Multiferroic LuFe2O4 Research Articles

Pressure-induced charge orders and their postulated coupling to magnetism in hexagonal multiferroic LuFe2O4

Hexagonal LuFe2O4 is a promising charge order (CO) driven multiferroic material with high charge and spin-ordering temperatures. The coexisting charge and spin orders on Fe3+/Fe2+ sites result in magnetoelectric behaviors, but the coupling mechanism between the charge and spin orders remains elusive. Here, by tuning external pressure, we reveal three charge-ordered phases with suggested correlation to magnetic orders in LuFe2O4: (i) a centrosymmetric incommensurate three-dimensional CO with ferrimagnetism, (ii) a non-centrosymmetric incommensurate quasi-two-dimensional CO with ferrimagnetism, and (iii) a centrosymmetric commensurate CO with antiferromagnetism. Experimental in situ single-crystal X-ray diffraction and X-ray magnetic circular dichroism measurements combined with density functional theory calculations suggest that the charge density redistribution caused by pressure-induced compression in the frustrated double-layer [Fe2O4] cluster is responsible for the correlated spin-charge phase transitions. The pressure-enhanced effective Coulomb interactions among Fe-Fe bonds drive the frustrated (1/3, 1/3) CO to a less frustrated (1/4, 1/4) CO, which induces the ferrimagnetic to antiferromagnetic transition. Our results not only elucidate the coupling mechanism among charge, spin, and lattice degrees of freedom in LuFe2O4, but also provide a new way to tune the spin-charge orders in a highly controlled manner.

Negative dynamic dielectric permittivity of a ceramic multiferroic LuFe-=SUB=-2-=/SUB=-O-=SUB=-4-=/SUB=- with oxygen nonstoichiometry under the combined influence of temperature and electric field

The temperature dependences (100-380 K) of the dielectric constant and conductivity of a ceramic multiferroic LuFe2O4 with oxygen nonstoichiometry, measured at alternating current (1, 10, 100 kHz, 1 MHz) and in a static electric field (0-7.1 V/mm), are presented. The regularity of the sign inversion of the dynamic permittivity caused by the appearance of solid-state inductance in the samples is discussed. Keywords: negative permittivity, solid-state inductance, multiferroic.

Отрицательная динамическая диэлектрическая проницаемость керамического мультиферроика LuFe-=SUB=-2-=/SUB=-O-=SUB=-4-=/SUB=- с кислородной нестехиометрией при совместном воздействии температуры и электрического поля

The temperature dependences (100 – 380 K) of the dielectric constant and conductivity of a ceramic multiferroic LuFe2O4 with oxygen nonstoichiometry, measured at alternating current (1, 10, 100 kHz, 1 MHz) and in a static electric field (0 – 8 V/1.4 mm), are presented. The regularity of the sign inversion of the dynamic permittivity caused by the appearance of solid-state inductance in the samples is discussed.

Terahertz Spectra Revealing the Collective Excitation Mode in Charge‐Density‐Wave Single Crystal LuFe 2 O 4

A low-energy collective excitation mode in charge-ordered multiferroic LuFe2O4 is reported via terahertz time-domain spectroscopy. Upon cooling from 300 to 40 K, the central resonance frequency showed a pronounced hardening from 0.85 to 1.15 THz. In analogy to the well-known low-energy optical properties of LuFe2O4, this emerging resonance was attributed to the charge–density–wave (CDW) collective excitations. By using the Drude–Lorentz model fitting, the CDW collective mode becomes increasingly damped with the increasing temperature. Furthermore, the kinks of the CDW collective mode at the magnetic transition temperature are analyzed, which indicate the coupling of spin order with electric polarization.

Resonant photoemission study of multiferroic LuFe2O4 across three-dimensional to two-dimensional charge ordering

Electronic structures modification of polycrystalline LuFe2O4 across 3 dimensional to 2 dimensional charge ordering (CO) has been studied by resonance photoemission spectroscopy. Constant-initial-state spectra measured across 3p → 3d resonance energy indicate its charge transfer insulating behavior. Valance band spectra study shows states lying closer to the Fermi level are due to Fe2+ ions. As the temperature is varied from 3D to 2D CO state, a weight transfer of DOS from low binding energy to higher binding energy is observed which is attributed to the modification in the Fe-O hybridized states.

