Anisotropy of hexagonal ferrites with M,W and Y structures containing Fe3+ and Fe2+ as magnetic ions

The ferrimagnetic saturation moment and hexagonal anisotropy constant K₁ have been measured at 4K on a Zn₂Y single crystal and on polycrystalline BaFe²⁺₂W and SrFe²⁺₂W samples. The moment of Fe₂W is in agreement with a collinear spin coupling and with the known site occupation for the Fe²⁺ ions. The moment of Zn₂Y is 9% lower than the value for a collinear configuration.The uniaxial anisotropy of Fe²⁺ in hexagonal ferrites is discussed and compared with that of Co²⁺. No noticeable Fe²⁺ anisotropy is found in Fe₂W in contrast to LaM = LaFe²⁺Fe³⁺₁₁O₁₉, in which the Fe²⁺ anisotropy is strong. The difference is attributed to the symmetry difference of the sites occupied by the Fe²⁺ ions in both compounds. The current theory does not satisfactorily explain the anisotropy and quadrupole splitting of Fe²⁺ in LaM. From this it is concluded that admixing of ⁵E states and (or) the influence of a dynamical Jahn-Teller effect cannot be neglected.The dipole-dipole anisotropy is computed for the M, W and Y structure and some deviation from the literature data is found. Using these results, a mean anisotropy of 1.3 to 2.3 cm^?1 per Fe³⁺ ion is found for the three structures.     

Backward differentiation type formulas for Volterra integral equations of the second kind

Summary Numerical integration formulas are discussed which are obtained by differentiation of the Volterra integral equation and by applying backward differentiation formulas to the resulting integro-differential equation. In particular, the stability of the method is investigated for a class of convolution kernels. The accuracy and stability behaviour of the method proposed in this paper is compared with that of (i) a block-implicit Runge-Kutta scheme, and (ii) the scheme obtained by applying directly a quadrature rule which is reducible to the backward differentiation formulas. The present method is particularly advantageous in the case of stiff Volterra integral equations.     

Electronic surface properties of uhv-cleaved III–V compounds

Combined CPD and photoemission measurements were performed on uhv-cleaved surfaces of the III–V compounds InAs, GaSb, GaAs and GaP with moderate p-type and n-type dopings. Except for n-type GaP these materials show practically no band bending. N-type GaP exhibits surface Fermi level stabilization at 0.55 eV below the conduction band edge. This is ascribed to an intrinsic empty surface state band in the forbidden zone. On the basis of our experiments together with available data from literature we propose an empirical model for the (110) plane of III–V compounds containing In or Ga as metal and Sb, As or P as non-metal atoms from which the lower edge of the empty surface state band can be predicted. The model indicates that for any of these compounds except for GaP no empty surface state band exist in the band gap on the (110) surface.     

13C?1H spin–spin couplings in cyclic α,β-unsaturated enones and their protonated derivatives. Measurements and quantum chemical calculations

Direct and long-range ¹³C? ¹H coupling constants are presented for the enone moieties of cyclopent-2-enone, cyclohex-2-enone and their protonated derivatives. A correlation is given between the experimental values and the results of quantum chemical calculations (CNDO/2, INDO). The existing calculations are modified in order to improve the relationship between the calculated and experimental results.     

Monte Carlo calculation of angular transmission functions for small angle scattering

We define an angular transmission function η in the center of mass system. The convolution of the differential cross section σ with η yields the signal in the laboratory system. For the case of elastic small angle scattering by spherically symmetric potentials we calculate η by a Monte Carlo method. Random positions are taken in the beam defining collimators, resulting in a trajectory with a deflection angle at the scattering centre. These deflection angles are transformed to the c.m. system with the small angle tranformation formulae. From the distribution we calculate η as a histogram and the central moments of η. The function η depends on the velocity ratio and on the mass ratio of the scattering partners. We store the results in such a way that the central moments can be calculated afterwards for all mass and velocity ratios. By using the central moments the convolution integral can be reduced to a simple weighted sum of σ-values at different scattering angles. The r.m.s. deviations of the central moments scale with N^{$\text{1}\text{2}$}, with N the number of Monte Carlo trajectories. A typical deviation is 1% in the second order moment for N = 2 × 10⁴, increasing slightly with increasing order of the moments. This method of calculation gives a large degree of freedom for optimisation of the collimation geometry. The use of an angular transmission function defined in the center of mass system gives a good insight in the experimental reflection of the physical events. As an example we apply the method to the case of small angle scattering of Ar as a primary beam by Kr as a secondary beam and the inverse configuration of Kr by Ar.     

A low noise heterodyne receiver for astronomical observations operating around 0.63 mm wavelength

A heterodyne receiver is described in which an InSb hot electron bolometer is used as a mixer. The local oscillator power is obtained by doubling the frequency of a backward wave oscillator. The receiver operates between 460 and 500 GHz (0.65–0.6 mm). Noise temperatures amount typically to 1000 K.     

An improved power supply for the magnet in a.c. modulated Zeeman atomic absorption spectrometry

This note describes a simple power supply for a magnet to be used in a.c. ZAA spectrometry. The modified sine wave shape of the 50 Hz 10 kG magnetic field allows a 0.5 ms period at zero field to perform the zero field intensity measurement. At lower maximum field strength, the zero field period can be extended.     

Deviations from the critical state model observed in niobium

The values of the critical current density of niobium slabs, measured with the help of trapezoidal ac magnetic fields are not in agreement with the critical state model. Extensions of this model are suggested to explain the observed deviation.     

The microwave spectrum of lactic acid

The microwave spectrum of lactic acid has been identified with aid of double resonance techniques. Satellite spectra have been tentatively assigned to the two CC torsional modes. From deuterium substitution into the two hydroxyl groups the conformation was determined: the molecule shows a hydrogen bond from the α-hydroxyl group to the carbonyl oxygen atom. The c coordinate of the hydrogen atom involved has the appalling imaginary value 0.31i, thus once more casting doubt on the reliability of the substitution method for accurate structure determination.