X-Band ESR and51V NMR study of the Haldane system PbNi2−xMgxV2O8

The temperature and angular dependence of the X-band electron spin resonance (ESR) and⁵¹V nuclear magnetic resonance (NMR) spectra have been measured in a recently discovered Haldenegap system, PbNi_2-xMg_xV₂O₈ (0≤x≤0.24). The angular dependence of the ESR signal suggests that both the spin diffusion as well as the magnetic anisotropy determine the electronic spin correlation functions. However, in doped samples the magnetic anisotropy increasingly dominates the spin dynamics on cooling. The huge broadening of the⁵¹V NMR spectra in doped samples at low temperatures provides evidence for localized magnetic moments in the vicinity of the Mg impurities. Locally distorted structure around each Mg impurity may slightly modify the magnetic interactions and be potentially responsible for the antiferromagnetic ordering (belowT _N≈ 3.5K) in doped compositions.     

ESR study of the undoped heavy-fermion compound YbRh2Si2

The electron spin resonance (ESR) of the heavy-fermion metal YbRh₂Si₂ has been studied. The angular variation and the temperature dependence of the ESR line width have been measured in YbRh₂Si₂ single crystals in the temperature range of 4–25 K. The characteristic spin-fluctuation temperatureT ^* ～ 17 K estimated from these studies coincides very well with other experimental data. A well-behaved ESR signal due to local Yb³⁺ moments strongly supports the localized moment scenario for heavy-fermion quantum critical points.     

The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

2-D electron spin transient nutation spectroscopy of Lanthanoid ion Eu2+(8S7/2) in a CaF2 single crystal on the basis of FT-pulsed electron spin resonance spectroscopy: Transition moment spectroscopy

This work demonstrates the usefulness of pulsed electron spin resonance (ESR)-based two-dimensional electron spin transient nutation (2-D ESN) spectroscopy for complete assignments of complicated fine-structure hyperfine ESR spectra including hyperfine forbidden transitions from electronic and nuclear high-spin systems. The 2-D ESN spectroscopy is termed transition moment spectroscopy as spectra are acquired as a function of transition moment instead of transition energy used in conventional spectroscopy. We have applied the novel spectroscopic technique to Eu²⁺ ion (S=7/2,I=5/2), which has two isotopes (¹⁵¹Eu [47.9%] and¹⁵³Eu [52.1%]), in a CaF₂ single crystal as a model system. We have completely identified the complicated fine-structure hyperfine ESR spectra by invoking the spectral simulation of the 2-D ESN spectra on the basis of transition moment analyses. The analyses are based on exact numerical calculations of the transition moments as well as a perturbation-based analytical approach combined with reduced rotation matrices for the nuclear part of the transition moment. This is the first example of the spectral simulation for 2-D ESN spectra including the hyperfine allowed and forbidden transitions in high-spin systems. In addition, we have made simulation of the fine-structure forbidden transitions, which reproduces the angular variations of the observed spectra at liquid helium temperatures.     

Half-field and normal electron spin resonance study of the quasi-two dimensional magnetic crystal Mn(CHOO)2·2H2O

We have studied the normal electron spin resonance at the Larmor frequency as well as the half-field resonance of the quasi two dimensional magnetic crystal Mn(CHOO)₂·2H₂O at 35 GHz and room temperature. There are two groups of Mn²⁺ ions referred as A and B in this compound. The A ions lie in parallel (100) planes and have a strong exchange interaction within the plane. On the other hand, the B ions which lie in the adjacent parallel planes have no exchange interaction in the plane. We have measured the line width of the normal resonance from the A ions as a function of θ and φ where θ is the angle between the d.c. magnetic field H₀ and the a-axis of the crystal, which is very nearly perpendicular to the (100) plane and φ is the azimuthal angle of H₀ in the bc plane with respect to the b-axis. We also measured the intensity of the half-field resonance as a function of φ. We have calculated the second moments of the normal resonance line as a function of θ and φ. There is a satisfactory agreement between the calculated and the observed θ dependence of the line widths. The intensity of the half-field resonance has a maximum at φ = 40° whereas in an ideal 2d magnet, the maximum intensity is expected at φ = 45°. The ESR line width, because of the spindynamics in a 2 dimensional magnetic crystal, has an angular dependence of the form (3 cos²θ ? 1)². The observed dependence of the line width in Mn(COOH)₂·2H₂O seems to follow the same angular dependence when corrected for the contribution from the nearest neighbor B ions.     

