Magnetic properties of novel FeSe(Te) superconductors

Abstract

A detailed magnetization study for the novel FeSe superconductor is carried out to investigate the behavior of the intrinsic magnetic susceptibility χ in the normal state with temperature and under hydrostatic pressure. The temperature dependencies of χ and its anisotropy ∆χ = χ − χ ⊥ are measured for FeSe single crystals in the temperature range 4.2 − 300 K, and a substantial growth of susceptibility with temperature is revealed. The observed anisotropy ∆χ is very large and comparable with the averaged susceptibility at low temperatures. For a polycrystalline sample of FeSe, a significant pressure effect on χ is determined to be essentially dependent on temperature. Ab initio calculations of the pressure dependent electronic structure and magnetic susceptibility indicate that FeSe is close to magnetic instability with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and especially on the height Z of chalcogen species from the Fe plane. The change of Z under pressure determines a large positive pressure effect on χ which is observed at low temperatures. It is shown that the literature experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with calculated behavior of the density of electronic states at the Fermi level.

Interrelation of superconductivity and magnetism in FeSe1−xTex compounds. Pressure effects

“…The Fermi level E F is situated in the region of a local flat plateau of N (E), where the main contribution to DOS comes from the d-states of iron. Such position of E F implies a weak temperature dependence of the spin susceptibility in FeSe 0.5 Te 0.5 , which is consistent with available experimental data for this compound [16,22,23].…”

“…[16,23]). This confirms the dominating role of the spin contribution to magnetism of FeSe 0.5 Te 0.5 compound, that is, apparently, characteristic for the whole FeSe 1−x Te x system [23,25,26].…”

“…To evaluate these derivatives we used the corresponding values of χ(T → 0 K) from Ref. [23] (Fig. 5b) and the average values of d lnχ/dP from Table II.…”

“…Magnetic properties of FeSe 1−x Te x system were investigated in a number of works [16][17][18][19][20][21][22][23][24], however, data on the magnetic susceptibility in the normal state remain incomplete and quantitatively inconsistent. This is caused not only by a different quality of the samples used, but also by the existence in them of impurities of iron and its secondary magnetic phases which considerably mask their intrinsic magnetic susceptibility and must be carefully taken into account [22].…”

Electronic Structure and Magnetic Properties of FeTe, BiFeO3, SrFe12O19 and SrCoTiFe10O19 Compounds

“…However, recently the new class of iron-based superconductors was discovered. The representatives of this class are iron-based compounds with group VI elements, FeS, FeSe and FeTe, which are distinguished by the simplest crystal structure among iron-based superconductors, and very different magnetic properties [1][2][3][4][5][6]. The characteristic feature of these compounds is interplay of antiferromagnetism (AFM) and superconductivity.…”