Session X9: Focus Session: Complex Bulk Oxides: Magnetic Phase Transitions

Temperature and pressure induced Cu-Fe intermetallic charge transfer in LaCu$_{3}$Fe$_{4}$O$_{12}$

An A-site ordered double perovskite LaCu$_{3}$Fe$_{4}$O$_{12}$ was prepared at high pressure and high temperature. At ambient condition, the charge combination was proved to be LaCu$^{3+}_{3}$Fe$^{3+}_{4}$O$_{12}$. When the temperature was increased to 393 K, however, an intermetallic charge transfer was found to occur between the A-site Cu and the B-site Fe ions, giving rising to the change of charge combination from the low-temperature LaCu$^{3+}_{3}$Fe$^{3+}_{4}$O$_{12}$ to the high-temperature LaCu$^{2+}_{3}$Fe$^{3.75+}_{4}$O$_{12}$.[1,2] Similarly, at room temperature, if high pressure was applied, the Cu-Fe intermetallic charge transfer could also be induced. This charge transfer led to a first-order isostructural phase transition with sharp reduction in unit cell, negative thermal expansion and unusual softening (decreased bulk modulus). Moreover, the material experienced antiferromagnetism-to-paramagnetism and insulator-to-metal transitions accompanying with the charge-transfer transition. \\[4pt] [1] Y. W. Long \textit{et al}, Nature, 458, 60-63 (2009). \\[0pt] [2] Y. W. Long and Y. Shimakawa, New J. Phys. 12, 063029 (2010).   

Spin-phonon coupling effect in $A$MnO$_3$ ($A$=Ca, Sr, Ba) and La$M$O$_3$ ($M$=Cr, Fe, Cr/Fe) from DFT+$U$ and hybrid functional methods

Spin-phonon coupling effects, as reflected in phonon frequency shifts between ferromagnetic (FM) and G-type antiferromagnetic (AFM) configurations in cubic CaMnO$_3$, SrMnO$_3$, BaMnO$_3$, LaCrO$_3$, LaFeO$_3$ and La$_2$(CrFe)O$_6$, are investigated using density-functional methods. The calculations are carried out using the DFT+$U$ method with a $U$ that has been extracted by comparing with hybrid-functional (HSE) calculations. The phonon frequency shifts $\Delta \omega = \omega_{\rm AFM} - \omega_{\rm FM}$ obtained in this way agree well with those computed directly from the more accurate HSE approach, but are obtained with much less computational effort. We find that in the $A$MnO$_3$ materials class with ($A$=Ca,Sr,Ba), the $\Gamma$ ($R$) phonon frequency shift $\Delta \omega$ decreases (increases) as the $A^{2+}$ size increases. In La$M$O$_3$ ($M$=Cr, Fe, Cr/Fe), the phonon frequencies at $\Gamma$ decrease as spin order changes from AFM to FM for LaCrO$_3$ and LaFeO$_3$, but they increase for double perovskite La$_2$(CrFe)O$_6$. We discuss the prospects for bulk and superlattice forms of these materials to be useful as multiferroics.   

Nd magnetic order in NdFe$_x$Ga$_{1-x}$O$_3$

The Nd magnetic order in NdFe$_x$Ga$_{1-x}$O$_3$ has been studied as a function of Fe content along the whole $0 [Preview Abstract]

Magnetic phase transitions in single crystalline Mn$_{0.68}$Ni$_{0.32}$TiO$_3$

The magnetoelectric MnTiO$_3$ has the ilmenite structure and order antiferromagnetically with neighboring Mn$^{2+}$ spins antiparallel to each other both within the ab-plane and along the c-axis. We have observed a magnetic field induced electric polarization in the 32$\%$Ni-doped MnTiO$_3$. To understand the origin of this magnetoelectric effect, we have carried out neutron diffraction study on single crystalline Mn$_{0.68}$Ni$_{0.32}$TiO$_3$. The Mn$^{2+}$ spins order below 27 K and arrange in the same antiparallel manner as the parent compound, but with different spin direction. On cooling the magnetic phase goes through a second transition at 15 K, below which the spins lock into a new direction. We have also determined the spin structure under a magnetic field applied along the c-axis.   

