Temperature Dependence Of Electron Spin Resonance Spectra Research Articles

Anisotropic microwave response in Cr1∕3TaS2 hosting chiral magnetic soliton

Chromium-intercalated hexagonal Cr1∕3TaS2 possesses exotic properties, such as high-temperature magnetic soliton, crossover of critical behavior, and nontrivial magnetism. In this study, we employ the electron spin resonance (ESR) technique to modulate and study the chiral magnetic soliton lattice (CSL) in Cr1∕3TaS2 using microwave at X-band. The experimental results show that Cr1∕3TaS2 has a particularly strong microwave response when H⊥c but very weak feedback when H//c, revealing an anisotropic microwave response. When H⊥c, the resonance peak at lower fields results from the surface barrier induced by the competition between Dzyaloshinsky–Moriya interaction, Zeeman energy, and exchange energy. The dominant resonance is caused by the ferromagnetic mode with the twisted region as the boundary condition, which transforms into a paramagnetic resonance above the phase transition temperature (TC). Furthermore, an unusual temperature dependence of ESR spectra is revealed. The crystalline electric field (CEF) excitation owing to large strong spin–orbit coupling results in the widening of the ESR linewidth above TC. Furthermore, an abrupt shift in the resonance field Hr2 and ESR linewidth ΔHPP2 is observed around TC, which is attributed to a sudden change in spin-exchange interactions and magnetic anisotropy around TC, as Cr1∕3TaS2 was previously observed for spin interaction transition from strong anisotropic 3D-Ising type to isotropic 3D-Heisenberg magnetic interaction below TC. This work demonstrates that the ESR can detect the CSL state and investigate materials with nontrivial magnetism.

Susceptibility, high-field magnetization and ESR studies in a spin-5/2 triangular-lattice antiferromagnet Ba3MnSb2O9

The polycrystalline Ba3MnSb2O9 system as a spin-5/2 quasi-two dimensional triangular-lattice antiferromagnet (TLAF), is investigated by means of magnetic susceptibility, specific heat, high field magnetization and electron spin resonance (ESR) measurements. The ac susceptibility and specific heat studies reveal that it undergoes an AFM transition at TN=10.2K rather than the spin-glass one. High-field magnetization at 4.2K exhibits a weak 1/3 magnetization step near ~16.5T indicative of the field-induced quantum phase transition behavior. The analysis of temperature dependence of ESR spectra suggests that the short-range antiferromagnetic (AFM) correlation appear at temperatures well above TN. Further high-field ESR measurement up to 25T at 4.2K shows a broad resonance line, and the obtained g value is 2.094±0.012. Combined with the magnetic susceptibility and specific heat, the magnetic state below TN is explained by an AFM long rang order or a well developed AFM short range order with small anisotropy.

Electron-Spin Resonance of a La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Slab

We have studied electron-spin resonance (ESR) spectra of a La0.7Sr0.3MnO3 slab. Experimental results reveal that ESR spectra in the ferromagnetic (FM) region are followed with a broadly resonant mode as θ = 90°, but several modes as θ = 0°. Their spectral parameters depend strongly on θ. When keeping θ = 0° and measuring the temperature dependence of ESR spectra, it is found that several resonant modes in the FM region become a single mode in the Dysonian line-shape at temperatures T >; Tmin ≈ 1.1 TC. At Tmin, the spectral linewidth reaches to a minimum value of ΔHmin= 283 Oe. Temperature dependences of the EPR intensity above Tmin obey a function I(T) = Io exp(Ea/ kB T). Having based on the relation of I(T) ∝ χ(T) = 1/(T-Θ) , we obtained the Curie-Weiss temperature Θ to be 376 K. If using a simple relation of g = hν/ μB Hr (Hr is the resonant position, and μB is the Bohr magneton), we have found that the effective Lande factor g increases from 1.91 (around Tmin) to ~ 1.98 (at 473 K) close to the value of g = 2.0 for spin-spin interactions of Mn ions. This could be due to the additional presence of magnetic interactions, besides spin-spin interactions, such as interactions between FM clusters and/or spin-orbital coupling. Interestingly, despite the sample in the PM regime, the spectral parameters depend on the angle θ. This is assigned to an anisotropic field related to Mn 3+-Mn 4 + FM clusters persisting above Tmin. At 470 K, the anisotropy constant obtained is Ku ≈ 57 erg/cm 3, much smaller than that in the FM region.

