Sr,Ca)CuO(2) and the five-leg ladder La(8)Cu(7)O(19). For (Sr,Ca)CuO(2), two absorption features around 0.4 eV in sig_c (chain direction) and sig_b (inter-chain direction) are identified as magnetic contributions to the optical conductivity. The analysis of sig_c enables the very precise determination of the nearest-neighbor exchange coupling Jc as a function of temperature and Ca substitution. We have found Jc = (227 +- 4) meV for SrCuO(2) at low temperatures and no effect on Jc upon Ca-substitution of 10%. Furthermore, we ascertained that the theoretically predicted two-spinon-plus-phonon contribution describes roughly 74-97% of the observed spectral weight. We suggest that the remaining weight has to be ascribed to multi-spinon excitations. To explain the small magnetic absorption features in sig_b we sketch a new mechanism which gives rise to a dipole moment perpendicular to the chains when holes are virtually exchanged along the chain. In particular, we point out the possible relevance of this contribution to the optical conductivity of many quasi-1D quantum spin systems. Our data of La(8)Cu(7)O(19) are the first experimental result on the magnetic excitations of n-leg ladders (n > 2) at high energies. Amongst others, we compared our data with the well-investigated spectrum of the two-leg ladder and found a surprising resemblance. Although a detailed understanding of the observed line shape of the 5-leg ladder is missing up to now, our data strongly suggest the existence of bound states of magnetic excitations also in this system. The comparison of different low-dimensional S = 1/2 systems suggests that the magnetic excitations in the high-energy continua are similar and develop smoothly from 1D to 2D

Investigation of structural, magnetic and optical properties of rare earth substituted bismuth ferrite

Polycrystalline BiFeO3 and rare earth substituted Bi 0.9R0.1FeO3 (BRFO, R=Y, Ho and Er) compounds were prepared by rapid solid state sintering technique. Structural phase analysis indicated that all the compounds stabilized in rhombohedral structure (R3c space group) and a small orthorhombic phase fraction was observed in BRFO compounds. From the Raman spectra results, the changes in the phonon frequencies (A1) and line widths suggested lattice distortion in the BRFO compounds as was evidenced in the XRD analysis. Compared to the linear variation of magnetization with magnetic field (M-H) shown by BFO, an obvious M-H loop was observed in BRFO compounds which could be due to the suppression of space modulated spin structure and was explained on the basis of weak ferromagnetism and field induced spin reorientation. UV-Vis spectroscopy evidenced a change in local FeO6 environment due to shift in the 6A 1g→4T2g energy transition band. BRFO compounds with improved remnant magnetization and coercive field are applicable for magnetoelectric devices

High-pressure structural stability of multiferroic hexagonal RMnO

Structural changes in RMnO 3 (R = Y, Ho, Lu) under high pressure were examined by synchrotron x-ray diffraction methods at room temperature. Compression occurs more readily in the ab plane than along the c axis. With increased pressure, a pressure-induced hexagonal to orthorhombic phase transition was observed starting at ∼22 GPa for Lu(Y)MnO 3 . When the pressure is increased to 35 GPa, a small volume fraction of Lu(Y)MnO 3 is converted to the orthorhombic phase and the orthorhombic phase is maintained on pressure release. High-pressure infrared absorption spectroscopy and Mn K-edge near-edge x-ray absorption spectroscopy confirm that the hexagonal P6 3 cm structure is stable below ∼20 GPa and the environment around the Mn ion is not changed. Shifts in the unoccupied p-band density of states with pressure are observed in the Mn K-edge spectra. A schematic pressure-temperature phase diagram is given for the small ion RMnO 3 system