Ferromagnetism below the Stoner limit in La-DopedSrB6

Abstract

Spin-polarized band calculations for LaSr 7 B 48 show a weak ferro-magnetic state. This is despite a low density-of-states (DOS) and a low Stoner factor. The reason for the magnetic state is found to be associated with a gain in potential energy in addition to the exchange energy, as a spin-splitting is imposed. An impurity like La DOS is essential for this effect. It makes a correction to the Stoner factor, and provides an explanation of the recently observed weak ferro-magnetism in doped hexaborides.Recent experimental works on doped hexaborides show many surprises [1,2]. Among the unexpected properties is the observation of a very weak ferromagnetic state that persists up to large temperatures when small amounts of Ca are replaced by La in the hexaboride system, La x Ca (1−x) B 6 [3]. This is surprising since the density-of-states at the Fermi energy, N(E F ), and the Stoner factor are expected to be small in this system. Band calculations [4] in the local density approximation (LDA) [5] on SrB 6 and CaB 6 confirm that the DOS is low, with E F falling in a valley of the DOS, and spin-polarized calculations for EuB 6 show a very weak spill-over of the spin-density from the Eu-f spin to the rest of the valence band. Electronic structure calculations based on the local (spin) density approximation, either by spin-polarized calculations or by calculations of the Stoner factorS, are in general able to detect if a system is ferromagnetic or not [6,7,8]. Because the Stoner factor for La x Sr (1−x) B 6 is far below the limit for magnetism, it is tempting to suggest that the observed magnetism is not a band magnetism, but something else, perhaps excitonic [9] or a polarization of the dilute electron-gas [3,10]. However, for Ce it is found that the Stoner factor underestimates the tendency towards magnetism, because the stabilization energy is not only coming from exchange, and the Stoner criterion has to be corrected for a potential energy [11]. Even if this correction is small in Ce, there is nothing in principle that prevents a large correction to the Stoner model in other systems. In this letter we show that La doped SrB 6 is such a system. According to the traditional understanding of ferromagnetism, the exchange energy is the source of a magnetic transition, but here we find that the band structure is such that the exchange splitting triggers off charge transfers that contribute to the total gain in energy via the Coulomb interaction.

CaB6: A New Semiconducting Material for Spin Electronics

“…This explanation requires that the exciton binding energy should be comparable in size to the single-particle band gap. Jarlborg suggested [6] that the magnetism may be conventional itinerant magnetism. Although magnetism in materials which do not have partly filled 3d, 4f, or 5f shells is rare, it is not unprecedented.…”

Electronic structure and magnetism in slightly doped SrB6

“…The 80-site cell calculation of LaSr 7 B 48 using 20 kpoints and the large basis, finds a small moment of 0.10 µ B /cell, which is about 10 mRy/supercell lower in total energy than the paramagnetic solution 10 . If fewer kpoints are used, one finds that the (paramagnetic) DOS and its energy derivative near E F are larger, cf Table 1.…”

“…The effects can be larger in a compound as long as the different sub-bands and the ionicity of the system show favorable conditions. In a recent work we showed that LaSr 7 B 48 is likely to be such a system 10 . Here we extend these calculations to larger unit cells, to different types of dopants, and give a more complete discussion of the results.…”

“…The results from these calculations can be found in ref. 10 . A reduced basis set for the large 270-site calculations includes s and p states for the B-atoms and only s-states for the empty sites in the direct basis.…”

“…Because the Stoner factor for La x Sr (1−x) B 6 is far below the limit for magnetism 10 , it is tempting to suggest that the observed magnetism is not a band magnetism, but something else. Electron-hole interaction for an asymmetric position of E F relative to the gap has been proposed to lead to excitonic magnetism 17 .…”