Electron spin resonance and quantum critical phenomena in VOx multiwall nanotubes

Basing on the high frequency (60 GHz) electron spin resonance study of the VO_x multiwall nanotubes (VO_x ‐NTs) carried out in the temperature range 4.2–200 K we report: (i) the first direct experimental evidence of the presence of the antiferromagnetic dimers in VO_x ‐NTs and (ii) the observation of an anomalous low temperature growth of the magnetic susceptibility for quasi‐free spins, which obey the power law χ (T)～1/T ^α with the exponent α ≈ 0.6 in a wide temperature range 4.2–50 K. We argue that the observed departures from the Curie–Weiss behaviour manifest the onset of the quantum critical regime and formation of the Griffiths phase as a magnetic ground state of these spin species. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron spin resonance and magnetic susceptibility suggest superconductivity in Na doped WO3samples

In WO₃ doped with Na (WO₃:Na) an Electron Spin Resonance (ESR) signal with unresolved fine and/or hyperfine structure is detected and used as a probe for the state of the carriers. Using the saturation method we determined the spin-lattice relaxation rate 1/ T ₁ of these paramagnetic centers. Upon cooling below 100 K, 1/ T ₁ decreases markedly, as known to occur in NMR when a gap opens in the superconducting state. At low temperatures, 1/ T ₁ quantitatively follows BCS behavior with a gap K or 2 . The magnetic susceptibility exhibits a strong difference between magnetic-field cooled and zero-field cooled regimes below which also supports a so far unknown superconducting phase resulting from Na doping. Received 14 April 1999 and Received in final form 6 September 1999     

The Peierls transition of Perylene radical cation salts with 2:1 stoichiometry containing tetrahydrofuran

Crystal structure, microwave conductivity and static magnetic susceptibility were analyzed for the conducting perylene (PE) radical cation salts of nominal composition (PE)₂(PF₆)_1–x (AsF₆) _x ·2/3 THF for 0x1. Crystallographic phase transitions in the high-temperature metal phase were also characterized by differential scanning calorimetry and electron spin resonance. The Peierls transition occuring at 100–120 K (depending onx) and the opening of the energy gap in the semiconductor phase were determined from the magnetic data. The dependence of these properties on the size of the complex anions and the packing of the PE molecules is discussed.     

Antiferroelectric spin wave resonance

Antiferroelectric spin wave resonance (AFESWR) and electric spin moment, mathematically predicted by Dirac, were identified for the first time. It has also been found, in addition to well-known SSH-model, that topological quasiparticles, called σ-quasiparticles, can be formed in CC σ-bonds of organic polymers. Topological σ-polaron lattice is proposed to be responsible for the observed AFESWR both in starting polyvinylidene fluoride (PVDF) films and in carbynoid films, which are the products of PVDF-dehydrohalogenation. Dynamical systems of σ-polarons and π-solitons, identified in the samples studied, are substantionally nonlinear and can be registered in stochastic regime by usual IR absorption measurements.     

EPR study of the low temperature ferroelectric phase transition in Cu doped Rb2ZnCl4 single crystals

Temperature dependent EPR measurements on copper doped Rb₂ZnCl₄ single crystals allowed us to evidence and study the P2₁cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu²⁺ centers localized at different anionic sites, called Cu²⁺(I) and Cu²⁺(II), which are formed in this compound, only the Cu²⁺(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition.     

Electron spin resonance study of bulk and nanosized La0.875Sr0.125MnO3

The structural, transport and electron spin resonance properties of bulk and nanosized La_0.875Sr_0.125MnO₃ prepared by a sol-gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below T_C. In addition to a sharp peak in the vicinity of T_C, another sharp peak close to is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.     

