Magnetic resonance in systems with equivalent spin-1/2 nuclides. Part 1

Electron paramagnetic resonance (EPR) spectra of S=1/2 systems XL(n) with n equivalent nuclei having spin I=1/2 have been simulated for microwave frequencies in the L-, X-, and W-bands. It has been shown that for n>2 nuclei, the EPR spectra have a more complicated form than anticipated from the usual oversimplified analysis, which predicts n+1 lines with intensity ratios given by the coefficients of the binomial expansion. For the XL(n) system with n=3, the EPR spectra in fact consist of six lines. The exact solution of the spin-hamiltonian for this case has been obtained, which gives four levels in zero magnetic field. For n>2 systems, the degeneracy of the energy levels cannot be completely removed by the Zeeman electronic and nuclear interactions. For n>4, certain spin states cannot occur, consistent with the (generalized) Pauli exclusion principle. Discussion of the underlying theory, invoking exchange degeneracy and the appropriate permutation group theory, is included in some detail. Analogous considerations hold for NMR spectroscopy of non-radicals.     

Spectroscopic investigation of neutral and reduced forms of Fe-monoazaporphyrins

The results of an investigation into neutral Fe-monoazaetioporphyrin (FeMAEP) and its reduced forms carried out using the methods of absorption spectroscopy, EPR, and Mössbauer spectroscopy are presented. It is shown that at room and liquid nitrogen temperatures in an initial complex a spin state of Fe(III) is implemented, which is the result of a quantum-mechanical mixing of two spin states (S=5/2, S=3/2). The absorption spectra of the reduced forms [(FeMAEP)^n?, n=1, 2, 3] are similar to those of anions of other Fe-porphyrins; in the EPR spectra of Fe(I)MAEP lines of a hyperfine structure are observed that are sensitive to a solvent.     

Landau-level splitting in graphene in high magnetic fields

The quantum Hall (QH) effect in two-dimensional electrons and holes in high quality graphene samples is studied in strong magnetic fields up to 45 T. QH plateaus at filling factors nu = 0, +/-1, +/-4 are discovered at magnetic fields B > 20 T, indicating the lifting of the fourfold degeneracy of the previously observed QH states at nu = +/-4(absolute value(n) + 1/2), where n is the Landau-level index. In particular, the presence of the nu = 0, +/-1 QH plateaus indicates that the Landau level at the charge neutral Dirac point splits into four sublevels, lifting sublattice and spin degeneracy. The QH effect at nu = +/-4 is investigated in a tilted magnetic field and can be attributed to lifting of the spin degeneracy of the n = 1 Landau level.     

[Cu2(O COO C-O-N H-O-CH3)4(DMF2)]·4DMF的EPR谱、磁性与电子结构研究

在室温、77K条件下,对·4DMF(1)簇合物的固态和溶液样品进行了EPR谱的测定,获得三套谱图(Ⅰ、Ⅱ、Ⅲ),其分别归属于簇合物中未配对电子的两种形式:(1)类似于自由的单铜离子的未配对电了(Ⅰ、Ⅱ两套谱).(2)双铜未配对电子偶合成的三重态(Ⅲ套谱). 文中用双铜的三重态自旋哈密顿H_S=βHgS+DS_z²+E(S_x²-S_y²)-(2)/3D公式计算三重态EPR谱的参数. 题目化合物(1)与双铜簇合物·4DMF(2)相比较,在配体结构上稍有不同(前者,甲苯胺中的甲基是连接于苯环的间位;而后者,甲基是连接于苯环的对位),由此引起一些磁性参数:有效磁矩μ_eff、磁交换相互作用参数J、相对的电子自旋浓度ρ和EPR谱的超精细结构(h.f.s)参数都有所不同.     

Synthesis and spectroscopic properties of LiIn(WO4)2 nanopowders and single crystals doped with chromium(III) ions

LiIn(WO₄)₂ single crystals and nanopowders doped with chromium(III) ions were synthesized and investigated, including their EPR spectra and magnetic properties. The EPR spectra have shown low and high field lines attributed to isolated chromium ions with an electron spin S=3/2 and complex chromium centers with higher spin value, respectively. Magnetic susceptibility measurements revealed an almost linear dependence on the magnetic field. The electron resonance and magnetic properties have been related to the structural and spectroscopic data of the studied material.     

