Paramagnetic Resonance Linewidth Research Articles

Paramagnetic spin correlations and spin dynamics in doped manganites as the precursors of their magnetic ordering

To confront the magnetic ordering in bulk and nanometer-sized La1−xCaxMnO3 manganites with the Ca doping level x=0.1 and 0.6, the fitting of the temperature dependences of electron paramagnetic resonance linewidth and intensity to the theoretical models was combined with the magnetic measurements. In the x=0.1 case, the magnetic ordering in nanosamples changes as compared to bulk due to transition from the confined state of charge carriers in chemically disordered bulk to a mobile one in an impuritylike band in homogeneous nanocrystals. While both bulk and nano-x=0.6 samples demonstrate stable antiferromagnetic ordering in spite of strong ferromagnetic correlations in the paramagnetic state. This may be associated with the localization of doped electrons, which (together with the elastic interaction between Jahn-Teller) ions may be a prerequisite for electron-hole doping asymmetry in the phase diagram of La1−xCaxMnO3.

Soliton contribution to the electron paramagnetic resonance linewidth in a two-dimensionalantiferromagnet with a staggered field

In this paper we study the interactions between magnons and a solitonin a classical and isotropic two-dimensional Heisenberg antiferromagnetin the presence of a staggered field applied perpendicularly to theXY plane. The temperature dependence of the linewidth is calculated using the dynamic spincorrelation function derived from soliton–magnon scattering.

NMR study of lineshifts and relaxation rates of the one-dimensional antiferromagnetLiCuVO4

NMR measurements were performed on single-crystalline samples of the one-dimensional Heisenberg antiferromagnet (1D HAF) ${\mathrm{LiCuVO}}_{4}$. We investigated $^{7}\mathrm{Li}$ and $^{51}\mathrm{V}$ NMR spectra, deduced the respective lineshifts $K(T)$, spin-lattice relaxation rates $1∕{T}_{1}$, and spin-spin relaxation rates $1∕{T}_{2}$ and $1∕{T}_{2G}$. The results are compared to theoretical predictions for the relaxation rates $1∕{T}_{1}(T)$ and $1∕{T}_{2G}(T)$ deduced from the dynamic susceptibility $\ensuremath{\chi}(\mathbf{q},\ensuremath{\omega})$ of a 1D HAF spin chain as proposed by Sachdev [Phys. Rev. B 50, 13006 (1994)]. We document a correspondence between the anisotropic relaxation rates derived from electron paramagnetic resonance linewidth $\mathrm{\ensuremath{\Delta}}H(T)$ and nuclear magnetic resonance $1∕{T}_{1}(T)$, respectively, the former being well described in the framework of symmetric anisotropic exchange interactions for ${\mathrm{LiCuVO}}_{4}$ by Krug von Nidda et al. [Phys. Rev. B 65, 134445 (2002)].

Spin Interactions in Cd<sub>1-x</sub>Mn<sub>x</sub>S Bulk Crystals

Electron paramagnetic resonance (EPR) linewidth in the Cd1-xMnxS bulk crystals with manganese concentrations x = 0.25, 0.33, 0.35 and 0.42 has been studied in the high-temperature region. An analysis of the infinite-temperature EPR linewidth performed within the exchange narrowing model shows that the contributions of the anisotropic superexchange and magnetic dipolar interaction to the linewidth are nearly the same due to weak anion spin-orbit coupling. The single-ion anisotropy originating from the crystal field of axial symmetry with the parameter Dsingle= 0.0295 cm-1, as observed in some CdS:Mn crystals, gives a contribution to the linewidth comparable with the influence of the anisotropic superexchange and magnetic dipolar interaction. The calculated infinite-temperature linewidths, including all anisotropic spin interactions examined in this work, are close to the experimental infinite-temperature linewidths estimated from the existing high-temperature data.

Orbital Order and Spin Relaxation in La0.95Sr0.05MnO3

We analyze the temperature and angular dependence of the paramagnetic-resonance linewidth of La 0.95 Sr 0.05 MnO 3. The observed anisotropy can be well described by the crystal field contributions originated by the Jahn–Teller distortion. The critical behavior of the linewidth at the Jahn–Teller transition is found to be directly related with the orbital order.

Phase Transitions and Spin Relaxation in La0.95Sr0.05MnO3

We present a model calculation of the paramagnetic-resonance linewidth in La 0.95 Sr 0.05 MnO 3. Both the temperature and the angular dependencies can be described by contributions from the crystal field due to the cooperative Jahn–Teller transition.

