Synthesis and Physical Properties of 3d, 4d and 5d Transition Metal Compounds

In this thesis I study the synthesis and basic physical properties characterization on 3 $d$ , 4 $d$ and 5 $d$ transition metal compounds. Great success has been obtained in 3 $d$ transition metal compounds, in which the electric states are well localized due to the large on-site Coulomb repulsion $U$ . Most stoichiometric 3 $d$ transition metal oxides are antiferromagnetic Mott insulators. Among them, low dimensional geometrically frustrated systems, such as $S$ = 1/2 Kagome lattice antiferromagnets, are at the forefront of condensed matter research. Recently, high-quality single crystals of $\mathrm{Cu_2OSO_4}$ , which are spin-1/2 antiferromagnets with low dimensional magnetism, have been successfully synthesized. The measurements of specific heat, susceptibility and magnetization were performed on this material. We found that the Kagome-like compound $\mathrm{Cu_2OSO_4}$ shows typical signatures for a canted-AFM ground state with a weak ferromagnetic component. On the other hand, $4d$ transition metal compounds were considered as weakly correlated systems because the electron correlation is expected to be weaker in $4d$ transition metal compounds compared with the $3d$ ones. The $4d$ ones naturally bridge two different regimes of the strongly correlated $3d$ compounds and the $5d$ compounds. Most notably, for instance, it is intriguing that seemingly similar $\mathrm{Ca_2RuO_4}$ and $\mathrm{Sr_2RuO_4}$ display totally different behavior: the former is a Mott insulator while the latter is metallic and becomes superconducting at low temperature. Here we report the synthesis of large single crystals of $\mathrm{MoPO_5}$ , and present their magnetic and thermodynamic properties. We found that the $4d^1$ compound $\mathrm{MoPO_5}$ is orbitally quenched and orders into an antiferromagnet with the moments along $c$ axis. Spin-flop transition is observed which indicates magnetic anisotropy. $5d$ orbitals are more extended and the Coulomb repulsion $U$ values are expected to be further reduced compared with those of $3d$ and $4d$ transition metal compounds. Thus, insulating behaviors in $5d$ transition metal compounds have been puzzling. A possible reason is the strong spin-orbit coupling. Here we show the ambient-pressure synthesis and physical properties of a new all-Ir $^{6+}$ iridate $\mathrm{Ba_8Al_2IrO_{14}}$ and a novel layered iridate $\mathrm{Ba_{21}Ir_9O_{43}}$ . The synthesis, crystal structure, transport, and magnetic properties of them have been reported. $\mathrm{Ba_8Al_2IrO_{14}}$ is a $p-$ type band insulator and shows antiferromagnetic couplings but display no order down to 2 K. $\mathrm{Ba_{21}Ir_9O_{43}}$ is an insulator with antiferromagnetic Curie-Weiss behavior, where a magnetic transition is suppressed down to low temperature of 9 K despite the large Curie-Weiss temperature of $-90$ K. We also performed the pressure-dependent resistivity measurements of the $5d$ compound $\mathrm{Ir_{0.95}Pt_{0.05}Te_{2}}$ and found that the charge order with $q$ =(1/5,0,1/5) dimer configuration is introduced and the superconductivity undergoes a dimensionality cross-over from 3 dimension to 2 dimension under pressure.

Synthesis, crystal growth and physical properties of the frustrated spin ladder material BiCu2PO6

