Clustering in a Precipitate-Free GeMn Magnetic Semiconductor

Abstract

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...

Magnetic and structural properties ofGexMn1−xfilms: Precipitation of intermetallic nanomagnets

“…For temperatures below 18 K, the precipitate-free T S = 60 • C sample exhibits magnetic signatures of a transition into a metastable state, which we proposed 16 to be a consequence of the formation of coherent clusters. These magnetic signatures are observed for samples with T S > 60 • C in the same temperature range.…”

“…A thorough analysis of the precipitatefree T S = 60 • C sample reveals that this behaviour can be interpreted as a magnetic signature of the coherent clusters. 16 The real part of the AC susceptibility of the T S = 85 • C sample presented in Fig. 4 shows two well separated peaks, one at low temperatures, and one which coincides with the characteristic temperature T B .…”

Inhomogeneous Cr distribution and superparamagnetic behavior in magnetic semiconductor (Zn, Cr)Te

“…It has been observed that the ferromagnetism with high transition temperatures originates from the formation of nano-scale regions containing magnetic elements in high contents [2][3][4]. In our previous publication, we reported that the ferromagnetic properties of Zn 1-…”

Comparison of the magnetic properties of GeMn thin films through Mn L-edge x-ray absorption

“…At T S = 60°C, thin films consist solely of selfassembled Mn-rich Ge 1−x Mn x nanoclusters embedded in a Ge matrix with diamond-type lattice. 4 Increasing the fabrication temperature beyond 60°C additionally leads to the precipitation of nanometer-sized inclusions of the intermetallic compound Mn 5 Ge 3 in the Ge matrix. 8 This is shown in trast corresponding to self-assembled Ge 1−x Mn x nanoclusters.…”

“…1-3 The latter was observed in epitaxially fabricated GeMn thin films, where specific epitaxy conditions far from thermodynamic equilibrium lead to an inhomogeneous distribution of Mn in the Ge host in the form of self-assembled, nanometer-sized, Mn-rich regions coherently embedded in the Ge-rich host matrix. 1,4 In addition to such thermodynamically metastable Ge 1−x Mn x nanoclusters, a small number of Mn x Ge y intermetallic compounds are known, like, for example, Mn 5 Ge 3 . Mn 5 Ge 3 is a magnetically hard compound 5 with a Curie temperature near room temperature 6 and a hexagonal lattice structure.…”