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Current advances in molecular magnetism are aimed at the construction of molecular nanomagnets and spin qubits for their utilization as high-density data storage materials and quantum computers. Mononuclear coordination compounds with low spin values of

Current advances in molecular magnetism are aimed at the construction of molecular nanomagnets and spin qubits for their utilization as high-density data storage materials and quantum computers. Mononuclear coordination compounds with low spin values of S=½ are excellent candidates for this endeavour, but their construction via rational design is limited. This particularly applies to the single copper(II) spin center, having been only recently demonstrated to exhibit slow relaxation of magnetisation in the appropriate octahedral environment. We have thus prepared a novel, modular organic scaffold that would allow one to gain in-depth insight into how purposeful structural differences affect the slow magnetic relaxation in monometallic, transition metal complexes. As a proof-of-principle, we demonstrate how one can construct two, structurally very similar complexes with isolated Cu(II) ions in an octahedral ligand environment, the magnetic properties of which differ significantly. The differences in structural symmetry effects and in magnetic relaxation are corroborated with a series of experimental and theoretical techniques, showing how symmetry distortions and crystal packing affect the relaxation behaviour in these isolated Cu(II) systems. Our highly modular organic platform can be efficiently utilized for the construction of various transition-metal ion systems in the future, effectively providing a model system for investigation of magnetic relaxation via targeted structural distortions.

Experimental spectroscopic and magnetic data for Co2+(3d7) ions in various systems are reviewed and critically examined. The focus is on Co2+ ions with the electronic spin S = 3/2, properties of which may be interpreted using the spin Hamiltonian with the effective S = 3/2 or the fictitious ‘spin’ S (S′) = ½. Possible distinct ground states of Co2+(3d7) ions arising from crystal field energy levels are discussed. Distinctions between the concepts of the effective spin S and the fictitious ‘spin’ S′ are outlined to clarify the terminological confusion encountered in literature. Sample cases of the ground state assignments and options for the ‘spin’ S′ = ½ origin are considered for better understanding of the Co2+ ions local environment in various systems, including low symmetry cases. Present study is motivated by potential applications of Co2+(S = 3/2) complexes exhibiting very large or moderate zero-field splitting as molecular nanomagnets.

The virtual lack of information on electronic spectra of divalent lanthanide elements (Ln2+) other than Sm2+, Eu2+, Tm2+, and Yb2+ has prompted us to set for synthesis and characterization of novel Ln2+ systems. First successful attempt concerned SrCl2/Nd2+ single crystals. Here, we report stabilization of divalent dysprosium in a chloride host. Importantly this has been accomplished with Dy ions introduced in a divalent state during synthesis, unlike by γ-irradiation of Dy3+ systems employed previously. This synthesis method yields good quality SrCl2/Dy2+ single crystals. The electronic absorption spectra of Dy2+ doped in SrCl2 have been recorded with high resolution at liquid helium temperature (4.2 K). Identification of the absorption bands occurring in the spectral range of 5000-45000 cm-1 is achieved. On the basis of theoretical calculations using semiempirical Hamiltonian model, assignment of bands and determination of the Hamiltonian parameters for Dy2+(4f95d1) configuration is carried out. The experimental and theoretical studies reveal fine structure and zero-phonon transitions and thus enable high-resolution assignment of spectral lines. It is shown that spin-forbidden transitions gain relatively high intensity due to significant admixing of low-spin character to nominally high-spin states.