Electron energy loss spectroscopy study of Sr$_{2-x}$Gd$_x$TiMnO$_6$

The newly synthesized double perovskite family Sr$_{2-x}$Gd$_x$TiMnO$_6$ (0$<$x$<$1) is magnetically frustrated system that orders magnetically at T=40K. In spite of the probable double exchange interaction around x=0.5, no metallic state is established and the magnetoresistance is weak in the whole family. The most interesting feature in this material is the giant electroresistance that persists even at room temperature. We have studied the microscopic composition of these polycrystals by the electron energy loss spectroscopy. We find that, is spite of some precipitations of Mn and Ti rich regions that exist in 0.25$\leq$x$\leq$0.75, the manganese and titanium ions are generally well intermixed in both interior of the grains and on the grain boundaries. We discuss these results in the frame of highly non-linear electrical conductivity found in these materials.   

Density functional modeling for a perovskite SrTi$_{1-x}$Co$_{x}$O$_3$ system: Beyond GGA+U functional

Understanding the exact mechanisms of spin-stabilization according to the oxygen stoichiometry and substituting metal-transition ions is essential to finding new perovskites-based technologies to augment silicon-based devices with room-temperature spintronic materials. We perform ab-initio modeling for the SrTi$_{1-x}$Co$_{x}$O$_3$ system with x = 0.0, 0.25, 1.0, and by using a HSE06 hybrid functional. Electronic structure for x=(0.0,1.0) predicts 3.1 eV band gap and a metallic behavior, respectively, predicting lattice parameters in agreement with experiments. Different positions for the cobalt ions are taken into account for x=0.25, and predicted ground states and Co energy-correlations suggest the structurally stabilized Co-site positions as fingerprints of whether possible intrinsic magnetic ordering or other phenomena is giving rise to the macroscopic magnetism. Passing from Stoichiometric SrTi$_{0.75}$Co$_{0.25}$O$_3$ to 1-O-vacancy SrTi$_{0.75}$Co$_{0.25}$O$_{3-\delta}$ produces a lattice parameter expansion due to the valence state distribution of the substituting B (ABO$_3$) ions, which is turn lead us to a band gap expansion. The stabilized valence spin states of the magnetic ions are discussed in comparison with the ligand-field theory.   

Role of cooperative structural distortions in the metal--insulator transitions of perovskite ferrates

Transition-metal oxides within the perovskite crystal family exhibit strong electron--electron correlation effects that coexist with complex structural distortions, leading to metal-insulator (MI) transitions. Using first-principles density functional calculations, we investigate the effects of cooperative octahedral rotations and dilations/contractions on the charge-ordering MI-transition in CaFeO$_3$. By calculating the evolution in the lattice phonons, which describe the different octahedral distortions present in the low-symmetry monoclinic phase of CaFeO$_3$ with increasing electron correlation, we show that the MI-transition results from a complex interplay between these modes and correlation effects. We combine this study with group theoretical tools to disentangle the electron--lattice interactions by computing the evolution in the low-energy electronic band structure with the lattice phonons, demonstrating the MI-transition in CaFeO$_3$ proceeds through a symmetry-lowering transition driven by a cooperative three-dimensional octahedral dilation/contraction pattern. Finally, we suggest a possible route by which to control the charge ordering by fine-tuning the electron--lattice coupling.   

Far infrared spectroscopy of magnons and phonons in TbFeO$_{3}$ single crystal

Far-infrared spectra of TbFeO$_{3}$ single crystals have been studied in the temperature range between 1.6 K and 300 K using transmission in high magnetic field and rotating analyzer ellipsometry. The symmetry of the IR optical phonons and their oscillator strengths were determined. Polarization and frequencies of two AFM resonances at around 18 and 23 cm$^{-1}$ were analyzed around the spin reorientation (SR) transition at $\sim $8K and magnetic fields up to 9 T. Intensity of the AFM resonances exhibit an unusual oscillator dependence on both temperature and magnetic field. The observed effects are analyzed taking into account main magnetic interactions in the system including exchange of the Fe$^{3+}$ spins with Tb$^{3+}$ paramagnetic moments as well as the geometry of the measurements.   

