Session Y9: Focus Session: Complex Bulk Oxide: Orbital Physics

Temperature dependence of Photoinduced dynamics in the orbital-ordered state of $A$V$_{10}$O$_{15}$ ($A$ = Ba, Sr)

In $A$V$_{10}$O$_{15}$ ($A$ = Ba, Sr), the V ions with mixed-valence states, V$^{2+}$/V$^{3+}$ ($3d^{3}$/$3d^{2}$), are located on the triangular lattice. BaV$_{10}$O$_{15}$ exhibits a structural phase transition with V trimerization caused by the orbital ordering of V ions at $T_{c}$ = 123 K, whereas SrV$_{10}$O$_{15}$ does not exhibit such a phase transition. We performed a femtosecond pump-probe reflection spectroscopy on BaV$_{10}$O$_{15}$ and SrV$_{10}$O$_{15}$ to clarify their photoinduced dynamics. For $A$ = Ba, a photoinduced melting of V trimerization, i.e. a photoinduced phase transition, was observed at 10 K ($< T_{c}$). At $T$ = 200 K ($>> T_{c}$), the photoinduced reflectivity change ($\Delta R/R$) for $A$ = Ba shows an oscillation with the period of several tens picoseconds, similarly to the behavior for $A$ = Sr at 10 K. This oscillation can be explained by assuming that the photoinduced state at the sample surface propagates into the inside of the sample. At $T$ = 135 K, immediately above $T_{c}$, we found that $\mid\Delta R/R\mid$ for BaV$_{10}$O$_{15}$ increases with time, suggesting that the area of the photoinduced state on the sample surface increases with time.   

Chemical pressure effects on structural, magnetic, and transport properties of Mn$_{1-x}$Co$_{x}$V$_{2}$O$_{4}$

The low-temperature x-ray diffraction, susceptibility, specific heat, and resistivity of the single crystal Mn$_{1-x}$Co$_x$V$_2$O$_{4}$ have been investigated. With increasing Co-doping, the chemical pressure related to the decreasing V-V distance drives the system towards the itinerant electron limit, accompanied with the increase of the ferrimagnetic transition temperature and the suppression of the structural distortion. These effects are compared to the effects from the application of physical pressure, and show that the V-V distance is the critical parameter controlling the properties of AV$_2$O$_4$.   

Thermal conductivity of spinel MnV$_{2}$O$_{4}$ with doping

Spinel MnV$_{2}$O$_{4}$ exhibits a structural phase transition and ferrimagnetic ordering simultaneously at 57 K. The crystal symmetry in the low-temperature phase obtained by the x-ray diffraction of a single crystal indicates an antiferro-orbital ordering of V $t_{2g}$ states [1]. It was also found that Al doping into the V site suppresses the orbital-ordering temperature ($T_{\rm oo}$) but barely affects the ferrimagnetic-ordering temperature ($T_{\rm N}$); thus two transition temperatures are separated in the Al-doped samples. We measured the thermal conductivity of Mn(V$_{1-x}$Al$_{x}$)$_{2}$O$_{4}$, and found that thermal conductivity sharply increases (thermal resistivity decreases) below $T_{\rm oo}$. It was also found that, with applied magnetic field, thermal resistivity increases above $T_{\rm oo}$ but decreases below $T_{\rm oo}$. These results indicate that thermal conductivity is dominated by the fluctuation of orbital ordering. [1] T. Suzuki {\em et al.}, Phys. Rev. Lett. 98, 127203 (2007).   

Magnetic properties driven by local structure in quasi-1D Ising chain system cobaltate system

Using {\it ab-initio} band structure method and the microscopic model calculation, the origins of the large orbital magnetic moment and unique magnetic anisotropy in the quasi-1D magnetic cobaltate, $\alpha$-CoV$_{2}$O$_{6}$, is investigated. Unique crystal electric field effect in $\alpha$-CoV$_{2}$O$_{6}$ is combined with the strong spin-orbit coupling, results in intriguing magnetic properties of the system. Based on the estimated strengths of the intra- and the inter-chain exchange interaction, experimentally found 1/3 magnetization plateau in the $MH$ curve can be attributed to spin-flop mechanism. Origin of the reduced magnetic entropy behavior is found to be the strong uniaxial magnetic anisotropy in the quasi-1D Ising chain system.   

A Comparative Study of Magnetic and Structural Transitions Focusing on Dielectric and RF Measurements

High frequency inductive and dielectric measurements were used to study the magnetic and structural transitions of a number of compounds, including a series of spinel vanadates of the form Mn$_{1-x}$Co$_x$V$_2$O$_4$. We then compared the results to those found using other measurement techniques, such as resistivity and specific heat. The high frequency inductive measurements were conducted using a tunnel diode oscillator (TDO), and proved to be an effective and simple way to observe the magnetic transitions. From capacitance and dissipation measurements we were able to observe both magnetic and structural transitions, but only in the more insulating samples.   

