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Formation process and superparamagnetic properties of (Mn,Ga)As nanocrystals in GaAs fabricated by annealing of (Ga,Mn)As layers with low Mn content

Janusz Sadowski, Jaroslaw Z. Domagala, Roland Mathieu, András Kovács, Takeshi Kasama, Rafal E. Dunin-Borkowski, and Tomasz Dietl
Phys. Rev. B 84, 245306 – Published 12 December 2011
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  • INTRODUCTION
  • EXPERIMENTAL DETAILS
  • STRUCTURAL PROPERTIES
  • MAGNETIC PROPERTIES AND DISCUSSION
  • CONCLUSIONS
  • ACKNOWLEDGEMENTS
    • References

    Abstract

    X-ray diffraction, transmission electron microscopy, and magnetization measurements are employed to study the structural and magnetic properties of Mn-rich (Mn,Ga)As nanocrystals embedded in GaAs. These nanocomposites are obtained by moderate-temperature (400 °C) and high-temperature (560 °C and 630 °C) annealing of (Ga,Mn)As layers with Mn concentrations between 0.1% and 2%, grown by molecular beam epitaxy at 270 °C. Decomposition of (Ga,Mn)As is already observed at the lowest annealing temperature of 400 °C for layers with initial Mn content of 1% and 2%. Both cubic and hexagonal (Mn,Ga)As nanocrystals, with similar diameters of 7–10 nm, are observed to coexist in layers with an initial Mn content of 0.5% and 2% after higher-temperature annealing. Measurements of magnetization relaxation in the time span 0.1–10 000 s provide evidence for superparamagnetic properties of the (Mn,Ga)As nanocrystals, as well as for the absence of spin-glass dynamics. These findings point to weak coupling between nanocrystals even in layers with the highest nanocrystal density.

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    • Received 16 February 2011

    DOI:https://doi.org/10.1103/PhysRevB.84.245306

    ©2011 American Physical Society

    Authors & Affiliations

    Janusz Sadowski1,2,*, Jaroslaw Z. Domagala2, Roland Mathieu3, András Kovács4,5, Takeshi Kasama4, Rafal E. Dunin-Borkowski4,5, and Tomasz Dietl2,6

    • 1MAX-IV laboratory, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden
    • 2Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, PL-02-668 Warszawa, Poland
    • 3Department of Engineering Sciences, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden
    • 4Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
    • 5Ernst Ruska-Centre and Peter Grünberg Institute, Research Centre Jülich, 52425 Jülich, Germany
    • 6Faculty of Physics, University of Warsaw, PL-00-681 Warszawa, Poland

    • *janusz.sadowski@maxlab.lu.se

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    Issue

    Vol. 84, Iss. 24 — 15 December 2011

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    Images

    • Figure 1
      Figure 1
      006 reflections in 2θ/ω x-ray diffractometer scans, measured for (a) as-grown and (b) 400 °C-, (c) 560 °C-, and (d) 630 °C-annealed (Ga,Mn)As layers containing 0.1%, 0.3%, 0.5%, 1%, and 2% Mn.Reuse & Permissions
    • Figure 2
      Figure 2
      Evolution of strain in the as-grown and annealed (Ga,Mn)As layers. Annealing temperatures (Ta) were set to 400 °C, 560 °C, and 630 °C. The dashed lines joining the experimental points are visual guides and do not necessarily follow the true values of strain in layers annealed at intermediate temperatures.Reuse & Permissions
    • Figure 3
      Figure 3
      Cross-sectional bright-field TEM images of Ga0.995Mn0.005As layers annealed at (a) 400 °C and (b) 630 °C. (c) Aberration-corrected ADF STEM image of a void, a hexagonal (Mn,Ga)As nanocrystal, and a rhombohedral As nanocrystal in a sample annealed at 560 °C. (d) Aberration-corrected ADF STEM image of a void and a cubic (Mn,Ga)As nanocrystal in a GaAs host. The ADF inner detector semiangle used was 47.4 mrad.Reuse & Permissions
    • Figure 4
      Figure 4
      High-resolution aberration-corrected TEM images of hexagonal and cubic (Mn,Ga)As nanocrystals in Ga0.98Mn0.02As layers annealed at (a) 560 °C and (b) 630 °C. (a) Dashed circles mark the location of the nanocrystals. (b) Dashed circles mark cubic (Mn,Ga)As nanocrystals, while arrows indicate the c-axes of hexagonal crystals that are parallel with {111} orientation of the GaAs host.Reuse & Permissions
    • Figure 5
      Figure 5
      Temperature T dependence of the (a) and (d) FC and (b) and (c) ZFC/FC magnetization M of all layers measured (a) before and (b)–(d) after heat treatment. (d) Temperature dependence of the FC magnetization at higher temperatures for the layers annealed at 630 °C. No significant magnetic signal is observed for the as-grown or annealed layers with 0.1% Mn. In both the as-grown and the annealed cases, M is converted to units of kiloamps per meter by considering the initial volumes of the (Ga,Mn)As layers.Reuse & Permissions
    • Figure 6
      Figure 6
      (a) Temperature (main frame) and time (inset) dependence of the ZFC, FC, and TRM magnetization of the layer with 2% Mn, plotted as M/H and [M − M(t = 0.3 s)]/H, respectively. (b) TRM relaxation recorded at measurement temperature Tm = 15 K for different magnetic fields.Reuse & Permissions
    • Figure 7
      Figure 7
      Time t dependence of the TRM of the layer with 2% Mn, plotted as [M − M(t = 0.3 s)]/H. The sample was cooled rapidly from a reference temperature Tref = 120 K to the measurement temperature Tm in a magnetic field H = 1 Oe. After a waiting time tw, the magnetic field was switched off and the magnetization was recorded as a function of time while keeping the temperature constant. (a) The TRM magnetization was recorded at Tm = 15 K for different waiting times (the inset shows examples of measurements recorded over longer time scales). (b) The TRM magnetization was recorded at different temperatures without a waiting time, i.e., for tw = 0 s.Reuse & Permissions
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