The reverse shift of the EPR line of paramagnetic centers coupled to species with a fast paramagnetic relaxation

The EPR spectrum of the spin 1/2 paramagnetic centers with a relatively slow relaxation is considered in the case when they are coupled via the Heisenberg exchange interaction to partners which have short times of the longitudinal and transverse paramagnetic relaxation. Under these conditions only the EPR line of paramagnetic centers with a relatively slow relaxation is detectable in experiment. The shape of this line is analyzed by solving numerically kinetic equations for the spin density matrix for simple model systems. Depending on a ratio between the exchange integral and the paramagnetic relaxation rates of partner spins, the EPR line shifts in opposite directions. For moderate relaxation rates, as the relaxation rates decrease, the EPR line shifts toward the gravity center of the total EPR spectrum. In the case of extremely fast relaxation, as the relaxation rates decrease, the reverse shift of the EPR line is expected, the line shifts away from the gravity center of the total EPR spectrum. This type of the non-monotonous line shift was experimentally observed for the monocrystal of [CuNd₂(C₄O₄)₄(H₂O)₁₆] · 2H₂O when relaxation rates were changed by temperature variation.     

Effect of oxygen on the NMR relaxation properties of crude oils

Nuclear magnetic resonance relaxation measurements of bulk fluids provide a sensitive probe of the dynamics of molecular motion. Dissolved oxygen can interfere with this technique as its paramagnetic nature leads to a reduction of the paramagnetic relaxation times of the fluids. We studied this effect for the relaxation properties of crude oils that are in general characterized by a distribution of relaxation times. The samples were stock tank oils that have been exposed to air. We comparedT ₁ andT ₂ relaxation time distributions and their correlation functions of the initial (oxygenated) samples with those from the deoxygenated samples. Oxygen was removed from the oils with a freeze-thaw technique. As expected, the effect of oxygen is most apparent in oils with long relaxation times. In these oils the effect of oxygen can be described by an additional relaxation rate 1/T _1,2 ^ox to the transverse and longitudinal relaxation rates that is sample dependent but does not vary within the relaxation time distribution of the oil. Values of 1/T _1,2 ^ox for different crude oils were found to be in the range of 2.5 to 8.3 s. For crude oils that have components with relaxation times less than 100 ms, no significant oxygen effect is observed.     

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr_1.6V_0.4Se₄ spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T _C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T _C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V³⁺ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.     

Electron spin resonance and longitudinal relaxation around phase transition in La0.9Ca0.1 MnO3

With original modulation technique, the longitudinal electron spin-relaxation timeT ₁ has been measured in the La_1-xCa_xMnO₃ manganite (x = 0.1) both in the paramagnetic state and around the temperature (T _c) of the ferromagnetic ordering. The data are compared with the evolution of the transverse relaxation time T₂ as determined from the electron spin resonance (ESR) linewidth. Well above T_c, theT ₁ =T ₂ equality was confirmed, whereas a steep slowing down ofT ₂ was observed asT _c was approached (theT ₁/T₂ ratio increased by two orders of magnitude). The temperature dependence ofT ₁ within the whole temperature range was found to be consistent with that ofT · χ(T), where χ(T) is the electron-spin susceptibility obtained from the ESR absorption area. The interpretation suggests that both the longitudinal and transverse electron-spin relaxation rates are governed by strong exchange interaction between the Mn ions, the ESR linewidth being inhomogeneously broadened in the vicinity of the phase transition.     

Magnetic interactions between Copper and RE in RE Ba2Cu3O7-δ

The antiferromagnetic ordering temperatures of the rare earth (RE) moments in RE Ba₂ Cu₃O_7-gd, obtained from specific heat measurements, are roughly in agreement with the de Gennes factors, i.e., the ordering mechanism can be mainly seen as an indirect spin-spin exchange. However, the oxygen dependence of T _N is found to be reversed for the light rare earths compared to the heavy rare earths. As origin for this systematic observation an indirect interaction between the 3d-moments of copper and the 4f-moments of rare earths is discussed as a second order effect. Such an interaction is supported by measurements of the 4f relaxation behavior on the Nd 1: 2: 3 cuprates by inelastic magnetic neutron scattering. Here, the usual thermally driven increase of the magnetic relaxation rate is suppressed up to about 80 K. This correlates with the appearance of a spin gap found by Rossat- Mignod in YBa₂Cu₃O₇ and therefore the 3d-4f coupling can be understood as an interaction of the 4f moments with a spin-fluctuation exchange in the CuO₂ planes. Furthermore, the quasielastic magnetic response has a Gaussian contribution at temperatures below 100 K, i.e., much above the long ranged ordering temperatures T _N. Magnon-like excitations appear already at slightly larger temperatures than T _N. In addition the paramagnetic inelastic spectra show only little dependence of the crystal field scheme on the oxygen concentration.     

