59Co Nuclear Magnetic Resonance Research Articles

NMR Study of LiCo0.96Al0.04O2 As a Positive Electrode Material for Li-Ion Batteries: Homogeneity and Role of Doping on Mechanisms

LiCoO2 (LCO) is one of the most widely used positive electrode materials in lithium-ion batteries due to its high volumetric energy density which can be further improved by charging at high voltages. However, working at high potential (above 4.3 V vs. Li+/Li) causes a deterioration in cycling performance induced by structural instabilities, electrolyte oxidation and Co dissolution. Extensive studies have been devoted to increasing energy density by increasing the cutoff voltage. The substitution of cobalt with various metals is one of the classic methods, the choice of aluminum as dopant for example is based on several criteria: low cost and non-toxicity, a similar radius for Al3+ compared to Co3+ (0.535 Å vs. 0.545 Å) facilitates the replacement of the latter and maintains the structure leading to the complete solid solution LiCo1-yAlyO2.We will focus here on LiCo0.96Al0.04O2, synthesized by the solid route and characterized by 7Li, 27Al and 59Co Nuclear Magnetic Resonance (NMR) and synchrotron X-Ray Diffraction (XRD). In addition to the characterization of the mean structure by XRD, the NMR allows us to study the local environments of the different nuclei. 27Al MAS NMR and synchrotron XRD studies revealed that the Al-doping in this material is homogeneous. 7Li MAS NMR indicates that its stoichiometry (Li/M=1.00) is ideal. The electrochemical study has shown that the phase transitions occurring during delithiation of LiCoO2, were partially eliminated by aluminum doping even with a low doping amount and the high potential mechanisms appear to be different. In order to better understand the role of Al-doping on the mechanisms involved during cycling, several phases LixCo0.96Al0.04O2 were prepared by electrochemical deintercalation and studied by 7Li, 59Co, and 27Al NMR and by XRD.During lithium deintercalation, Co3+ is oxidized to Co4+ then one unpaired electron is generated in the t2g band of Co atoms (t2g 5eg 0). For 0.8 ≤ x ˂ 1.00 the 7Li MAS NMR shows that the system behaves locally as two phase domain: domain with localized electrons and domain with delocalized ones on Con+ ions (Knight shift), even for very low amount of deintercalation (x=0.98), thus the XRD results confirm the existence of the biphasic domain. Whereas, a single phase domain is observed for the deintercalated materials with 0.4 ≤ x ≤ 0.8 and also at the NMR time of scale a single signal (Knight shift) is observed. This signal increases in intensity at the expense of 0 ppm and shifts to the higher isotropic displacement values up to x=0.5 then decreases, thus the evolution of the chex lattice parameter is correlated with the signal shift. For x = 0.3 and x = 0.2: a new broad signal appears probably due to formation of new Li environment in stacking faulted layers (O1 type). These different contributions will be more detailed on the presentation.

The Function of Co in Co-substituted M-type Ferrites: 59Co-NMR Study

The uniaxial magnetic anisotropy of the magnetoplumbyte-type (M-type) hard ferrite magnet is improved by the substitution of small amount of Co for Fe. The Co atoms occupy at least 3 sites among 5 crystallographically inequivalent Fe sites. Different magnetic functions of Co are expected at the different sites because of different oxygen coordinations around the sites. We have revealed from 59Co nuclear magnetic resonance experiment that only Co located at the tetrahedrally coordinated 4f1 site is responsible for the enhancement in the uniaxial anisotropy, demonstrating that the control of the Co occupation site is essentially important in the Co-doped M-type ferrite. If only the tetrahedrally coordinated 4f1 site were occupied by Co atoms, the magnetic performance can be improved more efficiently.

Epitaxial Co1-xMox thin film alloys studied by 59Co NMR

59Co NMR (Nuclear Magnetic Resonance) has been used to investigate the distribution of 59Co hyperfine fields in the Co1-xMox thin film alloys (0.01 < x < 0.1) co-deposited on V buffer. At x = 0 the films consisted of a mixture of fcc and hcp Co, and Mo was found to enter randomly the two Co fractions in the investigated concentration range. However, alloying with Mo was found to increase the amount of stacking faults, in agreement with the previously published results of theoretical calculations for binary alloys. A certain deviation from a random distribution of Mo, most probably due to the presence of magnetically induced miscibility gap for Co:Mo, has been identified for the alloys with Mo content larger than 1%. It was found that in the limit of a very small Mo concentration x ≈ 0, each Mo nearest neighbor lowers the 59Co NMR resonance frequency by 43.8 MHz, i.e. the corresponding hyperfine field is reduced by 4.34 T, implying that Co atoms would become nonmagnetic when 5 Mo atoms enter as the nearest neighbors. However, the effect increases with concentration, due to the contribution from Mo in the more distant shells. The extrapolation to Mo concentration higher than the one used in the experiment predicts that Co becomes nonmagnetic when surrounded by 3 Mo nearest neighbors for the concentration as low as x = 0.2–0.25.

