V. A. Atsarkin

Effect of diamagnetic dilution and non-stoichiometry on ESR spectra in manganites

ESR spectra are studied in a series of diamagnetically diluted manganites, single crystals of LaGa1?xMnxO3, in a broad range of the Mn ion concentration, . With increase in Mn concentration, a complex multi-line spectrum, typical for low Mn concentrations, evolves to an exchange-narrowed several-?or single-line spectrum at x>0.05. The temperature behaviour of the ESR line-width in the range of 80?300?K is found to be very different in the materials with high and intermediate concentration of Mn ions: a slight growth of the line-width is observed in the former case, while a steep line narrowing occurs at x = 0.05 and?0.1 with increase in temperature. Similar temperature dependence of the line-width is also observed for an additional ESR line arising in the non-stoichiometric oxygen-deficient LaMnO3?? manganite. We explain the experimental data by taking into account the effect of the thermo-activated Jahn?Teller reorientation process on spin dynamics in Mn clusters, and estimate its activation energy as 50?100?meV.

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

With original modulation technique, the longitudinal electron spin-relaxation timeT1 has been measured in the La1-xCaxMnO3 manganite (x = 0.1) both in the paramagnetic state and around the temperature (Tc) 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 Tc, theT1 =T2 equality was confirmed, whereas a steep slowing down ofT2 was observed asTc was approached (theT1/T2 ratio increased by two orders of magnitude). The temperature dependence ofT1 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.

Dipolar Broadening and Exchange Narrowing of EPR Lines from the Paramagnetic Centers Distributed on a Solid Surface

Experiments with char samples provide support for the theory of EPR line shape in a system of surface paramagnetic centers with dipole-dipole interactions. It is also demonstrated that, if a strong Heisenberg exchange dominates over the dipole-dipole interactions, the EPR line of a two-dimensional system narrows and acquires Lorentzian shape, similar to the three-dimensional case.

NMR and spin relaxation in systems with magnetic nanoparticles

The ¹H NMR spectra and spin dynamics of the host systems have been studied in liquid and solid suspensions of γ-Fe₂O₃ nanoparticles. Significant broadening of ¹H NMR spectra and growing relaxation rates were observed with increased concentration of nanoparticles in the liquid systems, with the relation T₁/T₂ depending on the particular host. Solid systems demonstrate inhomogeneous broadening of the spectra and practically no dependence of T₁ upon the nanoparticle concentration. We explain the experimental results taking into account the predomination of self-diffusion as a source of the relaxation in liquid suspensions, and estimate effective parameters of relaxation in the systems under study.

Electron spin-lattice relaxation in noncommon metals YBa2Cu3Ox and Rb1C60

Using an original modulation technique, the electron spin-lattice relaxation have been investigated in two noncommon metals: YBa2Cu3Ox, high-Tc material doped with 1% Gd, and Rb1C60, linear polymer phase fulleride. In the first case, the Korringa-like temperature dependence of the Gd3+ longitudinal relaxation time T1, is found forx = 6.59 in a wide temperature range 4.2 <T < 200 K, both above and below Tc = 56 K. Atx = 6.95 (Tc = 90 K), the T1 behavior within 50 <T < 200 K is evidently affected by spin gap opening with the gap value of about 240 K. At 200 K, an unexpected acceleration of the relaxation rate takes place, suggesting some change in the relaxation mechanism. The data are discussed in terms of the Barnes-Plefke theory with allowance made for microscopic separation of the normal and superconducting phases. In Rb1C60, the evolution of the ESR line and relaxation rates have been studied accurately in the range of the metal-insulator transition (below 50 K). Interpretation is suggested which takes into account breaking down the relaxation bottleneck due to opening of the energy gap near the Fermi surface. The gap value of about 100 cm−1 is estimated from the analysis of relaxation rates, lineshape and spin susceptibilities.

