Structural and magnetic characteristics of MnAs nanoclusters embedded in Be-doped GaAs

Abstract

We describe a systematic study of the synthesis, microstructure and magnetization of hybrid ferromagnet-semiconductor nanomaterials comprised of MnAs nanoclusters embedded in a p-doped GaAs matrix. These samples are created during the in situ annealing of Be-doped (Ga,Mn)As heterostructures grown by molecular beam epitaxy. Transmission electron microscopy and magnetometry studies reveal two distinct classes of nanoclustered samples whose structural and magnetic properties depend on the Mn content of the initial (Ga,Mn)As layer. For Mn content in the range 5% − 7.5%, annealing creates a superparamagnetic material with a uniform distribution of small clusters (diameter ∼ 6 nm) and with a low blocking temperature (T B ∼ 10 K). While transmission electron microscopy cannot definitively identify the composition and crystalline phase of these small clusters, our experimental data suggest that they may be comprised of either zinc-blende MnAs or Mn-rich regions of (Ga,Mn)As. At higher Mn content ( 8%), we find that annealing results in an inhomogeneous distribution of both small clusters as well as much larger NiAs-phase MnAs clusters (diameter ∼ 25 nm). These samples also exhibit supermagnetism, albeit with substantially larger magnetic moments and coercive fields, and blocking temperatures well above room temperature.

Structural, electronic, and magnetic properties of single MnAs nanoclusters in GaAs

“…These values agree with those found with transmission electron microscopy (TEM) studies discussed in Ref. 9. The histograms show that the size of the type I sample clusters is smaller than 8 nm, whereas in the type II sample clusters with a diameter larger than 20 nm were found.…”

“…This corresponds to the observations in Ref. 9, where the measured blocking temperature for material with on average smaller clusters is T B ¼ 10 K and that for material with on average larger clusters is above room-temperature. We chose the experimental temperature of T ¼ 77 K in the X-STM measurements to verify the different magnetic behavior for the smaller and larger nanoclusters.…”

“…An alternative approach is the use of nm-sized ferromagnetic clusters in a semiconductor host, e.g., MnAs nanoclusters in GaAs. [6][7][8] This material has recently been demonstrated to be ferromagnetic well above room-temperature 9 and the MnAs clusters have been shown to exhibit metallic behavior. 10 In this work, the aim is to investigate the MnAs nanoclusters in detail with crosssectional scanning tunneling microscopy (X-STM) and confirm their structural, metallic, and magnetic properties at the nanoscale.…”

“…9, depending on the Mn concentration, the samples exhibit different magnetic behavior that can be linked to the size of the clusters. For the type I sample with a low Mn concentration of x 7.5%, the cluster diameters are small, $6 nm, and have a zinc-blende crystal phase like the surrounding GaAs.…”

“…These concentrations agree with the initial growth parameters for the two types of samples. 9 The MnAs clusters give rise to both bright and dark contrasts with respect to the GaAs lattice. Because the metallic character corresponds to a good electrical conductivity, the MnAs appears brighter than the GaAs semiconductor in X-STM measurements.…”