Optical properties of iron oxides

Magnetoelectric coupling in materials like multiferroics, dilute magnetic semiconductors, and topological insulators has attracted a great deal of attention, although most work has been done in the static limit. Optical spectroscopy offers a way to investigate the dynamics of charge-spin coupling, an area where there has been much less effort. Using these techniques, we discovered that charge fluctuation in LuFe$_2$O$_4$, the prototypical charge ordered multiferroic, 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. At the same time, dramatic splitting of the LuO$_2$ layer phonon mode is attributed to charge-rich/poor proximity effects, and its temperature dependence reveals the antipolar nature of the W layer pattern. Using optical techniques, we also discovered that $\alpha$-Fe$_2$O$_3$, a chemically-similar parent compound and one of the world's oldest and most iconic antiferromagnetic materials, appears more red in applied magnetic field than in zero field conditions. This effect is driven by a field-induced reorientation of magnetic order. The oscillator strength lost in the color band is partially transferred to the magnon side band, a process that also reveals a new exciton pattern induced by the modified exchange coupling. Analysis of the exciton pattern exposes $C2/c$ monoclinic symmetry in the high field phase of hematite. Taken together, these findings advance our understanding of iron-based materials under extreme conditions. \\[4pt] Collaborators include: X. S. Xu, P. Chen, Q. -C. Sun, T. V. Brinzari (Tennessee); S. McGill (NHMFL); J. De Groot, M. Angst, R. P. Hermann (Julich); A. D. Christianson, B. C. Sales, D. Mandrus (ORNL); A. P. Litvinchuk (Houston); J. -W. Kim (Ames); Z. Islam (Argonne); N. Lee, S. -W. Cheong (Rutgers).   

Theory of Spectroscopy and Transport in Half-metallic Double Perovskites

Half-metallic double perovskites hold great promise in spintronics applications, hence we are motivated to understand the spectroscopy and charge transport in these materials. We present theoretical calculations of the temperature and disorder dependence of the spin-resolved density of states of the conduction electrons, the optical conductivity $\sigma(\omega)$, and the anomalous Hall conductivity for Sr$_2$FeMoO$_6$ (SFMO), a half-metal with 100\% spin-down polarized charge carriers at $T=0$. We build on the recent progress [1] in modeling magnetic properties of SFMO by using an exact diagonalization plus Monte Carlo scheme. We obtain $\sigma(\omega)$ as a function of disorder as well as temperature using the Kubo formula for linear response in the exact eigenstate basis. In agreement with experiment, we find a secondary peak in $\sigma(\omega)$ at $\omega\sim0.5\,eV$, and attribute it to a spin-up density of states at the chemical potential induced by disorder and/or thermal fluctuations. We propose that the size of the secondary peak can be used to determine the polarization of conduction electrons at the chemical potential, facilitating experimental measurements. \newline [1] O. Erten et al, arXiv:1107.0983; Phys. Rev. Lett. (to appear)   

High T$_c$ Ferrimagnetism, Multiband Mott Transition and Spin-Orbit Coupling in Double Perovskites

The ferrimagnetic insulator Sr$_2$CrOsO$_6$ (SCOO), which has the highest $T_c = 725$K among all double perovskites, raises several questions. Why is this material an insulator? What sets the scale for the high T$_c$? Why is there a net moment given that both Cr and Os have d$^3$ configurations? What is the role of spin-orbit coupling in Os? Finally, why does SCOO show a highly unusual, non-monotonic magnetization $M(T)$ as a function of temperature? We address all of these questions. First, we describe the charge sector using slave-rotor mean field theory and obtain an analytic Mott criterion $\sqrt{U_{Cr}U_{Os}} > 2.5W$ relating the Hubbard $U$'s to the bandwidth $W$. We argue that SCCO is a multiband Mott insulator. Next, we argue that the orbital moment on Os is quenched in SCOO and spin-orbit coupling does not play a major role in this material. Finally, we show that the effective spin Hamiltonian for SCOO has both Cr-Os and Os-Os superexchange interactions that are frustrated. This leads to a canted ground state with a net moment at $T=0$ and a nonmonotonic magnetization $M(T)$. Our results are in excellent agreement with available data and we make predictions to test our theory.   

Magnetically driven metal-insulator transition in NaOsO$_3$

The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials, enjoying interest both for its fundamental nature and technological application. Various mechanisms producing MITs have been extensively considered over the years associated with the names of their originators, including most especially Mott (electron localization through Coulomb repulsion) and Anderson (localization through disorder). An alternative route due to Slater dating back to 1951, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention, particularly from an experimental viewpoint. Using neutron and X-ray scattering we have shown that the MIT in NaOsO$_3$ is coincident with the onset of long-range commensurate three dimensional magnetic order at 410 K. NaOsO$_3$ thus encompasses all of the expected features of the long predicted Slater transition. Our results are the first definitive experimental example of a Slater MIT and we discuss them in the light of recent reports of a Mott spin-orbit insulating state in other $5d$ oxides.