Compositional dependence of ferromagnetism in (Al,Ga,Mn)As magnetic semiconductors

We report on a detailed study of the magnetic, electrical, and structural properties of the quaternary ferromagnetic semiconductor (Al,Ga,Mn)As. We investigate films with Al concentration $y$ varying from 0.05 to 1, with a fixed total Mn density. The ferromagnetic transition temperature $T_c$ decreases with increasing Al concentration, with no ferromagnetism observed at $y=0.5$ and $y=0.75$ for as-grown and annealed films, respectively. Detailed measurements identify three mechanisms giving rise to a suppression of $T_c$ on alloying with Al: an increased tendency for Mn to occupy compensating interstitial sites, an increased stability of interstitials against annealing, and an increased localization of carriers. These studies serve as a test of the validity of theories of ferromagnetism in III-V semiconductors across different chemical compositions and represent a starting point for the development of new GaAs/(Al,Ga)As ferromagnetic heterostructures.

Figure 1

(Color online) (a) Curie temperature obtained from remanence measurements versus Al concentration for as-grown films (triangles) and after annealing at $190°\text{C}$ for 24 h (squares) and 48 h (circles). (b) Maximum changes on annealing in $T_c$ (filled symbols, left axis) and relaxed lattice constant (open symbols, right axis).

Figure 2

Resistivity versus temperature for (Al,Ga,Mn)As films (a) as grown and (b) annealed. The Al concentration $y$ corresponding to each curve is displayed in the figure. The inset of (a) shows $1/\rho$ on a logarithmic scale versus $T^{-1/3}$ for $y=0.2$ and 0.3.

Figure 3

X-ray-diffraction $\omega -2\theta$ scans for the 25-nm-thick (Al,Ga,Mn)As films. The lines are experimental data, the points are fits, and the dashed lines indicate the 004 peak corresponding to the film. The curves are offset in order of increasing Al concentration $y$, indicated by the numbers shown to the right of the figure.

Figure 4

(Color online) Relaxed lattice constants versus Al concentration. The filled symbols are for as-grown films and the open symbols are for annealed films. The dashed lines indicate the expected expansion of the lattice on Al incorporation according to Vegard’s law, assuming a fixed offset due to the incorporation of Mn.

Figure 5

(Color online) Fractions of Mn at substitutional (triangles), interstitial (circles), and random (squares) locations, as obtained from ion channeling measurements, as a function of the Al concentration $y$. The filled symbols are for the as-grown films and the open symbols are for after annealing.

Figure 6

Density of uncompensated acceptors, estimated from ion channeling measurements for as-grown (filled symbols) and annealed (open symbols) thin films, versus Al concentration. The solid line indicates the critical acceptor concentration separating metallic and insulating films, predicted by a simple model which accounts for the valence-band offset in GaAs-AlAs alloys.