Magnetic-field induced multiferroicity in a quantum critical frustrated spin liquid

Dielectric spectroscopy is used to check for the onset of polar order in the quasi one-dimensional quantum spin system Sul-Cu2Cl4 when passing from the spin-liquid state into the ordered spiral phase in an external magnetic field. We find clear evidence for multiferroicity in this material and treat in detail its H-T phase diagram close to the quantum-critical regime.Comment: 5 pages, 4 figures. Revised according to suggestions of referee

Relaxor ferroelectricity and the freezing of short-range polar order in magnetite

A thorough investigation of single crystalline magnetite using broadband dielectric spectroscopy and other methods provides evidence for relaxor-like polar order in Fe3O4. We find long-range ferroelectric order to be im-peded by the continuous freezing of polar degrees of freedom and the formation of a tunneling-dominated glasslike state at low temperatures. This also explains the lack of clear evidence for a non-centrosymmetric crystal structure below the Verwey transition. Within the framework of recent models assuming an intimate relation of charge and polar order, the charge order, too, can be speculated to be of short-range type only and to be dominated by tunneling at low temperatures.Comment: 16 pages, 4 figures, final version with revisions according to referee demand

FeCr₂S₄ in magnetic fields: possible evidence for a multiferroic ground state.

We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC ≈ 170 K and the transition at TOO ≈ 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5 T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K

Chain Formation by Spin Pentamers in eta-Na9V14O35

The nature of the gapped ground state in the quasi-one-dimensional compound eta-Na9V14O35 cannot easily be understood, if one takes into account the odd number of spins on each structural element. Combining the results of specific heat, susceptibility and electron spin resonance measurements we show that eta-Na9V14O35 exhibits a novel ground state where multi-spin objects build up a linear chain. These objects - pentamers - consist of five antiferromagnetically arranged spins with effective spin 1/2. Their spatial extent results in an exchange constant along the chain direction comparable to the one in the high-temperature state.Comment: 6 pages, 5 figure

Multiferroicity in an organic charge-transfer salt: Electric-dipole-driven magnetism

Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view. A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order. There, similar to conventional ferroelectrics, the electrical dipoles arise from an off-centre displacement of ions. However, recently a different mechanism, namely purely electronic ferroelectricity, where charge order breaks inversion symmetry, has attracted considerable interest. Here we provide evidence for this exotic type of ferroelectricity, accompanied by antiferromagnetic spin order, in a two-dimensional organic charge-transfer salt, thus representing a new class of multiferroics. Quite unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly simultaneously. This can be ascribed to the loss of spin frustration induced by the ferroelectric ordering. Hence, here the spin order is driven by the ferroelectricity, in marked contrast to the spin-driven ferroelectricity in helical magnets.Comment: 8 pages, 9 figures (including 4 pages and 6 figures in supplementary information). Version 2 with minor errors corrected (legend of Fig. 3c and definition of vectors e and Q

The multiferroic phases of (Eu:Y)MnO3

We report on structural, magnetic, dielectric, and thermodynamic properties of (Eu:Y)MnO3 for Y doping levels 0 <= x < 1. This system resembles the multiferroic perovskite manganites RMnO3 (with R= Gd, Dy, Tb) but without the interference of magnetic contributions of the 4f-ions. In addition, it offers the possibility to continuously tune the influence of the A-site ionic radii. For small concentrations x <= 0.1 we find a canted antiferromagnetic and paraelectric groundstate. For higher concentrations x <= 0.3 ferroelectric polarization coexists with the features of a long wavelength incommensurate spiral magnetic phase analogous to the observations in TbMnO3. In the intermediate concentration range around x = 0.2 a multiferroic scenario is realized combining weak ferroelectricity and weak ferromagnetism, presumably due to a canted spiral magnetic structure.Comment: 8 pages, 8 figure

Switching the Ferroelectric Polarization by External Magnetic Fields in the Spin = 1/2 Chain Cuprate LiCuVO4

