The x-ray coherent scattering in nanosized MnAs crystallites embedded in a GaAs matrix has been detected. The room-temperature interatomic distances along three orthogonal directions of the crystallites are determined. The MnAs nanocrystals are found to exhibit an anisotropically distorted hexagonal structure as compared to unstrained bulk MnAs. Despite the crystallite lattice distortion, the granular GaAs:MnAs material exhibits robust ferromagnetism, with enhanced transition temperature. The

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... served magnetic behavior is consistently explained by a localized double-exchange model of MnAs ferromagnetism, where magnetic order appears for large enough Mn-As-Mn distances, i.e., for weak enough p-d hybridization. Epitaxial MnAs films on GaAs and granular GaAs:MnAs, consisting of MnAs nanocrystals embedded in a GaAs matrix, are promising hybrid ferromagnetsemiconductor materials for information storage, magnetooptical, and spin-electronics applications. 1-3 Bulk MnAs exhibits a complex magnetostructural phase diagram as a function of temperature and pressure. 4-7 Most important, at approximately 313 K, 7 bulk MnAs exhibits a first-order phase transition from a high-temperature paramagnetic orthorhombic MnP-type B31 structure ͑␤-phase͒ to a lowtemperature ferromagnetic hexagonal NiAs-type B8 1 structure ͑␣-phase͒. Upon cooling across the first-order phase transition, an abrupt volume expansion of about 2% occurs, with the nearest-neighbor distance in the basal plane increasing abruptly and the nearest-neighbor distance along the c axis remaining unchanged. The mechanism of ferromagnetism in MnAs is an old matter of debate, which extends to present days. Localized models of MnAs ferromagnetism have been confronted with itinerant models for many years. Although ␣-MnAs shows metallic conductivity and contains itinerant electrons, 8 there is increasing evidence 7,9,10 that ferromagnetic ordering within and in between the hexagonal Mn planes is transmitted by the strongly directional and localized Mn-As-Mn bonds via a double-exchange mechanism, 11,12 where the parallel spin alignment is promoted by an electron transfer between the localized d states of the Mn cations through extended d or p type states of the anionic As. 9 The Mn-As-Mn interaction/distance seems to be the key parameter driving the transition from the paramagnetic to the ferromagnetic state.