Orbital driven impurity spin effect on the magnetic order of quasi-3D cupric oxide

Abstract

Density functional calculations are performed to study the magnetic order of the severely distorted square planar cupric oxide (CuO) and local spin disorder in it in the presence of the transition metal impurities M (=Cr, Mn, Fe, Co and Ni). The distortion in the crystal structure, arisen to reduce the band energy by minimizing the covalent interaction, creates two crisscrossing zigzag spin-1/2 chains. From the spin dimer analysis we find that while the spin chain along [Formula: see text] has strong Heisenberg type antiferromagnetic coupling (J ~ 127 meV), along [Formula: see text] it exhibits weak, but robust, ferromagnetic coupling (J ~ 9 meV) mediated by reminiscent p-d covalent interactions. The impurity effect on the magnetic ordering is independent of M and purely orbital driven. If the given spin-state of M is such that the [Formula: see text] orbital is spin-polarized, then the original long-range ordering is maintained. However, if [Formula: see text] orbital is unoccupied, the absence of corresponding covalent interaction breaks the weak ferromagnetic coupling and a spin-flip takes place at the impurity site leading to breakdown of the long range magnetic ordering.

Mechanistic Insights into the Superexchange-Interaction-Driven Negative Thermal Expansion in CuO

“…It is known that the [101̅ ] direction has the strongest magnetic coupling among all the distinctive exchange interaction paths (refer to Figure 2a). 48,49 Therefore, the results here show the strong connection between the magnetic coupling and the NTE direction, consistent with a previous report. 38 Since the crystal structure of CuO is monoclinic (with space group C2/c), changes of the lattice parameters a, b, and c with T do not exactly reflect the strain (here, the thermal effect) of the system.…”

“…Among the directions analyzed, only the distance along [101̅] (Figure d, also refer to Figure a for path illustration) shows a clear increase with decreasing T below ∼150 K (refer to Figure S2 to Figure S5 for other paths inspected). It is known that the [101̅] direction has the strongest magnetic coupling among all the distinctive exchange interaction paths (refer to Figure a). , Therefore, the results here show the strong connection between the magnetic coupling and the NTE direction, consistent with a previous report …”

“…55 Additionally, Hubbard U was included to account for the strong correlation effect in this transition metal oxide. The value of U was chosen to be 5 eV, according to previous work by B. G. Ganga et al 48 Considering the ground state magnetic structure of CuO, the self-consistent calculations were carried out on a 2 × 1 × 2 supercell. A 2 × 4 × 1 k-mesh was used for the Brillouin-zone integration following the Monkhorst−Pack approach.…”

Universality in the electronic structure of 3d transition metal oxides

“…Similarly the conductivity can be large along the xy-plane for the d 9 system. However, at low temperature, these structure will be unstable and further distortion can occur which has been observed earlier in CuO by one of us [61].…”

“…11) or large scale structural transition (ST) to remove the degeneracy at E F which in turn opens a gap. In the literature, ST is observed in ZnMn 2 O 4 (cubic to tetragonal) [60] to break the e g ↑ degeneracy and in CuO (cubic to distorted square-planar) [61] to remove the t 2g ↓ degeneracy.…”