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Register a new userRobust charge and magnetic order under electric field and current in the multiferroic LuFe(2)O(4)
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We performed elastic neutron scattering measurements on the charge- and magnetically-ordered multiferroic material LuFe(2)O(4). An external electric field along the [001] direction with strength up to 20 kV/cm applied at low temperature (~100 K) does not affect either the charge or magnetic structure. At higher temperatures (~360 K), before the transition to three-dimensional charge-ordered state, the resistivity of the sample is low, and an electric current was applied instead. A reduction of the charge and magnetic peak intensities occurs when the sample is cooled under a constant electric current. However, after calibrating the real sample temperature using its own resistance-temperature curve, we show that the actual sample temperature is higher than the thermometer readings, and the intensity reduction is entirely due to internal sample heating by the applied current. Our results suggest that the charge and magnetic orders in LuFe(2)O(4) are unaffected by the application of external electric field/current, and previously observed electric field/current effects can be naturally explained by internal sample heating.
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The reflectivity of a large LuFe(2)O(4) single crystal has been measured with the radiation field either perpendicular or parallel to the c axis of its rhombohedral structure, from 10 to 500K, and from 7 to 16000 cm-1. The transition between the two-dimensional and the three-dimensional charge order at T_(CO) = 320 K is found to change dramatically the phonon spectrum in both polarizations. The number of the observed modes above and below T_(CO), according to a factor-group analysis, is in good agreement with a transition from the rhombohedral space group R{bar 3}m to the monoclinic C2/m. In the sub-THz region a peak becomes evident at low temperature, whose origin is discussed in relation with previous experiments.
We investigated the series of temperature and field-driven transitions in LuFe$_2$O$_4$ by optical and M{o}ssbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the order by fluctuation mechanism for the development of charge order superstructure. Bragg splitting and large magneto optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field.
In view of the recent experimental predictions of a weak structural transition in CoV$_{2}$O$_{4}$ we explore the possible orbital order states in its low temperature tetragonal phases from first principles density functional theory calculations. We observe that the tetragonal phase with I4$_1/amd$ symmetry is associated with an orbital order involving complex orbitals with a reasonably large orbital moment at Vanadium sites while in the phase with I4$_1/a$ symmetry, the real orbitals with quenched orbital moment constitute the orbital order. Further, to study the competition between orbital order and electron itinerancy we considered Mn$_{0.5}$Co$_{0.5}$V$_{2}$O$_{4}$ as one of the parent compounds, CoV$_{2}$O$_{4}$, lies near itinerant limit while the other, MnV$_{2}$O$_{4}$, lies deep inside the orbitally ordered insulating regime. Orbital order and electron transport have been investigated using first principles density functional theory and Boltzmann transport theory in CoV$_{2}$O$_{4}$, MnV$_{2}$O$_{4}$ and Mn$_{0.5}$Co$_{0.5}$V$_{2}$O$_{4}$. Our results show that as we go from MnV$_{2}$O$_{4}$ to CoV$_{2}$O$_{4}$ there is enhancement in the electrons itinerancy while the nature of orbital order remains unchanged.
Our results describe an unprecedented example of change in the mechanism of magnetically-induced electric polarization from spin current to spin-dependent p-d hybridization model. We have followed the evolution of the magnetic structures of (ND4)2[FeCl5 D2O] compound using single crystal neutron diffraction under external magnetic field. The spin arrangements change from incommensurate cycloidal to commensurate distorted-cycloidal and finally to quasi-collinear. The determination of the magnetic structures allows us to explain the observed electric polarization in the different ferroelectric phases. Two different magneto-electric coupling mechanisms are at play: the spin-current mechanism for external magnetic field below 5 T, and the spin dependent p-d hybridization mechanism for magnetic field above this value, being this compound the first example reported presenting this sequence of magneto-electric coupling mechanisms.
Using neutron diffraction, we have studied the magnetic field effect on charge structures in the charge-ordered multiferroic material LuFe$_2$O$_4$. An external magnetic field is able to change the magnitude and correlation lengths of the charge valence order even before the magnetic order sets in. This affects the dielectric and ferroelectric properties of the material and induces a giant magneto-electric effect. Our results suggest that the magneto-electric coupling in LuFe$_2$O$_4$ is likely due to magnetic field effect on local spins, in clear contrast to the case in most other known multiferroic systems where the bulk magnetic order is important.
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