Session W16: Focus Session: Bulk Properties of Complex Oxides - Other

Magnetic Force Microcopy of the Magnetostructural Phases of Mn3O4

In this talk, we report temperature- and field- dependent magnetic force microscopy (MFM) studies of Mn3O4. The spinel Mn3O4 has novel phase structure at low temperatures due to a three-way competition between spin-orbital coupling, geometric frustration and external magnetic field. This competition could lead to complex magnetic pattern formations. A particularly interesting phase exists where the crystal lattice undergoes a transition to resolve the spin frustration due to internal and external magnetic fields. In this phase the spins were expected to be in a disordered state, but local magnetic ordering has not been explored in previous studies. We present results from our investigation of Mn3O4 phase space using a variable temeprature MFM. Measurements were made at temperatures between 4K and 75K and in magnetic fields up to 5 tesla.   

Magnetic excitations in the geometric frustrated multiferroic CuCrO$_2$

The delafossite CuCrO$_2$, crystallizing in the rhombohedral $R\overline{3}M$ space group, is an interesting case of a multiferroic compound due to its apparent strong coupling of spin and charge. In contrast to other multiferroic compounds CuCrO$_2$ shows a spontaneous electric polarization upon antiferromagnetic ordering at \emph{T}$_{\textrm{N}}\approx$~24~K without an accompanying structural phase transition. Further, CuCrO$_2$ is a rare example where the magnetoelectric properties are tunable by both an electric and a magnetic field. In our contribution we present inelastic neutron scattering experiments on CuCrO$_2$ single crystals. The measured magnetic excitation spectra have been modeled by Monte-Carlo spin wave calculations and allowed the determination of the relevant exchange interaction and anisotropy terms. We will present evidence for a weak ferromagnetic Cr-Cr-interlayer exchange interaction and show that this interaction is relevant for the multiferroic properties of CuCrO$_2$.   

Charge dynamics in frustrated charge ordered system with strong electron correlation

Electronic ferroelectricity is known as phenomena where the electric polarization is caused by the electronic charge order without inversion symmetry. This is seen in some transition metal oxides, e.g. LuFe$_{2}$O$_{4}$, and charge transfer organic salts. It is suggested from the theoretical works [1,2] that large charge fluctuation and frustration are responsible for the electric polarization. This charge fluctuation is expected to govern dynamical properties and external field effects. Actually, the relaxer like large dielectric fluctuation in some electronic ferroelectricity is observed. Motivated by these experimental and theoretical results, we study charge dynamics in charge ordered fermion system on the layered triangular lattice. We adopt the V-t model where the inter-site electron transfers and the inter-site Coulomb interactions are taken into account. We analyze this model by utilizing the exact diagonalization method and focus on effects of frustration in the charge dynamics. In the 3-fold charge ordered phase associated with the electric polarization, the optical conductivity shows multiple-peak structure in a wide energy range. In the non-polar 2-fold charge ordered phase, a precursor of the 3fold charge order appears in the low energy charge fluctuation around the phase boundary. [1] A. Nagano et al. Phys. Rev. Lett. \textbf{99} 217202. [2] M. Naka et al. Phys. Rev. B. \textbf{77} 224441.   

Photo induced spin-state change in itinerant correlated electron systems

Recently developed ultrafast optical techniques open up a new frontier for research of the phase transition. This is the so-called photo-induced phase transition (PIPT). The spin-state transition between different magnitudes of the spin-angular moment is one of the targets. In particular, the photo-induced spin-state transition is seen in the cobalt oxides with a perovskite structure. The ultrafast optical measurements in the low-temperature low-spin insulators show transient metallic spectra which are completely different from the spectra in the high-temperature high-spin phase. We study theoretically the photo-induced spin-state change in itinerant correlated electron systems. We derive the model Hamiltonians where numbers of photo-excited electron-hole pair are fixed. A bound state of the photo-doped hole and the high-spin state are created inside of the low-spin sites. This bound state brings about a characteristic peak structure in the optical pump-probe spectra which are completely different from the spectra in thermally excited states. The present theory provides a possible scenario in recently observed photo-induced hidden state in perovskite cobaltites.   

Rapidly fluctuating orbital occupancy above the orbital ordering transition in spin-gap compounds

Low-dimensionality spin systems develop a spin-dimer phase within a molecular orbital that competes with long-range antiferromagnetism below TS. Very often, preferential orbital occupancy and ordering are the actual driving force for dimerization, as in the orbitally-driven spin-Peierls (MgTi2O4, CuIr2S4, La4Ru2O10, NaTiSi2O6, etc.). Through a microscopic analysis of the thermal conductivity in La4Ru2O10, we show that the orbital occupancy fluctuates rapidly above TS, resulting in an orbital-liquid state. Strong orbital-lattice coupling introduces dynamic bond-length fluctuations that scatter the phonons to produce a glass-like thermal conductivity above TS. This phonon-glass to phonon-crystal transition occurs in other spin-dimer systems, like NaTiSi2O6, pointing to a general phenomenon.   

