Session C40: Magnetic Structure in Bulk Oxides

Spin density wave order and its influence on the unconventional metallic states in carrier-doped Srn+1IrnO3n+1 systems

The discovery of spin-orbit assisted or J_eff=1/2 Mott insulating states in select 4d and 5d transition metal oxides has fueled a number of predictions of novel electronic phases that emerge once the parent Mott phase is quenched. Depending on the structure type, examples of these phases span from unconventional superconductivity to new manifestations of correlated topological electronic states. The seminal examples of J_eff=1/2 Mott states found in the homologous Ruddlesden-Popper series Sr_n+1Ir_nO_3n+1 are a versatile platform for understanding the mechanism for the collapse of the Mott state as the J_eff=1/2 band is driven away from half filling and for resolving the presence of nearby electronic instabilities. In this talk, I will present some of our recent work exploring the competing states accessed via chemically doping these compounds. Upon electron-doping into Sr₂IrO₄ and hole-doping into Sr₃Ir₂O₇, spin density wave states emerge and provide clues to the nature of the unconventional metals that form in each. A “diagonal” spin density wave state analogous to those formed in the underdoped high-T_c cuprates appears in electron-doped Sr₂IrO₄ and suggests a link between the competing ground states of La_2-xSr_xCuO4 and Sr_2-xLa_xIrO₄. In hole-doped Sr₃Ir₂O₇, the slow collapse of the Mott state reveals an intermediate strange metal regime where spin density wave order survives beyond the collapse of the Mott charge gap. I will discuss these two examples and the implications of each in understanding the phase diagrams of more strongly correlated Mott states.   

Evolution of Magneto-Orbital order Upon B-Site Electron Doping in Na1−xCaxMn7O12 Quadruple Perovskite Manganites

We present the discovery and refinement by neutron powder diffraction of a new magnetic phase in the Na_1−xCa_xMn₇O₁₂ quadruple perovskite phase diagram, which is the incommensurate analogue of the well known pseudo-CE phase of the simple perovskite manganites. We demonstrate that incommensurate magnetic order arises in quadruple perovskites due to the exchange interactions between A and B sites. Furthermore, by constructing a simple mean field Heisenberg exchange model that generically describes both simple and quadruple perovskite systems, we show that this new magnetic phase unifies a picture of the interplay between charge, magnetic, and orbital ordering across a wide range of compounds.   

Magnetic Properties of the Q1D Solid Solution Cax-1NaxCr2O4 Studied with Neutrons and Muons

In this work we present the results of the measurements carried out by neutron powder diffraction and muon spin rotation/relaxation (μ⁺SR) in ambient and high pressure on the x = 1 (NaCr₂O₄) and x = 0 (CaCr₂O₄) members of the solid soution Ca_1−xNa_xCr₂O₄ and on the intermediate compound x = 0.5 (Ca_0.5Na_0.5Cr₂O₄). The x = 1 results show a unique spin structure where the Cr moments in each zig-zag chain are aligned ferromagnetically along the c-axis, whereas antiferromagnetically along the a-axis between the adjacent zig-zag chains. The x = 0 results show the formation of a complex magnetic order below T_N, that is consistent with an incommensurate AF (IC-AF) order. The investigation on the intermediate compound Ca_1−xNa_xCr₂O₄ showes a fast-relaxing component, possibly indicating a spin-glass state. Moreover, the magnetic order was shown to be almost completely unsensitive to pressures (up to p » 25 kbar), hereby revealing that the effect from the tuning of the spin density of the chains is clearly dominant.   

Experimental observation on Jeff=1/2 state in CuAl2O4

The spin-orbit entangled pseudo-spin state, J_eff=1/2 has driven intriguing phenomena such as high-Tc superconductivity, quantum spin liquid, and topological semimetal. This studies have been conducted on 4d or 5d compounds but 3d systems haven't been focused on. The J_eff=1/2 states in 3d systems including cuprates haven't been considered to emerge in condensed matter physics. One reason is the spin-orbit coupling constant of 3d is smaller than that of Ru (4d) or Ir (5d) ions and the other is that many 3d systems have distorted-crystal structures which lead quenched orbital angular momentum. However, we suggest CuAl2O4 is a candidate material to induce Jeff=1/2 states. Cubic symmetry of the crystal structure verified by single crystal XRD supports Jeff=1/2 nature. Moreover, X-ray absorption spectroscopy also shows this nature following the electric dipole selection rule.   

Magnetism in spin-orbit coupled oxides

Transition metal oxides with 4d and 5d ions provide a glimpse into unusual facets of spin-orbit coupled magnetism. In this talk, I will discuss how a theoretical study of resonant inelastic X-ray scattering enables us to extract single-site effective Hamiltonian parameters relevant to ``bottom-up'' modelling of such materials, including iridates, rhenates, and osmates. Such single-site Hamiltonians are useful to extract spin-exchange interactions, study its sensitivity to lattice distortions, and to understand bulk magnetism. We illustrate this via the exploration of frustrated jeff=1/2 Mott insulators on the face-centered cubic lattice. These results are applicable to a wide class of materials called double-perovskites. Finally, we show how thin films of spin-orbit coupled metallic magnets may host tunable skyrmion crystals induced by strain and magnetic field, and potentially exhibit a phenomenon we term "topological quantum oscillations".   