Spin probe ESR studies of PEGxLiClO4 polymer electrolyte systems

The technique of Electron Spin Resonance (ESR) is shown to be useful in the study of dynamics of solid polymer electrolytes (SPE). Through the ESR of the nitroxide radical (2,2,6,6-tetramethyl-1-piperidine-1-oxyl; TEMPO) dispersed in the SPE PEG₄₆LiClO₄ temperature dependence of correlation time is found. The glass transition temperature T_g is estimated to be −51 °C from the measurement of T_50G, the temperature at which the extrema separation 2A_ZZ becomes 50G and is found to be close to that measured using DSC (−51.7 °C). T_g for pure PEG-2000, which could not be measured from DSC because of its high crystallinity, is determined to be −72 °C by spin probe ESR. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Anomalous pressure dependence of the Lamb-Mössbauerf-factor in the spin crossover system [Fe(2-pic-ND2)3]Cl2·EtOD

The pressure dependence of the Lamb-Mössbauer factor of the spin crossover compound [Fe(2-pic-ND₂)₃]Cl₂·EtOD(2-pic-ND₂=2-picolylamine, deuterated at the^?NH₂ group) has been measured at pressures up to 1500 bar and at three temperatures (115.7, 146.7, 185.6 K) around the transition temperatureT _t=135 K. The temperature dependence of the unit cell volume has been determined by X-ray diffraction. In the transition region (T=146.7 K), the pressure dependence of thef-factor shows an anomalous increase as compared to the dependence below and above the transition. The behaviour off(p, T), its anomaly and absolute value, can be consistently explained within a theoretical model which treats the compound as an isotropic elastic medium. The Poisson ratio of σ=0.4, the bulk modulusK (185.6 K)=1.26·10¹⁰ Nm^?2, and the Grüneisen constant γ^G=3.1 were determined.     

Low temperature ESR spectra of nickel — doped NaCl crystals

Paramagnetic resonance of Bridgman-grown NaCl crystals nominally doped with divalent nickel was studied over the temperature range from 4 K to 300 K. The broad isotropic EPR line with g = 2.12 ÷ 2.23 and ΔH _pp = 280 ÷ 780 G was detected at temperatures higher than 130 K for samples of various thermal history. The origin of these spectra was attributed to aggregated and precipitated Ni²⁺ ions. At temperatures lower than 40 K another ESR signal was registered. It was an anisotropic one, the intensity of which strongly depends upon the temperature. This spectrum results from substitutional Ni⁺ ions with tetragonally distorted surrounding along a S100T direction. The angular dependence of the ESR line positions was fitted to an appropriate spin-Hamiltonian with the g-factor components g _∥ and g _⊥ equal to 2.86 and 2.10, respectively. The temperature-dependence of the signal intensity at temperatures between 4 and 110 K has been interpreted in terms of a transition from the static to dynamic Jahn-Teller effect.     

Electron spin resonance analysis of magnetic structures in La2/3Ca1/3MnO3

Measurements of electron spin resonance (ESR) of La_2/3Ca_1/3MnO₃ (LCMO) in the ferromagnetic and paramagnetic phases were carried out. Phase transition and temperature dependence of the peak-to-peak ESR linewidth were determined. The transition temperature between ferromagnetic and paramagnetic phases was observed at 265 K. A prominent increase of the peak-to-peak linewidth with decreasing temperature below T_c was observed. Using the dynamic scale theory and block spin transformation in critical phenomenon, the quantitative calculation of peak-to-peak linewidth at near T_c was made, which was in good agreement with the experimental data. It was believed that the long interactions between the ferromagnetic microregions for LCMO played a key role in determining the ESR linewidth.     

EPR study of Mn2+ around the ferroelastic transition point of Pb3(PO4)2

The spin Hamiltonian parameters of Mn²⁺ have been measured above and below the transition point (180°C) of the lead phosphate. They show that Mn²⁺ substitutes a Pb_I ion. Between 175 and 180°C the principal axis OX of the fine tensor is parallel to the wave vector of the soft mode which condensates at the transition point. An exaltation of the linewidth is observed. The linewidth remains constant within 5°C of T_c; in this temperature range, the “static regime” is achieved, and the correlation time of the fluctuations is less than 10^?8 sec.     

