Local character of magnetic coupling in ionic solids

Magnetic interactions in ionic solids are studied using parameter-free meth ods designed to provide accurate energy differences associated with quantum stares defining the Heisenberg constant J. For a series of ionic solids in cluding KNiF3, K2NiF4, KCuF3, K2CuF4, and high- T-c parent compound La2CuO4 , the J experimental value is quantitatively reproduced. This result has fu ndamental implications because J values have been calculated from a finite cluster model whereas experiments refer to infinite solids. The present stu dy permits us to firmly establish that in these wide-gap insulators, J is d etermined from strongly local electronic interactions involving two magneti c centers only thus providing an ab initio support to commonly used model H amiltonians. [S0163-1829(99)51510-5].