EPR of Yb3+ ions in Ba1−xLaxF2+x mixed crystals

Electron paramagnetic resonance (EPR) spectra of impurity Yb³⁺ ions (about 0.1 at.%) in mixed crystals BaF₂(1-x) plus LaF₃(x) have been investigated for different values of the concentrationx at a frequency of about 9.5 GHz by both continuous-wave (CW) EPR and electron spin echo methods. A spectrum of trigonal symmetry with a complex hyperfine structure is observed in “pure” BaF₂:Yb³⁺ (x=0). Upon admixture of small amounts of LaF₃ (x=0.001), additional EPR lines arise with intensities increasing with the increase ofx up to 0.005. These lines are attributed to trigonal centers including two rare-earth ions and two compensating fluorine ions. A further increase ofx results in a decrease of the total EPR spectrum intensity, and atx≥0.05 the CW resonance becomes practically unobservable. This may be due to the formation of rare-earth ion clusters with paramagnetic Yb³⁺ ions occurring in domains with a disordered structure of surroundings resulting in very broad EPR lines, which cannot be registered by CW EPR. Indeed, very broad (not less than 1 KG) EPR lines were observed by the electron spin echo method for concentrationsx<-0.02.     

13C spin-lattice relaxation in natural diamond: Zeeman relaxation in fields of 500 to 5000 G at 300 K due to fixed paramagnetic nitrogen defects

¹³C Spin–lattice relaxation (SLR) times in the laboratory frame have been measured at room temperature as a function of field in the range of 500 to 5000 G on two natural type Ib and Ia diamonds after dynamic nuclear polarization. Each of the diamonds contains two types of fixed paramagnetic centers with overlapping inhomogeneous electron paramagnetic resonance (EPR) lines. EPR techniques have been employed to identify these defects and to determine their concentrations and relaxation times at X-band. Three different nuclear SLR paths, namely that due to electron SLR and two types of three spin processes, are discussed. The one three-spin process (TSP) (type 1) involves a simultaneous transition of two electron spins belonging to the same hyperfine EPR line and a ¹³C spin while the other process (type 2) involves two electron spins belonging to different hyperfine EPR lines and a ¹³C spin. It is shown that the thermal contact between the ¹³C nuclear Zeeman and electron dipole–dipole interaction reservoirs decreases with an increase in field intensity, thus forming a bottleneck in the ¹³C relaxation path due to the type 1 TSP. The contribution of TSP of type 1 dominates that due to electron SLR and the type 2 TSP in relaxing the ¹³C nuclei in type Ib diamond from about 1200 to 5000 G, while for type Ia diamond it dominates from 500 up to about 2200 G. In type Ia diamond over the range 2200 to 5000 G it seems that the type 2 TSP, which involves electrons of neighboring P2 hyperfine lines, dominates that of electron spin–lattice and the type 1 TSP. Over the range 500 to about 1200 G, a field-dependent electron SLR mechanism associated with N3 centers appears to dominate the ¹³C SLR.     

Geometrical frustration, spin ice and negative thermal expansion – the physics of underconstraint

The idea that some systems could have a thermodynamically large number of accessible ground states was presaged in the work of Pauling on ice (Pauling, Cornell University Press, Ithaca, NY, 1945) [1]. With the advent of spin glasses, the methodology for describing ground states changed dramatically, and in particular it was realized that the observed slow dynamics were due to relaxation among a large number of nearly degenerate ground states. Now the accepted wisdom is that both “frustration”, as well as structural disorder, is responsible for spin glass behavior. However, well before spin-glasses were identified as a distinct class of systems, it had been appreciated that even for structurally periodic systems, bond frustration could lead to a thermodynamically large number of states. There is now a well-defined class of magnets which display effects of macroscopic ground state degeneracy. This class of geometrically frustrated magnets presents some new paradigms with which to view condensed matter systems – marginal underconstraint and downward shift of spectral weight. We discuss possible realizations of these phenomena in both in spin ice and also outside the context of local-moment magnetism.     

Spin dynamics investigated by EPR in the paramagnetic regime of La2/3Ca1/3Mn1−x Me x O3 (Me = Al, In) manganites

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of the colossal magnetoresistive manganites La_2/3Ca_1/3Mn_1?x Me _x O₃ (Me=Al, In;x≤0.05). The temperature dependences of the EPR linewidth and integral intensity have been analyzed in terms of the bottleneck spin relaxation and small-polaron hopping models. The exchange coupling integral between Mn³⁺ and Mn⁴⁺ ions and the polaron activation energy decrease with increasing doping level. A discussion is given concerning the factors which could explain the observed changes.     

Electron spin relaxation in pseudo-Jahn-Teller low-symmetry Cu(II) complexes in diaqua(L-aspartate)Zn(II).H(2)O crystals.

