Microscopic analysis of insulating magnetism of La4Ba2Cu2O10 and Nd4Ba2Cu2O10

The rare-earth ions La and Nd evidently play a crucial role in the spin ordering of the Cu(2+) moments in (RE)(4)Ba2Cu2O10 ("RE422"), because La422 is a ferromagnetic (FM) insulator (a rare occurrence anyway) while Nd422 is antiferromagnetic (AF). Here we introduce explicit Wannier function (the "spin orbital") for the problem that illustrates the unusually strong effect of covalency and extends to (and beyond) the rare-earth ions. Material constants t, U, and (interatomic) J are calculated (not fitted) directly from the Wannier functions and show that the FM interatomic direct exchange can overwhelm the usual AF superexchange. Computationally, La422 is compressed to the volume of Nd422 to demonstrate that volume difference is not the determining factor; rather it is local "chemistry" on the rare-earth ion. This viewpoint is couched within a more general approach to the magnetic insulator problem. (C) 2003 Elsevier Science (USA). All rights reserved.