MicroRaman study of orbitonphonon coupling in YbVO3

Owing to their strong electron correlation, transition-metal oxides with perovskyte related structures display a variety of interesting properties such as Mott transition, high-T$_{c}$ superconductivity and colossal magnetoresistance. YbVO$_{3}$ belong to this family and exhibits magnetic and orbital orderings at low temperatures. In this communication, we present a study of its first order and multiphonon Raman active excitations as a function of temperature. Impact of various orbital, magnetic and structural transitions is analyzed and possibilities of orbiton-phonon coupling in the observed phonon combinations around 1400 cm$^{-1}$ are discussed.   

Controlling Magnetism in Multiferroic BiFeO$_3$

BiFeO$_3$ (BFO) is a room-temperature multiferroic combining large electric polarization with an antiferromagnetic structure with a superimposed long-wavelength (62 nm) cycloidal modulation. Large single crystals of BFO have become available recently, opening up new opportunities for experiments. In this talk, we discuss various ways of controlling magnetism in BFO single crystals using coupling between the ferroelectric and magnetic order parameters. Electric field can rotate the electric polarization and Fe spins simultaneously, and a chiral magnetic monodomain state can be obtained. Populations of the 3 equivalent cycloidal magnetic domains can be controlled by an electric field through piezoelectric coupling. Alternatively, they can be controlled via the inverse effect by applying uniaxial pressure. Very small ($\sim$50 bar) pressures producing tiny elastic strain ($\sim 10^{-5}$) are needed to move the magnetic domain walls. Using polarized small-angle neutron scattering, we show for the first time that the spins in the cycloid are tilted, producing local weak ferromagnetism (0.06 $\mu_B$ rms value), confirming a long-standing theoretical prediction. This shows that intrinsic macroscopic ferromagnetism could be expected in strained BFO, in which the cycloid is suppressed. Combined with the ability to control the magnetic domains by an electric field or tiny deformation, this observation accentuates the potential of BFO for room-temperarture applications involving magnetoelectric effects.   

Pressure-driven changes in electronic structure of BiCoO$_3$

Using first-principles DFT based calculations, carried out on the recently measured crystal structure data [Oka et.al. J.Am.Chem.Soc.132, 9438 (2010)], we study the changes in the electronic structure of BiCoO$_3$ between the ambient-pressure and the high-pressure conditions. Our study shows that the application of high pressure drives the high-spin-to-low-spin transition at the Co site. The obtained results for the ambient pressure phase shows C-type AFM alignment of Co high spins, while the electronic structure at the high-pressure phase shows the presence of a finite energy gap at E$_f$ in contrast with previously reported metallic or semimetallic character with low-spin state of Co. This semiconducting behavior in the nonmagnetic BiCoO$_3$ with LS state of Co is found to be driven by the presence of the GdFeO$_3$ type of orthorhombic distortion which arises due to finite mixing of Bi lone-pair states with O-p states, as opposed to previously predicted cubic or tetragonal symmetry of the high-pressure. The ambient pressure phase shows an order of magnitude larger energy gap arising due to the AFM alignment of Co in the high-spin state than the energy gap at the high pressure phase, explains the observed 3-order of magnitude jump in resistivity.   

Long Range Ordering in Oxygen Doped SrCoO$_{x}$

Recent investigations have shown magnetic phase separation in polycrystalline samples of SrCoO$_{x} (2.5 \le x \le 3.0)$.\footnote{Xie et al. Appl. Phys. Lett 99, 052503 (2011)} As the samples are oxidized electrochemically, distinct ferromagnetic phases are formed at SrCoO$_{2.75}$ (T$_{C}$ = 165 K), SrCoO$_{2.88}$ (T$_{C}$ = 220 K), and SrCoO$_{3}$ (T$_{C}$ = 280 K). In the polycrystalline bulk samples, two magnetic phases are seen at oxygen concentrations between 2.875 and 3.0, always occurring in only a single structural phase. The distinct phases are also evident in epitaxial 100 nm films, but the mixed magnetic phase at intermediate concentrations is suppressed. The occurrence of separate magnetic phases at concentrations of 2.75 [3-1/4], 2.88 [3-1/8], and 3.0 imply an electronic ordering commensurate with the lattice. Using resonant x-ray diffraction in thin film samples, we have discovered the existence of a superlattice peak at (1/4, 1/4, 1/4), revealing a previously hidden order with a periodicity four times the basic perovskite cell in all directions.   

