First, to find the distribution of Mn impurities in a GaAs (InAs) NW, we compare the total energies of NWs with one Mn ion substituting different nonequivalent cation sites in the wire, [4]. We incorporate the Mn ion into two types of GaAs (InAs) wires: wurtzite (WZ) NWs grown along the <0001> direction and zinc blende (ZB) NWs along the <111> axis. Segregation energy for an impurity in a given site of the wire is the energy difference between the NW with the impurity in this site and NW, in which the dopant occupies the site exactly in the center. The most important result of the study of segregation energy is that while in WZ NWs a nearly homogeneous distribution of Mn ions can be expected, in ZB NWs the Mn ions are trapped at the lateral surfaces of the wires.

 The comparison of the energies of the NWs with two Mn ions with parallel (ferromagnetic) and antiparallel (antiferromagnetic) spin alignment has shown that the ferromagnetism can be expected in III-V NWs only of WZ structure. In III-V NWs with ZB structure the ferromagnetic coupling is considerably suppressed. Moreover, in contrast to the (Ga,Mn)As layers but similarly to the situation observed in (Ga,Mn)As nanocrystals [5,6], this Mn-Mn interaction is a short-range one.When two Mn atoms in the wire are separated by more than one bridging As atom, the energy difference between FM and AFM phases decreases rapidly.

 The research leading to these results has received funding from the European Community's Seventh Framework Programme [FP7/2007-2013] under grant agreement n° 215368 and from the project of Polish Ministry of Science and Higher Education. Calculations were carried out at the Academic Computer Center in Gdańsk.

[1] J. Sadowski et al., Nano Lett 7, 2724 (2007).

[2] A. Rudolph  et al., Nano Lett. 9, 3860 (2009).

[3] M. Hilse  et al., Appl. Phys. Lett. 95, 133126 (2009).

[4] M. Galicka  et al., Nano Lett. 11, 3319 (2011).

[5] S. Sapra  et al., Nano Lett.  2, 605 (2002).

[6] H. Xiangyang  et al.,  Phys. Rev. Lett. 94, 236801 (2005).

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