Ferrimagnetism of the magnetoelectric compound Cu2OSeO3 probed by Se-77 NMR

We present a thorough Se-77 nuclear-magnetic-resonance (NMR) study of a single crystal of the magnetoelectric compound Cu2OSeO3. The temperature dependence of the local electronic moments extracted from the NMR data is fully consistent with a magnetic phase transition from the high-T paramagnetic phase to a low-T ferrimagnetic state with 3/4 of the Cu2+ ions aligned parallel and 1/4 aligned antiparallel to the applied field of 14.09 T. The transition to this 3up-1down magnetic state is not accompanied by any splitting of the NMR lines or any abrupt modification in their broadening, hence there is no observable reduction in the crystal symmetry from its high-T cubic P2(1)3 space group. These results are in agreement with high-resolution x-ray diffraction and magnetization data on powder samples reported previously by Bos et al. [Phys. Rev. B 78, 094416 (2008)]. We also develop a mean-field theory description of the problem based on a microscopic spin Hamiltonian with one antiferromagnetic (J(afm) similar or equal to 68 K) and one ferromagnetic (J(fm) similar or equal to -50 K) nearest-neighbor exchange interaction.

Magnetic properties of the magnetoelectric compound Cu2OSeO3: Magnetization and Se-77 NMR study

Jean-Philippe Ansermet, Helmuth Berger, Simon Granville

We present magnetization and Se-77 Nuclear Magnetic Resonance (NMR) measurements in single crystals of the magnetoelectric compound Cu2OSeO3. The temperature and field dependence of the magnetization suggest a ferrimagnetic ordering at T-c similar or equal to 60 K in a 3up-1down configuration. The easy axis of the magnetization is along the [100] crystallographic direction. The Se-77 NMR line shape data collected at 14.09 T are consistent with the symmetries imposed by the cubic P2(1)3 space group in the temperature range 20-290 K and confirm that the magnetic transition is not accompanied by any lowering of the crystal symmetry as has recently been proposed by Bos et al. [Phys. Rev. B 78 094416 (2008)].

Iop Publishing Ltd, Dirac House, Temple Back, Bristol Bs1 6Be, England2011

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

Lin Yang

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.

EPFL2017

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

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

Lin Yang

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}

\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}

\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.

EPFL2017

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

Shuang Wang

EPFL2013