Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H17: Focus Session: Bulk Properties of Complex Oxides - Ferrites |
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Sponsoring Units: DMP GMAG Chair: Peter Gehring, National Institute of Standards and Technology Room: D174 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H17.00001: Inelastic neutron scattering measurements on the triangular lattice antiferromagnet CuFe1-xGaxO2 in the paraelectric and multiferroic phases Invited Speaker: The intensive efforts to study multiferroic materials in recent years have led to a better understanding of the fundamental physical processes and interactions leading to the complex behavior in those compounds. In this talk, I will focus on the recent neutron scattering measurements and calculations of the spin dynamics of the triangular lattice antiferromagnet CuFe$_{1-x}$Ga$_{x}$O$_{2}$. In pure CuFeO$_{2}$ a low-temperature~collinear spin structure is stabilized by long range magnetic interactions. The spin wave spectra show dynamics precursory to the multiferroic phase. When this system is doped with a few percent of nonmagnetic gallium, its low-temperature phase has a complex noncollinear (CNC) spin order and becomes multiferroic. The CNC phase appears to have distorted screw-type magnetic configuration that is stabilized by the displacement of the oxygen atoms. The spin dynamics and the spin order in this material, as well as their implications to the origin of the ferroelectricity will be discussed [1,2]. \\[4pt] [1] J. T. Haraldsen, et al., \textit{Phys. Rev. B}, \textbf{82}, 020404 (2010). \\[0pt] [2]. Research was sponsored by the U.S. DOE/BES, Division of Materials Sciences and Engineering and the Division of Scientific User Facilities. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H17.00002: Investigating the spin dynamics of the non-collinear magnetic phase of 3.5\% Ga-doped CuFeO$_2$ T. Kimura, J.T. Haraldsen, F. Ye, R.S. Fishman, J.A. Fernandez-Baca, Y. Yamaguchi, K. Kimura We examine the evolution of the non-collinear phase of a hexagonal lattice antiferromagnet to help understand the inelastic neutron scattering measurements for the multiferroic ground state of 3.5\% Ga-doped CuFeO$_2$. With the complex ground state stabilized by the displacement of the oxygen atoms, the multiferroic coupling is explained by the predicted ``spin-driven'' model. By comparing the observed and calculated spectrum of spin excitations for multiple spin configurations, we conclude that the magnetic ground state is a distorted screw-type spin configuration with a distribution of turn angles produced by lattice distortions. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H17.00003: Long-period solitonic lattice in the rare earth orthoferrite TbFeO3 Niels Jensen, Sergey Artyukhin, Andrey Maljuk, Kim Lefman, Maxim Mostovoy, Dimitri Argyriou Rare earth orthoferrites show a variety of magnetic transitions and spectacular magneto-electric effects originating from the coupling between the iron and rare earth magnetic sublattices. Our recent single-crystal neutron diffraction measurements revealed the presence of an unusual incommensurate phase in TbFeO3, which is induced by the magnetic field along the c axis and has a period of about 100 unit cells. We also present the results of model calculations, which explain the origin of this novel phase and reproduce the magnetic phase diagram of TbFeO3. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H17.00004: Magnetic and Raman spectroscopic studies of yttrium substituted BiFeO$_{3}$ R. Naik, S. Talebi, Ambesh Dixit, V. Naik, G. Lawes As a room temperature multiferroic BiFeO$_{3 }$is attractive for a number of applications. However the relatively weak magnetism limits its suitability for applications. Bi$_{1-x}$Y$_{x}$FeO$_{3}$ (x=0, 0.05, 0.10, 0.15 and 0.20) ceramic samples have been prepared by sol-gel technique and thin films by metalorganic decomposition. X-ray diffraction, X-ray photoelectron and Raman spectroscopy measurements confirm that these ceramic and thin film samples are of single phase. More remarkably these Y modified BiFeO$_{3}$ samples exhibit enhanced room temperature magnetic and magnetodielectric properties, with the saturation magnetization approaching 25 emu cm$^{-3}$ for x=0.20. We discuss the correlation between the enhanced room temperature magnetization and two-phonon Raman modes in the context of investigating the origin of the magnetic properties in Bi$_{1-x}$Y$_{x}$FeO$_{3}$. