White Synchrotron Radiation Topography of Ba1.03Fe11.97O19Ba_{1.03}Fe_{11.97}O_{19} and Ba1.06Fe10.92Mn1.02O19Ba_{1.06}Fe_{10.92}Mn_{1.02}O_{19} Single Crystals

We describe some preliminary structural differences, observed between barium ferrite and man-ganese-barium ferrite single crystal topographs, recorded by white synchrotron radiation (WSR) at the HASYLAB, (DESY/Hamburg) site. The method emphasizes the advantage of WSR topography over the conventional Lang technique for obtaining full topographs of misorientated hexagonal ferrite (2110) crystal sections

The Self-Organized Critical Forest-Fire Model with Trees and Bushes

We present a forest-fire model with trees and bushes. Additional to the usual nearest-neighbour fire spreading both populations can ignite each other at the same lattice site. Simulations in two dimensions show that the bushes with the higher growth rate remain critical while the self-organization of the trees is destroyed by quick bush fires which ignite the tree clusters before they can become large. A mean-field treatment leads to coupled equations for the densities at the stationary point, depending on the ratio $k$ of the growth rates