Correlation between quantum charge fluctuations and magnetic ordering in multiferroic LuFe2O4

We examine the interplay between quantum charge fluctuations and magnetic ordering in multiferroic LuFe2O4 and show that this can couple spin and charge degrees of freedom in a LuFe2O4 bilayer below the Neel temperature TN. Our analysis supports the idea that the double exchange mechanism normally used in metallic systems can be applied to charge-ordered insulators. This causes ferrimagnetic spin order to reduce the transfer integrals between Fe2+ and Fe3+ in LuFe2O4, decreasing charge fluctuations and increasing the polarization in this system below TN. This work thus provides a more detailed understanding of the mechanism for spin-charge coupling in LuFe2O4.

Specific Heat Study of Multiferroic LuFe2O4 Single Crystal

Precise measurements of specific heat were performed on high quality single crystals of multiferroic LuFe2O4 by using an ac calorimeter. The thermal anomalies corresponding to four known phase transitions were observed at 220 K (Tg), 260 K (TN), 380 K (T3CO) and 590 K (T2CO). In addition to them, a new clear anomaly was detected at 305 K. No other distinct anomaly could be observed below 220 K down to 100 K. It is clarified that the temperature dependence of the order parameter associated with this new phase transition at 305 K is quite similar to those of spontaneous polarization reported previously by Ikeda et al.

Probing the Interplay between Quantum Charge Fluctuations and Magnetic Ordering in LuFe2O4

The mechanisms producing strong coupling between electric and magnetic order in multiferroics are not always well understood, since their microscopic origins can be quite different. Hence, gaining a deeper understanding of magnetoelectric coupling in these materials is the key to their rational design. Here, we use ultrafast optical spectroscopy to show that the influence of magnetic ordering on quantum charge fluctuations via the double-exchange mechanism can govern the interplay between electric polarization and magnetism in the charge-ordered multiferroic LuFe2O4.

Atomic-resolution chemical imaging of oxygen local bonding environments by electron energy loss spectroscopy

Electron energy loss spectroscopy on an aberration-corrected scanning transmission electron microscope was used to map the elemental composition and bonding in a thin film of the multiferroic LuFe2O4 with atomic resolution. A two-dimensional analysis of the fine structure of the O-K edge yielded distinct signals for the two inequivalent oxygen sites in the crystal. Comparison to an ab-initio simulation showed that these two components can be interpreted in terms of the differing hybridization of the O p orbitals to the Lu and Fe d orbitals, thus producing an atomic-resolution map of the local oxygen bonding environment.

Pulsed Laser Deposition as a Novel Growth Technique of Multiferroic LuFe2O4 Thin Films

AbstractGrowth of polycrystalline Lutetium Iron Oxide via pulsed laser deposition in thin film form is reported for the first time herein, and the multiferroic LuFe2O4 phase is stabilized. Fluence and pressure dependent phase growth is demonstrated, along with crystalline structure in vacuum (˜10-5 torr) conditions. Thermodynamic considerations at the laser-target interaction were investigated, as well as at the plume-substrate interface, which reveal that the necessary Gibbs free energy is available in the optimized growth environment to allow formation of the LuFe2O4 polycrystalline phase. The resulting growth rate is found to be related to the Gibbs free energy and concentration of nucleation sites on the substrate. Characterization of the multiferroic aspect of LuFe2O4 entailed direct measurement of the ferroelectricity in the thin film, as well as magnetic behavior, both at various temperatures. In particular, the ferroelectric polarization vs. voltage data yield values of 0.61 μC/cm2 at 300 K to 3.29 μC/cm2 at 183 K; moreover, these data are in agreement with those reported in the literature. Magnetization vs. applied field data shows the magnetization at 300 K to be 180 emu/cm3 and increasing to 200 emu/cm3 at 10 K.

Formation of Pancakelike Ising Domains and Giant Magnetic Coercivity in FerrimagneticLuFe2O4

We have studied quasi-two-dimensional multiferroic LuFe2O4 with strong charge-spin-lattice coupling, in which low-temperature coercivity approaches an extraordinary value of 9 T in single crystals. The enhancement of the coercivity is connected to the collective freezing of nanoscale pancakelike ferrimagnetic domains with large uniaxial magnetic anisotropy ("Ising pancakes"). Our results suggest that collective freezing in low-dimensional magnets with large uniaxial anisotropy provides an effective mechanism to achieve enhanced coercivity. This observation may help identify novel approaches for synthesis of magnets with enhanced properties.