Lanthanides have relatively recently been recognized as ideal candidates for the construction of advanced magnetic materials that would allow for their future applications in spintronics and high-density data storage. Despite enormous progress that deals with the control of magnetic anisotropy and slow relaxation of magnetization in Single Molecule Magnets (SMMs), further improvements are still indispensable to go beyond the ultra-low temperature regime. We have thus prepared four lanthanide complexes ([ErL2(OTf)(MeOH)2](OTf)2 (1), [YbL2(OTf)2](OTf) (2), [ErL(NO3)3(H2O)](3) and [YbL(NO3)3(MeOH)]·MeCN (4)) with a tridentate Schiff-base ligand L, to unravel magneto-structural correlations in this new family of field-induced Single Ion Magnets (SIMs). Interestingly, as revealed by the single crystal X-ray diffraction, their structures are synthetically tuned by the choice of the applied counterion. The static and dynamic magnetic properties of 1–4 were investigated revealing that all compounds behave as field-induced Single Ion Magnets (SIMs). Their energy barriers Ueff decrease in the sequence: 4, 2, 3, 1, with an order of magnitude difference between the highest and the lowest value. To correlate the observed magnetic properties with spectroscopic data, low-temperature absorption spectroscopy was performed. This has allowed the determination of the energy levels of the Ln(III) ions and the exact composition of the state vectors for the Ln(III) ground multiplets via crystal-field analysis (CFA) and semiempirical superposition model (SPM) approach. Theoretical and magneto-structural correlation studies indicate that one can modulate the heterotopic coordination spheres around the prolate Er(III) and Yb(III) solely with the counterions. This leads to rarely observed high-coordinate SIM species with the LnNxOy first coordination sphere (where Ln – Er or Yb, x = 3 or 6, y = 2, 3 or 6). Their performance can be related to the intricate interactions between the electron density on the Ln ion and the crystal field created by the surroundings.

Recent wide-band (65–850 GHz) electron magnetic resonance (EMR) study determined the zero field splitting (ZFS) parameters (ZFSPs) for Fe2+ in natural and synthetic forsterite (Mg2SiO4) based on the fictitious spin S′ = 1 approach. So-obtained ZFSPs (D′, E′) are incompatible with those available in literature for 3d6 (Fe2+) and 3d4 (Fe4+, Mn3+, Cr2+) ions with the electronic spin S = 2, which predominantly pertain to the effective spin S ˜ = 2. Background for the effective spin S ˜ versus the fictitious ‘spin’ S′ (J′) approaches is provided with focus on application to Fe2+ ions. To enable comparison of the S′ = 1 ZFSPs with the S ˜ = 2 ones methodology for conversions has been worked out and appropriate conversion relations derived for various combinations of the possible energy level schemes for the spin S ˜ = 2 and S′ = 1. The second-rank S′ = 1 ZFSPs (D′, E′) measured by high-frequency EMR for Fe2+ in Mg2SiO4 are converted to the S ˜ = 2 ZFSPs (D, E) and compared with literature data. Suitability of Fe2+:Mg2SiO4 systems for application as high-pressure probes for high-magnetic field and high-frequency EMR (HMF-EMR) is then considered. The results of this study may be applied to other cases of 3d6 and 3d4 (S = 2) ions in various hosts.

Survey of spectroscopic studies of trivalent rare-earth (RE3+) ions at orthorhombic or monoclinic sites in various crystals, which are important in view of promising technological applications, has revealed a number of crystal field (CF) parameter (CFP) sets belonging to different ranges in the CF parameter space. This indicates that the major properties of orthorhombic and monoclinic CF Hamiltonians have not been fully realized in literature. This includes the existence of alternative, i.e. physically equivalent, CF parameter (CFP) sets and incorrectness of comparison of such alternative yet disparate CFP sets, which due to their intrinsic features cannot be directly compared. Proper methodology for dealing with such CFP sets has been outlined earlier. In this paper we consider implications of these properties for interpretation of (i) spectroscopic data for rare-earth ions obtained from several experimental techniques and (ii) the axis systems in which the fitted CFPs are supposedly expressed. Since these aspects are not realized by many researchers, incorrect conclusions concerning, e.g. the strength of CF and structural implications, have often been drawn. The standardization approach is employed for meaningful comparative analysis of the CFP sets for RE3+ ions at monoclinic sites in selected laser crystals. The following ion-host systems are studied: Nd3+: LuAlO3, Nd3+: BaY2F8, Eu3+: Gd2O3, Eu3+, Pr3+, Tb3+, Ho3+, Tm3+, Nd3+, Sm3+, Dy3+, Er3+: Y2O3, Er3+: Er2O3, Pr3+, Sm3+, Gd3+, Er3+: LaF3, Er3+: YAlO3. A representative sample of CFP sets is analyzed, standardized, and presented in a unified way. The alternative CFP sets provided here may be utilized in the multiple correlated fitting technique to improve reliability of final fitted CFPs. The present approach may facilitate future spectroscopic determination of CFPs as well as increase reliability of fitted and theoretical CFPs for fN (RE3+) ions at lower symmetry sites.