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.     

The Challenge of Paramagnetism in Two-Dimensional <Superscript>6,7</Superscript>Li Exchange NMR

^6,7Li fast magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy is used to study LiMn₂O₄ and Li₃V₂(PO₄)₃. The presence of paramagnetic transition metal centers in these materials has a profound effect on the resulting NMR spectra. Lithium ion mobility has been studied by two-dimensional (2-D) exchange spectroscopy (EXSY) in Li₃V₂(PO₄)₃ but an absence of lithium ion exchange was observed for LiMn₂O₄. Several differences between the two materials are explored to explain these results. LiMn₂O₄ experiences a greater donation of electron spin density to the Li nucleus via the Fermi-contact interaction when compared with Li₃V₂(PO₄)₃. This contributes to a greater hyperfine chemical shift and a larger dependence of chemical shift on temperature. The delocalized electrons in LiMn₂O₄ cause temperature-independent T ₁ relaxation rates and shorter relative T ₂ values. The relative rates of ionic conductivity and spin–lattice or spin–spin relaxation in LiMn₂O₄ and Li₃V₂(PO₄)₃ are contrasted to illustrate the constraints on the use of 2-D EXSY to characterize ion dynamics in paramagnetic materials. Authors' address: Gillian R. Goward, Department of Chemistry and Brockhouse Institute for Materials Research, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4M1, Canada     

Probing interaction of paramagnetic electron with conduction electrons in high Tc superconductor LaSrCuO by (μ-O) spin relaxation

The paramagnetic (μ^-O) state formed at the oxygen site in high T_c LaSrCuO was used to probe an interaction between the localized moment of the paramagnetic electron and the conduction electrons via zero field μ^- spin relaxation. The enhanced relaxation rates consistently observed in the superconducting state of various Sr concentrations are explained as an effect of spin‐pairing in the high T_c supercurrent. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nuclear quadrupole spin-lattice relaxation in Bi4Ge3O12 single crystals doped with atoms of d or f elements. Crystal field effects in compounds exhibiting anomalous magnetic properties

The nuclear quadrupole spin-lattice relaxation was studied in the range 4.2–300 K for single crystals of Bi₄Ge₃O₁₂ doped with minor amounts (the tenth fractions of mol%) of paramagnetic atoms of Cr, Nd, and Gd. Unusual spin dynamic features were recently found for these crystals at room temperature: a dramatic (up to 8-fold) increase in the effective nuclear quadrupole spin-spin relaxation time T ₂* occurred upon doping the pure Bi₄Ge₃O₁₂ sample. Unlike T ₂*, the effective spin-lattice relaxation time T ₁* at room temperature differs insignificantly for both doped and pure samples. But at lower temperatures, the samples exhibit considerably different behavior of the spin-lattice relaxation with temperature, which is caused by different contributions to the relaxation process of the dopant paramagnetic atoms. The distinctive maximum in the temperature dependence of the spin-lattice relaxation time for the Nd-doped crystal is shown to result from the crystal electric field effects.     

Zur Theorie der Kernspinrelaxation bei einem Austausch zwischen zwei Bereichen

Using REDFIELD 's theory of relaxation and SILLESCU 's master equation treatment of molecular reorientation, the longitudinal and transverse nuclear spin relaxation functions have been calculated in a two phase system with different magnetic interaction energies. The interaction HAMILTON ian represents the dipolar coupling amongst the nuclei in region (a, 1) and between a nucleus and a paramagnetic ion in region (b, 2). Assuming a strong electron spin relaxation which is statistically independent from the nuclear relaxation, a situation realized in paramagnetic solutions and adsorbate systems, the problem simplifies considerably. If some correlation, expressed by a correlation coefficient c, is lost in each transfer between the regions, a longitudinal relaxation time T^(c)_1m can be defined, as long as the life time τ_b in the region with the lower mobility is not comparable with the correlation time τ₁ in the other phase. Without any restriction, however, one time constant T^(c)_2m should characterize the decay of transverse magnetization as a good approximation. The apparent correlation times, determined from experimental data without any knowledge of the coefficient c, differ only slightly from the effective correlation times (in general less than 10%), in contrast to the case of equal interaction energies in both regions. If the interaction HAMILTON ian does not vary under the exchange (e. g. dipolar interaction between the nuclei in both regions) the results of the wellknown statistical treatment of BECKERT and PFEIFER are obtained. Neglecting any correlation at each transfer (i.e. c = 0) the relaxation rates are weighted averages, which correspond to the fast exchange case in the theory of ZIMMERMAN and BRITTIN with the effective correlation times τ_ca = τ_a?τ₁/τ_a+τ₁ and τ_cb = τ_b?τ₁/τ_b+τ₁.     

