Session A19: Manganites and Cobaltites

Investigating short-range magnetism in strongly correlated materials via magnetic pair distribution function analysis and \textit{ab initio} theory

Short-range magnetic correlations are known to exist in a variety of strongly correlated electron systems, but our understanding of the role they play is challenged by the difficulty of experimentally probing such correlations. Magnetic pair distribution function (mPDF) analysis is a newly developed neutron total scattering method that can reveal short-range magnetic correlations directly in real space, and may therefore help ameliorate this difficulty. We present temperature-dependent mPDF measurements of the short-range magnetic correlations in the paramagnetic phase of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. We observe significant correlations on a $\sim$1~nm length scale that differ substantially from the low-temperature long-range-ordered spin arrangement. With no free parameters, \textit{ab initio} calculations using the self-interaction-corrected local spin density approximation of density functional theory quantitatively reproduce the magnetic correlations to a high degree of accuracy. These results yield valuable insight into the magnetic exchange in MnO and showcase the utility of the mPDF technique for studying magnetic properties of strongly correlated electron systems.   

Magnetic Order in the Mixed-Spin Triangular Lattice Antiferromagnet Na$_x$MnO$_2$

Na$_x$TMO$_2$ (TM = transition metal) materials consist of alternating layers of Na and TM ions with the TM ions arranged on a geometrically frustrated triangular lattice. Na can be easily and reversibly removed from these materials, making them of interest for application in rechargeable batteries and allowing for exploration of their rich phase diagrams as a function of Na concentration. Na ordering is an important factor in ground state selection, and is driven by electrostatic interactions in many Na$_x$TMO$_2$ systems. The TM = Mn series differs in that Na ordering is driven by a cooperative Jahn-Teller effect, due to the coexistence of Jahn-Teller active Mn$^{3+}$ and inactive Mn$^{4+}$ ions. This effect also results in an ordered arrangement of the Mn$^{3+}$ and Mn$^{4+}$ ions, and thus of spin-2 and spin-3/2 moments. For x = 5/8, we have recently shown the coexistence of charge and magnetic stripe orderings [1]. Here, we present the results of neutron diffraction measurements performed on single crystal samples of Na$_x$MnO$_2$ and discuss the details of the magnetic structure in the magnetically ordered phase. \\[4pt] [1] X. Li \textit{et al.} Nature Mater. \textbf{13}, 586 (2014).   

Spin polarized scanning tunneling microscopy of bilayer manganite \textit{La}$_{2-2x}$\textit{ Sr}$_{1+2x}$\textit{Mn}$_{2}O_{7}$ single crystals.

We employ spin-polarized scanning tunneling microscopy to investigate the (001) surface of bilayer manganite \textit{La}$_{2-2x}$\textit{Sr}$_{1+2x}$\textit{Mn}$_{2}O_{7}$ single crystals with $x=$\textit{0.32} at various temperature and different magnetic fields. A spin reorientation transition (SRT) at this doping level starts around 70K, when the ferromagnetic spins change from out of plane to in plane configuration. Tracing the SRT while applying magnetic field along the c axis we are going to investigate the corresponding magnetic domain wall structure and motion in detail by spin polarized tunneling. Moreover, we are also going to search for the emergence of biskyrmions near the same doping level.   

Electronic, structural and magnetic properties of LaMnO3 phase transition at high temperature

We have developed a theoretical approach for investigating systems that contain a range of correlation that varies with experimentally controlled parameters. We applied this method to the LaMnO3 compound[1,2,3] to accurately describe the antiferromagnetic (AFM) insulating ground-state, the metal-to-insulator transition and the high temperature ferromagnetic (FM) state, where we observe a half-metallic behavior. [1]: T. Saitoh, A. E. Bocquet, T. Mizokawa, H. Namatame, A. Fujimori, M. Abbate, Y. Takeda, and M. Takano, \textit{Phys. Rev. B, }, 13942 (1995). [2]: J. S. Zhou, and J. B. Goodenough, \textit{Phys. Rev. B, }, R15002 (1999). [3]: J. Rodriguez-Carvajal, M. Hennion, F. Moussa, A. H. Mouden, L. Pinsard, and A. Revcolevschi, \textit{Phys. Rev. B}, , R3189 (1998).   