Modification of magnetic fluctuations by interfacial interactions in artificially engineered heavy-fermion superlattices

Recent progress in the fabrication techniques of superlattices (SLs) has made it possible to sandwich several-layer-thick block layers (BLs) of heavy-fermion superconductor CeCoIn5 between conventional-metal YbCoIn5 BLs or spin-density-wave-metal CeRhIn5 BLs of a similar thickness. However, the magnetic state in each BL, particularly at the interface, is not yet understood, as experimental techniques applicable to the SL system are limited. Here, we report measurements of 59Co nuclear magnetic resonance, which is a microscopic probe of the magnetic properties inside the target BLs. In the CeCoIn5/YbCoIn5 SL, the low-temperature magnetic fluctuations of the CeCoIn5 BL are weakened as expected from the Rashba spin-orbit effect. However, in the CeCoIn5/CeRhIn5 SL, the fluctuations show an anomalous enhancement below 6 K, highlighting the importance of the magnetic proximity effect occurring near a magnetic-ordering temperature TN ~ 3 K of the CeRhIn5 BL. We suggest that the magnetic properties of the BLs can be altered by the interfacial interaction, which is an alternative route to modify the magnetic properties.

Co site preference and site-selective substitution in La–Co co-substituted magnetoplumbite-type strontium ferrites probed by 59Co nuclear magnetic resonance

In the La–Co co-substituted magnetoplumbite-type strontium (Sr–La–Co) ferrite, the base materials of the high-performance hard ferrite magnet, Co tends to occupy more than two crystallographically inequivalent sites. To reveal the Co site preference and the function of each site, 59Co nuclear magnetic resonance (NMR) spectrum was measured for the Sr–La–Co ferrites with various Co compositions. The intimate correlation between the anisotropy field and the relative intensity of the strongest NMR line clearly indicates that Co occupying the tetrahedrally coordinated 4f1 site is only responsible for the enhancement in the uniaxial anisotropy. In the samples with the Co concentrations much larger than commercial magnets, most Co selectively occupies the 4f1 site, indicating possible tuning of the magnetic performance with limited Co content in the future development. The close correlation between orbital moments and local strain is also demonstrated.

Disorder in Co-Cu granular alloys studied by 59Co NMR

We have performed 59Co zero field nuclear magnetic resonance (NMR) experiment at 4 K on CoxCu1-x (x = 0.10, 0.32 and 0.76) nanogranular alloys. The NMR signal arises from Co rich ferromagnetic clusters formed within the Cu matrix. The analysis of NMR spectra shows that the resonance peaks arise from both fcc and hcp phases of Co. However, Co nuclei experience magnetic hyperfine field from Co only environment as well as other environments in which nearest-neighbor (NN) Co atoms are replaced by Cu. Assuming that these two environments constitute the core and the shell of the Co clusters, it is estimated using previous magnetization measurements that the contribution of core dominates in the ferromagnetism of the particles though it occupies only about 20% of the volume of Co clusters even for high Co containing alloys.

Discussing the Homogeneity of Aluminum Distribution in LiCo1-­YAlyO2 for Low Doping Amounts (y ≤ 0.04)