Effect of magnetic nanoparticles to NMR and nuclear spin relaxation in liquid and solid hosts

The H1 NMR spectra and spin dynamics of the host system in liquid and solid suspensions of γ-Fe2O3 nanoparticles are reported. Significant line broadening of H1 NMR spectra and growing relaxation rates were observed with increased concentration of nanoparticles in liquid systems, with the relation T1∕T2 depending on the particular host. Solid systems demonstrate inhomogeneous broadening of the spectra depending on the nanoparticle concentration and practically no dependence of T1 upon the nanoparticle concentration. In variable temperature experiments, significant increase in the relaxation rates was observed in the liquid systems. The experimental results are explained in terms of the self-diffusion as a predominating source of the spin-lattice relaxation. The effective radius of relaxation in the systems in study is estimated.

Magnetic-field-controlled phase separation in manganites: Electron magnetic resonance study

The electron magnetic resonance spectra of Sm1−xSrxMnO3 (x = 0.30, 0.40, 0.45) manganites have been studied. At temperatures that are higher than the Curie point by several tens of kelvins, samples with x = 0.40 and 0.45 exhibit a ferromagnetic resonance (FMR) spectrum imposed on their usual EPR spectrum. The FMR spectrum appears as the applied magnetic field H exceeds a certain critical field Hc, which decreases upon cooling and becomes zero at T = TC. These results agree with the magnetic-measurement data and indicate the magnetic-field-induced nucleation and growth of ferromagnetic domains in a paramagnetic matrix. In the initial growth stage, the volume of the ferromagnetic domains is proportional to (H − Hc)β, where β = 4.0 ± 0.3, and it changes in phase with magnetic field modulation up to a frequency of 100 kHz. In the same field and temperature ranges, hysteretic phenomena and narrow unstable spectral lines are detected; these lines indicate a dynamic character of the phase separation. The results obtained are interpreted in terms of the competition of different types of magnetic and charge ordering.

Oxygen effect on spin relaxation of the surface paramagnetic centers in carbon chars

The original technique with the use of longitudinal electron paramagnetic resonance detection was applied to measure longitudinal and transversal electron relaxation times (T1 andT2) in specially prepared carbon chars containing paramagnetic centers (PCs) on the surface. The results not only confirmed the conclusions of our previous studies but enabled us to determine the exchange integral and clarify the mechanism of interaction between the PCs and adsorbed molecular oxygen O2. A spin relaxation mechanism was also suggested which takes into account both the exchange and dipolar interactions in the systems combining PC and O2. The value of the exchange integral between PCs as well as its functional dependence on the oxygen content were estimated.

Anomalies of the electronic spin-lattice relaxation of Gd3+ in YBa2Cu4O8 and YBa2Cu3O6+x near 200 K

A sharp kink in the temperature dependence of the electronic spin-lattice relaxation rate near 200 K was found in the high-temperature superconductors YBa2Cu4O8 and YBa2Cu3O6+x. The effect is correlated with opening of a spin gap and microscopic phase separation.

Direct measurement of electron spin-lattice relaxation time of paramagnetic centers and non-linear modulated responses in HTSC

The report consists of two parts. 1. The method is described to measure directly the electron spin-lattice relaxation timeT1 in high-Tc superconductors (HTSC). The technique is based on registering the oscillating longitudinal spin magnetization while saturating the EPR line by an amplitude-modulated microwave field. TheT1 values of the Cu2+ centers in YBa2Cu3O6+x (x=0.24−0.9) and Y2BaCuO5 materials are measured and found to be about 1.4·10−9 s independently ofx and temperature in the range 77–300 K. Besides much longer relaxation is displayed in degradated materials. The data obtained can be considered as an argument in favor of the “green-phase” origin of the “high-temperature” EPR spectra in the YBaCuO HTSCs. 2. Non-linear responses of the microwave (1010 Hz) and rf (105–107 Hz) absorption in HTSC materials to low-frequency magnetic modulation are studied and found to be quite different in the two frequency ranges. It is shown that at microwaves the effect is caused by non-linear interaction with shielding supercurrents whereas at lower frequencies it is due to jumps of fluxons over potential barriers. The models developed take into account the Josephson weak links and the thermo-assisted flux creep.