Spin Exchange Constants Research Articles

Measurement of the spin-exchange and chemical ionization rate constants in collisions of polarized metastable 23 S 1 helium atoms with 32 S 1/2 sodium atoms

Experimental data on the spin-exchange rate constants for the He(23S1)-Na(32S1/2) system are reported for the first time. Measurements show that the spin-exchange rate constant is Cse = (23 ± 11) × 10−10 cm3 s−1 and the chemical ionization rate constant is Csi = (29 ± 14) × 10−10 cm3 s−1 at a temperature of 420 K. The results are compared with the data calculated from the rate constants.

Characterization of theS=9excited state inFe8Br8by electron paramagnetic resonance

High Frequency electron paramagnetic resonance has been used to observe the magnetic dipole, $\Delta$ M$_s$ = $\pm$ 1, transitions in the $S = 9$ excited state of the single molecule magnet Fe$_8$Br$_8$. A Boltzmann analysis of the measured intensities locates it at 24 $\pm$ 2 K above the $S = 10$ ground state, while the line positions yield its magnetic parameters D = -0.27 K, E = $\pm$0.05 K, and B$_4^0$ = -1.3$\times$ 10$^{-6}$ K. D is thus smaller by 8% and E larger by 7% than for $S = 10$. The anisotropy barrier for $S = 9$ is estimated as 22 K,which is 25% smaller than that for $S = 10$ (29 K). These data also help assign the spin exchange constants(J's) and thus provide a basis for improved electronic structure calculations of Fe$_8$Br$_8$.

Orbital-spin structure and lattice coupling in RTiO3 where R=La, Pr, Nd, and Sm.

The origin of the G-type antiferromagnetism [AFM(G)] and puzzling properties of RTiO3 with R=La are studied. We clarify that the crystal field from La caused by the GdFeO3-type distortion lifts the t(2g) degeneracy at Ti 3d orbitals. The lift stabilizes the AFM(G) with spin-exchange constant in agreement with neutron-scattering results. The orbital-spin structures for R=Pr, Nd, and Sm are also consistent with experiments. We propose that the GdFeO3-type distortion has a universal mechanism of controlling orbital-spin structure competing with the Jahn-Teller (JT) mechanism.

Influence of local molecular motions on the determination of 1H– 1H internuclear distances measured by 2D 1H spin-exchange experiments

Analysis of spin-exchange build-up curves obtained by measurement of 2D 1H CRAMPS spectra of α-glycine was performed to evaluate the rate of 1H– 1H spin-exchange process with respect to the influence of variation in internal molecular motion. Differences in local motions significantly affect spin-exchange constants even in highly rigid organic solids with virtually uniform motion behavior. The polarization transfer between nonequivalent α-protons is described by the spin-exchange constant D=0.77 nm 2 ms −1, while the polarization transfer involving spin exchange between αH and NH 3 + protons is characterized by D=0.24–0.21 nm 2 ms −1. This significant decrease corresponds to rotation of hydrogen-bonded amino groups. Neglecting this variation in local spin-exchange constants the resulting calculated 1H– 1H distance can be overestimated by up to 100%. Complications following from relayed and back polarization transfer involving the nearest spins within one functional group (e.g., CH 2 and/or NH 3 +) and intermolecular spin exchange are discussed. It was shown that 2H quadrupolar splitting determined for selected sites directly correlates with the experimentally observed differences in spin-exchange coefficients. It is also demonstrated that a medium level quantum chemical calculation of molecular dynamics provides relevant data that can be used to estimate differences in molecular motions.

NonlinearσModel for Inhomogeneous Spin Chains

We derive a nonlinear $\ensuremath{\sigma}$ model (NLSM) that represents antiferromagnetic Heisenberg spin chains with inhomogeneous spin magnitudes and inhomogeneous nearest-neighbor exchange constants arrayed in finite periods. The only restriction is that the average spin magnitude on a sublattice is the same as that on another sublattice. The NLSM yields the gapless condition in terms of the spin magnitudes and exchange constants. We apply this condition to several cases, including systems with impurities. The result shows that the spin gap persists, despite impurities, in many cases.

Exact solution of the open Heisenberg chain with two impurities

We propose an integrable model of the spin-1/2 Heisenberg chain coupled to two impurity moments. With the open boundary conditions at the impurity sites, the model can be exactly solved for arbitrary impurity spin and arbitrary exchange constants between the bulk and the impurities. The absence of redundant terms in the Hamiltonian makes the model very reasonable. The Hamiltonian is diagonalized via algebraic Bethe ansatz. It is found that the impurity spins can only be screened (partially for $S>1/2$) for antiferromagnetic coupling between the impurity and the bulk. Otherwise the impurity spins cannot be screened. The residual entropy of the ground state and the Kondo temperature are also derived explicitly based on the Bethe ansatz and the local Fermi-liquid theory.

