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Register a new userInfrared study of the charge-ordered multiferroic LuFe(2)O(4)
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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.
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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.
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.
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.
We present the muon spin relaxation/rotation spectra in the multiferroic compound (Cu,Zn)$_{3}$Mo$_{2}$O$_{9}$. The parent material Cu$_{3}$Mo$_{2}$O$_{9}$ has a multiferroic phase below $T_{rm N}$ = 8.0 K, where the canted antiferromagnetism and the ferroelectricity coexist. The asymmetry time spectra taken at RIKEN-RAL pulsed muon facility show a drastic change at $T_{rm N}$. At low temperatures the weakly beating oscillation caused by the static internal magnetic fields in the antiferromagnetic phase was observed in Cu$_{3}$Mo$_{2}$O$_{9}$ and the lightly ($0.5%$) Zn-doped sample. From the fitting of the oscillating term, we obtain the order parameter in these samples: ferromagnetic moment in two sublattices of antiferromagnet. In the heavily ($5.0%$) Zn-doped sample, the muon-spin oscillation is rapidly damped. The frequency-domain spectrum of this sample suggests a formation of magnetic superstructure.
We report an infrared spectroscopy study of the axion topological insulator candidate EuIn$_2$As$_2$ for which the Eu moments exhibit an A-type antiferromagnetic (AFM) order below $T_N simeq 18 mathrm{K}$. The low energy response is composed of a weak Drude peak at the origin, a pronounced infrared-active phonon mode at 185 cm$^{-1}$ and a free carrier plasma edge around 600 cm$^{-1}$. The interband transitions start above 800 cm$^{-1}$ and give rise to a series of weak absorption bands at 5,000 and 12,000 cm$^{-1}$ and strong ones at 20,000, 27,500 and 32,000 cm$^{-1}$. The AFM transition gives rise to pronounced anomalies of the charge response in terms of a cusp-like maximum of the free carrier scattering rate around $T_N$ and large magnetic splittings of the interband transitions at 5,000 and 12,000 cm$^{-1}$. The phonon mode at 185 cm$^{-1}$ has also an anomalous temperature dependence around $T_N$ which suggests that it couples to the fluctuations of the Eu spins. The combined data provide evidence for a strong interaction amongst the charge, spin and lattice degrees of freedom.
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