Spintronics and Epitaxy

The development of suitable materials for spin-polarized injection into group IV semiconductors is a key step towards the full integration of spintronic devices into semiconductor technology. This approach presents itself as the most promising alternative to Si or Ge based dilute magnetic semiconductors (DMS) whose Curie temperatures (T_C) remain well below 300K.

We have shown that the ferromagnetic intermetallic (FM) alloy Mn₅Ge₃ can be epitaxially synthesized directly on Ge(111) with a low dislocation density (10^{^4}cm^-2, cf. figure 1). The efficiency of spin-polarized current injection by tunneling through the Schottky barrier formed at the interface between a ferromagnetic metal and a semiconductor relies on a very high crystal quality of the heterostructure and an abrupt interface. We have therefore developed different growth techniques (diffusive and non-diffusive) to synthesize by molecular beam epitaxy (MBE) the Mn₅Ge₃/Ge heterostructure. The study of the structural properties by RHEED, DRX, TEM and AFM reveals an excellent crystal quality and an abrupt interface at the atomic scale, making this system suitable for electrical injection.

The limited Curie temperature of Mn5Ge3 (296 K) is a hindrance to its use in devices. We have shown that the incorporation of a small amount of C allows maintaining the ferromagnetic order up to 430 K. The results of our experimental studies show for the first time that C atoms are located in the octahedral sites of the Mn5Ge3 lattice as shown in Figure 2.

By complementary magnetic characterizations (SQUID, VSM, FMR, NMR, MFM...) and micromagnetism studies by OOMMF, we have shown that this material exhibits a moderate uniaxial anisotropy leading to the presence of a critical thickness above which the film structures into ribbon-like magnetic domains with out-of-plane magnetization (figure 3).

We have determined the height and width of the Schottky barrier and shown that these values can be modulated by δ-doping of Ge near the Mn₅Ge₃/Ge interface. Based on these results, we are working with AIST (Tsukuba, Japan) to perform spin-polarized injection into Ge in lateral structures and vertical spin gates.

Antiferromagnetic (AF) materials are magnetic at the atomic scale and non-magnetic at the macroscopic scale. This results in robustness to stray magnetic fields and an absence of leakage fields which, in addition to a fast dynamic range (in the THz range), makes them unique for solving important problems in today's ICT field (information storage, cybersecurity, device operation speed... ).

Among them, the compound Mn5Si3 is particularly interesting because it exhibits a metamagnetic phase transition between two chiral-ordered spin structures below 65K and collinear above (figure 4). Remarkably, its isostructural equivalent Mn₅Si₃C_x is ferromagnetic. The Mn₅Si₃C_x compound thus presents itself as a model structure for identifying and exploiting new transport mechanisms in complex antiferromagnetic (AF) materials.

Diluted semiconductors (DMS) obtained by incorporating a magnetic element in the semiconductor matrix would allow to combine storage and manipulation of information in the current microelectronic processes. We are particularly interested in the growth and magnetic properties of Ge and Mn based DMS.

The first growth steps of Mn on Ge(001) have shown the presence of preferred adsorption sites for Mn atoms that react with Ge to form an alloy (see figure 5) and a very strong surface diffusion, even for temperatures as low as 80°C. These results corroborate the numerous studies reporting the difficulty of obtaining Ge(1-x)Mnx DMS thin films by co-depositing Mn and Ge on Ge(001). The Curie temperatures (T_C) remain limited by the low solubility of Mn in Ge (T_C < 150K) for a maximum concentration of 2-3% Mn in the thin films).

Hybrid organic/inorganic interfaces could pave the way for new chemically designed multifunctional electronic devices, especially in the field of spintronics where, for example, the interfacial spin polarization can be tuned by chemical interactions and surface modifications. In this pioneering work, we have studied the formation of self-assembled monolayers on the surface of Mn₅Ge₃. The first steps have thus shown the feasibility of the functionalization of the Mn₅Ge₃ surface by octanethiol molecules (Figure 6).