Session M32: Focus Session: Multiferroics II and Other 3d Transition Metal Oxides

Imaging and characterization of conducting ferroelectric domain walls by photoemission electron microscopy

Unusual electronic properties arise at ferroelectric domain walls due to the low local symmetry and hypersensitivity of these natural oxide interfaces to electrostatics and strain. A major challenge is to expand the experimentally accessible parameter space in order to better understand these interfacial phenomena and ultimately exploit them to design domain-wall-based next-generation devices. Here, we show that ferroelectric domain walls can be visualized based on photo-induced charging effects using high-resolution X-ray photoemission electron microscopy (X-PEEM). We probe local variations in the electronic conductance in the ferroelectric semiconductor ErMnO$_{3}$ contact-free and with nanometer resolution by mapping the kinetic energy distribution of photoelectrons. We find a pronounced domain-wall contrast which we assign to the local conductivity by a direct comparison with scanning probe microscopy data. Our experiments reveal a new and non-destructive pathway for element-specific studies of electronic and chemical domain-wall structures in ferroelectric and multiferroic bulk systems.   

Magnetochiral dichroism resonant with electromagnons in a chiral-lattice magnet Cu$_{2}$OSeO$_{3}$

Multiferroics, in which magnetic and ferroelectric properties coexist and are entangled, is one of the most promising candidates for materials that are capable of the control of the magnetization (M) by an electric field and the electric polarization (P) by a magnetic field. One direct consequence of the strong magnetoelectric (ME) coupling in multiferroics is an intriguing spin excitation endowed with electric-dipole activity. This remarkable spin excitation, termed as electromagnon, is ubiquitously found in multiferroics. In this study, we performed broadband microwave spectroscopy under magnetic fields in Faraday geometry to study the ME resonant character of the electromagnon in a multiferroic chiral-lattice magnet Cu$_{2}$OSeO$_{3}$. We successfully observed different transmittance for oppositely propagating microwaves upon the electromagnon--the behavior known as magnetochiral dichroism (MChD). By studying the MChD spectra for various configurations, we elucidate the relationship between the MChD spectra and the dynamics of M and P upon the electromagnon excitation. We also discuss the magnitude of the MChD within the framework of a quantum theory and find that the theory well reproduces the experiment.   

Giant thermal Hall effect in polar magnets (Zn,Fe)$_{2}$Mo$_{3}$O$_{8}$

Spin transport in magnetic insulators has been attracting much attention because of the fundamental and technological interest for future spintronics. Recently we have observed Hall effect of magnons in ferromagnetic insulators with pyrochlore and perovskite structures in terms of the thermal Hall effect. Observed thermal Hall conductivity can be well explained by the Berry curvature of magnons induced by the Dzyaloshinsky-Moriya spin-orbit interaction which reflects the lattice geometry, while the magnitude of the signal is small and Hall effects in other magnetic phases have been unknown. In this work, we have studied thermal Hall effect in magnetic insulators (Zn,Fe)$_{2}$Mo$_{3}$O$_{8}$. (Zn,Fe)$_{2}$Mo$_{3}$O$_{8}$ has the polar crystal structure and shows various magnetic phases by changing the composition ratio of Fe and Zn or by applying the magnetic field. We have observed giant thermal Hall effect in ferrimagnetic phase in which the thermal Hall conductivities are twenty times larger than those of the magnon Hall effect observed in the previous study. We discuss possible mechanism of the large thermal Hall effect in this systems.   

Magnetoelectricty in antiferromagnetic Ni$_3$TeO$_6$ with chiral and polar lattice

Ni$_3$TeO$_6$, crystallizing in a polar and chiral lattice structure ($R$3 space group), becomes antiferromagnetic below $T_N$ = 52 K. It was reported that magnetic field-induced spin flop transition below $T_N$ accompanies a record-large non-hysteretic magnetoelectric effect (1300 ps/m). Here we present comprehensive magnetic, electric and structural properties of Ni$_3$TeO$_6$ single crystals for magnetic fields up to 92 T and temperatures down to 1.5 K. We discovered an additional sharp field-induced phase transition at 55 T, accompanied by a colossal change of electric polarization - the largest magnetically induced polarization change observed to date. We interpret our data with the help of density-functional calculations and construct a microscopic model across the whole magnetic field range.   

