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Only a few four-coordinated Co 2+-complexes show single-molecule magnet (SMM) properties without an applied dc field. Common for those is a large magnetic anisotropy generated by the close proximity of the (Formula presented.) and (Formula presented.) orbitals. This type of magnetic anisotropy is strongly correlated with the extent of structural distortion from ideal tetrahedral towards linear coordination. Quantification of the governing magneto-structural correlations is hence crucial for the development of better SMMs. For this purpose, we synthesized and analyzed four significantly distorted tetrahedral cobalt complexes that exhibit extremely large magnetic anisotropies and slow relaxation of magnetization in zero field. The N−Co−N bite angles vary from 70.8 to 72.7°, and magnetic measurements show strong anisotropy with D-values in the range from (Formula presented.) 75 to (Formula presented.) 114 cm −1. Ab initio calculations supported the experimental results and highlighted a significant increase of D up to (Formula presented.) 141 cm −1, correlated with the reduction of the energy difference between the (Formula presented.) and (Formula presented.) orbitals. Based on these magneto-structural correlations and in contrast to the current assumption that the smaller bite angle is the better, we predict that with an ideal bite angle in the range from 76–78° in distorted Co(N 2R) 2 complexes, the energy difference between the two important d-orbitals is at a minimum and hence the magnetic anisotropy is maximized.

Nuclear Magnetic Resonance (NMR) is ubiquitous as a tool for investigating the structure of both molecules and materials. In most cases the best results are obtained by placing the sample under study in the largest possible magnetic field, since this maximizes both the sensitivity and the resolution of the experiment. This fact underlies the momentous engineering effor that has gone into developing ever larger super-conducting magnets for use in NMR experiments. However, there are many applications of NMR where the sheersize, not to mention cost, of a high-field instrument can not be accommodated. Therefore there has been a sustained research effort aimed at developing small mobile NMR systems that are not based on super-conducting magnets.NMR performed in zero-field, using magnetic shielding to screen out the ambient field of the earth, and detected using vapor-cell magnetometers, is one such approach. In zero-field NMR the signal originates entirely in the spin-spin coupling part of the Hamiltonian and chemically informative spectra are obtained even in the absence of chemical-shifts. The very large absolute field homogeneity obtainable at zero-field ensures that the spectral resolution is even better than in high-field spectrometers. Perhaps even more interestingly, the different geometry that results from not having a preferred axis imposed on the system enables experiments that are not possible in the presence of magnetic fields. Examples include the ability to distinguish enantiomers based on the phase of an NMR signal.This work extends directly detected zero-field NMR beyond pulse-acquire type experiments. New methods for spin control and excitation, as well as two-dimensional spectroscopy and various decoupling techniques are introduced and demonstrated experimentally. For completeness this dissertation also contains a review of zero-field J-spectroscopy, as well as a detailed guide to the relevant instrumentation. It is my hope that the techniques presented herein will find use in future NMR experiments performed in the regime of zero and ultralow field, whether the application is portable chemical analysis, fundamental physics, or anything in between.

Agradecimentos: This work was supported by the Brazilian agencies FAPESP (Project No.2012/51198-2, 2017/10581-1, and 2020/07397-7), as well CNPq (436573/2018-0). We thank the beam-time at the PGM beamline of the Brazilian Synchrotron Light Laboratory (LNLS) for the soft X rays and XMCD measurements. We would like to thank the Laboratory of Materials and Low Temperatures (LMBT) of the University of Campinas (Unicamp), for the VSM and SQUID measurements. We also thank the Microfabrication Laboratory of the Brazilian Nanotechnology National Laboratory (LNNano) for growing samples facilities Abstract: We demonstrate the observation of spin spirals and individual skyrmions in synthetic Pt/CoGd/Pt ferrimagnetic multilayers at room-temperature without the assistance of an external magnetic field. The magnetic coupling between the magnetic moments of Co and Gd, was investigated across various CoGd thicknesses by element specific X-ray magnetic circular dichroism (XMCD) performed around the Co L-2,L-3 and Gd M(4,5 )edges. It show s an uncompensated ferrimagnetic orientation between the Co and Gd atoms preserved at ambient conditions, even for sub-nanometer thick layers. In addition, with the tunable enhanced perpendicular magnetic anisotropy K-u and remnant magnetization M-R found for thinner CoGd & LE; (0.9 nm) isolated zero-field skyrmions were imaged by magnetic force microscopy (MFM) as the magnetic ground state, replacing the spin spirals observed for thicker CoGd layers & GE; (1.2 nm). Such stable skyrmions in perpendicularly magnetized materials with antiparallel exchange-coupling are potential candidates for energy-efficient data storage and processing in future antiferromagnetic-based devices FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ Fechado

