Energy Barrier For Reversal Research Articles

Energy barrier of magnetization reversal in a magnetic wire with defects

We consider the energy barrier to magnetization reversal in a one-dimensional (1D) magnetic wire with defects. By solving the variable coefficient double Sine-Gordon equation, we obtain the dependence of the energy barrier on the strength and the range of a defect potential. We find that there is a minimum-energy barrier for two defects placed at an optimal distance which is correlated with the width of a $360\ifmmode^\circ\else\textdegree\fi{}$ untwisted domain wall. The possible experimental realization of our results is discussed.

Atomic Oxygen Absorption into Pt-Based Alloy Subsurfaces

Density functional theory is used to study atomic oxygen absorption into subsurfaces of various platinum-based alloys to investigate the effects of alloyed metals on the thermodynamics and kinetics of this process. The results show that alloying Pt with Ir and Rh increases the absorption energy barrier compared with pure Pt, making the O migration to the subsurface more difficult, and that the addition of Co and Ru to Pt can decrease the reverse energy barrier of absorption making O unstable in the subsurface. By comparison of the atomic oxygen absorption in Pt3X alloys with that in PtX alloys, it is found that both the nature of the alloyed metal species and the alloy composition have effects on the atomic oxygen absorption kinetics. Analyses of the atomic oxygen absorption into the subsurface of ternary alloys such as Pt2IrRu show that compared with pure platinum the presence of the two added metals can increase the barriers for surface diffusion and that for O absorption while simultaneously decreasing...

Single molecule magnet: Heterodinuclear cyano-bridged cubic cluster [(Tp) 8Fe 4Ni 4(CN) 12] (Tp = hydrotris(1-pyrazolyl)borate)

The octanuclear cyano-bridged cluster [(Tp) 8Fe 4Ni 4(CN) 12] · H 2O · 24CH 3CN ( 1) (Tp = hydrotris(1-pyrazolyl)borate) showing magnetic properties of single-molecule magnet has been synthesized by reaction of [ fac-Fe(Tp)(CN) 3] − with {(Tp)Ni(NO 3)} species formed from an equimolar reaction mixture of Ni(NO 3) 2 · 6H 2O and KTp in MeCN. The X-ray analysis of 1 shows molecular cube structure in which Fe III and Ni II ions reside in alternate corners. The average intramolecular Fe⋯Ni distance is 5.124 Å. Out-of-phase ac susceptibility and reduce magnetization measurements show that 1 is a single molecule magnet with ground spin state S = 6 and spin reversal energy barrier U = 14 K. Magnetic hysteresis loops were also observed by applying fast sweeping field.

An ab initio and DFT study of some halogen atom transfer reactions from alkyl groups to acyl radical

Ab initio calculations using the 6-311G**, cc-pVDZ, and (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation, and density functional (BHandHLYP, B3LYP) calculations predict that the transition states for the reaction of acetyl radical with several alkyl halides adopt an almost collinear arrangement of attacking and leaving radicals at the halogen atom. Energy barriers (DeltaE(double dagger)) for these halogen transfer reactions of between 89.2 (chlorine transfer from methyl group) and 25.3 kJ mol(-1) (iodine transfer from tert-butyl group) are calculated at the BHandHLYP/DZP level of theory. While the difference in forward and reverse energy barriers for iodine transfer to acetyl radical is predicted to be 15.1 kJ mol(-1) for primary alkyl iodide, these values are calculated to be 6.7 and -4.2 kJ mol(-1) for secondary and tertiary alkyl iodide respectively. These data are in good agreement with available experimental data in that atom transfer radical carbonylation reactions are sluggish with primary alkyl iodides, but proceed smoothly with secondary and tertiary alkyl iodides. These calculations also predict that bromine transfer reactions involving acyl radical are also feasible at moderately high temperature.

Dissociation Dynamics of Sequential Ionic Reactions: Heats of Formation of Tri-, Di-, and Monoethylphosphine

The sequential ethene (C2H4) loss channels of energy-selected ethylphosphine ions have been studied using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy in which ion time-of-flight (TOF) distributions are recorded as a function of the photon energy. The ion TOF distributions and breakdown diagrams have been modeled in terms of the statistical RRKM theory for unimolecular reactions, providing 0 K dissociation onsets, E0, for the ethene loss channels. Three RRKM curves were used to model the five measurements, since two of the reactions differ only by the internal energy of the parent ion. This series of dissociations provides a detailed check of the calculation of the product energy distribution for sequential reactions. From the determined E0's, the heats of formation of several ethylphosphine neutrals and ions have been determined: Delta(f)H degrees 298K[P(C(2)H(5))3] = -152.7 +/- 2.8 kJ/mol, Delta(f)H degrees 298K[P(C(2)H(5))3+] = 571.6 +/- 4.0 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2] = -89.6 +/- 2.1 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2+] = 669.9 +/- 2.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)] = -36.5 +/- 1.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)+] = 784.0 +/- 1.9 kJ/mol. These values have been supported by G2 and G3 calculations using isodesmic reactions. Coupled cluster calculations have been used to show that the C2H4 loss channel, which involves a hydrogen transfer step, proceeds without a reverse energy barrier.