Although LiCoO2 has been now used for decades as positive electrode material in Li-ion batteries, the maximum x(Li+) exchanged between the electrodes in a commercial cell has remained quite unchanged. Indeed, only approximately 0.62 Li+ ions are reversibly de-intercalated from the lamellar structure of LiCoO2 (analogue to NaFeO2 structure, crystallizing in the R-3m space group), giving a capacity of 174 mAh/g and a typical cycling electrochemical window of 3.0 – 4.4 V vs. graphite (3.0 – 4.3 V vs. Li). One possible strategy to try to access more capacity in a LiCoO2//C cell is then to succeed in reversibly extracting the remaining 0.38 Li+ ions, that could lead to a potential capacity gain of roughly 100 mAh/g (272 mAh/g being the theoretical capacity). Even though the full extraction seems rather unlikely, as it was previously shown that the final CoO2 suffers from critical oxygen losses1, a partial de-intercalation of the remaining 0.38 Li+ looks reasonable enough. The number of publications dealing with the performance of LiCoO2 cycled up to 4.5 V vs. Li or even 4.7 V vs. Li has then increased in the last years. However, bare LiCoO2 has always proven to show a very poor cycling performance in such conditions. It is believed that the main causes for this are i) structural instabilities1–3, ii) electrolyte degradation4,5 and iii) cobalt dissolution6. Two main strategies have been successfully investigated to overcome these issues: coatings and doping. Aluminum ions Al3+ as dopants were among the first considered, by the means of theoretical calculations from Ceder’s group7, followed by experimental work from Jang et. al 8 and other groups9–11. The choice for aluminum was motivated by i) the low cost and non-toxicity of aluminum, ii) an increased overall voltage necessary to achieve higher power densities, iii) a similar radius for Al3+ compared to Co3+ (53.5 pm vs. 54.5 pm) facilitating the substitution of the latter and preserving the structure leading to the full solid solution LiCo1-yAlyO2. These phases were synthesized in many different ways from one article to another, but investigation on the aluminum distribution is often missing. Dahn’s group recently studied the interdiffusion of Co3+ and Al3+ in the case of coated LiCoO2 12, but what about their interdiffusion during the synthesis of Al-doped LiCo1-yAlyO2? In this talk, we will discuss the aluminum distribution homogeneity in Al-doped LiCo1-yAlyO2 phases obtained from different solid state syntheses with well-controlled low doping amount (y = 0.01 or 0.04) and particle size. We will show that synchrotron X-ray diffraction data (SXRD) and 27Al & 59Co nuclear magnetic resonance (NMR) techniques are powerful tools in that matter. A special focus will also be given to the relation between the homogeneity of the Al-doping and the electrochemical performances of the materials used as positive electrode in Li cells. G. G. Amatucci, J. M. Tarascon, and L. C. Klein, J. Electrochem. Soc., 143, 1114–1123 (1996).T. Ohzuku and A. Ueda, J. Electrochem. Soc., 141, 2972–2977 (1994).H. Xia, L. Lu, Y. S. Meng, and G. Ceder, J. Electrochem. Soc., 154, A337–A342 (2007).Q. Zhuang et al., Sci. China Ser. B Chem., 50, 776–783 (2007).A. Würsig, H. Buqa, M. Holzapfel, F. Krumeich, and P. Novák, Electrochem. Solid-State Lett, 8, A34–A37 (2005).G. G. Amatucci, J. M. Tarascon, and L. C. Klein, Solid State Ion., 83, 167–173 (1996).G. Ceder et al., Nature, 392, 694 (1998).Y.-I. Jang et al., J. Electrochem. Soc., 146, 862–868 (1999).S.-T. Myung, N. Kumagai, S. Komaba, and H.-T. Chung, Solid State Ion., 139, 47–56 (2001).Y. Lee et al., Electrochimica Acta, 50, 491–494 (2004).H.-T. Xu, H. Zhang, L. Liu, Y. Feng, and Y. Wang, ACS Appl. Mater. Interfaces, 7, 20979–20986 (2015).12. Y. Zhao, J. Li, and J. R. Dahn, Chem. Mater., 29, 5239–5248 (2017).