We demonstrate that heterogeneous/biphasic chemical reactions can be monitored with high spectroscopic resolution using zero‐field nuclear magnetic resonance spectroscopy. This is possible because magnetic susceptibility broadening is negligible at ultralow magnetic fields. We show the two‐step hydrogenation of dimethyl acetylenedicarboxylate with para‐enriched hydrogen gas in conventional glass NMR tubes, as well as in a titanium tube. The low frequency zero‐field NMR signals ensure that there is no significant signal attenuation arising from shielding by the electrically conductive sample container. This method paves the way for in situ monitoring of reactions in complex heterogeneous multiphase systems and in reactors made of conductive materials while maintaining resolution and chemical specificity., Authority zero: Process monitoring of a hydrogenation reaction using zero‐field NMR spectroscopy is possible if the reaction is performed with H2 gas enriched in the para nuclear spin isomer, which gives hyperpolarized NMR signals. Zero‐field NMR allows the reaction to be monitored through a metal tube during continuous bubbling of hydrogen into solution, while maintaining the specificity of high‐field NMR spectroscopy.

Single-molecule magnet materials owe their function to the presence of significant magnetic anisotropy, which arises from the interplay between the ligand field and spin-orbit coupling, and this is responsible for setting up an energy barrier for magnetic relaxation. Therefore, chemical control of magnetic anisotropy is a central challenge in the quest to synthesize new molecular nanomagnets with improved properties. There have been several reports of design principles targeting such control; however, these principles rely on idealized geometries, which are rarely obtained in crystal structures. Here, we present the results of high-pressure single-crystal diffraction on the single-ion magnet, Co(SPh) 4(PPh 4) 2, in the pressure range of 0-9.2 GPa. Upon pressurization a sequence of small geometrical distortions of the central CoS 4 moeity are observed, enabling a thorough analysis of the magneto-structural correlations. The magneto-structural correlations are investigated by theoretical analyses of the pressure-dependent experimental molecular structures. We observed a significant increase in the magnitude of the zero-field splitting parameter D, from -54.6 cm -1 to -89.7 cm -1, which was clearly explained from the reduction of the energy difference between the essential d xy and d x 2-y 2 orbitals, and structurally assigned to the change of an angle of compression of the CoS 4 moeity.

Herein, we report on investigations of magnetic and spectroscopic properties of three heterobimetallic Fe(ii)-Co(ii) coordination compounds based on the tetracoordinate {CoP2X2} core encapsulated by dppf metalloligand, where X = Cl (1), Br (2), I (3), dppf = 1,1 '-ferrocenediyl -bis(diphenylphosphine). The analysis of static magnetic data has revealed the presence of axial magnetic anisotropy in compounds (1) and (2) and this was further confirmed by high-frequency electron spin resonance (HF-ESR) spectroscopy. Dynamic magnetic data confirmed that (1) and (2) behave as field-induced Single-Ion Magnets (SIMs). Together with bulk studies, we have also tested the possibility of depositing (2) as thick films on Au(111), glass, and polymeric acetate by drop-casting as well as thermal sublimation, a key aspect for the development of future devices embedding these magnetic objects.