Scaling behavior of thermally assisted magnetization reversal in nanomagnets

Energy barriers and the corresponding magnetization configurations of a nanowire of arbitrary length are derived analytically. The results interpolate between the Néel-Brown theory and the soliton-antisoliton mechanism for long samples. It is shown that the reversal mechanism depends on the time scale being probed. For sufficient size, one and the same particle reverses uniformly for fields close to the anisotropy field at short time scales, but reverses nonuniformly at low fields and large time scales. Remarkably, the energy barrier for nonuniform reversal is considerably lower than that for uniform reversal at small time scales.

Field dependence of the barrier to magnetization reversal of a Stoner-Wohlfarth particle

It is well known that the energy barrier for magnetization reversal, EB, varies quadratically with the magnetic field for the Stoner-Wohlfarth model. However, the enthalpy H (switching energy) required to reverse the magnetization is the sum of EB and the work Wf done by the Néel fluctuation field. The sum of these two terms gives an enthalpy of reversal. If the fluctuation field is uniaxial and parallel to the holding field, then the enthalpy barrier for switching is linear in the holding field. The linearity is consistent with certain experimental results. If the fluctuation field is anisotropic, then for the same size field, the probability of the particle’s switching will depend upon the direction of that field. The paper discusses the holding field variation of the energy barrier for different fluctuation field directions.

Mn${{{{III}\hfill \atop 2\hfill}}}$(5‐Rsaltmen)2NiII(pao)2(L)]2+: An ST=3 Building Block for a Single‐Chain Magnet That Behaves as a Single‐Molecule Magnet

AbstractMnIII‐NiII‐MnIII linear‐type trinuclear complexes bridged by oximate groups were selectively synthesized by the assembly reaction of [Mn2(5‐Rsaltmen)2(H2O)2](ClO4)2 (5‐Rsaltmen2−=N,N′‐(1,1,2,2‐tetramethylethylene) bis(5‐R‐salicylideneiminate); R=Cl, Br) with [Ni(pao)2(phen)] (pao−=pyridine‐2‐aldoximate; phen=1,10‐phenanthroline) in methanol/water: [Mn2(5‐Rsaltmen)2Ni(pao)2(phen)](ClO4)2 (R=Cl, 1; R=Br, 2). Structural analysis revealed that the [MnIII‐ON‐NiII‐NO‐MnIII] skeleton of these trimers is in every respect similar to the repeating unit found in the previously reported series of 1D materials [Mn2(saltmen)2Ni(pao)2(L1)2](A)2 (L1=pyridine, 4‐picoline, 4‐tert‐butylpyridine, N‐methylimidazole; A=ClO4−, BF4−, PF6−, ReO4−). Recently, these 1D compounds have attracted a great deal of attention for their magnetic properties, since they exhibit slow relaxation of the magnetization (also called single‐chain magnet (SCM) behavior). This unique magnetic behavior was explained in the framework of Glauber's theory, generalized for chains of ferromagnetically coupled anisotropic spins. Thus, in these 1D compounds, the [MnIII‐ON‐NiII‐NO‐MnIII] unit was considered as an ST=3 anisotropic spin. Direct‐current magnetic measurements on 1 and 2 confirm their ST=3 ground state and strong uniaxial anisotropy (D/kB≈−2.4 K), in excellent agreement with the magnetic characteristic deduced in the study on the SCM series. The ac magnetic susceptibility of these trimers is strongly frequency‐dependent and characteristic of single‐molecule magnet (SMM) behavior. The relaxation time τ shows a thermally activated (Arrhenius) behavior with τ0≈1×10−7 s and Δeff/kB≈18 K. The effective energy barrier for reversal of the magnetization Δeff is consistent with the theoretical value (21 K) estimated from |D|${S{{2\hfill \atop {\rm T}\hfill}}}$. The present results reinforce consistently the interpretation of the SCM behavior observed in the [Mn2(saltmen)2Ni(pao)2(L1)2](A)2 series and opens new perspectives to design single‐chain magnets.