Structural and magnetic properties of Nd$_{1-x}$Ca$_{x}$BaCo$_{2}$O$_{5.5}$

R$_{1-x}$A$_{x}$BaCo$_{2}$O$_{5.5}$ (R = rare earth, A = alkaline metal) is a relatively new class of complex oxide materials that exhibit a wide range of magnetic attributes in addition to metal/insulator switching properties, structural transitions and superstructure order parameters. In many ways, this family exhibits behaviors similar to those of the famous colossal magnetoresistive manganites; however, more complex properties have also been identified owing to the fact that the oxidation state of the cobalt ions often behave in unpredictable ways depending on the chemical composition of the investigated material and the corresponding Co local environment. Thus, Co$^{3+}$ and Co$^{4+}$ ions with high, intermediate and low spin states may be produced offering an additional degree of freedom to be accounted for when designing new materials with tunable magnetic properties. In this talk, I will discuss the effects of calcium substitution at the Nd sites and the various structural and magnetic models as determined by neutron powder diffraction and complementary magnetic measurements.   

Peculiarities of spin and charge degrees of freedom in strongly correlated layered cobaltates

The layered cobaltate systems stand for the rare case of metallic quasi-2D triangular lattices at doping $x$. While $x$=0 corresponds to a half-filled scenario, $x$=1 marks the band-insulating limit. Surprisingly, the phase diagram displays especially for $x>$0.5 a rich competition between different spin-orderings as well as intriguing charge-ordering processes. Though already at high doping, strong electronic correlations are mainly responsible for the complex physics [1-4]. By means of combinations of band-structure techniques with many-body approaches it is shown that various cobaltate features may be tackled successfully. For instance the in-plane crossover from antiferromagnetic spin correlations towards the onset of ferromagnetism as well as charge ordering tendencies favoring an effective kagome lattice close to $x$=0.67 in agreement with experiment. The charge order also substantially effects the spectral and transport properties, giving rise to a specific low-energy scale susceptible to nonlocal correlations. \\[4pt] [1] F. Lechermann, Phys. Rev. Lett. 102, 046403 (2009).\\[0pt] [2] C. Piefke, L. Boehnke, A. Georges and F. Lechermann, Phys. Rev. B 82, 165118 (2010).\\[0pt] [3] L. Boehnke and F. Lechermann, arXiv:1012.5943.\\[0pt] [4] Oleg E. Peil, A. Georges and F. Lechermann, arXiv:1107.437   

Stability of Multiferroic Transition in FeVO$_{4}$ against Transition Metal Doping

FeVO$_{4}$ is a recently discovered multiferroic material which undergoes successive antiferromagnetic phase transitions at T$_{N1} \quad \sim $ 22 K and T$_{N2} \quad \sim $ 15 K, with ferroelectricity developing at the T$_{N2}$ transition. FeVO$_{4}$ is a type II multiferroic where the ferroelectricity is magnetically driven. We have studied the effect of transition metal doping on these two phase transitions in order to explore how the multiferroic order is affected by introducing perturbations into the lattice. We synthesized polycrystalline M$_{x}$Fe$_{1-x}$VO$_{4}$ samples (M = Zn, Mn, Cr) using a standard solid state reaction method, and we used magnetic, dielectric, and heat capacity measurements to track the transition temperatures. Both magnetic and heat capacity measurements show clear peaks at the two transitions, enabling us to map how the transitions are suppressed as the doping fraction is changed. On doping with non magnetic Zn, we find only a minimal suppression of both transition temperatures, indicating the magnetic interactions producing the multiferroic order are surprisingly robust against non-magnetic perturbations. We will also present preliminary results of the effects of magnetic dopants, specifically Mn and Cr.   

$^{51}$V single-crystal NMR Study on multiferroic FeVO$_4$

The multiferroicity in FeVO$_4$ is so far not well understood because its spin-orbit coupling is probably very weak. In this talk, we report our $^{51}$V single crystal NMR study on FeVO$_4$ under zero field and a finite field. The double magnetic transitions with $T_{SDW}\approx$ 19 K and $T_{helical}\approx$ 13 K are clearly shown by NMR spectra and the spin-lattice relaxation rate $1/T_1$. Two noneqivalent $^{51}$V sites are identified with different hyperfine couplings. Just below $T_{helical}$, a large RF enhancement is seen, which indicates rich magnetic domain walls formed in the helical state and not in the SDW state. Based on our results, we discuss the coupling between the magnetism and the ferro-electricity in FeVO$_4$.