Electron spin resonance study of single-walled carbon nanotubes

Studies of electron spin resonance (ESR) are reported on several types of single-walled carbon nanotubes (SWCNTs) synthesized by different methods. We work both with the obtained samples, which contained randomly distributed semiconducting and metallic SWCNTs, and with selected 99% semiconducting or metallic SWCNTs. Our aim is to use ESR spectroscopy in order to assess the quality of carbon nanotubes. We analyze the temperature dependence of ESR spectra in the range 4-300K and report the temperature dependence of the linewidth, the intensity and the asymmetry factor (I+/I-) of the ESR lines. The ESR response from the arc discharge SWCNTs produced using non-magnetic Pt/Rh catalyst and from the selected semiconducting tubes exhibits an asymmetric line with an intensity that remains almost constant with decreasing temperature, showing a constant linewidth of about 2.9 mT. For selected bucky paper (BP) metallic or semiconducting carbon nanotubes, we also study the ESR signal according to the BP parallel or perpendicular orientation to the magnetic field. The g-factor and the asymmetry factor show a clear anisotropy. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Influence of Chain End and Molecular Weight on Molecular Motion of Polystyrene, Revealed by the ESR Selective Spin-Label Method

A local segmental mobility was determined by electron spin resonance (ESR) spin-label method for a series of polystyrene (PS) with various molecular weights. Each PS specimen was selectively spin-labeled with stable nitroxide radicals at a chain end or inside sites. Molecular motion at the inside of the chain was compared with that at the chain end from the temperature dependence of ESR spectra of the nitroxide radicals. The transition temperature of molecular motion, T5.0mT, at which the extreme separation width due to 14N anisotropic hyperfine splitting is 5.0 mT, increased with an increase in molecular weight. The WLF equation confirmed that the T5.0mT correlated with a glass transition temperature, Tg, of PS. The T5.0mT for the spin-labeled PS at the chain end was ca. 5 K lower than that for the spin-labeled PS at the inside sites due to the enrichment of the specific free volume around the chain end. The transition temperature, T5.0mT, for both labeled PS depended on the molecular weight in accordance with the Unberreiter−Kanig equation for a glass transition. The T5.0mT for the spin-labeled PS at the chain end had a strong dependence on the molecular weight as compared with that at the inside sites because the molecular motion of the chain end was accelerated by an encounter of more than two chain ends. From the molecular weight dependence, we determined the short correlation time for segmental motion of the chain end, ca. 40 s, and the segment size undergoing the segmental motion at the Tg. The obtained segment size agreed well with the general segment size reported by others, 5−10 monomeric unit size.

Temperature dependence of ESR spectra of methyl methacrylate radicals from 77 K to 300 K

AbstractThe radicals formed in poly(methyl methacrylate) (PMMA) under vacuum by UV irradiation at room temperature were carefully examined from 77 K to 300 K by electron spin resonance (ESR). The conventional nine‐line spectrum was observed with significant overall intensity changes in contrast to previous reports. The intensity decreases greatly as the temperature increases from 77 K to 100 K. The intensity of the ESR spectrum increases as the temperature increases gradually from 100 K to 260 K. The spectral changes were reversible at all temperatures. Three different models are considered to interpret the temperature dependence of the intensity of the ESR spectrum. The results indicate that the ESR spectrum depends on (1) the steady‐state concentration of the propagating radical in the polymer, (2) the conformational distributions of the radicals, and (3) the environmental structures of the polymer matrix.