Glassy dynamics in the low-temperature inhomogeneous ferromagnetic phase of the quantum spin ice Yb2Sn2O7

Here we present a detailed muon spin relaxation (mu SR), neutron diffraction, and ac-susceptibility study of Yb2Sn2O7 that shows that despite the onset of ferromagnetic long-range order at T-c approximate to 0.140 K, the low-temperature phase of this material remains highly disordered with glassy dynamics that span various frequency decades. We suggest that the observed disordered ground state might be made of infinite, percolating clusters coexisting with smaller ones. This mesoscopic cluster glass state would be the result of the enhanced effect of even a small amount of disorder, capable of disrupting the "clean" ordered state near a quantum critical point.

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

Sc2Ga2CuO7: A possible quantum spin liquid near the percolation threshold

Paul Gregory Freeman, Henrik Moodysson Rønnow

Sc2Ga2CuO7 (SGCO) crystallizes in a hexagonal structure (space group: P63/mmc), which can be seen as an alternating stacking of single and double triangular layers. Combining neutron, x-ray, and resonant x-ray diffraction, we establish that the single triangular layers are mainly populated by nonmagnetic Ga3+ ions (85% Ga and 15% Cu), while the bilayers have comparable population of Cu2+ and Ga3+ ions (43% Cu and 57% Ga). Our susceptibility measurements in the temperature range 1.8–400 K give no indication of any spin-freezing or magnetic long-range order (LRO). We infer an effective paramagnetic moment μeff=1.79±0.09μB and a Curie-Weiss temperature θCW of about −44 K, suggesting antiferromagnetic interactions between the Cu2+(S=1/2) ions. Low-temperature neutron powder diffraction data showed no evidence for LRO down to 1.5 K. In our specific heat data as well, no anomalies were found down to 0.35 K, in the field range 0–140 kOe. The magnetic specific heat Cm, exhibits a broad maximum at around 2.5 K followed by a nearly power law Cm∝Tα behavior at lower temperatures, with α increasing from 0.3 to 1.9 as a function of field for fields up to 90 kOe and then remaining at 1.9 for fields up to 140 kOe. Our results point to a disordered ground state in SGCO.

2015

Ferrimagnetic 120 degrees magnetic structure in Cu2OSO4

Helmuth Berger, Virgile Yves Favre, Arnaud Magrez, Henrik Moodysson Rønnow, Romain Sibille, Gregory Scott Tucker, Lin Yang, Ivica Zivkovic

We report magnetic properties of a 3d(9) (Cu2+) magnetic insulator Cu2OSO4 measured on both powder and single crystal. The magnetic atoms of this compound form layers whose geometry can be described either as a system of chains coupled through dimers or as a kagome lattice where every third spin is replaced by a dimer. Specific heat and DC susceptibility show a magnetic transition at 20 K, which is also confirmed by neutron scattering. Magnetic entropy extracted from the specific heat data is consistent with an S = 1/2 degree of freedom per Cu2+, and so is the effective moment extracted from DC susceptibility. The ground state has been identified by means of neutron diffraction on both powder and single crystal and corresponds to an similar to 120 degrees spin structure in which ferromagnetic intradimer alignment results in a net ferrimagnetic moment. No evidence is found for a change in lattice symmetry down to 2 K. Our results suggest that Cu2OSO4 represents a type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored.

AMER PHYSICAL SOC2020

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

Shuang Wang

EPFL2013