Influence of particle size on the magnetic and magnetocaloric properties of nanocrystalline La$_{2/8}$Pr$_{3/8}$Ca$_{3/8}$MnO$_{3}$

Bulk manganites La$_{5/8-y}$Pr$_{y}$Ca$_{3/8}$MnO$_{3 }$(LPCMO) exhibit a complex phase diagram due to coexisting and competing charge-ordered (CO) and ferromagnetic (FM) phases. Of particular interest is the CO phase that is unstable under various perturbations, such as carrier doping, strain, magnetic and electric field. We report systematic studies of the influence of particle size on the magnetic and magnetocaloric properties of nanocrystalline LPCMO (y=3/8) synthesized by sol-gel method. The nanocrystalline samples with mean particle sizes of 30 nm, 150 nm, and 250 nm were structurally characterized by XRD, SEM, and TEM. Magnetic and magnetocaloric measurements were conducted using a Quantum Design PPMS. We find that the 150 nm and 250 nm samples exhibit features similar to their bulk counterpart. However, the case is very different for the 30 nm sample where only a paramagnetic to ferromagnetic transition occurs. Size reduction has been found to suppress the CO phase, decrease the magnetization, and strongly modify the magnetocaloric effect in LPCMO.   

Studies on manganese substituted cobalt ferrite prepared by autocombustion route

Compositions of Co1.2-xMnxFe1.8O4 (0 = x = 0.4) were synthesized by autocombustion route keeping oxidizer to fuel ratio at 1. Structural and compositional characterizations of all the samples were performed by XRD, SEM and EDS. Magnetization measurements showed that the Ms increases form 106.5 emu/g for x = 0.0 to 138.5 emu/g for x = 0.2 and then decreases from x = 0.3 (124.71 emu/g for x = 0.3 and 97.78 emu/g for x =0.4), whereas the coercivity (Hc) decreases with manganese (Mn) substitution, except for x = 0.3. Room temperature dielectric properties such as relative dielectric permittivity (er), dielectric loss and ac conductivity, were studied as a function of frequency in the range from 20 Hz to 1 MHz. These studies indicates that the relative dielectric permittivity increasing (from er = 600 for x =0.0 to er = 2400 for x = 0.4) with the increase of Mn content in cobalt ferrite and also all samples show the usual dielectric dispersion which may be due to the Maxwell-Wagner-type of interfacial polarization. Dr. Y. D. Kolekar gratefully acknowledges the award of BOYSCAST fellowship by Department of Science and Technology, India. *On leave from Department of Physics, University of Pune, Pune- 411 007, India.   

Single magneto-chiral domain observed in langasite Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ by non-resonant magnetic X-ray scattering

The helical magnetic ground state of the chiral langasite compound Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ has been investigated using a left-handed single crystal using non resonant x-ray magnetic scattering. This technique, when combined with circularly polarized x-ray and a full polarization analysis of the scattered beam, is sensitive to the chirality of the spiral order previously reported in this compound and it allows an unique determination of the chirality of the magnetic ground state. A topographic map of the sample surface shows that the crystal is made of a single magneto-chiral domain. Azimuthal scans revealed that the long range magnetic order with wave-vector k=(0,0,1/7) is characterized by an elliptical modulation rather than a circular one, as initially reported. We also discuss the possible spin-driven ferroelectric state in this compound.   

Simple point-ion electrostatic model explains the cation ordering in A2BO4 spinel oxides

The A2BO4 spinel oxides are distinguished by having either a normal or an inverse distribution of the A, B cations over the octahedrally and tetrahedrally coordinated sites. While normal spinel represents a single structure (A-octahedral, B-tetrahedral) the inverse spinel is similar to a 50-50 alloy with octahedral sublattice occupied randomly by A and B. We show that a simple point-ion electrostatic (PIE) model parameterized by the oxygen displacement parameter u and by the relative formal cation valencies ZA vs ZB provides a simple rule: if ZA$>$ZB the structure is normal for u$>$0.2592 and inverse for u$<$0.2578, while if ZA$<$ZB the structure is normal for u$<$0.2550 and inverse for u$>$0.2578. This rule is illustrated for the known spinel oxides, proving to be 98 \% successful (PRL 105, 075501). Moreover, in inverse spinels the PIE model also explains the origin of the experimentally observed ordered phase that emerges from the random alloy at low T.   

Anionogenic Magnetism: Mixed Alkali -- Alkaline Earth Metal Oxides

The scientific and technological potential of materials in which magnetism arises from p-electrons is little explored. Accordingly, we have synthesized the solid solution Ba$_{1-x}$K$_{x}$(O$_{2}^{2-})_{1-x}$(O$_{2}^{-})_{x}$ which contains a nominal mixture of magnetic superoxide and non magnetic peroxide anions. Magnetization measurements reveal short range antiferromagnetic ordering below $\sim $65 K for all compositions, with an increasing tendency toward ferromagnetic interactions at lower temperatures, characterized by the opening of magnetic hysteresis loops in applied fields.Field induced reorientation of the dioxygen dumbbells probably occurs, altering the magnetic exchange interactions between nearest neighbor anions.