59Co NMR analysis of CoFeB-MgO based magnetic tunnel junctions

59Co Nuclear Magnetic Resonance (NMR) and X-Ray Diffraction (XRD) has been used to investigate annealing effects on crystallographic structure and magnetic properties of ferromagnetic CoFeB layer in pseudo spin valves (P-SV): Ta/CoFeB/MgO/CoFeB and in an exchange bias spin valves (EB-SV): PtMn/CoFe/Ru/CoFeB/MgO/CoFeB magnetic tunnel junctions. It was found, that the as-deposited CoFeB layers in the P-SV and EB-SV are amorphous. The annealing treatment at 340 °C and 360 °C induces in all studied systems progressive phase segregation in CoFeB layer to areas of bcc CoFe regions displaying different degree of B2 order and to areas of an amorphous magnetic reminder with a higher B content. In the CoFeB layers grown on Ru (EB-SV) and Ta (P-SV) and covered by MgO in the upper interface, after annealing at 500 °C a state of substantially full phase segregation to B2 ordered bcc CoFe phase has been observed. The annealing at 500 °C of the CoFeB layers grown on MgO and covered by Ru in P-SV and EB-SV triggers extensive thermal diffusion and induces mixing at the CoFeB/Ru interface which transforms the CoFeB into a non-magnetic layer. Moreover, it has been found that CoFeB crystallizes better into the bcc CoFe (100) texture when deposited on Ta than on a Ru layer. XRD profiles show that while CoFeB layer is growing on Ru it crystallizes into bcc CoFe (110) oriented texture, and the one grown on Ta crystallizes into bcc CoFe (200) when annealed at 500 °C. 59Co NMR restoring field analysis (Hrest - magnetic stiffness) showed that the insertion of the antiferromagnetic PtMn layer into the EB-SV system increases the magnetic hardness of the CoFeB layer by an order of the magnitude. However, adding CoFeB/MgO layers between PtMn/CoFe/Ru and CoFeB layers in the EB-SV decouples the CoFeB from the hard-magnetic system making it magnetically soft.

Internal field 59Co NMR study of cobalt-iron nanoparticles during the activation of CoFe2/CaO catalyst for carbon nanotube synthesis

CoFe2/CaO catalysts for multi-walled carbon nanotubes (MWCNT) were characterized during the activation period (T = 670 °C; C2H4/Ar = 1:1, 400 sccm) by internal field 59Co nuclear magnetic resonance. During this induction period, although C2H4 is already reacting, the MWCNT yield remains negligible. The NMR spectra of the catalysts revealed the occurrence of several structural and magnetic species. They were not detectable by conventional X-ray diffraction but the heterogeneity of the metal particles was corroborated by high-resolution transmission electronic microscopy coupled with energy dispersive X-ray spectroscopy. Two samples quenched at different activation time contained, beside non-reacted pure metal Co particles, strong and weak ferromagnetic alloys. During activation, the pure Co metal content dropped and an increase of the ordering of the cobalt-iron alloy in the catalyst was evidenced thus revealing a multistage activation mechanism for the reaction. We propose this ordering process during the induction period to be an intermediate step between the strong ferromagnetic Co rich alloy and the weak ferromagnetic Fe rich one and that it favors the onset of MWCNTs growth.

Direct evidence for the source of reported magnetic behavior in “CoTe”

In order to unambiguously identify the source of magnetism reported in recent studies of the Co-Te system, two sets of high-quality, epitaxial CoTex films (thickness ≈ 300 nm) were prepared by pulse laser deposition (PLD). X-ray diffraction (XRD) shows that all of the films are epitaxial along the [001] direction and have the hexagonal NiAs structure. There is no indication of any second phase metallic Co peaks (either fcc or hcp) in the XRD patterns. The two sets of CoTex films were grown on various substrates with PLD targets having Co:Te in the atomic ratio of 50:50 and 35:65. From the measured lattice parameters c = 5.396 Å for the former and c = 5.402 Å for the latter, the compositions CoTe1.71 (63.1% Te) and CoTe1.76 (63.8% Te), respectively, are assigned to the principal phase. Although XRD shows no trace of metallic Co second phase, the magnetic measurements do show a ferromagnetic contribution for both sets of films with the saturation magnetization values for the CoTe1.71 films being approximately four times the values for the CoTe1.76 films. 59Co spin-echo nuclear magnetic resonance (NMR) clearly shows the existence of metallic Co inclusions in the films. The source of weak ferromagnetism reported in several recent studies is due to the presence of metallic Co, since the stoichiometric composition “CoTe” does not exist.

NMR studies of the helical antiferromagnetic compound EuCo2P2

In EuCo2P2, 4f electron spins of Eu2+ ions order antiferromagnetically below a Néel temperature TN=66.5K. The magnetic structure below TN was reported to be helical with the helix axis along the c-axis from the neutron diffraction study. We report the results of 153Eu, 59Co and 31P nuclear magnetic resonance (NMR) measurements on EuCo2P2 using a single crystal and a powdered sample. In the antiferromagnetic (AFM) state, we succeeded in observing 153Eu, 59Co and 31P NMR spectra in zero magnetic field. The sharp 153Eu zero field NMR (ZF NMR) lines indicate homogeneous Eu ordered moment. The 59Co and 31P ZF NMR spectra showed an asymmetric spectral shape, indicating a distribution of the internal magnetic induction at each nuclear position. The AFM propagation vector k characterizing the helical AFM state can be determined from the internal magnetic induction at Co site. We have determined the model-independent value of the AFM propagation vector k distributed from (0, 0, 0.86)2π/c to (0, 0, 0.73)2π/c, where c is the lattice parameter.

