Electronic structure and magnetism in slightly doped SrB6

Abstract

Spin-polarized band calculations for supercells of SrB6, where a La-, In-or Al-impurity or a vacancy is replacing one Sr, are performed within the local spin density approximation. Moderately large cells with 8 formula units (56 atoms) are studied for all dopings and large ones with 27 formula units (189 atoms) for the case of La-doping. The undoped system has a vanishing density-of-states (DOS) at the Fermi energy (EF ), while the additional La-d band makes EF to enter the bands above the gap. An Al (or In) impurity has the opposite effect, with a rigid-band like shift of EF to below the gap. In the former case, the addition of a d-electron makes a local, impurity-like modification of the electronic structure close to the La atom. As has been shown in a previous publication [Phys. Rev. Lett. 85, 186, (2000)], it can lead to a weak ferromagnetic state. This result is shown to persist at even lower doping, by the 189-atom cell calculations, with a moment of the order 0.1 µB per La impurity. An addition of a p-electron, by an In-impurity, makes no similar effect. The DOS at EF and the Stoner factors are in all cases low, and would not suggest ferromagnetism alone. The magnetic state can be understood from a charge transfer and an additional gain in potential energy, as a spin-splitting is imposed. A DOS with a localized impurity band is essential for this effect. Different models with modified localization of the La-band are made in order to show the correlation between charge transfer and the size of the magnetic moment.

High-temperature ferromagnetism of sp electrons in narrow impurity bands: application to CaB6

“…[11], they need not be intrinsically magnetic. Jarlborg [59,60] [64], that their CaB 6 samples were contaminated with Al granules. It therefore appears that Al does enter the bulk during Al flux growth and might also substitute for Ca.…”

Gd3+ and Eu2+ local environment in Ca1–xEuxB6 (0.0001 ≤ x ≤ 0.30) and Ca1–xGdxB6 (0.0001 ≤ x ≤ 0.01)

“…Over the last few years, enormous efforts were devoted, both, theoretically [2][3][4][5][6][7][8] and experimentally [1,[9][10][11][12][13][14][15][16][17] to establish the origin of this WF in Ca 1-x La x B 6 and its relationship with the actual conducting nature of R doped CaB 6 . However, the nature of the WF of the parent compound CaB 6 is still controversial.…”

Gradual transition from insulator to semimetal ofCa1−xEuxB6with increasing Eu concentration

“…The system Ca 1−x R x B 6 (R = rare-earths, specially La) has become the focus of extensive scientific investigations since weak-ferromagnetism (WF) at high-temperature (T c ≈ 600 − 800 K) has been reported in these materials by Young et al [1] Over the last few years, enormous efforts were devoted, both theoretically [2,3,4,5,6,7,8] and experimentally, [1,9,10,11,12,13,14,15,16,17] to establish the origin of the WF in Ca 1−x La x B 6 and its relationship with the actual conducting nature of R doped CaB 6 . However, the nature of the WF of the parent compound CaB 6 is still controversial.…”