A series of mononuclear tetrahedral Co-II complexes with a general molecular formula [CoL2X2] [L = thiourea and X = Cl (1), Br (2) and I (3)] were synthesized and their structures were characterized by single-crystal X-ray diffraction. Direct-current (dc) magnetic susceptibility [cMT(T) and M(H)] and its slow relaxation of magnetization were measured for all three complexes. The experimental dc magnetic data are excellently reproduced by fitting both chi T-M(T) and M(H) simultaneously with the parameters D=+ 10.8 cm(-1), g(1)= 2.2, g(2) = 2.2, and g(3) = 2.4 for 1; D= -18.7 cm(-1), g(iso) = 2.21 for 2; and D= -19.3 cm(-1), g(iso) = 2.3 for 3. The replacement of chloride in 1 by bromide or iodide (in 2 and 3, respectively) was accompanied by a change in both sign and magnitude of the magnetic anisotropy D. Field-induced out-of-phase susceptibility signals observed in 10% diluted sam-ples of 1-3 imply slow relaxation of magnetization of molecular origin. To better understand the magnetization relaxation dynamics of complexes 1-3, detailed ab initio CASSCF/NEVPT2 calculations were performed. The computed spin Hamiltonian parameters are in good agreement with experimental data. In particular, the calculations unveil the role of halide ions in switching the sign of D on moving from Cl- to I-. The large spin-orbit coupling constant associated with the heavier halide ion and weaker p donation reduces the ground state-excited state gap, which leads to a larger contribution to negative D for complex 3 compared to complex 1. Further magnetostructural D correlations were developed to understand the role of structural distortion in the sign and magnitude of D values in this family of complexes.

A simple two-level system is introduced to demonstrate the existence of the intermolecular mechanism of the appearance/disappearance of multiple maxima in molecular dynamic susceptibility at low temperature. With minimum two relaxation processes, quantum tunneling induced by the nuclear spin bath and the direct process due to the spin-phonon interaction, we prove that the existence of a sufficiently wide dipolar field distribution in the experimental sample is the main cause of this phenomenon in zero or weak applied dc field. The correlations between the phenomenon and the applied dc field, temperature, or magnetic dilution of the sample are also investigated. Application to several experimental systems has shown a good agreement between the proposed theory and experiments. Cases with more complex multiplet spectrum, more relaxation processes,and possibility of more maxima in the molecular dynamic susceptibility are discussed., 13 pages, 9 figures, 1 Supplementary Material

A series of mononuclear tetrahedral cobalt(II) complexes with the general molecular formula [Co(L-1)(2)X-2] [where L-1 = tetramethylthiourea ([(CH3)(2)N](2)C=S) and X = Cl (1), Br (2), and I (3)] were isolated, and their structures were characterized by single-crystal X-ray diffraction. The experimental direct-current magnetic data are excellently " reproduced by fitting both chi T-M(T) and M(H) simultaneously using the spin Hamiltonian (SH) parameters D-1 = 18.1 cm(-1) and g(1,iso) = 2.26, D-2 = 16.4 cm(-1) and g(2,iso) = 2.33, and D-3 = -22 cm(-1) and g(3,iso) 2.4 for 1-3, respectively, and the sign of D was unambiguously confirmed from X-band electron paramagnetic resonance measurements. The effective energy barrier extracted for the magnetically diluted complexes 1-3 (10%) is larger than the barrier observed for the pure samples and implies a nonzero contribution of dipolar interaction to the magnetization relaxation dynamics. The SH parameters extracted for the three complexes drastically differ from their respective parent complexes that possess the general molecular formula [Co(L)(2)X-2] [where L = thiourea [(NH2)(2)C=S] and X = Cl (la), Br (2a), and I (3a)], which is rationalized by detailed ab initio calculations. An exhaustive theoretical study reveals that both the ground and excited states are not pure but rather multideterminental in nature (1-3). Noticeably, the substitution of L by L, induces structural distortion in 1-3 on the level of the secondary coordination sphere compared to 1a-3a. This distortion leads to an overall reduction in vertical bar E/D vertical bar of 1-3 compared to 1a-3a. This may be one of the reasons for the origin of the slower relaxation times of 1-3 compared to 1a-3a.

