A considerable effort is currently devoted to the optimization of ferromagnetic III-V and II-VI semiconductors containing transition metal atoms, such as (Ga,Mn)As. The prospect of spintronic applications generates a search for new magnetic materials. In particular, Geshi et al. [1] predicted that CaAs and CaP are ferromagnetic (FM) compounds. This result is unexpected since in this case FM occurs in compounds that do not contain magnetic atoms, and thus Ca pnictides represent a new class of magnetic materials.

We analyzed the origin of magnetism of CaP and CaAs, as well as in MgP and MgAs. The calculations are based on the local spin density approximation, and the code developed in Ref. [2]. We find that both CaP and CaAs at equilibrium are half-metals with one free hole in the valence band per unit cell, and a full spin polarization of the hole gas. The paramagnetic (PM) phase of both crystals is higher in energy by about 30 meV/cell. In contrast, both MgP and MgAs are not magnetic.

By studying CaAs in a wide range of the lattice constants we show that its spin polarization is due to the spin polarization of the As atoms stemming from the Hund's rule. With the decreasing lattice constant an abrupt suppression of FM occurs. The effect is explained by observing that the kinetic energy of the fully spin-polarized FM free hole gas is higher than that of the unpolarized PM case. The energy difference between these two cases increases with the increasing hole concentration. Thus, the FM-to-PM transition corresponds to the crossover from low-density regime where exchange-correlation effects induce the FM state to the high-density regime where the kinetic energy dominates and stabilizes the PM state. This analysis holds for CaP as well. Similarly, the lack of FM in MgP and MgAs is due to their small equilibrium lattice constants.

The work is supported by grant PBZ-KBN-044/P03/2001

[1] M. Geshi et al, cond-mat. 0402641 (2004).

[2] http://www.pwscf.org

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