Susceptibility — Knight shift correlation and the band structure of VO2

The magnetic susceptibility and⁵¹V Knight shift have been measured in powdered VO₂ from the semiconductor-metal transition temperatureT _t=341°K to 478°K. The fact that both depend linearly on temperature enables the orbital and spin contributions to both quantities to be evaluated. The orbital and spin susceptibilities at 341°K are found to be 1.30±0.07 and 7.49±0.07μemu g^?1, respectively, and the corresponding shifts +0.61±0.04% and ?1.00±0.04%. The existence and magnitude of the orbital term are consistent with the currently accepted two band model of VO₂ aboveT _t.     

On the mechanism of the (n, 2n) reaction with 14MeV projectiles on127I and209Bi

The angular and spectral distributions of coincident neutrons from the reactions¹²⁷I(n, 2n)¹²⁶I and²⁰⁹Bi(n, 2n)²⁰⁸Bi have been measured with two time-of-flight detectors at the incident energy 14.1 MeV. Neutron emission has been studied for reaction anglesθ _lab between 10° and 150° (relative angles from 60° to 270°). Energy spectra, angular distributions and coincidence yields are compared with statistical model calculations including preequilibrium decay modes, and nuclear level density and spin cut off parameters are derived.     

Berechnung der streuspektren schwach ionisierter plasmen mit hilfe zweizeitiger wahrscheinlichkeitsdichten

ESR linewidth measurements have been performed on the quasi-two-dimensional Heisenberg magnet (C₂H₅NH₃)₂MnCl₄ at room temperature in the X-, K-, Q- and V-band. As expected from theory a logarithmic frequency dependence of the magic angle linewidth has been determined, which represents a characteristic feature of spin diffusion in a two-dimensional lattice. The spin diffusion constant is evaluated to be D/a² = (1700±250) rad GHz.     

Spin-Probe ESR Studies on Nanocomposite Polymer Electrolytes

The possibility of using spin-probe electron spin resonance (ESR) as a tool to study glass transition temperature, T _g, of polymer electrolytes is explored in 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL)-doped composite polymer electrolyte (PEG)₄₆LiClO₄ dispersed with nanoparticles of hydrotalcite. The T _g is estimated from the measured values of T _50G, the temperature at which the extrema separation 2A _zz of the broad powder spectrum decreases to 50 G. In another method, the correlation time τ_c for the spin probe dynamics was determined by computer simulation of the ESR spectra and T _g has been identified as the temperature at which τ_c begins to show temperature dependence. While both methods give values of T _g close to those obtained from differential scanning calorimetry, it is concluded that more work is required to establish spin-probe ESR as a reliable technique for the determination of T _g.     

Study of combined magnetic and electric hyperfine interactions for cadmium in Dysprosium by time differential perturbed angular correlations

The combined magnetic and electric hyperfine interaction at the site of a¹¹¹Cd impurity in magnetically ordered Dysprosium has been investigated as a function of temperature by time differential perturbed angular correlation measurements. Three different phases have been found in metallic Dy with transition temperatures of 85 and 179 °K in agreement with the results of bulk material measurements. In the paramagnetic phase above 179 °K a pure electric quadrupole interaction has been observed. The various contributions to the electric fieldgradient are analyzed and it is shown, that the dominant contribution comes from the conduction electrons. In the ferromagnetic phase which extends from 0 to 85 °K the magnetic hyperfine field at the site of¹¹¹Cd has the same temperature dependence as the spontaneous magnetization. The value of the hyperfine field at 4.2 °K is ¦H _eff¦=(221 ± 4) kG. At 85 °K a transition to the antiferromagnetic phase of Dy occurs, which shows a hysteresis of the transition temperature. In the antiferromagnetic phase the temperature dependence of the hyperfine field deviates considerably from the magnetization curve. It is suggested that this deviation might be due to a temperature dependence of thes-f exchange interaction.     

ESR of X-ray induced centre in CaF2:Gd crystals

An ESR centre with orthorhombic symmetry was induced in CaF₂:Gd crystals by X-ray irradiation after heat treatment at 900°C. The observed ESR spectrum has resolved shfs in particular crystal directions. Spin Hamiltonian analysis indicates that the ESR centre is a hole trapped at an oxygen substituted for a fluorine site and interacts mainly with two nearby fluorine ions located along the crystal [001] direction. The determined spin Hamiltonian parameters are g_xx = 2.0036, g_yy = 2.0159, g_zz = 2.0306, A_xx = 22.5, A_yy = 5.6 and A_zz = 5.6 (G), respectively. A probable model is discussed by considering both the angular dependence of the ESR lines and the local charge neutrality around the ESR centre.     