A systematic one-step one-pot multicomponent reaction of Co(ClO4)(2)center dot 6H(2)O, H3L (2,6-bis((2-(2-hydroxyethylamino)ethylimino)methyl)-4-methylphenol), and readily available carboxylate salts (RCO2Na; R = CH3, C2H5) resulted in the two structurally novel coordination aggregates [(CoCo4L2)-Co-II-L-III(mu(1,3)-O2CCH3)(2)(mu-OH)(2)](ClO4)(4)(center dot) 4H(2)O(1) and [Co(II)co(4)(III)L(2)(mu(1,3)-O2CC2H5)(2)(mu-OH)(mu-OMe)](ClO4)(4)center dot 5H(2)O (2). At room temperature, reactions of H3L in MeOH with cobalt(II) perchlorate salts led to coassembly of initially formed ligand-bound {Co-2(II)} fragments following aerial oxidation of metal centers and bridging by in situ generated hydroxido/alkoxido groups and added carboxylate anions. Available alkoxido arms of the initially formed {L(mu(1,3)-O2CCH3)(mu-OH/OMe)Co-2}(+) fragments were utilized to trap a central Co-II ion during the formation of [Co-5] aggregates. In the solid state, both complexes have been characterized by X-ray crystallography, variable-temperature magnetic measurements, and theoretical studies. Both 1 and 2 show field-induced slow magnetic relaxation that arises from the single pseudo-T-d Co-II ion present. The structural distortion leads to an easy-axis magnetic anisotropy (D = -31.31 cm(-1) for 1 and -21.88 cm(-)1 for 2) and a small but non-negligible transverse component (E/D = 0.11 for 1 and 0.08 for 2). The theoretical studies also reveal how the O-Co-O bond angles and the interplanar angles control D and E values in 1 and 2. The presence of two diamagnetic {Co-2(mu-L)} hosts controls the distortion of the central {CoO4} unit, highlighting a strategy to control single-ion magnetic anisotropy by trapping single ions within a diamagnetic coordination environment.

A monometallic non-Kramers Ho(III) complex with a strong uniaxial ligand field displays a record high energy barrier of 355 K with hysteresis opening up to 4 K at a sweep rate of 0.027 T s(-1), while the isomorphous, more anisotropic non-Kramers Tb(III) complex displays strong quantum tunnelling between the ground states. Ab initio calculations reveal that the presence of strong axial ligands and moderately weaker equatorial ligands is sufficient to quench the QTM between the ground pseudo doublets (m(J) = +/- 8) in the case of Ho(III) complex, where the relaxation is found to proceed via the first excited state. However, the equatorial field disrupts the stabilization of ground pseudo doublets in Tb(III), inducing QTM and inhibiting SIM behaviour. Further insights into the decisive role played by the secondary coordination sphere and hydrogen-bonding in the SIM characteristics of these two non-Kramers ions were also gained. This combined experimental and computational approach highlights that although strong axiality holds the key for designing high temperature SIMs based on oblate non-Kramers ions, the strength of the equatorial ligand field also cannot be ignored.