DFT Studies on Reaction Mechanism of the Double Bond Isomerization of Butene Catalyzed by 1-ethyl-3-methyl-imidazolium of the Ionic Liquid (II)

The possible reaction paths of butene isomerization catalyzed by 4-H and 5-H of 1-ethyl-3-methyl-imidazolium (EMIM(superscript +)) have been studied by using density functional theory (DFT) at B3LYP/6-3G(superscript **) levels. The geometries of the reactants, transition states and products are fully optimized by energy gradient technology and the equilibrium states and transition states are verified according to the number of imaginary frequency obtained through vibrational analysis. The computed results indicate that the butene molecule approaches to 4-H or 5-H of EMIM(superscript +) at the first and then forms a supermolecule due to physical adsorption. Whereafter the isomerization of butene double bond is catalytically performed. The forward energy barriers of the rearrangement reaction catalyzed by 4-H and 5-H atom of EMIM(superscript +) are 204.2 and 207.3 kJ•mol^(-1), and the reverse energy barriers are 220.9 and 223.8kJ•mol^(-1), respectively. The isomerization of outene catalyzed by the proton of the imidazolium ring may proceed through multi-channel at one-step process.

ＬＬＧシミュレーションによるサブミクロンサイズ・パーマロイ薄膜の熱安定性に関する検討

Thermal stability of submicron size permalloy thin films was investigated both by the Langevin LLG simulation and by the energy reconstruction method. The former gives the reversal time due to thermal fluctuation, and the latter, the potential energy profile of the films. The results were consistent with each other. The relation between the reversal time and energy barrier is well established by these methods. The magnetization behavior before reversal at finite temperature is visualized by a combination of these methods.

Lanthanide double-decker complexes functioning as magnets at the single-molecular level.

Double-decker phthalocyanine complexes with Tb3+ or Dy3+ showed slow magnetization relaxation as a single-molecular property. The temperature ranges in which the behavior was observed were far higher than that of the transition-metal-cluster single-molecule magnets (SMMs). The significant temperature rise results from a mechanism in the relaxation process different from that in the transition-metal-cluster SMMs. The effective energy barrier for reversal of the magnetic moment is determined by the ligand field around a lanthanide ion, which gives the lowest degenerate substate a large |Jz| value and large energy separations from the rest of the substates in the ground-state multiplets.

Thermal stability of the magnetization of 150 nm×230 nm Ni19Fe81 elements

The time- and temperature-dependencies of the switching fields of arrays of 230 nm×150 nm Ni19Fe81 rectangular elements have been investigated. The elements have thicknesses of 3 nm, 5 nm, and 10 nm. For temperatures from 100 K to 300 K, all three arrays show a linear dependence of switching field on temperature, with similar slopes, but below 100 K, a deviation from linearity occurs, indicating a change in the switching process. The energy barrier for reversal at zero field has been estimated as 70kT300, 125kT300, and 280kT300 for the 3-nm, 5-nm, and 10-nm thick elements, respectively, which suggests that the energy barrier scales approximately linearly with film thickness. For uncoupled permalloy elements with these lateral dimensions, superparamagnetism is expected at room temperature for thicknesses below about 1 nm.

Thermal Effects in the High-Speed Switching of the Magnetization of Fine Grains

A numerical study of thermal effects on the high-speed switching of the magnetization of perpendicular recording media is presented. The Langevin equation is solved numerically to determine the magnetic response of a single grain during motion through the field profile of a single pole head. We evaluate the effective time of the thermal relaxation process and the thermal reduction of the write coercivity. A transition from thermally activated to dynamic reversal occurs at some critical value v1 of the relative head-medium velocity. In the thermal regime (v<v1), the gyromagnetic precession enhances the energy barrier of reversal. In the dynamic reversal regime (v>v1), the thermal reduction of the write coercivity is independent of the velocity.

Thermally and field driven magnetization processes in nanometer-scale particle arrays

Nanometer-scale iron particles were fabricated by Scanning Tunneling Microscopy assisted chemical vapor deposition. With this, the particles can precisely be placed at a predetermined position and their dimensions can easily be controlled. The latter enabled us to perform a systematic study of the dependence of thermal activation on the particles' diameter (9-20 nm for the different arrays). Magnetic measurements included Hall magnetometry and variable field MFM. From the temperature dependence of the switching fields of our elongated particles we inferred a thermally activated nucleation-propagation type of magnetization reversal even for the smallest (9 nm) particles. The energy barrier for magnetization reversal in elongated particles is derived, from which dynamic coercivities can be calculated. The impact of thermal activation was found to scale with the particles' cross-section, in agreement with magnetic viscosity measurements.

Characterization of Perpendicular Recording Media of CoPtCrO Granular Thin Films.

Thermal stability of CoPtCrO granular thin films with perpendicular magnetic anisotropy is investigated. Magnetization decay with time M(t) exhibits almost a linear dependence with ln(t), similar to that for longitudinal recording media. In comparison with amorphous rare earth-transition metal alloys media which shows a non-linear decay with ln(t), the differences in magnetization reversal process and energy barrier distribution are discussed. From magnetic viscosity measurements, activation volume is determined and shows a minimum of about 2x10-18 cm3 for CoPtCrO at fields near its coercivity, leading to a coupled region of 10 nm average diameter. Also recording performance is investigated using a conventional merged GMR head and magnetic force microscope. The linear recording density dependence of signal amplitude reveals that the 50% rolloff density D50 is about 300 kfci.