EPR studies of linewidth anomalies at phase transitions in [N(C2H5)4]2MnCl4

We have studied [N(C₂H₅)₄]₂MnCl₄ crystal by X-band CW EPR spectra in the temperature range 170-300 K. The angular dependences of linewidth ΔH were measured and described in the light of a double-layer system (2D) with exchange interactions. Two temperature anomalies of linewidth ΔH were found at T₁=225 K and T₂=192 K on cooling. Different behaviors of ΔH anomalies recorded for an external magnetic field parallel and perpendicular to the ab crystallographic plane indicate ordering/disordering of MnCl₄ groups in this plane and their displacement along the c-axis which occurs in the temperature of about 225 K.     

Electron spin resonance studies on γ-irradiated KMnO4

Electron spin resonance (ESR) studies have been carried out on the radicals produced by the γ-irradiation of single crystals of KMnO₄.Analysis of the data yields at least two paramagnetic species. The first radical is produced by room temperature irradiation and gives a single line for each of two inequivalent magnetic sites. There is no resolvable hyperfine structure and the g tensor for the species is given by, g_b = g_μ′ = 2·0176, g_x′ = 2·0020, g_z′ = 2·0112. The spectrum is tentatively assigned to the O₃^? radical although MnO₂ and MnO₄ are also possibilities.The second radical is produced by irradiation at 77°K and the spectrum disappears irreversibly at slightly higher temperatures. Analysis of the very complicated spectra yields, g_b = 1·9617, g_x = 1·9452, g_u = 1·9940, A_b = 386 MHz, A₂ = 87 MHz, A₃ = 20 MHz. This spectrum is assigned to the MnO₄^?2 radical. The relationship of the radicals observed to the effect of high energy radiation on the decomposition of KMnO₄ in the light of current solid state theory is discussed.     

Magnetocaloric effect in ferromagnetic and ferrimagnetic systems under first and second order phase transition

In this work we present a model to describe the magnetocaloric effect (MCE) in ferrimagnetic arrangements. Our model takes into account the magnetoelastic interactions in the two coupled magnetic sublattices, which can lead to the onset of the first order magnetic phase transition and the giant-MCE. Several profiles of the MCE, such as: the inverse- and giant-MCE were systematically studied. Application of the model to the ferromagnetic compounds GdAl₂, Gd₅(Ge_1.72Si_2.28), Gd₅(Ge₂Si₂), and to the ferrimagnetic compound Y₃Fe₅O₁₂ was performed, showing a good agreement with the experimental data.     

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.     

Electron paramagnetic and muon spin resonance studies in fullerenes

This article reviews the various EPR (CW, pulsed and time-resolved) and μSR studies reported in C₆₀/C₇₀/C₈₂ fullerenes. Different techniques for preparing paramagnetic C₆₀/C₇₀ species giving an EPR signal have been included. This literature survey is complete from the beginning of fullerene research (1985) up to middle 1994.     

Electron spin resonance of chromia-yttria solid solutions

Electron spin resonance spectra of chromia-yttria solid solutions have been studied at room temperature for Cr concentrations between 0.20 and 2.00 mol%. Isolated Cr³⁺ ions in sites with two different symmetries were observed, as well as well as Cr³⁺ ions coupled by the exchange interaction. The relative concentration of isolated to coupled Cr³⁺ ions decreases with increasing chromium concentration. The results are consistent with the assumption that the chromium ions occupy preferentially the C₂ symmetry sites. A theoretical calculation based on this model yields an effective range of the exchange interaction between Cr³⁺ ions of 0.64 nm, of the same order as that of Cr³⁺ ions in MgO.     