Low-temperature (4-55 K) pulsed EPR measurements were performed with the magnetic field directed along the z-axis of the g-factor of the low-symmetry octahedral complex [(63)Cu(L-aspartate)(2)(H2O)2] undergoing dynamic Jahn-Teller effect in diaqua(L-aspartate)Zn(II) hydrate single crystals. Spin-lattice relaxation time T(1) and phase memory time T(M) were determined by the electron spin echo (ESE) method. The relaxation rate 1/T(1) increases strongly over 5 decades in the temperature range 4-55 K. Various processes and mechanisms of T(1)-relaxation are discussed, and it is shown that the relaxation is governed mainly by Raman relaxation processes with the Debye temperature Theta(D)=204 K, with a detectable contribution from disorder in the doped Cu(2+) ions system below 12 K. An analytical approximation of the transport integral I(8) is given in temperature range T=0.025-10Theta(D) and applied for computer fitting procedures. Since the Jahn-Teller distorted configurations differ strongly in energy (delta(12)=240 cm(-1)), there is no influence of the classical vibronic dynamics mechanism on T(1). Dephasing of the ESE (phase relaxation) is governed by instantaneous diffusion and spectral diffusion below 20 K with resulting rigid lattice value 1/T(0)(M)=1.88 MHz. Above this temperature the relaxation rate 1/T(M) increases upon heating due to two mechanisms. The first is the phonon-controlled excitation to the first excited vibronic level of energy Delta=243 cm(-1), with subsequent tunneling to the neighbor potential well. This vibronic-type dynamics also produces a temperature-dependent broadening of lines in the ESEEM spectra. The second mechanism is produced by the spin-lattice relaxation. The increase in T(M) is described in terms of the spin packets forming inhomogeneously broadened EPR lines.     

Thermally excited multiplet states in macerals separated from bituminous coal

Electron paramagnetic resonance searches of thermally excited multiplet states in macerals, exinite, vitrinite, and inertinite of Polish medium-rank coal (85.6 wt% C), were performed. Numerical analysis of lineshape indicates a multicomponent structure of the EPR spectra of macerals heated at 300 degrees and 650 degrees C. EPR spectra of exinite and vitrinite are a superposition of broad Gauss, broad Lorentz (Lorentz 1), and narrow Lorentz (Lorentz 3) lines. Two narrow Lorentz (Lorentz 2 and Lorentz 3) lines were observed in the resonance absorption curves of inertinite. The influence of the measuring temperature (100-300 K) on the EPR lines of the macerals was also studied. The experimentally obtained temperature dependence of the EPR line intensities were fitted by the theoretical functions characteristic for paramagnetic centers with ground doublet state (S = 12) and paramagnetic centers with thermally excited triplet (S = 1) and quadruplet (S = 32) states. Thermally excited multiplet states were found in exinite and vitrinite. Both paramagnetic centers with doublet ground state (S = 12) and paramagnetic centers with thermally excited states, probably quadruplet states (S = 32), exist in the group of paramagnetic centers of exinite and vitrinite with the broad Lorentz 1 lines. Intensities (I) of the broad Gauss and the narrow Lorentz 3 lines of exinite and vitrinite changes with temperature according to the Curie law (I = C/T). The existence of thermally excited multiplet states was not stated for inertinite. The two groups of paramagnetic centers of inertinite with Lorentz 2 and Lorentz 3 lines obey the Curie law. Copyright 2000 Academic Press.     

EPR of radiation defects in LiBaF 3 crystals

EPR spectra of LiBaF 3 crystals have been investigated after X-irradiation at RT. A spectrum consisting of approximately 35 nearly equidistant EPR lines has a strong angular dependence on the line intensities. The spectrum is caused by a hyperfine interaction (hfs) of a spin S =1/2 with neighbouring groups of nuclei. The observed large number of hfs lines required Li nuclei being in the first shell and fluorine nuclei in the more distant second shell. We analysed the spectrum in the F -centre model, taking reduced hfs values of the F -centre in LiF and found qualitative explanation of the number of hfs lines. The angular dependence of the line intensities could be explained by an anisotropy of the g -tensor with its main axis along the [1 v 0 v 0] axis of the crystal.     

