A ten-year perspective on dilute magnetic semiconductors and oxides

Journal name:
Nature Materials
Volume:
9,
Pages:
965–974
Year published:
DOI:
doi:10.1038/nmat2898
Published online

Abstract

Over the past ten years, the search for compounds combining the properties of semiconductors and ferromagnets has evolved into an important field of materials science. This endeavour has been fuelled by many demonstrations of remarkable low-temperature functionalities in the ferromagnetic structures (Ga,Mn)As and p-(Cd,Mn)Te, and related compounds, and by the theoretical prediction that magnetically doped, p-type nitride and oxide semiconductors might support ferromagnetism mediated by valence-band holes to above room temperature. Indeed, ferromagnetic signatures persisting at high temperatures have been detected in a number of non-metallic systems, even under conditions in which the presence of spin ordering was not originally anticipated. Here I review recent experimental and theoretical developments, emphasizing that they not only disentangle many controversies and puzzles accumulated over the past decade but also offer new research prospects.

At a glance

Figures

  1. Experimental data for p-type DMS films.
    Figure 1: Experimental data for p-type DMS films.

    a, Temperature dependence of the magnetization in (Ge,Mn)Te with high (circles) and low (triangles) hole concentrations. b, TC as a function of saturation magnetization for annealed (Ga,Mn)As films grown in various molecular beam epitaxy (MBE) systems. TC approaches 200 K for an effective Mn concentration of less than 10%. Figures reproduced with permission from: a, ref. 28, © 2008 AIP; b, ref. 30, © 2008 AIP.

  2. Experimental energies of Mn levels in the gaps of III-V compounds, with respect to the valence-band edges.
    Figure 2: Experimental energies of Mn levels in the gaps of III–V compounds, with respect to the valence-band edges.

    Bars show relative positions of valence band tops; points depict Mn acceptor levels. Reproduced with permission from ref. 56, © 2002 APS.

  3. Predictions of the p-d Zener model compared with experimental data for p-type (III,Mn)V DMSs.
    Figure 3: Predictions of the pd Zener model compared with experimental data for p-type (III,Mn)V DMSs.

    a, computed values of TC for various p-type semiconductors containing 5% Mn and 3.5 × 1020 holes cm−3 (the value for (In,Mn)Sb is taken from ref. 69). Reproduced with permission from ref. 21, © 2000 AAAS. b, Highest reported values for (Ga,Mn)P (ref. 65), (Ga,Mn)As (refs 29, 30), (In,Mn)As (ref. 66), (Ga,Mn)Sb (ref. 67) and (In,Mn)Sb (ref. 68).

  4. Dependence of TC on the concentration of magnetic impurities and density of hole states at the Fermi level for weak and strong coupling.
    Figure 4: Dependence of TC on the concentration of magnetic impurities and density of hole states at the Fermi level for weak and strong coupling.

    Higher values of TC are predicted within the virtual-crystal approximation (VCA) and the molecular-field approximation for strong coupling. However, the region where the holes are localized and do not mediate the spin–spin interaction is wider in the strong-coupling case. Reproduced with permission from ref. 57, © 2008 APS.

  5. Resistive indications of ferromagnetism in p-Zn0.981Mn0.019Te:N and n-Zn0.97Mn0.03O:Al.
    Figure 5: Resistive indications of ferromagnetism in p-Zn0.981Mn0.019Te:N and n-Zn0.97Mn0.03O:Al.

    a, The temperature dependence of the hysteresis widths at low temperatures and the magnetic susceptibility measurements above 2 K indicate that TC = 1.45 ± 0.05 K in p-Zn0.981Mn0.019Te:N with a hole concentration of 1.2 × 1020 cm−3. b, The temperature and field scales are an order of magnitude smaller in n-Zn0.97Mn0.03O:Al with an electron concentration of 1.4 × 1020 cm−3, where TC = 160 ± 20 mK. Solid lines show changes of longitudinal resistivity in the magnetic field, ΔRxx, as measured for decreasing (blue arrows) and increasing (red arrows) the field. Curves obtained at different temperatures are vertically shifted for clarity. The width of the hysteresis loops is seen to increase on lowering the temperature. Figures reproduced with permission from: a, ref. 80, © 2001 APS; b, ref. 81, © 2001 Springer.

  6. Evidence for crystallographic and chemical phase separations in DMSs.
    Figure 6: Evidence for crystallographic and chemical phase separations in DMSs.

    a, Synchrotron X-ray diffraction (main panel) and transmission electron microscopy (inset) results for (Ga,Fe)N, showing the precipitation of hexagonal ε-Fe3N nanocrystals. c.p.s., counts per second; Θ, diffraction angle. Modified from ref. 89, © 2008 APS. b, Element-specific synchrotron radiation microprobe analysis of (Ga,Mn)N showing aggregation of Mn cations. X-ray fluorescence spectra are shown for Mn-poor and Mn-rich regions (low (Mn) and high (Mn), respectively). Red, blue and green in the middle panels correspond to the spatially resolved Mn-Kα, Ga-Kα fluorescence line and inelastic (Compton) scattering signal, respectively. Ga (black) and Mn (red) profiles along the white scan line are shown in the lowest panel, indicating the formation of regions rich in Mn and Ga. Reproduced with permission from ref. 78, © 2005 AIP.

  7. Formation of nanocolumns DMSs by aggregation of transition-metal cations.
    Figure 7: Formation of nanocolumns DMSs by aggregation of transition-metal cations.

    a, Mn-rich nanocolumns in (Ge,Mn) shown by high-resolution transmission electron microscopy (left, plan view showing nanocolumns as dots on the surface) and Mn chemical maps (right, bright regions show Mn atoms substituting Ge in the invisible Ge lattice). b, Monte Carlo simulation of chemical phase separation in (Zn,Cr)Te with seeding to initiate the growth of nanocolumns and control of their diameter by Cr flux; pink points show positions of Cr atoms substituting Zn in the invisible ZnTe lattice. Figures reproduced with permission from: a, ref. 93, © 2006 NPG; b, ref. 95, © 2007 Wiley.

  8. Computed energy change Ed in ZnTe and GaAs.
    Figure 8: Computed energy change Ed in ZnTe and GaAs.

    Energy change (pairing energy) resulting from bringing two Cr impurities to the nearest-neighbour cation positions, as a function of the number of holes in the Cr d5 shell. Reproduced with permission from ref. 103, © 2008 IOP.

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  1. Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, PL-02-668 Warszawa, Poland

    • Tomasz Dietl
  2. Institute of Theoretical Physics, University of Warsaw, PL-00-681 Warszawa, Poland.

    • Tomasz Dietl

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