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.

No abstract

The temperature dependence and the effect of pressure P up to 2 kbar on the magnetic susceptibility χ of the tetraborides SmB4 and YbB4 was studied. For the compound CeB4, the electronic structure and magnetic susceptibility were calculated from first principles as a function of the atomic volume. The results show that in the studied tetraborides, rare-earth ions (Ce4+, Sm3+ and Yb2.8+) exhibit different valence states, which determines the specific features of their magnetic properties. In particular, the obtained pressure derivatives of susceptibility dlnχ/dP for cerium, samarium and ytterbium tetraborides are −2, −0.6 and +2.7 (in units of Mbar−1), respectively, which are characteristic for the exchange-enhanced itinerant paramagnetism, Van Vleck ionic paramagnetism with a stable f-shell, and the magnetism of rare-earth ions in the intermediate valence state.

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 structure and a number of thermodynamic characteristics of the ternary RRh4B4 (R = Y, Lu) systems of superconductors in the normal phase are calculated from first principles. The electronic states and interactions responsible for the superconducting and magnetic properties of these systems are analyzed. It is found that the Fermi level in the compounds YRh4B4 and LuRh4B4 is immediately adjacent to a peak in the density of electronic states and about 1 eV above a pseudogap in the electronic spectrum. The existence of a number of groups of quasi-degenerate electronic states with low effective masses near the Fermi level is revealed. These states may be the cause of a significant diamagnetic contribution of the conduction electrons to the magnetic susceptibility and may be responsible for the strong temperature dependence of the susceptibility in the rhodium borides.

Ab initio calculations of the electronic structures are carried out for the novel FeSe1–xTex superconductors to explain the experimentally found anomalous magnetic properties in the normal state. The calculations have shown that FeSe1–xTex systems are close to a magnetic instability with dominating enhanced spin paramagnetism. The magnetic susceptibility is found to increase gradually with the Te content. The temperature dependences of the magnetic susceptibility χ and its anisotropy ∆ χ = χ║ – χ┴ are investigated for FeSe, and a growth of the susceptibility with the temperature is revealed in the temperature range 4.2–300 K. For FeTe, a substantial increase of χ under pressure is found. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume V and especially the height of chalcogen species from the Fe plane. The calculations have explained the experimental data on χ(T) and χ(P) for FeSe and FeTe, respectively.

The magnetic and magnetocaloric properties of NdFe 12−x Mo x compounds