Magnetic clusters in Nd1−xSrxMnO3 (0.3⩽x⩽0.5): An electron-spin resonance study

The paramagnetic electron-spin resonance (ESR) linewidth of polycrystalline Nd1−xSrxMnO3 (x=0.3, 0.33, 0.4, and 0.5) increases in a quasilinear manner up to 400 K due to the formation of paramagnetic clusters, which is also confirmed by the observation of an activated behavior of the ESR intensity above TC. Upon cooling, a pair of ferromagnetic resonance lines appear well above the magnetic transition temperature (TC, TN) in all cases. This suggests the presence of ferromagnetic clusters at a temperature T*>TC. The two-line structure below T* is discussed in terms of phase separation and formation of magnetic clusters and their coexistence with a charge-ordered phase for x>0.4.

Electron paramagnetic resonance linewidths and line shapes for the molecular magnets Fe8 and Mn12

We study theoretically electron paramagnetic resonance (EPR) linewidths for single crystals of the molecular magnets Fe8 and Mn12 as functions of energy eigenstates Ms, frequency, and temperature when a magnetic field along the easy axis is swept at fixed excitation frequency. This work was motivated by recent EPR experiments. To calculate the linewidths, we use density-matrix equations, including dipolar interactions and distributions of the uniaxial anisotropy parameter D and the Landé g factor. Our calculated linewidths agree well with the experimental data. We also examine the line shapes of the EPR spectra due to local rotations of the magnetic anisotropy axes caused by defects in samples. Our preliminary results predict that this effect leads to asymmetry in the EPR spectra.

Crystal field, Dzyaloshinsky-Moriya interaction, and orbital order inLa0.95Sr0.05MnO3probed by ESR

We present a comprehensive analysis of Dzyaloshinsky-Moriya interaction and crystal-field parameters using the angular dependence of the paramagnetic resonance shift and linewidth in single crystals of La_{0.95}Sr_{0.05}MnO_3 within the orthorhombic Jahn-Teller distorted phase. The Dzyaloshinsky-Moriya interaction (~ 1K) results from the tilting of the MnO_6 octahedra against each other. The crystal-field parameters D and E are found to be of comparable magnitude (~ 1K) with D ~= -E. This indicates a strong mixing of the |3z^2-r^2> and |x^2-y^2> states for the real orbital configuration.

Small Impurity-Pinned Solitons in Layered Antiferromagnets

Layered manganese-halide compounds exhibit quasi-two-dimensional magnetic behavior in the temperature region immediately above the ordering temperature where soliton excitation can be experimentally detected as an Arrhenius, exp(E/T), temperature-dependent electron paramagnetic resonance linewidth, where E is the soliton energy. When a nonmagnetic impurity such as magnesium is introduced into the Mn lattice, experimental linewidth data indicate that the excitation energy is dramatically reduced and the temperature range over which Arrhenius behavior is observed is widened to include higher temperatures. These effects occur for very small (less than 1%) impurity concentrations.

Spin relaxation in the quasi-one-dimensional conductor Rb2Pt(CN)4(FHF)0.4

We analyze the electron paramagnetic resonance linewidth in Rb2Pt(CN)4(FHF)0.4 as a function of temperature. Above 60 K, the results are interpreted as a superposition of spin-phonon and spin-spin relaxation effects. The dominant contribution to the relaxation rate l/T2 has a dipolar origin in contrast to the spin-phonon origin observed in K2Pt(CN)4Br0.3 · H2O. This difference could arise from different anisotropy ratios of the transfer integrals. The influence of the interchain coupling is also discussed.

Electron paramagnetic resonance linewidth in a two-dimensional Heisenberg antiferromagnet (S=1/2) with skyrmeons

Electron magnetic relaxation was studied in a two-dimensional Heisenberg antiferromagnet (S=1/2) with skyrmeons. The electron paramagnetic resonance (EPR) linewidth (Г⊥) was calculated at temperatures T≤J (J is the nearest-neighbor interaction constant). The magnitude of Г⊥ caused by the Dzyaloshinskii-Moriya interaction is shown to grow as r0/a with decreasing temperature, whereas the anisotropic symmetric interaction leads to the (r0/a)3 dependence, where r0 is the average size of a skyrmeon and a is the lattice parameter. The results obtained agree qualitatively with calculations performed on the basis of a renormalization-group (RG) analysis of the nonlinear σ model at Open image in new window .