Exciton magnetic polarons in the semimagnetic alloys Cd1-x-yMnxMgyTe.

The semimagnetic semiconductors ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Mg}}_{\mathit{y}}$Te grown by molecular-beam epitaxy have been studied by photoluminescence and photoluminescence excitation spectroscopy. It is shown that the Zeeman splittings of exciton states depend on the Mn content only and are independent of the Mg content, i.e., the carrier--Mn spin-exchange constants are not affected by the Mg incorporation. Exciton magnetic polarons have been studied using the method of selective excitation of the exciton luminescence. The partial substitution of Cd by Mg leads to a strong increase of the magnetic polaron energy from 12 meV in ${\mathrm{Cd}}_{0.85}$${\mathrm{Mn}}_{0.15}$Te to 28 meV in ${\mathrm{Cd}}_{0.70}$${\mathrm{Mn}}_{0.14}$${\mathrm{Mg}}_{0.16}$Te. This result emphasizes the strong dependence of the polaron energy on the conditions of the primary localization of the excitons. Time-resolved photoluminescence measurements yield polaron formation times of about 100 ps without pronounced differences between the quaternary and ternary alloys. But the gain in polaron energy within a fixed time interval increases with increasing Mg content. The independently controllable variation of magnetic and nonmagnetic properties in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Mg}}_{\mathit{y}}$Te alloys allows us to demonstrate the importance of the primary localization of excitons for the magnetic polaron formation in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Mg}}_{\mathit{y}}$Te as well as in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te alloys.

OPTICAL BANDGAP OF HIGH PURITY SINGLE CRYSTAL EuSe IN HIGH MAGNETIC FIELD UP TO 30T

The optical bandgap of high purity EuSe single crystals were studied in high magnetic fields up to 30T in a wide temperature range from 2 K to 280 K. The optical bandgap increased monotonically with decreasing temperature down to a temperature around 20 K which is far above the Neel temperature TN=4.6 K, and started to derease at lower temperatures down to 2 K. We found the optical bandgap decreased from 1.969 to 1.862 eV with increasing magnetic field from 0 to 30 T at a fixed temperature T=12 K and from 1.950 to 1.870 eV at T=4.2 K. From these results, we obtained the spin exhange interaction constant of 5d band conduction electrons with 4f local spins, JCdf=61 meV and for donor electrons (activation energy=24 mY) JDdf=52 meV, using the value of JCdf-JCdf= 9 meV previously obtained by transport measurements. We also determined the spin exchange constants for trapped electrons distributed in the midgap. These values decreased with increasing activation energies which reflect the spatial extentions of the wave functions.

Theory of Nuclear Pseudo-Ferromagnetic Resonance of Solid 3He

The spin wave theory for the pseudo-ferromagnetic state of solid 3He is developed for the case which includes the nuclear dipole interaction beside the usual exchange and Zeeman interactions. The value of the nuclear magnetic resonance frequency shift, Δν, is calculated from the eigenenergy of the spin wave for the wave vector, k, being equal to zero and we verify that Δν∝cos θ, there θ is the angle between the orientations of the spin and the magnetic field. We also compute the energy of the spin wave for k ≠0. The range of the parameter values of the triple and quadruple spin exchange constants, Jt, KP and KF are determined to make the pseudo-ferromagnetic state metastable for the given experimental value of Hc, the critical magnetic field to the UUDD state, e2, the second coefficient of the inverse high temperature expansion of the specific heat and Δν.

Proton Spin Relaxation in Aqueous Solutions of Paramagnetic Ions. II. Cr+++, Mn++, Ni++, Cu++, and Gd+++

Nuclear magnetic relaxation times for protons in dilute aqueous solutions of chromic, manganous, nickel, cupric, and gadolinium ions were measured in the frequency range 1.9 to 60 Mc/sec. Results were interpreted in terms of Solomon's formulation of electron-nuclear dipole-dipole interaction and Bloembergen's expression for scalar coupling of electron and nuclear spins. In large magnetic fields relaxation times were found to be shorter than those expected on the basis of low field values, suggesting that the effective ion magnetic moments, electron spin relaxation times, and/or electron-nuclear spin exchange constants are field-dependent.