Magnetic Mn5Ge3 nanocrystals embedded in crystalline Ge: a magnet/semiconductor hybrid synthesized by ion implantation
© Zhou et al.; licensee Springer. 2012
Received: 14 July 2012
Accepted: 2 September 2012
Published: 25 September 2012
The integration of ferromagnetic Mn5Ge3 with the Ge matrix is promising for spin injection in a silicon-compatible geometry. In this paper, we report the preparation of magnetic Mn5Ge3 nanocrystals embedded inside the Ge matrix by Mn ion implantation at elevated temperature. By X-ray diffraction and transmission electron microscopy, we observe crystalline Mn5Ge3 with variable size depending on the Mn ion fluence. The electronic structure of Mn in Mn5Ge3 nanocrystals is a 3d6 configuration, which is the same as that in bulk Mn5Ge3. A large positive magnetoresistance has been observed at low temperatures. It can be explained by the conductivity inhomogeneity in the magnetic/semiconductor hybrid system.
KeywordsMn5Ge3 Ion implantation Magnetic nanocrystals Magnetoresistance
Due to its compatibility to Si technology, Ge has attracted special attention as a host semiconductor for diluted magnetic impurity atoms. However, due to the low solid solubility of transition metals in Ge, intermetallic compounds (mainly Mn5Ge3) tend to form in the Ge host[1–6]. Mn5Ge3 is a half metallic ferromagnet with a large spin polarization. By first principle calculation, large spin injection efficiency is expected by the integration of Mn5Ge3 within the Ge matrix. Electrical spin injection and detection in Ge have been experimentally demonstrated[8, 9]. Therefore, considerable work has been done to fabricate epitaxial Mn5Ge3 films as well as nanostructures[10–12]. The Curie temperature (TC) of Mn5Ge3 is 296 K, which can be effectively increased by carbon doping. Spiesser et al. reported the epitaxial growth of Mn5Ge3C x films on Ge(111). When x is around 0.6, TC can be as high as 430 K. On the other hand, some unknown nanoscale Mn-rich phases also form under particular conditions during molecular beam epitaxy (MBE) growth[14–19]. Those nanostructures can have a TC much higher than 300 K. Besides MBE, ion implantation has been used to prepare ferromagnetic semiconductors as well as hybrids of ferromagnets embedded in semiconductors[20–24]. The advantages of ion implantation include compatibility with conventional Si-chip technology and lateral patterning. Patterning by ion implantation allows the synthesis of magnetic structures comprising different magnetic phases. By carbon implantation into Mn5Ge3 and Mn5Si3, Sürgers et al. obtained lateral magnetic hybrid structures in the micrometer and submicrometer range. In this contribution, we report the preparation of magnetic Mn5Ge3 nanocrystals embedded inside the Ge matrix by Mn ion implantation at an elevated temperature. We identify the formation of nanocrystalline Mn5Ge3 by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The magnetic, electronic, and magnetotransport properties will be reported for this magnetic/semiconductor hybrid system.
Sample identification, structural, and magnetic parameters
1 × 1015 cm−2
1 × 1016 cm−2
5 × 1016 cm−2
Results and discussion
Mn5Ge3 nanocrystal formation
Figure3b shows the magnetization versus field reversal (M-H) of all samples measured at 5 K. Hysteretic behaviors were observed for samples 1E16 and 5E16. With increasing Mn concentration, the saturation magnetization is increased from 10.1 to 69.2 emu/cm3 (by assuming the implanted depth of 100 nm), and the coercivity is increased from 0.22 to 0.26 T. At 300 K, sample 5E16 only shows field-induced magnetization (see the inset of Figure3b). The saturation magnetization of the sample 5E16 (1E16) is 69.2 (10.1) emu/cm3, corresponding to around 1.5 (1.1) μB/Mn, which is smaller than 2.6 ± 0.5 μB/Mn as reported in the study of Bihler et al., which means that not all of the implanted Mn ions form the ferromagnetic Mn5Ge3 phase.
We also compared the magnetization between the in-plane and out-of-plane directions at 5 K for sample 5E16 (not shown). In contrast to the studies of Bihler et al. and Jain et al., there is no detectable magnetic anisotropy. For the bulk Mn5Ge3, the magnetic easy axis is . The absence of magnetic anisotropy in our samples is due to the random crystallographic orientation of the Mn5Ge3 nanocrystals.
As shown in Figure3b, the hysteresis loop is not square-like. The distribution of coercivity field is due to the size distribution of the nanomagnets, as evidenced by the TEM images, and is also possibly due to the random distribution of the nanomagnet easy axis. According to the Stoner and Wohlfarth model for single-domain magnetic nanoparticles, the maximum coercive field gives the anisotropy field μ0Ha 2 = 0.26 T for sample 5E16. Using the bulk saturation magnetization (M S ) for Mn5Ge3 (1,100 kA/m), one can deduce the anisotropy constant: K2 = μ0Ha 2M S /2 ≈ 1.4 × 105 J/m3, which is smaller than the value reported by Jain et al.. Based on the Néel-Brown model, the volume for a nanomagnet V = 25kBTmax/K2 (kB as the Boltzmann constant), we calculate the average diameter of Ge3Mn5 clusters in sample 5E16 to be approximately 10.8 nm (Tmax = 270 K). The average diameter is in good agreement with the results obtained by TEM. However, the average diameter for sample 1E16 deduced from the ZFC magnetization is as large as 9.5 nm, which is much larger than the value from the TEM observation.
The material parameters of the matrix are chosen to be,. The material parameters of the nanocrystal are as follows: and. There are two free parameters a and b which are the ratios of conductivity and Hall coefficient of the two phases, respectively. Both the conductivity and the Hall coefficient are functions of temperature. The resistance of the system is calculated by FEM where a constant current is applied and the corresponding voltages are measured in the geometry of the van der Pauw method. The calculated curves are presented in Figure5b. The experimental MR curves can be well reproduced by FEM calculations. The a and b values used in the FEM calculations are shown in Figure5c. The MR magnitude is sensitive to the ratio of conductivity of the two constitutes. Beside the magnetoresistance, the samples also show anomalous Hall resistance (i.e., the Hall resistance deviates from a linear behavior), which can be explained by two kinds of carriers with different mobilities. On the other hand, we have to note the rather large discrepancy in the MR magnitude between the experimental and modeled values. In the model, for simplifying, we neglect the anomalous Hall effect in the GeMn constitute, which may induce this discrepancy. Also, in order to account non-monotonic dependence of MR on temperature (see Figure5c), we have to vary parameters a and b accordingly. The decrease of a and b at temperature below 50 K cannot be understood and is the aim for the future work.
We have prepared magnetic Mn5Ge3 nanocrystals embedded inside the Ge matrix by Mn ion implantation into Ge substrates. The crystalline size of Mn5Ge3 can be tuned by varying the Mn fluence. The Mn ions in Mn5Ge3 nanocrystals are in the 3d6 configuration. Large positive magnetoresistance has been observed in the Mn5Ge3/Ge hybrid system. It could be due to the inhomogeneity in samples with constitutes having different transport properties.
The work was supported by the Helmholtz-Gemeinschaft Deutscher Forschungszentren (HGF-VH-NG-713) and by the International Science and Technology Cooperation Program of China (2012DFA51430). Heidemarie Schmidt thanks the financial support from DFG SCHM1663/4-1.
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