EPR linewidths inLa1−xCaxMnO3:0<~x<~1

We have analyzed recent results for the electron paramagnetic resonance linewidths in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ and related materials over the range $0l~xl~1.$ In all cases studied, the linewidth away from magnetic and structural transitions can be fitted to the expression $[{\ensuremath{\chi}}_{0}(T)/\ensuremath{\chi}(T)]\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty}),$ where the first factor is the ratio of the Curie susceptibility ${\ensuremath{\chi}}_{0}(T)$ to the measured susceptibility $\ensuremath{\chi}(T)$ and the second factor is a temperature-independent constant. Formally exact and approximate expressions for $\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty})$ are derived. In the case of ${\mathrm{LaMnO}}_{3},$ the linewidth in the distorted ${\mathrm{O}}^{\ensuremath{'}}$ phase arises from the interplay of crystal field (single ion) anisotropy and antisymmetric (Dzialozhinsky-Moriya) exchange together with isotropic superexchange. Above the Jahn-Teller transition to the pseudocubic O phase, the single-ion anisotropy is much weaker and the linewidth arises primarily from the antisymmetric interactions. Similar behavior is also found in both phases of the less-distorted, oxygenated material ${\mathrm{LaMnO}}_{3.04}.$ In the case of ${\mathrm{CaMnO}}_{3},$ the distortions of the oxygen octahedra are also small and the antisymmetric interactions are again dominant. In the intermediate region $0lxl1,$ $\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty})$ decreases with increasing Ca (i.e., ${\mathrm{Mn}}^{4+})$ concentration in contrast to the Curie-Weiss temperature, which peaks near $x=1/3,$ reflecting the importance of the double-exchange mechanism. It is proposed that the absence of a signature of the double-exchange interaction in the infinite-temperature linewidth is a consequence of the fact that the time scale associated with the changes in Mn valence arising from double exchange is too long to affect the decay of the correlation function in $\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty})$ so that the linewidth is determined primarily by the superexchange interactions between the Mn ions.

Exchange interactions ofEr3+in theabplane forErxY1−xBa2Cu3O7−δ:An EPR linewidth study

The electron paramagnetic resonance (EPR) linewidth of ${\mathrm{Er}}^{3+}$ in ${\mathrm{Er}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}} (\ensuremath{\delta}\ensuremath{\approx}1)$ is studied as a function of Er concentration. The EPR spectrum complies with the crystal-field ground doublet, while linewidth analysis shows the presence of exchange narrowing. Comparison of the theoretical EPR linewidth with the experimental data provide evidence for the presence of nearest-neighbor exchange interactions of the order of 1 K, most probably anisotropic along the a and b crystallographic axes.

Carbon-centered radicals as oxygen sensors for in vivo electron paramagnetic resonance: screening for an optimal probe among commercially available charcoals.

It is known that some charcoals possess paramagnetic centers with an electron paramagnetic resonance (EPR) linewidth which can be broadened by oxygen. In order to identify potential candidates as sensors for in vivo EPR oximetry, we carried out a systematic study among commercially available charcoals. A total of 34 charcoals were tested. The steps used for the screening were: (1) to check the presence of paramagnetic centers in the material; (2) to measure the EPR linewidth in nitrogen and in air on the dry material and on a aqueous suspension of particles; (3) to calibrate the oxygen sensitive materials (EPR linewidth vs. pO2); (4) to test the sensitivity and stability of the response to changes of pO2 in a simple model of hypoxia induced in mice. Seventeen charcoals contained paramagnetic centers detectable by low-frequency EPR (1.1 GHz). The EPR spectrum consist of one single line which is typical of carbon-centered radicals (g-factor approximately 2). Eight charcoals presented sufficient interesting EPR properties (linewidth in nitrogen < 0.1 mT, linewidth in air for an aqueous suspension of particles > 0.15 mT) to be further characterized in vivo. Only three charcoals presented a stable, reproducible, and sensitive response to pO2 for more than 2 months. These three coals should be considered as good candidates to be used as oxygen sensor using in vivo EPR spectroscopy.

