Hysteresis in the magneto-transport of Manganese-doped Germanium: evidence for carrier-mediated ferromagnetism

The III-V compound GaMnAs is considered as being the prototype diluted ferromagnetic semiconductor (FMS), exhibiting negative magneto-resistance (MR) and anomalous Hall-effect (AHE) related to carrier-mediated ferromagnetism. However, it would be very desirable to have a group-IV FMS, being compatible with silicon technology. In particular manganese-doped germanium prepared using low-temperature molecular beam epitaxy (LT-MBE) has been proven to be a very promising material [1]. Still, no direct correspondence between transport and magnetization data has been reported yet to date. We believe that the origin of these observations lies in the less effective substitution of Mn at Ge sites, which results in too low a hole concentration, making carrier-mediated ferromagnetism impossible. The hole concentrations realized in Ge:Mn grown by LT-MBE are mostly well below 10¹⁹ cm^-3, which indicates the possible unsuitability of LT-MBE to achieve a large hole concentration in Ge:Mn.
In this contribution, we show that the hole concentration can be increased by two orders of magnitude, from 10¹⁸ to 10²⁰ cm^-3, through Mn-ion implantation into Ge followed by pulsed laser annealing [2]. This non-equilibrium technique allows the preparation of metastable materials with interesting material properties, e.g. diluted ferromagnetic GaMnAs [3]. In Mn-doped Ge with a hole-concentration of around 2.1×10²⁰ cm^-3, we observe that the longitudinal and the Hall resistance exhibit the same hysteresis as the magnetization at temperatures below 10 K. This hysteresis in magneto-transport is usually considered as a direct evidence of carrier-mediated ferromagnetism. We will present a comprehensive correlation between the magnetic, transport and structural properties of Ge:Mn samples with different hole concentrations, as well as a comparison with literature. Note that ion implantation followed by pulsed laser annealing is an established scalable chip technology and may have a significant industry impact.

[1] Y. D. Park et al., Science 295, 651 (2002); M. Jamet et al., Nature Mater. 5, 653 (2006).
[2] S. Zhou et al., Phys. Rev. B 81, 165204 (2010).
[3] D. Bürger et al., Phys. Rev. B 81, 115202 (2010).