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.

Extended X-ray absorption fine structure study of mixed-ligand copper(II) complexes having analogous structures

We study the relationship between the circular polarization of photoluminescence and the magnetic-fieldinduced spin polarization of the recombining charge carriers in bulk Si and Ge/Si quantum dots. First, we quantitatively compare experimental results on the degree of circular polarization of photons resulting from phonon-assisted radiative transitions in intrinsic and doped bulk Si with calculations which we adapt from recently predicted spin-dependent phonon-assisted transition probabilities in Si. The excellent agreement of our experiments and calculations quantitatively verifies these spin-dependent transition probabilities and extends their validity to weak magnetic fields. Such magnetic fields can induce a luminescence polarization of up to 3%/T. We then investigate phononless transitions in Ge/Si quantum dots as well as in degenerately doped Si. Our experiments systematically show that the sign of the degree of circular polarization of luminescence resulting from phononless transitions is opposite to the one associated with phonon-assisted transitions in Si and with phononless transitions in direct-band-gap semiconductors. This observation implies qualitatively different spin-dependent selection rules for phononless transitions, which seem to be related to the confined character of the electron wave function. Silicon is an attractive materials platform for spin-based information-processing devices [1] due to properties favoring long spin lifetimes such as a weak hyperfine coupling [2], low spin-orbit coupling, and the absence of piezoelectricity [3,4]. For direct-band-gap semiconductors, the optical orientation of charge carrier spins by an interaction with circularly polarized light represents an important tool for the study of carrier spins [5,6]. For indirect-band-gap group IV materials, this concept has triggered recent work, highlighting, for example, the accessibility of optical spin orientation via the direct band gap in Ge [7,8]. In bulk Si, however, optical transitions relevant for spin orientation experiments involve phonon-assisted transitions across the indirect band gap. A quantitative and contact-free optical spin detection and analysis of spin dynamics in this material will depend on the knowledge of spin-dependent optical selection rules for the involved phonon-assisted transitions. The foundations of such selection rules have been discussed only recently [9,10], providing a theoretical framework. Moreover, the spin-dependent mechanisms governing phononless transitions in Si-based quantum-confined structures, which have been discussed in terms of enhanced optical properties compared to bulk Si [11,12], have yet to be established.In this contribution, we present a study on spin-dependent transition probabilities for radiative recombinations of photoexcited carriers in bulk Si and quantum-confined Ge/Si structures in photoluminescence (PL) experiments. The degree of spin polarization (DSP) of the photoexcited carriers is adjusted through static magnetic fields. To quantitatively connect the...

We present evidence that electrical transport studies of epitaxial p-type GeMn thin films fabricated on highresistivity Ge substrates are severely influenced by parallel conduction through the substrate, related to the large intrinsic conductivity of Ge due to its small band gap. Anomalous Hall measurements and large magnetoresistance effects are completely understood by taking a dominating substrate contribution as well as the measurement geometry into account. It is shown that substrate conduction persists also for well-conducting, degenerate, p-type thin films, giving rise to an effective two-layer conduction scheme. Using n-type Ge substrates, parallel conduction through the substrate can be reduced for the p-type epilayers, as a consequence of the emerging pn-interface junction. GeMn thin films fabricated on these substrates exhibit a negligible magnetoresistance effect. Our study underlines the importance of a thorough characterization and understanding of possible substrate contributions for electrical transport studies of GeMn thin films.

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We present an analysis of the electronic confinement properties of self-assembled islands forming via silicon and germanium co-deposition in molecular beam epitaxy. This approach allows the fabrication of laterally self-ordered three dimensional islands in the Stranski-Krastanow growth mode. Using a systematic structural analysis, we derive a realistic fit-parameter free island model for band structure simulations. A comparison between these band structure simulations and photoluminescence spectroscopy shows that such islands have a significant three dimensional spatial electron-hole wave function overlap. In addition, we show that this spatial wave function overlap overcompensates a weak wave function spreading in k-space.