Infrared survey of the carrier dynamics in III-V digital ferromagnetic heterostructures

K. S. Burch, E. J. Singley, J. Stephens, R. K. Kawakami, D. D. Awschalom, and D. N. Basov

Phys. Rev. B 71, 125340 – Published 31 March 2005

Abstract

We report on the electromagnetic response of digital ferromagnetic heterostructures (DFH): systems with $δ$ -doped MnAs layers separated by GaAs spacers of variable thickness $(y)$ . The gross features of the infrared conductivity of DFH samples are consistent with the notion that these digital structures are $Ga As ∕ {Ga}_{1 - x} {Mn}_{x} As$ superlattices. This conclusion is supported by a combination of spectral weight analysis and effective medium theory. The optical properties of DFH also provide insights into the evolution of their critical temperature with GaAs spacing. In DFH a low-lying gap materializes in the energy dependent conductivity, which is interpreted as a mobility gap resulting from Anderson localization.

Received 28 April 2004

DOI:https://doi.org/10.1103/PhysRevB.71.125340

Authors & Affiliations

K. S. Burch^1,*, E. J. Singley^1,†, J. Stephens², R. K. Kawakami^2,‡, D. D. Awschalom², and D. N. Basov¹

¹Department of Physics, University of California, San Diego, California 92093-0319, USA
²Center for Spintronics and Quantum Computation, University of California, Santa Barbara, California 93106, USA

^*Electronic address: burch@physics.ucsd.edu
^†Permanent address: Department of Physics, California State University, Hayward, CA 94542.
^‡Permanent address: Department of Physics, University of California, Riverside, CA 92521.

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Vol. 71, Iss. 12 — 15 March 2005

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Images

Figure 1
The raw transmission data for five of the samples in this study. Displayed is the transmission through the DFH samples that has been divided by the transmission through a GaAs substrate $(T_{meas} = T_{DFH} ∕ T_{s})$ at $292 K$ and $5 K$ . Deviations of $T_{meas} from 1$ is an indication of absorption in the DFH film. The left panel compares three DFH samples with increasing spacer thickness. While the dip in the midinfrared $(\approx 2000 {cm}^{- 1})$ grows at low temperatures, the far infrared absorption is dramatically reduced at $5 K$ . This effect is more accurately portrayed in the right panel for the three films with similar thickness. The weak oscillations observed throughout the frequency range in all samples are due to interference within the film.Reuse & Permissions
Figure 2
The real part of the frequency dependent conductivity $[σ_{1} (ω)]$ for all digital ferromagnetic heterostructures studied. Shown here are spectra at room temperature $(292 K)$ , the ferromagnetic transition temperature $(T_{C})$ , and the lowest temperature studied $(5 K)$ . All samples reveal a resonance at $ω \approx 2000 {cm}^{- 1}$ and band tailing at high frequencies due to defects. The free carrier component is reduced with increasing spacer thickness and decreasing temperature, until $T_{C}$ . For all digital samples with a GaAs spacer thickness greater than 10 ML, a new feature emerges: a low lying gap in $σ_{1} (ω)$ developing at low temperature.Reuse & Permissions
Figure 3
Temperature dependence of $σ_{1} (ω)$ for DFH with $y = 10$ ML (bottom panels) and ${Ga}_{.948} {Mn}_{.052} As$ random alloy (top panels). On the left we show data at temperatures from $292 K$ to $T_{C}$ , and on the right $σ_{1} (ω)$ spectra from $T_{C}$ to $5 K$ . A resonance near $2000 {cm}^{- 1}$ which grows as the temperature is lowered can be seen in all of the spectra. Additionally as the temperature is lowered until $T_{C}$ the free carrier part is reduced, and then grows slightly below $T_{C}$ .Reuse & Permissions
Figure 4
$N_{eff}^{meas}$ ratios, as defined in the text, of all DFHs studied versus the EMT prediction [see Eq. (2)] along with the $T_{C}$ ratios (Ref. 13). In the inset we show a schematic representation of the band critical points versus position along the growth axis. $E_{C, V}$ are the conduction (valence) band minima (maxima). Also displayed are the positions of the intragap levels produced by both Mn and As substitution on a Ga site.Reuse & Permissions
Figure 5
The experimental conductivity at $292 K$ of a MnAs thin film is compared with the intrinsic conductivity of $σ_{1}^{el} (ω)$ of the DFH with 15 ML spacing. Conductivity due to MnAs clusters in LT-GaAs has been simulated using the effective medium approximation (Refs. 12, 18. In extracting the conductivity for DFH 15 ML and generating the response of the MnAs clusters, it was assumed that the MnAs diffused 5 ML. Noting the log-log scale, there is a significant difference between the results for the clusters model and the extracted conductivity for a single doped layer in DFH.Reuse & Permissions
Figure 6
The effective conductivity of a doped layer $[σ_{1}^{el} (ω)]$ extracted using the EMT analysis of raw data as described in the text. The top panel shows the result at $292 K$ and in the bottom panel $5 K$ results are plotted. Also shown are $σ_{1} (ω)$ for ${Ga}_{0.948} {Mn}_{0.052} As$ and $σ_{1} (ω) \times 5$ for LT-GaAs.Reuse & Permissions

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