Shuang Wang

In recent years quantum antiferromagnets with an intrinsically disordered (“spin liquid”) ground state and an energy gap in the spin excitation spectrum have received a great deal of attention. In search for new experimental realizations of spin-ladder and related models, we have embarked upon a new interesting family of low-dimensional materials with the general formula BiCu2AO6 (A = P, V and As). Among them, BiCu2PO6 shows a structure where Cu spins appear to form a two-leg zigzag ladder, which is believed to be a valuable model of a quantum magnet. However, for detailed studies of quantum magnetism, single crystals are indispensible. Therefore, in this thesis, synthesis and characterization of the newly-discovered spin ladder material BiCu2PO6 (BCPO) and its derivatives in the form of powder and single crystal are presented. Polycrystalline samples of Bi(Cu1-xZnx)2PO6 (x= 0.01, 0.02, 0.03, 0.04, 0.05 and 1), Bi1-y(Ca, Sr, La )yCu2PO6 (y = 0.15, 0.5 and 1) and BiCu2(P1-zVz)O6 (z = 1) were synthesized in this work. The solid state synthesis reactions, phase purity and thermal properties of the samples, substitution solubility, and preliminary magnetic susceptibility have been studied by thermal analysis (TG-DTA), X-ray powder diffraction (XRD), and Scanning Electron Microscope (SEM) as well as DC magnetization measurements, respectively. The BiCu2PO6 structure shows a remarkable chemical flexibility and interesting substitution effects. For detailed studies of the magnetic properties, well characterized large single crystals are indispensable. Thus in this thesis, a number of single crystal growth experiments have been performed using the Travelling Solvent Floating Zone method (TSFZ). Under the elaborated crystal growth conditions (growth rate of ~1 mm/h, gas mixture of 20% O2 in Argon, total 3 pressure (6~6.5 bar), high quality cm -size single crystals of Bi(Cu1-xZnx)2PO6 (x = 0, 0.01, 0.03, 0.05) have been successfully grown for the first time. For these crystals phase purity, crystal morphology/orientation, defect density, chemical composition distribution, crystal structure (room temperature and 1.5 K) and single crystal quality were thoroughly studied based on X-ray and neutron powder diffraction, optical and scanning electron microscopy, X-ray/neutron Laue diffraction, ICP-OES/X-ray fluorescence and two-axis neutron diffraction. Moreover, the specific heat Cp (H,T), AC susceptibility χ(H,T) and DC magnetization M(H,T) measurements on well-characterized single crystal samples of the undoped BCPO and 5% Zn-doped BCPO with the field applied along main crystallographic axes were performed in order to study the unique low temperature magnetic behaviour exhibited in this system, such as anisotropy of the magnetic properties, impurity-induced magnetism by the substitution of non-2+ magnetic cations (e.g. Zn ) and frustration. The magnetic phase diagram (H-T) under low field has been constructed. Further results obtained by close collaborators, based on the measurements made on single crystal samples grown in this thesis, including field-induced quantum phase transitions, NMR, Raman studies are attached directly at the end of this thesis.

EPFL2013

Study of novel electronic conductors

Neven Barisic

This thesis presents the results of a concerted effort to understand the complex array of physical properties exhibited by the BaVS3 family of materials. As a 3d1 system, BaVS3 displays a unique collection of correlation-driven phenomena, including a metal-insulator transition driven by spin-density waves and or charge-density waves as well as a pressure-dependent crossover between the non-Fermi-liquid and Fermi-liquid behaviors. In an attempt to better understand these and many other properties, we have undertaken a systematic experimental study of BaVS3 and related compounds. The primary measurements carried out were those of the transport properties, resistivity and thermoelectric power (TEP). Through the construction of specialized measuring apparatuses, we were able to measure simultaneously these transport properties under conditions of variable temperature (from 2 to 600 K), pressure (up to 3 GPa) and magnetic field (up to 12 T). At ambient pressure and in the range of 250 to 600K, BaVS3 shows nearly isotropic but poor metallic behavior with linear temperature dependences of resistivity and TEP and a Curie-like magnetic susceptibility. The nearly isotropic conductivity contrasts with the 1D 2kF fluctuations observed by lowering the temperature below 250 K (at which the first Jahn-Teller structural phase transition occurs) deep in the metallic phase. The fluctuations reveal the 1D aspects of the electronic character, originating from the chain like structure of the material. The salient feature of BaVS3 at ambient pressure is the second order metal-to-insulator (MI) transition at TMI = 69 K, accompanied by the tetramerization (doubling of the 2V unit cell in the chain direction). In addition to the transport measurements, the strong changes in the electrical and magnetic properties of the system around TMI were followed by magnetic susceptibility, angle-resolved photoelectron spectroscopy and frequency-dependent conductivity. By increasing the pressure, the three-dimensionality of BaVS3 is enhanced and the MI phase transition is suppressed to lower temperatures. TEP and magneto TEP measurements in this pressure range revealed the existence of polarons and of spin fluctuations in the metallic phase. The latter can be identified as precursors to the MI transition. Around 1.8 GPa, where TMI ~15 K, the system enters a strongly fluctuating regime, highly sensitive to the magnetic field, the amplitude and the frequency of the measuring current and to a further increase of the pressure. Closely related to these features, the phase boundary collapses and a hysteretic behavior appears in the transport properties and their magnetic-field-dependent counterparts. At a critical pressure of ~2 GPa, a non-Fermi liquid (NFL) state arises (with n~1.5 in the Tn resistivity law between 1 and 15 K) in relation to the proximate Quantum Critical Point. The p-H-T phase diagram of BaVS3 in this region has been explored in some detail with particular emphasis placed upon the relevance of the spin degrees of freedom (on the insulator side) and the role of quantum fluctuations above the critical pressure. Finally, as the pressure is increased further, the conventional Fermi-liquid exponent of n = 2 is obtained. In order to delve further into the subtleties of the MI transition and the NFL behavior, a comparative study was carried out on the sister compounds Bax-1SrxVS3 and BaVS3 and on sulfur-deficient BaVS3. From this study it became apparent that the imposed chemical substitution manifests itself as an additional effective pressure. The inherent disorder of these samples was observed to affect the NFL behavior by reducing the exponent n towards a value of 1, while the apparent ferromagnetic order below 15 K was seemingly independent of pressure. All the observed features are consistent with a relatively simple two-band tight-binding, vanadium-based, model consisting of a wide quasi one-dimensional band hybridized with a rather isotropic narrow band. Within this context, it is concluded that the MI transition is controlled by the Coulomb-interacting electrons of the wide quasi one-dimensional band. Based on this model a new description of the NFL regime has been proposed.

