Effect of Mn concentration and atomic structure on the magnetic properties of Ge thin films

Abstract

This work reports on the magnetic properties of Ge100−xMnx (x=0–24 at. %) films prepared by cosputtering a Ge+Mn target and submitted to cumulative thermal annealing treatments up to 500 °C. Both as-deposited and annealed films were investigated by means of compositional analysis, Raman scattering spectroscopy, magnetic force microscopy, superconducting quantum interference device magnetometry, and electrical resistivity measurements. All as-deposited films (either pure or containing Mn) exhibit an amorphous structure, which changes to crystalline as the annealing treatments are performed at increasing temperatures. In fact, the magnetic properties of the present Ge100−xMnx films are very sensitive to the Mn content and whether their atomic structure is amorphous or crystalline. More specifically: whereas the amorphous Ge100−xMnx films (with high x) present a characteristic spin glass behavior at low temperature; after crystallization, the films (with moderate Mn contents) are ferromagnetic at room temperature. Moreover, the magnetic behavior of the films scales with their Mn concentration and tends to be more pronounced after crystallization. Finally, the semiconducting behavior of the films, experienced by previous optical studies, was confirmed through electrical measurements, which also indicate the dependence of the resistivity with the atomic composition of the films.

The composition, structure and optical properties of weakly magnetic Co-containing amorphous Si and Ge films

“…As previously observed for Mn-containing samples [16,17] and in accordance with the literature [34,35], the MFM measurements for the Co-free Si and Ge films (both amorphous and annealed up to crystallization temperature) did not exhibit any magnetic contrast. When annealed at high temperatures, the XRD results indicated that non-magnetic phases were present in the films containing Co.…”

“…Thermal treatment at temperatures ≥500 • C induced crystallization in the films, regardless of the Co concentration. Therefore, a well-defined signal was observed in the spectra at approximately 300 cm −1 , which indicated that Ge crystallites were present [15,17]. Based on the Raman results of the entire series of GeCo and SiCo films, crystallization occurred at temperatures ≥900 • C and ≥500 • C, respectively, which was independent of the Co concentration.…”

“…The measurements were performed at room temperature under air in the lift mode by means of a Co/Cr coated tip that was magnetized immediately before use. The MFM tip scanned a line that was approximately 20 m long across the c-quartz substrate, which was partially covered by the film [17]. By adopting this procedure, the MFM tip experienced a signal difference at the bare substratefilm edge, which is proportional to the magnetic response of the probed region.…”

“…In addition, similar to some of our previous work [16,17], we explored the magnetism of these materials using magnetic force microscopy. From a complementary and systematic approach, we investigated the properties of the films as a function of the following variables: (1) the inclusion of different Co concentrations and (2) the existence of different degrees of atomic ordering that resulted from post-deposition thermal treatments.…”

“…On the other hand, the inherent structural metastability of amorphous silicon (a-Si) and germanium (a-Ge) matrices increases the solid solubility of foreign species [6][7][8]. This property is the motivation of many investigations involving amorphous magnetic semiconductors [9][10][11][12][13][14][15][16][17][18]. Due to the amorphous (or disordered) nature of a-Si and a-Ge, a large amount of Mn can be incorporated in a homogeneous distribution [9,10,12,[15][16][17][18], which can eventually lead to magnetic activity [9,12,16,17].…”