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H17.00005: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H17.00006: Relation between electric phase and magnetic ordering of Y-type Hexaferrite Woo-suk Noh, H. Jang, K.-T. Ko, J.-H Park, S.H. Chun, K.H Kim, B.-G Park, J.-Y Kim Y-type hexaferrite Ba$_{0.5}$Sr$_{1.5}$Zn$_{2}$Fe$_{12}$O$_{22}$(BSZFO), one of multiferroic materials, we could acquire magnetic field-induced commensurate phase, changing of magnetic phase of BSZFO and Ba$_{0.5}$Sr$_{1.5}$Zn$_{2}$(Fe$_{1-x}$Al$_{x}$)$_{12}$O$_{22}$ (x = 0.08)(BSZFAO) using resonant soft X-ray scattering(RSXS) experiment. Also we could confirm that magnetic ordering changing has some relation with electric phase transition, q=1.5 at ferroelectric phase not only BSZFO but also BSZFAO. This research results were acquired by using 2A EPU beamline at PAL. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H17.00007: Magnetic Spectra on Charge Ordering in $R_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ ($R$ = La, Pr, and Nd) Jie Ma, Jiaqiang Yan, Souleymane Omar Diallo, Rebecca Stevens, Anna Llobet, Frans Trouw, Douglas Abernathy, Matthew Stone, Robert McQueeney $R_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ ($R$ = La, Pr, and Nd) compounds are reported to have the same charge ordering (CO) and Neel temperatures. Inelastic neutron scattering is applied to study the magnetic energy effect on the CO state in this system. Based on the ratio of the ferromagnetic exchange energy ($J_{F})$ and antiferromagnetic exchange energy ($\vert J_{AF}\vert )$, the magnetic exchange energy can stabilize the CO state in La$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ and Pr$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ alone; with the smaller $R^{3+}$ substitution, $\vert J_{AF}\vert $ increases a lot from La$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ to Nd$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$, but the CO of Fe ions could still be driven by magnetic energy itself with the correction on magnetic energy ratio. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H17.00008: First-principles calculations for XAS of infinite-layer iron oxides Mitsuru Kodera, Tatsuya Shishidou, Tamio Oguchi The oxygen defect perovskite SrFeO$_{3-x}$ shows various properties such as the giant magnetoresistance effect and the thermoelectric effect. It had been believed that the oxygen content in SrFeO$_{3-x}$ changes up to $x=0.5$. Recently, Tsujimoto $et$ $al.$ have succeeded in synthesizing the infinite-layer iron oxide SrFeO$_2$. SrFeO$_2$ has a square-planar oxygen coordination, while the iron oxides usually have the tetrahedral and octahedral coordination. CaFeO$_2$ has also infinite layer structure and the same magnetic ordering as SrFeO$_2$. However, it is suggested that the oxygen coordination of CaFeO$_2$ is different from that of SrFeO$_2$. In order to investigate the electronic structure of iron in (Ca, Sr)FeO$_2$, the x-ray absorption spectroscopy (XAS) spectrum has been measured. In this work, we perform the calculation for XAS spectrum near the Fe-K edge of (Ca, Sr)FeO$_2$ using the first-principles calculations. We compare the results with the experiment and discuss the electronic structure of iron in (Ca, Sr)FeO$_2$. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H17.00009: Canted-spin-caused electric dipoles: a local symmetry theory Thomas A. Kaplan, Subhendra D. Mahanti A pair of magnetic atoms with canted spins ${\rm {\bf S}}_a \,\mbox{and}\,{\rm {\bf S}}_b $can give rise to an electric dipole moment \textbf{P. } Several forms for the dependence of \textbf{P} on the spins have been derived from various microscopic models, some of which have been invoked to explain experimental results found in some multiferroic materials. The forms are ${\rm {\bf P}}_1 \propto {\rm {\bf R}}\times ({\rm {\bf S}}_{\rm {\bf a}} \times {\rm {\bf S}}_b ),\,{\rm {\bf P}}_2 \propto {\rm {\bf S}}_{\rm {\bf a}} \times {\rm {\bf S}}_b ,\,{\rm {\bf P}}_3 \propto {\rm {\bf S}}_{\rm {\bf a}} ({\rm {\bf S}}_{\rm {\bf a}} \cdot {\rm {\bf R}})-{\rm {\bf S}}_b ({\rm {\bf S}}_b \cdot {\rm {\bf R}})$, where \textbf{R} is the relative position of the atoms. To unify and generalize these various forms we consider \textbf{P }as the most general quadratic function of the spin components that vanishes whenever ${\rm {\bf S}}_{\rm {\bf a}} \,\mbox{and }{\rm {\bf S}}_b $are collinear. The study reveals new forms. We generalize to the vector \textbf{P}, Moriya's symmetry considerations on the (scalar) DM energy ${\rm {\bf D}}\cdot {\rm {\bf S}}_{\rm {\bf a}} \times {\rm {\bf S}}_b $(which led to restrictions on \textbf{D}). This provides a rigorous proof that \textbf{P}$_{1}$ is allowed no matter how high the symmetry of the atoms plus environment, and gives restrictions for all other contributions. The analysis leads to the suggestion of new terms omitted in the existing microscopic models, and predicts an unusual antiferroelectric ordering in the antiferromagnetically and ferroelectrically ordered phase of RbFe(MoO$_{4})_{2}$. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H17.00010: Structural and magnetic ordering in bulk Sr2FeMoO6 synthesized by planetary ball mill: The effects of grinding. J.M. Lucy, A.J. Hauser, B. Peters, F.Y. Yang The standard solid-state synthesis procedure has been widely used to make bulk complex oxides, including half-metallic double perovskite Sr2FeMoO6. However, although it is generally recognized that multi-step grinding and heating are crucial for synthesis of high quality materials, little has been done to quantitatively characterize the effect of grinding on the quality of the final products. We systematically varied the level of grinding, ranging from poor grinding by hand for a short period of time ($\sim $10 min) to very fine grinding and mixing by a planetary ball mill for many hours which produces uniform sub-micron particles. XRD, SEM and VSM were used to characterize the structural and magnetic properties. The Sr2FeMoO6 samples made by different grinding methods exhibit drastically different structural and magnetic ordering. The highest quality Sr2FeMoO6 is from the most thorough grinding and gives a close to ideal Fe/Mo ordering and magnetic moment close to 4 Bohr magneton per formula unit. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H17.00011: Theory of ferromagnetic double perovskites Oinam Nganba Meetei, Onur Erten, Anamitra Mukherjee, Mohit Randeria, Nandini Trivedi, Patrick Woodward We derive and validate an effective classical spin model which describes the magnetic properties of double perovskites (DP) like Sr$_{2}$FeMoO$_{6}$, including the effects of disorder and carrier concentration. This model generalizes the Anderson-Hasegawa model for manganites to DP's. We validate our effective spin model by making detailed comparisons with the results obtained from a quantum Hamiltonian of itinerant electrons interacting with spins on the Fe-sites. We show that the conduction electron polarization at the chemical potential $P(T)$ tracks the temperature-dependence of the total magnetization $M(T)$. We point out the importance of Coulomb correlation $U$ on Mo-sites and of direct Mo-Mo hopping $t^\prime$ on stabilizing the ferromagnetic phase as a function of electron doping (by La substitution of Sr). We show how the small parameters $U$ and $t^\prime$ are crucial in understanding the experimental results for T$_c$ as a function of carrier concentration. We predict how the ferromagnetic $T_c$ can be raised substantially (up to 40\%), without sacrificing the polarization $P$, by a combination of excess Fe and La-doping. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H17.00012: Role of disorder and doping in ferromagnetic double perovskites Anamitra Mukherjee, Oinam Nganba Meetei, Onur Erten, Mohit Randeria, Nandini Trivedi, Patrick Woodward We use an effective classical spin model, which we have recently derived and validated, to examine the effect of disorder on the magnetic properties of double perovskites like Sr$_{2}$FeMoO$_{6}$ (SFMO). We compute the effects of excess Fe, excess Mo, and anti-site disorder on the magnetization $M(T)$ and the ferromagnetic transition temperature $T_c$, and show that our results are quantitatively consistent with available experiments. We then use these results to propose routes to increase the ferromagnetic $T_c$ above that of pure SFMO (420K). We predict that, by adding excess Fe on Mo sites and compensating for the change in conduction electron density by La substitution on the Sr sites, we can raise $T_c$ by as much as 40\%, without substantially sacrificing the conduction electron polarization $P$ at the chemical potential. [Preview Abstract] |
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