Electric Control of the Local Magnetic Moment in Multiferroic Compound Ba2CoGe2O7

Ba$_{2}$CoGe$_{2}$O$_{7}$ is a multiferroics exhibiting a collinear antiferromagnetic structure with the easy axis along $<$100$>$ and an antiferroelectric one with the electric polarization along [001] at $T \le 6.7$ K. For a local CoO$_4$ tetrahedron, the relation between the polarization {\bf p} and the magnetic moment {\bf S} is explained by spin-dependent $d$ - $p$ hybridization mechanism. In previous study we reported that a spin-nematic interaction was responsible for the magnetic anisotropy and the structure of the polarization[1]. In the present study we report a neutron diffraction in the electric field to demonstrate the electric control of the magnetic moment. With the increase of the field along [001] the magnetic intensities at $Q = (h,~k,~0)$ with $h >$ 0 and $k >$ 0 decrease, and in contrast, those with $h >$ 0 and $k <$ 0 increase. Combination of the results, the relation between {\bf p} and {\bf S}, and the assumption that the {\bf p}-direction does not deviate from [001] leads to the constraint that the moment is confined in (001) plane. The electric field dependence of the Bragg intensities is explained by the model that the direction of the magnetic moment continuously rotates from $<$100$>$ to $<$110$>$. [1] M. Soda, et al., PRL {\bf 112}, 127205 (2014).   

Co-existence of {\it ac} and {\it bc}-cycloidal phases, giant magnetocapacitance and polarization in the multiferroic R$_{0.5}$Dy$_{0.5}$MnO$_3$ (R = Eu and Gd)

In the cycloidal RMnO$_3$ (R = Tb and Dy) multiferroic compounds, the cycloidal spin propagates along the bc plane and polarization appears along c-direction. In the case of GdMnO$_3$, it appears that the cycloidal plane is along the ab plane and the polarization points in the a-direction. In my talk, I will demonstrate the emergence of these two cycloidal ordered phases with temperature and their co-existence from the measurements of pyroelectric current and dielectric properties in mixed rare-earth compounds, Eu$_{0.5}$Dy$_{0.5}$MnO$_3$ and Gd$_{0.5}$Dy$_{0.5}$MnO$_3$. The obtained value of polarization is in the same order of magnitude as in the E-type magnetic multiferroic (HoMnO$_3$). Further, these compound show giant magnetic field enhancement of ferroelectric polarization (up to +150\% in Gd$_{0.5}$Dy$_{0.5}$MnO$_3$) in contrast to TbMnO$_3$ (up to -10\%). Also, the value of magneto-capacitance is nearly twenty times higher than that of TbMnO$_3$. We attribute this high value of polarization and magneto-capacitance to the coexistence of ab and bc cycloidal phases. The isothermal current observed upon change of magnetic field with time, demonstrates giant magnetoelectric coupling for a number of cycle without any training effect.   

Determination of the Mott-Hubbard gap in GdTiO3

The band gaps of rare-earth titanates are commonly reported to be 0.2-0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report theoretical and experimental results on GdTiO$_3$ (GTO) indicating that the gap is significantly larger. First-principles calculations, based either on density functional theory (DFT) with a hybrid functional or on DFT+U, show that the gap is close to 2 eV. We compare these results with photoluminescence (PL) measurements, which show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. We show that the previously reported optical absorption spectra arise from excitations related to small hole polarons. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials. This work was supported by ONR and NSF.   

Carrier Mediated Ferromagnetism in Fe-doped SrTiO$_{3}$

The discovery of III-V dilute magnetic semiconductors (DMSs) and the subsequent unsuccessful search for room temperature ferromagnetism in DMSs have motivated researches on alternate dilute magnetic systems. Recent progresses in thin flim growth techniques of perovskite oxides suggest that dilute magnetic oxides (DMOs) can be viable candidates to improve the magnetic properties of DMSs. In this talk we present an ab initio study of Fe-doped SrTiO$_{3}$. We find that a ferromagnetic ordering among localized Fe t$_{2g}$~spins is mediated by itinerant Fe e$_{g}$~electrons. The exchange interaction between t$_{2g}$~and e$_{g}$~electrons depends on crystal field splitting, on-site electron-electron interaction, and the relative energy of Fe d-ortbitals to oxygen p-orbitals. The exchange coupling and the majority-minority spin splitting decrease with decreasing carrier concentration, confirming that itinerant carriers mediate the ferromagnetism.   

