The modulated antiferromagnetic structures in multiferroic FeVO4: A 57Fe Mössbauer spectroscopy investigation

International audienceWe present an investigation of the two incommensurate magnetic phases in the multiferroic material FeVO4 using magnetic susceptibility and 57Fe Mössbauer spectroscopy measurements. The susceptibility shows anomalies at the phase transitions at 15.7 and 23 K. We devise Mössbauer lineshapes appropriate for planar elliptical and collinear modulated magnetic structures and show that they reproduce very well the Mössbauer spectra in FeVO4, in full qualitative agreement with a previous neutron diffraction study. Quantitatively, our spectra provide precise determinations of the characteristics of the elliptical and modulated structures which match rather well the neutron diffraction results. We find that the hyperfine field elliptical modulation persists as T→0, which we attribute to an anisotropy of the hyperfine interaction since a moment modulation is forbidden at T=0 for a spin only ion like Fe3+

Palmer-Chalker correlations in the XY pyrochlore antiferromagnet Er2Sn2O7

\ersn\, is considered, together with \erti, as a realization of the XY antiferromagnet on the pyrochlore lattice. We present magnetization measurements confirming that \ersn\, does not order down to 100 mK but exhibits a freezing below 200 mK. Our neutron scattering experiments evidence the strong XY character of the \er moment and point out the existence of short range correlations in which the magnetic moments are in peculiar configurations, the Palmer-Chalker states, predicted theoretically for an XY pyrochlore antiferromagnet with dipolar interactions. Our estimation of the \ersn\, parameters confirm the role of the latter interactions on top of relatively weak and isotropic exchange couplings

Magnetic anisotropy, unusual hysteresis and putative "up-up-down" magnetic structure in EuTAl4Si2 (T = Rh and Ir)

International audienceWe present detailed investigations on single crystals of quaternary EuRhAl4Si2 and EuIrAl4Si2. The two compounds order antiferromagnetically at TN1 = 11.7 and 14.7 K, respectively, each undergoing two magnetic transitions. The magnetic properties in the ordered state present a large anisotropy despite Eu2+being an S-state ion for which the single-ion anisotropy is expected to be weak. Two features in the magnetization measured along the c-axis are prominent. At 1.8 K, a ferromagnetic-like jump occurs at very low field to a value one third of the saturation magnetization (1/3 M0) followed by a wide plateau up to 2 T for Rh and 4 T for Ir-compound. At this field value, a sharp hysteretic spin-flop transition occurs to a fully saturated state (M0). Surprisingly, the magnetization does not return to origin when the field is reduced to zero in the return cycle, as expected in an antiferromagnet. Instead, a remnant magnetization 1/3 M0 is observed and the magnetic loop around the origin shows hysteresis. This suggests that the zero field magnetic structure has a ferromagnetic component, and we present a model with up to third neighbor exchange and dipolar interaction which reproduces the magnetization curves and hints to an “up-up-down” magnetic structure in zero field

Spin dynamics in the ordered spin ice Tb2_2Sn2_2O7_7

Geometrical frustration is a central challenge in contemporary condensed matter physics, a crucible favourable to the emergence of novel physics. The pyrochlore magnets, with rare earth magnetic moments localized at the vertices of corner-sharing tetrahedra, play a prominent role in this field, with a rich variety of exotic ground states ranging from the "spin ices" \hoti\ and \dyti\ to the "spin liquid" and "ordered spin ice" ground states in \tbti\ and \tbsn. Inelastic neutron scattering provides valuable information for understanding the nature of these ground states, shedding light on the crystal electric field (CEF) level scheme and on the interactions between magnetic moments. We have performed such measurements with unprecedented neutron flux and energy resolution, in the "ordered spin ice" \tbsn. We argue that a new interaction, which involves the spin lattice coupling through a low temperature distortion of the trigonal crystal field, is necessary to account for the data

Towards a model of a dynamical Jahn-Teller coupling at very low temperatures in Tb 2 Ti 2 O 7

International audienceWe present an interpretation of zero-field energy integrated (" diffuse ") neutron scattering and of high-field magnetization data at very low temperature in the frustrated pyrochlore system Tb 2 Ti 2 O 7. This material has antiferromagnetic exchange interactions, and it is expected to have an Ising character at low temperature. Contrary to expectations, it shows no magnetic ordering down to 0.05 K, being thus labeled a " spin liquid. " However, the ground state in Tb 2 Ti 2 O 7 is not a mere fluctuating moment paramagnet but, as demonstrated by very recent experiments, a state where the electronic degrees of freedom are hybridized with the phononic variables in an unconventional way. We show here that, by approximating this complex and still unraveled electron-phonon interaction by a dynamic Jahn-Teller coupling, one can account rather well for the diffuse neutron scattering and the low-temperature isothermal magnetization

EuNiGe 3 , an anisotropic antiferromagnet

International audienceSingle crystals of EuNiGe 3 crystallizing in the non-centrosymmetric BaNiSn 3-type structure have been grown using In flux, enabling us to explore the anisotropic magnetic properties which was not possible with previously reported polycrystalline samples. The EuNiGe 3 single crystalline sample is found to order antiferromagnetically at 13.2 K as revealed from the magnetic susceptibility, heat capacity and electrical resistivity data. The low temperature magnetization M(H) is distinctly different for field parallel to ab-plane and c-axis; the ab-plane magnetization varies nearly linearly with field before the occurrence of an induced ferromagnetic phase (spin-flip) at 6.2 Tesla; on the other hand M(H) along the c-axis is accompanied by two metamagnetic transitions followed by a spin-flip at 4.1 T. A model including anisotropic exchange and dipole-dipole interactions reproduces the main features of magnetization plots but falls short of full representation. (H,T) phase diagrams have been constructed for the field applied along the principal directions. From the 151 Eu Mössbauer spectra, we determine that the 13.2 K transition leads to an incommensu-rate antiferromagnetic intermediate phase followed by a transition near 10.5 K to a commensurate antiferromagnetic configuration

Resistive switching in ceramic multiferroic Bi 0.9Ca 0.1FeO 3

We report resistive switching effects in polycrystalline samples of the multiferroic Bi 0.9Ca 0.1FeO 3 with silver electrodes. Mössbauer spectroscopy shows that upon Ca-doping the Fe remains in a 3 valence state, suggesting charge compensation through the creation of large amounts of oxygen vacancies. Electrical characterization shows that the oxide/metal resistance can be switched between high and low resistance states by applying voltage pulses. This process was shown to be forming free and a strong relaxation after switching was found. We rationalize our results by considering oxygen vacancies migration to and from the metal-oxide interface, resulting in variations of the Schottky potential barrier height that modulate the interface resistance.Fil: Rubi, Diego. Comisión Nacional de Energía Atómica; ArgentinaFil: Gomez-Marlasca, Fernando. Comisión Nacional de Energía Atómica; ArgentinaFil: Bonville, Pierre. Centre National de la Recherche Scientifique; FranciaFil: Colson, Dorothee. Centre National de la Recherche Scientifique; FranciaFil: Levy, Pablo Eduardo. Comisión Nacional de Energía Atómica. Gerencia del Área Investigaciones y Aplicaciones no Nucleares; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin