Ab initiostudy ofMF2(M=Mn,

Abstract

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the groundstate structural, electronic, and magnetic properties of the rutile-type compounds M F 2 (M ϭMn, Fe, Co, and Ni͒. All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the ͓001͔, ͓111͔, and ͓100͔ ͑or ͓010͔͒ directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF 2 and CoF 2 is also discussed.

Periodic approach to the electronic structure and magnetic coupling in KCuF3, K2CuF4, and Sr2CuO2Cl2 low‐dimensional magnetic systems

“…Similar results have been previously found for other strongly correlated systems such as NiO, La 2 CuO 4 , and other TM compounds 62, 67, 69, 70. HF tends to overestimate the gap by a factor of ∼ 3 in highly ionic fluorides; this overestimation can be larger in oxides due to the highly correlated nature of these compounds 48, 50, 51, 54–60, 62. LDA and GGA usually predict a too small band‐gap, indicating either a metallic or, in the most favorable cases, a semiconducting behavior 22, 28.…”

“…The relevant magnetic coupling constants have been calculated by means of the supercell approximation 55–59 to generate several magnetic orderings or magnetic phases by fixing the S z value during the SCF procedure. The experimental crystal structures have been considered for all compounds.…”

“…The most important magnetic interactions ( J c in KCuF 3 and J ab in Sr 2 CuO 2 Cl 2 and K 2 CuF 4 ) are underestimated with respect to the experimental value in the UHF approach. However, the UHF describes correctly the effective dimensionality of the systems, providing useful relative values of the set of magnetic coupling constants defining their magnetic structure 9, 10, 52, 53, 55–58, 62. In the case of LDA and PW‐GGA, the magnetic coupling constants are largely overestimated, indicating the difficulties to describe these kinds of magnetic TM compounds.…”

“…An alternative strategy is to use the ab initio unrestricted Hartree–Fock (HF) periodic approach 43–46, which has previously been applied to several large‐gap TM oxides 47–53 and fluorides 9, 10, 54–60 including simple and complex 3‐D, 2‐D, or (quasi)1‐D magnetic systems with different kinds of ions in several structures, bearing different spin moments, symmetrical or strongly Jahn–Teller distorted. A qualitatively correct description of the insulating ground state is attained, and the magnetic dimensionality is well described on purely ab initio grounds.…”

Magnetic interactions and electronic structure of uvarovite and andradite garnets. An ab initio all‐electron simulation with the CRYSTAL06 program

“…In the last 15 years, CRYSTAL has been used to investigate the magnetic properties of a wide variety of TM solids (from NiO 29 to cuprates 30) at both the unrestricted Hartree–Fock (UHF) 31–34 and, more recently, the B3LYP 29, 30, 35–37 level of theory. From this body of work, it can be observed that the exchange coupling constants J i obtained with UHF or B3LYP are in qualitative agreement with experiments; UHF underestimates J i by about 70% as correlation is completely missing, whereas B3LYP, on the contrary, overstimates J i by about 50%, see Refs.…”