Spin waves and phonons in a paraelectric antiferromagnet EuTiO3

Perovskite titanates ATiO3 (A=Ba,Pb,Sr,Ca,Cd,or Eu) are widely studied for their interesting instabilities and broad applications. A ferroelectric (FE) transition occurs in Ba, Pb, and Cd titanates, but not in SrTiO3 (STO) or EuTiO3 (ETO). In the case of STO, fluctuations yield a quantum paraelectric state, but whether ETO is quantum paraelectric remains an open question. Despite a number of similarities with well-studied STO, ETO is also unique owing to the magnetic Eu ions. By applying a tuning parameter, such as bi-axial tension, ETO can be turned into a FE ferromagnet, the ideal multiferroic. [J. H. Lee, et al., Nature 466, 954 (2010)] Studies of spin-spin and spin-lattice couplings in ETO are of great interest not only from a fundamental standpoint, but also for technological applications. We successfully grew a large, high-quality isotopically-enriched ETO crystal for neutron scattering. The crystal and magnetic structures were characterized with single crystal diffraction at HB-3A at HFIR at ORNL. The spin waves and phonons were measured in the temperature range of 1.5-400 K with CNCS at SNS and HB-3 at HFIR at ORNL. In this presentation, we will discuss structural instabilities, spin-spin interactions, and spin-phonon couplings in ETO.   

Changes in magnetic properties of cobalt-iron-titanium oxide due to temperature variations

It has been found that the magnetic properties of cobalt ferrite such as magnetization, Curie temperature, including the electrical and structural properties can be tailored in a remarkably linear pattern by substituting titanium and controlling its concentration. The dependence of magnetocrystalline anisotropy and coercivity on temperature variations was found to be different compared to previous studies on cation substitution in cobalt ferrite. For example, we found a competition between magnetocrystalline anisotropy and microstructure in controlling coercivity; one dominating at higher concentration of substitution and the other dominating at lower concentration. It was also found that, while magnetocrystalline anisotropy controls the temperature dependence of coercivity, obstruction to domain wall processes by pinning sites in the microstructure controls the compositional dependence of coercivity. The physics of magnetism controlling the observed properties and how those depend on temperature variations will be presented. Such understanding is necessary for the application of the material in device development.   

The impact of small polarons on the properties of rare-earth titanates

Optical conductivity measurements are frequently applied to determine the band gaps of complex oxides, such as the rare-earth titanate (RTiO$_{\mathrm{3}})$ Mott insulators. The onset of the measured spectra, in the range of 0.2-0.7 eV, is commonly interpreted as the Mott-Hubbard gap. However, first-principles calculations that take strong electron-electron interactions into account [using either density functional theory (DFT) with a hybrid functional or DFT$+$U] produce band gaps close to 2 eV for GdTiO$_{\mathrm{3}}$ and YTiO$_{\mathrm{3}}$ [Himmetoglu et al., Phys. Rev. B 90, 161102 (2014)]. This raises the question of the origin of the absorption below 2 eV observed in optical experiments. We attribute this signal to excitation of small hole polarons. The rare-earth titanates commonly exhibit unintentional p-type conductivity, and we show that hole localization in the form of small polarons is energetically favorable. Configuration-coordinate diagrams enable us not only to assess the peak in optical absorption, but also to model the lineshape. Good agreement with experiment indicates that the infrared absorption is indeed likely to be polaron-related in GdTiO$_{\mathrm{3}}$. The results probably apply to other rare-earth titanates as well.   

