Enhanced electron correlations, local moments, and Curie temperature in strained MnAs nanocrystals embedded in GaAs

We have studied the electronic structure of hexagonal MnAs, as epitaxial continuous film on GaAs(001) and as nanocrystals embedded in GaAs, by $\text{Mn}2p$ core-level photoemission spectroscopy. Configuration-interaction analyses based on a cluster model show that the ground state of the embedded MnAs nanocrystals is dominated by a $d^5$ configuration that maximizes the local Mn moment. Nanoscaling and strain significantly alter the properties of MnAs. Internal strain in the nanocrystals results in reduced $p\text{−}d$ hybridization and enhanced ionic character of the Mn-As bonding interactions. The spatial confinement and reduced $p\text{−}d$ hybridization in the nanocrystals lead to enhanced $d$-electron localization, triggering $d\text{−}d$ electron correlations, and enhancing local Mn moments. These changes in the electronic structure of MnAs have an advantageous effect on the Curie temperature of the nanocrystals, which is measured to be remarkably higher than that of bulk MnAs.

Figure 1

Temperature dependence of the magnetization $M$ measured under low applied field, normalized by the value of the field-cooled magnetization at $T=5\text{K}$ $\left(M_0\right)$, for an epitaxial MnAs film grown on GaAs (gray curves) and for MnAs nanoclusters embedded in GaAs (black curves). The samples were first cooled from 395 K down to 5 K under zero applied field. Then, a low field was applied (30 Oe for the film and 50 Oe for the nanoclusters) and ZFC and FC curves were consecutively measured upon heating and upon cooling, respectively.

Figure 2

(a) and (b) $\text{Mn}2p$ raw photoemission spectra for a MnAs film (gray dots) and for MnAs nanoclusters embedded in GaAs (black dots), respectively. The secondary-electron backgrounds are shown by dashed curves. (c) The same spectra after background subtraction and area normalization. Binding energies are referenced to the Fermi level (the negative sign is omitted).

Figure 3

(Color online) Line shape analysis of the $\text{Mn}2p$ spectra shown in Fig. 2c for (a) MnAs film and for (b) MnAs nanoclusters embedded in GaAs. The vertical black (blue) bars are the calculated unbroadened spectral lines corresponding to elastic photoemission, either main (m) or satellite (s1, s2) contributions. The vertical gray (green) bars are unbroadened spectral lines corresponding to the excitation of bulk $\left({\text{p}}_{\text{b}}\right)$ or interface $\left({\text{p}}_{\text{i}}\right)$ plasmons. The corresponding broadened spectral contributions are shown by dotted black (blue) curves (elastic photoemission) and dashed-dotted gray (green) curves (plasmon losses), superimposed on the respective background contributions. The gray continuous curves represent the theoretical spectra obtained adding up all contributions.

Figure 4

Energy-level diagrams for the initial (before photoemission) and final (after $\text{Mn}2p$ photoemission) states of the ${\text{MnAs}}_6$ cluster, assuming a nominal ${\text{Mn}}^{+3}$ oxidation state.