The effect of pressure on magnetic properties of LaCoO3 is studied experimentally and theoretically. The pressure dependence of magnetic susceptibility χ of LaCoO3 is obtained by precise measurements of χ as a function of the hydrostatic pressure P up to 2 kbar in the temperature range from 78 K to 300 K. A pronounced magnitude of the pressure effect is found to be negative in sign and strongly temperature dependent. The obtained experimental data are analysed by using a two-level model and DFT+U calculations of the electronic structure of LaCoO3. In particular, the fixed spin moment method was employed to obtain a volume dependence of the total energy difference Δ between the low spin and the intermediate spin states of LaCoO3. Analysis of the obtained experimental χ(P) dependence within the two-level model, as well as our DFT+U calculations, have revealed the anomalous large decrease in the energy difference Δ with increasing of the unit cell volume. This effect, taking into account a thermal expansion, can be responsible for the temperatures dependence of Δ, predicting its vanishing near room temperature.
Electronic structure and magnetic properties of graphite-based systems with intercalated 3d-transition metal atoms (V, Cr, Mn, Fe, Co, Ni) were calculated ab initio using the density functional theory. The presence of different magnetic states depending on the type of inserted M atoms is revealed for hexagonal P6/mmm and
The de Haas-van Alphen (dHvA) effect in the antiferromagnetic compound FeGe2 is studied experimentally. A sharp suppression of the amplitude of the first harmonic of the dHvA oscillations is observed for a number of orbits on the Fermi surface of FeGe2. The change in the conditions for suppression of the amplitude ("spin zeroes") when FeGe2 is doped with cobalt is determined. A method for analyzing the experimental data is proposed in which the g-factors of the conduction electrons are computed on the basis of calculations of the spin-polarized electron structure of the band antiferromagnet taking spin-orbital interactions into account. The exchange enhancement factor for spin paramagnetism in FeGe2 is also calculated.
A detailed theoretical study of the anomalous magnetovolume effect in the exchange-enhanced itinerant paramagnet YCo2 was carried out based on DFT calculations of the electronic structure in an external magnetic field and further complemented with the experimental data on the behavior of the magnetic susceptibility χ under high hydrostatic pressure. The calculations of the magnetic susceptibility and magnetovolume effect dlnχ/dlnV are in reasonable agreement with the experimental data, indicating the proximity of YCo2 to the ferromagnetic instability.
The magnetic properties of Fe(1+y)Te single crystals (y ≃ 0.1 ÷ 0.18) were studied at temperatures 4.2 ÷ 300 K. At an ambient pressure, with decreasing temperature a drastic drop in χ(T) was confirmed at T ≃ 60 ÷ 65 K, which appears to be closely related to the antiferromagnetic (AFM) ordering. It is found that the magnitudes of the anisotropy of magnetic susceptibility Δχ in the AFM phase are close in the studied samples, whereas the sign of the anisotropy apparently depends on the small variations of the excess iron y in Fe(1+y)Te samples. The performed DFT calculations of the electronic structure and magnetic properties for the stoichiometric FeTe compound indicate the presence of frustrated AFM ground states. There are very close energies and magnetic moments for the double stripe configurations, with the AFM axes oriented either on the basal plane or along the [0 0 1] direction. Presumably, both these configurations can be realized in Fe(1+y)Te single crystals, depending on the variations of the excess iron. This can provide different signs of magnetic anisotropy in the AFM phase, presently observed in the Fe(1+y)Te samples. For these types of AFM configuration, the calculations for the FeTe values of Δχ are consistent with our experimental data.
The electronic energy structures and magnetic properties of iron-based compounds with group VI elements (FeTe, BiFeO3, SrFe12O19 and SrCoTiFe10O19) are studied using the density functional theory (DFT) methods. Manifestations of different types of chemical bonds in magnetism of these compounds are studied theoretically. Calculations of electronic structures of these systems were performed using the generalized gradient approximation (GGA) for description of the exchange and correlation effects within DFT. For SrFe12O19 and SrCoTiFe10O19 hexaferrites the GGA+U method was also employed to deal with strongly correlated 3d-electrons. The calculations have revealed distinctive features of electronic structure of the investigated iron-based compounds with strongly correlated 3d-electrons, which can be responsible for their peculiar structural and magnetic properties.
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