The interface between optical spectroscopy, electron magnetic resonance (EMR), and magnetism of transition ions forms the intricate web of interrelated notions. Major notions are the physical Hamiltonians, which include the crystal field (CF) (or equivalently ligand field (LF)) Hamiltonians, and the effective spin Hamiltonians (SH), which include the zero-field splitting (ZFS) Hamiltonians as well as to a certain extent also the notion of magnetic anisotropy (MA). Survey of recent literature has revealed that this interface, denoted CF (LF) ↔ SH (ZFS), has become dangerously entangled over the years. The same notion is referred to by three names that are not synonymous: CF (LF), SH (ZFS), and MA. In view of the strong need for systematization of nomenclature aimed at bringing order to the multitude of different Hamiltonians and the associated quantities, we have embarked on this systematization. In this article, we do an overview of our efforts aimed at providing a deeper understanding of the major intricacies occurring at the CF (LF) ↔ SH (ZFS) interface with the focus on the EMR-related problems for transition ions.

The virtual lack of information on electronic spectra of divalent lanthanide elements (Ln

This is fourth paper in a series devoted to reanalysis of the non-standard and standard crystal-field (CF) parameter (CFP) sets obtained from various experimental techniques and theoretical models for trivalent rare-earth (RE) ions in high-critical-temperature (high-T c ) superconductors and related systems. In this paper the experimental orthorhombic CFPs, obtained mostly by inelastic neutron scattering techniques and to a lesser extent by magnetic susceptibility and specific heat measurements as well as the respective theoretical CFPs for Er 3+ , Ho 3+ , Nd 3+ , and Pr 3+ ions in REBa 2 Cu 3 O 7− δ compounds, are considered. Several problems identified due to the comparative CFP analysis: (i) confusing notations used for operators and CFPs, (ii) different scaling factors and units used for CFPs, and (iii) lack of clear definitions of the axis systems used to express CFPs, were clarified. For each ion studied the available experimental and theoretical CFP sets are analyzed, standardized, and presented in a unified way. The large number of CFP sets prevents calculation of the five additional alternative sets for each originally reported CFP set. Instead, the alternative CFP sets are determined using the package CST for an average set calculated for selected compatible sets for each ion. These alternative CFP sets may be utilized in the multiple correlated fitting technique to improve reliability of the final fitted CFPs. The present results may facilitate future spectroscopic determination of CFPs as well as increase reliability of fitted and theoretical CFPs for RE 3+ ions in high-T c superconductors REBa 2 Cu 3 O 7− δ and related systems.

Absorption emission and excitation spectra are measured and analyzed to achieve assignments of energy levels for 4f(N) ions in Ba(2)YCl(7):RE(3+) (RE = Pr, Nd, Tb, Dy, Ho, Er, and Tm) crystals. The experimental energy levels were analyzed in terms of the usual free-ion parameters and the crystal field (CF) ones, B(kq), in the Wybourne notation. The orthorhombic C(2v) symmetry is shown to be a good approximation of the actual triclinic C(1) site symmetry at the metal center. The starting values of the CF parameters B(kq) used for fittings for the studied crystals were obtained from superposition model analysis. A good agreement between the calculated and experimental energy levels was obtained with rms deviations in the range from 6.8 cm(-1) (for Ho(3+)) to 14.1 cm(-1) (for Pr(3+)). The fitted sets of B(kq) parameters are, in general, consistent across the 4f(N) series. This study has enabled determination and discussion of the trends in variation of the free-ion parameters and CF ones across the 4f(N) series. The CF parameter set and energy level structure obtained for Nd(3+) ion in Ba(2)YCl(7) crystal are consistent with those for Nd(3+) in structurally related RbY(2)Cl(7) crystal. This systematic analysis of CF parameters is one of only few such studies encompassing nearly whole series of RE(3+) ions.