Magnetic properties of single crystalline Nd2O3

Triangular-lattice magnets have recently attracted interest due to their exotic phases like frustrated magnetism, quantum spin liquids, and topological spin textures that originate from competing interactions. The nearest-neighbor and next-nearest-neighbor magnetic interactions are crucial to realizing these exotic phases. Nd₂O₃ is a triangular-lattice compound that crystallizes in the trigonal centrosymmetric space group P m1. Very recently, long-range magnetic order below T_N = 0.55 K was revealed by studies on polycrystalline samples of Nd₂O₃ using neutron diffraction and thermodynamic measurements. The reported frustration ratio of = 43 suggests Nd₂O₃ is a frustrated system driven by competing interactions. We have synthesized single crystals of Nd₂O₃ to further characterize the intrinsic magnetic properties. This talk will emphasize the anisotropic magnetic properties of Nd₂O₃ single crystals and the temperature--magnetic field phase diagram.   

Muon spin rotation study of the rare-earth quantum dimer system Yb2Si2O7

In quantum dimer systems, a zero-field singlet state can give way to Bose-Einstein condensation (BEC) of triplons under applied magnetic field [1]. We will present work on the material Yb₂Si₂O₇, an effective spin-1/2 antiferromagnet where Yb ions form an anisotropic honeycomb lattice with two different bonds length. It is expected that Yb spins form dimers on the shortest Yb-Yb bonds, and the resulting singlet-triplet gap has been observed with specific heat and inelastic neutron scattering. Between critical fields H_C1 = 0.4 T and H_C2 = 1.4 T, an antiferromagnetic phase reminiscent of a BEC of triplons is observed. We will present a low-temperature µSR investigation of this Yb-based quantum dimer system. Below the critical field H_C1, our measurements confirm the lack of magnetic order, yet also exhibit anomalous spin relaxation, which might be explained through hyperfine enhancement or a perturbation of the local environment by the implantation of a muon. At longitudinal magnetic fields near and above H_C1, a dramatic increase in muon spin relaxation, associated with antiferromagnetic order, is observed.

[1] T. Giamarchi, C. Rüegg & O. Tchernyshyov. Nature Physics 4, 198 (2008).   

Effects of Oxygen Deficiencies in CaMn2O4-δ

Manganese oxides over the last few years have experienced a considerable increase in research interests due to their rich and complex phase diagrams, that are a result of the flexibility of the manganese ions oxidations states. The compounds in the Ca-Mn-O system provide an excellent platform, where theoretically predicted multiferroic behavior could exist. Many manganese oxides have already been thoroughly investigated; however, a potential candidate CaMn₂O₄ is still not well understood to date.

Polycrystalline CaMn₂O₄ samples were synthesized and annealed in oxygen and argon flows respectively to further explore the effect of oxygen deficiencies on the physical properties. From temperature dependent dc magnetization and heat capacity measurements, the previously reported antiferromagnetic ordering near 220 K was observed for both samples. However, a spontaneous polarization (P_S) was observed near 253 K for only the oxygen annealed sample without applied magnetic field, while no P_S was observed for the argon annealed sample. In addition, the dielectric and loss measurements revealed anomalies above 260 K with the time and magnetic field dependent phenomena in both samples. Detailed measurements for both samples are ongoing to gain greater insight into this complex system.   

Determining lattice distortion of Ba2NaOsO6

Exotic quantum phases have been proposed in magnetic Mott insulators with strong spin-orbit coupling. Our recent nuclear magnetic resonance (NMR) studies of the Mott insulator Ba2NaOsO6 established that local lattice distortions precede the onset of canted ferromagnetic order^1,2. It was suggested that lattice distortions are a manifestation of the orbital order that drives the magnetism in this class of materials³. To decipher the microscopic nature of the putative orbital order, we investigate different lattice distortion models and calculate their associated NMR observables. Results of various distortion models will be discussed in this talk.

[1] Lu, L., et al. "Magnetism and local symmetry breaking in a Mott insulator with strong spin orbit interactions." Nature communications 8 (2017): 14407.
[2] Liu, W., et al. "Nature of lattice distortions in the cubic double perovskite Ba 2 NaOsO 6." Physical Review B 97.22 (2018): 224103.
[3] Chen, Gang, Rodrigo Pereira, and Leon Balents. "Exotic phases induced by strong spin-orbit coupling in ordered double perovskites." Physical Review B 82.17 (2010): 174440.   

A strongly correlated polar metal LiOsO3

LiOsO₃ is metallic and undergoes a second-order transition to a polar phase at T_c≈140 K. This unusual property provides a unique opportunity to study the interplay between itinerant electrons and electric dipoles. We report a suite of measurements of the properties of LiOsO₃ versus temperature, including resistivity and magnetoresisitance on a single crystal sample down to 0.16 K; Seebeck coefficient, Hall coefficient, high-precision magnetization, and the specific heat were made on polycrystalline samples. The results indicate that electrons become strongly correlated in responding to ferroelectric ordering of dipoles. We have monitored how the thermal conductivity is changed by dipole fluctuations at the transition and how it is influenced by ferroelectric domain boundaries at low temperatures to obtain information on interaction between the electrons and the dipoles. The observation of a transition from glassy to phonon-like thermal conductivity on cooling through T_c provides a solid proof that the ferroelectric transition is an order-disorder, not a displacive transition.   

Ground state properties of Sr2RuO4: dynamical mean-field description based on spin-orbit entangled basis

We investigate the ground state wavefunction and dynamical properties of Sr₂RuO₄ at zero temperature, using the density functional theory plus dynamical mean-field theory with the exact diagonalization solver. We especially focus on the interplay between the spin-orbit and Hund’s coupling by considering the full rotationally invariant Kanamori interaction Hamiltonian.
Near the Fermi level, the quasi-particles and low-lying excitations are well described by spin-orbit coupled basis. They exhibit a distinguished self-energy behavior that reflects the signature of the magnetic response and the orbital dependent-correlations of the Hund’s metal. We will discuss how these responses are related to the two-fold degeneracy of ground state.