Ti：Al2O3晶体中Ti^3＋离子的电子顺磁共振研究

通过对Ｔｉ：Ａｌ２Ｏ３晶体的不同取向的电子顺磁共振（ＥＰＲ）研究，认为在９３Ｋ温度下观测到的ｇ＝２．００的强烈各向异性的共振线是来自于Ｔｉ：Ａｌ２Ｏ３晶体中的Ｔｉ３＋离子２Ｔ２ｇ能态的中间能级１Ｅ１／２的顺磁共振吸收。而室温下观察到的ｇ≈２．００的吸收线是由Ｔｉ３＋离子２Ｔ２ｇ能态上能级２Ａ１的共振吸收产生的。由晶体场理论进行的计算与上述结果基本符合。     

Analysis of EPR-time profiles of transient radicals with unresolved spectra

A method for analysing EPR-time profiles of transient radicals in solution with unresolved hyperfine structure is proposed. It is based on considering the magnetic field integral of the magnetization, i.e., the total EPR signal intensity, instead of single components of overlapping EPR transitions. For a radical system involving chemical kinetics, chemically induced electron polarization (CIDEP), and spin relaxation, an analytical solution is found for the evolution of the integral magnetization in the Laplace domain. The solution in the time domain is given for the case of negligible saturation, i.e., omega2(1)T1T2 < 1. The formulae presented are suitable to avoid equivocal multi-parameter fits when analysing the results of time-resolved continuous-wave EPR experiments for the observables, which characterize the chemical kinetics, CIDEP, and electron spin relaxation of radical systems.     

Magnetic properties of the tetranitrosyl-iron complex Fe<Subscript>2</Subscript>(SC<Subscript>3</Subscript>H<Subscript>5</Subscript>N<Subscript>2</Subscript>)<Subscript>2</Subscript>(NO)<Subscript>4</Subscript>

The magnetic properties of the binuclear nitrosyl-iron complexes Fe₂(SC₃H₅N₂)₂(NO)₄ are investigated. It is demonstrated that several types of particles, such as dimers with a pair of spins 1/2, dimers with a pair of spins 5/2, and paramagnetic particles with spin 3/2, make a contribution to the magnetic properties of the complexes. A decrease in the temperature below 25 K leads to a change in the shape of the EPR spectra corresponding to these dimers, so that Lorentzian lines (homogeneous broadening) transform into Gaussian lines (inhomogeneous broadening). This is accompanied by a stepwise change in the EPR line width and g factors. The change in the line shape indicates that complexes become asymmetric at low temperatures, possibly, due to the decrease in the spin exchange frequency below the frequency of the microwave field of the spectrometer.     

Interpretation of Mn2+ EPR spectra in disordered Materials

It is interesting to be able to estimate the values of the zero-field splitting parametersD andE appearing in the spin Hamiltonian of the Mn²⁺ ion in disordered materials, such as glass and polycrystalline samples. Since the electronic spin of the Mn²⁺ ion is 5/2 (>1/2), it is able to interact with the crystalline electric field of the environment to provide information on its surroundings. The present work attemps to interpret the intensities and positions of both the allowed and hyperfine forbidden EPR lines of the Mn²⁺ ion in disordered materials. First, a discussion is presented of the analytical expressions for the intensities and positions of the EPR lines in a monocrystal, which are then exploited by averaging to describe the expected behavior in disordered materials in order to estimate the values of the parametersD andE. The discussion is thereafter generalized to use computer simulations to generate Mn²⁺ EPR spectra in glassy and polycrystalline media. The effects of distribution ofD andE parameters and expected singularities in Mn²⁺ EPR spectra on the resulting EPR spectra in disordered materials are considered. Some published results are included to illustrate the application of the techniques described in this paper.     

Long-range spin-pairing order and spin defects in quantum spin- $${1 \over 2}$$ ladders

For w-legged antiferromagnetic spin-1/2 Heisenberg ladders, a long-range spin pairing order can be identified which enables the separation of the space spanned by finite-range (covalent) valence-bond configurations into w +1 subspaces. Since every subspace has an equivalent counter subspace connected by translational symmetry, twofold degeneracy, breaking translational symmetry is found except for the subspace where the ground state of w = even belongs to. In terms of energy ordering, (non)degeneracy and the discontinuities introduced in the long-range spin pairing order by topological spin defects, the differences between even and odd ladders are explained in a general and systematic way. Received 19 July 1999 and Received in final form 8 October 1999     

An effective quantum parameter for strongly correlated metallic ferromagnets

The correlated motion of electrons in metallic ferromagnets is investigated in terms of a realistic interacting-electron model with N-fold orbital degeneracy and intra-orbital (U) and inter-orbital (J) Coulomb interactions. Correlation-induced self-energy and vertex corrections are incorporated systematically to provide a non-perturbative Goldstone-mode-preserving scheme. An effective quantum parameter [U2+(N-1)J2]/[U+(N-1)J]2 is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large N, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. Correlation effects are investigated for spin stiffness, magnon dispersion, electronic spectral function, density of states, and finite-temperature spin dynamics using realistic bandwidth, interaction, and lattice parameters for iron.