This is third paper in a series devoted to reanalysis of the non-standard and standard crystal-field (CF) parameter (CFP) sets obtained from various experimental techniques for trivalent rare-earth (RE) ions in high temperature superconductors (HTSC) and related systems. Unlike the previous two papers, which dealt with orthorhombic CFPs, in this paper the monoclinic CFPs for Tm 3+ ions in Tm 2 BaXO 5 (X = Co, Cu, Ni) obtained in Mossbauer spectroscopy studies are reanalyzed. Additional aspects arising from lowering of the site symmetry from orthorhombic to monoclinic are considered. Using the package CST for each original experimentally determined CFP set, five additional alternative CFP sets are determined. These results provide deeper meaning to the fitted CFP sets and may be helpful in future spectroscopic studies of Tm 2 BaXO 5 and related systems.

The intrinsic properties of orthorhombic Hamiltonians, e.g. zero-field splitting (ZFS) ones or crystal-field (CF) ones, embodied in the standardization idea may be utilized to improve reliability and comparability of experimentally determined ZFS (or CF) parameters. These properties have enabled derivation of transformation relations for Hamiltonian parameters expressed in several related axis systems defined earlier by one of us. In this paper the standardization transformations are used to generate alternative physically equivalent yet numerically distinct parameter sets for Mn2+ ions in various hosts exhibiting orthorhombic or lower site symmetry. One parameter set out of six alternative sets satisfies the standardization criteria. Importantly, the standardized sets should be used for direct comparisons. Several non-standard ZFS parameter (ZFSP) sets for Mn2+ ions in crystals, identified in our literature survey of EMR studies, are here standardized. Thus the ratio of λ=E/D or λ′=B22/B20 is limited to the standard range (0, ±1/3) or (0, ±1), respectively. Our considerations provide useful correlations between the distinct and non-compatible yet physically equivalent ZFS parameter sets reported in literature. All calculated alternative sets are tabularized for ten Mn2+ complexes considered. These ZFSP sets may serve for application of the multiple correlated fitting techniques in follow-up EMR studies. Some misinterpretations yielding ambiguous and unreliable results are clarified. The intricate low symmetry aspects are also discussed, whenever applicable. The results of this paper enable more reliable analysis, comparison, and fitting of ZFSP sets from EMR spectra for Mn2+ ions in various crystals.

In this series of papers, implications of two major properties of orthorhombic crystal field (CF) Hamiltonians for interpretation of CF data for rare-earth ions obtained from various experimental techniques are considered. These properties, i.e. existence of alternative (physically equivalent) CF parameter (CFP) sets and incorrectness of comparison of such alternative yet disparate CFP sets, appear not fully utilized in literature. The CFP sets for Tm 3+ in orthorhombic high T c superconductors TmBa 2 Cu 4 O 8 and TmBa 2 Cu 3 O 7− δ are reanalysed applying standardization and closeness of CFP sets. Correlated CFP sets are transformed to the same nominal standard axis system. Standardization transformations are used to generate alternative (standard and non-standard) CFP sets, which may be utilized in the multiple correlated fitting technique to improve reliability of final fitted CFPs. The discrepancies concerning orthorhombic CFPs, identified in Mossbauer spectroscopy studies of Tm 3+ ion in TmBa 2 Cu 4 O 8 , are clarified. Misinterpretations of CF and structural data arising from usage of intrinsically incomparable CFP sets for similar ion–host systems are discussed.

This is second paper in a series devoted to reanalysis of the non-standard and standard crystal-field (CF) parameter (CFP) sets obtained from various experimental techniques for trivalent rare-earth (RE) ions in high temperature superconductors (HTSC) and related systems. Two intrinsic properties of orthorhombic CF Hamiltonians, not fully recognized in the literature, bear significantly on interpretation of CF data for transition ions. These properties are: (i) existence of alternative (physically equivalent) CF parameter (CFP) sets and (ii) incorrectness of comparison of such alternative yet disparate CFP sets. The orthorhombic CFP sets determined for RE 3+ ions in HTSC systems: Eu 3+ in Ln 2 BaZnO 5 (Ln = La, Nd, Y) and Eu 2 BaXO 5 (X = Co, Zn) and Er 3+ in Y 2 BaZnO 5 and Er 2 BaXO 5 (X = Ni, Cu, Zn) are reanalysed applying standardization and closeness of CFP sets. Correlated CFP sets are transformed to the same nominal standard axis system. Standardization transformations are used to generate alternative (standard and non-standard) CFP sets, which may be utilized in the multiple correlated fitting technique to improve reliability of final fitted CFPs. The discrepancies concerning orthorhombic CFPs, identified in spectroscopic studies under consideration, are clarified. Misinterpretations of CFPs and structural data arising from usage of intrinsically incomparable CFP sets for similar ion-host systems are discussed.

