Dilute magnetic nitride-based semiconductor
- Clustering behaviour in transition metal doped III-nitride
- Magnetic metastability in transition metal-doped GaN and AlN
- Importance of charged state study in DMS: Mn-doped GaN
Clustering behaviour in transition metal doped III-nitride
Figure shows optimized Cr clustering geometries and magnetic structures. The red circles with arrows represent substitutional CrAl,
the green circles Al atoms, and the gray circles N atoms. The Cr-Cr
distances before (in parenthesis) and after relaxation are in
angstroms, and the arrows indicate the directions of the local atomic
spin for the magnetic ground state.
Dilute magnetic semiconductor (DMS) present a unique type of promising materials for the emerging and ever closing spintronics devices. Understanding the origin of the ferromagnetism in the transition metal doped DMS presents an unexpected challenge in modern material sciences. To this end, knowledge of the spatial distribution and the magnetic coupling between magnetic dopants is a prerequisite to understanding the mechanisms of diluted magnetic semiconductors and the current controversy thereof. Traditionally, distribution of the dopants is widely assumed either random or homogeneous. Our extensive large-scale density functional theory calculations show in fact the ions tend to form embedded clusters. See for example Cr:AlN shown in Fig. 1) Such inhomogeneous distributions will dramatically change the ferromagnetism predicted on the basis of the assumption of a homogeneous or random distribution. Our results help to understand many puzzling experimental observations, such as low magnetic moment, lattice shrink, concentration effects, etc. We argue that such clustering behavior could be the rule rather than exception, originating from the attractive interactions among the dopants. Very similar behavior is predicted for Cr:GaN and Mn:AlN and Mn:GaN. More recently, embedded clustering behavior was experimentally characterized in precipitate-free (no secondary phases) Mn:Ge samples.
Magnetic metastability in transition metal-doped GaN and AlN
One
characteristic feature associated with TM-doped III-V semiconductors is
that the host band gap serves as the ‘‘arena’’ for the localized
TM-d-band electrons, in which the Fermi level is typically located.
This, in principle, facilitates the possibility to tune the magnetic
behavior by external perturbations. Results of density-functional
calculations for isolated transition metal (TM = V; Cr; Mn; Fe; Co; Ni
on cation sites) doped GaN demonstrate a novel magnetic metastability
in dilute magnetic semiconductors. In addition to the expected high
spin ground states, there are also metastable low spin states - a
phenomenon that can be explained in simple terms on the basis of the
ligand field theory. The transition between the high spin and low spin
states corresponds to an intraionic transfer of two electrons between
the t2 and e orbitals, accompanied by a spin-flip process.
Left: Relative total energy versus the total magnetic moment per cell for transition metal-doped GaN
Right: Local projected density-of-states of transition metal 3d (red solid line) and N-2p (blue dashed line) for the low spin, high spin, and ground magnetic states
Importance of charged state study in DMS: Mn-doped GaN
We confirm that for both neutral and charged state, the doped Mn atoms tend to form embedded clusters. And as expected, for a given structural configuration, various valence state can be stabilized by varying the Fermi energy. More interestingly, the magetic coupling is correlated with the valence states. For example, for pair-Mn, while for neutral pair-doping, the coupling is ferromagnetic regardless of the distance and orientation of the Mn atoms, the negatively charged states tend to weaken the parallel coupling.
Figure shows the formation energy (under N-rich condition) of the neutral and charged states for various poly-doping MnGa
configurations as calculated in a 96-atom cell. Dotted lines indicate
unstable valence states. Fermi energy corresponds to the top of the
valence band. For Ga-rich condition, the formation enthalpy of GaN
(1.04 eV) has to be substracted.