EPFL2005

Magnetism and Atomic Scale Structure of Bimetallic Nanostructures at Surfaces

Sergio Vlaic

his thesis reports on the magnetic properties of bi- and tri-metallic nanostructures at surfaces. The main goal is to build the smallest nanostructure ferromagnetic at room temperature and the assembly of those structures in high density arrays. To this purpose we have developed an approach consisting in a fine engineering of the nanostructure chemical structure at the atomic level. We grow core-shell nanostructures with atomically sharp interfaces between the different constituents and we investigate the effects produced by these interfaces as a function of their chemistry and structure. This approach is then used to grow organized arrays of bimetallic nanostructures that represent model systems for next generation high density magnetic storage media. The magnetic properties of the different experimental systems were investigated by magneto-optical Kerr effect (MOKE) and they were correlated to the morphology obtained by scanning tunneling microscopy (STM). The first part focuses on the understanding of the effects of atomically sharp interfaces among several elements. Two dimensional cobalt nanostructures have been grown on Pt(111) single crystal surface and subsequently their edges have been decorated by Fe, Pt or Pd. This one-dimensional decoration produces element dependent variations of the magnetic anisotropy energy with strong enhancement for Fe decoration and reduction for Pt and Pd. Furthermore we observe a crystallographic direction dependence on the effect of Pt and Pd decoration since Co/Pt or Co/Pd staking with {111}-orientation strongly enhances the magnetic hardness of the nanostructures. The effect of atomically sharp Co/Fe one-dimensional interface has been compared with direct alloying of the two elements finding the first one to give the highest effect for shell thicknesses smaller than 5 atoms. With the help of fully relativistic ab-initio we were able to reproduce the experimental results and unravel their pure electronic origin. The electronic hybridizations at the interface between the elements gives rise to "hot spots" in the electronic band structure responsible of a strong variation of the magnetocrystalline anisotropy resulting in the enhanced magnetic hardness. By growing Co-core Fe-shell Pd-capped nanostructures we were able to enhance the magnetization thermal stability of nanostructures by almost a factor of 3 with respect single-metal nanostructures of the same size. In the second part of this thesis, the first attempt of growing organized array of bimetallic nanostructures is presented. Taking advantage of the well-known self organized growth of Co nanodots on Au(111)-vicinal surfaces, we were able to grow Co-core Fe-shell nanodots on Au(11,12,12) single crystal. The thermal stability enhancement produced by the atomically sharp Co/Fe interface is higher than the direct alloying of the two elements as in the previous case. We argue that with this method, combined with three-dimensional growth of nanostructures arrays, it will be possible to produce multimetallic nanodots composed by ≈ 3000 atoms with magnetization thermal stability suitable for magnetic storage media. In the last part another model system for magnetic nanostructure superlattices is presented. Fe nanostructures have been grown on the Pd-seeded and unseeded alumina thin film formed by oxidation of a Ni3Al(111) single crystal surface at high temperature. We found that the sample stoichiometry near the surface evolves with the preparation cycles. This leads to the formation of a Ni-rich region, close to the alumina surface, ferromagnetic up to 250 K. By means of X-ray magnetic circular dichroism measurements we were able to estimate the amount of Ni in excess. Fe clusters deposited on the alumina surface are magnetically coupled to the Ni-rich region independently on the presence of Pd. Nevertheless, the system behavior cannot be explained by a simple ferromagnetic or antiferromagnetic coupling. We argue that the Fe nanostructures are coupled to the substrate by an interlayer exchange coupling across the alumina thin film. The coupling constant is expected to change sign depending on the distance between the Fe clusters and the Ni-rich region resulting in a peculiar magnetic behavior of the system.

EPFL2013

Study of novel electronic conductors

Neven Barisic

EPFL2005

Synthesis, crystal growth and physical properties of the frustrated spin ladder material BiCu2PO6

Shuang Wang

EPFL2013

Magnetism and Atomic Scale Structure of Bimetallic Nanostructures at Surfaces

Sergio Vlaic

EPFL2013