Our recent literature survey has revealed several crystal field parameter (CFP) datasets for rare-earth ions at orthorhombic, monoclinic, and triclinic symmetry sites in various hosts. Often CFP datasets are not directly comparable, even for the same ion-host system, whereas the low symmetry effects observed may be either actual or apparent. Careful considerations are required to distinguish the nature of the low symmetry effects and extract useful structural information inherent in the low symmetry CFPs. For this purpose, we propose a comprehensive approach comprising three methods. First method consists in finding the principal values of the second-rank CFPs and the orientation of their principal axis system w.r.t. the original or crystallographic axis system. Second method consists in extending the cubic/axial pseudosymmetry axes method to lower symmetry cases. Third method consists in considering for quantitative comparison of CFP datasets of the closeness factors C p and the norms ratios R p = N A / N B for the respective H CF terms: p = k = 2, 4, and 6, and the global ( p = gl) ones. Usefulness of the various physically equivalent CFP datasets generated by the first and second method as initial sets for the multiple correlated fitting technique is also discussed. This study benefits from cross-fertilization between the spin Hamiltonian theory used in the EMR area and the CF theory, since these methods have been used before in the EMR area. As an application of this approach, for the first time in the CF theory area, we reanalyze the triclinic-like CFPs and the low symmetry effects involved therein as well as the cubic CFPs for Pr 4+ in BaPrO 3 .

The exchange charge model (ECM) of crystal field is utilized to provide the theoretical explanation of the ground state absorption and the excited state absorption observed for the octahedrally coordinated Ni2+ ions in the spinel MgAl2O4. The ECM enables modeling of the crystal field parameters (CFPs) for the impurity ions based on the crystal structure data of the host lattice. To ensure the reliability of the CFPs, the convergence of the CFP values with the increasing number of the coordination spheres taken into account in the ECM calculations is considered. The trigonal CFPs B 2 0 , B 4 0 and B 4 − 3 determined by the ECM, together with the appropriate Racah parameters B and C, serve as input to two crystal field analysis computer packages, which compute the energy level schemes within the whole 3d8 configuration. The cubic approximation utilizing only one CFP Dq is also discussed. Basic features of the ground and excited state absorption spectra observed for MgAl2O4:Ni2+ are satisfactorily explained by our crystal field analysis. In order to model the pressure dependence of the CFPs (and thus of the absorption spectra when relevant experimental data become available), the variation of the CFPs induced by possible distortions of the lattice due to, e.g. overall relaxation of the ions or accommodation of the impurity ions in the lattice, is studied. Analysis of the experimental absorption spectra enables us to evaluate also the Huang–Rhys parameter, the effective phonon energy, and the zero-phonon line position.

Sol–gel method followed by calcination at temperatures of 300–900 °C was used to obtain a series of LiMn2O4 samples with varying amounts of chemical and structural defects while preserving a constant Li:Mn atomic ratio. The physicochemical and structural properties of the samples were characterized by X-ray diffraction (XRD), thermal gravimetry analysis–mass spectrometry (TGA–MS), differential scanning calorimetry (DSC), and Raman spectroscopy techniques. These results were correlated with separately performed high-frequency EMR measurements on the same samples. The oxidation number of Mn ions changes as compared with the stoichiometric spinel composition LiMn2O4. These changes, induced by the removal or incorporation of oxygen, were analyzed and correlated with the appearance or disappearance of the cubic to orthorhombic phase transition at around room temperature. The phase transition occurs only if the concentration of vacancies (cationic or anionic) is below the limit necessary to form sufficient majority of perfect MnO6 octahedra of the high symmetry, Oh7. Disturbance of this high local symmetry, being a condition for Jahn–Teller distortion, seems to also be the decisive factor in suppressing the phase transition that is regarded to be responsible for electrical capacity fading during cell cycling.

