Exchange-stiffness constant in cold-worked and nanocrystalline Ni measuredby elastic small-angle neutron scattering

We present a new method for determining the exchange-stiffness constant A o f a ferromagnetic bulk material by field-dependent elastic small-angle neut ron scattering (SANS). In the limit of high applied magnetic field H, for w hich the scattering volume is a single magnetic domain and the magnetizatio n is nearly aligned with the direction of the applied field, a combination of micromagnetics theory with neutron scattering formalism suggests closed- form expressions for the differential scattering cross section as a functio n of the scattering vector and of H. Based on these results it is suggested that the exchange-stiffness constant can be extracted from experimental SA NS data recorded as a function of H. At ambient temperature we have applied this method to polycrystalline cold-worked Ni and nanocrystalline electrod eposited Ni, finding exchange-stiffness constants of (8.2 +/- 0.2)x10(-12) and (7.6 +/- 0.3)x10(-12) J/m, respectively. Measurement at 5 K yields a va lue of (9.2 +/- 0.2)x10(-12) J/m for the nanocrystalline sample, a temperat ure dependence that agrees qualitatively with data in the literature. In ad dition to the value of A, the technique supplies information on the spatial structure of the magnetic anisotropy field. (C) 2000 American Institute of Physics. [S0021- 8979(00)16608-3].