We investigate the microscopic origin of the ferromagnetic and antiferromagnetic spin exchange couplings in the quasi one-dimensional cobalt compound Ca3Co2O6. In particular, we establish a local model which stabilizes a ferromagnetic alignment of the S = 2 spins on the cobalt sites with trigonal prismatic symmetry, for a sufficiently strong Hund's rule coupling on the cobalt ions. The exchange is mediated through a S = 0 cobalt ion at the octahedral sites of the chain structure. We present a strong coupling evaluation of the Heisenberg coupling between the S = 2 Co spins on a separate chain. The chains are coupled antiferromagnetically through super-superexchange via short O-O bonds.Recently there has been renewed interest in systems exhibiting magnetization steps. In classical systems such as CsCoBr 3 one single plateau is typically observed in the magnetization versus field curve at one third of the magnetization at saturation.1 This phenomenon attracted considerable attention, and Oshikawa, Yamanaka and Affleck demonstrated that Heisenberg antiferromagnetic chains exhibit such magnetization plateaus when embedded in a magnetic field.2 These steps are expected when N c (S − m) is an integer, where N c is the number of sites in the magnetic unit cell, S the spin quantum number, and m the average magnetization per spin, which we shall refer to as the OYA criterion. The steps can be stable when chains are coupled, for instance in a ladder geometry. In that case the magnetic frustration is an important ingredient to their stability.3 Plateaus according to the OYA criterion are also anticipated for general configurations, provided gapless excitations do not destabilize them.4 Indeed several systems exhibiting magnetization steps are now known;5,6 they all obey the OYA criterion, they are usually far from exhausting all the possible m values, they all are frustrated systems, and they all can be described by an antiferromagnetic Heisenberg model. Related behavior has been recently found in other systems. For example, up to five plateaus in the magnetization vs. field curve have been observed in Ca 3 Co 2 O 6 at low temperature 7,8,9 . However there is to date no microscopic explanation to this phenomenon, even though the location of the plateaus is in agreement with the OYA criterion.Ca 3 Co 2 O 6 belongs to the wide family of compounds A' 3 ABO 6 , and its structure belongs to the space group R3c. It consists of infinite chains formed by alternating face sharing AO 6 trigonal prisms and BO 6 octahedra -where Co atoms occupy both A and B sites. Each chain is surrounded by six chains separated by Ca atoms. As a result a Co ion has two neighboring Co ions on the same chain, at a distance of 2.59Å, and twelve Co neighbors on the neighboring chains at distances 7.53Å (cf. Fig. 3).10 Concerning the magnetic structure, the experiment points toward a ferromagnetic ordering of the magnetic Co ions along the chains, together with antiferromagnetic correlations in the buckling a-b plane.7 The transition into the ordered state...
The results of electronic structure calculations for the one-dimensional magnetic chain compound Ca 3 Co 2 O 6 are presented. The calculations are based on density functional theory and the local density approximation and used the augmented spherical wave (ASW) method. Our results allow for deeper understanding of recent experimental findings. In particular, alternation of Co 3d low-and high-spin states along the characteristic chains is related to differences in the oxygen coordination at the inequivalent cobalt sites. Strong hybridization of the d states with the O 2p states lays ground for polarization of the latter and the formation of extended localized magnetic moments centered at the high-spin sites. In contrast, strong metal-metal overlap along the chains gives rise to intrachain ferromagnetic exchange coupling of the extended moments via the d 3z 2 −r 2 orbitals of the low-spin cobalt atoms.
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