We present the first study relating structural parameters of precipitate-free Ge 0:95 Mn 0:05 films to magnetization data. Nanometer-sized clusters -areas with increased Mn content on substitutional lattice sites compared to the host matrix -are detected in transmission electron microscopy analysis. The films show no overall spontaneous magnetization at all down to 2 K. The TEM and magnetization results are interpreted in terms of an assembly of superparamagnetic moments developing in the dense distribution of clusters. Each cluster individually turns ferromagnetic below an ordering temperature which depends on its volume and Mn content. DOI: 10.1103/PhysRevLett.97.237202 PACS numbers: 75.50.Pp, 68.37.Lp, 75.75.+a, 81.15.Hi The development of a novel class of materials combining standard semiconductors with magnetic elements has recently been driven by considerable technological as well as fundamental scientific interest. While the possibility of a seamless combination of magnetic and semiconducting systems using spins as an additional degree of freedom opens stimulating perspectives in the field of electronics [1,2], reports on materials displaying both semiconducting and ferromagnetic properties have induced great theoretical and experimental efforts in the understanding of the underlying physics [3]. Ga(Mn)As today represents one of the best understood ferromagnets. This material is one example of a diluted magnetic semiconductor (DMS), meaning a dispersion of the magnetic elements without affecting the semiconducting properties of the matrix [4]. The realization of DMS with maximized ferromagnetic ordering temperatures T C represents the ultimate objective in this field.Special attention has been given to technologically important group IV semiconductor based magnetic materials, with a prominent position for GeMn. Since the first claim of the realization of a Ge based DMS [5], most publications [5][6][7][8] have concentrated on reporting high T C ranging from 116 [5] to 285 K [6] and on interpreting the observed ferromagnetism in terms of DMS theories [9]. It is only recently that several of the former GeMn reports have been questioned by structural proofs [10] and hints [11] for the formation of intermetallic ferromagnetic compounds through phase separation in single crystals and lowtemperature molecular beam epitaxy (MBE) fabricated films, respectively. Up to now, only Li et al.[11] presentindirect -indications for the realization of precipitate-free GeMn. Considering the current discussion on the magnetic properties of GeMn, a study of the crystal structure, exploring the degree of Mn dispersion that can be reached in Ge, would obviously be beneficial for the field.In this Letter, we present the first study relating structural parameters of precipitate-free Ge 0:95 Mn 0:05 films to magnetization data, providing new insights into the interpretation of the magnetic properties of GeMn. Although the incorporation of Mn does not induce explicit phase separation, nanometer-sized areas with increased Mn cont...

We present a comprehensive study relating the nanostructure of Ge0.95Mn0.05 films to their magnetic properties. The formation of ferromagnetic nanometer sized inclusions in a defect free Ge matrix fabricated by low temperature molecular beam epitaxy is observed down to substrate temperatures TS as low as 70 • C. A combined transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) analysis of the films identifies the inclusions as precipitates of the ferromagnetic compound Mn5Ge3. The volume and amount of these precipitates decreases with decreasing TS. Magnetometry of the films containing precipitates reveals distinct temperature ranges: Between the characteristic ferromagnetic transition temperature of Mn5Ge3 at approximately room temperature and a lower, TS dependent blocking temperature TB the magnetic properties are dominated by superparamagnetism of the Mn5Ge3 precipitates. Below TB, the magnetic signature of ferromagnetic precipitates with blocked magnetic moments is observed. At the lowest temperatures, the films show features characteristic for a metastable state.

Controlled nanoscale self-assembly of magnetic entities in semiconductors opens novel perspectives for the tailoring of magnetic semiconductor films and nanostructures with room temperature functionality. We report that a strongly directional self-assembly in growth direction in Mn-alloyed Ge is due to a stacking of individual Ge(1-x)Mn(x) clusters. The clusters represent the relevant entities for the magnetization of the material. They are formed of a core-shell structure displaying a Mn concentration gradient. While the magnetic moments seem to be carried by the shells of the clusters, their core is magnetically inactive.