Electron paramagnetic relaxation in aqueous solutions of manganese (II) ions

The processes of the electron paramagnetic relaxation, molecular motions and structural changes in aqueous solutions of manganese nitrate have been investigated by direct measurement of spin-lattice (T ₁) and spin-spin (T ₂) relaxation times for a wide range of concentrations, temperatures and viscosities. T ₁ and T ₂ were measured by a non-resonance absorption method.

It was discovered that some structural regions exist at the different concentrations of Mn(II) ions in solution. So, the structure of highly concentrated solutions may be considered as one of the corresponding crystallohydrate. The structural microinhomogeneities were observed also in the intermediate concentration range at definite temperatures. It is shown that the relaxation mechanism proposed by Bloembergen and Morgan is not effective in the concentration range studied by us.

The analysis of relaxation times and E.P.R. spectra has shown the formation of ‘liquid microphases’ at the freezing point of the solution. Such microphases can exist at temperatures a few tenths of a degree below the solvent freezing point, and its composition considerably differs from the initial solution.

The correlation times for intramolecular and intermolecular electron relaxation mechanisms are evaluated and their nature is discussed.     

Two-laser two-color time-resolved EPR study on higher-excited-state triplet-singlet intersystem crossing of porphyrins and phthalocyanines

Photoinduced effects on the electron spin polarization (ESP) in the lowest excited triplet (T₁) states of porphyrins (PORs) and phthalocyanines (PCs) have been observed with a two-color time-resolved (TR) electron paramagnetic resonance (EPR) technique in a glassy matrix at low temperatures. On single-color excitation with the wavelengths of the ground state absorptions of PORs and PCs, polarized EPR spectra due to the corresponding T₁ state were observed. The polarization patterns match well with interpretation as anisotropic intersystem crossing (ISC) induced by the spinorbit coupling between the singlet excited (S₁) and the triplet states. In contrast, two-color excitation led to a change of the phase of the T₁ state polarization pattern to the opposite. The observed ESP in the T₁ state resulting from the excitation to the upper triplet state (T _n ) was interpreted in terms of anisotropic ISC between the T _n and S₁ states. From the analysis of the ESP, changes in the quantum yields of the reverse ISC processes were determined at different temperatures. The results could be best interpreted by the existence of thermal pathways with small activation energy in the relaxation processes.     

In vivo oximetry by a pulsed longitudinally detected ESR spectrometer

By using a narrow single electron spin resonance (ESR) line agent, triarylmethyl, tris(8-carboxy-2,2,6,6-tetrahydroxyethylbenzo[1,2-d:4,5-d′] bis(1,3)dithiole-4-yl)methyl sodium salt (TAM OX063), pulsed longitudinally detected ESR (LODESR) measurements of a phantom or the chest of a living mouse at the operating frequency of ca. 300 MHz were taken and the effective longitudinal relaxation time (T ₁^*) was estimated for oximetry. Under irradiation of a pair of π-pulses with a variable interval between pulses (τ), in-phase LODESR signal intensities were obtained from the phantoms containing TAM dissolved in a physiological saline solution at a concentration of 1 mM and various concentrations of oxygen. TheT ₁^* of the phantom was calculated from the plotted curve of the LODESR signal intensity against τ. It was found that the reciprocal ofT ₁^*, i.e., the longitudinal relaxation rate, increased with the concentration of oxygen. In vivo pulsed LODESR measurements of the chest of living mice that had received a TAM injection via the intraperitoneal route were made. While the LODESR measurements were being made, the mice in one group breathed normal air and those in another group breathed 100% oxygen. It was found that the longitudinal relaxation rate of the mice breathing 100% oxygen was significantly greater than that of mice breathing normal air, indicating that breathing 100% oxygen elevates the thoracic longitudinal relaxation rate.     

