Heisenberg exchange in the magnetic monoxides

Abstract

The superexchange intertacion in transition-metal oxides, proposed initially by Anderson in 1950, is treated using contemporary tight-binding theory and existing parameters. We find also a direct exchange for nearest-neighbor metal ions, larger by a factor of order five than the superexchange. This direct exchange arises from V ddm coupling, rather than overlap of atomic charge densities, a small overlap exchange contribution which we also estimate. For FeO and CoO there is also an important negative contribution, related to Stoner ferromagnetism, from the partially filled minority-spin band which broadens when ionic spins are aligned. The corresponding J 1 and J 2 parameters are calculated for MnO, FeO, CoO, and NiO. They give good accounts of the Néel and the Curie-Weiss temperatures, show appropriate trends, and give a reasonable account of their volume dependences. For MnO the predicted value for the magnetic susceptibility at the Néel temperature and the crystal distortion arising from the antiferromagnetic transition were reasonably well given. Application to CuO 2 planes in the cuprates gives J=1220 o K, compared to an experimental 1500 o K, and for LiCrO 2 gives J 1 =4 50 o K compared to an experimental 230 o K.

Microemulsion Synthesis of Copper Oxide Nanorod-Like Structures

“…The structure of CuO is certainly an exception in the periodic table [18]: all neighbouring oxides (from MnO to NiO), in fact, show the cubic rocksalt structure and are correlated antiferromagnetic (AFM) insulators [19][20][21][22]. Cupric oxide, on the contrary, is monoclinic, ferroelectric [18] and recent studies seem to indicate it possesses at least three different magnetic polymorphs [23,24].…”

Verification of Anderson Superexchange in MnO via Magnetic Pair Distribution Function Analysis andab initioTheory

“…For example, various tight-binding models [2][3][4] and many important implementations of density functional theory (DFT) [5][6][7][8][9][10] that include the effects of strong electron correlations have been tested against MnO. Modern applications of these ab initio methods can successfully determine gross features of MnO, including the average atomic structure, magnetic ordering temperature, and local magnetic moment size.…”

Tight-binding theory of manganese and iron oxides

“…In Appendix A we describe the effects of electron correlations, motivated by the finding in Ref. 2 that simple one-electron theory, or "LDA+U", inevitably underestimates Heisenberg exchange by a factor of two. We want to be sure that such an error was not made here.…”

“…The simplest tight-binding description of these oxides is based only upon the oxygen atomic p states and the s and d states of manganese, the states we used in the treatment of Heisenberg exchange in the transition-metal monoxides 2 . The energies of these atomic states can be taken initially as the free-atom Hartree-Fock term values, given for example in our Elementary Electronic Structure 1 , and a good approximation to the removal energy of an electron from the corresponding state in the neutral atom For manganese the Hatree-Fock values were calculated for equal occupation of spin-up and spin-down d states, whereas a parallel alignment of spins is appropriate, so these were corrected to give majority-spin and minority-spin levels using an exchange coupling 1 77 eV.…”