Charge transport of the mesoscopic metallic state in partially crystalline polyanilines

Charge transport properties, including temperature-dependent dc conductivity, thermoelectric power, electron paramagnetic resonance, microwave frequency dielectric constant and conductivity, and electric-field-dependent conductance of partially crystalline (``physically'' cross-linked) HCl-doped polyaniline correlated with x-ray structure studies, demonstrate that charge delocalization in physically cross-linked polyaniline systems is structurally controlled. Further, we observe a positive dielectric constant at room temperature which increases (to values $g~{10}^{4})$ with increasing percent crystallinity, the size of crystalline regions, and polymer chain alignment in the disordered regions, supporting the establishment of mesoscopic metallic regions. We propose an inhomogeneous disorder model for this system in which ordered (crystalline) regions, described by three-dimensional metallic states, are connected through amorphous regions of polymer chains where one-dimensional disorder-induced localization is dominant. We utilize the metallic box, interrupted metallic strands, and Nakhmedov's phonon-induced delocalization models to account for the temperature dependence of charge transport properties of the various partially crystalline polyanilines. Analyses for the sample and temperature-dependent electron paramagnetic resonance linewidth and thermoelectric power are presented.

ELECTRON PARAMAGNETIC RESONANCE DETECTION OF SOLITONS IN TWO-DIMENSIONAL MAGNETS

It has previously been assumed that spin waves were the dominant excitations in lower-dimensional magnets. Recently, however, it has been shown that nonlinear excitations or solitons rather than spin waves influence the dynamic thermal quantities such as the spin correlation function which can be investigated experimentally through the electron paramagnetic resonance linewidth. In this review the influence of both spin waves and solitons on the temperature-dependent linewidth in the fluctuation region immediately above the ordering temperature is discussed. It is seen that both excitations result in a theoretical Arrhenius temperature-dependence, (∆H~ exp (E/T) where E=6πJs2 for spin waves and E=4πJs2 for solitons, J is the nearest neighbor exchange constant, and s is the value of the spin. In experiments, quantum (s=1/2) layered copper compounds exhibit the temperature dependence expected from spin waves even though nonlinear excitations have been shown to exist in these systems. On the other hand nearly classical (s=5/2) manganese compounds have the temperature dependence expected from solitons. The calculation of the linewidth from both spin waves and solitons is reviewed and compared with experimental data to show that solitons dominate the dynamics of the layered, nearly classical magnet.

The linear temperature dependence of the paramagnetic resonance linewidth in the manganate perovskites and

The temperature dependence of the static magnetic susceptibility (5 to 700 K) and that of the electron paramagnetic resonance linewidth (300 to 700 K) measured at 9.25 GHz is reported for bulk polycrystalline samples of and with negative giant magnetoresistance. For both systems, and the product increase linearly with temperature above . Following the analysis of Huber and Seehra for , it is argued that this linear temperature dependence of is most probably due to contributions from spin - phonon interaction. This result supports the recent suggestions of the importance of lattice effects in the magnetism of these oxides.

Ionic Association and Electron Spin Relaxation Rates in Aquo Gadolinium(III) Complexes

The electron paramagnetic resonance linewidth of aquo gadolinium(III) ion changes with the counter-ion identity and concentration in aqueous solutions. The EPR linewidth of 2 mMgadolinium(III) chloride increases from 49.2 to 89.0 mT when carbonate ion is added and decreases to 17.3 mT when nitrite ion is added. These observations suggest association reactions between aquo gadolinium(III) ion and anions that change the electron spin relaxation rates of the aquo ion. The concentration dependence of the gadolinium(III) EPR linewidth is consistent with binding constants for nitrite and nitrate ion with the aquo gadolinium(III) ion of 37 ± 6 and 2.3 ± 0.3 L mol−1respectively. The decreases in the EPR linewidth factors with these association reactions are difficult to understand unless the anion reactions increase the symmetry of the metal center. Although first-coordination reactions may not be ruled out, the decrease in EPR linewidth is more consistent with an outer-sphere association reaction that also reduces the coordination number of the metal center from 9 to 8.

Two-dimensional solitons in the classical Heisenberg antiferromagnet with nonmagnetic impurities

Experimental evidence is presented for the existence of nonlinear localized spin excitations (solitons) in the classical two-dimensional Heisenberg magnet doped with nonmagnetic impurities. These solitons are shown to provide the dominant mechanism for spin relaxation in electron paramagnetic resonance linewidths immediately above the ordering temperature for layered Mn compounds with a small impurity concentration. The new gapless excitations are pinned by the nonmagnetic impurities thereby becoming topologically unstable and lower in energy than the nonlinear excitations (skyrmions) that may exist in the pure material. Concentration-dependent results preclude a spin-wave mechanism for this effect.