Twofold spin reorientation and field induced incomplete phase transition in single crystal Dy$_{0.5}$Pr$_{0.5}$FeO$_{3}$

Recently, rare earth orthoferrites show emerging magnetoelectric effect, ultrafast optomagnetic effect, promising applications in multiferroics and ultrafast optomagnetic recording. All these properties come from its intrinsic coupling between R-4f and Fe-3d electrons, and strongly correlated with the spin reorientation (SR) transition. We report an intriguing twofold SR transition for the Fe- magnetic sublattice near SR temperatures TSR1$=$77 K and TSR2$=$45 K in Dy0.5Pr0.5FeO3 single crystal. Magnetic field-induced incomplete spin configuration transition was observed by measurement of magnetization as a function of temperature. The SR temperature of Dy0.5Pr0.5FeO3 single crystal can be controlled by changing the magnitude of the applied magnetic field. We also show that SR between TSR2 and TSR1 can be induced by an applied magnetic field along c axis. The origin of the magnetic behavior is ascribed to the anisotropic effective field whose strength is determined by the interactions with rareearth spins and can be modified by the external applied magnetic field. It provides deeper insights into the R-4f and Fe-3d magnetic interaction which dominate the sophisticated magnetic phase transitions in the rare earth orthoferrites.   

A-site magnetic ordering in quadruple perovskite oxides

Magnetic exchange interaction in A-site ordered quadruple perovskites AA'$_3$B$_4$O$_{12}$ is comprehensively investigated by using first-principles calculations. The ideal crystal structure ($Im{\bar 3}$) is characterized by the square-planer oxygen coordination around A' cations as well as the $a^+a^+a^+$ type tilting of BO$_6$ octahedra. Owing to these structural features, the compounds can include transition-metal ions both at the A' and B sites. Consequently, there are two magnetic sublattices with different oxygen coordination. Unlike the B-site magnetism that has been investigated for decades, detailed mechanisms for the A'-site magnetism and A'-B intersublattice magnetism are still unclear. In insulating compounds such as CaCu$_3$Ge$_4$O$_{12}$ and YMn$_3$Al$_4$O$_{12}$, it is found that the nearest-neighbor superexchange interaction between the A' sites determines the ground-state magnetic ordering. Furthermore, in our simulation, it is shown that magnetic phase transition from antiferromagnetism to ferromagnetism will occur along with insulator-to-metal transition in YMn$_3$Al$_4$O$_{12}$ by modulating the tilting of AlO$_6$ octahedra. Possible strategies are suggested to realize such a modulation, for example, by imposing chemical or physical pressure.   

Looking for multiferroics through oxygen deficiency in SrTi$_{1-x}$(Fe$_{x}$,Co$_{x}$)O$_{3-\delta}$

We present a theoretical/experimental study of AB$_{1-x}$$B'_{x}$O$_{3-\delta}$ magnetic perovskites obtained by solid solution of Fe and Co into SrTiO$_3$. We focus on the role of the oxygen deficiency as the factor triggering both a change of the saturation magnetization and the apparition of a macroscopic electric polarization. The magnetism is analyzed by calculating the t$_{2g}$e$_{g}$ occupancies of the magnetic cations for different O-deficiencies $(\delta)$, substitution values $(x)$ and FM/AFM orderings as well as the electric polarization is studied through the noncentrosymmetric resulting structures generated by the intrinsic strain related to the oxygen holes and changes of the ions radii. The optical properties are quantified by calculating the total and projected density of states while we take into account all the possible configurations of the O-vacancies respect to different B$'$ locations at the supercells. Ab-initio results are compared with experimental on es for SiTr(Fe,Co)O$_3$ and we show that a stable multiferroic state seems to be possible for intermediate values of the oxidation and magnetic composition as suggested by the agreement between modeling and experiments.   

Electronic structure of Fe$_{3}$O$_{4}$ revealed by RIXS

Magnetite (Fe$_{3}$O$_{4}$) is a prototypical example of a mixed valence compound. At 125 K, Fe$_{3}$O$_{4}$ undergoes the Verwey transition associated with charge ordering. The valence electrons order themselves over the octahedral sites (B sites) to form a Fe$^{2+}$ and Fe$^{3+}$ superstructure in the insulating phase below the transition temperature. Here we report measurements of resonant inelastic soft X-ray scattering (RIXS) to unravel the electronic structures of Fe$_{3}$O$_{4}$. By the advantage of high energy-resolution ($\Delta$E $\approx$ 80 meV) of the RIXS spectra, we found two distinctive features of magnetic excitations, coming from octahedral Fe$^{2+}$ and Fe$^{3+}$ separately. These magnetic excitations are described well by a local ionic model. The $dd$ excitations of the different Fe sites are also revealed in the RIXS spectra.