Multifrequency ESR in ET2 MnCu[N(CN)(2)](4): A radical cation salt with quasi-two-dimensional magnetic layers in a three-dimensional polymeric structure

The radical cation salt, ET2MnCuN(CN)(2), [ET=bis(ethylenedithio)tetrathiafulvalene] with an unusual three-dimensional anionic polymeric network is studied by x-ray diffraction, static susceptibility measurements, and electron spin resonance (ESR) at frequencies between 9 and 420 GHz. The magnetic properties are determined by the alternating two-dimensional layers of the Mn2+ ions of the network and the partially charged ET molecules. At ambient temperature the overlap between Mn2+ ions and ET molecules is weak and an exchange integral vertical bar J(Mn-ET)vertical bar approximate to 4.10(-2) K is estimated from their resolved ESR lines. At lower temperatures, ET2MnCuN(CN)(2) is not a simple system of weakly interacting paramagnetic ions in spite of the isotropic, Curie-like static susceptibility. There are first-order phase transitions at 292 K and in the range of 120-180 K. One of the lattice constants shows anomalous temperature dependence below 292 K. Anisotropic ESR shifts appear below 150 K, which we explain by demagnetizing fields of the platelike crystals and an exchange-narrowed fine structure. The latter contributes significantly to the shift when the populations of Zeeman levels are altered in high magnetic fields at low temperatures. We estimated the exchange coupling between Mn2+ ions within a layer, J(Mn-Mn)approximate to-48 K and determined the fine structure parameters below 150 K, showing a distortion in the plane of the Mn2+ ions.

Multi-frequency high-pressure electron spin resonance study of low dimensonal organic conductors

Bálint Náfrádi

Electronic and magnetic properties of quasi-one dimensional ormore widely low-dimensional systems aswell as their related correlated electron phenomena have been at the very frontier of condensed matter physics for quite some time. The reduced dimensionality in these systems offers a unique possibility of direct comparison with model calculations. Furthermore the competing ordered ground states in strongly correlated low-dimensional systems lead to rich phase diagrams comprising many electronic phases, such as superconductivity, charge/spin densitywaves, different electric charge distributions, or a novel formof metalicity. These phases are generically sensitive to a variety of parameters, such as temperature, magnetic field, pressure, a certain degree of irregularity, etc. In this thesis, we employ Electron Spin Resonance (ESR) spectroscopy using strong magnetic fields, high microwave frequencies and high hydrostatic pressure to study a recently discovered family of quasi-one-dimensional organic charge transfer salts, the δ-(EDT-TTF-CONMe2)2X; family to gain insight into the complex physics of strongly correlated low-dimensional compounds. ESR is a highly efficient technique to study organic conductors. This can be easily understood if one realizes that ESR is observable inmost organic conductorswhile only very few "ordinary" metals show detectable ESR signals, even at low temperatures. Indeed, this feature results principally from the electronic low-dimensionality of these systems which controls the spin relaxation process. This tendency is even more reinforced by the fact that most organic molecules only contain light chemical elements for which the effect of the spin-orbit coupling on the ESR linewidth remains small. For this reason, ESR spectroscopy is certainly amajor tool to investigate the paramagnetic state of organic conductors. Moreover, in samples where a magnetic order is present, the large frequency bandwidth of the spectrometers used in the experimental setup (4 GHz–420 GHz) makes the resonance signal accessible at low temperatures in the ordered state, where the resonance frequencywould be too high for conventional ESRspectrometers. Our device hence represents an efficient tool to probe different magnetic ground states and, in particular, the antiferromagnetic state, which is most commonly found in organic salts. Organic conductors exhibit a high degree of sensitivity with respect to magnetic fields and applied pressure. To combine the high sensitivity of the ESR method with the sensitivity and thus fine tuneability of these organic systems by magnetic field and pressure, we build a unique high-field, high-frequency and high-pressure ESR spectrometerwhich is well suited to the study of organic materials. Compared to other devices of its kind, the ESR spectrometer built in our laboratory as a part of this thesis enables a wide parameter space to be explored: magnetic fields from 0 – 16 T, microwave frequencies in the 105 – 420 GHz range and by making use of a custom-made pressure cell, the sample applied pressure can be elevated up to 1.6GPa. To the best of our knowledge, the addition of such a pressure regulating unit to an ESR is unique. Through varying the applied pressure, it can be used to investigate the competition of different ground states: bymodifying the lattice parameters, it can tune the interactions without introducing disorder. The first Part of the thesis is dedicated to a general overview about the physical properties of quasi-one-dimensional organic systems. It also contains a more specific introduction to the crystallographic and electronic properties of the δ-(EDT-TTF-CONMe2)2X family of charge transfer salts. In Part II, we focus on the theoretical description of electronic correlations with a special emphasis on lowdimensional electron systems and on the effect of dimensionality. The objective of this part is to assemble the necessary theoretical notions to understand the phenomena observed in the system under investigation. We show that Coulomb-repulsion in general leads to antiferromagnetism, and give a brief phenomenological description of the antiferromagnetically ordered state. Then, a standard description of the physicalprinciples of Electron Spin Resonance follows. We also give a briefdescription of antiferromagnetic resonance (AFMR) and Conduction Electron Spin Resonance (CESR) with a special focus on quasi-one-dimensional situations. Part III presents the technical details of the ESR spectrometer developed as a part of this thesis. The final Part describes the experimental observations collected during the thesis. In Chapter 9, the phase transitions at atmospheric pressure of a sample of δ-(EDT-TTF-CONMe2)2AsF6 as a function of temperature are presented and discussed. We found a spin-chain behavior at high temperatures and a complicated antiferromagnetic structure below TN=8 K. In the intermediate temperature range we found a suppression of the susceptibility what we explained by the opening of a spin-pseudogap. In Chapter 10, the same procedure is performed for a δ-(EDT-TTF-CONMe2)2Br sample and the findings are really similar to the case of the AsF6 salt. Finally, in Chapter 11, we describe and evaluate measurements performed under high hydrostatic pressure on the δ-(EDT-TTF-CONMe2)2Br sample. We followed the deconfinement transition of the salt and then identified a dimensional crossover from a quasi-one dimensional metal to a more conventional higher dimensional conducting state. At the end of the thesis we drown a general pressure temperature phase diagram of this family of charge transfer salts.

EPFL2008

Molecular and Spin Dynamics in the Paramagnetic Endohedral Fullerene Gd3N@C-80

László Forró, Bálint Náfrádi

Gd3N@C-80 endohedral fullerene, a starting material for a potential magnetic resonance imaging contrast agent, is investigated by high-frequency electron spin resonance (ESR) and SQUID magnetometry. The magnetic moments of the three Gd ions of the endohedral Gd3N molecule are ferromagnetically aligned at low temperatures and are uncorrelated at high temperatures. The 4 T broad 210 and 315 GHz ESR spectra measured at 2 K are well-described by a single transition between the lowest Zeeman levels of static molecules shifted by fine structure effects. At higher temperatures there is a gradual transition to a rotating state. At ambient temperatures the rotation frequency is much larger than the fine structure broadening, and a single ESR line is observed at a gyromagnetic ratio of g = 1.995.

Amer Chemical Soc2012

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.