Comparative studies of the NMR of thulium in Tm123 and Tm124 compounds: Evidence for structural and electronic phase separation

The nuclear magnetic relaxation of ¹⁶⁹Tm in TmBa₂Cu₃O_6+x (x=0.1–1.0, Δ x=0.1) and TmBa₂Cu₄O₈ is studied at temperatures below 5 K. In all the samples, the Tm spin-lattice relaxation proceeds via intrinsic paramagnetic centers (PCs) like Cu²⁺ or copper-oxygen spin-polarized clusters. The experimental data for TmBa₂Cu₃O_6+x support the idea of the structural (chemical) micro-phase separation in oxygen-deficient 123 compounds. Apparently, the samples with x⩾0.4 contain hole-poor nonsuperconducting regions, enriched with PCs, and hole-rich (PC-poor) superconducting regions. The volume fraction f _n of the PC-rich phase reaches a maximum value of 0.85 at x=0.4 and decreases monotonically with increasing x (f _n=0.5, 0.3, and 0.25 at x=0.5, 0.6, and 0.7, respectively). The Tm spin-lattice relaxation in the underdoped TmBa₂Cu₄O₈ compound indicates that this sample, in contrast to oxygen-deficient TmBa₂Cu₃O_6+x , has a homogeneous composition. However, the Tm spin-spin relaxation measurements reveal two sorts of the Tm nuclear spins in Tm124, having different NMR spectra and different relaxation times T ₂. The latter result is evidence of electronic phase separation in CuO₂ phases. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 365–370 (10 September 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Paramagnetic relaxation of Er in an Er single crystal

The paramagnetic relaxation of Er in an Er single crystal was measured atT≫T _N and compared to the relaxation of Er ions in a polycrystalline Pb host. The results indicate an enhancement of at most 50% of the f-conduction electron exchange interaction in Er, as expected for highly ionic f-orbitals. The ratio of the quadrupole moments of the¹⁵⁴ Er(I ^π=11⁻) and the¹⁵⁵ Er(I ^π=13/2⁺) isomers was measured to be 1.18(3) indicating onset of nuclear deformation at N=87. Deceased May, 1986     

Copper Defects and Conductivity in Bi—Pb—Sr—Ca—Cu—O Glasses

Glass ceramics of the composition (Bi_0.8Pb_0.2)₄Sr₃Ca₃Cu₄O₈ prepared by the melt quenching technique and the crystalline phases produced by the rapid thermal annealing have been studied by electrical resistivity and electron paramagnetic resonance (EPR) measurements in the temperature range from liquid helium up to room temperature. The concentration of the EPR active Cu^2? paramagnetic centers decreases as conductivity increases for the glass ceramics and disappears after crystallization and the growth of superconducting phases, similar to bulk high-T_c superconductors. The KPR spectra of both glass and crystallized ceramics after short-time annealing indicate the coexistence of Cu^2? paramagnetic ions and the exchange coupled clusters.     

Probing oxygen in high Tc superconductor LaSrCuO by negative muons

A novel microscopic method of muon spin rotation for negative muon (μ⁻) attached to oxygen is applied on highT _c materials LaSrCuO. The paramagnetic shift and relaxation rate are measured as a function of Sr concentration and temperature. Analysis shows the large difference in shift and relaxation between two different sites. A large anisotropy was also observed by preliminary measurements along different crystalline axes.     

EPR investigation of nanosized La0.67Ca0.33MnO3−δ manganites

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of nanosized La_0.67Ca_0.33MnO_3?δ manganites. The temperature dependences of the EPR line width and integral intensity have been analyzed in terms of the bottlenecked spin relaxation and small polaron hopping scenarios. The exchange coupling integral between Mn³⁺ and Mn⁴⁺ ions and the polaron activation energy decrease with the reduction of grain size. A discussion is given concerning the factors which could explain the observed changes.     

Proton magnetic relaxation in solutions of E. coli ribosomal RNA containing Mn2+ ions

A study has been made over a range of temperatures and magnetic field strengths of the spin relaxation of water protons in aqueous solutions of E. coli ribosomal RNA containing Mn²⁺ ions. The effects of the paramagnetic ions are enhanced in the presence of the RNA. As the temperature falls T ₁ passes through a minimum value, the magnitude of which is field dependent, and this is attributed to a change in dipolar relaxation mechanism from rotation of the aquocomplex to electron spin relaxation. The relevance of this work is assessed in relation to other work on proton relaxation enhancement in Mn²⁺-containing solutions of biopolymers.     

Spin physics in solid helium: Experimental results and applications

We have observed optical pumping signals from Cs atoms trapped in solid⁴He. While the longitudinal electronic spin relaxation timeT ₁ is found to be in the range of 1–2 s, the transverse relaxation timeT ₂, as inferred from magnetic resonance linewidths has a lower bound of 150 s, and is determined by magnetic field inhomogeneities. We present a quantitative discussion of how paramagnetic species trapped in solid He might be used in a highly sensitive search for permanent atomic electric dipole moments.