We present a detailed study of complex dielectric constant and ferroelectric polarization in multiferroic LiCuVO4 as function of temperature and external magnetic field. In zero external magnetic field, spiral spin order with an ab helix and a propagation vector along the crystallographic b direction is established, which induces ferroelectric order with spontaneous polarization parallel to a. The direction of the helix can be reoriented by an external magnetic field and allows switching of the spontaneous polarization. We find a strong dependence of the absolute value of the polarization for different orientations of the spiral plane. Above 7.5 T, LiCuVO4 reveals collinear spin order and remains paraelectric for all field directions. Thus this system is ideally suited to check the symmetry relations for spiral magnets as predicted theoretically. The strong coupling of ferroelectric and magnetic order is documented and the complex (B,T) phase diagram is fully explored.Comment: 6 pages, 5 figure

Microscopic mechanisms of spin-dependent electric polarization in 3d oxides

We present a short critical overview of different microscopic models for nonrelativistic and relativistic magnetoelectric coupling including the so-called "spin current scenario", ab-initio calculations, and several recent microscopic approaches to a spin-dependent electric polarization in 3d oxides.Comment: 8 pages, 3 figure

Physical properties of FeSe0.5_{0.5}Te0.5_{0.5} single crystals grown under different conditions

We report on structural, magnetic, conductivity, and thermodynamic studies of FeSe$_{0.5}$Te$_{0.5}$ single crystals grown by self-flux and Bridgman methods. The samples were prepared from starting materials of different purity at various temperatures and cooling rates. The lowest values of the susceptibility in the normal state, the highest transition temperature $T_c$ of 14.5 K, and the largest heat-capacity anomaly at $T_c$ were obtained for pure (oxygen-free) samples. The critical current density $j_c$ of $8 \times 10^4$ A/cm$^2$ (at 2 K) achieved in pure samples is attributed to intrinsic inhomogeneity due to disorder at the cation and anion sites. The impure samples show increased $j_c$ up to $2.3 \times 10^5$ A/cm$^2$ due to additional pinning centers of Fe$_3$O$_4$. The upper critical field $H_{c2}$ of $\sim 500$ kOe is estimated from the resistivity study in magnetic fields parallel to the \emph{c}-axis. The anisotropy of the upper critical field $\gamma_{H_{c2}} = H_{_{c2}}^{ab}/H_{_{c2}}^{c}$ reaches a value $\sim 6$ at $T\longrightarrow T_c$. Extremely low values of the residual Sommerfeld coefficient for pure samples indicate a high volume fraction of the superconducting phase (up to 97%). The electronic contribution to the specific heat in the superconducting state is well described within a single-band BCS model with a temperature dependent gap $\Delta_0 = 27(1)$ K. A broad cusp-like anomaly in the electronic specific heat of samples with suppressed bulk superconductivity is ascribed to a splitting of the ground state of the interstitial Fe$^{2+}$ ions. This contribution is fully suppressed in the ordered state in samples with bulk superconductivity.Comment: 11 pages, 11 figures, 3 table

Gap structure in the electron-doped Iron-Arsenide Superconductor Ba(Fe0.92Co0.08)2As2: low-temperature specific heat study

We report the field and temperature dependence of the low-temperature specific heat down to 400 mK and in magnetic fields up to 9 T of the electron-doped Ba(Fe0.92Co0.08)2As2 superconductor. Using the phonon specific heat obtained from pure BaFe2As2 we find the normal state Sommerfeld coefficient to be 18 mJ/mol.K^2 and a condensation energy of 1.27 J/mol. The temperature dependence of the electronic specific heat clearly indicate the presence of the low-energy excitations in the system. The magnetic field variation of field-induced specific heat cannot be described by single clean s- or d-wave models. Rather, the data require an anisotropic gap scenario which may or may not have nodes. We discuss the implications of these results.Comment: New Journal of Physics in press, 10 pages, 5 figure