LAUSR

Abstract The peculiarities of the magnetism of one of the first known and most studied multiferroic compounds remain a challenge for solid-state theory. The antiferromagnetic ordering of PbFe 1 / 2 Nb 1 / 2 O 3 (PFN) occurs at T N ≈ 150 K. On the one hand, this value is much larger than the T N value for most of the other double perovskites. On the other hand, it is significantly lower than theoretical estimates. The temperature dependence of magnetic susceptibility is substantially different from that one expected for conventional antiferromagnets. In order to find the solution of these puzzles, several density functional theory calculations of the magnetic interactions in PFN are reported. The interactions between first-, second-, third- and fourth-neighbor Fe 3 + ions ( S = 5 / 2 ) are calculated. The magnetic response of Fe spins is shown to depend both on the interaction value and the mutual spin arrangement. The magnetic susceptibility of various spin lattices is calculated using the tenth-order high-temperature expansion method and compared with the experiment. A substantial role of disorder for the understanding of the magnetic properties is discussed.