This study deals with the compatibility and reliability of the crystal field parameter (CFP) datasets for Nd 3+ (Pr 3+ ) ions at low symmetry sites in NdGaO 3 (PrGaO 3 ), which exemplify a number of apparently disparate and/or incompatible CFP datasets revealed by our recent literature survey. These considerations clarify three important aspects: (1) the definitions of the axis systems adopted in the pertinent CF studies; (2) the relevant coordination factors involved in the point charge model calculations; and (3) the multiplicative factors missing in some CFP's, e.g., those obtained by inelastic neutron scattering technique. Based on standardization theory, the reanalysis of the CFP's for Nd 3+ in NdGaO 3 and Pr 3+ in PrGaO 3 enables to resolve the discrepancies identified in the previous studies. A survey of CFP datasets for Nd 3+ ions in other structurally similar crystals is carried out for a meaningful comparison. Correlations between the physically equivalent CFP datasets for Nd 3+ ions in NdGaO 3 obtained by transformations of the nominal axis system are also considered. This work lays grounds for similar CF studies of rare-earth ions at low symmetry sites.

Our recent application of the Noether’s theorem for the crystal (ligand) field (CF/LF) Hamiltonians invariant under continuous rotational symmetry has revealed existence of several conserved quantities. In this paper we utilize the conserved quantities stipulated by the Noether’s theorem and other associated quantities to analyse the CF parameter (CFP) datasets for the trivalent rare-earth (RE 3+ ) ions at tetragonal type II S 4 symmetry sites in LiYF 4 . This host is selected since sufficient amount of data is available to warrant a comparative CFP analysis across the whole 4f N series. Other ion/host systems with S 4 symmetry are also briefly dealt with. Large number of CFP datasets for RE 3+ :LiYF 4 together with the associated quantities calculated by us are tabularised and their variations are presented in graphs. This systematics facilitates consideration of trends among the CFPs and the derived quantities across the 4f N series. Knowledge of such trends would enable identification of unreliable CFP datasets and CFP predictions for ions for which no or little data are available. The major observations for RE 3+ :LiYF 4 are: (i) the spread of the original CFPs is very large for most RE 3+ ions, which indicates several inconsistent CFP datasets; and (ii) the values averaged over the available RE 3+ data points show no well-defined trends across the 4f N series for the CFPs with q = 0, whereas weak diminishing trends with increasing N for the conserved quantities | v 44 | and | v 64 |. Various aspects bearing on the erratic CFP behaviour are discussed. Our considerations show the advantages of the Noether’s theorem stipulated conserved quantities over other derived quantities used in the literature and have implications for interpretation of the published CFP datasets and their reliability.

A close look at the definitions of “coercivity” reveals two distinct notions: one referring to the hysteresis loop for B vs H and the other to that of M vs H. These are confused in several books. The properties of the M vs H loop are often ascribed to the B vs H loop, giving rise to various misconceptions. In this note we highlight a number of misconceptions and misinterpretations in the textbook presentations of the hysteresis loop for ferromagnets. This brief report is aimed at helping faculty become aware of the existence of problems in the current literature.

Recent literature survey has revealed several crystal field parameter (CFP) datasets for rare-earth (RE) ions at orthorhombic, monoclinic, and triclinic symmetry sites, which cannot be directly compared. Triclinic CFPs include for a given rank ( k = 2, 4, 6) all components (− k ≤ q ≤ + k ). Depending on the axis system chosen, the low symmetry effects involved in the symbolic, fitted, or model CFPs may be only apparent and not actual . In order to extract useful structural information involved in the low symmetry CFPs, an approach comprising three methods is proposed: (1) the principal values of the 2nd-rank CFPs and the orientation of their principal axis system (AS) w.r.t. the original or crystallographic AS; (2) the pseudosymmetry axes method; (3) the closeness factors. This approach is applied to reanalyze the CFPs for Pr 4+ in BaPrO 3 .