Die Winkelabhängigkeit der Resonanzstreuung niederenergetischer Elektronen an N2

Results of scattering experiments with monochromatic electrons in the energy range from a few tenth of an eV to about 16 eV are reported. Below 1.8 eV collision energy no resonance structures have been found either in the total scattering (transmitted current) or in the elastic and inelastic differential cross sections. The resonances above 1.8 eV have been measured in the elastic and inelastic channels (up to vibrational quantum numberν=8 of the N₂-molecule) in the angular range from 10° to 110°. In the inelastic channelsν≧2 especially the first resonance peak appears asymmetric, as predicted by calculations of the associated Franck-Condon-factors. In all inelastic channels the angular dependence (due to the pure resonance scattering) shows maxima at 0° and 90° and a minimum around 55°, probably indicating gerade-symmetry of the associated N ₂ ^? -state. Excitation functions of three states of N₂ were measured between their thresholds (between 11 and 12 eV) and 15 eV, one of them showing several new resonance structures.     

Phase diagrams from spin probe studies for binary mixtures of non-mesomorphic solutes in 5CB

The effect of low concentrations of a non-mesomorphic solute impurity on the nematicto-isotropic transition of 4-cyano-4′-n-pentylbiphenyl (5CB), has been investigated by ESR spectroscopy, using predeuterated-2,2,6,6-tetramethyl-4-piperidone-N-oxide (PD-Tempone), at mole fractions of the order of 10^?5, as a probe. It has been found that at such low mole fractions PD-Tempone simply probes the nematic-to-isotropic transition without observably perturbing it. This fast approach relies on the fact that a smaller coupling constant is obtained for the spin probe in the orientationally ordered nematic phase relative to the isotropic phase. The method has been further refined in this study by the use of an especially designed sample holder which, in conjunction with a temperature control and monitoring system, allowed control of the temperature of the sample to within ±0.02°C. Additionally measurements were made, for each solute, at several solute mole fractions within the range from 0.01 to 0.07. The results were used to obtainβ _N andβ ₁, the moduli of the slopes of the nematic and isotropic boundary lines respectively in the reduced nematic-to-isotropic transition temperature (T ^*) versus solute mole fraction (x ₂) diagrams. The non-mesomorphic solutes used in this study are n-nonane, 3,3-diethylpentane, tetrapropyltin and tetrabutyltin.     

Electron spin resonance of Eu<Superscript>2 +</Superscript> in the Eu doped clathrate Ba<Subscript>6</Subscript>Ge<Subscript>25</Subscript>

We report electron spin resonance (ESR) investigations of the clathrate compound Ba_6-xEu_xGe₂₅ (x=0.03–0.4) which exhibits a temperature induced, two-step reconstructive structure transformation at temperatures between 185 K and 223 K. The linewidth of the Eu²⁺ ESR proves to be sensitive to the transformation. Another anomaly in the temperature dependence of the linewidth is found near T=60 K which points towards another possible structural transition. Both anomalies seen in the ESR linewidth are not sensitive to the Eu content in contrast to the strong Eu-concentration dependence of transport properties.     

The transition from dynamics to statics in the electron-spin-resonance spectra of impurity Mn2+ ions in strontium titanate

The electron-spin-resonance (ESR) spectra of SrTiO₃:Mn single crystals have been investigated. Results unambiguously indicate that the impurity center formed by an Mn²⁺ ion has a dynamic nature. In the high temperature range (T > 100 K), ESR spectra of Mn²⁺ ions reveal cubic symmetry; the spectrum is found to broaden significantly with a decrease in temperature. Upon cooling to T < 10 K, low-symmetry centers of Mn²⁺ ions with a strong orientational dependence emerge in the spectra. Temperature evolution of the ESR spectrum can be described within the model of a dynamic off-center Mn²⁺ ion substituting for the Sr²⁺ ion, with a transition to the static regime at low temperatures with an average localization energy of ～2.4 ± 0.4 meV for Mn²⁺ centers due to random deformations.