Incoherent Magnetization Model for Perpendicular Recording Media

To study the influence of thermal fluctuation in perpendicular recording media, we investigated a new incoherent magnetization model and combined it with the Langevin equation method. In our model, the energy barrier for magnetization reversal is reduced by using an incoherent reversal process. We simulated the M-H curve and the torque curve for perpendicular media with small Hc/Hk values, and calculated the time decay of Mr due to thermal fluctuation in the perpendicular media. The results roughly agreed with the experimental values of the time decay.

The heat of formation of CISO +

The heat of formation of CISO +, 802 ± 1 kJ mol −1 at 0 K, has been obtained by the dissociative photoionization of supersonically cooled thionyl chloride, Cl 2SO. Threshold photoelectron photoion coincidence spectroscopy was used to obtain the appearance energy from the cross-over point in the breakdown diagram for the dissociation. Also measured was the adiabatic ionization energy of thionyl chloride, 10.85 ± 0.05 eV. The experimentally measured kinetic energy release was found to agree with that calculated by statistical theory confirming that there is no reverse energy barrier to the Cl loss reaction. The interpretation was aided by ab initio MO calculations.

Molecular requirement for anticonvulsant activity in a series of thiazolo-1,4-benzodiazepine derivatives and comparison with classical benzodiazepines

1. 1. The behavioural and anticonvulsant effects of several thiazolo[3,2-d][1,4]benzodiazepines (TBZ) were studied after intraperitoneal administration in DBA/2 mice, a strain genetically susceptible to sound-induced seizures. 2. 2. Anticonvulsant effects on seizures evoked by means of auditory stimulation (109 dB, 12–16 kHz) were evaluated in DBA/2 mice placed singly under a perspex dome. 3. 3. Hypothermic activity was observed after the highest doses of the benzodiazepines studied. 4. 4. In addition, some TBZ were examined for anticonvulsant properties with respect to clonus induced by pentylenetetrazol. 5. 5. Our study demonstrated that some thiazolobenzodiazepine derivatives were more potent than clobazam, desmethylclobazam and chlordiazepoxide, and less potent than diazepam, desmethyldiazepam and alprazolam. 6. 6. In the series of tricyclic benzodiazepines, thiazole nucleus fusion to the “d” edge of the 7-membered ring results in an effective increase of the energy barrier for the heptatomic system reversal, and is probably responsible for, jointly with the lack of C=N double bonds, lower activity with respect to the 1,4-benzodiazepine precursors. 7. 7. The potency of various thiazolobenzodiazepine derivatives as inhibitors of specific [ 3H]flunitrazepam binding to membranes from cerebellum or hippocampus was evaluated. 8. 8. All tested compounds produced concentration-dependent inhibition of [ 3H]flunitrazepam binding. 9. 9. The pharmacological activity of TBZ2, the most active compound of this series, was significantly reduced by treatment with flumazenil (2.5 mg/kg i.p.), suggesting clear involvement of a benzodiazepine mechanism in the anticonvulsant activity of these compounds.

Thermal fluctuations in a pair of magnetostatically coupled particles

The influence of the thermal agitation on the switching dynamics for a pair of identical uniaxially anisotropic dipoles is studied for the case of the applied field parallel to the bond direction and the common anisotropy axis. A set of Langevin equations was derived from the micromagnetic energy expression and solved numerically. The switching behavior resembles a random walk over the energy barrier arising from the anisotropy of the system. The relaxation time is computed as a function of temperature, applied field, and coupling strength. The temperature dependence of the maximum energy of the fluctuations provides a method of evaluating the energy barrier of reversal. The thermal agitation is shown to reduce the symmetry of the ‘‘fanning’’ reversal mode.

Contributions of parent molecule fixed and excess energies to product energy partitioning in four-center elimination reactions

In four-center elimination reactions such as hydrogen halide elimination from halogenated hydrocarbons the energy barrier is higher than the difference in enthalpy of formation between the parent molecule and its fragments (HX and olefin). This determines that the energy available to products has two origins: the reverse reaction barrier (fixed energy), and the excess energy (energy above the barrier). Both types of energy are partitioned among products following different laws: more or less statistical for excess energy and non-statistical for fixed energy. In a study of CF 3-CH 3 decomposition, we describe a practical method, based on the variation of product energy partitioning with excess energy, to determine the partitioning of the fixed energy among different types of product energy, thus defining the exact nature of the reverse reaction energy barrier. We applied this model to other types of reactions, such as three-center molecular eliminations.