This paper deals with the analysis of the temperature dependence of high-frequency EMR (HF-EMR) spectra due to Mn3+ and Mn4+ ions in the lithium manganese spinel LiMn2O4. A range of powder samples obtained by the sol-gel method with calcinations in several temperature ranges were prepared for this study. Based on the initial characterization carried out by a number of techniques, the physicochemical and structural properties of the samples were earlier determined. Independently, temperature magnetization and HF-EMR measurements were carried out. The EMR spectra vary strongly between samples, indicating possible structural or chemical changes. Quantitative analysis of the temperature dependence of the HF-EMR spectra due to Mn3+ and Mn4+ ions in LiMn2O4 is presented in this paper. The spectral analysis concerns the line shape, linewidth, intensity and g-factors. Fittings using the Lorentzian spectral shape and, to a certain extent, the Gaussian spectral shape have been carried out in order to parameterize the temperature dependence of the HF-EMR spectra. This parameterization of the HF-EMR experimental data enables a deeper characterization of the samples. Subsequently, a better insight into the role of the Mn3+ and Mn4+ ions in accounting for the characteristics most suitable for application of LiMn2O4 as a cathode material may be gained.

Submillimetre and millimetre wave electron spin resonance (ESR) measurements of LiMn 2 O 4 powder samples have been performed using a pulsed magnetic field up to 16 T in the temperature region from 4.2 to 265 K. Although competition between spin and charge orderings of Mn ions on the octahedral B sites of lithium manganese spinel is expected, geometrical frustration is also expected in LiMn 2 O 4 because the octahedral B sites form the pyrochlore lattice. LiMn 2 O 4 calcined at 600 °C was employed in this study because it turned out to be stoichiometric and have fewer impurities. The detailed temperature dependence of the submillimetre and millimetre wave ESR of this sample will be given and its spin dynamics will be discussed.

High field ESR measurements of LiMn2O4 powder samples, which are obtained by the sol-gel method with calcinations at 700 and 850 °C, have been performed under the pulsed magnetic field up to 16 T and in the temperature region from 4.2 to 300 K. Analysis of the obtained data in connection with the magnetic susceptibility and our previous ESR data shows that the high field ESR is a powerful means for the characterization of LiMn2O4 powder samples. Our result shows that the impurity phase Li2MnO3 already exits in the sample obtained by calcinations at 850 °C and the sample obtained by calcinations at 600 °C is close to the stoichiometry. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the magnetization and high-frequency EMR studies of lithium manganese spinel LiMn2O4 powder samples obtained by the sol–gel method with calcinations at several temperature ranges. The results indicate that the low temperature magnetization and EMR spectra vary strongly between samples. Analysis of the experimental data enables a deeper characterization of the samples. It appears that a small amount of the impurity phase Li2MnO3 exists in the samples obtained by calcinations at 900°C.

The magnetocrystalline anisotropy theory developed earlier by us and based on a novel energy level model for Fe2+ in YIG:Si(Ge) is used here to study the mechanism of spin reorientation in YIG:Si. Single-ion anisotropy with a uniform distribution of Fe2+ ions indicate a broad spin reorientation which turns out to be due to the higher-order cubic terms K3 and K4. It is found that the completion temperature (T2) of the spin reorientation is strongly influenced by K4 thus indicating an inadequacy of the approach based on K1, K2 and K3 only. From the single Fe2+ ion anisotropy results we propose a two-center model which implies a new mechanism of the spin reorientation in YIG: Si. The bulk onset (T1′) and completion (T2′) temperatures are linked to the relative number (a) of the near Fe2+ sites. Spin orientation diagrams in the plane a - T are established for the total Fe2+-Fe3+ system for a wide range of the parameters involved. The two-center model accounts well for the bulk experimental temperatures T1′ and T2′ in YIG: Si and for the absence of spin reorientation in samples having Si-content less than 0.18. The total magnetization M is predicted not to rotate smoothly between the [100] and [111] direction. There exists an upper critical angle beyond which M switches sharply to [111] as the temperature approaches T2′. Comparison with the previous model which requires a nonuniform distribution of Fe2+ ions is also discussed.