A family of mononuclear tetrahedral cobalt(II) thiourea complexes, [Co(L-1)(4)](NO3)(2) (1) and [Co(L-x)(4)](ClO4)(2) where x = 2 (2), 3 (3), 4 (4) (where L-1 = thiourea, L-2 = 1,3-dibutylthiourea, L-3 = 1,3-phenylethylthiourea, and L-4 = 1,1,3,3-tetramethylthiourea), has been synthesized using a rationally designed synthetic approach, with the aim of stabilizing an Ising-type magnetic anisotropy (-D). On the basis of direct-current, alternating-current, and hysteresis magnetic measurements and theoretical calculations, we have identified the factors that govern the sign and magnitude of D and ultimately the ability to design a single-ion magnet for a tetrahedral cobalt(II) ion. To better understand the magnetization relaxation dynamics, particularly for complexes 1 and 2, dilution experiments were performed using their diamagnetic analogues, which are characterized by single-crystal X-ray diffraction with the general molecular formulas of [Zn(L-1)(4)](NO3)(2) (5) and [Zn(L-2)(4)]-(ClO4)(2) (6). Interestingly, intermolecular interactions are shown to play a role in quenching the quantum tunneling of magnetization in zero field, as evidenced in the hysteresis loop of 1. Complex 2 exhibits the largest U-eff value of 62 cm(1) and reveals open hysteresis loops below 4 K. Furthermore, the influence of the hyperfine interaction on the magnetization relaxation dynamics is witnessed in the hysteresis loops, allowing us to determine the electron/nuclear spin S(Co) = 3/2/I(Co) = 7/2 hyperfine coupling constant of 550 MHz, a method ideally suited to determine the hyperfine coupling constant of highly anisotropic metal ions stabilized with large D value, which are otherwise hard to determine by conventional methods such as electron paramagnetic resonance.

The new dinuclear (ZnDyIII)-Dy-II and trinuclear (ZnDyIII)-Dy-II-Zn-II complexes of formula [(LZnBrDy(ovan) (NO3)(H2O)](H2O)center dot 0.5(MeOH) (1) and [((LZnBr)-Zn-1)(2)Dy(MeOH)(2)](ClO4) (3) (L and L-1 are the dideprotonated forms of the N,N'-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato and 2-{(E)-[(3-{[(2E,3E)-3-(hydroxyimino)butan-2-ylidene ]amino}-2,2-dimethylpropyl)imino]methyl}-6-methoxyphenol Schiff base compartmental ligands, respectively) have been prepared and magnetostructurally characterized. The X-ray structure of 1 indicates that the Dy-III ion exhibits a DyO9 coordination sphere, which is made from four O atoms coming from the compartmental ligand (two methoxy terminal groups and two phenoxido bridging groups connecting Zn-II and Dy-III ions), other four atoms belonging to the chelating nitrato and ovanillin ligands, and the last one coming to the coordinated water molecule. The structure of 3 shows the central Dy-III ion surrounded by two L1Zn units, so that the Dy-III and Zn-II ions are linked by phenoxido/oximato bridging groups. The Dy ion is eight-coordinated by the six O atoms afforded by two L1 ligands and two O atoms coming from two methanol molecules. Alternating current (AC) dynamic magnetic measurements of 1, 3, and the previously reported dinuclear [LZnClDy(thd)(2)] (2) complex (where thd = 2,2,6,6-tetramethyl-3,5-heptanedionato ligand) indicate single molecule magnet (SMM) behavior for all these complexes with large thermal energy barriers for the reversal of the magnetization and butterfly-shaped hysteresis loops at 2 K. Ab initio calculations on 1-3 show a pure Ising ground state for all of them, which induces almost completely suppressed quantum tunnelling magnetization (QTM), and thermally assisted quantum tunnelling magnetization (TA-QTM) relaxations via the first excited Kramers doublet, leading to large energy barriers, thus supporting the observation of SMM behavior. The comparison between the experimental and theoretical magnetostructural data for 13 has allowed us to draw some conclusions about the influence of ligand substitution around the Dy-III on the SMM properties. Finally, these SMMs exhibit metal- and ligand-centered dual emissions in the visible region, and, therefore, they can be considered as magnetoluminescent bifunctional molecular materials.

The magnetism of DySc2N@C80 endofullerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temperatures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in DyIII ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc2N@C80 samples with C60 increases the zero-field 4f electron relaxation time at 2 K to several hours.