Nuclear magnetic resonance study of the magnetic line phases in SrCoOx (2.5 ≤ × ≤ 3)

A 59Co spin-echo nuclear magnetic resonance (NMR) study, along with complimentary magnetization and x-ray diffraction characterization, is reported for three of the four magnetic line phases which occur in the hole-doped perovskite system SrCoOx, 2.5 ≤ x ≤ 3. The magnetic line phases are the single-valence state end-member compounds SrCoO2.5 and SrCoO3, as well as the mixed-valence state compound SrCoO2.88. In this work, the Co valence was varied by changing the oxygen stoichiometry through electrochemical oxidation/reduction. Antiferromagnetic SrCoO2.5, with trivalent Co3+ (3d6), is characterized by a single strong, relatively narrow, 59Co NMR peak centered at 314 MHz. The 314 MHz peak frequency can be explained using a typical 3d hyperfine coupling constant and the Co moment value ≈3 μB obtained from neutron diffraction measurements. Ferromagnetic SrCoO3 with tetravalent Co4+ (3d5), is characterized by a single strong, relatively narrow, 59Co NMR peak centered at 93 MHz. However, the 93 MHz peak frequency is considerably smaller than expected from the 2.1 μB Co moment values obtained from the saturation magnetization indicating a significant orbital contribution. Ferromagnetic and mixed-valent (Co3+/Co4+) SrCoO2.88 is characterized by three broad 59Co peaks, centered at 47, 134, and 280 MHz, consistent with previous reports. Based on the NMR results from the end-member compounds, the 280 MHz peak is most likely due to Co3+ and not Co4+ as previously reported.

Site-dependent cobalt electronic state in La–Co co-substituted magnetoplumbite-type ferrite: 59Co nuclear magnetic resonance study

The nuclear magnetic resonance of 59Co was measured over a wide frequency range in a powder sample crushed from a well-characterized single crystal of La–Co co-substituted magnetoplumbite-type strontium ferrite (SrFe12O19), a familiar base material for the ferrite permanent magnet. The simultaneous observation of both high- and low-frequency resonances suggests the coexistence of both high- and low-spin states of the substituted Co or the presence of Co orbital moment at a particular site. The possible presence of trivalent Co was also investigated. The results suggest that the Co atoms are distributed across different crystallographic sites with different local environments, and that the electronic state of Co is much more subtle than the conventional understanding.

Nuclear magnetic resonance in magnetic nano-materials as an effective technique to test and/or to certificate local magnetic properties

The aim of the present paper concerns investigations of cobalt-containing nano-materials by means of nuclear magnetic resonance (NMR) technique. A comparison is made among the 59Co NMR spectra of nano-composites synthesised under different conditions and using different precursors. All studied nano-composite samples exhibit the hcp structure-type of α-Co and the existence of canted spins in the near-surface layers in spite of both the different type of stabilising matrixes and the proportion of Co-metal in the systems. The spectra analysis shows that the NMR method can be used for operative control and can lead to selection of the optimal conditions to design composite materials with desirable properties.

Impact of local order and stoichiometry on the ultrafast magnetization dynamics of Heusler compounds

Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics.Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 − xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals.

Mesoporous Co3O4 nanostructured material synthesized by one-step soft-templating: A magnetic study

A combined magnetization and zero-field 59Co spin-echo nuclear magnetic resonance (NMR) study has been carried out on one member of a recently developed class of highly ordered mesoporous nanostructured materials, mesoporous Co3O4 (designated UCT-8, University of Connecticut, mesoporous materials). The material was synthesized using one-step soft-templating by an inverse micelles packing approach. Characterization of UCT-8 by powder x-ray diffraction and electron microscopy reveals that the mesostructure consists of random close-packed Co3O4 nanoparticles ≈ 12 nm in diameter. The N2 sorption isotherm for UCT-8, which is type IV with a type H1 hysteresis loop, yields a 134 m2/g BET surface area and a 7.7 nm BJH desorption pore diameter. The effect of heat treatment on the structure is discussed. The antiferromagnetic Co3O4 nanoparticles have a Néel temperature TN = 27 K, somewhat lower than the bulk. A fit to the Curie-Weiss law over the temperature range 75 K ≤ T ≤ 300 K yields an effective magnetic moment of μeff = 4.36 μB for the Co2+ ions, indicative of some orbital contribution, and a Curie-Weiss temperature Θ = −93.5 K, consistent with antiferromagnetic ordering. The inter-sublattice and intra-sublattice exchange constants for the Co2+ ions are J1/kB = (−)4.75 K and J2/kB = (−)0.87 K, respectively, both corresponding to antiferromagnetic coupling. The presence of uncompensated surface spins is observed below TN with shifts in the hysteresis loops, i.e., an exchange-bias effect. The 59Co NMR spectrum for UCT-8, which is attributed to Co2+ ions at the tetrahedral A sites, is asymmetrically broadened with a peak at ≈55 MHz (T = 4.2 K). Since there is cubic symmetry at the A-sites, the broadening is indicative of a magnetic field distribution due to the uncompensated surface spins. The spectrum is consistent with antiferromagnetically ordered particles that are nanometer in size and single domain.

