We report density-functional calculations of the ferromagnetic (FM) stabilization energy δ=EFM-EAFM for differently oriented Mn pairs in Ⅲ-V's (GaN, GaP, GaAs) and chalcopyrite (CuGaS2, CuGaSe2, CuGaTe2) semiconductors. Ferromagnetism is found to be the universal ground state (δ＜0) in all cases. The order of FM stability in Ⅲ–V’s is GaN＞GaP＞GaAs, whereas in chalcopyrites it is CuGaS2＞CuGaSe2＞CuGaTe2. Considering both groups, the order is GaN →GaP→GaAs→CuGaS2→CuGaSe2→GaSb≈CuGaTe2. The stronger FM stabilization in Ⅲ-V's is attributed to the stronger covalent coupling between the Mn 3d and the anion p orbitals. In contrast to expectations based on Ruderman-Kittel-(Kasuya)-Yosida, (i)all Mn-Mn pair separations show FM, with no FM to antiferromagnetic oscillations and, (ii) FM is orientationally dependent, with <110> Mn-Mn pairs being the most FM.

The electronic and magnetic properties of Mn doping at either cation sites in the class of Ⅰ-Ⅲ-Ⅵ2 chalcopyrites are studied by first-principles calculation. It is found that Mn doping at the Ⅲ site provides holes and stabilizes the ferromagnetic interaction between neutral Mn defects; the neutral MnoCu also stabilizes the ferromagnetism, although it provides electrons to the conduction band, instead of holes. The ferromagnetic stability is generally weaker when the cation or the anion becomes heavier in these chalcopyrites, i.e., along the sequences CuAlS2→CuGaS2→CuInS2 and CuGaS2→CuGaSe2→CuGaTe2. Interestingly, CuAlO2 in the chalcopyrite structure is predicted to have lower FM energy than CuAlS2 despite its lighter anion and shorter bonds. In general, Ⅲ site substitution gives stabler ferromagnetism than Cu substitution. Thus, the preferred growth conditions are Cu-rich and Ⅲ-poor, which maximize MnⅢ replacement. In n-type samples, when MnⅢ is negatively charged, the antiferromagnetic coupling is preferred. In p-type samples, the ground state of positively charged Mn+Cu is also antiferromagnetism. The main feature of the calculated electronic properties of Mn defect at either Cu or Ⅲ site is explained using a simple picture of dangling bond hybride and crystal-field resonance.

The quest for combining semiconducting with ferromagnetic properties has recently led to the exploration of Mn substitutions not only in binary (GaAs, CdTe), but also in ternary semiconductors such as chalcopyrites ABⅢX2Ⅵ. Here, however, Mn would substitute any of the two metal sites A or B. The site preference of Mn doping in CuMⅢX2Ⅵ chalcopyrite is crucial because it releases different type of carriers: electrons for substitution on the Cu sites, and holes for substitution on the MⅢ sites. Using first-principles calculation we show that Mn prefers the MⅢ site under Cu-rich and Ⅲ-poor conditions, and the Cu site under Ⅲ-rich condition. We establish the chemical potential domains for pure CuAlS2, CuGaS2, CuInS2, CuGaSe2, and CuGaTe2 stability. We show that the solubility of Mn on the MⅢ (Cu) site increases (decreases) as the Fermi level moves toward the conduction-band minimum (n-type conditions). It is further found that domains of chemical stability of all these chalcopyrites may be largely reduced by Mn incorporation.

The nature and origin of ferromagnetism in magnetic semiconductors is investigated by means of highly precise electronic and magnetic property calculations on MnxGe1-x as a function of the location of Mn sites in a large supercell. Surprisingly, the coupling is not always ferromagnetic (FM), even for large Mn-Mn distances. The exchange interaction between Mn ions oscillates as a function of the distance between them and obeys the Ruderman-Kittel-Kasuya-Yosida analytic formula. The estimated Curie temperature is in good agreement with recent experiments, and the estimated effective magnetic moment is about 1.7 µB/Mn, in excellent agreement with the experimental values, (1.4–1.9) µB/Mn.

Ferromagnetism in Ⅰ-Ⅲ-Ⅵ2 chalcopyrite semiconductors is predicted to arise from holes provided by Mn doping in the illustrative case of CuGaSe2. The first-princi plescalculations within the generalized gradientapproximation to density functional theory demonstrate that Mn substitutes for Ga sites in CuGaSe2 with a formation energy that is～0.25 eV lower than that for Cu sites. Ferromagnetic CuGa1-xMnxSe2 is found to be a half-metallic material with a magnetic moment of 4 µB per Mn for x≥0.25, and its estimated Curie temperature is more than 110 K. We suggest that higher Curie temperatures may be achieved for this new class of ferromagnetic semiconductors based on Mn-doped Ⅰ-Ⅲ-Ⅵ2 chalcopyrites by employing other materials with a smaller lattice constant such as CuGaSe2 and CuAlSe2.