Spin wave damping in colossal magnetoresistive La$_{0.7}$Ca$_{0.3}$MnO$_{3}$

The hole-doped perovskite La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ is best known for the colossal magnetoresistance displayed at a combined ferromagnetic and metal-insulator phase transition ($T_c$=257~K). Previous studies have reported that the spin wave excitations in the ferromagnetic phase become anomalously damped near the Brillouin zone boundary, though a later work suggested that this was a measurement artifact due to an optical phonon branch. We have used the ARCS time-of-flight neutron spectrometer to investigate the spin wave excitations of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ at $T$=100~K and find a damping for spin waves at energies exceeding 20~meV that cannot be explained solely by proximity to the phonon branch. With additional measurements using the BT7 triple-axis neutron spectrometer, the spin wave damping is explored as a function of reduced wavevector, excitation energy, and temperature.   

Biquadratic and ring exchange interactions in orthorhombic perovskite manganites

We use ab initio electronic structure calculations within the GGA+U approximation to density functional theory (DFT) to determine the microscopic exchange interactions in the series of orthorhombic rare-earth manganites (o-RMnO$_3$). Our motivation is to construct a model Hamiltonian (excluding effects due to spin-orbit coupling), which can provide an accurate description of the magnetism in these materials. First we map the exchange couplings for several representatives of o-RMnO$_3$ series onto a Heisenberg Hamiltonian and find a clear deviation from the Heisenberg-like behavior. We demonstrate that this deviation can be explained only by the presence of relatively strong higher order exchange interactions (biquadratic and four-spin ring couplings) and show that they have the strongest effect in compounds, where nearest-neighbor exchange interactions are weakened due to the presence of large GdFeO$_3$-type distortion. Finally we discuss how these higher order terms determine magnetic ground states, influence magnetic excitations and define the multiferroic properties of o-RMnO$_3$.   

Role of Entropy and Structural Parameters in the Spin State Transition of $LaCoO_3$

The spin state transition in LaCoO$_3$ has eluded description for decades despite concerted theoretical and experimental effort. In this study, we approach this problem using fully charge consistent Density Functional Theory + Dynamical Mean Field Theory (DFT+DMFT). We show, from first principles, that LaCoO$_3$ cannot be described by a single, pure spin state at any temperature, but instead shows a gradual change in the population of higher spin multiples as temperature is increased. We explicitly elucidate the critical role of the lattice expansion and oxygen octahedral rotations in the spin state transition. We also show that the spin state transition and the metal-insulator transition in the compound occur at different temperatures. In addition, our results shed light on the importance of electronic entropy, which has so far been ignored in all first principles studies of this material.   

LaCoO$_{\mathrm{3}}$ (LCO) - Dramatic changes in Magnetic Moment in fields to 500T

LCO has attracted great attention over the years (\textgreater 2000 publications) because of its unusual magnetic properties; although in its ground state at low temperatures it is non-magnetic. A recent experiment$^{\mathrm{[1]}}$ in pulsed fields to 500T showed a moment of \textasciitilde 1.3$\mu_{\mathrm{B}}$ above 140T, and above \textasciitilde 270T the magnetization rises, reaching \textasciitilde 3.8$\mu_{\mathrm{B\thinspace }}$by 500T. We have performed first principles DFT calculations for LCO in high fields. Our earlier calculations$^{\mathrm{[2]}}$ explained the importance of a small rhombohedral distortion in the ground state that leads to a suppression of the 1.3$\mu_{\mathrm{B}}$ moment for fields below \textasciitilde 140T. By allowing fairly large atomic displacements in high fields, moments of \textasciitilde 4$\mu_{\mathrm{B}}$ are predicted. [1] V. V. Platonov \textit{et al}. Phys. Solid State \textbf{54}, 279 (2012) [2] Y. Lee and B. N. Harmon \textit{et al}. J. Appl. Phys. \textbf{113}, 17E145 (2013)   

Interplay between frustration, magnetism and sodium vacancy ordering in in Na$_{\mathrm{0.84}}$ CoO2

We have performed an extensive survey of low temperature specific heat of Na$_{\mathrm{0.84\thinspace }}$CoO$_{\mathrm{2\thinspace }}$. Heat capacity measurements were performed with an AC steady state method using a membrane nanocalorimeter. Thanks to the 10-30 nanogram sample mass we were able to perform well controlled ultra-fast cooling (500k/s) form high temperatures to temperatures where the sodium ions become immobile. This allowed us to take snapshots of different high temperature sodium configurations, relate them to particular structural transitions and at the same time establish their influence on the low temperature magnetic order. Through correlation with XRD data we demonstrate that the least ordered sodium configuration increases the Tc of the 22K transition by 2K.   