59Co nuclear magnetic resonance study of the local distribution of atoms in the Heusler compound Co2FeAl0.5Si0.5

In this work, the spin-echo nuclear magnetic resonance (NMR) technique is used to probe the local structure of Co2FeAl0.5Si0.5 bulk samples. The 59Co NMR spectrum of the Heusler compound Co2FeAl0.5Si0.5 consists of four main resonance lines with an underlying sub-structure. The splitting into the main resonance lines is explained by contributions of the B2 type structure. The sub-lines are attributed to a random distribution of Al and Si. By comparing the experimental results with an appropriate multinomial distribution, the fraction of the Al/Si intermixing and the ratio between the contributing structure types is assigned. The main structural contribution of as-cast bulk samples is of B2 type with 38% of L21 contributions. The L21 contribution can be enhanced to 59% by an appropriate annealing process. However, B2 contributions are still present after annealing. Additional foreign phases such as fcc-Co and Co-Al, with relative contributions of less than one percent, are also found in both as-cast and annealed samples. Resonance lines related to slight amounts of the ternary, parental Heusler compounds Co2FeAl and Co2FeSi are also observed.

Computational analysis of tris(1,2-ethanediamine) cobalt(III) complex ion: calculation of the 59Co shielding tensor using LF-DFT

The ligand-field density functional theory (LF-DFT) approach is employed for calculations of the 59Co nuclear magnetic resonance (NMR) shielding tensor of all four diastereoisomers (λλλ, λλδ, λδδ, and δδδ) of tris(1,2-ethanediamine) cobalt(III) complex ion, [Co(en)3]3+. The obtained values split into two groups according to the point group of the diastereoisomers. The influence of the individual ring conformation on the 59Co NMR shielding tensor is small. Comparisons with results obtained with conventional DFT and experimental values are given. The good agreement between calculated and experimental values demonstrates the validity of LF-DFT for calculating the shielding tensor for transition-metal complexes. .

Measurement of electron correlations inLixCoO2(x=0.0–0.35)usingC59onuclear magnetic resonance and nuclear quadrupole resonance techniques

CoO2 is the parent compound for the superconductor NaxCoO2\cdot1.3H2O and was widely believed to be a Mott insulator. We performed 59Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on LixCoO2 (x = 0.35, 0.25, 0.12, and 0.0) to uncover the electronic state and spin correlations in this series of compounds which was recently obtained through electrochemical de-intercalation of Li from pristine LiCoO2. We find that although the antiferromagnetic spin correlations systematically increase with decreasing Li-content (x), the end member, CoO2 is a non-correlated metal that well satisfies the Korringa relation for a Fermi liquid. Thus, CoO2 is not simply located at the limit of x->0 for AxCoO2 (A = Li, Na) compounds. The disappearance of the electron correlations in CoO2 is due to the three dimensionality of the compound which is in contrast to the highly two dimensional structure of AxCoO2.

A 59Co NMR study to observe the effects of ball milling on small ferromagnetic cobalt particles

To demonstrate the potential of nuclear magnetic resonance (NMR) spectroscopy for investigating detailed structural properties in ferromagnetic materials, three different particle sized cobalt (Co) powders have been ball milled for 24 h are accurately characterised by internal-field 59Co NMR. The 59Co NMR spectra show distinct resonance bands corresponding to the different Co sites, face-centred-cubic (fcc), hexagonal-close-packed (hcp) and stacking faults (sfs), in Co metal powders. The hcp+fcc→hcp phase transition encouraged by ball-milling was observed and quantitative values for each Co environment were obtained.