Average Energy Barrier Research Articles

Nonlinear temperature variation of magnetic viscosity in nanoscale FeOOH particles.

Temperature variations of the magnetization and magnetic viscosity [ital S] in 30-A particles of FeOOH are reported. An average blocking tempeature [ital [bar T]][sub [ital b]]=47 K is determined for this system from the temperature variation of the difference susceptibility between the field-cooled and zero-field-cooled cases. The measured decrease of [ital [bar T]][sub [ital b]] with applied field is used to determine an average magnetic moment [congruent]7150[mu][sub [ital B]] per particle. The relaxation data of the magnetization [ital M]([ital t]) follow the equation [ital M]([ital t])/[ital M]([tau][sub 0])=1 [minus][ital S] ln([ital t]/[tau][sub 0]), from which temperature variation of [ital S] is determined for the range 2 to 200 K. Below 7 K, [ital S] is temperature independent signifying quantum tunneling. Above 7 K, initially [ital S] increaes linearly with [ital T], but the variation becomes increasingly nonlinear, reaching an apparent maximum at [ital [bar T]][sub [ital b]]=47 K. The temperature variation of [ital S] is characteristic of a phase transition at [ital [bar T]][sub [ital b]] since the data fits well the equation [ital S]=[ital A][vert bar]([ital T][minus][ital [bar T]][sub [ital b]])/[ital [bar T]][sub [ital b]][vert bar][sup [minus][nu]] with [ital A]=0.040(0.0076) and [nu]=1.10(1.29) for [ital T][gt][ital [bar T]][sub [ital b]]([italmore » T][lt][ital [bar T]][sub [ital b]]). An average energy barrier [l angle][ital U][r angle]=2140 K and [tau][sub 0]=5[times]10[sup [minus]12] s are also estimated from the analysis of the data.« less

An NMR study of segmental motion in poly(isobutylene) and the relationship to translational diffusion of sorbed CO2

AbstractThe spin‐lattice relaxation times as well as nuclear Overhauser enhancements (NOE) are determined for the methylene carbon of polyisobutylene (PIB). Three different correlation functions are used to treat the relaxation data to describe the local motion of the PIB backbone rubber. The first is the Hall‐Helfand function combined with restricted anisotropic rotational diffusion, the second is the Dejean‐Lauprêtre‐Monnerie function, and the third is the Williams‐Watts function combined with restricted anisotropic rotational diffusion. Spin‐lattice relaxation times are also determined on PIB under pressure of CO2 gas to check for changes in segmental motion in the presence of this penetrant. All the models give a satisfactory description for the local dynamics of the backbone carbon of the polymer chain, yielding comparable values for the time scale of conformational changes, apparent activation energies and distribution of correlation times. In addition, a proton line shape experiment is performed on bulk PIB as a function of temperature. The results are interpreted in terms of two motional components in the polymer chains: fast and slow ones. Applying a revised version of Rössler's formalism to our data, an average energy barrier is found which is consistent with the apparent activation energies. The apparent activation energy obtained for PIB, from the nuclear magnetic resonance (NMR) measurements, about 35 kJ, is slightly higher than the apparent activation energy for the translational diffusion of CO2 in PIB from NMR data. The distribution of correlation times and the correlation times themselves for segmental motion of the PIB and translational diffusion of CO2 in PIB are very similar, indicating a close link between the two motional processes. The distribution of correlation times is close to a single exponential in PIB consistent with Angell characterization of this material as a strong liquid. © 1994 John Wiley & Sons, Inc.

Magnetic properties of icosahedral Al-Cr-Mn-Ge alloys.

Alloys in the series ${\mathrm{Al}}_{65}$${\mathrm{Cr}}_{20\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Ge}}_{15}$ (0\ensuremath{\le}x\ensuremath{\le}20) have been produced by melt spinning. These rapidly solidified alloys have been found to be single phase and of the icosahedral structure. Magnetic-susceptibility measurements reveal a moment of 0.45${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for Cr in the ${\mathrm{Al}}_{65}$${\mathrm{Cr}}_{20}$${\mathrm{Ge}}_{15}$ alloy. This is the first observation of a local moment on Cr in a quasicrystal. In the ${\mathrm{Al}}_{65}$${\mathrm{Mn}}_{20}$${\mathrm{Ge}}_{15}$ alloy a moment of 2.10${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for Mn is observed, which is the largest moment observed in Al-Mn-metalloid quasicrystal alloys. The compositional dependence of the magnetic moment is consistent with at least two classes of magnetic sites. Spin-glass behavior is observed below 8 K in the ${\mathrm{Al}}_{65}$${\mathrm{Mn}}_{20}$${\mathrm{Ge}}_{15}$ alloy. Measurements of the time dependence of the magnetization, in a field of 1 T, and from the zero-field-cooled state have been made for the ${\mathrm{Al}}_{65}$${\mathrm{Mn}}_{20}$${\mathrm{Ge}}_{15}$ alloy, revealing logarithmic relaxation. Analysis of this data using a first-order rate equation and a distribution of energy barriers reveals an average energy barrier of 6.8 meV.

Cluster-quench simulation of the two-dimensional random Ising model

A simulation technique is introduced which generated states stable against spin reversal of all spin clusters of n or fewer spins. The distribution of energy barriers for reversing m-spin clusters (m>n) and the distribution of energy gained are computed. The results are interpreted using a renormalisation-group picture with n1/2 as the length scale. The data scale with correlation length exponent nu =2.6 and an average energy barrier increasing approximately as 1-n-1/2 nu . These results provide a detailed test of the phenomenological scaling theory of spin glasses.

Long nonstationary transitions in particle deposition under external forces

A parabolic-type nonstationary transport equation was solved by applying a numerical method of inversion of the Laplace transform of this equation. Results are given in the case of particle sedimentation under gravity against an energy barrier. The influence of the energy barrier height and flow intensity on the relaxation time was theoretically investigated. The numerical calculations have shown that in such systems the relaxation time attained large values, i.e., of the order of minutes and hours, especially for larger barriers. A possibility of an experimental determination of an average energy barrier height is suggested.

Electronic Conduction in Bi–BiI3 Melts. A Theoretical Model

Experimental electrical conductivity data for the liquid Bi–BiI3 system are analyzed in terms of predominant conduction in the salt-rich region by an electronic ``jumping'' or exchange process between the mono- and tervalent states of bismuth. Calculations of the jump frequency from the conductivity data show the expected dependence on temperature and average distance between exchange sites. Results of the calculations are used to evaluate the average energy barrier height for the exchange process and the enthalpy of symmetrization of the ionic atmospheres about the exchange sites, as required by the Franck-Condon principle. The latter is found to be independent of the average distance between exchange sites.