Amorphous to crystalline phase transition in pulsed laser deposited silicon carbide

SiC thin films were grown on Si (1 0 0) substrates by excimer laser ablation of a SiC target in vacuum. The effect of deposition temperature (up to 950 °C), post-deposition annealing and laser energy on the nanostructure, bonding and crystalline properties of the films was studied, in order to elucidate their transition from an amorphous to a crystalline phase. Infra-red spectroscopy shows that growth at temperatures greater than 600 °C produces layers with increasingly uniform environment of the Si-C bonds, while the appearance of large crystallites is detected, by X-ray diffraction, at 800 °C. Electron paramagnetic resonance confirms the presence of clustered paramagnetic centers within the sp² carbon domains. Increasing deposition temperature leads to a decrease of the spin density and to a temperature-dependent component of the EPR linewidth induced by spin hopping. For films grown below 650 °C, post-deposition annealing at 1100 °C reduces the spin density as a result of a more uniform Si-C nanostructure, though large scale crystallization is not observed. For greater deposition temperatures, annealing leads to little changes in the bonding properties, but suppresses the temperature dependent component of the EPR linewidth. These findings are explained by a relaxation of the stress in the layers, through the annealing of the bond angle disorder that inhibits spin hopping processes.     

Electron paramagnetic resonance and optical absorption spectra of Cr ions in cadmium phosphate glass

EPR and optical absorption investigations of chromium doped cadmium phosphate glass are carried out. Crystal field, spin-Hamiltonian and bonding parameters are evaluated. From the results and analyses of the EPR and optical studies, the site symmetry of Cr³⁺ ion in the glass is ascribed to a distorted octahedron. The bonding parameters suggest the ionic nature of Cr³⁺ ion with the ligands.     

Electron paramagnetic resonance and magnetic susceptibility of rare-earth hydrazone compounds

The magnetic properties of the rare earth molecular compounds with hydrazone ligands containing Nd³⁺, Gd³⁺, and Yb³⁺ have been investigated by electron paramagnetic resonance (EPR) and magnetization measurements. For the Gd-compound, partially resolved fine structure due to Gd³⁺ and exchange narrowing effects at low temperatures are observed in the EPR spectra, suggesting, consistent with the EPR and dc magnetic susceptibility, weak antiferromagnetic exchange interactions. Paramagnetic behavior sustained down to low temperatures is derived for Yb³⁺ ions, whereas substantial ferromagnetic exchange coupling is inferred for the lighter Nd³⁺ ions, indicating significant variations of the exchange integrals along the lanthanide series. Received 29 April 2002 Published online 31 July 2002     

Unique magnetism observed in single-wall carbon nanohorns

The magnetic properties of single-wall carbon nanohorns (SWNH) were studied by electron spin resonance (ESR) and static magnetic susceptibility measurements. The SWNHs were ESR active with linewidth (ΔH) of ∼6 G in vacuo at room temperature. ΔH was susceptible to the partial pressure of O₂ and became 53 G at 1 atmospheric pressure of O₂, while the integrated ESR intensity was independent on O₂ pressure and behaved as Curie-like, suggesting an intrinsic ESR origin with localized electron spin character. The diamagnetic susceptibility for SWNHs indicated a value smaller than that of randomly oriented graphite by an order of magnitude, but showing a magnitude comparable to those of C₆₀ and C₇₀. It is suggested that the large diamagnetism expecting for sp² networked carbon materials will be canceled by the Van Vleck constant paramagnetism. Received: 20 November 2000 / Accepted: 21 November 2000 / Published online: 25 July 2001     

Electron spin resonance indication of the ferromagnet-spin glass transition in amorphous FeMn alloys

We find that ESR linewidth measurements of amorphous FeMn alloys indicate the ferromagnet-spin glass transition temperature without a scaling analysis. Comparison with neutron scattering data shows the importance of studying the long-wavelength limit.     

Magnetic resonances in one-dimensional spin systems NENP and NINO

We present Electron-Spin-Resonance experiments in the frequency range 35–420 GHz in magnetic fields up to 14 T and with temperatures down to 1.5 K. The quasi-onedimensionalS=1 Spin Systems NENP and NINO exhibit similar resonance-modes, which can be associated with transitions between excited states atq= and excitations from the groundstate. Groundstateexcitation can be explained with regard to a transverse staggered field due to inequivalent Ni²⁺-sites along the chains. Several features are discussed like polarisation-and temperature dependence of line-intensity and the angular and temperature-dependence of resonance-field.