Angle-resolved photoemission on the delafossite oxide metal PtCoO$_{2}$

The delafossite structural series of oxides has recently attracted considerable attention because of the remarkable and varied properties of the compounds in the series. Here we consider the Pt-based $5d$ delafossite oxide PtCoO$_{2}$, the most conductive oxide known [1]. From angle-resolved photoemission and density-functional theory, we show that the underlying Fermi surface is a single cylinder of nearly hexagonal cross-section, with very weak dispersion along k$_{z}$. Despite being predominantly composed of $d$-orbital character, the conduction band is remarkably steep, with an average effective mass of only 1.14$m_{e}$. Moreover, the sharp spectral features observed in photoemission remain well-defined with little additional broadening for over 500~meV below E$_{F}$, pointing to suppressed electron-electron scattering. Together, our findings establish PtCoO$_{2}$ as a model nearly-free electron system and an ideal testbed for elucidating the ultrahigh conductivity in delafossite oxides. [1] Kushwaha P. et al., Sci. Adv. {\textbf 1}, 9 (2015)   

Theory for the Spin State and Spectroscopic Modes of Multiferroic CaBaCo$_{\mathrm{4}}$O$_{\mathrm{7}}$

With alternating Kagome and triangular lattices, the type I multiferroic CaBaCo$_{\mathrm{4}}$O$_{\mathrm{7}}$ is highly frustrated. Magnetic frustration produces a non-collinear, ferrimagnetic spin state with a net magnetic moment of about 1 m$_{\mathrm{B}}$ along the $b$ axis below 60 K. Based on the the field dependence of the three observed spectroscopic modes between 0.8 and 2.7 THz and on the field dependence of the magnetization up to 14 T, we construct a microscopic model for this compound. Using the symmetry of the crystal, the model is constructed in terms of eight independent nearest-neighbor exchange interactions as well as both in-plane and easy-axis anisotropies. With three observed Co species (spins 1.45, 1.0, and 1.2), the magnetic unit cell contains 16 spins. Our results indicate that the easy-plane and hexagonal anisotropy in the triangular layers is far larger than the anisotropy in the kagome layers. The observed spin-induced polarization along the $c$ axis is produced by magnetostriction. We also predict other spin-wave modes outside the window of the spectroscopic measurements.   

\textbf{Investigation of the magnetic properties in double perovskite R}$_{\mathrm{\mathbf{2}}}$\textbf{CoMnO}$_{\mathrm{\mathbf{6}}}$\textbf{ single crystals (R}$=$\textbf{rare earth: La to Lu)}

We have successfully synthesized the series of the double-perovskite R$_{\mathrm{2}}$CoMnO$_{\mathrm{6}}$ (R$=$rare earth: La to Lu) single crystals and have investigated their magnetic properties. The ferromagnetic order of Co$^{\mathrm{2+}}$/Mn$^{\mathrm{4+}}$ spins emerges mainly along the $c$ axis. Upon decreasing the size of rare earth ion, the magnetic transition temperature decreases linearly from 204 K for La$_{\mathrm{2}}$CoMnO$_{\mathrm{6}}$ to 48 K for Lu$_{\mathrm{2}}$CoMnO$_{\mathrm{6}}$, along with the enhancement of monoclinic distortion. The temperature and magnetic-field dependences of magnetization reveal the various magnetic characteristics such as the metamagnetic transition in R$=$Eu, the isotropic nature of rare earth moment in R$=$Gd, and the reversal of magnetic anisotropy in R$=$Tb and Dy. Our results offer comprehensive information for understanding the roles of mixed-valent magnetic ions and rare earth magnetic moments on the magnetic properties.   

Enhanced magnetic coercivity and maximum energy product in double-perovskite Y$_{\mathrm{2}}$CoMnO$_{\mathrm{6}}$ single crystals

We have investigated the influence of different annealing conditions on the magnetic properties on the single crystals of double-perovskite Y$_{\mathrm{2}}$CoMnO$_{\mathrm{6}}$. The ferromagnetic moment along the c-axis with the large magnetic coercivity and high squareness ratio was observed. Particularly, in the quenched specimen, the magnetic functionality has been greatly improved compared to that of the as-grown crystal. The magnetic coercivity and maximum energy product have been increased by $\sim $120{\%} and $\sim $50{\%}, respectively, by comprising substantial disorders and defects. Our result renders an efficient route to improve the magnetic functionality in mixed-valent magnets.